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

Shape memory alloys (SMAs) are a functional material system, which can be used as an actuation device due to their high energy density. High temperature SMAs, a subset of SMAs, exhibit operative temperatures above 100°C which is attractive to aerospace, defense, and energy applications. One of the challenges for HTSMAs is to improve fatigue life, while also maximizing the actuation response. Based on a literature review and a larger study, we have down-selected to a Ni-lean NiTi- 10 at.% Hf HTSMA. Processing of these alloys can be difficult due to the formation of brittle intermetallic Ti₂Ni precipitates and Ti₄Ni₂O_x oxides. Thus, the focus of this project is to identify processing parameters which result in optimal microstructures for targeted applications. In this study, we investigate the effect of flash annealing on the thermo-mechanical properties of a Ni-lean Ni_49.8Ti_40.2Hf₁₀ HTSMA. The alloy produced as a large-scale melt by Fort Wayne Metals via plasma arc melting, homogenized for 72 hr at 1050°C then extruded at 900°C to 0.27 inch diameter, followed by cold drawing down to 0.01 inch diameter wire. To perform the flash-anneal, the wire was pulled through a tube furnace in an Ar gas environment in increments of 30 sec at temperatures of 550, 600, 650, or 700°C, respectively. The samples were characterized using scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy and thermo-mechanical testing. The results from this study show that the fatigue life of Ni_49.8Ti_40.2Hf₁₀ HTSMA can be significantly extended at flash annealing temperatures of 650°C and above.