Properties and Heat Treatment of High Transition Temperature Ni-Ti-Hf Alloys

Abstract

The high transition temperature Ni-Ti (Hf, Zr) alloys have long been of interest for actuators and other applications requiring transition temperatures greater than 100 °C. Unfortunately, the high hardness and poor fabricability of these alloys have prohibited the scale up to commercial production. Some of these alloys are so “hot short” that even modest size ingots cannot be cast without internal cracks formed by solidification shrinkage stresses. Hot rolling methods have recently been demonstrated that can produce crack free Ni-Ti-(6-10 at.%)Hf thin sheets having austenite transition temperatures up to approximately 170 °C. Since these alloys are soft martensite phase at room temperature, they can easily be formed and bent at ambient temperature but cold rolling can only be performed to a limited extent due to high work hardening rates which are typical for Ni-Ti alloys. Progress is now underway to scale up these methods to produce 500-600 mm wide sheets. The effects of composition variations, heat treatment and cold working on transition temperatures are discussed. Microstructural features unique to these ternary alloys and impurity effects are also discussed. The effects of stress on transition temperature have been determined. Austenite transition temperatures, as measured by DSC and bend-free recovery testing, can be controlled within 100-170 °C for these alloys.