Three theories exist which predict the influence of second phase particles on diffusional creep. Ashby considers grain boundary dislocations to be responsible for vacancy creation and annihilation. Interaction of these dislocations with grain boundary particles requires a stress σ0 = (2E/br)√V to be exceeded before creep can commence (E is the disloc'ation line energy, b the Burgers vector, r the particle radius, and V the volume fraction of second phase). The other two theories (Harris, Burton) consider the particle/matrix interface to be an inefficient sink and source for vacancies, so that vacancy condensation (or evaporation) at the adjacent boundary leads to stress concentration at particles. For continuing creep, plastic accommodation must occur at the particles to relieve the stress concentration. Burton considers the accommodation process to be the nucleation of interfacial defects and Harris considers accommodation by dislocation loop punching. Both theories predict σ0 ∝ <1>V</1> but the loop punching model predicts a stronger particle size dependence of σ0 The present work examines the effect of alumina particle size on the diffusional creep properties of sintered Cu-l vol.-%Al2O3. Interpretation of results is complicated by the presence of residual porosity but creep is shown to be more strongly inhibited by finer particles. The theory of creep limited by plastic accommodation is reexamined and the variation of creep rate with stress is calculated for stresses greater than σ0.
