Threshold stress and load partitioning during creep of metal matrix composites

Abstract

The threshold stress, σ o, included in the creep equation to explain the high-temperature behavior of discontinuously reinforced metal matrix composites (MMCs) is criticized on the basis of microstructural considerations and a new creep data analysis. An alternative interpretation, based on changes in the composite matrix microstructure and, in particular, a load transfer mechanism, is proposed. The resulting creep equation is similar to that in which σ o is used: in essence, σ o is simply replaced by the stress carried by the reinforcement (referred to as σ T). New creep data on 6061Al–15 vol.% SiC w composite and the corresponding unreinforced alloy, allowing direct experimental assessment of composite creep strengthening, Δ σ, are analyzed. The linear dependence found of Δ σ with the applied stress, σ, Δ σ( σ), correlates reasonably well with shear-lag and Eshelby model predictions of σ T, transferred during composite creep deformation. The possible occurrence of damage mechanisms and the complexity of modeling these mechanisms to predict the overall composite creep behavior are also discussed.