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R&D Review of Toyota CRDL, Vol.51 No.1 (2020) 1-30

Special Review
Review

Affordable High-performance Ti/TiB Metal Matrix Composites Tailoredfor Automobile Engine Components: Materials Design, Manufacturing
Process, and Remaining Issues
Takashi Saito
Report received on Mar. 20, 2020

In the autumn of 1998, Toyota Motor Corporation launched the world's firstmass-produced car that adopted a titanium-based composite material for the engine valves. First, this article
describes the development of an ultralow-cost manufacturing process for titanium parts, which began at
Toyota Central R&D Labs., Inc. in the mid-1980s, and the invention of high-performance titanium-based
composite materials using this process. This article then describes the process by which these materials
were applied to automotive components in cooperation with Toyota Group companies, and further explains
the issues that remain for widespread use of titanium as an automotive material.
 itanium Alloy, Metal Matrix Composites, MMC, TiB, Powder Metallurgy,
T
Blended Elemental Method, Sintering, Automobile, Engine Component,
Exhaust Valve, Formula 1

Contents
1. Introduction
2. Issues of Titanium Alloy as an Automotive Material
3. Taking on the Challenge of Developing Low-cost
Titanium Components
3. 1 Feasibility of Powder Metallurgy
3. 2 Density and Pores of Sintered Titanium Alloys
4. Taking on the Challenge of Developing
High-performance Sintered Titanium Alloys
4. 1 Development of New Process to Increase Density,
Reduce Pore Size, and Refine Microstructure
4. 2 Special Microstructural Control for Sintered
Titanium Alloy
4. 3 Improvement of Mechanical Properties by
Process and Alloy Composition Refinement
5. Development of High-performance Titanium Metal
Matrix Composites (MMC)
5. 1 Selection of Optimum Reinforcement
Compound for Ti-MMC
5. 2 Optimization of Matrix Alloy for TiB Particle
Reinforced Ti-MMCs
5. 3 Hot Workabilty of TiB Particle Reinforced
Ti-MMCs© Toyota Central R&D Labs., Inc. 2020

6. Development of Ti-MMC Engine Valves for
Mass-produced Passenger Vehicle
6. 1 Setting of Development Targets
6. 2 Extremely Low-cost Manufacturing Process
6. 3 Selection of Alloy Composition and
Optimization of Heat Treatment Conditions
6. 4 Effect of TiB Particle Quantity on Mechanical
Properties and Hot Workability
6. 5 Durability and Reliability of Ti-MMC
Exhaust Valves
6. 6 Performance of Ti-MMC as Engine Valves
7. Application of Developed Ti-MMC Valves to
Toyota Formula 1 Engine
8. Achieving Further Cost Reductions
8. 1 Issues of Mass-produced Ti-MMC Engine
Components
8. 2 Development of Ultra-high-density
Compaction Technology
8. 3 Improving Productivity Using Continuous
Sintering Furnace
9. Proposal for the General Adoption of Titanium (inPlace of a Conclusion)

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1. Introduction
Before the development of the materials describedin this paper, racing cars had already adopted a wide
range of titanium alloy components since titanium
alloy reciprocating engine components can operate atmuch higher rotational speeds than steel. The adoptionof titanium was also part of ongoing efforts to reduce
the weight of engine valves, one of the main types ofsuch reciprocating components. However, althoughtitanium alloy intake valves had been adopted on somecommercial sports cars, none had adopted titanium
exhaust valves. Although partly a reflection of thecost of titanium, this was principally due to the lack ofa titanium alloy with long-term durability when
exposed to high temperature exhaust emissions.
In 1998, twenty-two years ago, Toyota Motor
Corporation (abbreviated hereafter as Toyota) launched
the Altezza, a mass-produced passenger vehicle
installed with the 3S-GE engine, which adopted titaniummono-boride (TiB) particle reinforced titanium metal
matrix composites (TiB/Ti-MMC) exhaust valves andsinter-forged titanium alloy intake valves (Fig. 1).
This TiB/Ti-MMC technology originated at Toyota
Central R&D Labs., Inc. (abbreviated hereafter as
Toyota CRDL),(1-6) and was jointly developed andcommercialized by the Production Engineering
Development Div. at Toyota, Fine Sinter Co., Ltd., andAisan Industry Co., Ltd.(7-9) Nine years later, in 2007,
a high-performance variant of this technology was also
adopted in Toyota Formula 1 (F1) race car engines(10)with positive results that are still fresh in the mind.
This material was subsequently adopted in the engines
for the high-performance Lexus LFA sports car(Fig. 2),(11) which went on limited sale in 2010 and
the Lexus RC-F(12) launched in 2014. In 2000, almost
immediately after the launch of the Altezza, Yamaha
Motor Co., Ltd. also adopted this material in the engineof the YZ-250F motocross motorcycle, and continues
to use it for various motocross models.
This article describes why Toyota decided to adopt
titanium for both the intake and the exhaust valves ina mass-produced vehicle rather than a racing car. It
also describes why this material has not been adopted
on a wider range of vehicles, the remaining challenges
to be addressed, as well as potential solutions for thesechallenges.

IntakeExhaustFig. 1 T
 he first mass-produced car in the world tofeature TiB/Ti-MMC engine valves.

Lexus LFA

1LR-GUE

Fig. 2 A ppearance of Lexus LFA and its NA 4.8-liter
V10 1LR-GUE engine.

© Toyota Central R&D Labs., Inc. 2020

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2. Issues of Titanium Alloy as an Automotive Material
Figure 3 compares the specific strength (i.e., tensile
strength/specific gravity) of a range of typical structural
metals. As shown in the figure, the order of specificstrength is as follows: normal steel < high-strength
aluminum (Al) alloy < high-strength steel < high-strength
magnesium (Mg) alloy < Al alloy metal matrix
composite (MMC) < Maraging steel < titanium alloy.
Therefore, as titanium alloy is known to have higher
specific strength than any other practically available
metal, it is a widely adopted and indispensable
material for reducing weight in many fields, including
aviation, space, armaments, racing cars, and sporting
goods. Another important property of titanium alloy
is its unmatched corrosion resistance. Applications
that take advantage of this property include weather
resistant building materials, marine vessels, marine
structures, as well as cooking and eating utensils. In
addition, since titanium has extremely low toxicity
for humans, it also plays an irreplaceable role in the
medical field for artificial bones, implants, catheters,
dental corrective devices, and so on.
However, despite its wide-ranging advantages asa structural metal, the extremely high cost of titanium
components has restricted its actual adoption. In
terms of the abundance of metals in the Earth’s crust,titanium is more abundant by an order of magnitude
than chromium (Cr), nickel (Ni), copper (Cu),manganese (Mn), carbon (C), phosphorous (P), sulfur (S),and only lower in abundance than particularly common
metals such as silicon (Si), Al, iron (Fe), and Mg.
Despite this, however, there are two reasons whytitanium components are so costly. The first is thattitanium is highly reactive with oxygen (O) and is
not easily reduced. As a result, titanium is difficult
to isolate or refine from oxide mineral ores. Anotherreason is its inferior formability of titanium, as definedin the following terms. First, titanium is difficult tocast due to the extremely high reactivity of molten
titanium with melting crucibles and casting dies.
Second, titanium has a poor forging workability due
to its crystalline structure (hexagonal close-packed)
and extremely rapid grain growth rate at an elevatedtemperature. Finally, titanium is difficult to machining
since it has a clear tendency to adhere to and react with
cutting tools. The poor formability of titanium causes
substantial reductions in material yields and puts
further upward pressure on cost. As an example, Fig. 4
shows photographs of a titanium alloy component(a connecting rod used in a Toyota F1 racing car
engine) at different stages of production. On the rightis the connecting rod after forging and deburring.
The final part after machining is shown on the left.
The final machined part weighs approximately 250 g,much less than the approximately 850 g of the asforged part. This means that the machining yield of
this component is only around 30%. Machining yields
are even lower for aircraft parts and other componentsthat require complex shapes, and it is not unusual forthese yields to fall below 10%.

Strength converted to specific gravity of steel (MPa)
0

1000

2000

3000

4000

Normal steel
High-strength Al alloy
High-strength steel
High-strength Mg alloy
Al alloy MMC
Maraging steel
Titanium alloy

0

100

200

300

400

500

Specific tensile strength (strength/specific gravity) (MPa)Fig. 3

 omparison of specific strength (tensileC
strength/specific gravity) of various structural
metals.

© Toyota Central R&D Labs., Inc. 2020

Fig. 4 T itanium alloy (SP-700) connecting rod for
Toyota F1 engine (left) machined from hotforged preform (right).

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Overcoming this cost barrier was the first issue to beresolved before titanium alloy could be adopted for themoving parts of mass-produced vehicles. In addition,
despite the overwhelming advantage of titanium alloyin terms of specific strength compared to conventional
steel, this advantage actually created several issues forthe application of titanium alloy to moving parts ina vehicle. The first is the issue of stiffness. The specificstiffness (i.e., Young’s modulus/specific gravity) of
all structural metals (iron, titanium alloys, aluminumalloys, magnesium alloys, cupper alloys, and the like)is coincidentally around 25 GPa/cc/g. This meansthat achieving a substantial reduction in weight
through material substitution is not a principal aim
for components that prioritize stiffness over strength
as a design criterion. The second is the issue of wear
resistance. To a varying degree, excellent sliding
characteristics are a requirement of engine components.
However, titanium alloy has lower wear resistance thansteel and no low-cost surface treatment technologies
were available. The third is the issue of heat resistance.
Although titanium alloys have higher heat resistancethan aluminum and magnesium alloys, no heat-resistanttitanium alloys had been developed with superior
high-temperature properties to the heat-resistant steel
generally adopted in vehicle exhaust systems.
Consequently, two objectives had to be realized before
titanium alloy could be adopted in mass-produced
vehicle components. The first objective was toreduce manufacturing costs to a level comparable toconventional steel (i.e., the cost of the raw material,
forming processes, machining yield, surface refinements,and so on). The second objective was to drastically
improve material characteristics other than specificstrength (such as specific stiffness, wear resistance,and heat resistance). Unless both objectives could be
realized at the same time, titanium alloy could not be
considered as a replacement for conventional steel
components in mass-produced vehicles.
3. Taking on the Challenge of Developing Low-cost
Titanium Components(13-15)
3. 1 Feasibility of Powder Metallurgy
The previous section outlined the reasons forthe high cost of titanium alloy components. It was
realized that drastic cost improvements could not be
achieved without making fundamental changes. First,© Toyota Central R&D Labs., Inc. 2020to address the raw material cost, one issue was to
identify the maximum limit of impurities that could
be permitted in the raw material without adversely
affecting the material properties. Second, to achieve
far-reaching improvements in material yield, it
was necessary to take on the challenge of adopting
low-cost net shaping technology. The final issue was
the development of low-cost surface treatment andmachining technologies. At the same time, to help
realize these dramatic improvements in manufacturing
technologies, the research team also understood thenecessity of developing more refined alloy composition
and microstructure control techniques suitable forindustrial processes.
Research focused on powder metallurgy toaccomplish these objectives. Powder metallurgy is
a production method in which metal powder ismolded by compression in a die and then sinteredat high-temperature. It started to attract a great dealof attention in the 1970s as a low-cost forming
technology for steel components, and was alreadyin use for automotive parts. Toyota CRDL began
studying the applicability of titanium alloy vehicle
components in the mid 1980s. At the time, research
into titanium powder metallurgy had begun in the
U.S., primarily for military applications.(16-18) Twomain technological approaches were in practical use
around the world. The first was the alloy powder
method. In this method, titanium alloy powder with thetarget chemical composition is fabricated in advanceby gas atomizing(18) or via the rotating electrodeprocess (REP).(19) The powder is then enclosed ina metallic can and densified by hot isostatic pressing(HIP).(20) The second was the blended elemental (B/E)
method.(21) In this method, pure titanium powder ismixed with a master alloy powder selected to reinforce
the material, before being pressed into the shape of thecomponent in a metallic die and densified by vacuum
sintering.
In addition to the high cost of fabricating the alloy
powder and the HIP process, the material yield of thealloy powder method was also extremely low because
net shaping techniques could not be adopted. Despite
the lack of any cost advantage over conventional
forging technology, it attracted attention as a wayof forming high-performance titanium alloys witha uniform composition and microstructure. In contrast,
the B/E method involved the blending of low-cost
pure titanium powders (such as sponge fine or

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hydride-dehydride (HDH) powder) with low-costmaster alloy powders (primarily pulverized powders).
The resulting blended powder is compacted into the
product shape in a metallic die and further densified
by vacuum sintering. This process directly allows
the manufacture of near net shape components andis a far lower-cost technique than ingot forging or
the alloy powder method. However, as evidenced
by the expression “cheap and nasty”, components
manufactured by the B/E method have far inferiormaterial properties to forged components. In particular,
one fatal issue of components manufactured by theB/E method compared to ingot forged components
is extremely low fatigue strength, a very importantmaterial property under practical usage conditions.(22)Figure 5 compares the fatigue properties of a typicaltitanium alloy (Ti-6Al-4V: mass%) manufacturedby the B/E method (, ) with the same alloy
manufactured by sintering and HIP () aftera tensile/compression fatigue test (stress ratio R = 0.1).
The reasons why the B/E method was not regarded asreliable are obvious: first, the properties of sinteredmaterials fabricated by the B/E method differ widely
depending on the raw material powder. Then, there isa clear difference between these alloys and the alloy
after HIP (). It should also be noted that the fatigue
properties of a sintered titanium alloy after HIP arecomparable to those of an ingot forged material.(17)Therefore, Toyota CRDL began research in themid 1980s motivated by the idea of utilizing the costadvantage of the B/E method while finding a way
to improve fatigue properties. It was felt that thismight enable the development of a usable titanium
alloy for mass-produced automotive components.
The inferior fatigue properties of sintered titanium
alloys manufactured by the B/E method are due tothe notching effect of pores in the sintered titanium
alloy. Therefore, it was initially posited that thismaterial could not be used to manufacture importantcomponents unless these pores could be eliminatedby applying HIP to the sintered product.(17) However,there was a total lack of fundamental research into
this matter. No reports described the sizes of poresin sintered titanium alloys and to what extent thesepores affected fatigue strength. Therefore, Toyota
CRDL started its research by examining why sinteredmaterials have low fatigue strength.
3. 2 Density and Pores of Sintered Titanium Alloys
Figure 6 shows the microstructure of a typicalsintered titanium alloy (Ti-6Al-4V) manufactured
using different types of powder. The titanium powder
used for the alloy shown in Fig. 6(a) is low-cost
Hunter sponge fine (HSF) powder with a high impurity(a)Maximum stress (MPa)Ti-6Al-4V alloy

HDH sinter and HIP(b)

HDH as-sintered
HSF as-sintered
R = 0.1
f = 30 Hz

40 μm

Cycles to failure

Fig. 5

 omparison of fatigue properties of B/E as-sinteredC
Ti-6Al-4V alloy using different type of titanium
powders and post sinter HIP’ing.

© Toyota Central R&D Labs., Inc. 2020

Fig. 6 hange in sintered microstructure of Ti-6Al-4V
C
alloys using different type of titanium powders:(a) HSF powder and (b) HDH powder.

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content. In comparison, the titanium powder used forthe alloy shown in Fig. 6(b) is HDH powder, a relatively
high-cost powder with a lower impurity content.
Both were compacted under a pressure of 392 MPain a metallic die and vacuum sintered at 1300℃ for
four hours. Although both sintered compacts havea density of approximately 97% with respect to thetrue density, the distribution and pattern of the poresare completely different. With the HSF powder, thedistribution of pores is uneven and pore size is irregular.
In Fig. 5, the fatigue properties of the HDH powder
component are shown by the  symbols and the fatigue
properties of the HSF powder component are shownby the  symbols. These results indicate that the sizeand distribution of pores have a major effect on fatigue
properties. It also shows that the fatigue properties ofeven a high-cost HDH powder component with thisdensity and pore distribution cannot approach those ofa regular forging or a sintered alloy manufactured by
the alloy powder method.
Therefore, this research took on the challenge ofusing low-cost HSF powder to create an alloy structurewith pores that are as uniform and finely distributed
as possible. Table 1 compares the chemical analysis
results and price (in 1985 U.S. dollars) of HSF andHDH powders. The following three major differences
are evident: (1) HSF powder is clearly less expensive
than HDH powder, (2) HSF has a higher sodium andchlorine content than HDH powder, and (3) the oxygen
content of HSF powder is approximately half that of
HDH powder. These differences are caused by the
powder manufacturing process and can be explained
simply as follows.
First, HSF powder is a by-product of the Hunter method(a sodium reduction method) used to manufacture
sponge titanium, and is a fine powder with very few
alternative uses. For this reason, the price of HSF
powder is very low and it contains little oxygen
because it is a form of sponge titanium. However,since the Hunter method involves reducing titanium
tetrachloride using metallic sodium, large quantities
of sodium chloride (NaCl) are generated that haveto be washed off by water. As a result, HSF powder
unavoidably contains some residual NaCl. Figure 7
shows a photograph of particles (mainly NaCl and ferricoxide (Fe2O3)) sieved out of commercially available
HSF powder. In contrast, HDH powder is fabricatedas follows. First, sponge titanium manufactured by
the Kroll method (a magnesium reduction method)© Toyota Central R&D Labs., Inc. 2020

Table 1 C hemical analysis of applied titanium powders.
Powder

O

Cl

Na

Mg

Fe

Ti

Price (U.S. $/kg)

HSF

0.11

0.08

0.078

---

0.02

99.6

5 to 10HDH

0.19 < 0.002---< 0.001 0.029

99.8

20 to 50is embrittled by hydrogenation. The powder is thenfabricated by grinding at room temperature before
a further vacuum dehydrogenation treatment at around
500℃. Unlike the Hunter method, the final vacuum
treatment of the Kroll method is capable of removing
the magnesium chloride (MgCl2) by-product generated
during refining. Therefore, the amount of residual
chlorides is extremely low even without washing.
However, the multiple heat treatments required to
manufacture this powder push up the manufacturing
cost and generate a relatively high oxygen content.
Figure 8 shows scanning electron microscope (SEM)
images comparing the appearance of HSF and HDH
powders. The HSF powder shares the distinctive
irregular appearance of sponge titanium, whereas
the HDH powder has the distinctive squarish clumps
associated with crushed powders.
The graph in Fig. 9 compares the tap, green, andsintered densities of a titanium alloy manufactured
using HSF and HDH powders. Since HSF powder
has lower flowability than the HDH powder, its tap
density is also substantially lower. In contrast, since
HSF powder is easily plastic deformed due to its lower
oxygen content, its green density is higher. However,since the density of HSF powder is less easy toincrease by sintering, the final density of both powders
is roughly the same (approximately 97%). The poor
sintering properties of HSF powder materials is
because the NaCl particles included in the powder do
not melt into the titanium during sintering and becometrapped between the titanium particles, hampering thesintering shrinking process. It is thought that the large
pores exceeding 100 μm shown in Fig. 6(a) are causedby these NaCl particles.

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4. Taking on the Challenge of Developing
High-performance Sintered Titanium Alloys

NaCl

4. 1 Development of New Process to Increase Density,
Reduce Pore Size, and Refine Microstructure
Fe 2O 3Fig. 7100 μmI nclusion particles extracted from the
HSF titanium powder.

(a)

(b)

50 μm
Fig. 8 A ppearance of (a) HSF and (b) HDH titanium
powders (−#150).

Titanium powders: −#150, Compacting: 392 MPa, Sintering: 1300℃ for 4 hours

Tap
Green

HSF

Sinter
Tap
HDH

Green
SinterDensity (g/cm3)

Fig. 9Theoreticaldensity omparison of tap, green, and sintered densities ofC
Ti-6Al-4V alloy manufactured by conventional B/E
method using different type of titanium powders.

© Toyota Central R&D Labs., Inc. 2020

The large pores that remain in sintered bodies
manufactured by low-cost HSF powder are thought
to be caused by the intrusion of large NaCl particles.
Based on this presumed mechanism, reducing the sizeof these NaCl particles was thought to be a way of
eliminating large pores. Therefore, a kneading processusing a high-energy ball mill was adopted to crush the
NaCl particles included in the titanium powder. Since
finer master alloy powders are known to promote thesintering of titanium alloy, the high-energy ball mill
was used first to finely crush the master alloy. The
HSF powder was then introduced into the mill toblend the materials within a short time frame while
also kneading the titanium powder. Figure 10 shows
the appearance of the HSF powder after kneading
treatment. Compared to the HSF powder before
kneading as shown in Fig. 8, the kneaded HSF powderappears as rounded clumps. The large NaCl particles
have been broken up and are adhered on the surfaceof the titanium powder as fine particles alongside
the master alloy powder. The kneading treatment
substantially improved the flowability of the powder,thereby raising the tap density of the powder 1.5 times
higher than that of non-treated powder.
Figure 11 shows the sintered microstructure of
Ti-6Al-4V alloys manufactured using the new
mixing with kneading process. Regardless of thetype of titanium powder, both the quantity and sizeof the pores were drastically reduced under the same
compacting and sintering conditions compared withthe conventionally manufactured alloys shown in
Fig. 6, and a relative density of 99% was achieved.
This figure clearly indicates the effect of the powder
mixing process on the titanium alloy microstructure.
Specifically, in the case of the titanium alloy
manufactured using HSF powder, since NaCl neverdissolve into the titanium alloy at elevated temperature,
NaCl particles pulverized by the kneading treatment
are stably dispersed in the titanium matrix during
sintering process showing an important effect ofpinning the β phase grain boundaries. This pinningeffect suppresses β phase grain growth and changes theacicular α phase inside the β grain structure to granular

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α phase. In contrast, since there is no pinning effect ofincluded particles using HDH powder, β phase grains
grow significantly with densification during sintering,which forms a coarser acicular α phase inside the βgrain structure during cooling stage.

30 μm
Fig. 10 Surface morphology of HSF powder after kneading.

4. 2 Special Microstructural Control for Sintered
Titanium AlloyAs shown in Figs. 6 and 11, (α + β) type titanium
alloys as typified by Ti-6Al-4V form a rough acicular(α + β) microstructure if slowly cooled from thesintering temperature (i.e., the stable β phase region).
Since this type of directionally aligned coarse
acicular microstructure has an adverse effect onmechanical properties such as fatigue and ductility,
a further heat treatment is generally required toadjust the microstructure. However, to avoid a costly
post-sintering heat treatment, it is necessary to createa fine uniform microstructure through the slowly
cooling conditions after sintering.
An important aspect of creating a finer microstructure
though slowly cooling is to delay the transformationfrom the β to the α state. This can be accomplishedby lowering the β to α transition temperature (i.e.,stabilizing β phase). An effective means of achieving
this objective is to add a β stabilizing element having
small diffusivity in β titanium. After analyzing the
diffusion coefficients of elements in β titanium,(23) it

(a)

(a)

(b)

(b)

30 μm

30 μm
Fig. 11 C hange in sintered microstructure of Ti-6Al-4V
alloys using new mixing process and different
titanium powders: (a) HSF and (b) HDH. The
sample preparation conditions were the same as
the alloys shown in Fig. 6.

© Toyota Central R&D Labs., Inc. 2020

Fig. 12 ffect of Mo and B addition on theEmicrostructure of HDH Ti-6Al-4V
alloy: (a) Ti-6Al-4V-1Mo and(b) Ti-6Al-4V-1Mo-0.3B. Other
conditions are the same.

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Table 2 C omparison of impurity diffusivity of several β-stabilizing elementsin β-titanium at 900℃, normalized by the self-diffusivity of titanium.

Element

Ni

Co

Fe

Mn

Cr

Ti

V

Nb

Mo

Diffusivity

56

42

18

4.9

1.8

1

0.7

0.6

0.2

was proposed that the addition of molybdenum (Mo),an element with a particularly low diffusion coefficientin β titanium, would be most effective (Table 2).
As predicted, the addition of a small amount of Mo
was extremely effective in creating a fine acicular
α phase microstructure (Fig. 12). However, it was
found that adding Mo facilitated the precipitation of
network-like α phase structures along the β phasegrain boundaries during cooling (Fig. 12(a)). In
response, dispersing the fine precipitates that inhibit
β phase grain growth was proposed as an effective wayof suppressing α phase precipitation along the grainboundaries. Consequently, the research team searched
for a particle additive with low mutual solubility withtitanium that would not dissolve into β titanium evenat high temperatures. As a result, several candidate
additives were identified that effectively suppressed
abnormal grain growth during sintering and formed
equiaxed acicular α phases, including NaCl (describedabove), yttrium oxide (Y2O3), yttrium chloride (YCl3),and boron (B). As a typical example, Fig. 12(b) shows
the microstructure of a sintered Ti-6Al-4V alloy
containing small amounts of both Mo and B after
slowly cooling. The fine acicular α phases created by
the addition of Mo were equiaxed by the co-addition of
B. The network-like α phase structures along the grainboundaries have disappeared and an extremely fineand uniform equiaxed structure has been formed. The
noteworthy phenomenon here is that the B additive isdispersed as fine TiB grains with stable thermodynamic
properties within titanium alloys. It is thought thatthese finely dispersed TiB particles help to preventcoarsening of the β phase crystalline grains during
sintering and act as nuclei for α phase precipitation
during cooling, thereby promoting the formation ofan equiaxed fine α + β structure. This discovery wasa major hint for titanium-based composite material
development in the future.

© Toyota Central R&D Labs., Inc. 2020

4. 3 Improvement of Mechanical Properties by
Process and Alloy Composition Refinement
Table 3 compares the sintered density, maximum
pore size, and tensile properties of typical Ti-6Al-4V
sintered alloys manufactured using HSF or HDH
powder with two different master alloy powder sizes(9 and 40 μm in average diameter), with and without
kneading treatment, and with and without the additionof elements to refine the microstructure (Mo + B).
Figure 13 compares the fatigue properties of the six
samples (A to F) shown in Table 3.
To summarize the results, kneading the powder
enabled static and fatigue strength properties equivalent
to a titanium alloy manufactured using high-cost HDH
titanium powder and HIP, even when low-cost HSF
titanium powder was used. Based on these results, it
was found that a maximum pore diameter of less than
20 μm would not have an adverse effect on fatigue
properties. In addition, combining kneading withthe addition of elements to refine the microstructure
enabled substantially improved mechanical properties
compared to those achieved by HIP using high-purity(expensive) HDH titanium powder, even whenas-sintered condition using low-purity (cheap) HSF
titanium powder.
In conclusion, it was found that even the low-cost
B/E method could be used to easily achieve material
properties equal in every way to high-cost conventional
forged materials by improving the powder treatment
and additive minor alloying elements. However, as
described in the introduction, this performance wasstill insufficient to enable the application of titanium
alloys to automotive structural materials, and further
performance gains were required. Therefore, thefocus of the research switched to the development ofa low-cost and high-performance titanium-based
composite material.

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Maximum stress (MPa)

Table 3 C omparison of static mechanical properties of B/E titanium alloys and manufacturingconditions.

R = 0.1
f = 30 Hz

Cycles to failure

Fig. 13

 omparison of fatigue properties of blendedC
elemental Ti-6Al-4V alloys prepared under thevarious conditions listed in Table 3.

5. Development of High-performance Titanium
Metal Matrix Composites (MMC)(1-6)
5. 1 Selection of Optimum Reinforcement
Compound for Ti-MMC
Assuming application of the B/E method withhigh-temperature sintering, the conditions for an ideal
reinforcement compound for titanium alloy include thefollowing: (1) high strength, hardness, heat resistance,and stiffness, (2) remarkable thermodynamic stability
in titanium alloys throughout the temperature range,(3) constituent elements of the compounds neversoluble into the titanium matrix, (4) difference inthermal expansion coefficient between the titanium
matrix and the reinforcement compound is small, and© Toyota Central R&D Labs., Inc. 2020(5) high bonding strength between the titanium matrix
and reinforcement compound. However, at the time(the second half of the 1980s), all the reinforcement
compounds for titanium-based composite materials
that were being researched primarily in the U.S. (suchas titanium carbide (TiC), silicon carbide (SiC), titanium
nitride (TiN), titanium diboride (TiB2), silicon nitride(Si3N4), boron carbide (B4C), alumina (Al2O3), and soon) either resulted in low thermodynamic stability inthe titanium alloy, or were fundamentally unsuitablefor use as a reinforcement compound due to mutual
large solubility between the titanium matrix and thecompounds. Such research was attempting to boost
performance through the suppression of interface
reactions by, for example, modifying the interfaces
between the reinforcement compound and titanium
matrix.(24)
In contrast, the research at Toyota CRDL had
focused on the dispersing of TiB compound particles.
The advantage of this compound compared to thereinforcement compounds listed above in terms ofthermodynamic properties can be easily imagined fromthe Ti-B phase diagram shown in Fig. 14. Specifically,
the boron forms virtually no solid solution with thetitanium matrix in a range from room temperature up
to just below the melting point. In addition, the TiBphase forms a stoichiometric compound with a Ti:B
ratio of 50:50, which means that it has virtually no solid
solubility. This strongly suggests that TiB particles ina titanium alloy would be extremely thermodynamicallystable and have no adverse effects on the mechanicalproperties of the titanium matrix. Furthermore, as
shown in Table 4, TiB has extremely high strengthand elastic modulus, as well as virtually the same
coefficient of linear thermal expansion as titanium
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alloys. As a result, TiB has the ideal properties for use
as a titanium alloy reinforcement compound.
Based on the previous analysis, the research teamTemperature (℃)

Mass % BL

β Ti

Ti 3 B 4

TiB2

TiB

βB

αTi

Ti

Atomic % B

BFig. 14

Phase diagram of Ti-B binary system.

was sure that using TiB as a reinforcement compound
for titanium alloys manufactured using the B/E
method could achieve dramatic improvements inmaterial properties. Although stable when present intitanium alloy, TiB itself is an unstable compoundand cannot be obtained in powder form. However,stable TiB particles can be created from various
boride powders by recombining the boron atoms inthe boride with titanium atoms, which have a stronger
chemical affinity, in the sintering process. Figure 15
shows microstructural images of an alloy (10 vol%TiB/Ti-6Al-4V) manufactured using HSF titanium
powder, Al-40 mass%V master alloy powder, and TiB2
powder stirred and mixed together in a high-energy
ball mill, compacted in a metal die at 392 MPa, andvacuum sintered at 1300℃ for 16 hours. Figure 15(a)shows that the squared-off TiB particles are dispersed
uniformly, the enlarged image in Fig. 15(b) shows thatthe interfaces between the TiB particles and matrix

Table 4 A daptability of reinforcement compounds for titanium matrix composites.

(a)

(b)

20 μm

Fig. 15

(c)

2 μm

2 nm

 icrostructure of B/E 10 vol% TiB/Ti-6Al-4V MMC: (a) macro image (SEM), (b) TiB particle
M(TEM) and (c) TiB/b Ti interface (TEM).

© Toyota Central R&D Labs., Inc. 2020

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alloy are linear, and the lattice image in Fig. 15(c)shows that the β titanium matrix/TiB particle interface
is extremely consistent. Figure 16 compares the fatigue
properties of a Ti-6Al-4V sintered alloy manufacturedby the new process using HSF titanium powder andan MMC containing 10 vol% of TiB in a Ti-6Al-4V
alloy. These results illustrate the dramatic improvementin fatigue properties achieved by dispersing TiB
particles into the titanium alloy. For normal metallic
materials, it is relatively simple to improve static
strength, stiffness, wear resistance and the like bycomposite particle dispersing. However, fatigue
properties and impact characteristics are regarded as
particularly difficult to improve by the same method
due to the insufficient bonding strength of the interface
between the matrix alloy and reinforcement compound.
These interfaces are susceptible to becoming the origin
point of fatigue fractures due to concentrations instress. Fractures that occur are also likely to propagate
along these interfaces. However, it was estimated thatthe significant improvement in fatigue properties wasdue to reasons such as the extremely high consistency
and strong bonding of the TiB particle/titanium
alloy matrix interface (Fig. 15(c)) and the almost
identical coefficients of thermal expansion of the bothconstituent materials (Table 4). These results verified
that TiB is the ideal reinforcement compound fortitanium alloy.
5. 2 Optimization of Matrix Alloy for TiB Particle
Reinforced Ti-MMCs
The data shown in Fig. 16 was obtained based ona Ti-6Al-4V alloy. However, one of the advantagesof the B/E method is the capability to freely alter
the composition of the matrix alloy by adjusting themaster alloy powder. Table 5 shows the combinationsof master alloy powders adopted to manufacture
various titanium alloys for trial. The compositionsof the analyzed master alloys were selected basedon grindability. Some of the combinations included
pure metallic powders. For example, Ti-6Al-4V and
Ti-3Al-2V alloys were analyzed as (α + β) type MMCs
with a good balance of strength and stiffness. In
addition, presuming the application of the material
to the manufacture of connecting rods, an extremely
strong β type Ti-7Mo-4.5Fe-1.5Al-1.5V alloy MMC
for forging was also studied.(25)
Figure 17 compares various mechanical properties© Toyota Central R&D Labs., Inc. 2020

Fig. 16I ncrease in fatigue strength of Ti-6Al-4V
sintered alloy by addition of 10 vol% TiB
particles obtained by the new B/E process.

Table 5 C ombinations of master alloy compounds for
manufacturing B/E titanium alloys.
(mass%)

Target titanium alloy composition

Combinations of master alloy compounds

Ti-6Al-4V, Ti-3Al-2V

Al-40V

Ti-6Al-4V-1Mo-0.2B

Al-40V + pure Mo + TiB2

Ti-8Al-1Mo-1V
Ti-6Al-2Sn-4Zr-2Mo-0.2Si
Ti-6.5Al-4.5Sn-4.5Zr-1Nb-1Mo-0.35Si
Ti-5Al-12Cr-3.3V
Ti-10V-2Fe-3Al
Ti-4.5Fe-7.0Mo-1.5Al-1.5V
Ti-33.5Al

Al-38Ti + Al-40V + pure Mo
Al-14Sn-28Zr-1.4Mo-1.4Si
Al-25Sn-25Zr-5.6Nb-5.6Mo-1.9Si
Al-40V + pure Cr
Al-40V + Fe-80V (ferro-V)
Al-50V + Fe-62Mo (ferro-Mo)
Al-38Ti

of sinter-forged 20 vol% TiB/Ti-7Mo-4Fe-1.5Al-1.5V
MMC with those of B/E Ti-6Al-4V sintered alloy. The
static and fatigue strength of the MMC are twice ashigh as the conventional alloy. The Young’s modulusof the MMC is also around 1.5 times higher at
160 GPa and the wear resistance is also extremelyhigh.
The research also studied a (near-α) type
Ti-6Al-4.5Sn-4.5Zr-1Mo-1Nb-0.3Si alloy MMC tomaximize heat resistance for application as engine
exhaust valves. Although 650℃ was conventionallyregarded as the limit of heat resistance for the best heat
resistant titanium alloy, this material demonstrated
superior fatigue properties to heat-resistant steel at
850℃ (Fig. 18).
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Tensilestrength

Fatiguestrength

Wear loss

Wear loss (mg)Young’s modulus (GPa)

20 vol% TiB β Ti-MMC

Ti-6Al-4V

Tensile / Fatigue strength (MPa)R&D Review of Toyota CRDL, Vol.51 No.1 (2020) 1-30

Young'smodulusFig. 17 C omparison of mechanical properties of 20 vol% TiB reinforced
β Ti-MMC with conventional Ti-6Al-4V alloy.

Fig. 18

 ffect of TiB particle content on fatigueE
strength at 850℃ of the developed
TiB/Ti-6.6Al-4.6Sn-4.6Zr-0.9Nb-1.0Mo-0.35Si
MMC.

5. 3 H ot Workabilty of TiB Particle Reinforced
Ti-MMCs(25)Normally, titanium alloys have a crystal structure ofhexagonal close-packed (HCP) α phase at close to room
temperature, which transforms into a body centeredcubic (BCC) β phase at high temperatures. This β phasehas a high self-diffusion coefficient and generates
rapid grain growth. Therefore, if the titanium alloyis heated above the β transus temperature, the crystal
grains become coarser instantly and the titanium alloy
becomes susceptible to forging cracks. For this reason,
forging has to be carried out in the relatively low© Toyota Central R&D Labs., Inc. 2020

(α + β) dual phase temperature region, which has
high deformation resistance. However, if grain growth
can be suppressed even after heating the titanium alloyat the single β phase region, forging can be carriedout at ultra-high temperatures with low deformation
resistance and high deformability. This would also
have synergistic effects such as enabling net shaping(thereby improving material yield and minimizing therequirement for machining) and extending the lifetimeof the forging dies, helping to achieve major reductionsin processing costs.
Figure 19 compares the hot deformabilityof (a) (α + β) type Ti-6l-4V alloy, (b) β type
Ti-7Mo-4.5Fe-1.5Al-1.5V alloy, and (c) 20 vol%TiB/Ti-7Mo-4.5Fe-1.5Al-1.5V MMC. To illustratethese results, Fig. 20 shows side view images ofcylindrical test specimens after 75% compression(upset forging) at 800℃. The β phase grains of the
Ti-6Al-4V alloy (a), which contains no reinforcing
particles, became significantly larger due to the
1300℃ sintering process and formed a coarse (α + β)microstructure in the cooling process after sintering.
As a result, this alloy has large deformation resistanceand extremely low deformability, which explains thecracks in the sides of the specimen formed at a lower
compression range. In contrast, the single β phasemicrostructure of the β type alloy (b) and the β typealloy matrix MMC (c) at the process temperature(800℃) resulted in substantially lower deformation
resistance and higher deformability than the (α + β)
type Ti-6Al-4V alloy. In particular, although the MMC(c) contains 20 vol% of TiB particles, its deformation
resistance was virtually identical to the matrix alloy (b).

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Furthermore, the extreme smoothness of the side viewof the specimen suggests that the deformability of the
MMC is even superior to the matrix alloy.
Figure 21 compares the microstructure of the β typematrix alloy and the 20 vol% TiB MMC before hot
working. The figure shows that β grain growth has been
suppressed by the pinning effect of the TiB particles,
which are dispersed in large quantities throughout
the microstructure. These results demonstrate thatthe dispersing of TiB particles enable hot working
in the ultra-high-temperature β region, which had
been considered unrealistic for conventional titanium
alloys up to that point. Figure 22 shows an image ofa connecting rod formed by one-heat-forging in air attemperatures of 1200℃ or higher using a bar-shaped
sintered blank manufactured from the 20 vol%TiB/Ti-7Mo-4.5Fe-1.5Al-1.5V MMC. Regardless ofthe high TiB particle content, an extremely thin and

(a)

(a)
(c)
(b)

(b)

Fig. 19 hange in hot deformability ofC
β Ti-7Mo-4.5Fe-1.5Al-1.5V alloy byaddition of 20 vol% TiB particles
compared with α + β Ti-6Al-4V alloy.

100 μmFig. 21 inning effect of the TiB particle on the β grainP
growth during sintering. (a) β matrix alloy,(b) 20 vol% TiB MMC.

(a) Ti-6Al-4V alloy (α + β type)(a) Sintered blank(b) Hot-forged product(b) Ti-7Mo-4.5Fe-1.5Al-1.5V alloy (β type)(c) 20 vol% TiB/Ti-7Mo-4.5Fe-1.5Al-1.5V
MMC (β type)

50 mm

Fig. 20

ide view of upset forging test specimens
S
compressed at 800℃: (a) Ti-6Al-4V alloy,(b) β Ti matrix alloy, and (c) 20 vol% TiB MMC.

© Toyota Central R&D Labs., Inc. 2020

Fig. 22

 onnecting rod formed by one-heat-forging
Cat 1200℃ using sintered 20 vol%TiB/Ti-7Mo-4.5Fe-1.5Al-1.5V β-Ti MMC.

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R&D Review of Toyota CRDL, Vol.51 No.1 (2020) 1-30

clean flash has been formed by the hot forging process
without cracking, demonstrating the extremely highhot workability of this MMC. The fact that dispersing
TiB particles in the titanium alloy actually improved
hot workability, rather than having an adverse effectas might be expected, was a particularly noteworthy
discovery. As described in the following section, thischaracteristic became a decisive advantage when
applying TiB/Ti-MMC to manufacture valves.
In conclusion, the developed TiB particle reinforced
Ti-MMC manufactured using the B/E method wasfound to have the potential for commercialization ata much lower cost and with clearly superior material
characteristics to conventional titanium alloys.
6. Development of Ti-MMC Engine Valves(7-9,26)
6. 1 Setting of Development Targets
Engine intake valve has the minimum cost hurdle
among reciprocating engine parts for the applicationof titanium alloy to mass-produced vehicle. Althoughintake valves were the most likely candidate, exhaust
valves were also regarded as potentially viable,
providing the newly developed Ti-MMC could be
adapted for mass production. Therefore, a feasibility
study was conducted in 1991 with the aim of
simultaneously developing both intake and exhaust
valves. The target engine was a naturally aspirated (NA)4-cylinder 2000 cc engine for sporty vehicles, which
was due to start production in the second half of the
1990s. The vehicle was positioned as a potential rival
to a 2000 cc class high-performance sports car from
Honda Motor Co. Ltd. (Honda) that was rumored to be
due for release in the near future (the car that became
the Honda S-2000). Since it was expected that Honda
would develop an ultra-high revving short-stroke NA
engine using its variable valve timing & lift electronic

control (VTEC) technology, Toyota began work on
an engine with a rev limit of around 8500 to 9000 rpm
that could match the expected performance of Honda’s
engine. To realize this type of ultra-high revving
engine, it was absolutely necessary to substantiallyreduce the weight of the valves. However, since
only a minimum increase in costs could be tolerated
to manufacture these lightweight valves, it was notpossible to greatly exceed the cost of the current
heat-resistant steel valves. For these reasons, the engine
design division was very keen to use titanium valves if
the manufacturing cost could be kept sufficiently low.
Development started with this mission.
The manufacturing cost of heat-resistant steel
engine valves was between 150 and 350 yen (JPY)per valve. In comparison, the cost of existing titanium
alloy intake valves well exceeded 1000 yen (JPY)per valve. In addition, heat-resistant titanium alloy
had never been developed anywhere in the world
to manufacture exhaust valves in a mass-produced
engine. Therefore, the aim of this development was
to develop high-performance titanium valves with
sufficient potential to be adopted as both intake andexhaust valves, which could be manufactured at
a maximum cost of 300 yen (JPY) per valve.
6. 2 Extremely Low-cost Manufacturing Process
Figure 23 shows the final valve manufacturing
process. For the raw material titanium powder, a cheaphydrogenated titanium powder was selected. For thealloying additive, an Al-25Sn-25Zr-6Nb-6Mo-1.2Si(mass%) master alloy powder was developed withthe aim of achieving the ultra-high heat resistancerequired by exhaust valves. In addition, TiB2 powder
was selected as the source of boron for forming the TiBparticles. The manufacturing process was as follows:
division of each powder into the predetermined weightFig. 23 The developed low-cost engine valve manufacturing process for the TiB particle reinforced B/E Ti-MMC.

© Toyota Central R&D Labs., Inc. 2020

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R&D Review of Toyota CRDL, Vol.51 No.1 (2020) 1-30

proportions ⇒ stirring and mixing ⇒ compaction intocylindrical billets (dia. 16 × 42 mm) in a parting die
system ⇒ heating and dehydrogenation in a vacuum
furnace ⇒ densification to a relative density of at least
95% at 1300℃ ⇒ high-frequency induction heatingof the sintered billets in air to 1250℃ ⇒ extrusion of
the stem portion of the valves in a ceramic (SIALON:
invented 50 years ago at Toyota CRDL(27)) die ⇒forging into the final valve shape ⇒ annealing ⇒machining into the final product shape ⇒ oxidation
treatment of the seat contact surface and stem byheating in air at approximately 900℃ ⇒ correction ofcurvature ⇒ inspection ⇒ final product.
Adoption of this developed powder metallurgy
process enabled the commercialization of titanium
valves at much lower cost than conventional titanium
valves. A cost of 280 yen (JPY) per valve was achieved,
which is in line with the original target and similar tothe cost of heat-resistant steel exhaust valves. These
cost savings can be summarized into the following
four points: (1) the adoption of low-cost hydrogenatedtitanium powder, (2) the continuous powder mixing ⇒compaction ⇒ dehydrogenation ⇒ sintering process,which achieved a material yield of roughly 100% andenabled the manufacture of extrudable forging billets,(3) the dispersion of TiB particles, which enabled
ultra-high-temperature forging so that the valves could
be extruded into the final form, and (4) the formationof wear resistance due to the adoption of an extremely
productive and low-cost oxidation treatment in air.
6. 3 Selection of Alloy Composition and
Optimization of Heat Treatment Conditions
The excellent material properties of the developed
Ti-MMC are the result of the synergistic effectbetween an ultra-heat-resistant matrix and thereinforcement compound particles (TiB). The basic
composition of the matrix is determined by the
Ti-6Al-4Sn-4Zr-1Nb-1Mo-0.2S (mass%) alloy, which
was selected to achieve high-temperature fatigue andcreep properties, supplemented by 5 vol% of TiB
particles, which was selected to achieve high-temperaturestrength as well as important properties for component
manufacturing, such as ductility, hot workability,
machinability, and cost.
The material properties of titanium alloy are closely
dependent on its microstructure. In particular, themicrostructure of the matrix has a major impact on© Toyota Central R&D Labs., Inc. 2020

high-temperature creep resistance and high-temperature
fatigue properties. Specifically, titanium alloys witha coarse acicular microstructure have excellent creep
resistances, and titanium alloys with a fine acicular or
equiaxed structure have excellent fatigue properties.
Therefore, in the case of a normal heat-resistanttitanium alloy microstructure, the heat treatment(i.e., the solution treatment and aging) conditions
are carefully adjusted to ensure the optimum balance
between creep and fatigue properties.
In the case of the developed material, the dispersed
TiB particles maximized the β grain growth suppression
effect, thereby enabling annealing in the single β phase
temperature region, which is difficult to accomplish
with ordinary titanium alloys due to the rapid graincoarsening effect in this temperature region. Dispersing
the TiB particles also enabled the elimination of theunproductive aging process that tends to drive up costs.
6. 4 Effect of TiB Particle Quantity on Mechanical
Properties and Hot Workability
Figure 24 summarizes the effects of the TiB particles
on material properties at room temperature and elevatedtemperature of 800 or 850℃. Higher quantities of TiB
increase strength and fatigue strength at both room andhigh temperatures. High-temperature (850℃) fatigue
strength is an extremely important property of exhaust
valves. TiB particles have a significant effect on thisproperty and a blending amount of around 5 vol%of TiB is sufficient to outstrip the fatigue strength ofvalve steel (21-4N).
In contrast, dispersion of TiB particles has a slightnegative effect on high-temperature creep resistance.
This is mainly because the acicular α phase in thematrix becomes finer and adopts an equiaxed structure
as the TiB particle content increases. However, theeffect of the TiB content is only minor, and superior
creep resistance to 21-4N steel can be maintained evenat a blending ratio of 10 vol% of TiB.
Room temperature ductility remains virtuallyconstant regardless of the TiB particle content andhigh-temperature ductility is substantially improvedby adding a low amount of TiB particles. Specifically,
5 vol% of TiB has the effect of improving ductility
the maximum elongation of approximately 12%.
In addition, even 10 vol% of TiB containing alloy
maintains ductility by approximately 10%.
Next, this section describes the effects of TiB particles

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R.T.

1200
1100
1000

Strain rate: 4.55 x 10−4/sec

900

Tensile elongation (%)

R.T.

15
10
5

Strain rate: 4.55 x 10−4/sec

0

R.T.
800
700
600
500

Stress ratio: −1
0

2

4

6

TiB content (vol%)

800

800℃

600
400

21-4N

200

Strain rate: 10%/sec

0

800℃
15
10
5

Strain rate: 10%/sec

0

Rotating bending
fatigue strength (MPa)

Rotating bending
fatigue strength (MPa)0.2% proof strength (MPa)

1300

8

10

100 hr creep deflection (mm)Tensile elongation (%)

0.2% proof strength (MPa)R&D Review of Toyota CRDL, Vol.51 No.1 (2020) 1-30

850℃
175

21-4N

150
125

Stress ratio: −1

100

800℃
30
20

21-4N

10
0

Max. stress: 50 MPa
0

2

4

6

8

10

TiB content (vol%)Fig. 24 hanges in mechanical properties of the developed Ti-MMC in accordanceCwith TiB particle content at room and elevated temperatures.

© Toyota Central R&D Labs., Inc. 2020

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R&D Review of Toyota CRDL, Vol.51 No.1 (2020) 1-30

Tensile elongation (%)

140

1200℃

120
100
Maximum impact stress applied at high engine
speed (10000 rpm with valve jumping)80

Strain rate: 500%/s
60

0

2

4

6

8

10

TiB content (vol%)Fig. 25

 ffect of TiB particle content on high-temperatureE
ductility at a rapid strain rate.

on rapid hot deformability in the actual valve forging
temperature region (between 1200 and 1250℃).
Figure 25 shows the relationship between ductility
and TiB content when the material is rapidly
deformed (500%/s) in the valve forging temperature
region. The high-speed ductility at high temperature
is virtually consistent with the trend for low-speed(1%/s) elongation at 800℃ shown in Fig. 24,
which demonstrates that an appropriate TiB particle
content has the potential to substantially improve
hot workability. This is because the blending of TiB
particle suppresses β grain growth during heating,thereby extending the hot working window towardhigher temperatures at which working is easier.
6. 5 Durability and Reliability of Ti-MMC
Exhaust Valves
The previous sections described the development ofan MMC with high-temperature strength properties
equivalent or superior to the conventional valve
steel of the time. However, it was also crucial toensure wear resistance, which is a vital characteristic
for valves. Conventional titanium valves required
extremely high-cost surface treatments such as
molybdenum spray coating or chromium nitride(CrN) coating. In contrast, this research developed
a high-temperature oxidation treatment in air as
an effective low-cost alternative. The purpose ofoxidation treatment is to improve wear resistance byforming a solid solution with an appropriate quantityof oxygen in the titanium alloy matrix close to the© Toyota Central R&D Labs., Inc. 2020

Fig. 26

xample of reliability test results for theEdeveloped exhaust valve (impact strength after
long term exposure to air at high temperature).

surface. Combining the dispersion of TiB particles withoxidation treatment in air enabled the developmentof valve seat contact surfaces with high-temperature
wear resistance, valve stems with sliding and wear
resistance, and seizure-resistant valve heads.
Next, since titanium alloys have lower oxidation
resistance than heat-resistant steel, the researchteam was concerned about oxidation when used forextended periods in high-temperature combustiongas. The primary reliability requirement of engine
valves is that fractures must not occur under any
circumstances. As described above, when used underhigh temperatures, the excellent strength and ductilityof this material meant that absolutely no issues
occurred in an ultra-high-speed durability test (up to9500 rpm) designed to generate valve jumping. The
issue was reliability under strong impact stress at
close to room temperature. An index was created togauge the reliability of the material impact resistanceat room temperature. Figure 26 shows the results ofan impact test conducted at room temperature after
exposure to air at 900℃ for 1000 hours, assuming
the generation of high-temperature oxidation due
to usage in a severe driving environment. Although
the impact strength decreased substantially due tolong-term exposure to high-temperature oxidation in
air, the room temperature impact strength of the valve
was approximately five times the maximum impact
stress likely to cause abnormal valve behavior suchas juddering. Consequently, the developed Ti-MMC
exhaust valve was verified to be sufficiently reliable
for actual usage environments.
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6. 6 Performance of Ti-MMC as Engine Valves
Although the maximum speed of the new engine
installed with the newly developed sintered titanium
valves was substantially lowered from 9000 to
7700 rpm for various reasons, this was still 500 rpm
higher than the engine it replaced. The developed
engine generated a maximum of 210 hp, the highestof any 2000 cc NA engine at the time. The weight ofthe valve and valve springs was reduced by 40% and16%, respectively. This lowered high engine speed
noise by 30% and camshaft rotational torque by 20%,
thereby helping to boost the performance and fueleconomy of the engine. In comparison, however, theHonda S-2000 that was launched the following year
achieved a maximum engine speed of 9000 rpm andgenerated 250 hp with heat-resistant steel valves dueto the adoption of an ultra-short stroke 2000 cc NA
engine incorporating its VTEC technology.
7. Application of Developed Ti-MMC Valves to
Toyota Formula 1 Engine(10,28,29)
Racing car engines operate at extremely highrotational speeds, and reducing the weight ofreciprocating parts remains an important requirement
to the present day. Since racing cars tend to havelower cost restraints than ordinary vehicles, titanium
alloy had already been widely adopted in racing
engines. The developed Ti-MMC components were
designed to be manufactured at substantially lower
cost than conventional titanium alloy components for
adoption on mass-produced cars. However, the true
potential of this material could not be fully realized ina mass-produced engine. It was thought that moreinnovative designs that made greater use of theexcellent specific strength, specific stiffness, and heat
resistance of this material would enable even greater
component weight reduction, performance gains, andfuel economy.
In 1999, the year after the launch of the mass-produced
Altezza installed with these titanium-based valves,
Toyota announced its decision to participate in F1and started up a constructor team (Panasonic Toyota
Racing) to develop both a racing engine and chassis
under the auspices of Toyota Motorsport GmbH
(TMG) located in the outskirts of Cologne, Germany.
Even after the International Automobile Federation(FIA) changed the regulated engine from a V10 to© Toyota Central R&D Labs., Inc. 2020

19

a V8 configuration, further weight reduction in thevalve train was required to realize engine speedsin excess of 20000 rpm. Dr. Luca Marmorini, theTechnical Director of TMG’s F1 engine division,
noticed the new material (Ti-MMC) developed formass-produced vehicles by Toyota CRDL. At that time,the engines of every F1 team, including Toyota, usedtitanium valves supplied by the U.S. company Del
West Engineering, Inc. It was judged that the Toyota
original material (Ti-MMC) had a higher potential than
the titanium alloy manufactured by Titanium Metals
Corporation (TIMET) and used by Del West in termsof strength, stiffness, and heat resistance. Therefore,
the new material was seen as having a higher potential
ceiling for improving valve performance.
Since the cooling performance of the cylinder headin an F1 engine is far superior to the cylinder heads inmass-produced cars, the temperature rise of the exhaust
valves can be minimized. As a result, the developed
exhaust valve material, which was primarily aimed formass-produced cars, was found to be perfectly adequate
for the exhaust valves of an F1 engine. In contrast,
since the intake port of an F1 engine is much hotter
than in a mass-produced vehicle, the intake valves are
exposed to comparatively higher temperatures. For thisreason, the heat resistance of the Ti-6Al-4V alloy used
for the mass-produced intake valves was insufficient.
In addition, F1 engine intake valves are designed witha larger head diameter and smaller stem diameter for
greater intake efficiency. Consequently, the intake
valve material for an F1 engine must have higher
strength and creep properties than a mass-produced
material. Therefore, the composition of the intake
valve matrix alloy was changed to the same
TI-6Al-4Sn-4Zr-1Mo-1Nb-0.2Si (mass%)-based near-α
alloy composition with high heat resistance that wasused to develop the exhaust valves. The TiB particle
content was also doubled (10 vol%) compared to the
MMC developed for the exhaust valves. Figure 27
compares the appearance of the Ti-MMC intake valves
used on the mass-produced Altezza and in the F1
engine. There are clear differences in head and stem
diameters.
Figure 28 compares the relative mechanicalproperties of the new MMC developed for the F1 engine
intake valves with those of the previous Ti-6Al-4V
alloy used in the Altezza. The figure shows that theF1 material has far greater strength, stiffness, andheat resistance. Increasing fatigue strength and creep

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TF-109RVX-9AltezzaFig. 27

Intake valve
Exhaust valve

F1 ifferent appearances of the intake valves forD
the mass-produced 3S-GE engine (left) and
RVX-09 racing engine (right).

Fig. 29 A ppearance of Panasonic-Toyota Racing F1car (TF-109) and cutaway view of 2398 cc
NA V8 engine (RVX-09).

The engine installed with the developed Ti-MMC
valves was adopted in 2007. The performance of theengine was superior to that of other teams. In 2009,
immediately before Toyota decided to withdraw from
F1, this engine helped Toyota to achieve one poleposition, two fastest laps, and five podium finishes(Fig. 29). Unfortunately, Toyota was unable to achieve
its aim of standing on the center of the podium.
Fig. 28

Comparison of the mechanical propertiesof the developed MMC for F1 intake valves
relative to those of the Altezza intake valve.

8. Achieving Further Cost Reductions
8. 1 Issues of Mass-produced Ti-MMC Engine
Components

properties are particularly important ways of creating
a greater margin in terms of component durability. In
addition to reducing weight and enabling designs withhigher efficient intake geometry, this approach also
helped respond to increasingly stringent regulations.
Specifically, in 2006, when the regulated V10 engine
was changed to a V8 configuration, the minimum
permitted engine life was only two races (a total driving
distance of around 500 km). In 2009, teams wereprohibited from changing engines for eight races (atleast 2200 km), tough conditions that were difficult forevery team to satisfy. The Ti-MMC valves developed
all inside Toyota Group companies (including original
material and manufacturing process) were recognizedas making a major contribution to Toyota’s substantial
improvement in engine power under these conditions.
© Toyota Central R&D Labs., Inc. 2020

The manufacturing cost of the developed Ti-MMC
valves satisfied the original targets and helped to realize
a unit price of less than 300 yen (JPY) per valve. In
conclusion, although the initial objective of adopting
titanium alloy in a mass-produced car was achieved,
titanium alloy has unfortunately yet to establish itself
as a multi-purpose automotive material. Titanium isa feasible alternative for engine valves made fromhigh-cost heat-resistant steel, but further cost reductions
will be necessary before titanium alloy can be adopted
in a wider selection of parts requiring specific strengthand specific stiffness.
The previous sections have described a comparatively
straightforward engine valve manufacturing process
in which a simple cylindrical billet is extruded andforged at ultra-high temperatures of 1200℃ or higher,
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8. 2 Development of Ultra-high-density Compaction
Technology(30)The fastest way to increase the density of thesintered material to as close as possible to the true
density is to raise the density of the green compact.
However, the compacting pressure cannot simply beraised indiscriminately. Since titanium is susceptible
to adhesion to the die, high pressure compaction
is likely to result in a phenomenon known as die
galling. Ordinarily, small amounts of lubricant are
mixed with the powder to prevent seizure. However,increasing the lubricant content has an adverse effecton the properties of the product since the heatingprocess to eliminate the wax lubricant causes reactions
with the titanium and generates impurities in thematerial. Therefore, the lubricant content must be
minimized and it is difficult to increase the compacting
pressure above 600 MPa. In response to these issues,
innovative die lubrication technology was developedthat enabled ultra-high-density powder molding with

© Toyota Central R&D Labs., Inc. 2020

compacting pressures up to 2000 MPa. An outline ofthese technologies is described below.
The key aspect of this technology is the combinationof lithium stearate (Li-Ste.) as a die lubricant withwarm compaction at around 150℃. This approach
was discovered by chance in the second half of the
1990s by a visiting student researcher from Fraunhofer
IFAM Germany, who was working at Toyota CRDL to
identify lubricants for powder forming dies. Although
the initial research was focused on ferrous powders die
lubrication methods, it was discovered that applying
this approach with titanium powder achieved a far
greater lubrication effect than with ferrous powders.
Figure 30 shows an outline of the developed
compaction method. In this method, an aqueous
solution containing 1 mass% of Li-Ste. is sprayed
into a die warmed to approximately 150℃, forming
an Li-Ste. film on the walls of the die. The die isthen filled with raw material powder and compacted.
Figure 31 compares the relationship between the die
ejection force after compaction with the compacting
pressure for a coarse titanium powder (HDH: −#150).
The compacting conditions are (a) compacted at room
temperature lubricated with the conventional zincstearate (Zn-Ste.) and (b) compacted at 150℃ lubricatedwith Li-Ste., respectively. With the conventional
compaction method, the ejection force continues toincrease in accordance with the compacting pressure.
In contrast, with the developed warm compaction
method, the increase in ejection force tops out ataround a compacting pressure of 600 MPa before
decreasing as the compacting pressure increases. The
ejection force then stabilizes at around 1000 MPa andremains extremely low (approximately 5 MPa) evenat a compacting pressure of 1960 MPa. Although thedata for ferrous powders is almost identical to theSpray gunCarbide die

1 mass% Li-Ste.
water solutionBandheater(150℃)

Water evaporates
instantaneously

something that had not previously been thought possible
for titanium. This is a special case where high-yield
net shaping is feasible. However, general mechanicalcomponents often have complex shapes that require
the application of multiple machining processes(Fig. 4) to a forging to achieve net shaping, which
results in a substantial reduction in yield. In contrast,
the B/E method enables the raw material powder to becompacted into a net shape in a die. Converting this
into the finished component by sintering alone would
enable massive cost savings. This approach is identicalto the current manufacturing process for sinteredsteel components, and to realize it with titanium, thefollowing minimum conditions had to be satisfied.
(1) The sintered part must satisfy requirements formechanical properties and reliability without
further processing such as heat treatment.
(2) The sintered part must have sufficient dimensional
accuracy for it to function properly without
further machining.
(3) Mass-production using a continuous sintering
furnace must be possible.
The developed process is capable of achieving highdensities in excess of 97% after sintering and reducing
the size of the pores in the material. However, further
densification would be preferable to ensure sufficientreliability.

T.C.
10 µm
SEM image of Li-Ste. deposit

Fig. 30

 onfiguration of the metallic mold for theCultra-high-pressure warm compaction.

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results shown in Fig. 31, the amount of spring backwith titanium is close to three times that of steel,which means that the lubricating effect of the Li-Ste.
is stronger with titanium powder than the steel powder.
Figure 32 shows the results of the same experimentconducted with a fine titanium powder (HDH: −#350).
The ejection force after compaction did not exceed
1 MPa at almost all compacting pressures up to1960 MPa. In summary, the developed method iscapable of extracting particle compacts compressedat ultra-high pressures from the die using an ejection

pressure of only several kg/cm2. Considering the hugeelastic stress generated by spring back in the compacted
titanium powder, this is regarded as a super-lubrication
phenomenon with a frictional force of virtually zero.
Figure 33 compares the appearance of die sliding
surfaces of pure titanium powder compacts. The
compact on the left was molded normally at a compacting
pressure of 686 MPa. The compact on the right was
molded by the developed method at a compacting
pressure of 1960 MPa. The conventional green compact(pressure: 686 MPa) shows clear evidence of titanium
adhesion (die galling), indicating that the compacting
pressure limit was exceeded. In contrast, the surfaceof the 1960 MPa particle compact is glossy and shows
absolutely no evidence of adhesion.
Figure 34 illustrates the likely mechanism of thissuper-lubrication phenomenon. Time of flight-secondary
ion mass spectroscopy (TOF-SIMS) was carried out

Conventional method

Developed method

ρ = 4.01 g/cc (88.9%)

ρ = 4.49 g/cc (99.5%)(686 MPa at room temperature)

Fig. 31

 elationship between ejection force andRcompacting pressure for conventional
compaction and the developed warmcompaction using a coarse HDH titanium
powder (TS-150).

(1960 MPa at 150℃)

Fig. 33 A ppearance of pure titanium green compacts(HDH: TS-150).

Fig. 32

 elationship between ejection force andRcompacting pressure for conventional
compaction and the developed warmcompaction using a fine HDH titanium
powder (TS-459).

© Toyota Central R&D Labs., Inc. 2020

Fig. 34

 utline of lubricating film transformation onO
the surface of titanium particles during highpressure warm compaction (TOF-SIMS).

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on the side wall surface of the particle compacts.
Analysis results showed that the Lt-Ste. die coating
adhered to the surface of the room temperature particle
compact without transformation. In contrast, the
Li-Ste. transformed into a titanium stearate (Ti-Ste.)at the surface of the warm compaction/high-pressurecompact. Specifically, the interaction of the appropriate
temperature (150℃) and high pressure (higher than
600 MPa) causes a mechano-chemical reaction between
the Li-Ste. coated on the die wall and the titanium
powder surface, transforming the Lt-Ste. into Ti-Ste.
This Ti-Ste. is strongly chemically adsorbed by
the surface of the titanium powder (i.e., the particle
compact), and the super-lubrication phenomenon
described above appears between the Li-Ste., which is
physically adsorbed by the surface of the die.
Figure 35 shows the relationship between relative
density and compacting pressure in the case ofa Ti-6Al-4V alloy green compact molded using thedeveloped method, and the same particle compact after
sintering at 1300℃ for four hours. The relative densityof the sintered compact increases in accordance withthe compacting pressure, rising from around 96% at
490 MPa to over 99% at compacting pressures atand above 1000 MPa. Although achieving a sinteredcompact with close to the true density will naturally
help to improve reliability, the important point of theseresults is the density gap between the green compact andsintered compact decreases as the compacting pressure
increases. Figure 36 shows the relationship between
dimensional change after sintering and compacting
pressure. At normal compacting pressures (under
500 MPa), sintering causes dimensional shrinkage ofmore than 3%. In contrast, with the developed method,
dimensional change decreases as the compacting
pressure rises, falling to an extremely low level ofless than 1% at pressures over 1000 MPa. This meansthat post-sintering machining can be minimized
by increasing the compacting pressure to as higha value as possible. Although reducing the requirement
for post-sintering machining helps to greatly lower
costs, the ideal situation is to eliminate machining
entirely and use the sintered skin without further
processing. However, the fatigue strength and otherproperties of the sintered skin must be sufficient
to allow the sintered skin to be used. Figure 37 shows
the results of a four-point bending fatigue test using
thin-plate sintered components (Ti-6Al-4V alloy),with the top and bottom surfaces used as the die press© Toyota Central R&D Labs., Inc. 2020

Fig. 35

 elationship of green and sintered densitiesRwith compacting pressure for the developed
warm compaction using a coarse HDH
titanium powder (TS-150).

Fig. 36

 elationship between dimensional changeRafter sintering and compacting pressure
for the developed warm compaction using
a coarse HDH titanium powder (TS-150).

surfaces. The graph compares the results of sinteredskins compacted conventionally at a pressure of
392 MPa and the developed method at a pressure of
1568 MPa. A clear difference between the conventionaland high-density compacts can be seen at 105 cycles. At
107 cycles, the durability of the high-density compact
is five times higher than the conventional compact.
This graph also compares the high-density compact
sintered skin with an ingot metallurgy forged plateafter machining. Virtually almost the same results
were achieved at all cycles, which demonstrates thata high-density compact sintered skin has comparable
fatigue properties to a machined I/M forge product.

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8. 3 Improving Productivity Using Continuous
Sintering Furnace(31,32)

Surface roughness (Ra/μm)

Fig. 37

 omparison of four-point bending fatigue
C
property of as-sintered Ti-6Al-4V alloy via
different method carried out on non-machined
surface.

Conventional method

Developed method

Compacting pressure (MPa)Fig. 38

 omparison of the surface roughness
Cand morphology of sintered compacts
manufactured using the conventional anddeveloped compacting methods.

The research described in the previous section
demonstrated that a sintered skin manufactured by theB/E method has equivalent mechanical properties toa high-cost ingot forged and machined part. However,the sintering is carried out in a vacuum furnace,
which is a batch process with low-productivity anda major obstacle to reducing cost. Adopting the same
type of belt-type continuous sintering furnace as
conventional sintered steel parts might enable further
substantial cost reductions. In the second half of the
1990s, this research focused on the ultra-low-oxygen
partial-pressure continuous-furnace(33) developed by
Kanto Yakin Kogyo Co., Ltd. under the name “Oxynon”.
In summary, this is a belt furnace with an inert gas
atmosphere. Critically, the interior of the furnace iscompletely manufactured from carbon, including
the heater, mesh belt and muffle. By includinga small amount of oxygen (< 10−3 atm) in the inert gas
atmosphere, this oxygen reacts with the carbon in the
furnace to maintain the Boudoir equilibrium, which
can be expressed as 2C + O2 ⇒ 2CO. As a result,the inside of the furnace maintains an incredibly
low oxygen partial pressure of between 10−20 and
10−27 atm, which forms deoxygenation conditions fortitanium at sintering temperatures between 1200 and
1300℃. Figure 39 shows an outline of the Oxynon
furnace configuration. Figure 40 compares the tensile
properties of a Ti-6Al-4V sintered alloy manufactured
in a vacuum furnace (< 10−5 torr) and the Oxynon
continuous furnace (in an argon atmosphere). Thereis virtually no difference in mechanical properties atany sintering temperature. This research demonstrated
that titanium parts can be sintered to the same quality
in a mass-production continuous belt furnace as ina vacuum furnace of batch operations.

Figure 38 compares the surface of sintered compacts
manufactured at 392 and 1562 MPa. Compared tothe surface roughness of the conventional compact(Ra = approximately 3.0 μm), the surface roughnessof the high-pressure compact was only Ra = 0.5 μm orless, which is almost equivalent smoothness to that ofa machined part.

© Toyota Central R&D Labs., Inc. 2020

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Specimen
Carbon belt

ExhaustHeaterSpecimenN2 or Ar
gas inlet

ExhaustCarbon muffle

Cooling
chamber
CarbonheaterCarbon belt

Longitudinal section

Ductility

Vacuum furnaceStrength

Tensile elongation (%)

Compacting: 490 MPa
Sintering: 30 min

Sintering temperature (℃)

Fig. 40

Specimen

Cross section

Configuration of the ultra-low oxygen partial pressure continuous furnace (Oxynon).

Gas furnace (Oxynon)

Ultimate tensile strength (MPa)Fig. 39

Carbon belt

 omparison of tensile properties of B/E
C
Ti-6Al-4V alloy with different types ofsintering furnace.

9. Proposal for the General Adoption of Titanium(in Place of a Conclusion)
To cast aside the preconceived idea that titanium is too
high-cost for application to mass-produced automotive
parts, Toyota CRDL worked for about fifteen years
from the middle of the 1980s to the beginning of the
2000s to drastically reduce the cost and substantially
improve the performance of titanium components.
These efforts successfully resulted in materials far© Toyota Central R&D Labs., Inc. 2020

superior to conventional titanium alloys in termsof cost and performances (Fig. 41). In addition tolower cost, in 1998, Toyota CRDL helped to develop
a mass-produced engine exhaust valves, somethingthat was thought to be impossible in terms of
performance. Even higher performance valves werethen adopted by the F1 engine used by PanasonicToyota Racing from 2007.
Through the development of ultra-high-density powder
compaction and continuous sintering technologies,
which was carried out alongside the practical adoptionof the titanium-based engine valves, major progress
was made toward further lowering the manufacturing
cost of titanium components to a level far beyond theconventional conception of titanium alloy. However,
the original goal of this research, which was to achieve
manufacturing costs equivalent to steel components
has yet to be achieved. The price of raw materialtitanium powder is too high to consider titanium asa multi-purpose automotive material. Machining costs
were also minimized by achieving a material yieldof almost 100% through extreme densification anddimensional accuracy. The application of continuous
sintering also helped to realize drastic improvements in
productivity. The final cost of manufacturing a titanium
component is the total of the raw material powder
cost and the process running costs (powder mixing

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⇒ compacting ⇒ sintering). These running costs
decrease as the production volume increases, bringing
the product price close to the price of the raw material
powder. When research into sintered titanium began in
the 1980s, it was possible to obtain extremely low-cost(lower than U.S. $10/kg) and high-performance HSF
titanium powder manufactured by the Hunter method.
However, in the first half of the 1990s, titanium smelting
technology using the Hunter method was expelled ona global scale by the Kroll method. As a result, fromthe 1990s, it was only possible to obtain high-cost(higher than U.S. $20/kg) HDH powder manufactured
from sponge titanium by the Kroll method. By
chance, the low-cost hydrogenated titanium powder
used to manufacture these valves was particularly
suited to the green compact ⇒ dehydrogenation⇒ vacuum sintering ⇒ hot extrusion forging
manufacturing process. However, this powder is notparticularly suited to the innovative combination ofultra-high-density powder compaction and sintering in
an innert gas continuous furnace. If the price of puretitanium powder falls comparable to that of the Huntersponge fine (around U.S. $10/kg), it is not beyond therealms of possibility that high-performance sintered
Ti-MMC can be adopted for a wide range of automotive
components beyond valves. It is hoped that low-cost
pure titanium powder equivalent to HSF will once
again appear on the market.

In the first half of the 1980s, Toyota launched theworld’s first powder forged steel connecting rod.(34)
Since then, sinter forging technology has become
widely used by automakers around the world and is
now firmly established as a high-quality low-cost
process with extremely high product weight accuracy
compared with conventional ingot forging method. It
is thought that sinter forging can be adopted as-is tothe manufacture of Ti-MMC components. Specifically,
this would entail sintering a near net-shaped sinteredpreform in an Oxynon continuous furnace, transportingit to the press while maintaining the sintered compact
at the sintering temperature, and then directly forming
the product by hot forging. This approach has major
potential as a low-cost manufacturing process forultra-high-performance Ti-MMC products.
Finally, this article concludes by introducing
an example application that combines sintering and hot
working. Figure 42 shows several prototype engine
parts made of the TiB/Ti-MMCs. In addition to thecommercialized engine valves, feasibility studies wereconducted on spur gears, connecting rods, valve lifters,
valve retainers, hollow valves, and so on. The spur
gear shown in Fig. 42 was manufactured from 20 vol%TiB/Ti-7Mo-4.5Fe-1.5Al-1.5V MMC by applying
a high frequency induction heating to the circumferenceof a disk-shaped ring sintered compact in air and thencarrying out roll-forming. Although it is very important

Specific strength
Developed TiB/Ti MMC
Conventional titanium alloy
Conventional steel

Specific modulus
Wear resistanceHeat resistanceFig. 41

Affordability

 adar chart comparing typical properties of the developed TiB/Ti-MMC,
R
conventional titanium alloy, and conventional steel.

© Toyota Central R&D Labs., Inc. 2020

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to reduce the weight of gears, the extremely high stressapplied to the gear teeth means that both high stiffness
and rolling wear fatigue properties of the material are
equally important. For this reason, there have been no
proposals to use materials other than steel for theseparts. However, with a high TiB content, the developed
Ti-MMC can achieve the mechanical properties
equivalent to gear steels with high fatigue strength androlling wear resistance, at substantially lower weight(less than 60% of steel). In addition, by optimizing thematrix alloy composition, it may be possible to realize

Sintered density (%)

Fig. 42a sintered compact with a high density of at least 98%of the true density even if the TiB particle content
increases up to 40 vol% (Figs. 43 and 44).(35) This
extreme densification phenomenon at high volume ofTiB particles was verified to be achieved via a uniqueactivated sintering mechanism brought by coexistenceof B, Fe, and Mo.(35) In addition, since this MMC also
has excellent hot workability,(25) it can be easily hotroll-formed into the gear shape shown in Fig. 42. This
prototype gear is an indication of the future wider
potential of titanium.

 rototype automotive parts made of TiB/Ti-MMC produced by the developed B/E
P
process: spur gear, connecting rod, valve lifter, valve retainer, and hollow valve, etc.

TiB/Ti-7Mo-4.5Fe-1.5Al-1.5V

TiB/Ti-6Al-4V
Compacting: 392 MPa
Sintering: 1300℃, 4 hours

20 μm

TiB particle content (vol%)Fig. 43

Relationship of sintered density of
Ti-7Mo-4.5Fe-1.5Al-1.5V based andTi-6Al-4V based MMCs with TiB particle
content obtained by the developed B/Eprocess.

© Toyota Central R&D Labs., Inc. 2020

Fig. 44 intered microstructure of 30 vol%S
TiB/Ti-7Mo-4.5Fe-1.5Al-1.5V MMC
compacted at 392 MPa followedby sintering at 1300℃ for 4 hours(ρ/ρ0 = 99%).

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No. 11 (1971), pp. 1637-1642.
(28) Suzuki,T., Ikehata, H., Takamiya, H. and Saito, T.,“Ryūshi Kyōka-gata Chitan Gōkin no F1 Enjin e no
Tekiyō”, Proc. 23rd Meet. 123rd Comm. Heat Resisting
Materials and Alloys (123 HiMAT) (in Jpapanese),
Vol. 54, No. 3 (2013), pp. 357-365, JSPS.
(29) “Toyota RVX-09 (2009)”, Motor Fan Illustrated,
Special Edition (in Japanese) (2010), pp. 16-23.

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R&D Review of Toyota CRDL, Vol.51 No.1 (2020) 1-30(30) Takamiya, H., Kondoh, M. and Saito, T., “Ultra-highPressure Warm Compaction for P/M Titanium
Components”, Cost-affordable Titanium (2004),
pp. 185-192, TMS.
(31) Saito, T., “Ti-B Alloy for Automobile Use: Remaining
Issues”, presented at: Titanium Alloys Modified with
Boron Workshop; 2005 October 11, Dayton, OH, TMS.
(32) Saito, T., “New Titanium Products via Powder
Metallurgy Process”, Ti-2003, Science andTechnology: Proc. Conf. 10th World Conf. Titanium(2003), pp. 399-410, Wiley-VCH.
(33) Kanda, K., “Development and Application of
the Continuous Atmosphere Controlled Furnace‘OXYNON’”, J. Advanced Science (in Japanese),
Vol. 16, No. 1 (2004), pp. 7-11.
(34) Ohnishi, T., Takahashi, A., Tsumuki, C. and Nagare, I.,“Shōketsu Tanzō Konroddo no Kaihatsu”, Bulletin of
the Japan Institute of Metals (in Japanese), Vol. 22,
No. 6 (1983), pp. 537-539.
(35) Saito, T., Furuta, T. and Takamiya, H., “Sintering
Behavior and Thermal Stability of a TiB Particle
Reinforced PM Beta-Titanium Matrix Composite”,
Titanium ’95: Science and Technology: Proc. 8thWorld Conf. Titanium (1995), pp. 2763-2770.
Fig. 1 (upper row)
Reprinted from Altezza Qualitat catalog (2000), © 2000
TOYOTA CUSTOMIZING & DEVELOPMENT Co., Ltd.,
with permission from TOYOTA CUSTOMIZING &DEVELOPMENT Co., Ltd.
Fig. 2 (upper row)
Reprinted from LFA photo gallery (https://lexus.jp/models/lfa/gallery/), © 2020 LEXUS.

Fig. 29 (upper row)
Reprinted from Toyota Gazoo Racing, TOYOTA F1Archive 2002-2009, TF109 (https://toyotagazooracing.
com/archive/ms/jp/F1archive/team/tf109/index.html),© 2009 Toyota Motor Corporation with permission fromToyota Motor Corporation.
Fig. 29 (lower row)
Reprinted from Motor-Fan TECH (https://motor-fan.jp/
tech/10012932), © 2019 SAN-EI CORPORATION, withpermission from SAN-EI CORPORATION.
Fig. 38
Reprinted from Cost-affordable Titanium (2004),
pp. 185-192, Takamiya, H., Kondoh, M. and Saito, T.,
Ultra-high Pressure Warm Compaction for P/M Titanium
Components, © 2004 TMS, with permission from John
Wiley & Sons.
Fig. 43
Reprinted from Titanium ’95: Science and Technology:
Proc. 8th World Conf. Titanium (1995), pp. 2763-2770,
Saito, T., Furuta, T. and Takamiya, H., Sintering Behavior
and Thermal Stability of a TiB Particle Reinforced PM
Beta-Titanium Matrix Composite, © 1995 The Institute ofMaterials, with permission from Taylor & Francis.
Tables 4-5
Reprinted from Recent Advances in Titanium Metal Matrix
Composites: Proc. Symp. Held during Materials Week
(1995), pp. 33-44, Saito, T., Furuta, T. and Yamaguchi, T.,
Development of Low Cost Titanium Matrix Composite,© 1995 TMS, with permission from The Minerals, Metals& Materials Society.

Fig. 2 (lower row)
Reprinted from Motor Fan Illustrated, Special Edition(2010), pp. 16-23, Toyota RVX-09 (2009), © 2010
SAN-EI CORPORATION, with permission from SAN-EICORPORATION.
Figs. 5, 10, 13-14 and Tables 1-3
Reprinted from R&D Rev. Toyota CRDL, Vol. 29, No. 3(1994), Saito, T. and Furuta, T., Development of High
Performance Titanium Matrix Composite, © 1994
TOYOTA CRDL., INC.
Figs. 24-25
Reprinted from SAE Tech. Pap. Ser., No. 2000-01-0905(2000), Yamaguchi, T., Morishita, H., Iwase, S., Yamada, S.,
Furuta, T. and Saito, T., Development of P/M Titanium
Engine Valves, © 2000 SAE International, withpermission from SAE International.

© Toyota Central R&D Labs., Inc. 2020

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R&D Review of Toyota CRDL, Vol.51 No.1 (2020) 1-30

Takashi Saito
Research Fields:
		 - Physical Metallurgy
		- Alloy Design
		 - Powder Metallurgy
Academic Degree: Dr.Eng.
Academic Societies:
		 - The Japan Institute of Metals and Materials
		 - The Iron and Steel Institute of Japan		 - The Institute of Materials, Minerals and Mining
		 - Society of Automotive Engineers of Japan
Awards:		 - Technical Development Award, The Japan Institute of
Metals and Materials, 1994, 1995, 1996, 1998, 2004
		 - R&D 100 Award, 1996, 1998
		 - JSMS Award for Technical Developments, The
Society of Materials Science, Japan, 1999
		 - Process Development Award, Japan Powder
Metallurgy Association, 2000
		 - Technical Award, The Japan Titanium Society, 2000
		 - Dissertation Award, Gas Turbine Society of Japan,
2002
		 - Persons of Scientific and Technological Merits,
Commendation by the Minister of Education, Culture,
Sports, Science and Technology, Ministry of
Education, Culture, Sports, Science and Technology,
2003© Toyota Central R&D Labs., Inc. 2020

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