A brief review of shape memory effects and fabrication processes of NiTi shape memory alloys

Abstract

After the discovery of martensitic in steel, people started research on the martensitic transformation of different materials. In 1932 discovery of reversal of this transformation was discovered in AuCd alloys. Due to this phenomenon, materials were able to gain their original shape and size before mechanical deformations and the concept of shape memory effect (SME) and shape memory alloy (SMA) was developed. After that, so many SMAs are developed but the incident which discovered Nitinol in 1961 enhanced the applicability of SMAs in real-life applications. It happened because of its cost effectiveness and its SME for a suitable range of working temperature (Generally in between −50 °C and 110 °C). Earlier liquid metallurgy route was used to produce the products. But in a few decades, powder metallurgy and additive manufacturing processes are dominated. NiTi phase is the only responsible phase in the NiTi alloys, which is capable of conversion of martensite to austenite and vice versa. At higher service temperature thermal-induced SME is not possible because it operates only when martensite can change to austenite. Service temperatures above austenitic finish temperature can only exhibit stress-induced SME.