The Effects of Nickel in Ni-rich NiTiHf15 High-Temperature Shape Memory Alloys during Thermomechanical Processing

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. One of the current challenges which HTSMAs face is difficulties associated with processability, especially in Ni-rich alloys, which limits actuator forms. In this study, the effects of nickel content on the processability of Ni-rich NiTiHf₁₅ HTSMAs during thermo-mechanical processing was analyzed. Three NiTiHf₁₅ compositions (Ni_50.1Ti_34.9Hf₁₅, Ni_50.3Ti_34.7Hf₁₅, and Ni_50.5Ti_34.5Hf₁₅ in at. %) were created using vacuum arc melting. After melting, the samples were encapsulated in an Argon atmosphere and then homogenized for 1hr at 1050°C. The samples were thermo-mechanically processed by hot rolling at 800°C in air to a reduction in thickness of 25% and 50%, respectively. The samples were characterized using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), synchrotron-radiation x-ray diffraction (SR-XRD), Vickers hardness, and thermo-mechanical testing. The effects of the Ni concentration on the microstructure of the HTSMA and its processability are discussed herein.