Weak-beam TEM study of the h.c.p. to f.c.c. martensitic phase transformation lamellae in CoNi

Abstract

The thermically induced h.c.p. to f.c.c. martensitic phase transformation was studied in Co-32 at.% Ni single crystals by the weak-beam method of transmission electron microscopy (TEM). In this alloy the mean transformation temperature is at RT and therefore both phases are stable at RT. At the beginning of the transformation new f.c.c. lamellae grow from the thick into the thin region of the specimen. At the front of the lamella a Shockley partial dislocation must lie on every other close-packed plane. The dislocation contrast of the front observed by weak-beam images is similar to that of a single partial dislocation, even when the lamella is up to 50 atom layers thick. The minimum thickness of the lamellae is 6 to 10 close-packed planes; the lamellae grow in thickness by ledges. The lack of long range stress fields and the observation of lamellae of discrete height seem to indicate that at the front of the lamellae all three coplanar Burgers vectors of the partial dislocations are present and cancel each other to a large extent. Therefore, in the present case of a h.c.p. to f.c.c. transformation the experimental results do not support models based on a polar dislocation mechanism, whereas the models favoured are those based on the nucleation of lamellae which have an initial thickness of a few atom layers. The results seem to be in agreement with recent neutron-scattering experiments that showed the occurrence of satellites corresponding to a modulation of six layers of close-packed planes.