Ti-50.8Ni-0.5V形状记忆合金的相变、组织与性能

Phase transformation,microstructure and properties of Ti-50.8Ni-0.5V shape memory alloy

利用XRD、示差扫描热分析仪(DSC)、光学显微镜和拉伸试验研究退火态Ti-50.8Ni-0.5V(摩尔分数,%)形状记忆合金的相变行为、组织特征和力学性能。结果表明:400~700℃退火态Ti-50.8Ni-0.5V合金室温组成相为母相B2和马氏体B19′。随退火温度升高,该合金冷却/加热时的相变类型由B2→R/R→B2型向B2→R→B19′/B19′→R→B2型向B2→R→B19′/B19′→B2型向B2→B19′/B19′→B2型转变;R相、马氏体相变温度先升高后降低,极大值28.1℃、-58.5℃分别在400℃、500℃退火态合金中取得;显微组织由纤维状变为等轴状,再结晶温度在550~600℃之间;抗拉强度降低,塑性提高;应力诱发马氏体临界应力ζM先降低后升高,极小值325MPa在450℃退火态合金中取得;残余应变先升高后降低再升高,极大值2.55%在450℃退火态合金中取得,极小值0.65%在500~650℃退火态合金中取得。随形变温度升高,合金由形状记忆效应(SME)向超弹性(SE)转变,SME→SE转变温度可用DSC曲线上马氏体逆相变峰温度tMr+16℃估算。随循环次数增加,400℃退火态合金的ζM稳定,500和600℃退火态合金的ζM先降低后趋于稳定。

The phase transformation behaviors,microstructure characteristics and mechanical properties of Ti-50.8Ni-0.5V(mole fraction,%)shape memory alloy were studied by XRD,differential scanning calorimetry(DSC),optical microscope and tensile test.The results show that the constituent phases at room temperature of Ti-50.8Ni-0.5V alloys annealed at 400−700℃are parent phase B2 and martensite B19′.With the increase of the annealing temperature,the phase transformation type of the alloy changes from B2→R/R→B2 to B2→R→B19′/B19′→R→B2 to B2→R→B19′/B19′→B2 to B2→B19′/B19′→B2 upon cooling/heating.The R,martensitic transformation temperatures of the alloy increase first and then decrease,and the maximum 28.1℃,−58.5℃are obtained in the alloys annealed at 400℃and 500℃,respectively.The microstructure of the alloy changes from fibrous to equiaxed,and the recrystallization temperature of the is among 550−600℃.The strength of extension decreases and the plasticity increases in the alloy.The critical stress for inducing martensitic transformation(σM)of the alloy decreases first and then increases,and the minimum 325 MPa ofσM is obtained in the alloy annealed at 450℃.The residual strain increases first and then decreases and then increases,the maximum 2.55%and 1.84%are obtained in the alloys annealed at 450℃and 700℃,respectively,and the minimum 0.65%is acquired in the alloys annealed at 500−650℃.With the increase of the deforming temperature,a transformation of shape memory effect(SME)→superelasticity(SE)occurs in annealed Ti-50.8Ni-0.5V alloy,and the SME→SE transformation temperature can be approximately estimated by tMr+16℃(tMr is the peak temperature of the martensitic reverse transformation in DSC curve).With the increase of the cyclic number,theσM of the alloy annealed at 400℃is stable,and theσM of the alloys annealed at 500 and 600℃decreases first and then tend to be stable.