Creep of aluminum strengthened by alumina particles

Abstract

Creep of aluminum strengthened by various amounts of alumina particles (4, 7, 10 and 14 wt.%) was investigated in the temperature interval 295–870°K by isothermal tests technique and by microscopy techniques. The major part of alumina particles, having a form of flakes, was situated on grain boundaries, the dislocation density in the grains was very low. The whole temperature interval investigated can be divided into two regions, low temperature Region 1 and high temperature Region 2, separated from each other by a transition region. In both Region 1 and Region 2 the temperature and stress dependence of steady state creep rate ϵg3 s, can be described by the equation ϵ ̇ g3 s = A(T) exp[B(T,f)σ)] , where A = A 0 exp [ −H sd RT ] , A 0 being a function of structure only, H sd meaning the activation enthalpy of lattice self-diffusion in aluminum, f the volume fraction of alumina and σ the applied tensile stress. The dependence of the parameter B on f in the range of f investigated can be approximately described by the equation B( T, f) = B 0.( T) f − m where B 0 depends on temperature only and m is equal to about 1 2 In Region 1 (∼295 to ∼500°K) the parameter B decreases with increasing temperature, the apparent activation energy Q c = ∂ In ϵ ̇ g3 s/∂( −1 RT ) is temperature independent but it decreases l increasing stress. Lattice self-diffusion probably controls creep in this region, however, the rate controlling dislocation mechanism has not been identified. In Region 2 (above ∼620°K) B increases with temperature. The apparent activation energy Q c increases with increasing temperature and increasing stress and achieves values up to about 250 kcal mol −1. Creep is probably controlled by lattice self-diffusion in aluminum in Region 2, and, as it is not connected with any observable change in dislocation substructure, grain boundary slidings were suggested to be the main deformation mechanism of creep in this region. A model was proposed explaining the strong creep rate dependence on the tensile stress axis orientation with respect to the rolling direction.