BAIN STRAIN RELAXATION DURING EARLY-STAGE DECOMPOSITION OF A HYPER-EUTECTOID CUBE ALLOY

Decomposition of a hyper-eutectoid beta (b.c.c.) phase CuBe alloy into the two phase [gamma(B2) + alpha(fc.c.)] mixture involves composition separation, b.c.c. to B2 atomic ordering and b.c.c. to fc.c. crystal lattice rearrangement. In this investigation, the relaxation of the Bain transformation strain associated with the b.c.c. to fc.c. lattice rearrangement is studied by TEM during the early stages of decomposition and experimental findings are interpreted within the framework of an elasticity theory of plate-like precipitates. We have found that relaxation of the Bain strain proceeds gradually with the advancement of decomposition. A decomposition sequence which is consistent with the microstructures is: b.c.c. to B2 ordering --> isostructural secondary decomposition and the formation of {001} plate-like G. P. zones (b.c.t.) --> b.c.c. to f.c.c. crystal lattice rearrangement and the formation of plate-like alpha'(f.c.t.) precipitates with {001} habit planes --> gradual relaxation to alpha(f.c.c.) phase. From habit plane stability analysis, we demonstrate that the intermediate alpha' (f.c.t.) state is elastically unstable. Driven by this instability, the relaxation was found to occur mainly through the interplay of two basic mechanisms, that is, habit rotation and the formation of polytwin morphology. The interplay was manifested in the form of characteristic saw-tooth type polytwin plates. From a careful analysis of as-quenched states, we have found that the formation of the saw-tooth morphology initiates by plate intersection through the formation of stacking faults. We have explained this in terms of a dislocation model.