Effect of the loading mode on the evolution of the deformation mechanisms in randomly textured magnesium polycrystals – Comparison of experimental and modeling results (original) (raw)
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Acta Physica Polonica A, 2015
The evolution of the dislocation structure in randomly textured cast magnesium as a function of loading mode is studied using whole neutron diraction pattern line prole analysis and elasto-plastic self-consistent modeling. Both the experimental results and the theoretical data indicate the onset of basal slip at low stresses and the key role of prismatic slip in the macroscopic yield. Dependence of the second-order pyramidal slip on the loading mode is revealed.
In-Situ Study of the Influence of the Strain Path Change on the Deformation Processes in Magnesium
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The twinning activity and internal stress evolution in random textured polycrystalline magnesium were studied as a function of strain path changes using in-situ neutron diffraction method. The pre-deformation to different stresses in compression and tension, respectively was followed by application of load with opposite sign. The results indicates that the deformation behavior significantly depends on the level of the predeformation.
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.
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.
Advances in Materials Science and Engineering, 2018
The deformation behavior and texture evolution of pure magnesium were investigated during plane strain compression, simple compression, and uniaxial tension at room temperature. The distinctive stages in the measured anisotropic stress-strain responses and numerically computed strain-hardening rates were correlated with texture and deformation mechanisms. More specifically, in plane strain compression and simple compression, the onset of tensile twins and the accompanying texture-hardening effect were associated with the initial high strain-hardening rates observed in specimens loaded in directions perpendicular to the crystallographic c-axis in most of the grains. The subsequent drop in strain-hardening rates in these samples was correlated with the exhaustion of tensile twins and the activation of pyramidal slip systems. The falling strain-hardening rates were observed in simple compression and plane strain compression with loading directions parallel to the c-axis where the secon...
Strain hardening due to {10 1̄ 2} twinning in pure magnesium
2008
The effect of grain segmentation by f10 " 1 2g mechanical twinning on the strain-hardening behaviour of textured and random polycrystals is assessed using the Kocks-Mecking method of analysis. Profuse twinning in the first 6-8% strain in textured polycrystals has relatively small strengthening effects despite the large volume fraction of grains undergoing twinning. This is due to the small value of the Hall-Petch constant in textured polycrystals. For random polycrystals, the Hall-Petch constant is much larger but the overall hardening effect is reduced due to the small volume fraction of grains undergoing twinning. Additional hardening effects due to the twinning crystallographic transformation on dislocation mobility are deemed small in cast polycrystals due to their low dislocation density, but may be more important in textured polycrystals with higher dislocation densities. Grain size-independent storage of dislocations accounts for the strain hardening at large strains.
Investigation of deformation dynamics in a wrought magnesium alloy
International Journal of Plasticity, 2014
In the present research, the real-time in-situ neutron diffraction measurements under a continuous-loading condition and elastic-viscoplastic self-consistent (EVPSC) polycrystal modeling were employed to study the deformation dynamics and the effect of the deformation history on plastic deformation in a wrought magnesium alloy. The experimental results reveal that pre-deformation delays the activation of the tensile twinning during the subsequent compression, mainly resulting from the residual strains. Detwinning does not occur until the applied stress exceeds the tensile yield strength during the reverse loading. It is believed that the grain rotation plays an important role in the elastic region during the reverse loading. The EVPSC model, which has been recently updated by implementing the twinning and detwinning model, was employed to characterize the deformation mechanism during the strain-path changes. The simulation result predicts well the experimental observation from the real-time in-situ neutron diffraction measurements. The present study provides a new insight of the nature of deformation mechanisms in a hexagonal close-packed (HCP) structured polycrystalline wrought magnesium alloy, which has significant implications for future work on studying the deformation mechanisms of HCP-structured materials.
Texture effects on plastic deformation of magnesium
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2005
Plastic deformation of hexagonal metals such as magnesium is strongly affected by the hexagonal crystal structure, and thus, for an understanding of the deformation behaviour of magnesium polycrystals, texture must be taken into account. Plane-strain compression tests on pure magnesium as well as on magnesium alloy AZ31 were carried out at 100 and 200 • C. A special emphasis was laid upon the influence of different starting textures on the deformation characteristics and the development of texture and microstructure. The measured flow curves and the microstructure investigation reveal that plastic deformation of magnesium at these temperatures is generally inhomogeneous and dominated by the appearance of shear bands. However, if the initial texture is chosen such that the formation of a basal texture is slowed down or even suppressed, substantial ductility can be achieved at temperatures as low as 100 • C. The texture development due to crystallographic slip can be reasonably modelled by a relaxed constraints Taylor simulation and yields information on the activated slip systems.
On the strain hardening behaviour of magnesium at room temperature
Materials Science and Engineering: A, 2007
The strain hardening behaviour of randomly oriented polycrystals deformed in tension and compression is analysed using the Kocks-Mecking phenomenological approach. At low stresses, an extended regime of linear hardening consistent with an athermal forest hardening mechanism, akin to that observed in face-centered cubic metals, is observed. The increase in yield strength with decreasing grain size can also be accounted for by relating the mean free path of dislocations to the grain size. Profuse twinning in compression seems to have little or no effects on the overall strain hardening behaviour.