Modeling The Inhomogeneous Deformation And Twinning Behaviors During Bending Of Magnesium Alloy With Crystal Plasticity FEM

Lightweight magnesium(Mg)alloys have great potential for improving energy efficiency across the vehicle and aerospace industries.However,the applications have been limited by the poor formability of Mg alloy due to its inherent hexagonal close packed(hcp)lattice.Therefore,an in-depth understanding of the deformation behavior and mechanisms for Mg alloys at room temperature is of great significance to the development of high-performance Mg alloys.The traditional experimental approaches are often time consuming and costly.Moreover,they often have limitations in understanding the mechanisms of some complex deformation behaviors of Mg alloys.The crystal plasticity finite element method(CPFEM)that incorporates the crystal plasticity theory into the framework of finite element method provides a new research approach for the study of complex deformation behaviors of Mg alloys.A rate-dependent crystal plasticity model considering both twinning and slip was established based on the classical crystal plasticity theory.Combined with commercial finite element package ABAQUS,a full filed CPFEM was developed,which can simulate the complex twinning behaviors and evolution of microstructure and texture of Mg alloys.Based on the independently developed CPFEM,the texture effect on the the shift of neutral layer,twinning and strain localization and twin variants selection during the three point bending deformation of Mg alloys were systematically studied.Then the bending and transverse tensile tests of friction stir welded(FSW)AZ31 Mg alloys with complex micro-texture were simulated by CPFEM.The mechanisms of the un-uniform deformation and fracture behaviors of FSW Mg joint during the two deformation modes were revealed.The specific research conclusions are as follows:(1)The shift of neutral layer strongly depends on the initial texture of bent AZ31 Mg alloys.It located in the outer,middle and inner region for S0,S45 and S90,respectively.This is attributed to the asymmetry of plasticity between the inner and outer regions.In particular,it is correlated to the texture-dependent activity of extension twinning.The shifting behavior of neutral layer is also sensitive to the applied indenter displacement(IDt).This could be related to the texture strengthening as induced by twinning and the competition between twinning and slip systems in the inner and outer regions.CPFEM successfully predicted the high dependence of neutral layer shift on the initial texture and applied IDt during bending of Mg alloys.Additionally,the spatial distribution of neutral layer in bended sample was also simulated via a novel approach based on CPFEM.(2)CPFEM successfully predicted the experimentally observed twin bands during bending of AZ31 Mg alloys with different initial textures,and the localization degree of twin bands is closely related to intensity of the initial texture of the bent sample.The formation of these twin bands in the bending process is the natural outcome for coordinating the plastic deformation along the direction of maximum shear stress.(3)Profuse {1012}-{1012} secondary twins took place in the outer zone of S90 at the later stage of bending test.The formation mechanism of them can be understood as the strain-path change under monotonic loading.The activation of unfavorable extension twins(ETWs)in both the inner zone of S90 and the outer zone of S0 is to accommodate the compressive strain along transverse direction(TD).Therefore,the unusual ETWs in both cases can be characterized as geometrically necessary twins.The difference in twinning plane trace between the inner and outer regions of S45 can be ascribed to the accommodation of different external bending strains and twin variant selection.Combining the generalized Schmid‘s law and CPFEM,the twin variant selection is accurately predicted.(4)The observed twinning behavior,texture evolution,?concave-convex?appearance and crack nucleation sites in FSW Mg alloys during bending can be successfully predicted by CPFEM.The strong localized texture in conjunction with the different deformation modes in each sub-region of the FSW AZ31 Mg alloy causes severe strain heterogeneity in welding direction(WD),which results in the?concave-convex? appearance observed on the longitudinal section.The huge differences in the activities of slip and twinning modes in each sub-region cause plastic deformation incompatibility,which initiates fracture in the area with maximum strain in the FSW Mg joint.(5)The observed non-uniform deformations in width and thickness direction of FSW Mg joints were successfully simulated by CPFEM.Especially,the ?embossed?appearance in SZ-center was predicted without considering the distribution of the sheared layers.The mirror-symmetrical distribution of basal planes in EABS regions determined the ?concave-convex? appearance via activating profuse basal slip during the tensile test of FSW Mg joint.Varying the texture distribution in stir zone(SZ)largely affected the asymmetrical feature of plastic deformation.With the c-axis rotated away from the WD-TD plane,the ?embossed? feature became more obvious in SZ-center first,and then gradually weakened.