Study On Plastic Deformation Behavior And Mechanism Of AZ31 Magnesium Alloy Sheet At Low Temperature

The seeking for low mass structure materials in transportation,space and aero industry grows as results of increased looming of energy and environment issue.Magnesium alloy has great potential as its advantage in mechanical properties,physical and chemical properties,environment compatibility,and technical and efficient.Recently,great efforts are made by researchers to overcome the poor plasticity at room temperature and secondary process difficulty of magnesium alloys.As part of fundamental work on utility expanding of magnesium alloy,this work choose AZ31 alloy as subject to discuss low temperature plastic deformation behavior and mechanism,by using mechanical experiments,theoretical analysis and numerical simulation.Meanwhile,the influence of grain size,initial orientation and pre-deformation on low temperature plastic deformation behavior and mechanism.Therefore,this work would support the understanding of deformation mechanism and using of magnesium.Uniaxial tension test shows that AZ31 alloy exhibits classical deformation characteristic even deformed from room temperature to 77 K.As decreasing of tension temperature,AZ31 alloy shows increasing yielding stress and ultimate tensile strength,but decreasing elongation.Yielding stress increases from 155 MPa at room temperature to 252 MPa at 77 K,and ultimate tensile strength increases from 253 MPa to 364 MPa,but elongation decreases from 25% to 5% correspondingly.AZ31 alloy does not experience highly strain rate sensitivity in the strain rate rage of 10-2/s to 10-3/s,but temperature correlation is clear.The hardening behaviors of AZ31 are different from deformed at temperature above room temperature.The increasing hardening rate along with deformation temperature decreasing is evidently.Based on analysis of activation energy and activation volume,the micro-mechanism of AZ31 deformation at low temperature is transforming form single pass over barriers into forming kink to pass barriers at low temperature,because the activation energy decreases from 110 k J/mol at room temperature to 20 k J/mol and activation volume decreases form 176b3 into 20b3.Thus,deformation mechanism of AZ31 alloy can be summarized as transition from a slip and twinning dominated into a slip dominated supplemented by non-basal slips.Microstructure affects the mechanical behavior of magnesium strongly.Asymmetry of yielding behavior in compression and tension is mainly from the strong fiber texture as rolling proceeding.The Hall-Petch relationship is still fitting as tension temperature dropping into 77 K.As temperature descending,grain size effect becomes intense,0 and k increasing accordingly.Hardening rate increases with grain size decreasing and strain rate sensitivity increases as well and increases intensely at 77 K.Pre-deformation in AZ31 alloy shows consequences on dislocation density and twinning.Pre-tension causes the difference in dislocation density and components of dislocation,the tactical stored dislocation in pretensioned at room temperature is more than pre-tensioned at 77 K,and the geometric necessary dislocation and forest dislocation are more in 77 K pre-tensioned sample than room temperature pre-tensioned.Pre-compression effects is reflection of detwinning behavior in following tension.Based on above-mentioned researches and studies,VPSC model is used to simulate plastic deformation of AZ31 alloy at low temperature.The results show different reaction of deformation modes to temperature change.In tensile simulation,?0 of basal a slip does not change with temperature decreasing from room temperature to 77 K and keep around 28 to 35 MPa,but prismatic a slip and pyramidal c+a slip change dramatically from 125 MPa into 220 MPa,?0 of contraction twinning also keep still and above 200 MPa.In compression simulation,?0 of basal a slip and tension twinning are temperature independent and are about 25 MPa and 50 to 59 MPa,?0 of prismatic a slip has the value of 55 MPa during the temperature dropping,?0 of pyramidal c+a slip is around 80 to 86 MPa.VPSC model is also suitable for discussion of microstructure influence on mechanical properties.During the simulation,the shortage of VPSC simulation is also revealed as the low ?0 of tension twinning weight the contribution and makes the computation ignor the contribution of c+a slip.