MARTENSITIC-TRANSFORMATION IN THIN FOIL SPECIMEN OF A SHAPE-MEMORY TINI ALLOY

Since shape memory alloys are applied in various fields, especially in the micro-machine field, and the dependence of size effect on shape m emory phenomena of the alloys has been reported, it should be importan t for using the alloys actually to clarify the quantitative relationsh ip between the thickness and the occurrence of martensitic transformat ion of the alloys. In the present paper, the relationship for TiNi (it s M(s) temperature is 248 K) thin film specimens suitable for TEM obse rvation 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 nn; (2) The M(s) temperatur e of the thin film is considerably lower than that of the alloy in the bulk-state; and (3) The interval between A(s) and A(f) temperatures d ecreases with decreasing specimen thickness, and in the case of a thin film specimen suitable for TEM observation, the interval becomes almo st zero.