A COMBINED ELECTRON MICROSCOPY AND LASER FLASH ANALYSIS STUDY OF THE EFFECTS OF HEAT TREATMENTS ON THE 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 austenite-to-martensite phase transition and can be induced by a temperature change. Heat transport has, therefore, a key role in the function of the shape memory effect.

This research focuses on the effects of heat treatments on the microstructure and 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 120-200ºC depending on nickel concentration and heat treatment, as well as high yield strength. These features make it an attractive candidate for high-temperature aerospace actuators.

Microstructures of NiTi-20Hf sheet specimens after several heat treatments were characterized by high-resolution electron microscopy. Hf-rich nano-precipitates were observed and identified using high-resolution scanning electron microscopy (HRSEM), high-resolution transmission electron microscopy (HRTEM), and scanning/transmission electron microscopy (STEM) operated in the high-angle annular dark field (HAADF) mode, focusing on the analysis of the precipitate size distribution and number density. In turn, the thermal conductivity is measured applying the laser flash analysis (LFA) technique, and the electrical conductivity is measured using a four-point probe apparatus. We observe direct influence of the precipitate size and number density on both thermal and electrical conductivities. Moreover, the temperature dependent thermal conductivity exhibits a hysteresis behavior, in which the phase transition temperatures are clearly identified. Combination of advanced electron microscopy with LFA is found, for the first time, to be a useful tool for evaluation of the performance of SMAs.