A Quantitative Three-Dimensional Investigation of Cyclic Deformation Mechanisms in Mg Alloys

Abstract

Mg alloys are promising materials for lightweighting due to their low density and high strength-to-weight ratio. Increased integration of Mg in the transportation industries can provide a pathway for improvements in fuel economy and emission reductions, however accelerated use of Mg in structural parts requires a more detailed understanding of deformation behavior during the cyclic loading conditions experienced in-service. In this dissertation, the role of alloying and heterogeneities in three-dimensional microstructures and local micromechanical states on cyclic deformation in Mg was investigated. The cyclic evolution of grain-scale twinning and detwinning and the impact of microstructural features on short-crack growth were characterized using high energy X-ray diffraction microscopy (HEDM). Experimental reconstructions of grain-scale mechanical response were further combined with crystal plasticity finite element (CPFE) simulations to characterize the impact of alloying on slip and twin activity. First, the evolution of grain-scale twinning and detwinning during uniaxial compression-tension displacement was characterized in a binary Mg-Al alloy using combined far-field HEDM and near-field HEDM. An automated technique for quantitative reconstruction of grain-scale twin nucleation, twin growth, and detwinning was developed, and the role of grain and neighborhood orientation, stress states, and grain size on the cyclic evolution of twinning and detwinning and twin variant selection was analyzed. Variability in twin nucleation and detwin stress states were correlated to heterogeneities in the orientation, resolved stress, and geometric compatibility of grain neighborhoods, and twinning and detwinning were linked to differences in the cyclic evolution of slip system strength. Twin nucleation and growth induced stress relaxation and the evolution of stress in twinned and parent lattices were further characterized during cyclic displacement. The grain-scale twin reconstruction methods were also applied to a binary Mg-Nd alloy to investigate the impact of rare-earth alloying on mechanical response during fully-reversed tension-compression cyclic straining. Rare-earth alloying induced a basal texture that was rotated and weak in the extruded bar starting material. This texture resulted in modest twinning during both tensile and compressive loading parallel to the extrusion axis and favorable grain alignment for basal slip. The impact of Nd alloying on the critical resolved shear stresses for the operative slip modes and twinning was characterized by combined HEDM experimental reconstructions and CPFE simulations. Finally, the three-dimensional crystallography of fatigue short-crack paths in Mg alloy WE43 was characterized using combined post-mortem X-ray computed tomography (CT) and near-field HEDM. The reconstructed three-dimensional grain structure in the short-crack growth regions of cylindrical and thin-foil fatigue specimens were aligned and mapped to high resolution CT reconstructions of the fracture surface. The intragranular and intergranular nature of the crack paths were characterized by mapping top-to-bottom fracture lattice misorientations across the crack path, and crack growth behavior at crack-grain-boundary intersections was correlated to crystallographic and geometric parameters of the three-dimensional grain boundary plane, trailing and leading grains, and advancing short-crack front. The insights from this dissertation demonstrate the applicability of three-dimensional microstructural characterization and micromechanical modeling for investigating grain-scale cyclic deformation behavior in Mg. This framework can be further combined with experimental and simulation tools across broader length scales to accelerate the development and adoption of lightweight structural metals through an integrated computation materials engineering approach.