Studying HT-SMAs in compression

The addition of Hf and further elements to Ni-Ti for the development of high-temperature shape memory alloys makes the control of microstructure and transformation properties multiple times more complex. Due to the low processability and machinability of HTSMA, high efforts might be necessary for converting the alloy in a testable shape. For prescreening and studying the transformation characteristics of HTSMA formulations, a feasible strategy could lie in testing these materials in compression. By this approach, simple cylindrical shapes can be tested and several transformation properties can be characterized already on ingot or semi-finished product level without the need of spending excessive resources in metal working or sample machining. In this study, a multi-functional compression and quenching dilatometer instrument with the possibility to apply controlled heating and cooling rates is used for the characterization of cast Ni-Ti-Hf-Nb alloys which underwent a minimum of processing steps. By this experimentation, important transformation characteristics as transformation temperatures, Clausius-Clapeyron relation, recoverable strain, martensitic plateau height, super-elastic response and others can be qualitatively assessed. The results were than compared to those measured on a NiTiHfNb wire for validating their significance.