COMBUSTION SYNTHESIS OF TINI INTERMETALLIC COMPOUNDS .3. MICROSTRUCTURAL CHARACTERIZATION

The microstructural characteristics of NiTi intermetallic shape memory compounds produced by combustion synthesis have been investigated. It was found that the microstructure consists mainly of NiTi parent phase and NiTi2 second phase. The faceted or coarse dendritic NiTi2 phase, which is produced using a low cooling rate of the synthesized liquid product, can be modified to provide an evenly distributed, fine dendritic structure by increasing the cooling rate. This fine dendritic product can be readily hot-rolled into plate exhibiting the shape memory effect (SME). The initial nickel particle size has an important influence on the microstructure and also on the SME transition temperature. The morphology of the microstructure can also be modified to that of conventionally produced alloy by solution treatment. It is proposed that the synthesis reaction mechanism occurs by two combustion stages, i.e. precombustion and combustion. The particle size plays a key role in the precombustion stage.