Reactive processing of ultra-light iron foam with high specific stiffness���・���������������������
Makoto Kobashi, Department of Materials Science Engineering, Nagoya University
名古屋大学

小橋

眞

1 introduction
Porous metals have unique properties such as high-specific stiffness, damping capacity, heatinsulating effect and so forth. By these characteristics, they are expected to be applied to impact
absorbers, sound absorption panels and compact heat exchangers [1]. Porous aluminum alloys havebeen studied by many researchers [2-4], and nowadays iron based porous metals are also researchedsince they possess several attractive properties such as high strength, high stiffness, high melting
temperature and compatibility with steel components [5-9]. However, there are some difficulties tomanufacture iron based porous materials. For example, high temperature near the melting point of ironis required for the fabrication, and the drainage occurs easily because the specific gravity of iron islarger than that of aluminum.
The applicants have developed an innovative manufacturing process, “reactive precursor method”,
in which the precursors consisting of at least two elemental powders are used. The elemental powders
should react with each other and release high heat of formation to let the combustion reaction happen[10-15].

Once the combustion reaction occurs, inorganic compounds (ceramics or intermetallics) are

synthesized and the temperature of the reaction products increases rapidly due to the heat of formation.
It is known that the specimens after the combustion reaction are usually porous. The residual
atmospheric gas originally existed in the precursor and the released gases originally adsorbed at orabsorbed in the elemental powders are the main sources of the pores.

Since the combustion reaction

generates large amount of heat, the specimen’s temperature is much higher than the atmospherictemperature during the reaction. Therefore the cooling rate is very fast and the solidification occurs
rapidly. This is also an advantage to avoid the severe drainage.
In this research, we chose three elemental powders (iron, titanium and boron) as starting powders.
By heating the blended [Fe+Ti+B] powder, a combustion reaction shown below occurs.
αFe + Ti + 2B → αFe + TiB� �323kJ mol�� TiB2�・・・(1)By changing the α in eq.(1), the volume fraction of TiB2 in iron can be controlled. It is reported that
the TiB2 is thermodynamically stable in iron and suitable as the strengthening ceramics in iron. Asidefrom releasing the large amount of heat of reaction, TiB2 particles play another important role toincrease the viscosity of molten iron, which can stabilize the foam structure.

Additionally, theinterface bonding between iron and TiB2 is reported to be strong and the dispersion of TiB2 particles
improves the stiffness of iron.
The objective of this research is to develop a processing technique to manufacture porous Fe/TiB2－ 119 －

composites from the reactive precursors. In this research, the carbon powder was added in order tocontrol the pore morphology and porosity (foaming agent). We investigated the effect of the volume
ration of TiB2 addition on the foaming behavior and also the effect of various foaming agent on
porosity.
2 Experimental Procedures
Iron powder (3-5µm, 99.99%), titanium powder (<45µm, 99.9%) and boron powder (<45µm, 99%)
were blended to synthesize 20 to 70vol% TiB2 particles in Fe/TiB2 composites after the reaction (1). A
foaming agent powder was added to the blended powder. The additive amount of carbon was varied
from 1.0 to 5.0mass%. The blended powder was compressed into a cylindrical shape (φ10×h10 mm)
by a uniaxial cold-pressing under 200MPa. The relative density of the precursors was about 0.67. The
precursor was heated in an induction furnace under an argon atmosphere to induce the combustion
reaction (heating rate: 3.5℃･min-1).

The specimen was cooled in the furnace immediately after theglow of the combustion reaction was observed. The porosity of the specimen was measured by anArchimedes method. The cross section of the specimen was observed by a scanning electron
microscope (SEM). The temperature profile during the combustion reaction was measured by insertinga thermocouple in the specimen. The quantitative analysis of a gas phase in the pore was carried out by
an inert gas fusion infrared absorption method (LECO TS 600).

The carbon concentration in the ironpowder was measured by a combustion-infrared absorptiometric method (LECO CS444).
3 Results and Discussion
3.1 Reaction between Fe, Ti and B powder
Fig.1 shows adiabatic temperatures of the combustion synthesized specimens with various volume
fractions of TiB2. The adiabatic temperature is the calculated maximum attainable temperature of thespecimen during the combustion reaction on the assumption that the specimen is under the adiabatic
state. In this experiment, the adiabatic temperature was calculated by the following equation.
T

T

T∆H� + �T � C���� dT + ∆H������� + �T �� C���� dT + �T �� C��T��� dT = 0
�

�

�

(2)

Where ΔHf is the heat of formation (J), ΔHmelt is the heat of fusion (J), T0 is the initial temperature(K), Cp is the heat capacity (J･mol-1･K-1), Tm is the melting temperature of iron and Tad is the adiabatic
temperature of combustion (K).
In order to fabricate closed-cell-type porous metals by combustion reactions, the adiabatic
temperature must exceed the melting point of the metal.
The maximum temperature did not exceed the melting point of iron, when TiB2 volume fraction wasblow 20 vol%. When TiB2 volume fraction reached to 70vol%, the maximum temperature exceeded
the boiling point of iron. This extremely high adiabatic temperature often ended up with the explosion－ 120 －

of the sppecimen duriing the combbustion reactiion. According to this caalculation, wee decided to carry out
this expeeriment withh TiB2 volumme fractions inn between 20
0 and 70vol%%.
Fig.2 showss
the crooss sections of the specim
mens with 20
0~70vol% TiiB2. With 20vol% TiB2 dispersion,d
the glow
w of the com
mbustion reaction was veery weak annd the closedd pores weree not observ
ved. With
30~60vool% TiB2 disspersion, thee glow of thhe combustio
on reaction was
w clearly observed an
nd closedpores weere formed (pore(sizes area about 1mmm). With 70
0vol% TiB2 addition, thee specimen exploded.
e
The expllosion occurrred because the adiabaticc temperaturee exceeded thhe boiling pooint of iron.

Fig.1 Adiaabatic temperature of thee specimens withw various TiB2 volumee fractions.

TiB2

20vol%

30
0vol%

40vol%

50vol%

60vol%

70vvol%―Cross secttionof thenspecimen

5mm
Fig.2 CrossC
sectionss of the speciimens with variousv
volum
me fractions of TiB2.
50vol%TiB2). TiB2 partticles (2-3µm) were
Fig.3 shows the microstructuure of the cell wall (5uniformlly dispersed in the iron matrix. Fig.4 shows thee temperaturre profile duuring the com
mbustionreaction.. Ignition tem
mperature waas confirmedd as 960℃. ThisT temperature is aboutt much lowerr than themelting point of iroon. Ignition temperatures were nearrly constant (about 950℃℃) regardless of theT 2. Althouugh the maxiimum tempeerature couldd not be meaasured correectly, it isvolume fraction of TiBapparentt that the coombustion reeaction occuurred in a shhort time. The
T temperatture of the specimens
exceededd the meltinng point of iron for aboout only 2 seconds.
sBassed on thesee results, We
W mainly－ 121 －

researched the effect of foaming agents for the specimens with 30~60vol%TiB2 in the following
sections.
2400
2200

TiB2

5mm

2000
1800
Temperature, T/℃

1600
1400
1200
1000
800
600
400
200
0
150

10µm

200

250

300
Time, t/s

350

400

450

Fig.3 SEM micrograph of the specimen of

Fig.4 Temperature profile of the specimen

50vol%TiB2 without foaming agent.

of 50vol%TiB2 without foaming agent.

3.2 Effect of carbon additive amountIn this section, the effect of the carbon additive amount is discussed. Fig.5 shows cross sections of
the specimens with 0~5.0mass%C additions (TiB2 volume fractions: 30~60vol%). When carbon
additive amounts were 1.0 and 2.0mass%, both the number of pores and the pore size were increased
in comparison with the non-added specimen. The porosity of the specimen prepared with 50vol%TiB2
and 3.0mass%C showed the highest value (60%). The closed pores were no longer formed by adding
3.0 and 4.0 mass%C when TiB2 volume fractions were 30, 40% and 50%.
Fig. 6 shows the porosities of the specimens with 1.0~3.0mass%C additions. Porosities of thespecimens with 1.0, 2.0 and 3.0mass%C additions were 48%, 51% and 60%, respectively. Fig. 7
shows the macroscopic and microscopic cross-sections of the specimen with the 3.0mass%C addition.
The pore size was about 800µm and the closed cell-structure was apparent (Fig. 7(a)). TiB2 particles(2-3µm) were uniformly dispersed in the iron matrix (Fig. 7 (b)). Fig. 8 shows the microstructure of
the specimen with 5.0mass%C addition. The cell structure of the specimen changed to an open-cell
type with the pore size under 100µm. Fig. 9 shows calculated adiabatic temperatures of the
combustion synthesized specimen with various carbon additive amounts. Fig. 10 shows temperature
profiles of the specimens with 0, 3.0 and 5.0mass%C additions. According to Fig. 9, it is apparent thatthe theoretical adiabatic temperature is not significantly affected by the carbon additive amount.
Nevertheless, with respect to the practical temperature profiles, the maximum temperatures exceed themelting point of iron when carbon additive amounts are 0 and 3.0mass%, but the exothermic peak of
the specimen with 5.0mass%C addition was lower than the melting point of iron. In this case, thepresence of the carbon powder between titanium and boron powders, and might prevent the reaction.

－ 122 －

C

4.0mass%

5.0mass%

30vol%--

40vol%--

TiB2

0mass%

1.0mass%

2.0mass%

3.0mass%

50vol%

60vol%

5mm
Fig.5 Cross sections of the specimens with 0~5.0mass%C additions (30~60vol%TiB2)

70
60%

60
51%
48%

Porosity, %

50

40

30

28%

20

10

0
0%C

1.0mass%C

2.0mass%C

3.0mass%C

Fig. 6 Porosities of the specimens with 0~3.0mass%C additions (TiB2 50vol%).

－ 123 －

(a)(b)
solid section

pore

TiB2

500µm

50μm

5µm

Fig. 7 SEM micrographs of the specimen with a

Fig. 8 SEM micrograph of

3.0mass%C addition (50vol%TiB2) (a)Macroscopic

thecross-section,

5.0mass%C(b) microstructure

specimenwithaddition

3500

2200

3000

2000

2500

1800

Temperature, T/℃

Temperature, T/℃(50vol%TiB2)

2000
1500
1000
500

1600
1400
1200
1000
800

0
0

1

2

3

4

5

Additive carbon amount（ mass%）

60mass%C

3.0mass%C

5.0mass%C

600

100s

Fig. 9 Adiabatic temperature of theFig.10combustion synthesized specimens withspecimens with various amounts ofvariouscarbon (50vol%TiB2).

carbonadditiveamounts

Temperature

profilesofthe(50vol%TiB2)

4 Conclusions
To fabricate porous Fe/TiB2 composites by chemical reaction, the reactive precursor method using
iron, titanium, boron powders was investigated. The following results were obtained through theexperiment.
1) For making porous Fe/TiB2 composites, the carbon powder turned out to be a suitable foaming
agent. The reason why the porosity increases are (i) the melting point of Fe decreases by carburizing
and (ii) carbon is oxidized during the combustion process and forms gaseous CO.

The oxygen was

originally dissolved in the elemental powders.
2) When TiB2 volume fraction was 50% and carbon additive amount was 3.0mass%, the porosity
reached the maximum (60%) and the pore size was about 800µm.

－ 124 －In the solid section, TiB2 particles

(2-3µm) were uniformly dispersed in iron matrix.
3) By increasing carbon additive amount (>4~5mass%), the closed-cell porous material could nolonger be obtained.

The accessibly added carbon prevented a contact between iron, titanium andboron powders and disturb the reaction.

Acknowledgement
This work was supported by JFE 21st Century Foundation.

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