Mehdi Sanjari

Texture weakening and static recrystallization in rolled Mg–2.9Y and Mg–2.9Zn solid solution alloys

The microstructure and texture evolution in the Mg–2.9Y and Mg–2.9Zn solid solution alloys were investigated following rolling and subsequent isothermal annealing. The Mg–2.9Y alloy was hot rolled, and the Mg–2.9Zn alloy was rolled at room temperature in order to evaluate the possibility of attaining texture weakening by the suppression of dynamic recrystallization (DRX) and promotion of static recrystallization (SRX). It was found that texture weakening can be attained in Mg even in the absence of Y when there is no DRX, and SRX occurs during annealing. In solid solution, Y suppresses DRX during hot rolling, and retards the kinetics of SRX and grain coarsening in Mg. In the two alloys, the orientation of statically recrystallized grains at bands/twins (TSRX grains) is close to that of double and compression twins, exhibiting a much more evenly distributed and slightly wider orientation than that of basal parent grains and twins. In both Mg–2.9Zn and Mg–2.9Y alloys, a continuous texture weakening is observed with the progress of SRX, which results in a bimodal microstructure consisting of small TSRX grains and larger ones. With the increase in grain size during coarsening, the maximum intensity of basal pole figures rises linearly, with the slope of the lines being nearly identical in the two alloys. This texture strengthening was ascribed to the consumption of small TSRX grains by larger ones.

Promotion of texture weakening in magnesium by alloying and thermomechanical processing: (I) alloying

The recrystallization and texture evolution of four Mg–Zn–Ce sheets with a warm-rolled microstructure obtained through two stages that can be characterised as rough rolling and finish rolling were investigated at different stages of post-rolling annealing. On annealing, the same regions of the microstructure, located by hardness indentations, were examined and tracked by electron backscatter diffraction (EBSD). Furthermore, intragranular misorientation axes (IGMA) analysis was used to investigate the associated deformation mechanisms in the as-deformed material. By combining these two methods, the development of the recrystallization microstructure was investigated and important aspects, such as preferential nucleation sites, correlation between activated deformation mechanism and initial orientation of the recrystallized grains, were studied. The results showed that the Mg–1Zn–1Ce alloy, which had the highest Ce/Zn ratio, showed the weakest as-rolled texture and the most homogenous distribution of shear banding/twinning. The IGMA analysis also showed that in Mg–1Zn–1Ce, other types of dislocations rather than basal 〈a〉 were activated; in particular, prismatic 〈a〉 type was activated during deformation. Therefore, the weakening of recrystallization texture during rolling resulting from the addition of RE elements was linked with a change in dynamic recrystallization (DRX) behaviour. Since the Mg–1Zn–1Ce alloy corresponds to the highest level of Ce in solid solution, the observed texture weakening was possibly due to decreasing grain boundary mobility as a result of solute partitioning of RE elements to dislocations and grain boundaries.

Influence of strain rate on hot deformation behaviour and texture evolution of AZ31B

Abstract In the present work, the effects of strain rate on the flow behaviour and microstructure evolution of AZ31 Mg alloy were studied by compression testing over a wide range of strain rates (0·01–100 s−1) and temperatures (300–450°C). In addition, the influence of different strain rates on the dynamic recrystallisation (DRX) mechanisms and texture evolution was investigated. The results showed that with increasing strain rate, the twin induced DRX fraction increased at a constant temperature, and the contribution of continuous DRX decreased. On increasing the strain rate, the formation of twins and subsequent twin induced DRX intensified the basal texture in the deformed sample. In addition, the recrystallised volume fraction increased significantly with strain rate. The flow behaviour was fitted to two types of constitutive equations: power law and hyperbolic sine. Average activation energies of about 162 and 135 kJ mol−1 were obtained for the peak and steady state strain respectively.

High speed rolling of Mg-3Al-1Zn alloy: texture and microstructure analysis

Abstract High speed rolling (HSR) of 1000 m min−1 was employed to successfully roll AZ31 alloy in one pass with 65% reduction in thickness at 300 and 450°C. The rollability, texture and microstructure after HSR, in comparison with low speed rolling (15 m min−1), improved significantly. It is suggested that the double peak and weaker basal texture obtained after HSR are attributed to the activation of compression and double twins. After annealing, the double peak basal texture is replaced by a single peak one, which may be due to preferential grain growth of basal grains.

Accumulative Roll Bonding of Pure Copper and IF Steel

Severe plastic deformation is a new method to produce ultrafine grain materials with enhanced mechanical properties. The main objective of this work is to investigate whether accumulative roll bonding (ARB) is an effective grain refinement technique for two engineering materials of pure copper and interstitial free (IF) steel strips. Additionally, the influence of severely plastic deformation imposed by ARB on the mechanical properties of these materials with different crystallographic structure is taken into account. For this purpose, a number of ARB processes were performed at elevated temperature on the materials with 50% of plastic deformation in each rolling pass. Hardness of the samples was measured using microhardness tests. It was found that both the ultimate grain size achieved, and the degree of bonding depend on the number of rolling passes and the total plastic deformation. The rolling process was stopped in the 4th cycle for copper and the 10th cycle for IF steel, until cracking of the edges became pronounced. The effects of process temperature and wire-brushing as significant parameters in ARB process on the mechanical behaviour of the samples were evaluated.

Influence of Static Precipitation on Microstructure and Texture of Annealed Cold-Rolled Mg-Al-Sn Alloys

The final mechanical properties of wrought magnesium alloys are mostly controlled by its microstructure and crystallographic orientation or texture. In the sheet form of common magnesium alloys, grain coarsening occurs during annealing, which only serves to strengthen the undesirable basal texture. One method to alleviate this problem is by stopping grain coarsening. Hence the aim of this work is to investigate the effect of static precipitation on microstructure and texture evolution during annealing at various temperatures after cold rolling. Mg-Al-Sn alloys were designed using thermodynamic modeling software, FactSageTM. It was found that static precipitates can only form extensively, after static recrystallization, at the recrystallized grain boundaries, and these retard grain growth during annealing. Presence of precipitates at the recrystallized grain boundaries also retard the strengthening of basal texture during annealing, related to grain coarsening.

Thermodynamic Modeling and Experimental Measurement of Precipitation Formation during Dynamic Recrystallization for Magnesium Alloys

Magnesium alloys have low formability at room temperature associated with its hexagonal closed pack structure. Formation of precipitates during deformation may pin the grain boundary and reduce the final grain size, which literally means the improvement of formability. The aim of this study is to design magnesium alloys that are capable of forming precipitate during hot deformation. Thermodynamic modeling software, FactSage, has been used to design Mg-Al-Sn alloys based on forming target Mg2Sn precipitate at hot deformation temperatures between 250 and 350oC. Uniaxial compression at elevated temperatures has been performed to simulate the hot deformation behaviour as well as to enhance the formation of precipitates. Strain rates used in this study were in the range of 1.0 to 0.001s-1 for a constant deformation degree of 90%. It was found that the formation of precipitate depends on deformation temperature and strain rate. Measured amounts of precipitate were compared with the calculated equilibrium results from FactSage.

Effect of Precipitation on Texture Evolution during Dynamic Recrystallization in Mg-Al-Sn Alloys

In common magnesium alloys, recrystallization is usually not accompanied with a noticeable change of deformation texture resulting in, strong mechanical anisotropy. The aim of this work is to investigate the effect of dynamic or strain induced precipitates on texture evolution during hot deformation of Mg-Al-Sn alloys. Mg-Al-Sn alloys have been designed using thermodynamic modeling software, FactSage, based on forming precipitates at hot deformation temperatures. Two alloys have been chosen in a way that one forms precipitates during the hot deformation process, another does not at a certain temperature. Uniaxial compression has been introduced at different strain rates at this elevated temperature to simulate the hot deformation behaviour. Two alloys have been compared in terms of the intensity of basal texture. It was found that the formation of dynamic precipitates during deformation weakens the texture compared to the case where no precipitates were formed.

Effect of Dynamic Recrystallization on Microstructure Evolution and Texture Weakening During Annealing of High Speed Rolled AZ31 Magnesium Alloy Sheets

Magnesium AZ31 (Mg-3 wt. % Al-1 wt. % Zn) alloy sheets were rolled at a high speed of 1000 m/min at 100 °C to reductions of 30% and 49%. Annealing was then conducted on the as-rolled specimens at temperatures of 200 °C and 350 °C for different times. The microstructure was characterized by optical microscopy and the macrotexture was analyzed by X-ray diffraction. The as-rolled microstructure of the specimen subjected to the reduction of 30% was heavily twinned and shear banded, while a partially dynamically recrystallized and twinned microstructure was observed at the reduction of 49%. Effect of the initial microstructure on static recrystallization behavior and texture evolution during annealing was studied at different temperatures. Texture weakening was found during annealing at both reductions. However, the weaker texture can be achieved in the specimen subjected to the reduction of 30% than that of 49%.

Age-Hardening Response of Mg-Al-Sn Alloys

Precipitation hardening has been used before as one of the most effective strengthening methods for many metallic alloys. However, this method has not been studied completely in magnesium alloys, and the numbers of precipitation hardenable wrought Mg alloys are still very limited compared to aluminum alloys and steels. The age hardening responses of Mg-Al-Sn alloys in cast-homogenized condition were investigated by isothermal aging at 200°C for prolonged time. It was found that hardness can be improved significantly for the alloy with higher amounts of tin. The improvement in hardness was reasoned by the formation of precipitates. The shapes and morphology of the precipitates were different depending on the orientations of the grains. The precipitates were characterized by scanning electron microscope.