Influence of pre-heating temperature and ultrasonic vibration treatment on the structure and martensitic transformations in NiTi foams produced by SHS

The structure and martensitic transformations in NiTi foams produced by self-propagating high-temperature synthesis under the action of ultrasonic vibrations (USV) at different pre-heating temperatures of the powder mixture are studied. It has been found that with an increase in the pre-heating temperature, the porous structure changes from layered to isotropic one in the NiTi foams, but cavities appear. The USV treatment reduces the number of cavities and increases the maximum pre-heating temperature at which they are not formed. It has been found that the USV treatment reduces the size and volume fraction of precipitates and makes the chemical composition of the NiTi phase more uniform that affects the martensitic transformation. It has been assumed that an increase in the pre-heating temperature compensates the heat transferred from the powder mixture outside the thermal chamber by the titanium waveguide, which increases the length of the reacted zone. This allowed USV to influence the structure formation in the NiTi foams in the same way as it was observed during conventional crystallization of Ti or Al-based alloys.