Effect Of Grain Size Distribution On Microstructure Evolution And Mechanical Response Of AZ80 Magnesium Alloy During Deformation Process

Magnesium alloys have the characteristics of low density,high specific strength,good biocompatibility,and environmental friendliness,and have broad application prospects in fields such as transportation,3C products,biomedicine,and new energy.Due to the hexagonal closed packed crystal(HCP)structure,magnesium alloys have fewer slip systems during deformation,which can easily form strong deformation textures.As a result,their room temperature plastic forming ability is poor,severely limiting the large-scale industrial application of deformed magnesium alloys.Regulating the mechanical and formability properties of magnesium alloys from a micro perspective,such as grain refinement and texture distribution,has achieved certain results.This article focuses on commercial AZ80 magnesium alloys with different grain size distribution characteristics,and uses a combination of experimental measurement and crystal plasticity simulation to systematically and deeply study the microstructure evolution and mechanical performance response during plastic deformation.The main research content and conclusions are as follows:The main research content and experimental results are:(1)AZ80 magnesium alloy with different grain size distribution characteristics was prepared using Equal Channel Angular Pressing(ECAP).The effects of pre-treatment state and deformation conditions on grain size distribution were investigated,and the plastic deformation behavior of magnesium alloy with bimodal and unimodal grain size distribution characteristics was studied.The results show that the AZ80 magnesium alloy after solution treatment can obtain a uniform unimodal structure after 4 passes of ECAP deformation at 320℃and 0.2mm/s,while the cast material still retains the original cast structure after 4 passes of ECAP deformation at 360℃ and 0.2mm/s.The mixed structure of coarse and fine grains formed at this time can be called a bimodal structure,Its room temperature strength(226.1 MPa,454.3 MPa)and plasticity(17.5%)are higher than those of small unimodal magnesium alloys(215.4 MPa,438.0 MPa)and plasticity(14.0%).(2)AZ80 magnesium alloy was subjected to 1,2,4,and 6 passes of extrusion using ECAP to track the evolution laws and mechanisms of the microstructure and mechanical properties of the magnesium alloy during the ECAP process.Research has found that as the number of passes increases,coarse grains gradually refine under the combined effects of strong shear deformation and DRX,and small angle grain boundaries gradually transform into large angle grain boundaries.The strengthening mechanism of bimodal grain structure can be explained from the perspectives of grain boundary strengthening,grain orientation strengthening,and grain size distribution.After ECAP processing,the grain boundary density increases,and recrystallization becomes more complete.The grain boundary between recrystallized fine grains and unrecrystallized coarse grains is strengthened,improving the ability to resist dislocation slip.At the same time,due to the sharp change in grain size from the fine grain region to the coarse grain region,the coarse and fine grain boundaries have stronger resistance to dislocation slip compared to ordinary grain boundaries.In addition to the strengthening effect of grain boundaries and grain orientation on performance,it is also believed that the good plasticity of bimodal structures is due to the stabilizing effect of coarse grains on deformation and the increase in fine grain volume fraction.The excellent bimodal structure is composed of coarse grains uniformly embedded in a large number of fine grains.(3)Characterization and full field crystal plasticity simulation of bimodal and unimodal structures before and after stretching were carried out using quasi in-situ EBSD experiments.Research has shown that compared to samples with unimodal structure,bimodal structure samples are more prone to activation and expansion of tensile twins during plastic deformation,and larger grain sizes are more prone to tensile twins,with most of the twins penetrating through the matrix grains.When twinning occurs,the basal slip within the grain is significantly suppressed,resulting in the Basinski effect.The stress distribution in the unimodal structure is relatively uniform,and stress concentration occurs in the fine grain area surrounded by coarse grains in the bimodal structure;The quantitative study of twinning and different slip modes simulated to promote deformation indicates that basal slip has always been the main deformation coordination mechanism in the plastic deformation process of magnesium alloys.