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AZ31 마그네슘 합금 압연재의 변형거동 및 저주기 피로 특성

AZ31 마그네슘 합금 압연재의 변형거동 및 저주기 피로 특성
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Magnesium (Mg) alloys have recently attracted a great deal of research interest because of their potential applications for lightweight structural components. However, their applications are restricted because they exhibit a high directional anisotropy and are hard to deform at room temperature due to their limited number of deformation modes. In this regard, deformation twinning plays an important role in the deformation of Mg alloys by helping to satisfy the von Mises criterion, which requires five independent deformation systems for homogeneous deformation. Although several types of deformation twins have been reported for Mg alloys, the {10-12} twin is recognized to occur most easily and frequently. The {10-12} twinning can be active under two loading conditions (i.e., two activation modes)
compression perpendicular to the c-axis of the hexagonal close-packed (HCP) lattice or tension parallel to the c-axis. The role of {10-12} twinning in the deformation can be understood by considering following three factors: (1) accommodation of plastic deformation, causing a low flow stress and strain hardening rate
(2) Hall-Petch hardening by twinning-induced grain size change
(3) twin texture induced change in activities of slips. Furthermore, such effects of {10-12} twinning on the deformation behavior would influence the fatigue properties of the material, which are considered a key factor for securing the reliability of products. In this study, the differences in {10-12} twinning characteristics between two activation modes (compression perpendicular to the c-axis and tension parallel to the c-axis) and their effects on the deformation characteristics (flow stress and strain hardening) were investigated using a rolled AZ31 Mg alloy. Moreover, the effects of material variables, such as stress/strain ratio, loading direction, and initial {10-12} twin, on the low-cycle fatigue behavior were also investigated. The results revealed that the active twin variants during {10-12} twinning are dependent on the activation mode and their section mechanism is governed by the Schmid law
although the nucleation of {10-12} twins was governed by the Schmid law, their growth characteristics during further deformation were significantly dependent on the activation mode, causing a totally different twinning contribution to the deformation. The intersection between different twin variant pairs was found to retard the twin growth and promote the nucleation of new twins. The activation of specific twin variants depending on the activation mode induced a significant difference in twinning characteristics, such as twin morphology, volume fraction of twins with strain, and twin texture, and consequently gave rise to a totally different effect on the deformation. The differences in the deformation characteristics (flow stress and strain hardening) between both activation modes were successfully explained in relation with activation stresses for twinning and slips, activities of twinning and slips in the deformation, Hall-Petch effect by twinning-induced grain size change, and twinning-induced change in activities of slips. Fatigue characteristics were investigated by performing the low-cycle fatigue (LCF) tests under various stress/strain conditions along the rolling direction (RD). The alloy was found to have a strong basal texture so that the fatigue deformation was dominated by the alternation of {10-12} twinning and detwinning during each cycle, and this made the cyclic stress response unstable and introduced a mean stress and/or strain depending on the loading condition. The effect of loading direction on the fatigue resistance was also examined by conducting the LCF tests along the normal direction (ND) to the rolling plane and comparing the results with those of the RD. Because of the intense basal texture, which facilitates the generation of {10-12} twinning under two loading conditions mentioned above, the material yielded easily in compression along the RD, while it was easy to yield in tension along the ND. Such a plastic anisotropy caused a superior fatigue resistance along the ND by introducing a beneficial compressive mean stress. In addition, the improvement of the fatigue resistance was achieved by reducing the tensile mean stress developed during fatigue deformation, which was available by tailoring the {10-12} twinning-detwinning characteristics of the material through the pre-compression process. The modification of the {10-12} twinning-detwinning characteristics made it possible to control the plastic deformation mechanisms activated during fatigue deformation so that the imposed tensile strain could be fully accommodated by the detwinning alone and this led to a significant reduction of tensile flow stress, finally resulting in the reduction of mean stress.
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