H. E. Karaca, G. Ded, B. Basaran, S. Saghaian, University of Kentucky, Lexington, KY; R. D. Noebe, NASA Glenn Research Center, Cleveland, OH
There is an increasing interest in High Temperature Shape Memory Alloys (HTSMAs) that have the ability to produce high actuation strain, stress, and work output at temperatures above 100 °C for industrial applications. It has been reported that addition of Pd, Pt, Au, Hf and Zr to NiTi increases its transformation temperatures. Among those, Ni-Ti-Hf is the most promising alloy family due to its relatively low raw material cost, high transformation temperatures, and fair ductility. However, its low strength and high transformation hysteresis results in unstable shape memory behavior and lack of superelasticity, which has restricted its potential application.
In this study, aging and chemical alloying are utilized to improve the mechanical and shape memory properties of the Ni-Ti-Hf alloys by forming nano-sized precipitates. Extensive thermal, microstructural and mechanical characterization studies have been conducted to determine the shape memory behavior of aged NiTiHf single and polycrystalline alloys. Effects of heat treatments, orientation, stress and temperature on transformation strain, stress and hysteresis as well as cyclic stability are revealed. It has been determined that precipitation hardening is a very effective tool to increase the material strength resulting in stable superelastic and shape memory behaviors with low transformation hysteresis. Moreover, the transformation temperatures are found to be highly heat treatment dependent in these Ni-rich NiTiHf alloys.
