Asymmetric and symmetric rolling of magnesium: Evolution of microstructure, texture and mechanical properties (original) (raw)
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Acta Materialia, 2009
Asymmetric rolling of commercially pure magnesium was carried out at three different temperatures: room temperature, 200°C and 350°C. Systematic analysis of microstructures, grain size distributions, texture and misorientation distributions were performed using electron backscattered diffraction in a field emission gun scanning electron microscope. The results were compared with conventional (symmetric) rolling carried out under the same conditions of temperature and strain rate. Simulations of deformation texture evolution were performed using the viscoplastic self-consistent polycrystal plasticity model. The main trends of texture evolution are faithfully reproduced by the simulations for the tests at room temperature. The deviations that appear for the textures obtained at high temperature can be explained by the occurrence of dynamic recrystallization. Finally, the mechanisms of texture evolution in magnesium during asymmetric and symmetric rolling are explained with the help of ideal orientations, grain velocity fields and divergence maps displayed in orientation space.
Materials Science and Engineering: A, 2012
The microstructure and texture evolution during hot rolling (350 • C) and subsequent annealing are compared in Mg-2.9Y and Mg-2.9Zn (in wt%) solid solution alloys. It is shown that Y in solid solution, unlike Zn, suppresses dynamic recrystallization. Nevertheless, the two alloys exhibit similar, strong basal textures. Upon isothermal annealing of the rolled material at 400 • C, the texture is weakened only in the Mg-2.9Y alloy. The texture weakening is attributed to static recrystallization in basal parent grains. During rolling of Mg-2.9Y, basal parent grains store a high amount of basal dislocations that are organized in a very fine substructure. Such a highly energetic configuration gives rise to fine recrystallized grains with a wide orientation spread during annealing. On the other hand, off-basal parent grains with c-axes close to the RD-TD plane develop coarser substructure accommodating prismatic slip during rolling. The energy stored in these grains is significantly lower than that in basal ones (∼20 times smaller). Static recrystallization at off-basal parent grains leads to coarse new grains and involves rotations about caxis. The texture weakening induced by static recrystallization in Mg-2.9Y was found to result in higher ductility and lower anisotropy when compared to Mg-2.9Zn.
Materials Science and Engineering: A, 2009
The contribution of twinning to the evolution of texture and microstructure during hot rolling of pure magnesium was investigated using 2-dimensional and 3-dimensional EBSD-based orientation microscopy. A very sharp 0 0 0 1 fibre texture component parallel to the sheet normal direction is observed after hot rolling, independent of the rolling degree. Tensile twins are observed in case that the c-axis of the matrix grains are inclined to the sheet normal direction by an angle larger than 45 •. Compression twins, on the other hand, are found in grains whose c-axis is inclined to the normal direction by less than 30 •. The formation of secondary twins is observed in most compression twins. At the intersection of two secondary twins new grains are formed provided that the matrix and twins have a common crystallographic rotation axis. These new grains have a specific orientation relation to the matrix grains, characterized by a rotation about an axis close to the c-axis. The newly formed grains grow effectively during the subsequent annealing. An estimation of their frequency shows that they may contribute every third to second grain in the recrystallised microstructure. Consequently, these new grains contribute to the persistence of strong basal texture of rolled sheet.
Thermal Response on the Microstructure and Texture of ECAP and Cold-Rolled Pure Magnesium
Metallurgical and Materials Transactions A, 2015
This paper deals with dynamic recrystallization (DRX), static recrystallization, and grain growth phenomena of pure magnesium after equal channel angular pressing (ECAP) by route A and B C at 523 K (250°C) followed by 80 pct cold rolling. The ECAP-deformed and the subsequently rolled samples were annealed at 373 K and 773 K (100°C and 500°C). The associated changes in the microstructure and texture were studied using electron back-scattered diffraction. ECAP produced an average grain size of~12 to 18 lm with B and C 2 fiber textures. Subsequent rolling led to an average grain size~8 to 10 lm with basal texture fiber parallel to ND. There was no noticeable increase in the average grain size on annealing at 373 K (100°C). However, significant increase in the average grain size occurred at 773 K (500°C). The occurrence of different DRX mechanisms was detected: discontinuous dynamic recrystallization was attributed to basal slip activity and continuous dynamic recovery and recrystallization to prismatic/pyramidal slip systems. Only continuous static recrystallization could be observed on annealing.
Microstructural evolution during recrystallization of magnesium alloys
2012
Microstructural evolution during the annealing of cold rolled Mg, Mg-1.5Nd and Mg-3Y sheets has been examined. The experimental results show a significant difference in recrystallization kinetics and grain growth between pure Mg and Mg-RE alloy sheets. Pure Mg sheet shows rapid recrystallization and grain growth, whereas recrystallization is considerably retarded in the Mg-RE alloys. Although recrystallized grains which are triggered at shear bands in the cold rolled pure Mg sheet show a relatively weak texture with a basal pole split into the sheet rolling direction, rapid grain growth is accompanied by re-strengthening of the basal-type texture. In contrast, a weak texture appears in the early recrystallization stage in Mg-RE alloys and is retained during annealing due to retarded recrystallization and grain growth.
Archives of Metallurgy and Materials, 2015
Magnesium alloys are the lightest structural materials, which makes them particularly suitable for use in the aircraft and automotive industry. However, due to hexagonal close-packed crystal structure, resulting in insufficient number of independent slip systems, magnesium alloys exhibit poor formability at room temperature. Conventional methods of work hardening of magnesium alloys requires the temperature about 300°C, which favours simultaneously processes of thermal recovery and grain growth, but decreases beneficial microstructure strengthening effect. Thus, it is a crucial to undertake development of a technology for semi-finished magnesium alloys elements, which will ensure better mechanical properties of the final products by forming desirable microstructure. In the paper we present the development of crystallographic texture of the Mg-based alloy (Mg-AZ31) in the form of pipe extruded at 430°C and subjected to pilger rolling at relatively low temperature.
Materials Science and Engineering: A, 2008
The combined effects of lithium additions (1-3 wt%) and processing parameters (rolling temperature, annealing) on the microstructural and texture evolution of pure Mg and Mg-3 wt% Al-1 wt% Zn alloy have been studied. Following rolling the basal planes were aligned with the sheet surface, although the basal poles were split and rotated towards the rolling direction. Lithium additions increased the rotation of basal poles in the rolling and transverse directions; an increase in the rolling temperature was associated with decreased rotation in the rolling direction and some broadening of texture in the transverse direction. Recrystallization during rolling varied between alloys, but had little influence on the texture. Recrystallization, and particularly grain growth, during annealing resulted in a single peak in the basal poles replacing the split observed following rolling. Texture is interpreted in terms of deformation, recrystallization and grain growth. Microstructural and texture evolution during industrial forming processes are also discussed.
Structure characteristics after rolling of magnesium alloys
2005
The presented article shows selected physical-metallurgical characteristics of magnesium alloys after hot rolling. The attention has been focussed on the analysis of mutual relations existing between the deformation conditions, micro structural parameters, and the achieved mechanical properties. The possibility of the method of equal channel angular pressing is also discussed for obtaining of new properties of magnesium alloys.
2012
The focus of the present research is to develop an integrated deformation and recrystallization model for magnesium alloys at the microstructural length scale. It is known that in magnesium alloys nucleation of recrystallized grains occurs at various microstructural inhomogeneities such as twins and localized deformation bands. However, models need to be developed that can predict the evolution of the grain structure and texture developed during recrystallization and grain growth, especially when the deformation process follows a complicated deformation path such as in asymmetric rolling. The deformation model is based on a crystal plasticity approach implemented at the length scale of the microstructure that includes deformation mechanisms based on dislocation slip and twinning. The recrystallization simulation is based on a Monte Carlo technique that operates on the output of the deformation simulations. The nucleation criterion during recrystallization is based on the local stored energy, and the Monte Carlo technique is used to simulate the growth of the nuclei resulting from local stored energy differences and curvature. The model predictions are compared with experimental data obtained through electron backscatter analysis and neutron diffraction.
The present research is focused on studying the evolution of microstructure and texture of a magnesium based alloy with the target composition Mg–3Al–1Zn–(0.5AgIn). Three samples A, B, and C were warm rolled at 300 °C to a cumulative reduction of 33% in 1, 2, and 8 passes, respectively. The optical microstructures and scanning electron microscopy (SEM) results revealed that sample A possessed more dynamic recrystallization (DRX) as compared to samples B and C. A split of basal pole from normal direction (ND) toward transverse direction (TD) was observed for sample A. However, as the number of passes was increased, the basal pole split was converted into a single peak for samples B and C. The basal intensity of sample C became almost double than that of sample A. It was concluded that a higher reduction per pass resulted in a larger volume fraction of DRXed grains and a weaker basal texture.