High-resolution transmission electron microscope (HRTEM) study of the transformation interface and substructure in NiTiHf 40 melt–spun ribbons

Abstract

The fine structures of self-accommodation spear-like martensite variant groups and the interface between the martensite region and the amorphous phase region in NiTi–Hf 40 melt–spun ribbons were studied by using high-resolution transmission electron microscopy (HRTEM). In the self-accommodation lath plate martensite group, the variants prefer to form orthogonal morphology of spear-like martensite on both sides of melt–spun ribbons. Some variant pairs were of the (011) B19′ Type I twin related with straight and coherent interface while the interface between other variants is curved and incoherent. The main substructure in variants of the group was not the (001) B19′ compound twin but the (011) B19′ Type I microtwin and (011) stacking faults. The interface between the amorphous phase and the martensite region showed high energy and nonequilibrium configuration. Some area in the interface is straight but not smooth and exhibited an irregular configuration and some area in the interface has an obvious zigzag configuration. Energy-dispersive spectroscopy (EDX) analysis of the composition in NiTi–Hf 40 ribbon showed that the Ni and Ti content were different in the martensite region and in the amorphous phase. The amorphous phase and the martensite coexisted in NiTi–Hf ribbons when the cooling rate was high enough. The higher hardness and brittleness of Hf 40 compared to those of lower Hf content NiTi–Hf alloys were due to the not retained austenite, the very small variant pair and the coexisting amorphous phase.