Study on damage evolution beneath spherical indenter through experiments and mechanism-based strain gradient crystal plastic simulations

Damage evolution of AA7075-T6 rolling plate with 2 mm thickness under highly compressive stress state, which is generated by spherical indentation, is studied through experiments and simulations. On the cross section of spherical deformation region, micro voids induced by crush of MgZn2 particles are observed by scanning electron microscopy (SEM) and some special characteristics are revealed including ductile tearing of the brittle particle, huddling between neighboring voids and an in-void crack, but no clue of void closure under such heavy compression. Through Electron Back-Scattered Diffraction (EBSD), strain gradient around void was found quite high (70 mm⁻¹) and much higher than the theoretical strain gradient induced by spherical indentation (1.8 mm⁻¹), which will result in a significant hardening effect and preventing the further shrink and closure of micro void. More insights are provided through mechanism-based strain gradient crystal plastic simulations which couples a polycrystalline, full-scale model, and a single crystal submodel. Simulation results verify the elevated strain gradient around microvoid and find strain gradient varies dramatically along the small range of void edge, which is validated by nanoindentation test around the edge of voids. Microvoid evolution controlled by strain gradient shows strong anisotropy on different cross sections. For the void at position D focused in present paper, its area on section perpendicular to Y axis (SecY) increases at the early stage of indentation, despite the compressive stress state. Influence of the orientations of adjacent grains is also studied. Void areas on SecZ and SecY are found more sensitive to the change of orientation of adjacent grain, however, the volume variation show much less sensitivity.