Non Equiatomic NiTi Alloy Produced by Self Propagating High Temperature Sinthesys

In the present work experimental and numerical studies, finalized to obtain porous NiTi alloys by Self propagating High temperature Synthesis (SHS) and to define thermo-mechanical properties of the product are presented. Samples were prepared starting from elemental powders of Ni,  Ti, and hydrated Ti, homogeneously dry mixed with stoichiometric ratio between Ni and Ti close to 1:1. In order to lower ignition temperatures, mechanical activation obtained through ball milling was also applied. Powders were then pressed to obtain samples having different initial density.

Different sample sizes were chosen on the basis of the analyzed phenomena namely heating, ignition and reaction processes. Tests aimed to obtain volumetric heating of sample were performed on small cylindrical sample (diameter 8 mm, height 1-2 mm): the effect of the heating rate on the ignition parameters could be evaluate. Self-propagating high-temperature synthesis phenomenon was investigated to define the dependence of reaction front velocity and reaction temperature on the pre-heating conditions (sample diameter 8-10 mm and length 10-20 mm).

Porous products were characterized both from a morphological and a functional point of view. Total porosity, as well as pore size, shape and distribution, was analyzed. Sample microstructure was also investigated: the main phase produced during the self-propagating high-temperature synthesis reaction was NiTi phase.

Differential scanning calorimetry was performed on as reacted as well as on solution treated specimens to determine transformation temperatures. Thermal analysis was completed defining the thermal conductivity dependence on temperature and porosity from 300K up to 1400 K: thermal conductivity increases with the temperature while it decreases with porosity increase.

Selected samples were also subjected to compression tests in order to highlight the influence of porosity on mechanical behavior of the sample.