Understanding Structure-Property Linkages in Magnesium Alloys via Size-Dependent Crystal Plasticity Modeling

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Metallic alloys with a low symmetry hexagonal close packed (HCP) crystal structure, such as pure magnesium (Mg) and its alloys, are widely used in various industrial applications such as defense, nuclear, automotive, and aerospace mainly due to their high strength-to-weight ratio. In these materials, texture and grain size (their microstructure) play important roles in determining their strengthening, hardening, plastic anisotropy, tension-compression asymmetry and damage (i.e., their properties) . While microstructure-property linkages are better understood in more common engineering metals (e.g., aluminum), it is not well characterized for Mg alloys. This research employs a size-dependent single crystal plasticity finite element modeling (CPFEM) in a finite deformation setting to model the macroscopic behaviors of polycrystalline Mg and its alloys. There are four focal aspects of this thesis: (1) Extracting the role of grain size-texture interaction on the uniaxial tensile and compressive macroscopic material responses under multiple loading orientations via high-resolution polycrystal modeling, (2) Projecting the effect of the macroscopic plastic anisotropy obtained from these uniaxial response on ductile failure via micromechanical theory of damage, (3) Elucidating the combined effect of grain size and texture in boundary value problems involving multiaxial tensile stress states, and (4) Understanding the micromechanics of texture-dependent ductile damage under multiaxial tensile stress states via high-resolution unit cell models of voided polycrystals. The main observations from each of these investigations provide an understanding of the emergent responses for a prescribed set of constitutive behaviors. The resulting rich data sets can be employed in developing improved homogenization-based continuum plasticity models of strengthening and failure.

Mg alloys, Microstructure-property relations, Grain-size effect, Crystal Plasticity, Damage micromechanics