The Influence of Alumina Dispersions on the Diffusion-Creep Behaviour of Polycrystalline Copper

Abstract

The creep behaviour of powder-compacted copper and copper containing 0.5, 1.0 and 1.5 vol.-%alumina has been investigated in the range where the stress-directed lattice diffusion of vacancies is expected to control. Addition of alumina particles is shown to change the initial flow characteristics from Newtonian, where the creep rate έ is proportional to the applied stress σ, to Bingham type, where έ is proportional to an effective stress σέ. This effective stress is defined by σέ = σ − σ₀, where σ₀ is a threshold value that increases linearly with volume fraction of alumina and below which no deformation occurs. The proportionality constant between έ and σ_E is shown to be identical to that between έ and σ and is given by the Nabarro-Herring equation as BΩD/d² kT, where B is a numerical constant, Ω the atomic volume, D the lattice self-diffusion coefficient, D the grain size, and kT has its usual meaning. Above the threshold stress, creep rate decreases with strain and this curvature is more pronounced at lower stress levels. Arrhenius plots of creep data for the alloys are non-linear and this is attributed to a temperature-dependence of σ₀ which is described by the relationship σ₀ = A(1 − T/T_M ), where A is a constant having the value 10.9 MN/m² and T/T_M is the homologous temperature. It is suggested that the presence of particles on boundaries inhibits emission and absorption of vacancies and gives rise to the observed effects.