Effect of Aging Heat Treatment on the High Cycle Fatigue Life of Ni50.3Ti29.7Hf20 High-Temperature Shape Memory Alloy

Shape memory alloys can be utilized as actuators for several applications in aerospace industry which require high strength and stable actuation cycles together with the transformation temperatures above 100 degrees C. Aging is one of the methods for Nickel rich NiTiHf alloys that adjusts the transformation temperatures and enhances the cyclic stability due to the formation of nano-sized precipitates. In this study, the high cycle functional fatigue life and behavior of the extruded and aged Ni50.3Ti29.7Hf20 high-temperature shape memory alloy were investigated in order to reveal the effect of aging on the stability of the actuation strain and transformation temperatures. The aging was conducted at 550 degrees C for 3h. 200MPa was chosen in the functional fatigue experiments since no irrecoverable strain was determined under this stress magnitude for the extruded and the aged samples in the load-biased heating cooling experiments. The fatigue experiments were conducted twice to check the repeatability of the shape memory properties of the samples and it was observed that the life cycle of the aged sample was determined as 20,337 and the extruded sample completely lost the shape recovery ability after 5000 cycles.