Damage accumulation and mechanical properties of particle-reinforced metal–matrix composites during hydrostatic extrusion

The effects of hydrostatic extrusion on particle cracking and on the subsequent tensile properties of some prototypical particle-reinforced metal–matrix composites are investigated. In most cases, tensile failure occurs through a plastic instability in accordance with the Considere criterion for necking. The corresponding failure strain is therefore dictated by the global flow and hardening characteristics of the composites, as influenced by the intrinsic flow properties of the matrix as well as the extent and rate of particle cracking. Such cracking leads to significant reductions in the hardening rate and thus causes a reduction in the failure strain relative to that of the neat matrix alloy. Extrusion prior to tensile testing has the effect of saturating the flow stress of the matrix and limiting the tensile ductility to low values, largely because of the very low hardening rate of the matrix. Particle cracking during extrusion causes a further reduction in ductility. The dominant role of the matrix hardening is demonstrated through re-tempering treatments of extruded billets prior to tensile testing. A micromechanical model of particle cracking is developed, taking into account the effects of both the hydrostatic and the deviatoric stress components in axisymmetric loadings. The model is used to rationalize the observed trends in damage accumulation with particle content, particle type, and loading configuration (tension vs. extrusion).