| AZ91 magnesium alloy has the advantages of low density,high strength weight ratio and good damping performance.The application of AZ91 magnesium alloy in power battery shell material can not only meet the requirements of static and dynamic characteristics,but also reduce the weight of power battery pack to the greatest extent and realize the lightweight of new energy vehicles.However,AZ91 magnesium alloy exhibits poor creep resistance in the environment with temperature higher than 125℃,which seriously restricts the application of AZ91 magnesium alloy in some special conditions.Therefore,it is very important to develop magnesium alloys with excellent high temperature creep resistance.In this paper,AZ91 magnesium alloy was used as the matrix alloy.Starting from the practical application of engineering,in order to enhance the high-temperature creep resistance of the power battery shell material.AZ91-xY(x=0,0.5,1,2 wt.%,the same below)magnesium alloy was generated by adding different contents of rare earth element Y.The changes in the microstructure,mechanical properties and high-temperature creep properties of the alloy were analyzed,and the high-temperature creep resistant magnesium alloy with the best Y content was obtained.Based on this alloy,the high-temperature creep resistance of magnesium alloy was further strengthened by pre-deformation and heat treatment,and the mechanism and strengthening mechanism of high-temperature creep were analyzed.The conclusions are as follows:(1)The effect of Y content on high temperature creep properties of AZ91-xY magnesium alloy was studied.Rare earth element Y can refine the grain structure of magnesium alloy,and generate metal compound Al2Y during the melting process,which increases the comprehensive mechanical properties of AZ91-xY magnesium alloy.However.when the content of Y exceeds 0.5wt.%,Al2Y segregation occurs gradually,which makes the grain of magnesium alloy gradually rectangular,the microstructure gradually uneven,and the creep performance decreases.Overall,the high temperature creep resistance of magnesium alloy with Y element is better than that of AZ91 magnesium alloy.Among the three Y-containing magnesium alloys,AZ91-0.5Y magnesium alloy has the best high temperature creep resistance.0.5wt.%Y is the optimal content of AZ91 magnesium alloy in this study.The creep resistance of the four alloys is AZ91-0.5Y>AZ91-1Y>AZ91-2Y>AZ91.(2)The effect of Y content on the dynamic precipitation and creep mechanism of AZ91xY magnesium alloy during creep was studied.Dynamic precipitation of Mg17Al12 during creep of magnesium alloys with different Y contents,and the dynamic precipitates of AZ91 magnesium alloy are all lamellar Mg17Al12 with the highest volume fraction,some spherical Mg17Al12 will be dynamically precipitated in magnesium alloys with Y addition.The volume fraction of spherical Mg17Al12 in AZ91-0.5Y magnesium alloy is the highest among AZ91-0.5 Y,AZ91-1Y and AZ91-2Y magnesium alloys.The morphology and volume fraction of precipitates have great influence on the creep mechanism of AZ91-xY magnesium alloy.The precipitates of AZ91 magnesium alloy are all lamellar Mg17Al12,and the creep mechanism is dominated by basal slip,accompanied by dislocation climb.The dynamic precipitation of partial spherical Mg17Al12 of A291-1Y magnesium alloy hinders the formation of dislocation by pinning,and its creep mechanism is accompanied by a small amount of cross slip in addition to the basal slip.AZ91-2Y magnesium alloy due to more Y content,segregation phenomenon is obvious,creep mechanism for basal slip with a small amount of dislocation climb;the microstructure of AZ91-0.5 Y magnesium alloy is uniform,and the volume fraction of spherical Mg17Al12 is high,and its creep mechanism is cross-slip.(3)Based on the optimal Y content AZ91-0.5Y magnesium alloy,the effects of heat treatment and pre-deformation on the high temperature creep properties of AZ91-0.5Y magnesium alloy were studied.It was found that twins were introduced into the alloy by predeformation,and Mg17Al12 was precipitated in the alloy by aging treatment.Overall,the solution pre-deformation sample(SP)has the best creep resistance,followed by the predeformation aging sample(SPA),solution aging sample(SS)and solution aging sample(SA).(4)Dynamic precipitation during creep of AZ91-0.5Y magnesium alloy after predeformation and heat treatment was studied.It is pointed out that the twins generated by predeformation treatment of magnesium alloy will significantly affect the morphology of precipitates during thermal aging,and then affect the dynamic precipitation effect during creep.Under the combined action of stress and temperature,the volume fraction of spherical Mg17Al12 precipitates is the largest when they are dynamically precipitated in SP sample.The dislocation mechanism of SS,SPA,SS and SA samples is also affected by the type and volume fraction of Mg17Al12 precipitates.For example,in SS and SA samples,lamellar precipitation is more,basal slip and cylindrical slip are easily activated by lamellar structure.Due to the influence of twins,spherical Mg17Al12 precipitates in SP and SPA samples,which makes the conical slip and twins become the main creep mechanism. |
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