Investigation On The Tension-compression Asymmetry Of Extruded Mg-Ysheets

Compared with cast magnesium(Mg)alloy,wrought Mg alloy has better development prospects due to its higher and more varied mechanical properties,which can be achieved by controlling hot working process and heat treatment process.However,traditional wrought Mg alloy show strong basal texture indicates that basal plane was parallel to extrusion direction/rolling direction(ED/RD).These Mg alloy exhibit remarkable tension-compression asymmetry and will hinder the wide application of wrought Mg alloy in component which bear tensile stress and compressive stress meanwhile.Many studies indicate that the addition of Y element can effectively weaken basal texture,and then dramatically improve plasticity and formability.Nevertheless,the effect of Y addition on tension-compression yield asymmetry of extruded Mg-Y sheets is still unclear.This investigation comparatively studied and illuminated the effect of Y content(0-5wt.%),deformation temperature(25?-300?)and load direction(ED,DD,TD)on tension compression deformation behaviour and microstructure of extruded Mg-Y sheets.Discussing and revealing the key factor which effect the tension-compression yield asymmetry.Based on in-situ tension/compression,slip trace analysis and EBSD twinning analysis,this paper quantitatively studied and definitized the activation rule of various deformation modes(basal/prismatic/pyramidal slip,tension/compression twinning)in different deformation way(tension/compression),Y content and strain,and built the relationship between room temperature tension-compression yield asymmetry and activation rule of deformation mode.Damage and fracture behavior of Mg-Y sheets during tension and compression was investigate by Quasi-in-suit SEM too.Meanwhile,the microstructure stability,grain growth kinetics and texture evolution during annealing at 300-450? for 0-48 hours were studied,and built the relationship between grain size and texture,microhardness and tension-compression asymmetry.Main result as follows:Y content dramatically effected the room temperature tension-compression yield asymmetry.With increasing of Y content in room temperature,tension-compression yield asymmetry reduced and Mg-Y sheets exhibited tension-compression yield symmetry when Y content up to 3%-5%.The pure Mg sheet deformed mainly by tensile twinning when tension and compression at room temperature while the dominant deformation mode of Mg-Y sheets at room temperature during tension and compression was slip.With increasing of Y content(from 0.5% to 5%),the frequency of grains appeared slip trace increased from ~10% to ~43% and the observed dominant deformation mode transitioned from basal <a> slip(75%)combined with prismatic <a> slip(15%)to basal <a> slip(66%)combined with pyramidal <c+a> slip(26%)when tension at room temperature.The fraction of grains appeared slip trace increased from ~9% to ~24% and the observed dominant deformation mode transitioned from basal <a> slip(82%)combined with prismatic <a> slip(13%)to basal <a> slip(73%)combined with pyramidal <c+a> slip(17%)when compression at room temperature.For Mg-5Y sheet,2.1% and 1.6% 1st order pyramidal <c+a> slip were observed when tension and compression at room temperature.With increasing of Y content,the effect of slip became prominent and the frequency of non-basal slip increased.Besides,the fraction of twins in after tension and compression microstructure decreased,and there were various type twins in after compression microstructure resulting in the weakened of dominant effect of tension twinning.The combined effect of these two reasons lead to gradually decreased tension-compression yield asymmetry with increasing of Y content at room temperature.The crack behavior of pure Mg and Mg-Y sheets during tension and compression at room temperature was intergranular fracture.The addition of 1% content Y can achieve tension-compression yield symmetry in 150? and then the increasing of Y content had less effect on tension-compression yield asymmetry.All sheets exhibited tension-compression yield symmetry in 300? and Y content had less effect on it.Deformation temperature dramatically effected tension-compression yield asymmetry of lower Y content(?1%)sheets,but it had less effect on the tension-compression yield asymmetry of higher Y content(?3%)sheets.With increasing of temperature,the tensioncompression yield asymmetry of pure Mg dramatically decreased;the tension-compression yield asymmetry of Mg-1Y sheet obviously decreased in 25?-150? while it almost never changed in 150?-300?;the tension-compression yield asymmetry of Mg-3Y and Mg-5Y sheets almost never changed.The addition of 1% content Y can dramatically decrease tension-compression yield asymmetry in all directions,and the tension-compression yield asymmetry in all directions(ED,DD,TD)constantly decreased with increasing of Y content.With the changing of load direction(increasing of angle with ED),the tension-compression yield asymmetry of lower Y content sheets(?1%)gradually enhanced while the tension-compression yield asymmetry of higher Y content sheets(?3%)kept unchanged indicating that the effect of loading direction on tension-compression yield asymmetry gradually minished with Y content increasing.Fine-grained Mg-Y sheets had excellent thermostability at 300? annealing that grain size and microhardness almost unchanged.The grain growth during annealing can weaken basal texture.The basal texture weakening during grain growth can be ascribed to preferential growth of non-basal oriented grains,which were induced by segregation of Y atoms at grain boundaries.The measured grain growth activation energy(Q)of Mg-1Y at 300?-450? was 91±3 kJ/mol,suggesting that the growth was controlled by grain boundary diffusion.For Mg-5Y at lower temperatures(300-400°C),the Q value(59 kJ/mol)indicated that the grain growth was controlled by grain boundary diffusion,while at higher temperatures(400-450°C)the Q value(175 kJ/mol)implied that lattice self-diffusion controlled the grain growth.