Modeling the effect of twinning and detwinning during strain-path changes of magnesium alloy AZ31 (original) (raw)
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Dislocation transmutation by tension twinning in magnesium alloy AZ31
International Journal of Plasticity, 2016
The interaction between dislocations and twins appears to play an important role in the strain hardening behavior of Mg. Detailed transmission electron microscopy study was performed to investigate the concept of dislocation "transmutation" across twin boundaries. A previously proposed dislocation transmutation reaction is confirmed. For twins, the transmutation reactions involve or matrix dislocations resulting in dislocations, which populate the vicinity of the twin boundary. No other slip systems are observed within the twins, despite the fact that the dislocations have similar or lower resolved shear stress, as compared to other slip systems. This suggests the slip mode is source limited, since the observed slip systems are the only ones that can result from transmutation. The formation of a unit dislocation in the twin is proposed to involve two consecutive reactions, necessitating dislocation pileup in the matrix, and the associated dislocation configurations are evaluated in terms of elastic strain energy considerations. Dislocation reactions are proposed which could explain the presence of basal stacking faults of either or type inside twin. At the low stress levels typical of twinning dominated flow of textured polycrystals, the observed dislocations are most likely sessile. However, this could serve as a source mechanism for later deformation, and as a forest hardening mechanism against other slip systems. The interfacial reaction products could result in a dragging effect on twin boundary advancement, and establish a basis for subsequent rapid hardening.
2011
When a strongly textured hexagonal close packed (HCP) metal is loaded under an orientation causing profuse twinning or detwinning, the stress-strain curve is sigmoidal in shape and inflects at some threshold. Authors have largely attributed the dramatic stress increase in the lower-bound vicinity of the inflection point to a combined effect of a Hall-Petch mechanism correlated to grain refinement by twinning, and twinning-induced reorientation requiring activation of hard slip modes. We experimentally and numerically demonstrate that these two mechanisms alone are unable to reproduce the stress-strain behaviors obtained under intermediate loading orientations correlated to in-between profuse twinning and nominal twinning. We argue based on adopting various mechanistic approaches in hardening model correlations from the literature. We used both a physics dislocation based model and a phenomenological Voce hardening model. The HCP material is exemplified by an extruded AM30 magnesium alloy with a 1010 -fiber parallel to the extrusion direction.
Acta Materialia, 2023
Twin, dislocation, and grain boundary interaction in hexagonal materials, such as Mg, Ti, and Zr, has critical influence on the materials' mechanical properties. The development of a microstructure-sensitive constitutive model for these deformation mechanisms is the key to the design of high-strength and ductile alloys. In this work, we have developed a mechanical formulation within the finite strain framework for modeling dislocation slip-and deformation twinning-induced plasticity. A dislocation density-based crystal plasticity model was employed to describe the dislocation activities, and the stress and strain distributions. The model was coupled with a multi-phase-field model to predict twin formation and twin-twin interactions. The coupled model was then employed to study twin, dislocation, and grain boundary interactions in Mg single-and polycrystals during monotonic and cyclic deformation. The results show that twin-twin interactions can enhance the strength by impeding twin propagation and growth. The role of dislocation accommodation on twin-twin interactions was twofold. Dislocation slip diminished twin-twin hardening by relieving the development of back-stresses, while it effectively relaxed the stress concentration near twin-twin intersections and thus may alleviate crack nucleation. The plastic anisotropy in each grain and the constraints imposed by the local boundary conditions resulted in stress variations among grains. This stress heterogeneity was responsible for the observed anomalous twinning behaviour. That is, low Schmid factor twins were activated to relax local stresses and accommodate the strain incompatibility, whereas the absence of high Schmid factor twins was associated with slip band-induced stress relaxation.
International Journal of Plasticity, 2017
In this work, we employ the recently developed framework for the explicit modeling of discrete twin lamellae within a three-dimensional (3D) crystal plasticity finite element (CPFE) model to examine the effects of dislocation densities in the twin domain on twin thickening. Simulations are carried out for 1 012 〈101 1〉 extension twins in a magnesium AZ31 alloy. The model for the twin lamellae accounts for the crystallographic twin-matrix orientation relationship and characteristic twin shear transformation strain. The calculations for the mechanical fields as a result of twinning consider that one of three types of twin-dislocation density interactions have occurred. One case assumes that the expanding twin retains in its domain the same dislocation density as the parent. The second one considers that twin expansion has lowered the dislocation density as the twin thickens, and the last one, the Basinski effect, assumes that when twin sweeps the region, the dislocation density incorporated in the twin domain is amplified. In the modeling approach, the twin is thickened according to a criterion that maintains the stress state in the vicinity of the grain at a pre-defined characteristic twin resistance. The calculations show that most of the averaged properties, such as the rate of dislocation storage in the entire twin grain, the twin growth rate, the stress field in the twinned grain and neighboring grains, and the slip activity in the parent matrix are not significantly altered by dislocation storage in the twin. The results indicate that, however, the slip activity in the twinned domain is affected. In particular, in the increased dislocation density case, the rate of dislocation density in the twin domain increases at low strains when the twin is first growing from 2% to 5% volume fraction. This initial boost in the dislocation density storage rate causes the newly expanded dislocation twin to contain more stored dislocations than the other cases for all strain levels. Another interesting difference concerns the preference for one or two twins for the same total twin volume fraction; for the increased dislocation twin or twin that retains the dislocation density as it grows, formation of two twins is favored. For a twin that removes dislocation density, only one twin is preferred. The results imply that in the case with reduced dislocation density leads to lower stored dislocations and dislocation storage rates, and lower pyramidal slip activity.
In this work, the viscoplastic self-consistent based All Twin Variant (ATV) polycrystal modelling was employed to decipher the deformation behaviour of Mg-3Al-0.3Mn Magnesium alloy that develops f1012g1011-extension twins profoundly during ambient temperature compression. Twinning was considered by taking into account all the potential f1012g twin variants, and hence called here as the 'ATV' approach. The model treats each twin variant as a grain with increasing volume fraction transferred from the respective parent grain according to its pseudo-slip shear-rate. The slip and twin-induced strain hardening were simulated by adopting a classical phenomenological hardening model while assigning a higher hardening coefficient for the twins relative to the parent matrix. The viscoplastic self-consistent polycrystal homogenisation scheme combined with the ATV approach permitted to reproduce with high precision the experimentally measured strain hardening behaviour, crystallographic texture and twin volume fraction evolution. Beyond these average measures, the activities of twin variants in individual grains could be predicted in good agreement with Electron Back-Scattered Diffraction measurements. The ATV approach permits also to examine the matrix and twin phases separately in terms of textures and misorientation distributions.
Internal strain and texture evolution during deformation twinning in magnesium
Materials Science and Engineering: A, 2005
The development of a twinned microstructure in hexagonal close-packed rolled magnesium compressed in the in-plane direction has been monitored in situ with neutron diffraction. The continuous conversion of the parent to daughter microstructure is tracked through the variation of diffraction peak intensities corresponding to each. Approximately 80% of the parent microstructure twins by 8% compression. Elastic lattice strain measurements indicate that the stress in the newly formed twins (daughters) is relaxed relative to the stress field in the surrounding matrix. However, since the daughters are in a plastically "hard" deformation orientation, they quickly accumulate elastic strain as surrounding grains deform plastically. Polycrystal modeling of the deformation process provides insight about the crystallographic deformation mechanism involved.
International Journal of Plasticity, 2019
This work adapts a recently developed multi-level constitutive model for polycrystalline metals that deform by a combination of elasticity, crystallographic slip, and deformation twinning to interpret the deformation behavior of alloy WE43 as a function of strain rate. The model involves a two-level homogenization scheme. First, to relate the grain level to the level of a polycrystalline aggregate, a Taylor-type model is used. Second, to relate the aggregate level response at each finite element (FE) integration point to the macro-level, an implicit FE approach is employed. The model features a dislocation-based hardening law governing the activation stress at the slip and twin system level, taking into account the effects of temperature and strain rate through thermally-activated recovery, dislocation debris formation, and slip-twin interactions. The twinning model employs a composite grain approach for multiple twin variants and considers double twinning. The alloy is tested in simple compression and tension at a quasistatic deformation rate and in compression under high strain rates at room temperature. Microstructure evolution of the alloy is characterized using electron backscattered diffraction and neutron diffraction. Taking the measured initial texture as inputs, it is shown that the model successfully captures mechanical responses, twinning, and texture evolution using a single set of hardening parameters, which are associated with the thermally activated rate law for dislocation density across strain rates. The model internally adjusts relative amounts of active deformation modes based on evolution of slip and twin resistances during the imposed loadings to predict the deformation characteristics. We observe that WE43 exhibits much higher strain-hardening rates under high strain rate deformation than under quasi-static deformation. The observation is rationalized as primarily originating from the pronounced activation of twins and especially contraction and double twins during high strain rate deformation. These twins are effective in strain hardening of the alloy through the texture and barrier hardening effects.
Rate-dependent hardening due to twinning in an ultrafine-grained magnesium alloy
Acta Materialia, 2012
An ultrafine-grained (UFG) ZK60 Mg alloy with an average grain size of 1.0lmwasprocessedbyextrusionatrelativelylowtemperature(488K)withahighareareductionratio(1.0 lm was processed by extrusion at relatively low temperature (488 K) with a high area reduction ratio (1.0lmwasprocessedbyextrusionatrelativelylowtemperature(488K)withahighareareductionratio(25). The mechanical behavior of the UFG Mg alloy is investigated over strain rates spanning nearly eight decades (10 À4 -10 4 s À1 ). The stress-strain responses in the quasi-static ($10 À4 s À1 ) and high rate (10 4 s À1 ) regimes exhibit the characteristic sigmoidal profile that is a signature of f1 0 1 2gh1 0 1 1i extension twinning. Further, this sigmoidal profile is accentuated at high rates, suggesting a rate effect of twinning induced hardening. X-ray diffraction (XRD) and analysis of the as-received and deformed microstructures indicate the occurrence of twinning even at the quasi-static rates of loading. This observation is contrary to some of the theoretical predictions that suggest suppression of twinning in Mg below critical grain sizes much larger than in the present work. From the XRD analysis we infer that the twin volume fraction increases with increasing applied strain rate. Transmission electron microscopy observations of the tested specimens reveal high density non-basal dislocations that may result from the activation of these slip systems following twinning-induced lattice reorientation.
International Journal of Plasticity, 2014
Low thermoplastic formability is a key factor limiting the usage of magnesium alloys, which otherwise can have broad application in automotive industry for their competitive strength to density ratio. Combining with experimental calibration and validation, we report a systematic numerical investigation about the plastic deformation of magnesium alloy AZ31B at different temperatures and subjected to different boundary conditions. By employing 3D Voronoi grains based microstructure and the crystal plasticity constitutive model developed by Staroselsky and Anand (2003), which accounts for both dislocation slip and deformation twinning in polycrystalline magnesium, we estimate the dependence of critical resolved shear stresses (CRSS) of different slip/twinning systems on temperature. We further obtain the fractional plastic strains contributed by individual slip/twinning systems at different loading conditions. Grain level deformation analysis indicates that there exists significant stress and plasticity inhomogeneity among grains.