Microstructure and mechanical properties of severely deformed AX41 magnesium alloy

The object of the present paper is the study of mechanical properties and microstructural evolution of AX41 magnesium alloy, severely deformed using a combination of hot extrusion and equal channel angular pressing. Equal channel angular pressing processing was performed at 250°C following route Bc. Mechanical properties of the ultrafine-grained alloy were investigated in tension at a constant strain rate of 10-4 s-1 at room temperature and 100 °C. The dislocation density was determined by X-ray line profiles analysis. Microstructural observations performed by electron backscattering diffraction after 8 passes of equal channel angular pressing revealed very fine and homogeneous microstructure with a grain size of 0.3-6 μm. It has been found that the room temperature mechanical properties such as yield stress and tensile strength reach their maximum value even after the first pass which is in good agreement with the evolution of the dislocation density. Further processing by equal channel angular pressing led to the decrease in both the yield strength and the dislocation density, despite the slight grain size refinement

Activation Energy for Grain Growth of the Isochronally Annealed Ultrafine Grained Magnesium Alloy after Hot Extrusion and Equal-Channel Angular Pressing (EX-ECAP)

Magnesium alloy AZ31 prepared by hot extrusion and 4 passes of equal-channel angular pressing (EX-ECAP) has ultra-fine grained microstructure with an average grain size of 900 nm. Grain growth is analysed using a general equation for the grain growth and an Arrhenius equation. The calculated value of the activation energy for grain growth differs with the annealing temperature. The fitted value of activation energy for grain growth in the intermediate temperature range (210-400°C) is in accordance with the results of other authors, but it is shown in this study that such value is abnormally low and physically meaningless. More real values of apparent activation energy in this temperature range were calculated from the model assuming a linear increase of activation energy with increasing annealing temperature. Result of this linear model of evolution of activation energy in the temperature range between 210-400°C is expressed by the interval estimation of apparent activation energy values. It is concluded that the evolution of apparent activation energy can be explained by a change in the mechanism underlying the grain boundary migration. In the low temperature range, the grain boundary diffusion is dominant since the material is ultra-fine grained, whereas at higher temperatures, the lattice self-diffusion is more important

Twinning Evolution as a Function of Loading Direction in Magnesium

The twinning activity in random textured cast magnesium during monotonic, room temperature tension and compression tests was monitored by neutron diffraction. Decrease of integrated intensity which characterizes the twinned volume fraction of selected reflections was compared to its Schmid factor. The comparison shows that twinned fraction correlates with the maximum value of the Schmid factor with high precision during tensile test and with the average value of the Schmid factor during compression test

Microstructure and Properties of Spark Plasma Sintered Al-Zn-Mg-Cu Alloy

The microstructure of an aluminum alloy containing 53 wt% Zn, 2.1 wt% Mg and 1.3 wt% Cu as main alloying elements has been studied with the focus on the precipitation behavior during the spark plasma sintering process. The starting material was an atomized Al-Zn-Mg-Cu powder with the particle size below 50 μm. The particles showed a solidification microstructure from cellular to columnar or equiaxed dendritic morphology with a large fraction of the alloying elements segregated in form of intermetallic phases, mainly (Zn,Al,Cu)₄₉Mg₃₂ and Mg₂(Zn,Al,Cu)₁₁, at the cell and dendrite boundaries. The microstructure of the sintered specimens followed the microstructure of the initial powder. However, Mg(Zn,Al,Cu)₂ precipitates evolve at the expense of the initial precipitate phases. The precipitates which were initially continuously distributed along the intercellular and interdendritic boundaries form discrete chain-like structures in the sintered samples. Additionally, fine precipitates created during the sintering process evolve at the new low-angle boundaries. The large fraction of precipitates at the grain boundaries and especially at the former particle boundaries could not be solved into the matrix applying a usual solid solution heat treatment. A bending test reveals low ductility and strength. The mechanical properties suffer from the precipitates at former particle boundaries leading to fracture after an outer fiber tensile strain of 3.8%

Microstructure evolution in ultrafine-grained interstitial free steel processed by high pressure torsion

Commercial interstitial free steel was processed by high pressure torsion (HPT) at room temperature up to 5 revolutions. HPT resulted in strong grain refinement. The microstructure after HPT was inhomogeneous with refined grains mainly in regions near the specimen periphery, while coarse only slightly fragmented grains were observed in specimen centre. The microstructure inhomogeneity was continuously smeared out with increasing number of rotations by extending the fine grain region from specimen periphery towards its centre. However, even after 5 revolutions the microstructure remained inhomogeneous characterized by slightly coarser grains in central regions as compared to peripheral regions of the specimen. Positron annihilation spectroscopy (PAS) and X-ray line profile analysis (XLPA) were employed to characterize the structure inhomogeneity in individual specimens. Microstructure and dislocation density evolution were correlated with mechanical properties characterized by a detail microhardness measurement throughout the individual specimens. © Published under licence by IOP Publishing Ltd