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

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.

Key words:V content;NiTi shape memory alloys (SMAs);microstructure;martensite transformation;microhardness