Research On Plastic Deformation Mechanism And Fracture Mechanism Of Rare Earth-magnesium Alloy

Since the 21st century,the surface resources have become increasingly barren,and the problems of resources and environment have become the primary issues in the development of human society.In order to reduce energy consumption,lightweight has become the main development direction of aircraft,automobile and other industries.Magnesium alloy is one of the least dense metal materials in current engineering applications has the advantages of high specific strength,high specific stiffness,good toughness,etc.,which can meet the demand of lightweight.However,the plasticity of magnesium alloy at room temperature is poor,which limits its application in aerospace,automobile and other industries.Therefore,it is necessary to study the mechanism of plastic deformation and fracture of magnesium alloy.In this paper,the hot compression experiments of as-extruded WE43 rare earth magnesium alloy at 150-500? and 0.001-1s^-1 were carried out on a thermophysical simulator,and the intermittent compression experiments at 350? and 0.1s^-1 were carried out,with the reduction of 1.75%,32.8%,54.9% and 77.5%respectively.Combined with microhardness tests,OM,SEM,EBSD and EDS,the plastic deformation mechanism and fracture mechanism of as-extruded WE43 magnesium alloy in the process of hot compression deformation were analyzed.In order to obtain accurate fracture strain,this paper proposes an asymptotic approach.The specific research results are as follows:?1?When temperature lower than 350?,fracture occurs at a lower strain.For example,at 150??0.1s^-1,the strain at break is only 0.278.The strain rate is not sensitive to the strength and hardness of the alloy,but it has an effect on the toughness of the material.Observed from the microstructure,the main deformation mechanism of as-extruded WE43 magnesium alloy when deformed below 350? is twinning.The second phase particles induce twin nucleation,and the second phase particles and twins promote dynamic recrystallization.Grain size of the alloy increases with the increase of temperature,the strength and hardness of the alloy increase too,because there are more ridges at 250?.The number of twins and the second phase decreases with the increase of temperature,and the direction of twins changes.The fracture strain is higher at 150??1s^-1 and 250?,?0.001s^-1,because there are more twins and non-basal slip activation.In the microstructure there are two main types of twins,one is wider in the form of lens,the other is narrower in the form of line,and the number of lens twins is more than that of line twins.The fracture mode of the alloy is ductile brittle mixed mode,and the ratio of transgranular fracture increases with the increase of temperature.The second phase and twins are the location of the crack nucleation,and at different temperatures,the effect of the second relative fracture is different.?2?When temperature higher than 350?,WE43 magnesium alloy shows pseudo superplasticity instead of fracture.For example,at 450??0.1s^-1,the true compression strain even reaches 1.48.The strength and hardness of the alloy show strain rate sensitivity at high temperature,that is,at a certain temperature,at the same strain,the strength and hardness increase as the strain rate increases;at a certain strain rate,the strength and hardness decrease with the increase of temperature,which is the result of the competition between strain hardening and softening.From the observation of the microstructure,no twins were found in the microstructure when the deformation temperature higher than 350?,which indicated that the deformation process was dominated by slip,and the nucleation of continuous dynamic recrystallization was induced by the second phase particles such as Mg₂₄Y₅ phase and Mg₁₂Nd phase,which indicated that the particle excited nucleation?PSN?mechanism stimulated the continuous dynamic recrystallization during the whole deformation process.Grain size increases and the number of the second phase decreases with the increase of the temperature.Temperature has a significant effect on the second phase type.The second phase particles precipitated at 500?are mainly massive Mg₂₄Y₅ phase,while Mg₁₂Nd precipitates less.However,the two kinds of second phase particles are more precipitated when deformed at 450?.At lower strain rates(0.01s^-1,0.001s^-1),the dynamic recrystallization fraction reaches 100%,and the recrystallized grains are easy to grow;at higher strain rates(1s^-1,0.1s^-1),There will be a phenomenon that the dynamic recrystallization fraction decreases with increasing temperature.Based on the above research results,it is concluded that the microstructure is optimal at 450??0.1s^-1.At this time,the dynamic recrystallization fraction reaches 100%,the recrystallized grains are fine?only 3.52?m?,and the structure is uniform.?3?The critical transition temperature of WE43 magnesium alloy is 350?.At 350?,the strain rate of 0.1s^-1 and 1s^-1 are 89%different from the outer circumferential elongation.At 0.1s^-1,the strain reaches 1.48 and it does not break,while the strain reaches 0.363 at 1s^-1happens.This shows that the strain rate at this temperature is extremely sensitive,and the fracture at 350? is caused by ductile fracture of the second phase particles.In order to further explain the microstructure evolution mechanism and deformation mechanism,intermittent compression experiments at 350??0.1s^-1reductions of 1.75%,32.8%,54.9%,and 77.5%were carried out.The results show that:As the amount increases,the strength and hardness of the material increase first and then decrease,and the average grain size decreases from 9.2?m to 5.2?m.Analysis of the strain hardening rate under different strains shows that the alloy undergoes dynamic recrystallization in advance before reaching the peak stress.Observed from the microstructure:This is because the presence of second phase particles plays an important role in promoting early dynamic recrystallization,and the second phase particles provide potential for local shear to occur.Motive force,which means that sub-grain sprout around the second phase particles.In addition,from the observation of EBSD,it is known that the small-angle grain boundary gradually changes into the large-angle grain boundary,which indicates that continuous dynamic recrystallization promotes plastic deformation.Combined with the above research results,the optimal initial reduction is 54.9%.At this time,the alloy strength,hardness,and average grain size are 160MPa,116.13HV,and 6.6?m respectively.When the comparison reduction is 1.75%,the alloy strength is increased by 40MPa,the hardness is increased by 32.84HV,and the average grain size is reduced by 2.6?m.?4?In order to study the plastic forming behavior of magnesium alloy under different forming processes,the compression experiments of as-extruded and as-cast WE43 magnesium alloy at 350??0.1s^-1 were carried out.The results show that the original morphology of the two alloys is quite different,the former has"ridge"and a small amount of spherical second phase in the crystal,the latter has no"ridge"and there are a large number of strip eutectic phases on the grain boundary and in the crystal.The average grain size of the latter is far larger than that of the former,and the difference between them is as high as 71.2?m.The effect of fine grain strengthening makes the hardness of the former 9.43HV higher than that of the latter.After plastic deformation,the former reaches the peak stress later,and the yield strength of the former is higher.After deformation,the mechanical properties of the latter are not much lower than that of the former.The difference between the two is only 4MPa in strength and 4.37HV in hardness.This is because the broken eutectic phase in the latter plays a role of dispersion strengthening.The continuous dynamic recrystallization was induced by PSN mechanism,but only Mg₂₄Y₅ phase was found in the latter,and no Mg₁₂Nd phase was observed.The recrystallization degree of the latter is higher than that of the former,but the grain size of the latter is still much larger than that of the former?the difference is 47.17?m?.These results show that the optimal processing mode is extrusion+forging.?5?According to the above analysis,extrusion+forging is the best processing method.At this time,the best processing technology is deformation temperature of 450?,strain rate of 0.1s^-1 and initial reduction of 54.9%.