Materials Science & Engineering A 739 (2019) 491–498

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Materials Science & Engineering A
journal homepage: www.elsevier.com/locate/msea

Microstructural and mechanical properties of α-titanium sintered material
via thermal decomposition of additive chromium oxide particles

T

Kondoh Katsuyoshia, , Ikemasu Ryuhoa, Umeda Junkoa, Kariya Shotaa, Anak Khantachawanab⁎ab

Osaka University, Japan
King Mongkut's University of Technology Thonburi, Thailand

ARTICLE INFOABSTRACT

Keywords:
Powder metallurgy
Titanium
Chromium oxide
Solid solution
Grain refinement
Zener pinning
Solute dragThe pre-mixed pure Ti and Cr2O3 powder was consolidated by spark plasma sintering (SPS) and hot extrusion tofabricate α-titanium (Ti) materials with oxygen (O) and chromium (Cr) elements by powder metallurgy (PM)process. The Cr2O3 particles were completely decomposed during SPS, and then O and Cr atoms were dissolvedin α-Ti matrix. Oxygen atoms were remarkably improved the mechanical strength of PM Ti-O-Cr alloys by theirsolid solution hardening effect. The Cr solution and Ti4Cr precipitates had important roles to obstruct the grainscoarsening behavior by the solute drag and Zener pinning effects, respectively. The solid solution strengthening
effect by Cr atoms, however, was very limited due to a small Cr solubility of about 0.35 at% in α-Ti phase of Ti-OCr alloys. Since Ti4Cr precipitates with 10–20 µm diameters were not so fine, they hardly contributed to theprecipitation hardening of Ti-O-Cr alloys.

1. Introduction
Titanium (Ti) and its alloys are widely used as aircraft components,
chemical products plant and medical devices due to their high specificstrength, excellent corrosion resistance and biocompatibility [1–3]. For
the further improvement of their mechanical properties, the alloying
design by using the expensive rare metals was mainly discussed in theprevious studies. From a viewpoint of the sustainable use of rare metals,
however, a long-term strategy in the novel materials design and processing must be established [4]. For example, carbon (C), oxygen (O),nitrogen (N), hydrogen (H) and silicon (Si), which are ubiquitous resources, are employed as the alloying elements to improve the mechanical properties of Ti materials by PM process [5–12]. In particular,
O and N atoms are representative α-Ti phase stabilizer, and effectively
improve the mechanical strength of Ti materials by their solid solution
hardening behaviors because of a large solubility of both elements inαTi phase shown in Ti-O and Ti-N binary phase diagrams [13]. At thesame time, it is well known that the ductility significantly decreases
with increase in their contents. In the recent studies, however, PM Ti-O
and Ti-N materials revealed a good balance between high strength andexcellent ductility. In preparation of these Ti materials, the pre-mixed
pure Ti and TiO2 or TiN powder was used as the starting materials. The
additive TiO2 or TiN particles were decomposed during sintering, and O
or N atoms originated from them were dissolved intoα-Ti matrix. In this⁎study, the elemental mixture of pure Ti powder and chromium oxide(Cr2O3) particles was employed to fabricate PM Ti-O-Cr alloys. In SPS
process [14], the additive Cr2O3 particle was decomposed, and O and Cr
elements were dissolved in α-Ti phase. The lattice constant changes bythese solution atoms were quantitatively evaluated by using XRD (X-ray
Diffraction) profiles of PM Ti-O-Cr and Ti-O alloys. Increasing in the
Cr2O3 content, not only Cr solution, but also Ti-Cr precipitates at α-Ti
grain boundaries were effective to obstruct the grain coarsening. Microstructures changes were investigated through SEM (Scanning Electron Microscope) with EBSD (Electron Back Scattering Diffraction),
EPMA (Electron Probe Microanalysis) and TEM (Transmission Electron
Microscope) with EDS (Energy Dispersive X-ray Spectrometry) analysis.
Tensile properties of PM Ti-O-Cr alloys were evaluated, and thestrengthening mechanism was discussed by using their microstructures
analysis results.
2. Experimental procedure
A mean particle size of pure Ti powders (TC-450, Toho Technical
Service Co.) and Cr2O3 particles (KOJUNDO Chemical Laboratory CO.)are 27.8 µm and 2.5 µm, respectively. Both raw powder materials havea purity of 99.9%. The elemental mixture powder of them was preparedby ball milling process for 3.6 ks, where the mixing ratio of Cr2O3
particles to the pre-mixed powder was 0–5 mass%. They were

Corresponding author.
E-mail address: kondoh@jwri.osaka-u.ac.jp (K. Katsuyoshi).

https://doi.org/10.1016/j.msea.2018.10.081
Received 28 February 2018; Received in revised form 16 October 2018; Accepted 16 October 2018
Available online 18 October 2018
0921-5093/ © 2018 Elsevier B.V. All rights reserved.

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consolidated by SPS at 873 K～1273 K in vacuum (6 Pa) to fabricate thesintered billets with 42 mm diameter. The applied pressure of 30 MPa
and sintering time of 3.6 ks were used in SPS process. The homogenization heat treatment at 1273 K for 3.6ks in vacuum (20 Pa) was
additionally applied to these Ti sintered materials. Finally, hot extrusion was carried out to prepare the dense Ti materials, where the preheating temperature of the samples was 1273 K and an extrusion ratioof 12.8 (Φ43 mm→Φ12 mm) were employed. Oxygen, nitrogen, hydrogen and carbon contents of the extruded rods were analyzed, andmicrostructures observation was conducted by using optical microscope, SEM and TEM-EDS. EBSD with Orientation Imaging Microscopy(OIM) analysis was useful to evaluate their textures and grain size
distribution. XRD analysis was also applied to quantitatively measure
the lattice constant changes ofα-Ti crystals in a-axis and c-axis directions because O and Cr solution atoms significantly influenced the lattice expansion. Two tensile test specimens were machined from each
extruded rod, and used to evaluate the tensile properties at room
temperature under a strain ratio of 5.0 × 10−4 /s.

Fig. 2. Dependence of lattice constants in c-axis direction of Ti-5.0 mass%
Cr2O3 and pure Ti sintered materials on sintering temperature.

increased and reached 4.716Å with increasing the temperature up to1073 K. Furthermore, a very small lattice constant difference in sintering at between 1073 K and 1273 K was detected. The previous studies suggest that O solution causes the lattice constant increase in c-axisof hcp-Ti crystal because O atom is one of the interstitial solution elements of α-Ti [7,17]. In addition, a relative intensity of Cr2O3 diffraction peak decreased with increasing the sintering temperature as shownin Fig. 1(a). These results indicate that O atoms solution into α-Ti
crystal occurred after decomposition of Cr2O3 particles in SPS and resulted in the increment of the lattice constant in c-axis when the sintering temperature increased from 873 K to 1273 K. Therefore, the SPS
temperature of 1273 K was employed to consolidate the pre-mixed TiCr2O3 powder in this study.
XRD analysis was applied to Ti-X mass% Cr2O3 (X = 0–3) sinteredmaterials after the homogenization heat treatment at 1273 K for 3.6 ks
in vacuum. Fig. 3 shows a dependence of the lattice constants of α-Ti in
c-axis (a) and a-axis (b) direction on the oxygen content of Ti-X% Cr2O3
sintered materials. At the same time, those of Ti-Ymass%TiO2(Y=0–1.5) materials prepared in the previous study [7] were also used
as comparison data. These lattice constants were calculated from thediffraction peaks of (1 0−1 3) and (1 1−2 2) planes in α-Ti crystals ofeach material. As shown in (a), in the case of Ti-Cr2O3 sintered materials, the lattice constant in c-axis direction proportionally increased
with increase in the oxygen content. An increasing ratio (a gradient of
the approximation straight line) is 5.6 × 10−3 Å/O atom), and has agood agreement with that of Ti-TiO2 materials (5.7 × 10−3Å/O atom).
This is because Ti-Cr binary phase diagram [13] reveals a Cr solubility
limit inα-Ti phase is about 0.6 at%, and then Cr solution effect on thelattice expansion of α-Ti crystal is very small. That is, the lattice

3. Results and discussion
3.1. Oxygen and chromium distribution in Ti sintered materials
Ellingham diagram [15] shows the TiO2 formation free energy,
ΔG[TiO2] is much small compared to ΔG[Cr2O3] in a wide temperature
range from 300 K to 1500 K. It means Cr2O3 particles of the pre-mixed
raw powder are completely reduced during SPS at 873 K～1273 K.
When using the pre-mixed Ti-5mass% Cr2O3 powder, Fig. 1 shows XRD
profiles of Cr2O3 raw particle, the mixture powder, the sintered materials at 873 K~1273 K and pure Ti material sintered at 1273 K. The
analysis results of Cr2O3 peak at 2θ = 33.2–34.0 degree and that of Ti(0 0 0 2) peak at 37.0–39.0° are (a) and (b), respectively. A very small
diffraction peak of Cr2O3 was detected in the profiles of Ti materials
sintered at 873 K and 973 K as shown in Fig. 1(a), while the same peak
was never detected in Ti materials sintered at 1073 K or more. Regarding Ti (0 0 0 2) diffraction peak in Fig. 1(b), increasing in thesintering temperature up to 1073 K, the peak gradually shifted to thelower diffraction angle. It means a distance between the basal planes of
α-Ti crystal, having a hexagonal close-packed (hcp) structure, increased, and the lattice expansion of α-Ti crystal in c-axis direction
occurred. Fig. 2 reveals the dependence of the lattice values in c-axis
direction of Ti-5mass% Cr2O3 and pure Ti sintered materials on thesintering temperature. They were calculated from the XRD profiles
shown in Fig. 1(b) by using Bragg law [16]. Pure Ti sintered materials
have a constant lattice value (~ 4.685Å) when the sintering temperature changed from 873 K to 1273 K. On the other hand, in the case of Ti5.0 mass% Cr2O3 sintered materials, the lattice constant remarkably

Fig. 1. XRD profiles of Cr2O3 particle, Ti-5.0 mass% Cr2O3 raw mixture powder, their sintered materials at 873 K~1273 K and pure Ti sintered material at 1273 K:
2θ = 33.2°~34.0° (a) and 37.0°~39.0° (b).
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Fig. 3. Dependence of lattice constants in c-axis (a) and a-axis (b) directions on oxygen content of Ti-Cr2O3 and Ti-TiO2 sintered materials.

constant increment is mainly due to O solid solution in the case of TiCr2O3 sintered materials as well as Ti-TiO2 materials. As for the lattice
constant change in a-axis direction shown in (b), it shows a constantvalue and independent of the oxygen content. This tendency is similar
to the results in the previous studies on Ti materials with O solid solution [7,18,19].

easily occurs by plastic deformation during extrusion. Therefore, theextruded Ti-Cr2O3 materials with fine grains revealed the randomized
orientation as shown in Fig. 4(c)~(e).
Fig. 5 reveals a dependence of mean grain size of Ti-Cr2O3 and TiTiO2 extruded materials on the oxygen content. The mean grain size ofeach specimen was calculated by using IPF maps in EBSD analysis. In
both, the grain size becomes constant when O content reaches about
1.5 at%. Ti-Cr2O3 materials, however, indicate a remarkable grain refinement compared to Ti-TiO2 materials. It is obvious that Cr distribution in the Ti matrix is significantly effective for the grain refining
behavior, and the role of Cr elements on α-Ti grains refinement will be
discussed in later.
Fig. 6 shows EMPA and TEM-EDS analysis results of Ti-5.0 mass
%Cr2O3 extruded material. The point analysis by TEM-EDS reveals thatabout 3.0 at% O atoms uniformly exist inside ofα-Ti grains and theirboundaries. On the other hand, Cr elements are locally detected at the3.2. Grains refining behavior of Ti extruded materials
Fig. 4 shows inverse pole figure (IPF) maps in the cross-section of
extruded Ti-Cr2O3 materials, where the Cr2O3 content is 0 mass% (a),0.25 mass% (b), 0.5 mass% (c), 1.0 mass%(d) and 3.0 mass% (e). The αTi mean grain size calculated from EBSD results significantly decreased
with increase in the Cr2O3 content. All specimens mainly consist ofequiaxed grains and a texture with (0 0 0 1) basal planes parallel to the
extrusion direction. When the grains are refined, the crystal rotation

Fig. 4. Inverse pole figure (IPF) maps of extruded Ti-Cr2O3 materials and mean grain size, where Cr2O3 contents is 0 mass% (a), 0.25 mass% (b), 0.5 mass% (c), 1.0
mass%(d) and 3.0 mass% (e).
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almost equivalent to the composition ratio of Ti4Cr compounds. It is
concluded that a small amount of Cr atoms uniformly dissolved in α-Tiphase and most of them precipitated as Ti4Cr compounds at the grainboundaries of Ti-5.0 mass%Cr2O3 material.
In order to investigate the effect of Cr elements on the grain refining
behavior of Ti-Cr2O3 extruded materials shown in Fig. 5, the microstructural analysis by EBSD was conducted on Ti-X mass% Cr2O3(X = 0, 0.25, 1.0) materials before and after heat treatment (H.T) in
vacuum. The H.T condition at 823 K for 86.4 ks was employed inconsideration of α-Ti recrystallization temperature of 723 K. Fig. 8
shows the changes in IPF maps and a mean grain size of Ti-Cr2O3 materials before and after H.T, where the Cr2O3 content is 0 mass% (a),0.25 mass% (b) and 1.0 mass% (c). In the case of as-extruded Ti0.25%Cr2O3 material before H.T shown in (b), it has fine equiaxed α-Ti
grains (2.91 µm) compared to pure Ti material (5.82 µm) shown in (a).
The addition of 0.25mass%Cr2O3 particle means 0.15 at% Cr elements
contained in the material, which is smaller than Cr solubility in α-Ti
(0.35 at%) as mentioned above. That is, most of Cr elements are dissolved as the solution atoms in the grains, and their solute drag was
active to obstruct the grains coarsening. This is because the extruded
rods were rapidly cooled in the atmosphere after hot extrusion, andthere was not enough time for their coarsening behavior in cooling. On
the other hand, pure Ti material with no Cr2O3 addition shows a remarkable grain growth after H.T (9.63 µm) as shown in (a). When 0.25
mass%Cr2O3 particle was contained, its specimen also revealed a large
increment of the mean grain size from 2.91 µm to 7.20 µm via H.T in(b). Furthermore, in the case of Ti-1.0 mass%Cr2O3 material in (c), thegrain coarsening behavior was completely stopped, and no change in αTi grain size was observed in the specimen after H.T at 823 K. Fig. 9
indicates Cr mapping results and backscattered electron (BSE) images
by EDS of Ti-Xmass%Cr2O3 extruded materials, where X is 0.25% in (a)and 1.0% in (b), respectively. As mentioned above, most of Cr elements
are dissolved in the matrix, and a very few Cr precipitates locally exist
at the grain boundaries of (a) Ti-0.25 mass%Cr2O3 material. It meansthat both Cr solute drag [21,22] and Zener pinning [23–25] by theprecipitates are limited to obstruct the grains growth during a long-time
H.T at 823 K for 3.6 ks. On the other hand, a lot of Ti4Cr precipitates
uniformly dispersed along the grain boundaries of (b) Ti-1.0 mass
%Cr2O3 material, and resulted in the complete control of grains coarsening behavior by Zener pinning in the specimen after H.T at 823 K
shown in Fig. 8(c). When Zener pinning effect is active to obstruct thegrain growth, its growing force (σ) balances the dragging force by theFig. 5. Dependence of mean grain size of extruded Ti-Cr2O3 and Ti-TiO2 materials on oxygen content.

grain boundaries, and their average content of 16.1 at% is much large
compared to the inside of α-Ti grains with 0.35 at% Cr. By considering
the Ti-Cr binary phase diagram showing that Cr solid solubility limit in
α-Ti phase is about 0.6 at%, Cr solution content of 0.35 at% inα-Tigrains is smaller than this solubility in the phase diagram. The previous
study reported that an experimental solubility of Cr elements in α-Tiphase significantly decreases compared to that in the Ti-Cr phase diagram when both Cr and N atoms dissolve in α-Ti at the same time [20].
Therefore, this material also shows a smaller Cr solubility of about
0.35 at% due to the solid solution of both Cr and O atoms in α-Ti. On
the other hand, as mentioned above, Cr content of 16.1 at% at grainboundaries extremely exceeds Cr solubility limit, and then Cr elements
locally precipitate as Ti-Cr compounds at the boundaries after sintering.
An electron diffraction analysis by TEM was conducted for phase
identification of these Cr elements shown in Fig. 6(e). The diffraction
pattern successfully detected the above precipitates as Ti4Cr intermetallic compounds as shown in Fig. 7(b), and α-Ti structures wereobserved inside the grains in (c). In addition, as shown in Fig. 6(e),TEM-EDS analysis on Cr elements segregated at the grain boundary(No.6-13) indicates Ti: Cr = 80:16（at%）, and this atomic ratio isFig. 6. EPMA results of extruded Ti-5.0 mass%Cr2O3 material; Ti (a), Cr (b), O (c), backscattered (BS) electron image (d) and TEM-EDS analysis results (e) of O, Ti
and Cr elements content (at%) inside α-Ti grains and compounds near grain boundary. No.1–5 and 14–18 (+): Inside of α-Ti grains, No.6–13 (•): Inside of compounds
at grain boundary.
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Fig. 7. Electron diffraction patterns of extruded Ti-5.0 mass% Cr2O3 material, TEM image (a), diffraction patterns of Ti4Cr near grain boundary (b) and α-Ti (c) ingrain.

Fig. 8. Changes in IPF maps and mean grain size of extruded Ti-Cr2O3 materials before and after heat treatment at 823 K for 86.4 ks in vacuum, where Cr2O3 content
is 0 mass% (a), 0.25 mass% (b) and 1.0 mass% (c).

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Fig. 9. Chromium mapping and backscattered electron images of extruded Ti-Cr2O3 materials with Cr2O3 content of 0.25 mass% (a) and 1.0 mass% (b).

precipitates at the boundaries ( Gpta ), which is express as Eq. (1) by
Zener-Smith model [23].

Gpta

d
nV
2

Therefore, according the above Eq. (1), it is concluded that Zener drageffect by Ti4Cr compounds at the grain boundaries is dominant to obstruct α-Ti grains coarsening behavior of Ti-1.0 mass%Cr2O3 material.

(1)

3.3. Tensile properties of Ti-Cr2O3 materials and their strengthening
mechanism

where, d, n and V is a diameter, a number density per unit area, andmolar volume of precipitates, respectively. As mentioned above in
Fig. 9, a number density of Ti4Cr precipitates of Ti-1.0 mass%Cr2O3
material is much larger than that of Ti-0.25 mass%Cr2O3 one.

Fig. 10 shows a dependence of yield stress (a) and elongation to
failure (b) of Ti-Cr2O3 and Ti-TiO2 extruded materials on the O content

Fig. 10. Dependence of yield stress (a) and elongation to failure (b) of extruded Ti-Cr2O3 and Ti-TiO2 materials on oxygen content.
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at the ambient temperature. The yield stress (YS) of the former remarkably increases with increase in O content as well as Ti-TiO2 materials, which are strengthened by O solid solution mechanism [26]. As
shown in (b), a high enough elongation more than 20% is obtained until
the addition of 1.0 mass% Cr2O3 particles (oxygen content; 1.90 at%).
In the case of Ti-3.0 mass% Cr2O3 material (O content; 3.53 at%),however, it is remarkably reduced to about 2%, and the specimen isvery brittle due to a lot of Ti4Cr compounds dispersed in the matrix. On
the other hand, focusing on the gradient of YS line of each specimen
shown in Fig. 10(a), Ti-Cr2O3 material has two different slopes in therelationship between YS and oxygen content even though the gradientof Ti-TiO2 material is constant (276.2 MPa/at% O). In particular, theslope of Ti-Cr2O3 materials with 1.90 at% O or less (512.9 MPa/at% O)
is significantly large. It means these materials are strengthened by both
O and Cr elements originated from Cr2O3 particles. On the other hand,when O content is over 1.90 at%, the slope of Ti-Cr2O3 materials(272.5 MPa/at% O) is almost equal to that of Ti-TiO2 materials. It indicates the main strengthening factor is O solid solution hardening
when Ti-Cr2O3 materials contain more than 1.90 at% O elements.
In consideration of both α-Ti grains refinement and solid solution
behaviors of the extruded Ti-Cr2O3 materials with O content less than
1.90 at% shown in Fig. 5, the strengthening factors by the grain refinement, O and Cr solid solution are effective. The quantitative evaluation of each factor of Ti-Cr2O3 materials is carried out as follows;First of all, the grain refining effect (σy[gr], gr; grain refining) can be
estimated by using Hall-Petch equation express as shown below[27,28];y [gr ] =1y0 + Kd 2

Fig. 11. YS increment of theoretical using Labusch model and revised experimental* as function of oxygen content of extruded Ti-Cr2O3 materials. * Effectof grain refinement on YS increment was removed from original experimental
values.

specimen employed in the previous works [9] are used in this study.
Fig. 11 shows Δσy[sss] by O solution using the above Eq. (3) shown as“Theoretical” and the revised experimental as function of O content of
Ti-Cr2O3 extruded materials, where “revised experimental” means the
YS increment after removing the grain refining effect (Δσy[gr]) from theoriginal experimental values. The theoretical and revised experimental
results increase with increasing the O content, and the theoretical results are also generally consistent with the experimental values. The
strict analysis, however, indicates that the revised experimental is
40 MPa larger than the theoretical. Fig. 12 reveals a difference between
the above theoretical and experimental results (Δσy[d], d; difference) as
function of the Cr content of Ti-Cr2O3 extruded materials. Δσy[d] isconstant in the Cr content more than about 0.35 at%, which means anexperimental solid solubility of Cr atoms in α-Ti shown in Fig. 6(e). As
mentioned above, the grains refining behavior, O and Cr elements solid
solution are the main strengthening factors of Ti-Cr2O3 extruded materials. In addition, when Ti4Cr precipitation hardening is active toimprove their mechanical property, the estimated diameter of such
effective precipitates is less than about 25 nm [31]. The size of Ti-Cr
precipitates is, however, about 2–10 µm as shown in Fig. 9, and then
their precipitation hardening effect is very limited. On the other hand,

(2)
−1/2

where, K is Hall-Petch constant (18.6 MPa/mm[17]) and d is amean grain size of the specimen. Table 1 shows the oxygen content, ameanα-Ti grain size, and YS increment (Δσy[gr]) calculated by Eq. (2)of Ti-Cr2O3 and Ti-TiO2 extruded materials. The YS increment(94–232 MPa) by grains refining of Ti-Cr2O3 materials is much larger
than that (20–32 MPa) of Ti-TiO2 materials due to the remarkable grain
refinement by Zener pinning effect of the former as shown in Fig. 5.
With regard to the solid solution strengthening effect (Δσy[sss], sss;
solid solution strengthening), O and Cr elements are active as the dissolved atoms to increase the lattice constant in c-axis of hcp α-Ti
crystals of Ti-Cr2O3 materials. At first,Δσy[sss] by oxygen solution is
calculated by using Labusch limit shown as follows [29];y [sss ] =

0

SF

=

1 Fm4 c 2w
SF 4Gb913(3)

where, Schmid factor (SF) of 0.44–0.45 is measured by SEM-EBSD
analysis on each specimen. A maximum attractive force (Fm) of
6.22 × 10−10 [30], which was experimentally and theoretically determined in the previous study regarding Ti extruded materials with O
solution, is employed. The O solution concentration (c), four parabolicarcs of width (w), shear modulus (G), and Burger's vector (b) of each
Table 1Oxygen content, mean grain size and YS increment estimated by Hall-Petchequation of extruded Ti-Cr2O3 and Ti-TiO2 materials.

Cr2O3

TiO2

Oxygen content(at%)

Mean grainsize, d/μm

YS increment by grain
refinement, Δσy[GR]/MPa

0.98
1.16
1.35
1.90
3.53
0.65
1.35
1.76
2.34

5.8
2.9
2.3
1.5
1.4
11.6
9.3
8.2
8.7

－
94.0
136.2
230.2
232.1
－
19.5
31.7
25.9

Fig. 12. Relationship between experimental YS increment* and chromium
contents of extruded Ti-Cr2O3 materials. * Effects of grain refinement andoxygen solid solution on YS increment were removed from original experimental values.
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in consideration of the Cr solubility in α-Ti matrix of 0.35 at% and thesaturated Δσy[d] of about 40 MPa in Cr content of 0.35 at% or more, it
is concluded that the difference ofΔσy[sss] between the theoretical andexperimental values shown in Fig. 11 is due to Cr solution strengthening effect, not by Ti4Cr precipitation hardening behavior.

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4. Conclusions
Microstructural and mechanical properties of PM Ti-Cr2O3 materials, containing the dissolved O and Cr atoms and Ti4Cr precipitates
after a thermal decomposition of Cr2O3 particles in sintering, were investigated. The Cr dissolved atoms and Ti4Cr compounds were effective
to obstruct the grain growth by the solute drag and Zener pinning, respectively, and resulted in the mechanical strength improvement by αTi grains refinement. Both O and Cr elements had important roles toimprove yield stress by each solution hardening behavior. The former,
however, was much effective to the strength improvement compared to
Cr solution atoms. Ti4Cr precipitates with 10–20 µm diameters were notso fine, and then hardly contributed to their precipitation hardening forthe mechanical improvement of PM Ti-O-Cr materials.
Acknowledgement
A part of this study was financially supported by Japan Science andTechnology Agency under Industry-Academia Collaborative R&D
Program “Heterogeneous Structure Control: Towards Innovative
Development of Metallic Structural Materials” and JSPS KAKENHI
Grant Number JP16H02408. The authors also show thanks to Mr.
Yoshinori Muraki, Mitsuo Horie, Ryoji Minamitani and Hisashi Imai for
their technical supports and operation in the experiments.
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