PLASTIC DEFORMATION MECHANISM IN ROLLED AZ31 MAGNESIUM ALLOY AT ROOM TEMPERATURE

Yuzhi Zhu

Magnesium, as the lightest structural metal, is attracting researchers’ attention in recent years. However, its further application is limited by some shortcomings, like mechanical anisotropy and poor formability, which is ascribed to the low-symmetric HCP structure.The main objective of this dissertation is to unveil the deformation mechanisms of dislocation slip and deformation twinning in rolled AZ31 magnesium alloy during plastic deformation at ambient temperature. Characterization studies by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), and theoretical calculations with MATLAB are used to analyze the deformation behaviors. \nQuasi in-situ EBSD analysis shows that under an unfavorable loading condition for twinning activation, \\left\\{10\\bar{1}2\\right\\} tensile twins still can be activated but most of them can merely further thicken. While under repeated tensile and compressive strains, the twinning/detwinning process occurs with the sequence of nucleation, propagation, thickening, thinning, shortening, and vanishing. HRTEM analysis shows an intense atomic misalignment near \\left\\{10\\bar{1}2\\right\\} tensile twin boundaries, especially near the twin tip, ascribed to the mobility of twinning dislocations. TEM analysis also shows that <a> type basal slips are activated at the early deformation stage. <c+a> type pyramidal dislocations are not observed at low deformation strain but proved to be activated at the late deformation stage. During the whole deformation process, no <a> type prismatic slip is observed by TEM. The Schmid factor analysis explains the phenomena. GND calculation gives us a possibility to evaluate the slip behavior and make it visual in bulk samples. The slip activations depend on both the grain orientations and the strain conditions in the adjunct grains. The increased GND density is accompanied by an increased strain. <a> type basal slips and <c+a> type pyramidal slips dominate during the deformation process. <a> type prismatic GND density only increases at the late deformation stage. Furthermore, prismatic GNDs prefer to accumulate near grain boundaries and twin boundaries. IGMA analysis explains the slip behaviors in the matrix region and the twinning area. The activation of <a> type prismatic slips inside \\left\\{10\\bar{1}2\\right\\} tensile twins is easier than that in the matrix region.

PLASTIC DEFORMATION MECHANISM IN ROLLED AZ31 MAGNESIUM ALLOY AT ROOM TEMPERATURE

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