Effects Of Deformation Texture And The Distribution Of Grain Size On The Plastic Deformation Behavior Of Magnesium Alloys

Magnesium alloy is the lowest density structural metal,which has the advantages of high specific strength,specific stiffness and so on,so that,magnesium alloy has a broad application potential in the aerospace,automobile,medical,communication and other fields.However,the hexagonal close-packed crystal results the poor room-temperature mechanical properties of magnesium alloys.Plastic processing could improve the mechanical properties of materials by regulating texture and controlling microstructure.Exploring the influence of deformation texture and the distribution of grain size on the plastic deformation behavior of magnesium alloy has important theoretical value and engineering value significance for designing reasonable plastic processing technology and preparing high performance magnesium alloy This paper is mainly divided into two parts.On the one hand,the influence of deformation texture on the deformation behavior and the deformation mechanism of magnesium alloy at high temperature is investigated by using two kinds of pretreatment state Mg-4Zn alloy with uniform microstructure of extrusion state and ECAP state.And,and with the microstructures,the effect of deformation texture on the plastic processability is analyzed.On the other hand,in order to explore the effect of the distribution of grain size on the plastic deformation behavior of magnesium alloys at room temperature,deformed magnesium alloys with various grain size distribution were prepared by high temperature ECAP processing of AZ80 magnesium alloys under suitable deformation conditions.The microstructures(including second phase particles)of magnesium alloys were analyzed by metallographic microscopy(OM),scanning electron microscopy(SEM),etc.Meanwhile,the high temperature and room temperature plastic deformation ability of magnesium alloys were tested by Gleeble thermal compression and room temperature tensile experiments,respectively.The main research contents and experimental results are as follows:(1)The flow stress curves of the temperature between 200 to 350? with the strain rate ranged 0.001 to 1s-1 of the extruded and ECAPed Mg-4Zn magnesium alloys are studied.The deformation stress index and activation energy of extruded and ECAP state alloys are obtained based on hyperbolic sinusoidal phenomenological constitutive model calculation.The deformation texture and relative activity of each deformation mechanism are obtained by VPSC simulation.The experimental results show that the microstructure of the extruded and ECAPed alloys are ranther uniform,with average grain sizes of 46?m and 14?m,respectively.However,under the same conditions,the extruded alloy has higher flow stress than the ECAPed alloy and higher work hardening index at low temperature.Under the same conditions,the stress index and deformation activation energy of the two types of alloys are close.And at low temperature,it is numerically reflected that the deformation mechanism is mainly basal slip.The VPSC simulation found that ECAPed alloy has inclined texture,which is more conducive to the start-up of basal plane slip when axial compression,while extruded alloy has basal plane texture,which is more conducive to the activation of tensile twinning The tensile twinning makes the grain rotate?90° into hard orientation,which is more unfavorable to basal plane slip.Therefore,the extruded alloy exhibits higher deformation resistance(2)The influence of stress variables,temperature,strain rate and texture on the machinability and microstructure evolution(characteristics of grain size distribution)was analyzed by using the Murty plastic processing instability criterion based Mg-4Zn dynamic material model.The results show that although the average grain size of the ECAPed Mg-4Zn alloy is small,but affected by the deformation texture,the ECAPed alloy processing diagram forms a relatively large instability zone.The microstructure of the extruded alloy and the ECAPed alloy under the same deformation conditions is similar.At medium or low temperatures(200? and 250?),a large number of twins are observed in the microstructure,and with the increased temperature and the decreased strain rate,the dynamic recrystallization increases.The fine recrystallized grains preferentially nucleate at the twin boundary and constantly encroach on the original masterbatch.However,because the temperature is too low,recrystallization can not be complete,resulting in the microstructure of fine recrystallized grains,twins and original coarse grains.The recrystallization is complete at high temperature(300? and 350?),when the temperature is high and the strain rate is small,the recry stallized grains grow up,and the microstructure is generally homogeneous.While at high temperature and low strain rate,grain boundary sliding is easy to occur,resulting in bow-out(3)The effect of grain size distribution on the plastic deformation behavior of magnesium alloy at room temperature was studied by preparing AZ80 deformed magnesium alloy by heat treatment and high temperature ECAP processing.The results show that the deformation microstructure characteristics are closely related to the nucleation position of dynamic recrystallization and the proportion of dynamic recrystallization;the bimodal microstructure can significantly strengthen the room temperature mechanical properties of the alloy.The room-temperature strength(226.1MPa,454.3MPa)and plasticity(17.5%)of the bimodal microstructure magnesium alloy were higher than those of fine grain magnesium alloy(215.4MPa,438.0MPa,14.0%),with the average grain size?80?m and?4?m,respectively.It was found that the resistance of dislocations was greater when they moved at the interface between large and small grains.Moreover,the fine grains around the large grains of bimodal microstructure can coordinate the plastic deformation,and the second phase particles greatly enhance the plasticity of the alloy.(4)The influence of deformation conditions on the strength and plasticity of bimodal magnesium alloys was studied.At the A path,the bimodal microstructure has a significant mechanical property strengthening effect.This is because,the deformation of the A path is easy to form the basal plane texture.When stretched at room temperature,the basal plane texture is not conducive to the start of the basal plane slip,and the alloy shows high strength.In addition,the deformation temperature and the deformation pass can adjust the microstructure of alloys.Moderate increased the content of the small grains and decreased the size of large grains can increase the strength of the bimodal structure magnesium alloy.Conversely,excessive grain refinement would destroy the strengthening effect of bimodal structure characteristics on the material strength.