V含量对等原子比NiTi形状记忆合金微观组织、相变行为和显微硬度的影响

Effect of V Content on Microstructure, Phase Transforma-tion Behavior and Microhardness of Equiatomic NiTi Shape Memory Alloy

摘要:

采用光学显微镜、扫描电子显微镜、X射线衍射仪、差示扫描量热仪和显微硬度计等测试手段，研究了V含量对等原子比NiTi形状记忆合金微观组织、相变行为和显微硬度的影响规律。结果表明：当V含量为0.5at%时，具有等轴晶组织的NiTiV形状记忆合金包含B19' 和 Ti₂Ni相；当V 含量大于0.5at%时，NiTiV形状记忆合金形成B19'相、Ti₂Ni相和V的富集相，并且随着V含量增加，V的富集相越来越多聚集于晶界。进一步分析表明，Ni_49.75Ti_49.75V_0.5和Ni_49.25Ti_49.25V_1.5 形状记忆合金发生了B2?B19'的一级相变，而Ni_48.75Ti_48.75V_2.5和Ni_48.25Ti_48.25V_3.5形状记忆合金发生了B2?R?B19'的二级相变，尽管降温过程中同时发生了部分的R相变与B19'马氏体相变。随着V含量增加，相变温度随着V含量增加逐渐降低，其主要原因是Ni/Ti比例的增加。此外，随着V含量增加，合金的显微硬度值呈现先降低然后几乎保持不变的变化规律。

Abstract:

The effect of vanadium (V) content on the microstructure, phase transformation behavior, and microhardness of the as-cast equiatomic NiTi shape memory alloy was investigated by optical microscope (OM), scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and Vickers microhardness tester. Results indicate that the as-cast Ni_50-x/2Ti_50-x/2V_x alloys with equiaxed grains consist mainly of B19' and Ti₂Ni phases when V content is 0.5at%, above which Ni_50-x/2Ti_50-x/2V_x (x=1.5~3.5, at%) alloys exhibit a three-phase structure consisting of B19', Ti₂Ni and V-rich phases, and the V-rich phases are more segregated at grain boundaries with the increase of V content. Further analysis reveals that both Ni_49.75Ti_49.75V_0.5 and Ni_49.25Ti_49.25V_1.5 alloys show a one-stage B2?B19' transformation. However, a two-stage B2?R?B19' transformation occurs in Ni_48.75Ti_48.75V_2.5 and Ni_48.25Ti_48.25V_3.5 alloys although R-phase transformation partially overlaps B19' martensitic transformation upon cooling. The transformation temperatures drop down with increasing the V content, which is attributed to the increase of Ni/Ti ratio in the matrix. In addition, as V element increases from 0.5at% to 3.5at%, the microhardness of the alloys first decreases and then remains almost unchanged.