Research On Plastic Deformation Mechanism And Recrystallization Of AZ31Magnesium Alloy

Magnesium alloys have a wide range of potential applications, on account of theirexcellent mechanical properties, good thermal performance and ease of recycling.However, the plastic formability of magnesium alloys is poor, due to the lack of asufficient number of independent slip systems at room temperature. Therefore, studiesof the twinning behavior of magnesium alloys during low temperature deformation, andof non-basal slip system activation, has an important guiding significance for industrialproduction.In this study the effect of strain state and strain path on twinning behavior of the Mgalloy AZ31has been investigated, making use of a micro-grid method to measure localstrains, combined with electron backscatter diffraction (EBSD) measurements tomeasure local changes in orientation. In addition, the effect of sample orientation anddeformation temperature on the transition in dominant deformation mechanism fromtwinning to slip was also investigated. Based on the the results, an attempt was made toinvestigate the effect of sample orientation on the static recrystallization (SRX)behavior of the AZ31magnesium alloy.The main conclusions obtained are as follows:(1)The strain distribution on thesample surface can be measured using a micro-grid method. The effect ofinhomogeneous deformation on twinning behavior was thereby investigated and theresults show that the effect on the twin area fraction decreases with increasingmacroscopic strain. A statistical analysis of twin variants at regions experiencingdifferent strain states was made using both a Schmidt factor (SF) and strain tensorapproach. The results indicate that in all regions more than80%of {101ˉ2} twin variantscan be accounted for by a Schmid factor criterion. A SF model based on a considerationof the local stress state has been proposed for explaining the twin variant selectionphenomena physically. An analysis of twin variant selection based on a strain tensorapproach shows that individual twin variants can exhibit a wide range of local straintensor values, though the average values over many grains match well locally measuredvalues.(2) The deformation behavior during two compression paths, designed toinvestigate the process of detwinning, was investigated. In one case compression wasapplied first along the transverse direction (TD) and then along normal direction (ND). In the second case an intermediate deformation along a direction midway between TDand ND was applied. It was found that {101|-2} twinning dominates the deformationprocess during initial deformation along TD, basal slip dominates the paldeformationprocess during the intermediate compression, and detwinning dominates thedeformation process during the final compression along ND. In both cases a smallerstrain is required for detwinning than for the initial twinning. Moreover the interactionbetween slip and twinning accelerates the detwinning behavior.(3) The transition from{101|-2} twinning to prismatic <a> slip-dominated deformation has been investigatedthrough compression testing of samples cut from the sheet with the compression axisaligned at different angles to the sheet normal direction. The data show a clear transitionbetween twinning-and slip-dominated deformation in the range150℃to200℃forsamples cut with the compression axis at80°to ND. EBSD orientation maps of themicrostructure and slip trace observations have been used to verify the dominantdeformation mode for each sample and test temperature. Model calculations have alsobeen carried out to examine the sensitivity of this transition in deformation mode. Theresults have been used to estimate the critical resolved shear stress (CRSS) values forthe {101|-2} twin and prismatic <a> slip of the AZ31magnesium alloy as a function oftemperature, based on the experimentally collected stress-strain curves and the averageSchmid factors for each sample.(4) A comparative study of the SRX behavior betweensamples compressed along ND and TD has been carried out on the basis of the aboveinvestigations of deformation mechanisms. The results indicate that due to the differentdeformation mechanisms, both the initiation and the completion of SRX aresignificantly retarded in samples compressed along TD compared to samplescompressed along ND.