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Aspects of plastic anisotropy in damage accumulation are considered for a class of hexagonal crystals that deform by combined slip and twinning. Focus is placed on crystallographic aspects that are currently absent from constitutive formulations of ductile damage. To this end, three-dimensional finite-element calculations are carried out using a cubic unit cell containing a single void embedded in a crystal matrix. Plastic flow in the latter is described using crystal plasticity with parameters representative of single crystal pure magnesium. The effect of void oblateness is analyzed in some detail, as voids often form as blunted microcracks in Mg alloys. The analyses reveal two aspects peculiar to twinning-mediated void growth: (1) insensitivity of the effective stress-strain response to void oblateless and (2) a plastic auxetic effect. Both aspects manifest under certain circumstances. Some implications in terms of incorporating the uncovered crystallographic aspects in coupled plasticity-damage formulations of anisotropic materials are discussed.

 

This work investigates the microstructure-property linkages in magnesium (Mg) with an emphasis on understanding interaction effects between the grain size, texture, and loading orientation. A single crys- tal plasticity framework endowed with experimentally informed micro Hall-Petch type relations for the activation thresholds for slip and twinning is adopted to resolve polycrystalline microstructures over a broad texture-grain size space. The macroscopic trends from the simulations corroborate with experi- ments. The synergistic effects of microstructural engineering on the micromechanical characteristics are mapped, which reveal their role in the emergent macroscopic behaviors. The simulations predict reduced extension twinning with grain size refinement even though the micro Hall-Petch coefficient for twinning is smaller than that for the non-basal slip modes. While grain refinement and textural weakening gen- erally reduce the net plastic anisotropy and tension-compression asymmetry, the degree to which these macroscopic behaviors are tempered depends on the loading orientation. The results offer preliminary insight into the roles that texture and grain size may play in the damage behavior of engineered Mg microstructures. 

The remarkable crystallographic plastic anisotropy of magnesium and its alloys reflects in its polycrystal response via texture. While texture-strength linkages have been studied, the role of textural variability on damage remains elusive. The challenge is to obtain relevant metrics that relate the net plastic anisotropy to macroscopic modes of damage. A possible approach is to adopt mechanistic descriptions of the damage. Motivated by the recent experimental and theoretical works in this direction, here we appeal to the Hill yield function to characterize the net plastic anisotropy of polycrystalline magnesium via the Hill plastic anisotropy tensor h. Metrics based on the components of h offer a way to predict damage as a possible damage predictor. Using the results from our recent extensive three-dimensional crystal plasticity simulations for a wide range of textures, we map the net plastic anisotropy onto the coefficients of h, separately for the tensile and compressive responses. Metrics based on these coefficients serve as indicators for the propensity of textured polycrystals to damage by: (i) porosity evolution, or (ii) shear instability. An attempt is made to understand the potential roles textural variability and crystallographic plastic anisotropy play in damage under different loading conditions.