A mechanism for the strain-induced nucleation of martensitic transformations

The previous work of Professor W. G. Burgers and Dr. A. J. Bogers is used to develop a mechanism of strain-induced martensitic nucleation, involving two intersecting shear systems. We distinguish between strain-induced nucleation and stress-assisted nucleation, the latter involving the same sites and embryos as does the regular spontaneous transformation. The strain-induced nucleation, on the other hand, depends on the creation of new sites and embryos by plastic deformation; this phenomenon may also contribute in a major way to autocatalytic nucleation during the course of martensitic transformation. For the case of strain-induced nucleation, it is possible to focus on specific intersecting-shear systems when the austenitic stacking-fault energy is low and ε (h.c.p.) martensite can form as a part of the shear displacements. It then becomes feasible to extend the intersecting-shear mechanism from this special case to alloys of higher stacking-fault energy, where ε is no longer stable relative to the austenite. It should be noted, however, that these events are very early stages in the formation of martensitic plates and relate primarily to the genesis of embryos; the actual growth start-ups which determine the operational (measured) nucleation rates may be controlled by subsequent processes.