Body temperature NiTi alloys: effect of the heat treatment on the functional thermo-mechanical properties

The design process of devices based on shape memory alloys (SMA) is particularly demanding. It is fundamental the investigation of parameters describing the thermo-mechanical behavior of these materials, such as the phase transformation temperatures and the Clausius-Clapeyron coefficient (CCC). The aim of this study is to determine how heat treatment (HT) affects the above-mentioned parameters on two commercially available body-temperature NiTi alloys. This analysis is accomplished through differential scanning calorimetry (DSC) and strain recovery (SR) tests. Finally, the two NiTi alloys have been compared to determine whether exists a significant difference between different casting batches of the same commercial alloy conformed to the ASTM F2063. It results that both alloys present a similar overall trend of Af as a function of HT temperature and time, even if the values significantly differ for treatment at temperatures higher than 773 K for all HT times. Furthermore, it is noted that the HT time presents a minor effect on the value of Af. After the SR analysis, it has been possible to highlight the effect of the HT on the values of CCC for As, Ms and Rs. The overall trend is similar between the two NiTi alloys. These results lead to the conclusion that HT highly affect not only the transformation temperatures but also the transformation path of the alloy, thus its mechanical properties.