简介概要

Phase equilibria of low-Ni side in Nb-Ni-Ti system at 1100℃

来源期刊：Rare Metals2020年第8期

论文作者：Hong-Xiao Li Zhi-Chao Li Yu-Ping Ren Min Jiang Gao-Wu Qin

文章页码：936 - 941

摘要：The phase equilibria related to liquid phase in low-Ni side of Nb-Ni-Ti system at 1100℃ were investigated through scanning electron microscope（SEM）,electron probe micro-analyzer（EPMA）,X-ray diffraction（XRD） and differential scanning calorimetry（DSC）.The results show that there exists liquid phase in equilibrium with continuous solid solution（Nb,βTi） and compound TiNi in low-Ni side of Nb-Ni-Ti system at 1100℃.The liquid phase region originates from low-Ni side of binary TiNi system and extends to 9.4 at% Nb in isothermal section of Nb-Ni-Ti phase diagram.The solid solubility of Nb in binary compound TiNi is about 10.6 at%.At 1100℃,there exist two three-phase regions consisting of liquid+（Nb,βTi）+TiNi and（Nb,βTi）+TiNi+X_B in Nb-Ni-Ti system,where X_B is the compound with hexagonal structure and composition of 32.8 Nb-44.5 Ni-22.7 Ti.The wide two-phase region（Nb,βTi）+TiNi exists between these two three-phase regions.In this two-phase region,Ti content in continuous solid solution（Nb,βTi） varies from6.6 at% to 25.7 at%,but Ni content is almost the same,which is about 4.0 at%.The eutectic transformation L■（Nb,βTi）+TiNi takes place at about 1138℃ in Nb-Ni-Ti ternary system.

稀有金属(英文版) 2020,39(08),936-941

Phase equilibria of low-Ni side in Nb-Ni-Ti system at 1100℃

Hong-Xiao Li Zhi-Chao Li Yu-Ping Ren Min Jiang Gao-Wu Qin

Key Laboratory for Anisotropy and Texture of Materials,Ministry of Education,Northeastern University

作者简介：*Hong-Xiao Li,e-mail:lihx@atm.neu.edu.cn;

收稿日期：2 November 2015

基金：financially supported by the National Natural Science Foundation of China (No.50971037);the Supporting Program for the Key Laboratories in the Universities of Liaoning Province (No.LS2010062);

Phase equilibria of low-Ni side in Nb-Ni-Ti system at 1100℃

Hong-Xiao Li Zhi-Chao Li Yu-Ping Ren Min Jiang Gao-Wu Qin

Key Laboratory for Anisotropy and Texture of Materials,Ministry of Education,Northeastern University

Abstract：

The phase equilibria related to liquid phase in low-Ni side of Nb-Ni-Ti system at 1100℃ were investigated through scanning electron microscope(SEM),electron probe micro-analyzer(EPMA),X-ray diffraction(XRD) and differential scanning calorimetry(DSC).The results show that there exists liquid phase in equilibrium with continuous solid solution(Nb,βTi) and compound TiNi in low-Ni side of Nb-Ni-Ti system at 1100℃.The liquid phase region originates from low-Ni side of binary TiNi system and extends to 9.4 at% Nb in isothermal section of Nb-Ni-Ti phase diagram.The solid solubility of Nb in binary compound TiNi is about 10.6 at%.At 1100℃,there exist two three-phase regions consisting of liquid+(Nb,βTi)+TiNi and(Nb,βTi)+TiNi+X_B in Nb-Ni-Ti system,where X_B is the compound with hexagonal structure and composition of 32.8 Nb-44.5 Ni-22.7 Ti.The wide two-phase region(Nb,βTi)+TiNi exists between these two three-phase regions.In this two-phase region,Ti content in continuous solid solution(Nb,βTi) varies from6.6 at% to 25.7 at%,but Ni content is almost the same,which is about 4.0 at%.The eutectic transformation L■(Nb,βTi)+TiNi takes place at about 1138℃ in Nb-Ni-Ti ternary system.

Keyword：

Nb-Ni-Ti system; Phase equilibrium; Liquid phase; Solid solubility;

Received： 2 November 2015

1 Introduction

Research onphase diagrams andphase equilibriaof Nb-NiTi system was originated from the development of Ni-based superalloys.The pseudo-binary phase diagram of TiNi₃-NbNi₃ was obtained in 1958,and the isothermal sections of phase diagram of Ni-rich corner in Nb-Ni-Ti system at 900and 1000℃were acquired in 1966 [ 1] .The development of wide hysteresis shape memory alloys promoted further research on phase diagram of Nb-Ni-Ti system in 1900s.The isothermal sections of Nb-Ni-Ti system at 700,800 and900℃were determined,and the ternary compound X_B was found at the same time [ 2, 3, 4] .However,these experiments were mainly focused on solid-state phase equilibria [ 5] .

The eutectic transformation L (βTi)+Ti₂Ni takes place at 942℃in low-Ni side of Ti-Ni binary system [ 6] .Above this temperature,liquid phase should exist in NbNi-Ti ternary system,and the analysis on phase equilibria and compositions becomes complex.Therefore,only one eutectic temperature and corresponding eutectic composition were obtained in pseudo-binary phase diagram of NbTiNi determined by Piao et al. [ 7] ,but the compositions of Nb-rich phase and TiNi are doubtful.However,the phase equilibria related to liquid phase are necessary for the alloy design of novel Nb-TiNi hydrogen permeation membrane and Nb-Ni-Ti amorphous alloys [ 8, 9, 10, 11] .

The thermodynamic assessment of Nb-Ni-Ti system was done in 2005 [ 12] .The re-assessment was done in2015 by combining the first-principle total energy calculations with the CALPHAD approach on the basis of the almost the same experimental data [ 13] .Owing to lack of accurate experimental data at high temperature,these thermodynamic descriptions of Nb-Ni-Ti system should not be suitable for the alloy design related to liquid phase.Therefore,the purpose of the present work is to determine the equilibrium phase relationship,especially,the phase equilibria related to liquid phase in low-Ni side of Nb-NiTi system at 1100℃by means of the equilibrated alloy method and differential scanning calorimetry (DSC).It is expected to provide more reliable experimental data for the thermodynamic assessment and for the alloy design of hydrogen permeation membrane of Nb-NiTi system.

2 Experimental

Complex phase transformations should take place in the alloy with large content of liquid phase in volume fraction during quenching after equilibrium treatment,and it is difficult to determine the corresponding equilibrium composition of liquid phase.So,according to Ti-Ni,Ti-Nb binary systems [ 6] and some results of Nb-Ni-Ti ternary system [ 2, 3, 4, 7] ,the experimental alloys Nb₁₈Ni₃₇Ti₄₅,Nb₈Ni₂₂Ti₇₀,Nb₈Ni₃₂Ti₆₀,Nb₃₅Ni₃₃Ti₃₂ and Nb₃₂Ni₄₀Ti₂₈ (nominal composition,at%,the same as follows) in low-Ni side to be expected with small content of liquid phase in volume fraction after equilibrium treatment at 1100℃were designed to determine the phase equilibria related to liquid phase.The alloys were prepared from high-purity Ti(>99.990%),Ni (99.999%) and Nb (99.960%) in watercooled copper crucible by arc melting with magnetically controlled non-consumable tungsten electrode in a pure argon atmosphere.The weight of each ingot was about 40 g.

All the samples cut from the ingots were sealed in a quartz tube with the vacuum of 1×10^-2 Pa under the protection of high pure Ar atmosphere,then kept at 1100℃for equilibrium treatment and finally quenched by water or iced brine.The phase transition temperatures were determined through DSC(Netzsch STA 449F3) at heating rate of 10 K·min^-1.The microstructural observation was carried out by scanning electron microscope (SEM,SSX-500) with voltage of 20 kV.The equilibrium phase compositions of the alloys were analyzed by electron probe micro-analyzer (EPMA,Shimadzu EPMA-1600) with voltage of 15 kV and spot size of 1μm.The high pure Nb,Ni and Ti were used as standard samples.The equilibrium phase constituents were determined by X-ray diffractometer (XRD,Philips PW3040/60 X'Pert PROXRD) with Cu Kαradiation at voltage of 40 kV and current of 40 mA.

3 Results and discussion

3.1 Relative phase equilibria with liquid phase at 1100℃

The equilibrium microstructure of Nb₁₈Ni₃₇Ti₄₅ alloy at1100℃is shown in Fig.1a.The blocky white phase and black phase distribute in gray matrix.There are thin white lamellas in black phase with larger size.It could be deduced by the contrast of backscattered electron image that the thin lamellas are with more Nb atoms.If the lamellas have existed at 1100℃,they should be relatively thicker.And according to Gibbs phase rule,except for the invariant reactions,there exist no more than three phases in ternary system at constant pressure.Therefore,it might be concluded that the lamellas with more Nb atoms form during quenching.

The first peak with large endothermic effect emerges at1041.6℃on DSC curve of as-cast alloy Nb₁₈Ni₃₇Ti₄₅(Fig.1b),which should be caused by the melting of the sample.It means that liquid phase exists in Nb₁₈Ni₃₇Ti₄₅alloy kept at 1100℃.By combining morphology and composition of the lamellas,it could be derived that the thin lamellas crystallize from liquid,and then,the liquid transforms to black phase during quenching from 1100℃.So,the black phase and the thin lamellas should be in liquid state at 1100℃.The composition of the liquid phase at 1100℃should be the average composition of black phase and lamellas.Measured by EPMA,the equilibrium composition of gray phase is 1.9Nb-50.6Ni-47.5Ti,that of white phase is 70.8Nb-3.5Ni-25.7Ti,and the equilibrium composition of liquid phase is 9.4Nb-33.2Ni-57.4Ti (Table 1).

Fig.1 a Equilibrium SEM image of Nb₁₈Ni₃₇Ti₄₅ alloy at 1100℃,and b DSC curve of as-cast Nb₁₈Ni₃₇Ti₄₅ alloy and c corresponding XRD pattern of Nb_18Ni₃₇Ti₄₅ alloy at 1100℃

下载原图

Table 1 Equilibrium phases and compositions at 1100℃in Nb-Ni-Ti system measured by EPMA (at%)

The corresponding XRD pattern of equilibrium micros true ture of Nb₁₈Ni₃₇Ti₄₅ alloy at 1100℃is shown in Fig.1c.The characteristic diffraction peaks of TiNi,Nb and Ti₂Ni phase could be indexed.Combined with the composition analysis,it could be deduced that the gray matrix phase is TiNi phase with the solution of Nb atoms;the white phase is (Nb,βTi) phase with solution of Ni atoms.The black phase is Ti₂Ni phase formed from liquid during quenching,and the primary white lamellas might be(Nb,βTi) phase.So,Nb₁₈Ni₃₇Ti₄₅ alloy is in TiNi-(Nb,βTi)-liquid three-phase equilibrium,i.e.,the three-phase region TiNi+(Nb,βTi)+liquid exists in isothermal phase diagram of Nb-Ni-Ti system at 1100℃.

Similarly,Nb₈Ni₂₂Ti₇₀ alloy is in two-phase region (Nb,βTi)+liquid at 1100℃.The equilibrium composition of(Nb,βTi) phase is 21.3Nb-6.3Ni-72.4Ti and that of liquid phase is 5.5Nb-26.6Ni-67.9Ti.Nb₈Ni₃₂Ti₆₀ alloy is in liquid state at 1100℃.Because Nb₈Ni₃₂Ti₆₀ alloy is in(Nb,βTi)-liquid two-phase equilibrium at 1050℃,it should locate near liquid boundary at 1100℃.

3.2 Relative solid-state phase equilibria at 1100℃

The microstructure of as-cast Nb₃₅Ni₃₃Ti₃₂ alloy is shown in Fig.2a.The coarse white phase is surrounded by lamellar eutectic microstructure.The composition of white phase is 83.8Nb-3.8Ni-12.4Ti,and the mean composition of lamellae is 18.4Nb-43.8Ni-37.8Ti.According to the microstructure morphology and phase compositions,it could be deduced that the white phase with high Nb content crystallize from the melt first,and then,with temperature decreasing,the lamellar micro structure forms through eutectic transformation.DSC curve of as-cast Nb₃₅Ni₃₃Ti₃₂alloy is shown in Fig.2b,showing that the first endothermic peak starts at 1138.4℃.It means that no phase transformation takes place from room temperature to1100℃.Especially,no melting takes place within this temperature range,because the melting usually causes obvious endothermic effect.So Nb₃₅Ni₃₃Ti₃₂ alloy is in solid-state phase equilibrium at 1100℃.

Fig.2 SEM image a and DSC curve b of as-cast Nb₃₅Ni₃₃Ti₃₂ alloy;equilibrium SEM image c and corresponding XRD pattern d of Nb₃₅Ni₃₃Ti₃₂ alloy at 1100℃

The equilibrium micros true ture of Nb₃₅Ni₃₃Ti₃₂ alloy at1100℃is shown in Fig.2c.The white phase with different sizes distributes in gray matrix.The composition of gray phase is 8.0Nb-49.7Ni-42.3Ti and that of white phase is 84.8Nb-4.0Ni-11.2Ti.The corresponding XRD pattern is shown in Fig.2d.The characteristic peaks of TiNi phase with monocline structure and Nb with bcc structure could be indexed.So the gray phase is TiNi with solution of8 at%Nb and the white phase is Nb phase with solution of Ni and Ti.Because of the solution of Ni and Ti,the structure parameter of Nb-based solid solution is smaller than that of pure Nb.The coarser Nb phase comes from the primary phase whose composition changes to equilibrium composition at 1100℃.The small Nb particles form through spheroidization of lamellae at 1100℃.The other phase in eutectic microstructure form gray TiNi matrix.Therefore,it could be derived that the eutectic transformation L (Nb,βTi)+TiNi takes place at 1138.4℃.So,Nb₃₅Ni₃₃Ti₃₂ alloy is in solid-state equilibrium of (Nb,βTi)+TiNi at 1100℃.

The equilibrium microstructure of Nb₃₂Ni₄₀Ti₂₈ alloy at1100℃is shown in Fig.3a,and the corresponding XRD pattern is shown in Fig.3b.The characteristic peaks of Nb,TiNi and ternary compound X_B could be indexed,where X_B is the compound with hexagonal structure(a=0.492 nm,c=2.664 nm) [ 14, 15] .The composition of white phase is 89.2Nb-4.2Ni-6.6Ti which is Nb-based solid solution with Ni and Ti.The composition of the black phase is 10.6Nb-51.0Ni-38.4Ti which is binary compound TiNi with solution of Nb.The gray phase is ternary compound X_B with composition of 32.8Nb-44.5Ni-22.7Ti.DSC curve of as-cast Nb₃₂Ni₄₀Ti₂₈ alloy is shown in Fig.3c,showing that the obvious endothermic peak starts at 1133.8℃.Therefore,Nb₃₂Ni₄₀Ti₂₈ alloy is in solidstate phase equilibrium of (Nb,βTi)+TiNi+X_B at1100℃,i.e.,the three-phase region consisting of (Nb,βTi)+TiNi+X_B exists in isothermal phase diagram of NbNi-Ti system at 1100℃.

3.3 Partial isothermal section of phase diagram of Nb-Ni-Ti system at 1100℃

According to the composition of TiNi phase in (Nb,βTi)+TiNi+X_B three-phase equilibrium of Nb₃₂Ni₄₀Ti₂₈alloy at 1100℃,10.6Nb-51.0Ni-38.4Ti,the solubility of Nb in TiNi phase is at least 10.6 at%.It could be deduced from atomic ratio of Ti to Ni in TiNi phase that Nb atoms replace some Ti atoms.It is consistent with that the crystal structures of Ti and Nb are the same,and the crystal parameters are similar.

The partial isothermal section of phase diagram of lowNi side in Nb-Ni-Ti system at 1100℃was constructed according to above experimental data and is shown in Fig.4.The continuous solid solution (Nb,βTi) could form at 1100℃in Nb-Ni-Ti ternary system.The liquid phase region originates from low-Ni side of binary Ti-Ni system and extends to 9.4 at%Nb.The solid solubility of Nb in binary compound TiNi is about 10.6 at%.There exist two three-phase regions consisting of liquid+(Nb,βTi)+TiNi and (Nb,βTi)+TiNi+X_B in isothermal section of low-Ni side in Nb-Ni-Ti system at 1100℃.The wide two-phase region (Nb,βTi)+TiNi exists between these two threephase regions.In this two-phase region,Ti content in continuous solid solution (Nb,βTi) varies from 6.6 at%to25.7 at%,but Ni content is about 4 at%.

The vertical section at 28 at%Ni in Nb-Ni-Ti system was calculated separately by Matsumoto et al. [ 12] and Santhy and Hair Kumar [ 13] through CALPHAD approach.

Fig.3 Equilibrium SEM image a,corresponding XRD pattern b of alloy Nb₃₂Ni₄₀Ti₂₈ at 1100℃and DSC curve of as-cast Nb₃₂Ni₄₀Ti₂₈ alloy c

Fig.4 Isothermal section of low-Ni side in Nb-Ni-Ti ternary system at 1100℃(opened circle representing compositions of experimental alloys,filled square representing equilibrium phase compositions from experimental alloys,and filled circle representing data from binary alloy phase diagrams)

The two results are not the same in phase constitutes,phase boundaries and characteristic temperatures.The significant differences exist above 900℃.At 1100℃,the phase regions liquid,liquid+(Nb,βTi),liquid+(Nb,βTi)+ternary compound X_A,(Nb,βTi)+X_A and (Nb,βTi)+X_A+X_B line up with the increase in Nb content in vertical section at 28 at%Ni,as calculated by Santhy and Hair Kumar [ 13] .However,the phase regions liquid,liquid+(Nb,βTi) and liquid+(Nb,βTi)+X_B line up with the increase in Nb content in vertical section calculated by Matsumoto et al. [ 12] .

From the experimental results of Nb₃₅Ni₃₃Ti₃₂ alloy(Fig.2),it could be derived that the primary phase (Nb,βTi) crystallizes from the melt,and then the remaining liquid changes to (Nb,βTi)+TiNi through eutectic transformation.The eutectic microstructure (Nb,βTi)+TiNi keeps until～1138℃,so there exists phase equilibrium of (Nb,βTi)+TiNi at 1100℃.

It could be concluded from isothermal section in Fig.4that the phase regions liquid,liquid+(Nb,βTi),liquid+(Nb,βTi)+TiNi,(Nb,βTi)+TiNi and (Nb,βTi)+TiNi+X_B line up with the increase in Nb content at1100℃and 28 at%Ni.So,the precise experimental data at high temperature are the foundation for obtaining accurate thermodynamic parameters which are useful for the related alloy design.

The hydrogen permeability of dual-phase Nb-Ni-Ti alloys with microstructure consisting of Nb-based solid solution (Nb,βTi) and TiNi phase is equivalent to that of Pd alloys [ 10, 16, 17] .(Nb,βTi) phase contributes to the hydrogen permeation,and hydrogen permeability is also affected by the composition of (Nb,βTi) phase.TiNi phase suppresses the hydrogen embrittlement.So,the higher hydrogen permeability and resistance to hydrogen embrittlement could be achieved in dual-phase Nb-Ni-Ti alloys by controlling the contents in volume fraction,compositions and morphology of (Nb,βTi) and TiNi phases.The wide two-phase region (Nb,βTi)+TiNi could provide more possibilities for the dual-phase alloy design.

Even with the coexistence of TiNi phase,the compound X_B increases hydrogen embrittlement of the alloys [ 16] .The compound X_B easily forms in the alloys whose compositions are located in three-phase region (Nb,βTi)+TiNi+X_B.This should be considered for the alloy design of Nb-based hydrogen permeation membrane.

4 Conclusion

There exists liquid phase which could be in equilibrium with compound TiNi and continuous solid solution (Nb,βTi) in low-Ni side of Nb-Ni-Ti system at 1100℃.The liquid phase region originates from the low-Ni side of binary Ti-Ni system and extends to 9.4 at%Nb in Nb-NiTi system.The solid solubility of Nb atoms in binary compound TiNi is about10.6 at%at 1100℃.The eutectic transformation L (Nb,βTi)+TiNi takes place at about 1138℃.At 1100℃,there exist two three-phase regions consisting of liquid+(Nb,βTi)+TiNi and (Nb,βTi)+TiNi+X_B in isothermal section of low-Ni side in Nb-NiTi system.The wide two-phase region(Nb,βTi)+TiNi exists between these two three-phase regions.In this two-phase region,Ti content in continuous solid solution (Nb,βTi) varies from 6.6 at%to 25.7 at%;however,Ni content is almost the same,about 4.0 at%.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No.50971037) and the Supporting Program for the Key Laboratories in the Universities of Liaoning Province (No.LS2010062).