Thermally activated martensite in copper alloys

The mechanisms of the thermally induced f c c-h c p transformation in Cu-Ge alloys have been investigated by hot-stage microscopy and TEM techniques. The growth and thickening processes for the transformation are best described in terms of isothermal martensite growth in which the h c p phase formation is controlled by the rapid propagation of fine platelets having thicknesses ranging from 5 to 30 nm. The transformation progresses by the repeated nucleation of thin platelets often in close proximity to existing platelets, thereby leading to a morphology termed “fault bundles” by other investigators. Individual h c p plates form by the rapid movement of a transformation interface defined by groups of partial dislocations emanating from grain boundaries and non-coherent twin boundaries, and gliding parallel to a given {1 1 1} matrix orientation. The nucleation kinetics are controlled by the thermally activated propagation of partial dislocations originating from boundary networks. It is concluded that short-range diffusion is necessary for the h c p phase to achieve an equilibrium composition, but does not control the rate at which platelets nucleate or propagate.