Effect Of Master Alloy On Microstructure And Mechanical Properties Of Mg-RE-Zn Alloys

The Mg-RE-Zn magnesium alloy reinforced with long-period stacking order structure?LPSO?has superior strength,plasticity and other comprehensive mechanical properties and controllable bio-corrosion resistance,has broad application prospects in the fields of high-performance and high-temperature deformation magnesium alloys and biological medical field.However,the as-cast Mg-Gd-Zn alloy contains little or almost no LPSO and coarse reticulate brittle W-phase on the grain boundary and the LPSO phase in Mg-Y-Zn alloy system is characterized by coarse microstructure and unevenly distributed,which severely impairs the mechanical properties of the alloy.In this paper,two kinds of Mg-10Al-27Ca and Al-10Sr master alloys were designed and prepared in advance.Mg-Y-Zn-?Ca/Al?prepared by copper die suction casting method,Mg-Gd-Zn-?Ca/Al?alloy and Mg-Gd-Zn-?Al/Sr?alloy prepared by graphite crucible casting process,respectively.The microstructures were analysed by OM,SEM,XRD and TEM methods.The effect of Mg-10Al-27Ca and Al-10Sr master alloys on the formation of LPSO phase in as-cast Mg-Y-Zn or as-cast Mg-Gd-Zn were studied.The structural evolution,morphology,number,size and distribution of the phases of Mg matrix and LPSO during the process of solid solution,aging heat treatment and large plastic deformation at different ECAP routes and extrusion passes.The results of the study as follows:?1?The as-cast Mg-Y-Zn-?Ca/Al?alloys mainly consist of?-Mg phase,18R LPSO phase and W phase.With the increase of Ca/Al content,?Y,Al?₂Zn₁phase and Al₂Y emerged.The grains have been refined greatly with Ca/Al coaddition,promote the formation of 18R LPSO phase and suppress the precipitation of W phase in the alloy.With 1.37 wt.%Ca/Al coaddition,the alloy owned the mechanical properties with tensile strength of 247 MPa and elongation of 7.2%,respectively.?2?In the solid-solution treated Mg-Y-Zn-?Ca/Al?alloys,partial massive block 18R LPSO phase at the grain boundary gradually dissolved with the time and precipitates into the grain along the grain boundary with fine layer 14H LPSO phase.High Ca/Al content addition inhibited the precipitation of 14H LPSO phase and 18R LPSO gradually dissolved in the?-Mg matrix merged and grown at grain boundaries during the solution heat treatment in the alloy.Additionally,the dendritic?Y,Al?₂Zn₁ phase in the matrix was spheroidized.?3?In high temperature and high shear stress,massive block 18R-LPSO phases layered with each other,kinked or broken and refined at the grain boundary of MgY₃Zn₁-1.37?Ca/Al?alloy.A large number of fine dispersed fine-lamellar 14H phase precipitation from?-Mg matrix.The room-temperature tensile properties of 4 pass ECAPed B_C-UD3-UD3 alloys solid solution treated at 500?for 20 h showed the highest mechanical properties at room temperature with mechanical property of 335 MPa,207 MPa and 7.4%,respectively.?4?The as-cast Mg-Gd-Zn-?Al/Sr?alloy mainly consist of?-Mg matrix phase,18R LPSO phase(Mg₁₀GdZn),?Mg,Gd?Al₂ and W-phase?Mg,Zn?₃Gd.With a small amount of Al/Sr addition promotes the formation of 18R-LPSO structure and promote the transformation of the W eutectic phase to the 18R LPSO phase,the volume fraction of the second phase increased significantly and the magnesium grains refined.Higher density fine-lamellar 14H phase precipitation from super-saturated?-Mg matrix in MgGd_2.5Zn₁-0.6?Al/Sr?alloy compared with other alloys.The addition of high content of?Al/Sr?suppressed the precipitation of the 14H LPSO phase and the transformation of the?Mg,Zn?₃Gd eutectic structure.?5?Grainrefinement,solidsolutionstrengthening,precipitation strengthening and a large number of different forms of LPSO phase act together to improve the mechanical properties of 1 pass post-ECAP aged Mg Gd_2.5Zn₁-0.6?Al/Sr?alloys.Its tensile strength and elongation are as high as 303 MPa and6.7%,respectively.