Mechanism-based understanding of the plastic deformation of a magnesium alloy under multiaxial loading

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Johns Hopkins University
Low density and high specific strength make magnesium (Mg) an ideal candidate as a structural metal for automobile and aerospace applications, where light weight and energy efficiency are emphasized. Structural metals are likely to experience extreme loading such as car crashes, bird strikes, ballistic impacts and high speed machining. It is well known that materials behave differently when undergoing extreme loading in comparison to quasi-static loading. However, polycrystalline magnesium alloys usually develop strong textures after processing, resulting in anisotropy during plastic deformation. In this thesis, we seek to offer a comprehensive mechanism-based understanding of the rate dependence and anisotropy of a magnesium alloy under multiaxial loading. The mechanical properties of a textured polycrystalline AZ31B magnesium alloy are first investigated over a wide range of strain rates (0.0001~10000 1/s) under uniaxial compression along different loading orientations relative to the material texture. We then enrich our dataset by escalating the strain rate to ~100000 1/s using the pressure-shear plate impact technique. The macroscopic flow stress and work hardening rate exhibit strong rate dependence and anisotropy. The material behaves like a transversely isotropic material with negligible difference in the rolling plane. Stronger rate dependence is observed during plastic flow driven by pyramidal slip and contraction twinning than in deformation mediated by extension twinning. These comprehensive experimental measurements under various loading conditions are supplemented by examinations of the underlying deformation mechanisms. The deformation modes are observed to depend on the loading direction and stress state, and the evolution of the deformation mechanisms is heavily affected by the loading rate. Boundaries between different extension twin variants are found to have considerable contributions to the late stage deformation of the material. Although the volume-average adiabatic heat is negligible compared to the melting temperature of magnesium, localization frequently occurs in compression along the ND, which evolves with the plastic strain as well as the strain rate. This localization is attributed to both geometric softening by double twins and thermal softening. We also interrogate the rate dependence and anisotropy in the failure behaviors of this material. The compressive 'ductility' in the RD/TD is much higher than that in the ND. It is also observed that intermetallic inclusions play an important role in the macroscopic failure. We also develop a mechanism-informed crystal plasticity model for magnesium by introducing more physics about the dislocation driven plasticity. Plasticity at low strain rates is described by dislocation activities through thermal activation mechanism while deformation at high strain rates is characterized by dislocation drag mechanism. Special attention is paid to the plasticity at intermediate strain rates. Different approaches are proposed to bridge the low-rate and high-rate deformation.
Magnesium, Deformation mechanisms, Twinning, Rate sensitivity, Anisotropy, Pressure-shear, Localization, Failure