Babak Kondori

Effect of Stress Triaxiality on the Flow and Fracture of Mg Alloy AZ31

The microscopic damage mechanisms operating in a hot-rolled magnesium alloy AZ31B are investigated under both uniaxial and controlled triaxial loadings. Their connection to macroscopic fracture strains and fracture mode (normal vs shear) is elucidated using postmortem fractography, interrupted tests, and microscopic analysis. The fracture locus (strain-to-failure vs stress triaxiality) exhibits a maximum at moderate triaxiality, and the strain-to-failure is found to be greater in notched specimens than in initially smooth ones. A transition from twinning-induced fracture under uniaxial loading to microvoid coalescence fracture under triaxial loading is evidenced. It is argued that this transition accounts in part for the observed greater ductility in notched bars. The evolution of plastic anisotropy with stress triaxiality is also investigated. It is inferred that anisotropic plasticity at a macroscopic scale suffices to account for the observed transition in the fracture mode from flat (triaxial loading) to shear-like (uniaxial loading). Damage is found to initiate at second-phase particles and deformation twins. Fracture surfaces of broken specimens exhibit granular morphology, coarse splits, twin-sized crack traces, as well as shallow and deep dimples, in proportions that depend on the overall stress triaxiality and fracture mode. An important finding is that AZ31B has a greater tolerance to ductile damage accumulation than has been believed thus far, based on the fracture behavior in uniaxial specimens. Another finding, common to both tension and compression, is the increase in volumetric strain, the microscopic origins of which remain to be elucidated.

Ductility Enhancement in Mg Alloys by Anisotropy Engineering

A mean-field theory suggests that certain forms of plastic anisotropy hinder ductile damage accumulation. Here, a proof-of-concept is presented in the case of Mg–Al–Zn alloys. Textures produced by severe plastic deformation are compared with the as-received rolling texture in terms of their anisotropy-ductility correlations at ambient temperature. The 3D plastic anisotropy is characterized in each material using compression specimens. The ductility is characterized using tensile bars. A micromechanical model is introduced to rationalize the trends in terms of the anisotropy effect on ductility (AED) index. Here, this index is tuned via texture manipulations at fixed chemical composition and grain size. The main finding suggests that plastic anisotropy can be engineered to aid ductility.