Grain shape effects on interface-controlled diffusional creep under multiaxial stresses

Abstract

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 analyses of interfacially controlled diffusional creep of polycrystals with equiaxed grains under a uniaxial stress are extended and further developed to determine approximately the behaviour of material with anisotropic grain shapes under multiaxial stresses. Whilst all details of this creep process are not fully understood, it is shown that, by noting a set of conditions to be fulfilled, the form of equation required to describe anisotropic creep strength can be identified. Although interfacial control implies a power law dependance on stress, it is noted that the directionality in creep deformation is governed by grain shape orientation and by linear functions of the applied stresses and so anisotropic aspects of creep response under multiaxial stresses can be described by “creep compliance coefficients” analogous to those representing anisotropic elastic behaviour. Since anisotropy is influenced by three parameters, representing the principal orthogonal grain dimensions, only three of these coefficients are independent. From this approach a Von Mises type function can be formulated through which anisotropic interface-controlled diffusional creep strength may be evaluated.