Plastic Deformation Behavior of Magnesium Single Crystals

Abstract

Magnesium (Mg) alloys, having a density of about 1.74 g/cm3, are the lightest structure materials and their superior physical and mechanical properties make Mg alloys extremely attractive for applications requiring light-weight materials. Especially, in the automotive industry, magnesium alloys have become the key materials for increasing the fuel efficiency due to their low density, excellent specific strength and stiffness, exceptional dimensional stability, high damping capacity, and high recycle ability. Magnesium alloys can be divided into cast magnesium alloys and wrought magnesium ones in terms of difference in processing. At present, the casting magnesium alloys are utilized to produce the majority of magnesium alloy products. However, the cast magnesium alloys fail to meet most of the requirements and the wrought Mg alloys show both better strength and toughness. Unfortunately, there are several fundamental and technical issues preventing the development of high performance of wrought Mg alloys. For example, Mg alloys usually develop strong basal texture during rolling, resulting in anisotropic mechanical properties and poor formability. Additionally, fundamental observations such as temperature and orientation dependency on the slip and twinning modes, as well as interactions between deformation modes are still rarely reported. The first objective of this study is to determine the critical resolved shear stress (CRSS) values for various major slip and twin systems, by preparing single crystal specimens with different orientations and deformed at various temperatures. The deformed samples were systematically examined by optical microscopy and electron microscopy. The single slip oriented specimens were used to directly derive the CRSS values for the corresponding deformation modes by using Schmid factor (SF) criterion. Viscoplastic self-consistent (VPSC) simulations were utilized to obtain the best fitted CRSS and hardening parameters for various deformation modes when more than one deformation mode was involved due to the loading direction of the single crystal specimens. From experimental and simulated results, it was found that the CRSS for basal <a> slip and {10-12} twin showed a weak temperature dependence, whereas the CRSS for prismatic <a>, <c+a> slip and {10-11} twin exhibited a strong temperature dependence. The resolved shear stresses that activate the prismatic <a>, <c+a> slip and {10-11} twin modes are much greater than those required to initiate the basal <a> slip and {10-12} twin in magnesium at room temperature. Therefore, the predominant deformation modes of magnesium at room temperature are the basal slip and {10-12} twin. It was generally known that the {10-12} twin strongly affects plastic deformation of the Mg alloys since twining can accommodate the c-axis strain, but large strains require additional deformation by slip. The interactions between dislocations and twin boundaries are, therefore matters of practical interest because the deformation-induced twins may play as barriers to further slip, the source of the slip, and twin dislocations at the twin boundaries. The second objective of this study is investigate systematically the effects of the {10-12} twin on the deformation and the recrystallization behavior of magnesium single crystals. To date, there are so many works that have been carried out to modify the strong basal texture, which means spreading the basal planes from the sheets normal direction. However, there is still a lack of understanding of what kind of textures are desired textures because systematic research regarding the effects of the initial orientation on the mechanical behaviors has been rarely reported. The third objective of this study is to characterize the initial orientation effects on the deformation behavior of magnesium single crystals. The results show relative activity of slip and twinning modes changed dramatically due to different orientations. The <c+a> slip plays a dominant roles in the 0° rotated specimen. Basal slip is a main deformation mode in the range of 10 to 60° and the {10-12} twin mode dominates from 70 to 90° in the rotated samples from the C-axis. The second objective of this study is to examine the limited interactions between the basal slip and {10-12} twin. In this study, the effects of {10-12} twin on various slip and twinning behavior were observed in a systematic manner. The Mg single crystals with various crystallographic orientations from [0001] and [10-10] were prepared for two-step compression (TSC) tests. It was found that {10-12} twin has limited hardening effects on yield strength for the 0° rotated specimen owing to a high CRSS value for the <c+a> slip. In case of 10 ~ 60° specimens, the incorporation of basal dislocation into a {10-12} twin reveals that basal dislocations in the matrix cross-slip onto the basal plane in the twinned crystal. The percentage increase of yield strength for the 70 ~ 90° rotated specimens is relatively higher than other specimens because the Hall-Petch slop for the twinning-dominated flow is frequently greater than that for the slip-dominated flow.