Microstructural Response of Magnesium Alloys: 3D Crystal Plasticity and Experimental Validation

Abstract

Lightweight materials such as Aluminum are prevalent in aerospace and automotive vehicles, but the use of lighter Magnesium alloys will significantly increase fuel efficiency and cut emissions. Magnesium alloys present a wide array of unsolved scientific challenges, such as the deformation response of the slip and twin systems and the influence of dislocation interactions and twinning on tensile and fatigue behavior. In this thesis, a parallel three-dimensional(3D) crystal plasticity finite element open-source code was developed based on the deal.II finite element framework as part of PRISMS-Plasticity. Rate-independent crystal plasticity was implemented by developing a nonlinear algorithm which enables all the slip systems to lie on or inside the yield surface, and a consistent tangent modulus ensures convergence for small loading increments. A twin activation mechanism was incorporated into the framework based on a quadrature point sensitive scheme. Furthermore, by bounding the L2-norm of the plastic-slip, load-step adaptivity is enabled. The code demonstrates parallel performance and scaling on large-scale problems running on hundreds of processors. Using experimental microstructure images as input, the code has been used to compute, validate and investigate response of crystalline aggregates to mechanical loading; this leads to insights on slip and twin activity. Boundary value problems were set up to compare the displacement and strain fields obtained by Scanning Electron Microscope - surface Digital Image Correlation (DIC) experiments for Magnesium alloy WE-43 T5 and T6 tempers with the crystal plasticity finite element simulations. The results indicate a strong correlation between experiments and crystal plasticity finite element simulations.

For further insight into the material behavior and to interpret the surface observations better, it is important to know the subsurface effects on the surface behavior. 3D reconstruction of microstructures is growing to be a major topic of interest in the field of modeling and simulation for comparison with experiments. An inverse Voronoi problem approach is used to construct an approximate Voronoi representation of the surface microstructure by generating a convexified representation of the microstructure. The output is combined with random sections of Electron backscatter diffraction observations to build a 3D microstructure. Comparisons are made with surface DIC measurements for random samples of 3D microstructures and they indicate the effect of the underlying microstructure on the surface plastic strain. These developed methods will serve as powerful tools in an Integrated Computational Materials Engineering framework towards accelerating alloy development and in better understanding the mechanical behavior of materials.