GRAIN SHAPE EFFECTS ON INTERFACE-CONTROLLED DIFFUSIONAL CREEP UNDER MULTIAXIAL STRESSES

Interface control of the emission and absorption of vacancies reduces, but does not eliminate, the dependency of diffusional creep on grain size and shape and it increases the sensitivity to stress. Previous an alyses of interfacially controlled diffusional creep of polycrystals w ith equiaxed grains under a uniaxial stress are extended and further d eveloped to determine approximately the behaviour of material with ani sotropic grain shapes under multiaxial stresses. Whilst all details of this creep process are not fully understood, it is shown that, by not ing a set of conditions to be fulfilled, the form of equation required to describe anisotropic creep strength can be identified. Although in terfacial control implies a power law dependance on stress, it is note d that the directionality in creep deformation is governed by grain sh ape orientation and by linear functions of the applied stresses and so anisotropic aspects of creep response under multiaxial stresses can b e described by ''creep compliance coefficients'' analogous to those re presenting anisotropic elastic behaviour. Since anisotropy is influenc ed by three parameters, representing the principal orthogonal grain di mensions, only three of these coefficients are independent. From this approach a Von Mises type function can be formulated through which ani sotropic interface-controlled diffusional creep strength may be evalua ted.