DMA characterization of a Ni50.5at%Ti shape memory alloys

It is well-known that the special properties of NiTi shape memory alloys strongly depend on their microstructure and their thermomechanical state. DSC and mechanical tests (stress-strain vs temperature) do provide extensive information on the transformation temperatures and on the pseudoelastic effect: a Dynamic Mechanical Analyzer (DMA) is further able to investigate even more transition effects, because it makes it possible to study the transition temperatures in both stressed and unstressed conditions over a wide temperature range (-150 degrees C; 600 degrees C). Moreover, through dynamic experiments one can to analyse the linear mechanical response and the viscoelastic properties in different ranges of strains and frequency. In this work we measure the response of NiTi in the austenitic and rhombohedral phase. We test the material both in isothermal condition over a frequency range and at a constant frequency over a temperature ramp. The NiTi samples used were Ni50.5at%Ti wires, 80 mu m in diameter, thermally treated at 450 degrees C for 1.8 Ks.