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 Ba in transformation strain associated with the b.c.c. to fc.c. lattice r earrangement is studied by TEM during the early stages of decompositio n and experimental findings are interpreted within the framework of an elasticity theory of plate-like precipitates. We have found that rela xation of the Bain strain proceeds gradually with the advancement of d ecomposition. A decomposition sequence which is consistent with the mi crostructures is: b.c.c. to B2 ordering --> isostructural secondary de composition 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 i s 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 hav e found that the formation of the saw-tooth morphology initiates by pl ate intersection through the formation of stacking faults. We have exp lained this in terms of a dislocation model.