Martensitic transformation in thin foil specimen of a shape memory TiNi alloy

Abstract

Since shape memory alloys are applied in various fields, especially in the micro-machine field, and the dependence of size effect on shape memory phenomena of the alloys has been reported, it should be important for using the alloys actually to clarify the quantitative relationship between the thickness and the occurrence of martensitic transformation of the alloys. In the present paper, the relationship for TiNi (its Ms temperature is 248 K) thin film specimens suitable for TEM observation was investigated by in-situ TEM and SEM methods using cooling stages, and a convergent beam electron diffraction method. The results obtained are as follows: (1) At 98 K, martensitic transformation does not occur in areas thinner than about 100 nm; (2) The Ms temperature of the thin film is considerably lower than that of the alloy in the bulk-state; and (3) The interval between As and Af temperatures decreases with decreasing specimen thickness, and in the case of a thin film specimen suitable for TEM observation, the interval becomes almost zero.