Porous NiTi by thermal explosion mode of SHS: processing, mechanism and generation of single phase microstructure

For the first time, porous yet single phase NiTi was successfully prepared using the thermal explosion mode of SHS in combination with a controlled post-reaction heat treatment. The effects of the two crucial processing variables, namely the heating rate and the green density, were investigated and the reaction mechanism was identified by intercepting the combustion at different stages. There was a threshold heating rate that varied with the green density, above which the product melted. This molten-cast product was single phase but devoid of the requisite porosity. Below this threshold the product was porous but multiple phase. Interestingly, the post-reaction heat treatment needed for the conversion to single phase porous NiTi also showed strong dependence on the heating rate. A faster rate resulted in loss of both the porosity and the shape as the strong interaction with an intermediate liquid phase collapsed the porous structure. However this interaction could be controlled and heating up to 1150 °C at a very low rate (1 °C/min) could produce single phase NiTi without sacrificing the porosity.