Preparation of strong and ductile pure titanium via two-step rapid sintering of TiH2 powder

Article abstract of DOI:10.1016/j.jallcom.2016.05.020

The present work demonstrates the feasibility of preparing bulk-Ti, with high strength and good ductility, via spark plasma sintering of TiH₂ powders. The microstructure and mechanical properties of bulk titanium prepared under two different pr

Full text of DOI:10.1016/j.jallcom.2016.05.020

Accepted Manuscript
Preparation of strong and ductile pure titanium via two-step rapid sintering of TiH
powder
2
Bhupendra Sharma, Sanjay Kumar Vajpai, Kei Ameyama
PII:
S0925-8388(16)31325-1
10.1016/j.jallcom.2016.05.020
JALCOM 37525
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 24 February 2016
Revised Date: 20 April 2016
Accepted Date: 5 May 2016
Please cite this article as: B. Sharma, S.K. Vajpai, K. Ameyama, Preparation of strong and ductile pure
titanium via two-step rapid sintering of TiH powder, Journal of Alloys and Compounds (2016), doi:
2
10.1016/j.jallcom.2016.05.020.
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Preparation of Strong and Ductile Pure Titanium via Two-Step Rapid Sintering of
TiH Powder
2
a
b
a
Bhupendra Sharma , Sanjay Kumar Vajpai , Kei Ameyama
a
Department of Mechanical Engineering, College of Science and Engineering, Ritsumeikan
University, 1-1-1 Noji-Higashi, Kusatsu-city, Shiga 525-8577, Japan
b
Research Organization of Science and Technology, Ritsumeikan University, 1-1-1 Noji-
Higashi, Kusatsu-city, Shiga 525-8577, Japan
*
Corresponding author: Bhupendra Sharma, Department of Mechanical Engineering,
Ritsumeikan University, Biwako Kusatsu Campus, 1-1-1 Noji-Higashi, Kusatsu, Shiga,
25-8577, Japan. Tel.: +81 090-1222-1772; fax: +81 77-561-2665.
E-mail: gr0185fx@ed.ritsumei.ac.jp
5
1

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Abstract
The present work demonstrates the feasibility of preparing bulk-Ti, with high strength
and good ductility, via spark plasma sintering of TiH powders. The microstructure and
2
mechanical properties of bulk titanium prepared under two different processing conditions,
i.e. (i) Simultaneous dehydrogenation and sintering under one-step spark plasma sintering
(SPS), and (ii) dehydrogenation in vacuum followed by Spark Plasma Sintering i.e. two-
step SPS , are presented and discussed. The one-step sintering process, i.e. process (i),
resulted in Ti specimens with very high tensile strength, but poor ductility. On the other
hand, two-step sintering process, i.e. process (ii), resulted in bulk Ti with high strength and
high ductility.
Keywords
Titanium hydride, Pure-Ti, Spark Plasma Sintering
1
.
Introduction
Titanium alloys have exceptional potential for expanded usage in various industries.
However, initial high cost of Ti and the complex fabrication and subsequent machining
process often limit their applications. However, powder metallurgy (P/M) processing has
emerged as an efficient method with a possibility to produce titanium alloys with wide
range of compositions, controlled properties, and near net shape characteristics. Particularly,
P/M processing approach based on elemental powders are of particular interest due to
relatively lower processing cost as well as its higher maneuverability with respect to
materials, microstructural, and product designing [1, 2]. However, the usage of pure-Ti
powder poses several problems, such as high affinity toward oxygen, nitrogen, and carbon,
sticking with vials and balls during blending and milling, and higher cost associated with
preparing
compositional/microstructural control together with less than expected cost advantages [3,
]. In comparison to pure-Ti powder, use of titanium hydride powder has several
high-purity
Ti
powders.
These
factors
lead
to
lack
of
4
advantages, including lower initial price, less impurity contents, and relative ease of
handling during mechanical mixing/milling without encountering the problem of sticking
due to its inherent brittleness, leading to higher powder yield, smaller particle size, and
better compositional control. The fine-sized titanium hydride powder can be utilized to
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obtain sintered products with high density, fine grained microstructure, and relatively
higher strength at a relatively lower sintering temperature [5, 6].
Various techniques are used for the consolidation of powders [7-10]. Particularly,
spark plasma sintering technique has several advantages over the conventional hot pressing,
including higher heating/cooling rates and a greater applied mechanical load. Hence, SPS
technique offers a possibility to achieve dehydrogenation and consolidation of titanium-
hydride powder in a comparatively shorter period of time, together with high density
products with controlled microstructure and properties [11]. However, a very small amount
2
of information is available in the literature on the TiH based powder metallurgy
technology and mechanical properties of bulk-Ti alloys prepared using titanium hydride
powders [12-16, table-2]. Therefore, it would be interesting to carry out a feasibility study
about the microstructure and mechanical properties of bulk-Ti prepared via SPS of TiH2
powder, to realize the full potential of elemental powder based P/M approach using
titanium hydride powders. Hence, the present study is an initiative to evaluate the
feasibility of fabricating high purity bulk-Ti via spark plasma sintering using titanium
hydride powder as starting material. The microstructure and mechanical properties of the
fabricated Ti compacts, thus prepared, are presented and discussed.
2
.
Materials and Methods
TiH powder (45µm pass, ~99 % pure, supplied by “Kojundo Chemical Laboratory
2
Co. Ltd.” Japan) and pure-Ti (45µm pass, 99.98% pure, Gas-atomized low oxygen titanium
powder, supplied by “Osaka Titanium Technologies Co. Ltd”, Japan) were used as starting
materials.. The sintering was carried out using graphite die and punch under high vacuum
-3
atmosphere (~10 Pa) in DR.SINTER Spark Plasma Sintering machine (“Sumitomo Coal
Mining Co. Ltd.”, Japan). The powders were consolidated using two different approaches:
(i) One-Step SPS process: the pressure and temperature were increased simultaneously
from zero pressure to 50 MPa and room temperature to 1200 °C, respectively. The holding
time at 1200 °C was 1 h. Subsequently, the furnace was cooled to room temperature. High
purity Ti powder was also sintered under similar conditions. (ii) Two-Step SPS process:
The first step involved dehydrogenation at 800 °C for 0.5 h without any applied external
pressure for maximum dehydrogenation. In the second step, temperature and pressure were
increased, simultaneously, from 800 °C to 1200 °C and zero pressure to 50MPa,
respectively. Subsequently, the powder mass was held at 1200 °C for 0.5 h followed by
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furnace cooling up to room temperature. After sintering, disc-shaped compacts with
dimensions 15 mm (diameter) x 5mm (height) were obtained. Henceforth, in the subsequent
discussion, the specimens prepared by one-step sintering of TiH , two-step sintering of
2
2
TiH , and one-step sintering of pure Ti powders will be referred as Ti-1, Ti-2, and Ti-P
-4
specimens, respectively. Tensile tests were carried out at an initial strain rate of 5.6x10
using specimens with gauge size 3mmx1mmx1mm. Average Vickers micro-hardness was
obtained at a load of 980.7 mN (Hv0.1) and dwell time 10 sec. Moreover, dehydrogenation
temperature of TiH was analyzed by TG/DTA (Thermo gravimetric and differential
2
-1
thermal analysis) and at a constant heating rate of 20 Kmin under argon atmosphere.
3
3
.
Results and discussion
.1. Initial Powders
The results of XRD analysis of the initial powders are shown in Fig. 1(a). For pure Ti
and TiH powders, hexagonal close-packed (hcp) and cubic phases were identified,
2
respectively. The XRD pattern of the titanium hydride powder was similar to the standard
data corresponding to the hydride powder containing 95.96 mass% titanium and 4.04
mass% hydrogen. Fig.1 (b) and 1(c) shows DTA and TGA plots, respectively, obtained
from as received TiH powder. The presence of two overlapping endothermic events clearly
2
suggests that the decomposition process occurs in two stages. The DTA plot shows one
1
small peak (indicated by T ) at 469 °C due to low enthalpy change followed by second
large peak (indicated by T ) at 536 °C due to comparatively larger enthalpy change.
2
Thermo-gravimetric analysis confirms that dehydrogenation starts above approximately
4
40 °C and completes at approximately 700 °C (Fig.1c). A first hand estimation from TG
analysis shows that approximately 3.53 mass% hydrogen was lost, which is slightly lower
than the theoretical hydrogen content, i.e. 4.04 mass%, in the initial hydride powder.
Similar findings have also been reported by other researchers [17, 18].
3
.2. Microstructural Characteristics of the Sintered Bulk -Ti Specimens
In Fig.2 (a), the XRD profiles of Ti-1 and Ti-2 specimens are compared with the Ti-
P specimen. The diffraction patterns of Ti-P specimen consist of hcp α-phase only.
However, Ti-1 specimens indicated the presence of δ-phase (tetragonal hydrides) along
with α-phase. Interestingly, the XRD profile of Ti-2 specimen exhibit the presence of
primarily α-phase with minor amounts of δ and γ-phases. In the Ti-H system, following
hydride phases are known to exist; (i) δ-phase (51.22-66.67 at% hydrogen) with f.c.c.
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structure of CaF
2
type in which hydrogen atoms occupy tetrahedral sites randomly and (ii)
the γ hydrides (1-2.9 at% hydrogen) with f.c.t. structure and having c/a ~ 1.09 [19]. These
results indicate that a significant quantity of hydrogen is retained in the Ti-1 specimen. In
the Ti-2 specimen, the presence of γ-phase indicates the retention of relatively lower
amounts of hydrogen as compared to that of Ti-1 specimen. Therefore, it is apparent that
the two-step process is more effective in achieving sintered compacts with significantly
small amounts of retained hydrogen. In titanium, for hydrogen concentrations lower than 20
at%, in addition to the strong diffraction peaks from α phase, the weak diffraction peaks i.e.
{
111} {002} and {200} from the γ-phase and very weak diffraction peaks i.e. {111} from
δ-phase was also reported by Nakamura et al. [19].
In addition to the above results, the relative amounts of hydrogen in the pure
titanium compacts, prepared from titanium hydride powders, was also estimated by
calculating lattice parameters using the XRD profiles (shown in Table-1). It can be
observed that the c/a ratio of α-phase in Ti-1 specimen is larger than that calculated for Ti-2
specimen. In general, increasing the hydrogen concentration leads to an increase in c/a ratio
and to a negligible increase in the specific lattice volume [20, 21]. The calculated lattice
parameters for δ and γ-phases were found in good agreement with the results presented by
A. San-Martin et al. [20].
The Ti-P specimen exhibited lath-type microstructure with basket-weave type
morphology, having lath-width ~20µm and lath-length in the range of 50 to 200 µm
(
Fig.2b). Generally, in Ti alloys, such microstructure is developed due to slow cooling from
β-region. The α-phase begins to form below the beta transus, which is about 882 °C. The α-
phase forms in laths, with a crystallographic relationship, i.e. most close-packed plane of α-
phase parallel to a special plane in the β-phase, to the β-phase, in which it forms. Slow
cooling from beta phase leads to the nucleation of alpha nuclei, which grows faster along
the common plane and grows very slowly perpendicular to this plane [22]. As a result, such
laths are developed. Fig.2c shows the microstructure of Ti-1 specimen, which is also lath-
type; however, it contains comparatively finer laths (lath width <5µm). The finer lath size
can be attributed to the finer grain size due to a possibly higher hydrogen concentration in
the specimen [22]. The α-Ti (H) phase (darker in Fig.2c) + δ-TiH (needle shaped, brighter
x
in Fig.2c) can be observed. This observation is further correlated with the XRD results, as
shown in Fig. 2a, in which only α and δ phases can be observed. The similar results about
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2
the phase transformation during dehydrogenation of TiH were observed by P. Sun et al.
[23]. It can be observe that the microstructure of Ti-2 specimen was also lath type with
lath-width ~ 15µm (Fig.2d), and its appearance is more or less similar to the microstructure
obtained from Ti-P specimen (Fig.2b). These microstructural characteristics further
substantiate the findings that the two-step sintering process is more efficient for
dehydrogenation as compared to one-step sintering.
3
.3. Mechanical Properties
Figure 3 presents the mechanical properties and morphology of the fracture surfaces
of titanium specimens. The average hardness of Ti-1 specimen is significantly higher as
compared to that of the Ti-2 and Ti-P specimens (Fig.3a). The relatively higher average
hardness values of Ti-1 compacts can be associated with the higher relative amounts of
retained hydrides. The tensile engineering stress-strain curves of the various Ti samples are
shown in Fig. 3(b). For comparison, the mechanical properties of the bulk Ti, under tensile
2
loading, prepared from pure Ti and TiH powders are also presented in Table-2. It can be
observed that bulk-Ti prepared from TiH powder resulted in higher yield strength (YS)
2
and ultimate tensile strength (UTS) as compared to those of Ti specimen prepared by pure-
Ti powder. Moreover, the YS and UTS of Ti-1 specimen are also higher to those of the Ti-2
specimen. However, the strain-to-failure is lowest for the Ti-1 specimens, and it shows
almost brittle nature. On the other hand, the Ti-2 specimen demonstrates significant
ductility, wherein the strain-to-failure is comparable to that of the Ti-P specimens. Briant et
al. [24] have shown that increasing hydrogen content leads to increasing strength and
decreasing ductility in Ti alloys due to enhanced susceptibility for hydrogen embrittlement.
The fracture surfaces of Ti-P, Ti-1 and Ti-2 specimens are shown in Fig. 3c, 3d, and 3e,
respectively. The Ti-P specimen exhibit micro-void-coalescence type ductile nature of
fracture, as demonstrated by large dimpled morphology. However, Ti-1 specimens exhibit
faceted morphology, indicating brittle nature of fracture. Interestingly, Ti-2 specimens
consisted of comparatively small-sized dimples. The change in mechanical properties and
fracture mode is consistent with the changes in the hydrogen contents in Ti-based materials,
as demonstrated in other researches [25, 26]. Hence, it is apparent that the two-stage
sintering process is extremely effective in providing bulk Ti with a combination of high
strength and high ductility.
4
.
Conclusions
6

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Titanium hydride powder was successfully utilized to prepare Ti compact with high
strength and good ductility. One-step sintering resulted in high hardness, high strength, but
low strain to failure under tensile loading, whereas two-step sintering resulted in moderate
hardness, high strength, and comparatively higher strain-to-failure, i.e. ductility. The high
hardness, high strength values, low ductility were attributed to the relatively higher
hydrogen retention in the one-step sintered specimens. It was demonstrated that a novel
two-stage spark plasma sintering strategy is extremely effective in delivering bulk titanium
with high strength and high ductility.
Acknowledgment
This research was supported by the Grant-in-Aid for Scientific Research on Innovative
area, Bulk Nanostructured Metals through MEXT, Japan (contract No. 22102004). These
supports are gratefully appreciated.
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Table-1: Calculated lattice parameters of Ti-1 and Ti-2 Specimens
Ti-1 Specimen
c (Å)
Ti-2 Specimen
Lattice parameters
a (Å)
c/a (Å)
a (Å)
c (Å)
c/a (Å)
α-phase
δ-phase
γ phase
2.94977
4.6932
1.591
2.9471
4.39
4.6884
-
1.590
-
4.41
-
-
-
-
-
4.235
4.623
1.091
Table-2: Comparison of mechanical properties of bulk-Ti prepared by TiH
2
powder
Sintering
Initial
Method
Vickers
YS
UTS Elongation
Temp.
K)
Ref.
10
Material
Of Preparation
Hardness (Hv) (MPa) (MPa)
(%)
(
TiH
2
Press & sinter
1200
N/A
N/A
265
N/A
310
N/A
Powder
Hot
Ti+TiH2
Powder
50±2 HRB
Isostatic
Pressing
1020
N/A
N/A
N/A
11
(Rockwell B)
(
1)900
170-225
290-320
320-340
TiH2
Press & Sinter
(2)1200
N/A
N/A
N/A
N/A
12
Powder
(
(
(
(
3)1300
1)1000
2)1050
3)1100
TiH
2
Spark Plasma
Sintering
360-420
13
19
Powder
Pure-Ti
Powder
Pure-Ti
Powder
TiH2
P/M compact,
Annealed
N/A
1200
1200
1200
N/A
180
267
243
370
280
575
360
480
365
690
530
18
24
3.5
16
Present
Study
One-Step SPS
One-Step SPS
Two-step SPS
Present
Study
Powder
TiH
2
Present
Study
Powder
(N/A=data not reported)
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Figure Captions
2
Fig.1. (a) XRD of as received Pure Ti and TiH powders (b) differential thermal
analysis (DTA) and (c) thermo-gravimetric analysis (TG) of initial titanium hydride
powder
Fig.2. (a) XRD analysis and respective micrographs of (b) Ti-P, (c) Ti-1 & (d) Ti-2
specimens
Fig.3. (a) Hardness, (b) Stress-Strain curve and respective fracture surfaces of (c)
Ti-P, (d) Ti-1 & (e) Ti-2 specimens
1
0

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Highlights
•
•
High density pure titanium compacts were prepared from titanium hydride powders.
Two-step spark plasma sintering was extremely effective in dehydrogenation of
2
TiH .
•
The proposed method resulted in bulk-Ti with high strength and good ductility.