Sintering of titanium in argon and vacuum: Pore evolution and mechanical properties

Journal Pre-proof Sintering of titanium in argon and vacuum: Pore evolution and mechanical properties

S.D. Luo, B. Liu, J. Tian, M. Qian PII:

S0263-4368(20)30102-5

DOI:

https://doi.org/10.1016/j.ijrmhm.2020.105226

Reference:

RMHM 105226

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date:

22 December 2019

Revised date:

18 February 2020

Accepted date:

1 March 2020

Please cite this article as: S.D. Luo, B. Liu, J. Tian, et al., Sintering of titanium in argon and vacuum: Pore evolution and mechanical properties, International Journal of Refractory Metals and Hard Materials(2020), https://doi.org/10.1016/j.ijrmhm.2020.105226

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2020 Published by Elsevier.

Journal Pre-proof Sintering of titanium in argon and vacuum: Pore evolution and mechanical properties S.D. Luoa, B. Liub, J. Tianc, M. Qiana,* a

Centre for Additive Manufacturing, School of Engineering, RMIT University, Melbourne

3000, Australia b

Research Institute for New Material Technology, Chongqing University of Arts and

Sciences, Yongchuan, Chongqing 402160, China c

MicroNano Research Facility, RMIT University, Melbourne VIC 3000, Australia

oo

f

* Corresponding author:

pr

E-mail address: [email protected] (M. Qian)

e-

Abstract: Reducing porosity can effectively improve the mechanical properties of an as-

Pr

sintered material or part. An alternative is to tailor the size, distribution and morphology of the residual pores without having to reduce porosity. This study investigates the sintering of

al

commercially pure titanium (CP Ti) in a flowing argon (Ar) atmosphere and compares it with

rn

sintering in a vacuum of 10-2 Pa. The CP Ti sintered in Ar at 1300 °C for 2-3 h exhibited a

Jo u

marginally lower density but clearly better tensile ductility than Ti sintered in vacuum. The sensitivity of ductility to residual pores was analyzed. Samples sintered in Ar exhibited a much lower sensitivity than those sintered in vacuum. The superior ductility arises from the beneficial effect of sintering in Ar, which resulted in a greater number but smaller residual pores with lower pore aspect ratios and finer matrix grains due to the grain- growth inhibiting role of residual pores. The reason is attributed to the entrapped Ar in the closed pores during sintering, which is insoluble in Ti at the sintering temperature. The increased internal pore pressure prevents further pore shrinkage after reaching a critical size. The final pore size range is predicted and compared with experimental observations. Sintering of titanium in Ar is advantageous over sintering in vacuum in terms of both pore size distribution and pore

Journal Pre-proof morphology within a reasonable isothermal sintering period (e.g., 3 h at 1300 C). Therefore, it can be more attractive for non-fatigue critical applications than sintering in vacuum. Keywords: Titanium; Powder metallurgy; Argon; Microstructure; Ductility

1.

Introduction

Die compaction combined with pressureless sintering is the least expensive near- net-shape

f

powder metallurgy (PM) route to produce titanium (Ti) components for structural

oo

applications [1–3]. This approach, however, is difficult to achieve full densification. In

pr

general, a relative density of less than 98% of the theoretical density (TD) is attainable from

e-

using metallic Ti powders or less than 99.5%TD from using Ti hydride powders, when compacted and sintered under typical conditions [4–6]. Incompletely densified PM Ti

Pr

exhibits inferior mechanical properties because the residual pores not only reduce the

rn

premature fracture [6–8].

al

effective cross-section area for load bearing but can also act as crack initiation sites to trigger

In general, reducing porosity can improve the inferior properties of PM Ti. Removing pores

Jo u

open to the surface is particularly effective because their presence substantially affects not only the strength but also the ductility of Ti materials [9]. With open pores evolving into closed pores, the influence of the porosity becomes insignificant on strength but remains significant on ductility. This has been well demonstrated on additively manufactured Ti-6Al4V, where the presence of less than 1 vol.% of porosity resulted in differences of 20% in tensile strain to fracture and 50% in reduction of area but only less than 3% in tensile strengths [10]. Nevertheless, reducing porosity has attracted much research effort, e.g., by introducing fast diffusers [8], transient liquid phase [11,12], and hot isostatic pressing [13]. A different strategy is to place the emphasis on tailoring the residual pore size, distribution and morphology rather than focusing on reducing the level of porosity. The rationale is clear, i.e.,

Journal Pre-proof crack initiation and propagation are related to the pore size, spatial distribution and morphology. This means that for a given porosity, cost-effective measures should be taken to make pores smaller, smoother, rounder, and more evenly distributed if practical. Tailoring of pore structures has been explored in the fabrication of Ti foams or porous Ti [14,15], but few studies have been reported for dense PM Ti. This article investigates the sintering of commercially pure (CP) Ti in a flowing argon (Ar)

f

atmosphere and compares the results with sintering vacuum. We acknowledge that sintering

oo

of Ti in Ar has long been investigated, starting from Kroll in 1937 [16], as a cost-effective or

pr

more practical approach for the fabrication of PM Ti materials or components. Occasionally, Ar-sintered Ti exhibits better tensile ductility than the vacuum-sintered due to lower impurity

e-

uptakes [17–19]. However, to the authors’ knowledge, sintering in Ar has not been purposely

Pr

used as an effective way of tailoring microstructure for improved ductility and/or strengths for PM Ti. We show that sintering of CP Ti in Ar results in a preferred pore structure and

al

consequently better ductility than vacuum-sintered, despite a lower sintered density. The

rn

influence of the Ar atmosphere on the residual pore structure is discussed by considering the

2.

Jo u

critical pore size determined by the internal pore pressure during sintering.

Experimental procedure

Hydride-dehydride (HDH) Ti powder (<150 μm particle size, 99.4wt.% purity, 0.20wt.%O, 0.05wt.%Cl) supplied by Kimet Special Metal Precision Casting Co., Ltd, China, was used. Rectangular bars of ~50 × 8.2 × 3 mm3 were pressed uniaxially in a floating die at 600 MPa, giving a green density of (84.5 ± 0.5)%TD. In addition, Ti sponge particles (<2 mm, 99.3wt.% purity, 0.20wt.%O, 0.075wt.%Cl) from the same supplier were pressed into thin plates of ~55 × 10 × 2 mm3 at 50 MPa, serving as a gas getter during sintering.

Journal Pre-proof Isothermal sintering was carried out in a tube furnace in either a flowing Ar (purity 99.999%) atmosphere or a vacuum of 10-2 Pa. For sintering in Ar, after the samples were loaded, the furnace chamber was evacuated by a rotary pump and purged subsequently with Ar. After repeating the evacuation-purging cycle three times, an Ar flow of 1.0 litre/min was chosen and kept until sintering was complete. The sintering cycle consisted of heating samples at 4 °C/min to 1300 °C, isothermal holding 1300 °C for 2-3 h, and furnace cooling to room temperature at 4 °C/min, typical for the sintering of PM Ti [4]. For consistency, in each

pr

plates mentioned earlier to mitigate contamination.

oo

f

sintering operation (in either vacuum or Ar), all the samples were covered with Ti-sponge

The density was measured by the Archimedes method as described previously [20]. Samples

e-

for metallography were cut from as-sintered bars, ground, polished and etched with Kroll’s

Pr

etchant. Microstructure was examined using an optical microscope and a scanning electron microscope (SEM, XL 30). The pore size (maximum length), pore aspect ratio (maximum

al

length divided by the maximum width perpendicular to the length direction) and pore number

rn

density were measured from as-polished sample surfaces using software Image-Pro plus 6.0.

Jo u

Restricted by the resolution of the optical microscopy image (100× magnification), pores that are smaller than 12 µm2 in area were excluded. 500-900 pores in each sample were analysed. The pore attachment ratio, defined as the percentage of pores attached to grain boundaries, and grain size were measured from the etched surfaces. 600-800 grains were counted from each sample. Dog-bone-like tensile specimens with a cross section of 2.7 × 3 mm2 and a gauge length of 15 mm were machined from the as-sintered bars and tested at a cross- head speed of 0.5 mm/min or strain rate 5.6 × 10-4 s-1 on an Instron tester (Model 5584). The tensile properties and sintered density presented were an average of measurements for a minimum of four samples.

Journal Pre-proof Oxygen (O) content in the as-sintered bars was determined using Leco TC436, while the chlorine (Cl) content was determined using a Ratio Beam Spectrophotometer Hitachi U-1900.

3.

Results and discussion

3.1 Density and tensile properties from sintering in Ar and vacuum

f

Figure 1 shows the tensile properties of the CP Ti samples sintered in Ar and vacuum, while

oo

Table 1 lists the relative density data. No clear difference was observed in the ultimate tensile

pr

strength (UTS) (~510 MPa) and yield strength (YS) (~405 MPa). In addition, increasing

e-

sintering time from 2 h to 3 h led to little difference in both UTS and YS. However, there is a clear difference in the tensile ductility, especially when the isothermal sintering was

Pr

increased to 3 h from 2 h.

al

Samples sintered at 1300 °C in Ar had a lower density than those sintered in vacuum for

rn

either 2 h and 3 h of isothermal holding (see Table 1). The inferior sintering densification in Ar arose from gas entrapment that occurred at pore closure at about the sintered density of

Jo u

~92%TD [21] (see subsequent discussion). As the density further increased to 94-95%TD as measured in this study, the entrapped gas exerted a limited influence, thus leading to a marginal (<0.3%TD) but clearly discernible discrepancy in the sintered density compared with sintering in vacuum. As such, the better ductility obtained from sintering in Ar was not due to higher sintered density. Interstitial O can be another determinant of the ductility of assintered Ti and Ti alloys [22–24]. Table 2 details the O and Cl content in the samples sintered in this study. The difference in O content is limited to 0.03wt.%, which is negligible. This is corroborated by the very similar tensile strengths shown in Fig. 1. Excluding the influence of density and O content, it can be inferred that the difference in tensile ductility between

Journal Pre-proof sintering in Ar and vacuum arises from the residual pore structure and related grain size, as will be shown and discussed below. 600

500

Ar-2h

Vac-2h

400

Vac-3h

Ar-3h

oo

f

300

CP Ti sintered at 1300 °C in vacuum and Ar for 2-3 h

pr

200

100

0 2

4

6

8

10

12

14

Pr

0

e-

Engineering stress (MPa)

(a)

16

18

20

22

24

26

28

30

Elongation (%)

500 450 400 350

(b)

UTS

YS

Elongation

rn

550

al

600

Jo u

UTS & YS (MPa)

Engineering strain (%)

25 20 15 10 5 0

Vac-2h

Vac-3h

Ar-2h

Sintering conditions

Ar-3h

Journal Pre-proof Fig. 1. (a) Representative tensile stress-strain curves of CP Ti sintered at 1300 °C in vacuum and Ar, and (b) a comparison of tensile properties between Ar- and vacuum-sintered Ti.

Table 1. Summary of density and tensile properties of CP Ti sintered in Ar and vacuum. UTS (MPa) 505±7 516±9 515±5 509±16

YS (MPa) 397±9 407±19 407±10 405±15

Elongation (%) 16.9±1.1 17.1±1.9 18.4±3.8 21.0±5.2

pr

Ar-Ti

Sintered density (%TD) 94.70±0.18 94.92±0.21 94.46±0.16 94.63±0.30

f

Vac-Ti

Holding time at 1300 °C 2h 3h 2h 3h

oo

Samples

2 h in 3 h in 2 h in 3 h in

vacuum vacuum Ar Ar

Oxygen (wt.%) 0.20 0.20 0.20 0.22 0.23 0.20

Chlorine (wt.%) 0.05 0.075 0.032 0.028 0.032 0.034

Jo u

rn

HDH Ti powder Ti sponge particles Sintered CP Ti

Pr

Sintering conditions

al

Samples

e-

Table 2. O and Cl in HDH Ti powder, Ti sponge and as-sintered CP Ti from HDH Ti powder.

3.2 Influence of residual pores on tensile ductility The ductility of an as-sintered PM material is known to be sensitive to the residual pores in the sintered microstructure. The sensitivity can be assessed using Eq. (1) below, which describes the dependence of the relative ductility of an as-sintered material on porosity [21]: 3

𝑍=

⁄ (1−𝜀) 2 1

⁄ (1+𝑐𝜀 2 ) 2

(1)

where 𝑍 is the relative ductility or the ratio of the ductility of the as-sintered material to the ductility of the wrought material of the same composition, 𝜀 is porosity, and 𝑐 is a coefficient,

Journal Pre-proof which collectively represents the sensitivity of ductility to the pore size, shape, spacing and displacement of the residual pores in the as-sintered microstructure. Figure 2 plots the calculated coefficient 𝑐 versus porosity in samples sintered in Ar and vacuum in this work. In the calculation, the tensile ductility of wrought CP Ti containing (0.20-0.23)wt.%O was taken as 30% [25,26]. It is clear that samples sintered in Ar have a lower value of 𝑐 than those sintered in vacuum, suggesting that the ductility of samples sintered in Ar is less affected by the residual pores than those sintered in vacuum, despite having a higher level of porosity

pr

oo

f

(Table 1).

e-

1000 900

Pr

800

al

600

rn

500 400

200 100

Jo u

Coefficient c

700

300

Sintered in vacuum Sintered in Ar

0 4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

Porosity (%) Fig. 2. Ductility-sensitivity coefficient 𝑐 vs. porosity as defined by Eq. (1) for samples sintered in Ar and vacuum in this study. A small coefficient 𝑐 is preferred.

Journal Pre-proof To reveal the difference in residual pores, the as-sintered microstructures were examined quantitatively. Figure 3 compares representative microstructures obtained from sintering in Ar and vacuum, while Table 3 summarises the number density, size and aspect ratio of the residual pores in the as-sintered samples, along with the measured grain size and pore attachment ratio. Samples sintered in Ar contain a greater number density of residual pores than those sintered in vacuum. In addition, the residual pores in the Ar-sintered samples are finer and exhibit a lower pore aspect ratio than in the vacuum-sintered samples. In particular,

oo

f

the largest residual pore size in the Ar-sintered samples, which is often adversely indicative of the ductility of a PM Ti material [6,27], is noticeably smaller than that in the vacuum-

pr

sintered samples. Also, clearly different is the pore attachment ratio as shown in Table 3.

e-

Pores migrate slower than grain boundaries; their presence at grain boundaries restricts grain

Pr

boundary migration, thus suppressing grain growth. A higher pore attachment ratio favours a finer grain size. This is consistent with the finer grain size observed in the Ar-sintered

Jo u

rn

al

samples, which may have, to a limited extent, contributed to the better ductility.

Pr

e-

pr

oo

f

Journal Pre-proof

Fig. 3. Representative microstructures (SEM) of CP Ti samples sintered at 1300 °C for 2-3 h

rn

al

in vacuum and Ar.

Samples

Vac-Ti Ar-Ti

Jo u

Table 3. Relative density, pore parameters and grain size of CP Ti sintered in vacuum and Ar. Holding time at 1300 °C 2h 3h 2h 3h

Pore density (/mm2 ) 225±12 192±5 300±14 262±17

Mean pore size (µm) 16.7±0.7 18.5±0.3 16.2±0.4 17.2±1.0

Largest pore size (µm) 105.6±0.3 124.0±2.7 85.9±1.5 87.3±3.2

Pore aspect ratio 1.52±0.03 1.51±0.03 1.46±0.01 1.40±0.03

Pore attachment ratio (%) 59.9±1.6 53.0±3.0 62.0±0.5 61.6±1.1

Grain size (µm) 72.0±5.0 74.8±4.6 61.0±3.1 68.9±3.9

Note: Pore size, number density and aspect ratio were measured on as-polished samples while pore attachment ratio and grain size were measured on etched samples.

3.3 Sintering behaviour and pore evolution with entrapped gases

Journal Pre-proof The sintering of a PM material in a non-vacuum atmosphere involves gas entrapment, which occurs when the interconnected pore network collapses into isolated pores at a porosity level of ~8% [21]. In this study, sintering of the HDH Ti powder compact in Ar was carried out at the atmospheric pressure. Both Ar and chloride vapour can be entrapped in the isolated pores. Argon entrapment has been observed in sintered TiAl. After sintering in Ar of 9 × 104 Pa (900 mbar), the resultant TiAl (95.5%TD) contained 5.9 ± 0.15 ppm Ar [28]. The residual Ar

oo

𝑃𝑉 = 𝑛𝑅𝑇

f

can be estimated using the ideal gas law below: (2)

pr

where 𝑃 is the pressure, 𝑉 is the pore volume, 𝑛 is the number of moles of the entrapped gas,

e-

and 𝑅 is the gas constant (0.08250 atm·L/mol·K). Using 𝑃 = 1.01× 105 Pa and 𝑉 = 8 vol.% (when the pores just become isolated during sintering), the residual Ar content in the sintered

Pr

Ti is calculated to be ~6 ppm, which is consistent with Gerling’s experimental measurement.

rn

mass governs the process.

al

This is not surprising as Ar does not dissolve into Ti alloys and the law of conservation of

Jo u

Residual chlorides (MgCl2 ) always exist in the HDH Ti powder made from Kroll sponge. They will volatilize at elevated temperatures [29]. However, chlorides can dissolve in β- Ti with a Cl solubility of ~400 ppm at 1300 °C [30], higher than the Cl content measured in this work (Table 2). Therefore, the effect of chloride vapour on the internal pore pressure during sintering at 1300 C can be ignored in this study. Figure 4 is a schematic illustration of the sintering process. Chlorides are soluble in Ti at the sintering temperature and their vapour entrapped in the pores can progressively dissolve into the Ti matrix at the sintering temperature. Consequently, their impediment to pore shrinkage and coalescence diminishes with their dissolution into the Ti matrix. In contrast, Ar is insoluble in Ti [31] to persistently inhibit the evolution of each pore.

pr

oo

f

Journal Pre-proof

e-

Fig. 4. (a) SEM image of HDH Ti powder used in this study, and a schematic illustrating the

Pr

pore evolution during sintering from interparticle voids in powder compact (b), to isolated and closed pores from collapsed pore networks (c), and to final pores interacting with grain

al

boundary (GB) (d). During sintering, chloride vapour entrapped in closed pores dissolves in

rn

Ti matrix, but insoluble Ar persistently inhibits the pore evolution and detachment from GB.

Jo u

The inhibition of the entrapped Ar to pore evolution depends on pore size. During sintering, the surface free energy of the pore or the curvature effect is counteracted by the internal pore pressure (𝑃) according to Eq. (3) below [21]: 𝑃=

4𝛾𝑆𝑉 𝑑

(3)

where 𝛾𝑆𝑉 is the solid- vapour specific surface energy between Ti and Ar, which is about 1.73 J/m2 at 1300 °C [32], and d is the pore diameter. When a pore just becomes closed or isolated, its internal pressure due to the entrapped Ar equals the atmospheric pressure (1.01× 105 Pa). Accordingly, Eq. (3) gives d = 68.5 µm for sintering of Ti in Ar. For spherical pores (irregular in reality), this gives an indication of the approximate largest theoretical closed

Journal Pre-proof pore size and is close to the observed maximum pore size of 87 m (Table 3). Pore shrinkage from 68.5 µm downwards will progressively raise the internal pore pressure, which in turn slows down pore shrinkage. As the surface free energy of a pore is balanced by the internal pore pressure, the pore will tend to reach a final size 𝑑𝑒𝑛𝑑 defined by [21]: 𝑃0

1 2

3 2

𝑑𝑒𝑛𝑑 = (4𝐾 𝛾 ) ∙ 𝑑𝑖𝑛𝑖

(4)

𝑆𝑉

f

where 𝐾 is the compressibility factor of Ar (~1 at 1300 °C under 105 -107 Pa [33]), 𝑃0 is the

oo

internal pore pressure (1.01× 105 Pa) and 𝑑𝑖𝑛𝑖 is the initial pore size at pore closure.

pr

Figure 5(a) plots the final pore size dend vs. initial pore size 𝑑𝑖𝑛𝑖 for sintering of CP Ti at

e-

1300 °C in the Ar atmosphere. The final pore size is closely related to its initial size due to

Pr

the balance between the internal pore pressure and the curvature. The entire final pore size range predicted using Eq. (4) was observed in the as-sintered microstructure (Fig. 5(b)),

al

showing good consistency between theory and experiments. Clearly, owing to the high

rn

internal pressure at the equilibrium state, it is impractical to achieve a pore-free microstructure when sintered in Ar or a microstructure that contains very small pores (e.g., <

Jo u

5 m) unless the initial pores are all small. This explains the formation of a higher number density of pores in Ar-sintered Ti samples than in vacuum-sintered samples. In that regard, sintering of PM Ti in Ar is only suitable for non-fatigue critical applications (applying subsequent hot isostatic pressing may only be able to reduce the pore to a certain extent due to the increased internal pressure). However, for such non- fatigue critical applications, the results of this study suggest clear advantages over sintering in vacuum in terms of both pore size distribution and pore morphology.

Journal Pre-proof 70 60

0.20 0.15 0.10

50 0.05

40

0.00 -0.05

30

0.0 0.2 0.4 0.6 0.8 1.0

20

Pore shrinking follows: 3/2 dend=0.121×dini when dini<68.5 m

10

f

Final pore diameter dend (m)

(a)

0

10

20

oo

0 30

40

50

60

70

e-

pr

Initial pore diameter dini (m) 40

(b)

Pr

35

al

25 20 15

Note: pores smaller than 12 m in area were not included.

2

rn

Fraction (%)

30

o

Sintering at 1300 C: 2 h in vacuum 3 h in vacuum 2 h in Ar 3 h in Ar

5 0 0

Jo u

10

10

20

30

40

50

60

70

80

90

100

>110

Pore size (m)

Fig. 5. (a) Final pore size dend vs. initial pore size 𝑑𝑖𝑛𝑖 for sintering of CP Ti at 1300 °C in the Ar atmosphere. The inset is for 𝑑𝑖𝑛𝑖 ≤1 µm. The critical pore size defined by Eq. (4) with the internal pore pressure being equal to 1 atmospheric pressure is 68.5 m, which defines the largest theoretical closed pore size. (b) Pore size distribution measured in CP Ti sintered in either Ar or vacuum.

Journal Pre-proof The internal pore pressure affects pore coalescence as well. Similarly, because of the high internal pore pressure, pore coalescence occurs to a much less extent in Ar-sintered Ti than in vacuum-sintered Ti. This is corroborated by the observations that the maximum pore size is much smaller in Ar-sintered samples than in vacuum-sintered samples (Table 3) and that the number of small pores in Ar-sintered samples is far more than that in vacuum-sintered samples (Fig. 3 and Table 3). The pores in vacuum-sintered Ti have no or negligible internal gas pressure at the sintering temperature and are in a non-equilibrium state. Accordingly,

oo

f

pore coalescence can easily occur leading to large pore sizes (Table 3). In addition, the entrapped Ar inside the pores can restrain pore detachment from the grain boundary and

Pr

e-

pr

retard grain boundary migration for grain growth.

4. Conclusions

al

1) HDH Ti powder compacts were sintered at 1300 °C in vacuum and Ar. Samples sintered

rn

in Ar exhibit slightly lower density (≤0.3%TD), comparable tensile (~510 MPa) and yield

vacuum.

Jo u

(~405 MPa) strengths but clearly higher tensile ductility (21% vs. 17%) than sintered in

2) CP Ti samples sintered in Ar consists of smaller but far more residual pores with lower pore aspect ratios and finer grains than those sintered in vacuum. Such microstructural features make the tensile ductility less sensitive to porosity. 3) Ar is insoluble in Ti at the sintering temperature and therefore can persistently inhibit pore evolution and detachment from grain boundaries during sintering. As a result, sintering in Ar can lead to significant difference in the residual pore size, distribution and morphology compared with sintering in vacuum.

Journal Pre-proof 4) In principle, the internal pressure arising from Ar entrapment a llows only pores of <68.5 m in diameter to shrink during sintering at 1300 C and the shrinkage will cease at a final pore size that is dependent on the balance between the internal pore pressure and curvature, as well as the initial pore size. 5) Sintering of titanium in Ar is advantageous over sintering in vacuum in terms of both pore size distribution and pore morphology within a reasonable isothermal sintering

f

period (e.g., 3 h at 1300 C). Therefore, it can be more attractive for non- fatigue critical

pr

oo

applications than sintering in vacuum.

Acknowledgements

e-

This research is supported by the Australian Research Council (ARC) through LP

Pr

LP140100608.

rn

al

References

[1] M. Qian, Cold compaction and sintering of titanium and its alloys for near- net-shape or

Jo u

preform fabrication, Int. J. Powder Metall. 46 (2010) 29–44. [2] F.H. Froes, A historical perspective of titanium powder metallurgy, in Titanium Powder Metallurgy: Science, Technology and Application, ed. M. Qian, F.H. Froes, Elsevier, Oxford, 2014, pp. 1–19. [3] Z.Z. Fang, J.D. Paramore, P. Sun, K.S. Ravi Chandran, Y. Zhang, Y. Xia, F. Cao, M. Koopman, M. Free, Powder metallurgy of titanium – past, present, and future, Int. Mater. Rev. 63 (2018) 407–459. [4] I.M. Robertson, G.B. Schaffer, Review of densification of titanium based powder systems in press and sinter processing, Powder Metall. 53 (2010) 146–162.

Journal Pre-proof [5] O. Ivasishin, V. Moxson, Low-cost titanium hydride powder metallurgy, in Titanium Powder Metallurgy: Science, Technology and Application, ed. M. Qian, F.H. Froes, Elsevier, Oxford, 2014, pp. 117–148. [6] P. Kumar, K.S. Ravi Chandran, F. Cao, M. Koopman, Z.Z. Fang, The nature of tensile ductility as controlled by extreme-sized pores in powder metallurgy Ti-6Al-4V alloy, Metall. Mater. Trans. A 47 (2016) 2150–2161. [7] H. Wang, Z.Z. Fang, P. Sun, Critical review of mechanical properties of powder

oo

f

metallurgy titanium, Int. J. Powder Metall. 46 (2010) 45–57.

[8] A. Amherd Hidalgo, R. Frykholm, T. Ebel, F. Pyczak, Powder metallurgy strategies to

e-

2017, DOI: 10.1002/adem.201600743.

pr

improve properties and processing of titanium alloys: A review, Adv. Eng. Mater., Feb.

Pr

[9] Y.Y. Sun, S. Gulizia, C.H. Oh, D. Fraser, M. Leary, Y.F. Yang, M. Qian, The influence of as-built surface conditions on mechanical properties of Ti-6Al-4V additively

al

manufactured by selective electron beam melting, JOM 68 (2016) 791–798.

rn

[10] J. Elambasseril, S.L. Lu, Y.P. Ning, N. Liu, J. Wang, M. Brandt, H.P. Tang, M. Qian,

Jo u

3D characterization of defects in deep-powder-bed manufactured Ti-6Al-4V and their influence on tensile properties, Mater. Sci. Eng. A 761 (2019) 138031. [11] Y.F. Yang, S.D. Luo, G.B. Schaffer, M. Qian, The sintering, sintered microstructure and mechanical properties of Ti-Fe-Si alloys, Metall. Mater. Trans. A 43 (2012) 4896–4906. [12] S.D. Luo, Y.F. Yang, G.B. Schaffer, M. Qian, The effect of a small addition of boron on the sintering densification, microstructure and mechanical properties of powder metallurgy Ti-7Ni alloy, J. Alloy Compd. 555 (2013) 339–346. [13] P. Kumar, K.S. Ravi Chandran, Strength-ductility property maps of powder metallurgy (PM) Ti-6Al-4V alloy: A critical review of processing-structure-property relationships, Metall. Mater. Trans. A 48 (2017) 2301–2319.

Journal Pre-proof [14] Y.H. Chen, D. Kent, M. Bermingham, A, Dehghan-Manshadi, M. Dargusch, Manufacturing of biocompatible porous titanium scaffolds using a novel spherical sugar pellet space holder, Mater. Lett. 195 (2017) 92–95. [15] C. Torres-Sanches, J. McLaughlin, R. Bonallo, Effect of pore size, morphology and orientation on the bulk stiffness of a porous Ti35Nb4Sn alloy, J. Mater. Eng. Perform. 27 (2018) 2899–2909. [16] W. Kroll, Verformbare legierungen des titans, Z. Metallkunde 29 (1937) 189–192.

oo

f

[17] D.F. Heaney, R.M. German, Advances in the sintering of titanium powders, Proceedings of the PM2004 Powder Metallurgy World Congress (Vienna, Austria), vol.

pr

4, European Powder Metallurgy Association, Shrewsbury, UK, 2004, pp. 222–227.

e-

[18] T. Sidambe, I. A. Figueroa, H. G. C. Hamilton, I. Todd, Taguchi optimization of MIM

Pr

titanium sintering, Int. J. Powder Metall. 47 (2011) 21–28. [19] S. Banerjee, C.J. Joens, Sintering powder metal injection molded (MIM) titanium alloys:

al

in vacuum or argon? Key Eng. Mater. 704 (2016) 113–117.

rn

[20] S.D. Luo, M. Yan, G.B. Schaffer, M. Qian, Sintering of titanium in vacuum by

Jo u

microwave radiation, Metall. Mater. Trans. A 42 (2011) 2466–2474. [21] R.M. German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, Princeton NJ, 1994, pp. 244–385. [22] H. Conrad, Effect of interstitial solutes on the strength and ductility of titanium, Prog. Mater. Sci. 26 (1981) 123–403. [23] Q. Yu, L. Qi, T. Tsuru, R. Traylor, D. Rugg, J.W. Morris Jr., M. Asta, D.C. Chrzan, A.M. Minor, Origin of dramatic oxygen solute strengthening effect in titanium, Science 347 (2015) 635–639.

Journal Pre-proof [24] S.D. Luo, T.T. Song, B. Liu, J. Tian, M. Qian, Recent advances in the design and fabrication of strong and ductile (tensile) titanium metal matrix composites, Adv. Eng. Mater., 2019, DOI: 10.1002/adem.201801331. [25] W.L. Finlay, J.A. Snyder, Effects of three interstitial solutes (nitrogen, oxygen, and carbon) on the mechanical properties of high-purity alpha titanium, JOM 2 (1950) 277– 286. [26] D.S. Kang, W.J. Lee, E.P. Kwon, T. Tsuchiyama, Variation of work hardening rate by

oo

f

oxygen contents in pure titanium alloy, Mater. Sci. Eng. A 632 (2015) 120–126. [27] T. Saito, A cost-effective P/M titanium matrix composite for automobile use Adv.

pr

Perform. Mater. 2 (1995) 121–144.

e-

[28] R. Gerling, E. Aust, W. Limberg, M. Pfuff, F.P. Schimansky, Metal injection moulding

Pr

of gamma titanium aluminide alloy powder, Mater. Sci. Eng. A 423 (2006) 262–268. [29] Luxel Vapor Pressure Chart: https://luxel.com/wp-content/uploads/2013/04/Luxel-

al

Vapor-Pressure-Chart.pdf, accessed on Dec. 3, 2019.

rn

[30] Z. Fan, H.J. Niu, B. Cantor, A.P. Miodownik, T. Saito, Effect of Cl on microstructure

Jo u

and mechanical properties in situ Ti/TiB MMCs produced by a blended elemental powder metallurgy method, J. Microsc. 185 (1997) 157–167. [31] S. Shao, M.J. Mahtabi, N. Shamsaei, S.M. Thompson, Solubility of argon in laser additive manufactured α-titanium under hot isostatic pressing condition, Comput. Mater. Sci. 131 (2017) 209–219. [32] K. Zhou, H.P. Wang, J. Chang, B. Wei, Experimental study of surface tension, specific heat and thermal diffusivity of liquid and solid titanium, Chem. Phys. Lett. 639 (2015) 105–108. [33] W.J. Little, Tables of Thermodynamic Properties of Argon from 100 to 3000 K, Arnold Engineering Development Center, Air Force Systems Command, U.S. Air Force, 1964.

Jo u

rn

al

Pr

e-

pr

oo

f

Journal Pre-proof

Journal Pre-proof CRediT author statement

S.D. Luo: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Resources; Validation; Roles/Writing - original draft B. Liu: Funding acquisition; Project administration; Writing - review & editing J. Tian: Methodology; Data curation

Jo u

rn

al

Pr

e-

pr

oo

f

M. Qian: Conceptualization; Formal analysis; Funding acquisition; Project administration; Supervision; Writing - review & editing

Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Jo u

rn

al

Pr

e-

pr

oo

f

None

Journal Pre-proof Highlights:

Jo u

rn

al

Pr

e-

pr

oo

f

 Titanium sintered in argon exhibits better tensile ductility than sintered in high vacuum and similar strengths despite lower density.  Sintering in argon produces smaller but more residual pores and finer grains, benefiting ductility.  The initial pore size and entrapped insoluble argon dictate the final pore size.  Existing theoretical models for pore evolution containing entrapped gas agree with experimental observations.  Sintering titanium in argon offers advantages over vacuum sintering for non-fatigue critical applications and may need to be further revisited.