Abstract
Ti₃₆Ni_49−xZr₁₅Ta_x (x = 5, 10, 15 at.%) alloys were prepared using a lab-scale arc-melting furnace and homogenized at 1173 K under an argon atmosphere for 2 hours, followed by furnace cooling. The buttons were sliced into 1−2 mm-thick sheets using wire cutting. These sheets were solution-treated under an argon atmosphere, followed by aging and annealing. The results revealed an increase in hardness with Ta addition, due to solid-solution strengthening. With aging and annealing, the hardness decreased due to the softening effect of Ta diffusing from the matrix to form beta phase (β), whose peaks were observed in XRD for the quaternary samples, as well as the development of B2 peaks for the 10 Ta and 15 Ta samples in the diffractogram. Microstructure analysis showed grain refinement with 5 Ta addition. At higher Ta content of 10 and 15 at.%, the martensite structure coarsened due to decreased grain boundary mobility.

Key words
shape memory alloys, martensite, phase transformation, microstructure, hardness

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