EFFECTS OF HEAT TREATMENT ON PHYSICAL PROPERTIES OF A NITI-20HF SHAPE MEMORY ALLOY

Shape memory alloys (SMAs) constitute a large group of alloys with unique properties that allow technological applications in various fields. SMAs have been utilized for a variety of applications such as couplings, valves, medical guide wires, sensors and actuators owing to their ability to restore their original shape. The shape memory effect is related to the structural transformation from austenite to martensite, and can be induced by a temperature change. As a consequence, heat transport is a key process for the function of the shape memory effect.

This research focuses on the effects of heat treatments on the physical properties (including thermal and electrical conductivity) of the novel NiTi-20Hf alloy. This alloy has a relatively high transition temperature in the range of 120-200 ºC depending on the nickel concentration and heat treatment. In addition, it has high yield strength that is comparable with that of the binary NiTi. These features make it an attractive candidate for high-temperature aerospace actuators. Designing an SMA-based actuator cannot, however, rely on transformation temperature and yield strength only. Rather, it must take into consideration additional physical properties such as density, latent heat, specific heat and thermal conductivity, as well as electrical conductivity (e.g. for actuators heated by current). These properties, measured for samples of NiTi-20Hf in the forms of rod and sheet, will be presented in comparison to the binary NiTi alloy. The knowledge and the ability to control thermal and electrical properties of NiTi-20Hf SMAs will endorse their use for high temperature aerospace applications in the future.