Phase transformations and mechanical properties of nitial shape memory alloys with equal NiTi atom ratio

Phase transformation, microstructure and mechanical property of Ni _50-x/2Ti_50-x/2Al_x (x=1, 2, 3 and 4 at.%) alloys have been investigated by electrical resistance (ER) measurement, scanning electron microscope (SEM), X ray diffraction (XRD) and tensile tests. The results show that a two-step phase transformation occurs from parent phase to R phase and subsequently to martensite upon cooling, and the transformation reverses upon heating. Both the martensitic transformation and R phase transformation temperatures decrease with increasing of Al content. The microstructure analysis indicates that the NiTi-Al alloys are composed of NiTi as the predominant phase and Ti₂Ni as the second phase mainly distributing at grain boundaries. The solid solution hardening effect of Al at room temperature has been demonstrated by the remarkable increasing of yielding strength and decreasing of tensile strain with increasing of Al content. Ni ₄₉Ti₄₉Al₂ alloy deformed at the liquid temperature exhibits a maximum recoverable strain of 6.3%.