Micropyretic synthesis of NiTi in propagation mode

The microstructure of NiTi formed by micropyretic synthesis, carried out in the propagation mode, was investigated with preheat and 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 and their morphology. The microstructure essentially comprised the martensitic phase, the B2 parent phase and 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 and Ti11Ni14 phases and 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 and carrying out detailed microstructural characterization around the arrested front. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.