In situ tem study of the h.c.p. to f.c.c. martensitic phase transformation in CoNi single crystals

Abstract

A single crystal of Co + 32% Ni was grown and transformed by deformation from the f.c.c. to the h.c.p. phase at room temperature. TEM specimens made from this h.c.p. crystal were used to study the h.c.p. to f.c.c. martensitic phase transformation by in situ annealing experiments. At about 180°C the growth of new f.c.c. lamellas is observed; they are several atom layers thick, emerge from the bulk region of the specimen and are not in connection with the lamellas of retained austenite present in the thin foil. Several new lamellas have the orientation of the f.c.c. twin variant. When the temperature is increased more lamellas grow into the thin foil and at about 350–400°C the transformation is complete since hardly any retained martensite is encountered. From a comparison of the results with those of bulk annealed specimens and those of pure Co it can be concluded that the results could be generally valid for thermally induced transformations from h.c.p. to f.c.c. The experimental observations can be compared with the various models of the phase transformation found in the literature. There is no agreement with models based on the random nucleation of individual stacking faults since the lamellas expand at a temperature when the stacking fault energy is still positive. Also a polar dislocation model seems unlikely. The results are in favour of the models based on the formation of lamellas; at least there seems to be no contradiction to these models.