DOI: 10.11817/j.ysxb.1004.0609.2020-35717

时效温度对Ti-50.8Ni-0.1Nb合金相变和形状记忆行为的影响

刘康凯，贺志荣，冯  辉，杜雨青，冀荣耀

(陕西理工大学 材料科学与工程学院，汉中 723001)

摘  要：采用X射线衍射仪、示差扫描量热仪、激光扫描共聚焦显微镜、TEM和拉伸实验研究了时效温度(t_ag)对(300，400，500，600 ℃；1 h)时效态Ti-50.8Ni-0.1Nb形状记忆合金相变、组织和形状记忆行为的影响。结果表明：300~600 ℃时效态Ti-50.8Ni-0.1Nb合金的相组成为B2和B19′。随t_ag升高，合金冷却/加热相变类型发生由A→R→M/M→R→A型向A→M/M→A型转变(A—母相B2，CsCl结构；R—R相，菱方结构；M—马氏体相B19′，单斜结构)，R相变温度先升高后降低，M相变温度先降低后升高再降低；R和M相变热滞分别在3.8~12.0 ℃和8.7~131.7 ℃之间变化。随t_ag升高，合金中Ti₃Ni₄析出相的形态发生由点状→针状→透镜状→片状转变。400、    500 ℃时效态合金的抗拉强度高于300、600 ℃时效态合金的，其伸长率则低于后者。300、400、600 ℃时效态合金呈超弹性，500 ℃时效态合金呈形状记忆效应。随应力-应变循环次数增加，合金的应力-应变曲线平台应力下降并趋稳，300、400 ℃时效态合金的超弹性稳定性良好。

关键词：Ti-50.8Ni-0.1Nb合金；形状记忆合金；时效温度；相变；形状记忆行为

文章编号：1004-0609(2020)-08-1802-09　　     中图分类号：TG113.25　　     文献标志码：A

形状记忆合金(Shape memory alloys，简称SMA)是集感知和驱动功能于一体的新型智能材料，Ti-Ni基SMA是SMA中性能最优者^[1-3]。Ti-Ni基SMA不仅具有独特的形状记忆效应(Shape memory effect，简称SME)、超弹性(Superelasticity，简称SE)，还表现出优异的高阻尼性、生物相容性、耐腐蚀性以及耐磨性^[4-6]，已广泛的应用于生物医学、微机电系统、耐磨涂层、机械制造、航空航天以及建筑工程等领域^[7-11]。SME指将处于马氏体相(M)状态的合金进行适度变形，然后加热到母相(A)状态后，变形合金可恢复到变形前形状的现象；SE指将处于A状态的合金加载适度变形，卸载后变形合金可恢复到变形前形状的现象^[12-14]。与Ti-Ni二元SMA相比， Ti-Ni-Nb三元SMA具有较好的加工塑性和较宽的相变温度滞后，能在低温环境下变形处理，马氏体相的稳定性良好^[15-16]。Ti-47Ni-9Nb、Ti-36Ni-10Nb等合金已用于制造管接头、紧固件、管道的连接件等，为室温下运输、储存及其他工程的应用带来了便利^[17-19]。目前，对Ti-Ni-Nb系SMA的研究主要集中在Nb添加量较多(5%~10%，

摩尔分数)的合金上，对Nb添加量较少的Ti-Ni-Nb合金研究较少。事实上，Nb含量较低的Ti-Ni-Nb合金能克服Nb含量较高合金恢复力不足的缺点，同时也能保持Nb含量高的相变滞后宽的优点^[20-21]。为了进一步改善Ti-Ni-Nb系SMA的性能，拓展用途，增加品种，本研究通过在形状记忆性能良好的Ti-50.8Ni二元SMA中添加0.1%Nb，得到了Ti-50.8Ni-0.1Nb(摩尔分数)三元合金。贺志荣等^[22]系统研究了退火温度对Ti-50.8Ni-0.1Nb合金组织、相变和形状记忆行为的影响，给出了该合金获得良好SME和SE的退火热处理工艺。Ti-50.8Ni-0.1Nb合金属于富镍Ti-Ni基合金，富镍Ti-Ni合金具有显著的时效处理效应^[23]，通过固溶时效处理，获得含有Ti₃Ni₄型析出物的显微组织，进而改善合金的形状记忆性能^[24-27]。本论文旨在研究时效温度对Ti-50.8Ni-0.1Nb合金相变行为、显微组织、形状记忆效应和超弹性影响规律，为开发性能优异的Ti-Ni-Nb 系SMA及其热处理工艺提供理论依据和实验支撑。

1  实验

实验材料是直径1 mm的冷拉Ti-50.8Ni-0.1Nb(摩尔分数)合金丝材。合金原料是纯度分别为99.7%、99.9%和99.9%的海绵Ti，电解Ni和Nb粉。经熔炼、旋锻、拉丝等工序制成合金丝，每道次变形量为20%。用SK-GO6J23K型真空管式电阻炉对合金进行(800 ℃，30 min)固溶处理+(300，400，500，600 ℃；1 h)时效处理。用Rigaku Ultima IV型X射线衍射仪(XRD)分析合金的相组成。用TA-Q2000型示差扫描热分析仪(DSC)分析合金的相变行为，冷却/加热温度范围为-150~100 ℃，冷却/加热速率为10 ℃/min。用LSM800型激光扫描共聚焦显微镜(CLSM)分析合金的显微组织形貌，腐蚀剂为V(HF):V(HNO₃):V(H₂O)= 1:4:5的溶液。用JEM-200CX型透射电子显微镜(TEM)分析合金的显微组织，操作电压160 kV，相机长度60 cm，双喷减薄液成分(体积分数)为6%高氯酸+94%甲醇。用CMT5105型微机控制电子万能试验机测定合金在室温下的形变行为、形状记忆效应、超弹性和应力-应变循环特性，标距为50 mm，加载/卸载速率为2 mm/min，应力-应变循环试验应变量取6%。

2  结果及讨论

2.1  相组成

图1所示为(300，400，500，600 ℃；1 h)时效态Ti-50.8Ni-0.1Nb合金的XRD谱。由图1可以看出，室温下，时效态Ti-50.8Ni-0.1Nb合金由母相B2和马氏体B19′组成。衍射强度表明，合金组织中B2相较多，B19′相较少。由于B2相与SE对应，B19′相与SME对应^[28]，故该合金室温下呈SE+SME特性，以SE为主。

图1  时效温度对Ti-50.8Ni-0.1Nb合金相组成的影响

Fig. 1  Effect of aging temperature on phase composition of Ti-50.8Ni-0.1Nb alloy

2.2  显微组织

图2所示为(300，400，500，600 ℃；1 h)时效态Ti-50.8Ni-0.1Nb合金的激光扫描共聚焦显微组织。从图2中可以看出，随时效温度(t_ag)升高，合金的显微组织呈等轴状，晶粒尺寸在20~30 μm之间，表明t_ag对合金晶粒尺寸影响不大。图3所示为同状态合金的TEM显微组织。从图3中可以看出，时效态Ti-50.8Ni- 0.1Nb合金存在Ti₃Ni₄相析出，(300，400，500，600 ℃；1 h)时效态Ti-50.8Ni-0.1Nb合金Ti₃Ni₄析出相分别呈弥散分布点状、弥散分布针状、透镜状和粗片状，亦即随t_ag升高，该合金中Ti₃Ni₄析出相的形态发生由点状→针状→透镜状→片状转变，且尺寸增加。

2.3  相变行为

图4(a)所示为(300，400，500，600 ℃；1 h)时效态Ti-50.8Ni-0.1Nb合金的DSC曲线，给出了t_ag对该合金相变行为的影响规律。图中M、M_r分别代表正、逆马氏体相变峰，R和R_r分别代表正、逆R相变峰。从图4(a)中可看出，当300 ℃≤t_ag≤500 ℃时，合金在冷却/加热的过程中发生A→R→M/M→R→A(A—母相，CsCl型结构；M—马氏体，单斜结构；R—R相，菱方结构)型可逆相变，即合金冷却发生A→R和R→M两阶段正相变，加热时发生M→R和R→A两阶段逆相变；当t_ag=600 ℃时，R相变峰消失，合金冷却/加热时发生A→M/M→A型一阶段可逆相变。此外，与退火态Ti-50.8Ni-0.1Nb合金相变峰相比^[22]，时效态Ti-50.8Ni-0.1Nb合金相变峰强度比较微弱，原因是时效态合金中存在弥散分布的Ti₃Ni₄析出相，该析出相对基体的相变具有阻抑作用，使相变峰弱化甚至多阶段化^[29]。

图2  时效温度对Ti-50.8Ni-0.1Nb合金激光扫描共聚焦显微组织的影响

Fig. 2  Effects of aging temperature on LSCM (Laser scanning confocal microscope) microstructure of Ti-50.8Ni-0.1Nb alloy aged at 300 ℃ (a), 400 ℃ (b), 500 ℃ (c) and 600 ℃ (d), respectively

图3  时效温度对Ti-50.8Ni-0.1Nb合金TEM显微组织的影响

Fig. 3  Effects of aging temperature on TEM microstructures of Ti-50.8Ni-0.1Nb alloy aged at 300 ℃ (a), 400 ℃ (b), 500 ℃ (c) and 600 ℃ (d), respectively

图4  时效温度对Ti-50.8Ni-0.1Nb合金相变类型、相变温度和相变热滞的影响

Fig. 4  Effect of aging temperature on transformation type (a), transformation temperature (b) and temperature hysteresis (c) of Ti-50.8Ni-0.1Nb alloy

图4(b)和(c)所示分别为时效温度对Ti-50.8Ni- 0.1Nb合金相变温度和热滞的影响。由图4(b)可以看出，随t_ag升高，R相变温度t_R(R峰温度)先升高后降低，最低温度10 ℃在300 ℃时效后取得，最高温度35.2 ℃在400 ℃时效后取得；R逆相变温度t_Rr(R_r峰温度)升高，最低温度17.5 ℃在300 ℃时效后取得，最高温度38.3 ℃在500 ℃时效后取得；M相变温度t_M(M峰温度)先降低后升高再降低，最低温度-127.5 ℃在400 ℃时效后取得，最高温度-8 ℃在500 ℃时效后取得；M逆相变温度t_Mr(M_r峰温度)先升高后降低，最低温度-26.5 ℃在300 ℃时效后取得，最高温度4.2 ℃在400 ℃时效后取得。

由图4(c)可知，随t_ag 升高，R相变热滞△t_R(即t_Rr-t_R之值)较小，当t_ag从300 ℃升至500 ℃时，△t_R先降低后升高，当t_ag=400 ℃时，△t_R达到最小值3.8 ℃，当t_ag=500 ℃时△t_R达到最大值12 ℃。随t_ag 的升高，M相变热滞△t_M(即t_Mr-t_M之值)先升高后降低再升高，当t_ag=400 ℃时△t_M达到最大值131.7 ℃，当t_ag=500 ℃时△t_M达到最小值8.7 ℃。因此，若想得到较小的△t_M，t_ag应取500 ℃。相变温度决定了形状记忆合金元件的开关温度，高、低相变温度合金可分别用于制作在高、低温场合使用的控温形状记忆元件或超弹性减震、阻尼元件。相变热滞反映了SMA元件动作温度范围的宽窄，Δt越小，动作温度范围越窄，亦即元件对温度的反应越灵敏^[20]。小热滞SMA可用于制作传感器，大热滞SMA可用于制作连接元件。

2.4  形变行为

图5所示为t_ag对Ti-50.8Ni-0.1Nb合金室温拉伸曲线、抗拉强度σ_b和伸长率δ的影响。由图5可以看出，合金在拉伸过程中经历了母相弹性变形，应力诱发M相变和应力诱发M弹性变形、塑性变形、断裂等阶段^[21]。随t_ag升高，合金的强度σ_b先升高后降低，400 ℃和500 ℃时效态合金σ_b最大，达1272 MPa，300 ℃时效态合金的σ_b较小，600 ℃时效态合金的σ_b最小，为886 MPa。要想获得高强度Ti-50.8Ni-0.1Nb合金，合金的时效温度应选取400~500 ℃。随t_ag升高，合金的塑性δ先降低后升高，300 ℃时效态合金的塑性较好，400 ℃和500 ℃时效态合金δ最差(16.8%~19.4%)，600 ℃时效态合金的塑性最好，δ高达50.9%。要想对Ti-50.8Ni-0.1Nb合金进行大应变塑性加工，合金可在300 ℃或600 ℃时效处理。原因是400~500 ℃时效态合金中针状和透镜状Ti₃Ni₄析出相与基体呈共格关系，使基体强度提高，塑性受损；600 ℃时效态合金的Ti₃Ni₄析出相粗化，对基体强化效果减弱，使基体塑性得以发挥。

图5  时效温度对Ti-50.8Ni-0.1Nb合金拉伸曲线、抗拉强度σ_b和伸长率δ的影响

Fig. 5  Effect of aging temperature on tensile curves (a), tensile strength σ_b and elongation δ (b) of Ti-50.8Ni-0.1Nb alloy

2.5  形状记忆行为

图6所示为t_ag对Ti-50.8Ni-0.1Nb合金形状记忆行为、加载/卸载应力-应变曲线平台应力和残余应变的影响。由图6(a)可知，300 ℃和400 ℃时效态Ti-50.8Ni-0.1Nb合金呈SE，500 ℃时效态合金呈SME，600 ℃时效态合金呈SE。与SE对应的应力平台的物理意义是应力诱发马氏体相变，与SME对应的应力平台的物理意义为马氏体再取向。500 ℃时效态合金呈SME的原因是该合金相变温度较高，室温下处于马氏体状态，加载时处于马氏体状态的合金发生马氏体再取向，加热时单取向马氏体逆转变为母相，因而合金呈现SME。600 ℃时效态合金呈SE的原因是该合金相变温度较低，室温下处于母相状态，加载时发生应力诱发马氏体相变，卸载时马氏体逆转变为母相，故合金呈现SE。此外，图6(a)中锯齿状应力-应变曲线与SMA合金孪生变形方式有关。因为孪生变形产生孪晶是形核、长大过程，孪晶形核所需应力远高于长大所需应力，故当孪晶出现时伴随载荷突然下降现象，在变形过程中孪晶不断形成，从而形成了锯齿状应力应变曲线^[30]。 由图6(b)可以看出，随t_ag升高，合金加载/卸载应力-应变曲线平台应力先下降后上升，分别在500 ℃和600 ℃时效态取得最小值(195.6 MPa)和最大值(510.9 MPa)；残余应变先降低后升高再降低，300 ℃和400 ℃时效态合金的残余应变最小，仅为0.09%~ 0.3%，SE优异。500 ℃时效态合金的残余应变最大，为3.5%，加热后该残余应变降为0，合金形状恢复，呈现SME。可见，要使Ti-50.8Ni-0.1Nb合金具有优异的SME，t_ag应取500 ℃；要使合金具有优异的SE，t_ag应取300~400 ℃。

图6  时效温度对Ti-50.8Ni-0.1Nb合金形状记忆行为和应力-应变曲线平台应力、残余应变的影响

Fig. 6  Effect of aging temperature on shape memory behavior (a) and platform stress, residual strain (b) in stress-strain curves of Ti-50.8Ni-0.1Nb alloy

2.6  循环变形行为

图7所示为应力-应变循环对300、400、500、600 ℃时效态Ti-50.8Ni-0.1Nb合金形状记忆行为的影响，其中300、400、500 ℃时效态合金循环了50次，600 ℃时效态合金循环了2次。由图7可以看出，随应力-应变循环次数增加，合金的应力-应变曲线平台应力下降并趋稳，300 ℃和400 ℃时效态合金保持超弹性行为，且由部分超弹性(残余应变不为0)转变为完全超弹性(残余应变为0)^[31-32]；500 ℃时效态合金在1~6次循环后呈现形状记忆行为，第7次循环往后呈现超弹性行为，600 ℃时效态合金在循环次数超过2次后试样断裂。可见，300 ℃和400 ℃时效态Ti-50.8Ni-0.1Nb合金超弹性的稳定性良好。

图7  应力-应变循环对不同温度时效态Ti-50.8Ni-0.1Nb合金形状记忆行为的影响

Fig. 7  Effect of stress-strain cycle on shape memory behavior of Ti-50.8 Ni-0.1Nb alloy aged at different temperatures

3  结论

1) 300~600 ℃时效态Ti-50.8Ni-0.1Nb合金的组成相为B2和B19′。随t_ag升高，该合金冷却/加热时的相变类型由A→R→M/M→R→A型向A→M/M→A型转变；R相变温度先升高后降低，M相变温度先降低后升高再降低；R相变热滞在3.8~12 ℃之间变化，M相变热滞在8.7~131.7 ℃之间变化。

2) 300~600 ℃时效态Ti-50.8Ni-0.1Nb合金的组织形态呈等轴状，存在Ti₃Ni₄析出相，随t_ag升高，Ti₃Ni₄析出相的形态发生由点状→针状→透镜状→片状转变，尺寸增加。 3) 400、500 ℃时效态Ti-50.8Ni-0.1Nb合金的抗拉强度高于300、600 ℃时效态合金，伸长率则低于后者；300、400、600 ℃时效态合金呈SE，500 ℃时效态合金呈SME。随应力-应变循环次数增加，时效态Ti-50.8Ni-0.1Nb合金的应力-应变曲线平台应力下降并趋稳，300、400 ℃时效态合金的SE稳定性良好。

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Effect of aging temperature on phase transformation and shape memory behaviors of Ti-50.8Ni-0.1Nb alloy

LIU Kang-kai, HE Zhi-rong, FENG Hui, DU Yu-qing, JI Rong-yao

(School of Materials Science and Engineering, Shaanxi University of Technology, Hanzhong 723001, China)

Abstract: The effect of aging temperature (t_ag) on the phase transformation, microstructure, and shape memory behaviors of Ti-50.8Ni-0.1Nb shape memory alloy aged at 300, 400, 500 and 600 ℃ for 1 h, respectively, were investigated by means of X-ray diffractometry, differential scanning calorimetry, laser scanning confocal microscope, TEM and tensile test. The results show that the phase compositions of Ti-50.8Ni-0.1Nb alloy aged at 300-600 ℃ are B2 and B19′. With increasing t_ag, the transformation types of the alloy change from A→R→M/M→R→A to A→M/M→A (A—parent phase B2, CsCl; R—R phase, rhombohedral; M—martensite B19′, monoclinic) upon cooling/heating; the R phase transition temperature increases first and then decreases, and the M transition temperature decreases first, then increases and then decreases; the R and M temperature hysteresis change in the range of 3.8-12.0 ℃ and 8.7-131.7 ℃, respectively. With increasing t_ag, the morphology of Ti₃Ni₄ precipitates in the alloy changes from granular → acicular → lenticular → flaky. The tensile strength of the alloy aged at 400 ℃ and 500 ℃ is higher than that of the alloy aged at 300 ℃ and 600 ℃, while the percentage elongation of the former is lower than that of the latter. The alloys aged at 300, 400, 600 ℃ show superelasticity, and the alloy annealed at 500 ℃ shows shape memory effect. With increasing stress-strain cycling number, the platform stress in the stress-strain curve of the alloy decreases. The stability of the superelasticity in the alloy aged at 300 ℃ and 400 ℃ is excellent.

Key words: Ti-50.8Ni-0.1Nb alloy; shape memory alloy; aging temperature; phase transformation; shape memory behavior

Foundation item: Project(2016YFE0111400) supported by the National Program on Key Basic Research

Received date: 2018-12-23; Accepted date: 2020-06-22

Corresponding author: HE Zhi-rong; Tel: +86-13892611307; E-mail: hezhirong01@163.com

(编辑  何学锋)

基金项目：国家重点研发计划资助项目(2016YFE0111400)

收稿日期：2018-12-23；修订日期：2020-06-22

通信作者：贺志荣，教授，博士；电话：13892611307；E-mail：hezhirong01@163.com