PM Processing of Single-Phase NiTi Shape Memory Alloys by VPCR Process

Single-phase nickel–titanium alloys were successfully synthesized by using a powder metallurgical process based on the use of a calcium reductant source during sintering in argon atmosphere (VPCR process). This process allows avoiding secondary phase formation during the NiTi compound forming reaction. The experimental results show that both heating rate and sintering temperature play a significant role on the final porosity. Different sintering stages at temperatures below TE(Ti2Ni), between TE(Ti2Ni) and TE(Ni3Ti), and above TE(Ni3Ti) were investigated in order to elucidate the influence of the two liquid eutectics on the densification. By choosing a slow heating rate of 0.5 K/min and a long time sintering at 1193 K, an almost dense single-phase NiTi compact was obtained with austenite <= ftrightarrow martensite transformation heats comparable to those found in melt-cast NiTi alloys.