An electron-microscopical investigation on the martensitic transformation in TiNi

Abstract

An electron-microscopical investigation of the morphology of equiatomic TiNi martensite has revealed at least two types of martensite: the twinned plate martensite and a wavy martensite. To our knowledge the latter has never been reported in the literature before. In a thin foil, the plate martensite can be transformed into the wavy martensite within a few weeks at ambient temperature or by temperature cycling between +100° C and liquid nitrogen temperature.

The transformation into the wavy martensite has been studied dynamically with the aid of a video-recording system attached to the microscope. This revealed the step-wise migration of the irregular boundary between the wavy martensite and the high temperature phase. The transformation from the B2 phase to the wavy martensite is preceded by the formation of a superstructure having three times the lattice parameter of the CsCl (B2) structure. The transformation is very sensitive to the cooling rate during temperature cycling and can even be totally suppressed if the cooling rate is too high. The transformation characteristics are not affected by the thermal history of the sample.