Interaction of single variant martensitic transformation with small γ type precipitates in CuZnAl

Abstract

Thermally- and stress-induced single-interface and single-variant martensitic transformation, in specimens containing a fine distribution of γ precipitates, was studied. The average precipitate size was 7 nm and the density 8 × 10 21 prec./m 3. The hysteresis width, considerably larger than the one observed for specimens without precipitates, decreases with the number of cycles, from an initial value of 7.2 K to an asymptotic value of about 1.5 K. A simple model shows that the interaction can be explained essentially by considering the deformation due to the shape change upon transformation of the matrix cavities enclosing the precipitates. The energy to create dislocations and to produce the subsequent plastic deformation of the martensite around the precipitates is of the same order of magnitude as the excess of free energy associated to the hysteresis. Nucleation of the β phase inside a martensite plate was observed with a nucleation energy of about 1.3 J/mol.