Micropyretic synthesis of NiTi in propagation mode

Abstract

The microstructure of NiTi formed by micropyretic synthesis; carried out in the propagation mode; was investigated with preheat; Ti particle size as the two process variables. These variables were found to exert a significant influence on the microstructure. as well as the process. Unstable combustion was observed in specimens combusted without preheat. The microstructure of specimens undergoing unstable combustion showed some features which were similar to those developing during rapid solidification of NiTi in terms of the nature of the phases; their morphology. The microstructure essentially comprised the martensitic phase; the B2 parent phase; the Ti₂Ni intermetallic phase. In some specimens the Ti₁₁Ni₁₄ phase was found to form in an unusual lamellar morphology. The microstructural features of the micropyretically synthesized alloy were compared with those of the conventionally processed alloy. The structures of the B2 Ti₂Ni; Ti₁₁Ni₁₄ phases; of their interfaces with the matrix phase were examined by high resolution electron microscopy (HREM). The mechanism of synthesis of the alloy from elemental powders was studied by stopping the combustion front; carrying out detailed microstructural characterization around the arrested front.