The Effect of DSC Heating and Cooling Rate on the Evolution of Transformation Characteristics in NiTi-Based Shape Memory Alloys

摘要

Calorimetric measurement is a widely applied method to determine the transformation temperatures in NiTi-based shape memory alloys and the release and uptake of heat during a phase change which provides an estimation of the quantity of transformed material. As recommended by the standard practice ASTM F2004-10, differential scanning calorimetry (DSC) experiments on NiTi alloys are performed at a moderate heating and cooling rate of 10°C/min which allows for a decent signal sensitivity and testing reproducibility [1]. However, it was shown that the transformation peak position and morphology depends on the applied heating/cooling rate and thus, a faster or slower rate may alter the resulting key temperature parameters like onset and offset points of transformation and also the recorded heat dissipation like demonstrated by Nurveren et al.[2]. In their work they show that the different temperature parameters like austenite and martensite start, peak and finishing temperatures are diversely affected. Another observation is, that the sensitivity of phase transformation temperatures (TT) and hysteresis to the rate depends on the alloy composition [3], but is not well established how the introduction of further alloying elements modifies the DSC behavior by using a rate deviating from 10°C/min. E.g. Wang et al. showed that there is a significant impact of the rate on the TT in Ti-Ni-Cu consisting of 43 at. % Ni and 7 at. % Cu varying it between 1,5 and 20 °C/min [4] and that the hysteresis gets smaller by applying lower heating/cooling velocities without providing a comparison to binary Ni-Ti. Open questions remain also on how heating/cooling rate affects the measurement in educated wires like they are used in application. This research work presents a comparative study with the aim to bring out the effect of varying heating/cooling rate on the most important key parameters like transformation temperature, thermal hysteresis and transformation enthalpy obtained by DSC measurements for NiTi-based alloys with binary and ternary compositions in as-cast or processed conditions. By expanding the research on DSC rate effect to a wider range of materials and processing conditions, it is shown that composition, sample morphology and thermo-mechanical treatment affect DSC results to different extent and thus all significantly impact the interpretation of shape memory effect.