Microstructure Evolution And Mechanical Properties Of Mg-3Al-6RE Alloy

Mg-Al alloys are currently the most widely used magnesium alloys with excellent overall mechanical properties.The content,distribution,and morphology of the second phase in the alloy have an important influence on the properties of the alloy.Alloying and heat treatment can change the organization and mechanical properties of magnesium alloys.In order to obtain different microstructure states,this paper takes Mg-3Al alloy as the object of study,and adds rare earth elements Y,Nd,Ce and composite rare earth elements Y,Nd and Y,Ce to Mg-3Al alloy to study the tissue evolution of the alloy and analyze its mechanical properties;meanwhile,the effects of solid solution treatment and solid solution+aging treatment on the tissue and mechanical properties of the alloy are studied.The results show that:As-cast Mg-3Al-6Nd（AN36）,Mg-3Al-3Y-3Nd（AYN333）,Mg-3Al-3Y-3Ce（AYC333）alloys are composed ofα-Mg matrix,acicular Al₁₁RE₃ phases distributed on the grain boundaries,and granular Al2RE phases distributed inside the grain;The as-cast Mg-3Al-6Y（AY36）alloy is composed ofα-Mg matrix,Al2Y phases of short bar distributed on the grain boundaries,,and granular Al2Y phases distributed inside the grain;the as-cast Mg-3Al-6Ce（AC36）alloy is composed ofα-Mg matrix and acicular Al₁₁Ce₃ phases distributed on grain boundaries without granular phases within the grain.The as-cast AN36 alloy has the smallest average grain size and the highest percentage of second phase,the AC36 alloy has the highest content of acicular phase,and the AY36 alloy has the highest content of granular phase.Besides,according to the Miedema model,the enthalpy of generation of each phase in the alloy is calculated,and the its relationship is Al2Y<Al2Nd<Al11Nd3<Al11Ce3<Al2Ce.According to the solidification curve analysis of the alloy,the generated phase sequence in AN36,AYN333 and AYC333 alloys is Al2RE→α-Mg→Al11RE3;the generated phase sequence is Al₂RE→α-Mg in the AY36 alloy;the generated phase sequence isα-Mg→Al₁₁RE₃ in the AC36 alloy.The mechanical performance test analysis shows that the strength of the as-cast alloy is closely related to the grain size of the alloy.The finer the grain,the better the strength of the alloy.The as-cast AN36 alloy has the best room temperature mechanical properties,and its tensile strength is 186.2±6.5 MPa,the yield strength is 154.3±4.5MPa;the elongation of the as-cast alloy is closely related to the morphology,distribution and content of the phase.The more acicular phase content,the better the elongation of the alloy,and the as-cast AC36 alloy has the best elongation.Its elongation is 10.7±0.4%.The as-cast AN36 alloy has the best mechanical properties at 175℃.Its tensile strength is149.2±5.4MPa,yield strength is 117.2±2.7MPa,and elongation is 29.3±0.6%.After the solid solution treatment,the phase content in the five components of the alloy was reduced compared to the cast state,the acicular and rod phases became intermittent,the acicular phase became short rod phase and spherical phase,and the rod phase became shorter and smaller.According to the XRD phase detection and EDS analysis,it is found that the acicular Al11RE3 phase decomposed into short rod and spherical Al2RE phase in AN36 and AYN333alloys.A small amount of acicular Al11RE3 phase in AYC333 and AC36 alloys transforms into granular Al2Ce phase,and Al11RE3 phase still exists in the alloy.After solid solution treatment,the grain growth trend of the compounded rare-earth elements alloy is smaller than that of the alloy with rare-earth elements alone,suggesting that the compounded rare-earth elements have a more pronounced inhibitory effect on grain growth at high temperature.After solution treatment,the room temperature strength of AN36 alloy is the best.Its tensile strength is 192.8±7.8MPa,and its yield strength is 156.8±5.4MPa.The mechanical properties of other alloys are all reduced;the solid solution AN36and AC36 alloys have the best elongation,and their elongations are 13.3±0.9%and 13.3±0.4%,respectively.After solution treatment,the mechanical properties of the alloy at 175°C have little change compared to the as-cast state.After the aging treatment,acicular phases appear in AN36 alloy,granular and acicular phases appear in AY36 alloy,and plate strip phases appear in AC36alloy.AN36,AYC333 and AYN333 alloys have good strength at room temperature with tensile strengths of 210.7±7.8MPa,210.4±16.8MPa and206.9±15.8MPa respectively.The best elongation in the aging state is AN36 with12.8±0.8%.After the aging treatment,the mechanical properties of the alloy do not change much at 175℃,but the elongation of the alloy changes significantly,the best elongation of AYN333 alloy is 31.0±1.1%,AYC333 alloy also has a good elongation,whose elongation is 27.3±0.9%.