Numerical Assessment of the Role of Slip and Twinning in Magnesium Alloy AZ31B During Loading Path Reversal
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lloys are the lightest structural materials but have poor formability at room temperature because of insufficient available slip systems. Experimental and modeling efforts have made tremendous progress in understanding the correlation between mechanical behavior and deformation mechanisms.[1–4] Basal hai slip (f0001gh1120i) is the major dislocation-based plastic deformation mechanism at room temperature. Non-basal slip deformation mechanisms, Prismatic hai slip (f1010gh1120i) and Pyramidal hc þ aislip (f1122gh1123i), are relatively difficult to be activated at room temperature because of the high resistance and the low mobility associated with the nonplanar core of the respective dislocations.[5] To accommodate arbitrary deformation under mechanical loading, f1012gh1011i extension twins are commonly activated at room temperature. Differing from slips that take place regardless of the direction of the resolved shear stress, the extension twin can only be activated by an effective tension stress along c-axis. Consequently, HUAMIAO WANG, Postdoctoral Research Associate, is with the Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Contact e-mails: wanghm@ lanl.gov; [email protected] PEIDONG WU, Professor, is with the Department of Mechanical Engineering, McMaster University, Hamilton, ON L8S 4L7, Canada. JIAN WANG, Scientist, is with the Materials Science and Technology Division, Los Alamos National Laboratory, and also Professor with the Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588. Manuscript submitted November 17, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
detwinning occurs in a twin when the load is reversed.[6] Thus, the activity of twinning and detwinning is correlated to loading direction and materials texture. Under complex strain paths, such as cyclic tension– compression, shear, and torsion, twinning/detwinning will induce the characteristic phenomena: unsymmetrical or symmetrical hysteresis loops of stress–strain curves,[1–9] inelasticity during unloading,[1,2,10,11] and the low yield stress upon load reversals,[12] etc. To understand the role of different deformation mechanisms in developing characteristic deformation phenomena with respect to loading, deformation history, and texture, quantitative modeling efforts are essential but lag behind experimental studies because of the lack in the modeling capability of describing both twinning and detwinning. In the early models, slip is considered as the major plastic deformation mechanism.[13] Twinning (detwinning was missed) was considered to be responsible for the reorientation and hardening evolution under monotonic loading conditions.[14–24] Several phenomenological models have been developed to simulate both twinning and detwinning.[25– 28] For example, Li et al.[25] proposed a phenomenological approach to study the mechanical behavior of AZ31B sheet under cyclic loading. However, this approach gives an abrupt transition during strain path change instead
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