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Ni powder mixture
Materials Letters 191 (2017) 89–92
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Rapid synthesis of dense NiTi alloy through spark plasma sintering of a TiH2/Ni powder mixture Bolu Liu, Shuigen Huang, Liugang Chen, Jan Van Humbeeck, Jef Vleugels ⇑ KU Leuven, Department of Materials Engineering (MTM), Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium
a r t i c l e
i n f o
Article history: Received 28 August 2016 Received in revised form 4 January 2017 Accepted 14 January 2017 Available online 16 January 2017 Keywords: Biomaterials NiTi Titanium hydride Spark plasma sintering Microstructure
a b s t r a c t Dense NiTi alloy was prepared by liquid phase spark plasma sintering from a TiH2 and Ni powder mixture at 1050 °C for 4 min. Dehydrogenation of TiH2 in the powder mixture was rapidly completed after heating to 850 °C. The sintered alloy showed no obvious transformation peaks, whereas the homogenized alloy, thermally treated in Ar at 1150 °C for 2 h, displayed clear transformation peaks with a high transformation enthalpy of 10.6 J/g. The hardness of the sintered and homogenized alloys were statistically the same. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction NiTi alloys have attracted wide interest for practical applications due to their excellent functional and mechanical properties [1,2]. Powder metallurgy (PM) processing, which can fabricate a near-net-shape component at a lower sintering temperature compared to casting, has been used to prepare NiTi alloys [3]. However, fabrication of dense NiTi alloys by PM remains challenging, particularly when starting from a Ti and Ni elemental powder mixture. This is mainly due to the Kirkendall voids originating from the diffusivity difference between Ti and Ni, and the liquid capillary effect resulting from the presence of a Ti2Ni eutectic at 942 °C [4]. TiH2 powder, as a pore-forming agent due to its dehydrogenation, has been widely used to prepare porous NiTi alloys by PM [5]. TiH2 could also reduce the amount of Ni3Ti phase and decrease the oxide content of sintered products [6]. Moreover, the price of TiH2 powder is around half that of Ti powder with an equivalent particle size. Fine TiH2 can be easily produced by mechanical milling since it is extremely brittle [7]. However, the usage of TiH2 powder to prepare dense NiTi alloys is hardly investigated [8]. To decrease the high porosity (29–34%) after pressureless sintering, the materials needed to be hot isostaticpressed. Moreover, a low heating rate (e.g. 5 °C/min) was required to complete the dehydrogenation process.
⇑ Corresponding author. E-mail address: [email protected] (J. Vleugels). http://dx.doi.org/10.1016/j.matlet.2017.01.060 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
Spark plasma sintering (SPS) is an effective method to prepare fully dense NiTi alloys from elemental and pre-alloyed powders with a fast heating/cooling rate and short dwell time [9,10]. The present study attempts to rapidly synthesize dense NiTi alloys by SPS of a TiH2/Ni powder mixture. The microstructure and properties including the transformation behavior and hardness after SPS and thermal homogenization were characterized. 2. Experimental procedure Ni (Jiangmen GEM Co. Ltd) and TiH2 powders (grade VM, Chemetall) with an average particle size of 1 lm and 1.8 lm, respectively, were used to prepare Ti-50.8 at.% Ni alloy. The powder mixture was prepared by wet-mixing in isopropanol in a polyethylene bottle on a multi-directional mixer (Turbula) for 24 h using ZrO2 (grade TZ-3Y, Tosoh, Japan) milling balls. The suspension was dried in a rotating evaporator at 60 °C. SPS (Type HP D25/1, FCT Systeme, Frankenblick, Germany) was performed at 1050 °C for 4 min under a pressure of 7 MPa in vacuum (50 Pa). A cylindrical graphite die with an inner and outer diameter of 30 and 65 mm was used. The heating and initial cooling rate were 50 °C/min and 200 °C/min, respectively. Homogenization was conducted at 1150 °C for 2 h in flowing argon, followed by furnace cooling. The density of the materials was determined by the Archimedes method in deionized water. The measured relative density of the SPS-processed alloy was 100%. Phase identification was conducted on a X-ray diffractometer (3003-TT, Seifert). Rietveld refinements of
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the diffraction patterns were performed using the Materials Analysis Using Diffraction (MAUD) software [11]. The microstructure was examined by scanning electron microscope (SEM, XL30-FEG, FEI) and elemental maps were obtained by electron probe microanalysis (EPMA, JXA-8530F, JEOL). The transformation behavior was measured on a differential scanning calorimeter (DSC, Q2000, TA) at a cooling/heating rate of 10 °C/min. The Vickers hardness, HV10, was measured on a hardness tester (Model FV-700, Future-Tech Corp.) with an indentation load of 98 N. The reported values are the mean and standard deviation of five indentations. 3. Results and discussion 3.1. Microstructure The XRD patterns of the SPS-processed TiH2-Ni powder compacts and homogenized alloy are shown in Fig. 1(a). Apart from Ni-Ti phases (NiTi, Ti2Ni and Ni3Ti), the TiH2, Ni and a-Ti were also present when heated to 700 °C without dwell. Only the Ni-Ti phases were found after heating to 850 °C without dwell. Previous studies have reported that a complete dehydrogenation of pure TiH2 powder was difficult to achieve in a short time, even at a high temperature (e.g., SPS at 1300 °C for 20 min) [7,12]. Therefore, the rapid complete dehydrogenation of TiH2 should be related to the reaction between Ni and a-Ti/TiH2. The diffusion of Ni atoms into
TiH2 resulted in an expansion of the TiH2 lattice and the formation of Ni-Ti phases, which contributed to the release of hydrogen atoms. The SPS-processed (1050 °C for 4 min) and homogenized (1150 °C for 2 h) NiTi alloys had the same phase constitution as the powder compact SPS at 850 °C without dwell. To determine whether the Ti2Ni was oxidized and to assess the amount of Ni-Ti phases, Rietveld refinement of the XRD patterns of the SPS-processed and homogenized alloys was performed. For clarity, only the former is presented in Fig. 1b. The calculated lattice parameter of the cubic Ti2Ni of the SPS-processed (11.3360 ± 0.0028 Å) and homogenized (11.3434 ± 0.0041 Å) alloys were both higher than for the standard pattern (11.3193 Å, PDF card No. 01-072-0442), indicating the formation of Ti4Ni2Ox (0 < x 6 1). The x for the SPS-processed and homogenized NiTi alloys were 0.33 and 0.48, respectively, which were estimated from the linear relationship between x and the lattice parameter of Ti4Ni2Ox (L = 11.3193 + 0.0507x) according to Vegard’s law [13]. The calculated volume fraction of NiTi, Ti4Ni2Ox and Ni3Ti phases in the SPS-processed alloy were 65 ± 1.6, 23 ± 2.8 and 12 ± 1.7 vol.%, respectively, while their volume fractions in the homogenized alloy were 61 ± 2.1, 24 ± 2.2 and 15 ± 2.3 vol.%, respectively. This demonstrates that NiTi was the main phase in both SPS-processed and homogenized alloys, whereas the amount of Ni3Ti slightly increased and the amount of NiTi concomitantly decreased after homogenization.
Fig. 1. XRD patterns of the SPS-processed TiH2-Ni powder compacts and homogenized alloys (a), and the Rietveld refinement (Rwp = 11.5%, sig = 1.5) of the XRD data of the SPS NiTi alloy (b).
B. Liu et al. / Materials Letters 191 (2017) 89–92
The microstructure of the SPS-processed and homogenized NiTi alloys, as well as an EPMA elemental mapping of the SPS-processed alloy are shown in Fig. 2. For the SPS-processed alloy, a dark and light grey phase is homogeneously distributed in a medium grey matrix. The matrix was the NiTi phase based on the Rietveld Refinement. The composition analysed by wavelength dispersive spectroscopy (WDS) analysis at the cross symbol point (Fig. 2c) was Ti-52 at.% Ni, indicating the NiTi was quite Ni-rich. This was due to the fast cooling during SPS. The dark and light grey phase were Ti4Ni2Ox and Ni3Ti respectively, as indicated by the Ti, Ni and O elemental EPMA maps (Fig. 2d-f). For the homogenized alloy, an apparent grain growth of the Ni-Ti phases was observed. A dense NiTi alloy was obtained after SPS at 1050 °C for 4 min due to the liquid phase sintering. The liquid was formed by melting of Ti4Ni2Ox at around 984 °C according to the Ni-Ti binary and NiTi-O ternary phase diagrams [14]. Since the Gibbs free energy of formation for the Ti4Ni2Ox and Ni3Ti is higher than for the NiTi phase, the two phases were commonly observed in the sintered NiTi alloys [10]. The formation of Ti4Ni2Ox was mainly due to the remnant oxygen in the sintering vacuum, which was also observed in previous studies [9]. The Ti4Ni2Ox phase was difficult to remove after homogenization at 1150 °C for 2 h due to its stabilization by oxygen. This in return led to a Ni-rich NiTi at high temperature and concomitantly induced the precipitation of Ni3Ti.
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Fig. 3. DSC curves of the SPS-processed and homogenized NiTi alloys.
3.2. Transformation characteristics and hardness The DSC curves of the SPS-processed and homogenized (1150 °C/2 h) NiTi alloys are presented in Fig. 3. No obvious phase
Fig. 2. Backscattered electron images of the NiTi alloys SPS-processed at 1050 °C for 4 min (a) and homogenized at 1150 °C for 2 h (b), and EPMA micrograph of the NiTi alloy SPS-processed at 1050 °C for 4 min (c) with corresponding Ti (d), Ni (e) and O (f) elemental maps. The colour bar on the elemental maps represents the intensity level.
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Table 1 Vickers hardness of the SPS-processed and homogenized NiTi alloys. Materials
1050 °C/4 min (SPS)
1050 °C/4 min (SPS)-1150 °C/2 h (Homogenization)
HV10 (GPa)
5.3 ± 0.1
5.5 ± 0.1
transformation peaks during cooling and heating in the SPSprocessed alloy were observed due to the fact that the NiTi matrix was quite Ni-rich, as discussed above. Clear transformation peaks were found in the homogenized alloy, indicating a decrease of Ni content in the NiTi by means of Ni3Ti formation. The reverse transformation (A ? M) enthalpy, 10.6 J/g, of the homogenized alloy was higher than that reported in literature for NiTi alloys prepared from an elemental powder mixture (5 J/g [15] and 9.9 J/g [16]). The Vickers hardness (HV10) of the SPS-processed and homogenized NiTi alloys were 5.3 ± 0.1 and 5.5 ± 0.1 GPa respectively, as summarised in Table 1. They were the same considering the standard deviation although the grain size of the Ni-Ti phases in the homogenized alloy was bigger.
4. Conclusions Rapid liquid phase SPS (4 min at 1050 °C) of a TiH2/Ni powder mixture resulted in a fully dense NiTi alloy. A complete dehydrogenation of TiH2 in the powder compact was rapidly achieved after heating to 850 °C. The SPS-processed alloy contained NiTi, Ti4Ni2O0.33 and Ni3Ti phases with NiTi as a main phase. After homogenization (2 h at 1150 °C in Ar), the amount of NiTi and Ni3Ti were decreased and increased respectively, while the amount of the Ti4Ni2O0.48 was comparable.
No obvious phase transformation peaks were observed for the SPS-processed alloy, whereas clear transformation peaks were found for the homogenized alloy. The homogenized alloy showed a high transformation enthalpy of 10.6 J/g. The hardness of the SPS (5.3 ± 0.1 GPa) alloy was statistically the same as for the homogenized (5.5 ± 0.1 GPa) alloy. Acknowledgements Bolu Liu thanks the China Scholarship Council (CSC) for financial support (No. 201306830005). We gratefully acknowledge support of the Hercules Foundation (project ZW09-09) for the acquisition of the EPMA. Refereces [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
J. Van Humbeeck, Adv. Eng. Mater. (2001) 837–850. W. Guo, H. Kato, Mater. Lett. 158 (2015) 1–4. S.L. Zhu, X.J. Yang, F. Hu, S.H. Deng, Z.D. Cui, Mater. Lett. 58 (2004) 2369–2373. N. Zhang, P. Babayan Khosrovabadi, J.H. Lindenhovius, B.H. Kolster, Mater. Sci. Eng. A 150 (1992) 263–270. H.H. Mohd Zaki, J. Abdullah, Mater. Lett. 121 (2014) 36–39. W. Hongtao, Z.Z. Fang, S. Pei, Int. J. Powder Metall. 46 (2010) 45–57. Y. Zou, Z. Sun, S. Tada, H. Hashimoto, Scr. Mater. 56 (2007) 725–728. B. Bertheville, M. Neudenberger, J.-E. Bidaux, Mater. Sci. Eng. A 384 (2004) 143–150. C. Shearwood, Y.Q. Fu, L. Yu, K.A. Khor, Scr. Mater. 52 (2005) 455–460. A.M. Locci, R. Orrù, G. Cao, Z.A. Munir, Intermetallics 11 (2003) 555–571. L. Lutterotti, S. Matthies, H. Wenk, Newsl CPD 21 (1999) 14–15. H.H.-D. Linh, E.-R. Tamer, W.B. Michel, J. Alloy Comp. 349 (2003) 264–268. L. Vegard, Z. Phys. 5 (1921) 17–26. A.T. Qiu, L.J. Liu, W. Pang, X.G. Lu, C.H. Li, Trans. Nonferrous Met. Soc. China 21 (2010) 1808–1816. E. Schüller, O.A. Hamed, M. Bram, D. Sebold, H.P. Buchkremer, D. Stöver, Adv. Eng. Mater. 5 (2003) 918–924. M. Bram, A. Ahmad-Khanlou, A. Heckmann, B. Fuchs, H.P. Buchkremer, D. Stöver, Mater. Sci. Eng. A 337 (2002) 254–263.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Rapid synthesis of dense NiTi alloy through spark plasma sintering of a TiH2/Ni powder mixture Bolu Liu, Shuigen Huang, Liugang Chen, Jan Van Humbeeck, Jef Vleugels ⇑ KU Leuven, Department of Materials Engineering (MTM), Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium
a r t i c l e
i n f o
Article history: Received 28 August 2016 Received in revised form 4 January 2017 Accepted 14 January 2017 Available online 16 January 2017 Keywords: Biomaterials NiTi Titanium hydride Spark plasma sintering Microstructure
a b s t r a c t Dense NiTi alloy was prepared by liquid phase spark plasma sintering from a TiH2 and Ni powder mixture at 1050 °C for 4 min. Dehydrogenation of TiH2 in the powder mixture was rapidly completed after heating to 850 °C. The sintered alloy showed no obvious transformation peaks, whereas the homogenized alloy, thermally treated in Ar at 1150 °C for 2 h, displayed clear transformation peaks with a high transformation enthalpy of 10.6 J/g. The hardness of the sintered and homogenized alloys were statistically the same. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction NiTi alloys have attracted wide interest for practical applications due to their excellent functional and mechanical properties [1,2]. Powder metallurgy (PM) processing, which can fabricate a near-net-shape component at a lower sintering temperature compared to casting, has been used to prepare NiTi alloys [3]. However, fabrication of dense NiTi alloys by PM remains challenging, particularly when starting from a Ti and Ni elemental powder mixture. This is mainly due to the Kirkendall voids originating from the diffusivity difference between Ti and Ni, and the liquid capillary effect resulting from the presence of a Ti2Ni eutectic at 942 °C [4]. TiH2 powder, as a pore-forming agent due to its dehydrogenation, has been widely used to prepare porous NiTi alloys by PM [5]. TiH2 could also reduce the amount of Ni3Ti phase and decrease the oxide content of sintered products [6]. Moreover, the price of TiH2 powder is around half that of Ti powder with an equivalent particle size. Fine TiH2 can be easily produced by mechanical milling since it is extremely brittle [7]. However, the usage of TiH2 powder to prepare dense NiTi alloys is hardly investigated [8]. To decrease the high porosity (29–34%) after pressureless sintering, the materials needed to be hot isostaticpressed. Moreover, a low heating rate (e.g. 5 °C/min) was required to complete the dehydrogenation process.
⇑ Corresponding author. E-mail address: [email protected] (J. Vleugels). http://dx.doi.org/10.1016/j.matlet.2017.01.060 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
Spark plasma sintering (SPS) is an effective method to prepare fully dense NiTi alloys from elemental and pre-alloyed powders with a fast heating/cooling rate and short dwell time [9,10]. The present study attempts to rapidly synthesize dense NiTi alloys by SPS of a TiH2/Ni powder mixture. The microstructure and properties including the transformation behavior and hardness after SPS and thermal homogenization were characterized. 2. Experimental procedure Ni (Jiangmen GEM Co. Ltd) and TiH2 powders (grade VM, Chemetall) with an average particle size of 1 lm and 1.8 lm, respectively, were used to prepare Ti-50.8 at.% Ni alloy. The powder mixture was prepared by wet-mixing in isopropanol in a polyethylene bottle on a multi-directional mixer (Turbula) for 24 h using ZrO2 (grade TZ-3Y, Tosoh, Japan) milling balls. The suspension was dried in a rotating evaporator at 60 °C. SPS (Type HP D25/1, FCT Systeme, Frankenblick, Germany) was performed at 1050 °C for 4 min under a pressure of 7 MPa in vacuum (50 Pa). A cylindrical graphite die with an inner and outer diameter of 30 and 65 mm was used. The heating and initial cooling rate were 50 °C/min and 200 °C/min, respectively. Homogenization was conducted at 1150 °C for 2 h in flowing argon, followed by furnace cooling. The density of the materials was determined by the Archimedes method in deionized water. The measured relative density of the SPS-processed alloy was 100%. Phase identification was conducted on a X-ray diffractometer (3003-TT, Seifert). Rietveld refinements of
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the diffraction patterns were performed using the Materials Analysis Using Diffraction (MAUD) software [11]. The microstructure was examined by scanning electron microscope (SEM, XL30-FEG, FEI) and elemental maps were obtained by electron probe microanalysis (EPMA, JXA-8530F, JEOL). The transformation behavior was measured on a differential scanning calorimeter (DSC, Q2000, TA) at a cooling/heating rate of 10 °C/min. The Vickers hardness, HV10, was measured on a hardness tester (Model FV-700, Future-Tech Corp.) with an indentation load of 98 N. The reported values are the mean and standard deviation of five indentations. 3. Results and discussion 3.1. Microstructure The XRD patterns of the SPS-processed TiH2-Ni powder compacts and homogenized alloy are shown in Fig. 1(a). Apart from Ni-Ti phases (NiTi, Ti2Ni and Ni3Ti), the TiH2, Ni and a-Ti were also present when heated to 700 °C without dwell. Only the Ni-Ti phases were found after heating to 850 °C without dwell. Previous studies have reported that a complete dehydrogenation of pure TiH2 powder was difficult to achieve in a short time, even at a high temperature (e.g., SPS at 1300 °C for 20 min) [7,12]. Therefore, the rapid complete dehydrogenation of TiH2 should be related to the reaction between Ni and a-Ti/TiH2. The diffusion of Ni atoms into
TiH2 resulted in an expansion of the TiH2 lattice and the formation of Ni-Ti phases, which contributed to the release of hydrogen atoms. The SPS-processed (1050 °C for 4 min) and homogenized (1150 °C for 2 h) NiTi alloys had the same phase constitution as the powder compact SPS at 850 °C without dwell. To determine whether the Ti2Ni was oxidized and to assess the amount of Ni-Ti phases, Rietveld refinement of the XRD patterns of the SPS-processed and homogenized alloys was performed. For clarity, only the former is presented in Fig. 1b. The calculated lattice parameter of the cubic Ti2Ni of the SPS-processed (11.3360 ± 0.0028 Å) and homogenized (11.3434 ± 0.0041 Å) alloys were both higher than for the standard pattern (11.3193 Å, PDF card No. 01-072-0442), indicating the formation of Ti4Ni2Ox (0 < x 6 1). The x for the SPS-processed and homogenized NiTi alloys were 0.33 and 0.48, respectively, which were estimated from the linear relationship between x and the lattice parameter of Ti4Ni2Ox (L = 11.3193 + 0.0507x) according to Vegard’s law [13]. The calculated volume fraction of NiTi, Ti4Ni2Ox and Ni3Ti phases in the SPS-processed alloy were 65 ± 1.6, 23 ± 2.8 and 12 ± 1.7 vol.%, respectively, while their volume fractions in the homogenized alloy were 61 ± 2.1, 24 ± 2.2 and 15 ± 2.3 vol.%, respectively. This demonstrates that NiTi was the main phase in both SPS-processed and homogenized alloys, whereas the amount of Ni3Ti slightly increased and the amount of NiTi concomitantly decreased after homogenization.
Fig. 1. XRD patterns of the SPS-processed TiH2-Ni powder compacts and homogenized alloys (a), and the Rietveld refinement (Rwp = 11.5%, sig = 1.5) of the XRD data of the SPS NiTi alloy (b).
B. Liu et al. / Materials Letters 191 (2017) 89–92
The microstructure of the SPS-processed and homogenized NiTi alloys, as well as an EPMA elemental mapping of the SPS-processed alloy are shown in Fig. 2. For the SPS-processed alloy, a dark and light grey phase is homogeneously distributed in a medium grey matrix. The matrix was the NiTi phase based on the Rietveld Refinement. The composition analysed by wavelength dispersive spectroscopy (WDS) analysis at the cross symbol point (Fig. 2c) was Ti-52 at.% Ni, indicating the NiTi was quite Ni-rich. This was due to the fast cooling during SPS. The dark and light grey phase were Ti4Ni2Ox and Ni3Ti respectively, as indicated by the Ti, Ni and O elemental EPMA maps (Fig. 2d-f). For the homogenized alloy, an apparent grain growth of the Ni-Ti phases was observed. A dense NiTi alloy was obtained after SPS at 1050 °C for 4 min due to the liquid phase sintering. The liquid was formed by melting of Ti4Ni2Ox at around 984 °C according to the Ni-Ti binary and NiTi-O ternary phase diagrams [14]. Since the Gibbs free energy of formation for the Ti4Ni2Ox and Ni3Ti is higher than for the NiTi phase, the two phases were commonly observed in the sintered NiTi alloys [10]. The formation of Ti4Ni2Ox was mainly due to the remnant oxygen in the sintering vacuum, which was also observed in previous studies [9]. The Ti4Ni2Ox phase was difficult to remove after homogenization at 1150 °C for 2 h due to its stabilization by oxygen. This in return led to a Ni-rich NiTi at high temperature and concomitantly induced the precipitation of Ni3Ti.
91
Fig. 3. DSC curves of the SPS-processed and homogenized NiTi alloys.
3.2. Transformation characteristics and hardness The DSC curves of the SPS-processed and homogenized (1150 °C/2 h) NiTi alloys are presented in Fig. 3. No obvious phase
Fig. 2. Backscattered electron images of the NiTi alloys SPS-processed at 1050 °C for 4 min (a) and homogenized at 1150 °C for 2 h (b), and EPMA micrograph of the NiTi alloy SPS-processed at 1050 °C for 4 min (c) with corresponding Ti (d), Ni (e) and O (f) elemental maps. The colour bar on the elemental maps represents the intensity level.
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Table 1 Vickers hardness of the SPS-processed and homogenized NiTi alloys. Materials
1050 °C/4 min (SPS)
1050 °C/4 min (SPS)-1150 °C/2 h (Homogenization)
HV10 (GPa)
5.3 ± 0.1
5.5 ± 0.1
transformation peaks during cooling and heating in the SPSprocessed alloy were observed due to the fact that the NiTi matrix was quite Ni-rich, as discussed above. Clear transformation peaks were found in the homogenized alloy, indicating a decrease of Ni content in the NiTi by means of Ni3Ti formation. The reverse transformation (A ? M) enthalpy, 10.6 J/g, of the homogenized alloy was higher than that reported in literature for NiTi alloys prepared from an elemental powder mixture (5 J/g [15] and 9.9 J/g [16]). The Vickers hardness (HV10) of the SPS-processed and homogenized NiTi alloys were 5.3 ± 0.1 and 5.5 ± 0.1 GPa respectively, as summarised in Table 1. They were the same considering the standard deviation although the grain size of the Ni-Ti phases in the homogenized alloy was bigger.
4. Conclusions Rapid liquid phase SPS (4 min at 1050 °C) of a TiH2/Ni powder mixture resulted in a fully dense NiTi alloy. A complete dehydrogenation of TiH2 in the powder compact was rapidly achieved after heating to 850 °C. The SPS-processed alloy contained NiTi, Ti4Ni2O0.33 and Ni3Ti phases with NiTi as a main phase. After homogenization (2 h at 1150 °C in Ar), the amount of NiTi and Ni3Ti were decreased and increased respectively, while the amount of the Ti4Ni2O0.48 was comparable.
No obvious phase transformation peaks were observed for the SPS-processed alloy, whereas clear transformation peaks were found for the homogenized alloy. The homogenized alloy showed a high transformation enthalpy of 10.6 J/g. The hardness of the SPS (5.3 ± 0.1 GPa) alloy was statistically the same as for the homogenized (5.5 ± 0.1 GPa) alloy. Acknowledgements Bolu Liu thanks the China Scholarship Council (CSC) for financial support (No. 201306830005). We gratefully acknowledge support of the Hercules Foundation (project ZW09-09) for the acquisition of the EPMA. Refereces [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
J. Van Humbeeck, Adv. Eng. Mater. (2001) 837–850. W. Guo, H. Kato, Mater. Lett. 158 (2015) 1–4. S.L. Zhu, X.J. Yang, F. Hu, S.H. Deng, Z.D. Cui, Mater. Lett. 58 (2004) 2369–2373. N. Zhang, P. Babayan Khosrovabadi, J.H. Lindenhovius, B.H. Kolster, Mater. Sci. Eng. A 150 (1992) 263–270. H.H. Mohd Zaki, J. Abdullah, Mater. Lett. 121 (2014) 36–39. W. Hongtao, Z.Z. Fang, S. Pei, Int. J. Powder Metall. 46 (2010) 45–57. Y. Zou, Z. Sun, S. Tada, H. Hashimoto, Scr. Mater. 56 (2007) 725–728. B. Bertheville, M. Neudenberger, J.-E. Bidaux, Mater. Sci. Eng. A 384 (2004) 143–150. C. Shearwood, Y.Q. Fu, L. Yu, K.A. Khor, Scr. Mater. 52 (2005) 455–460. A.M. Locci, R. Orrù, G. Cao, Z.A. Munir, Intermetallics 11 (2003) 555–571. L. Lutterotti, S. Matthies, H. Wenk, Newsl CPD 21 (1999) 14–15. H.H.-D. Linh, E.-R. Tamer, W.B. Michel, J. Alloy Comp. 349 (2003) 264–268. L. Vegard, Z. Phys. 5 (1921) 17–26. A.T. Qiu, L.J. Liu, W. Pang, X.G. Lu, C.H. Li, Trans. Nonferrous Met. Soc. China 21 (2010) 1808–1816. E. Schüller, O.A. Hamed, M. Bram, D. Sebold, H.P. Buchkremer, D. Stöver, Adv. Eng. Mater. 5 (2003) 918–924. M. Bram, A. Ahmad-Khanlou, A. Heckmann, B. Fuchs, H.P. Buchkremer, D. Stöver, Mater. Sci. Eng. A 337 (2002) 254–263.