Constraints on diffusional cavity growth rates

AbstractThere have been a number of rate equations proposed in the literature for the growth of grain-boundary cavities during creep. Implicit in all these analyses is the assumption that growth occurs freely and that no constraints exist that would attenuate the predicted rates. It is shown that a constraint on growth rate can occur in polycrystals because creep cavities are inhomogeneously distributed amongst the various grain boundaries. This attenuation of growth rate is predicted to be greatest when high-strength alloys with large cavity populations are deformed slowly and least when pure metals with low cavity populations are deformed quickly. The application of these ideas to multiaxial stress states and extrapolation procedures is discussed in the knowledge that the rate of unconstrained cavity growth is directly proportional to the maximum principal tensile stress, whereas constrained growth is proportional to the nth power of the octahedral shear stress.