Effects of Ni in Ni-Rich NiTiHf High-Temperature Shape Memory Alloys during Thermo-mechanical Processing

摘要

Shape memory alloys (SMAs) are an active or multifunctional material due to their ability to recover a designed shape upon heating. High temperature SMAs (HTSMAs) are a subset of SMAs which have transformation temperatures above 100°C, which makes them attractive candidates for many aerospace, defense, and energy applications [1]. The transformation temperatures can be controlled via composition, processing, and heat treatments. NiTi is a well-studied binary SMA system with a long history of diverse and successful applications but is limited to a maximum austenite finish temperature of 115°C [1, 2]. The substitutions of Au, Pd, Pt, Hf, and Zr have been shown to increase transformation temperatures making the ternary alloys [1, 3]. The elements Hf and Zr offer a more cost-effective solution as compared to Au, Pd, and Pt. Previous researchers have found both NiTiHf and NiTiZr to be challenging to process due to the brittle nature of the material as well as the formation of complex oxides during the hot-rolling process. More recently, researchers have found NiTi with 15 at.% Hf to exhibit promising properties when processing by conventional methods such as vacuum arc melting and hot-rolling [4].