Study On Microstructure And Mechanical Properties Of Casting Mg-Sm/Yb-Zn-Zr Magnesium Alloys

As the lightest metal structure materials,magnesium alloy is widely used in aerospace,automotive,electronic communications and other industries.Mg-RE alloys have raised immensely attention both at home and abroad due to the fact that their excellent mechanical properties,especially high temperature service performance.China’s ZM6（Mg-Nd-Zn-Zr）alloy and the former Soviet Union ML10 alloy by the addition of Nd have already been commercial heat-resistant magnesium alloy The ZM6 industrial applications is extremely limited mainly because Nd is comparatively expensive.Therefore,it is a purpose that Nd is substituted by cheap Sm in this paper.Firstly,the microstructure and mechanical properties of Mg-Sm-Zn-Zr alloy were studied thoroughly,and the alloying component is optimized as well as strengthening mechanism is analyzed.In addition,the effect of Yb on the microstructure,mechanical properties and creep behavior of Mg-Sm-Zn-Zr alloy was studied.The strengthening mechanisms of Yb in Mg-Sm-Zn-Zr alloy and effect rules of Yb for microstructure and properties are explored.Finally,in order to study the strengthening mechanism of Yb in Mg-Zn-Zr alloy,the Mg-Yb-Zn-Zr alloy was prepared.The microstructure and mechanical properties of as-cast and aging were studied systematically.The reasons of high ductility and strong work hardening in the as-cast Mg-Zn-Zr alloys by the addition trace Yb were analyzed thoroughly.The main results are as follows:（1）After substitution Nd in the ZM6 alloy by Sm,the grain was effectively refined and the second phases also were modified.In ZM6 alloy,the dominate second phases is Mg₁₂Nd phase（body centered tetragonal,a=b=1.031 nm and c=0.693 nm）in the triangular grain boundaries,and the dominate second phases is Mg₄₁Sm₅ phase（body centered tetragonal,a=b=1.477 nm and c=1.032 nm）.In addition,both ZM6 and Mg-Sm-Zn-Zr alloys were carried out T4 treating（515 ^oC+16 h）and T6 treating（200 ^oC+12 h）.The dominate precipitates in peak-aging ZM6 alloy areβ1 andβ’,and they morphology and structure are similar to that WE43 or WE54 alloy.However,the dominate precipitates in peak-aging Mg-Sm-Zn-Zr alloys areβ’andβ’’.Under the same aging conditions,more precipitated precipitates were precipitated in the Mg-Sm-Zn-Zr alloys,which was the direct cause of the improved mechanical properties.Finally,by optimizing the alloy composition,the as-cast Mg-4Sm-0.6Zn-0.4Zr alloy exhibits the best comprehensive mechanical properties.（2）The effects of heavy rare earth Yb with different contents on the microstructure and properties of metal mold cast Mg-4Sm-0.6Zn-0.4Zr alloy were investigated.The Yb can obviously refine the grain of the alloy.When the content of Yb is 3 wt.%,the average grain size of the alloy is only 23.3μm.There are mainly two kinds of phases in as cast Mg-4Sm-xYb-0.6Zn-0.4Zr（x=0,0.5,1.5,3 wt.%）alloy.One is the dendritic Mg₂₄RE₅phase（body centered cubic（FCC）,lattice constant,a=1.12nm）and the other is the strip Mg₅RE phase（face centered cubic structure（FCC）,a=2.22nm）,and its orientation relationship with Mg matrix is（200）_Mg5RE|（-110-2）_Mg,[012]_Mg5RE||[-24-23]_Mg.Once again,the optimization solid solution processes of alloys are 515 ^oC-4 h+520 ^oC-2 h.After hot water（80-100 ^oC）quenching treatment,the elongation of alloy increased significantly,but the yield strength decreased slightly;Finally,after aging at 200 ^oC,the peak time of alloy was shortened due to more precipitated precipitates that is generated by the addition of Yb.TEM characterization of alloy shows that the precipitates in the peak aging state are mainly dispersedβ’phase precipitated and a small amount ofβ’’phase.The aged 1.5 Yb alloy exhibits the best comprehensive mechanical properties,which is superior to general commercial WE43 alloy.In addition,the effects of Yb on high temperature creep properties in as-cast Mg-4Sm-1.5Yb-0.6Zn-0.4Zr alloy were investigated.The creep mechanism is analyzed based on creep stress exponent and activation energy,that is dislocation climb under low and high temperature stress（225 ^oC/70¹00MPa and 250 ^oC/70¹00MPa）conditions;are dislocation climb and cross slip creep at high temperature and high stress（250 ^oC/70¹00MPa）;are grain boundary sliding and dislocation climb at high temperature and low stress（250 ^oC/<60MPa）.（3）In the study of Mg-4Sm-xYb-0.6Zn-0.4Zr alloy,Yb has good grain refinement and strengthening effect.Therefore,the microstructures,mechanical properties and aging behavior of Mg-xYb-0.6Zn-0.4Zr（x=0.5,1,2,3 wt.%）Alloys were studied systematically.The as-cast alloy has an obvious grain refinement effect and consists of eutectic flaky Mg₂Yb phase and a small amount of lath shaped Mg₄₁Yb₅ located in interdendritic regions.The Mg₂Yb phase is hexagonal crystal structure and the Mg₄₁Sm₅ phase belongs to the tetragonal crystal structure.After T5（200°C+4 h）artificial aging treatment,Mg-Yb-Zn-Zr alloy precipitats three kinds of precipitation phases.During aging treatment,obvious precipitation of T phase formed on grain boundaries can be observed in the alloys with Yb is less than 2 wt.%,while Mg₂Yb precipitate chains in the grain center and discrete Mg₂Yb particles homogeneously distributed near grain boundaries in the samples with Yb addition over 2 wt.%.Finally,ultra-fine needle-like precipitates were detected in all peak-aged samples,although with distinct difference on sizes and distribution densities.Based on energy dispersive spectrometer mappings of Mg,Yb,Zn and Zr and the SAED patterns,the needle-like precipitates were identified asg’phase（MgZnRE containing,hexagonal structure,a=0.65 nm and c=0.62 nm）and followed an orientation relationship as（1-100）γ’||（11-20）Mg,[0001]γ’||[0001]Mg.In addition,the studied alloys also exhibit good aging hardening response when aged at 200°C.The tensile strength（UTS）,the yield strength（YS）and the elongation of the peak-aging Mg-2Yb-0.6Zn-0.4Zr alloy is 246MPa、119MPa and 13.5%,respectively.In addition,the creep properties of 3 wt.%Yb content alloy was tested at the temperature of 180²20°C,the stress of 40⁸0MPa and 100h,and its the dominant creep mechanism is dislocation climb.Moreover,it is found that as-cast Mg-1Zn-0.4Zr-0.2Yb alloy that possesses excellent ductility（δ_f=38.5%）and strong work hardening effect（n=0.38）at room temperature,which is mianly due to the lower stacking fault energy induced via trace Yb addition.