Effect of Flash Annealing on the Microstructure and Fatigue Life of a Ni-rich NiTi-20 at.% Hf High Temperature Shape Memory Alloy

Abstract

Typical processing techniques involve thermo-mechanically treating the material such as cold working and subsequent annealing to control grain and precipitate size, shape, orientation, and morphology. Shape memory alloy (SMA) mechanical properties rely heavily on microstructural features such as precipitates and grain size to extend fatigue life. Novel approaches to control microstructural features have used laser anneal on amorphous NiTi thin films to recrystallize grains and short-time annealing on NiTi after angular extrusion and cold-drawn fine wires. A recent study examined rapid thermal annealing (RTA) on Ni-lean NiTi- 10 at.% Hf wires as an effective method for controlling grain size and extending actuation fatigue; however, flash annealing or RTA on Ni-rich NiTiHf high-temperature SMA (HTMSA) wires has not been investigated. Based on a larger study, Ni-rich NiTi-20 at.% Hf HTSMA was down-selected for further processing. This study investigates the effect of flash annealing on the thermo-mechanical properties of a Ni-rich Ni50.3Ti29.7Hf20 HTSMA. Microstructural changes were examined using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Actuation fatigue properties were also evaluated at 300 MPa. The results indicate that flash annealing HTSMA wires is an effective method for controlling grain size and extending fatigue life. The heating rate and time held are crucial parameters to control microstructural features such as grain size and coherency.