Thermomechanical Properties of Porous NiTi Alloy Produced by SHS

Samples obtained from relatively large powders (<150 µm), with total porosity in the range 30-68%, were characterized mainly from a morphological point of view. Total porosity, as well as pore size, shape and distribution, was analyzed. Sample microstructure was also investigated, indicating that the main phase produced during the self-propagating high-temperature synthesis (SHS) reaction is Ti reach NiTi phase, as confirmed by calorimetric analyses. Moreover, the presence of secondary phases, suggested by the low transformation enthalpy, was confirmed by SEM observations. In fact, EDS microanalyses and EBSD mapping helped in the identification of such secondary phases, such as Ni₃Ti, Ti₂Ni and Ti₄Ni₂O_x. Other samples were successively produced starting from the same powders but introducing a different powder compression methodology and operating conditions. In this way, the obtained samples showed higher porosity featured by more uniform size, shape and distribution while, from a micro-structural point of view, no significant differences were observed. Mechanical compression tests were carried out at room temperature and, on selected samples, also above A_f in order to highlight the influence of pore shape and distribution. Results obtained at room temperature show that the mechanical properties decrease with the porosity augmentation. For higher temperatures, the samples presented a pseudoelastic behavior. Dilatometric tests were also performed and the results well indicated the martensite to austenite transformation at the same temperature showed by the DSC analyses. Thermal analysis was completed by evaluating the thermal diffusivity temperature and porosity dependence using an experimental-numerical approach especially developed.