Influence Of Ca And Ca/Al Coaddtion On Microstructure And Mechanical Properties Of Mg-Gd-Zn Alloys Containing LPSO Phase

Mg-RE-Zn alloys with long-period ordered stacking structure?LPSO?are expected to be widely used in automobile and electronic products with the overcoming the shortcomings of their low absolute strength and poor plasticity.Among them,Mg-Gd-Zn alloy has attracted the researcher's interests due to the unique microstructure and superior mechanical properties.However,the as-cast Mg-Gd-Zn alloy has little or no LPSO phase.After solid solution treatment,the massive LPSO phases were precipitated at the grain boundary and difficult to be broken,which seriously deteriorates the mechanical properties of the alloy.In this paper,a large amount of 18R-LPSO phase was formed in the as-cast Mg-Gd-Zn alloy by adding non-rare earth element Ca and Ca/Al alloying elements by magnetic levitation melting and graphite crucible casting.The effects of Ca and Ca/Al coaddition on the microstructure of the alloy were investigated.Through solid solution treatment and hot extrusion,the alloy structure is refined and uniform.The transition rules from ?-?Mg,Zn?₃Gd phase to LPSO phase in the alloy was investigated.And the effect of Ca and?Ca/Al?coaddion on the microstructure and mechanical properties of extrudedMg-Gd-Zn alloys were investigated.The results of the study are as follows:?1?Various eutectic microstructure are formed in the as-cast Mg_83.65-x Gd₁4.02Zn₂.33 Cax alloy with the different amounts of Ca element addition.When the Ca content is less than 0.5%,??-Mg+?-?Mg,Zn?₃Gd?eutectic structure forms in the grain boundary of the alloy.The eutectic structure changes from the network structure to the coarse fish bone structure with increasing Ca addition.When the Ca content up to 1.0%,??-Mg+?-Ca₂Mg₆Zn₃?eutectic structure and small amount of?Mg,Zn?₃Gd are formed.?2?Solid solutioned Mg_83.65-xGd₁4.02Zn₂.33 Cax alloy is composed of a-Mg matrix,intergranular block LPSO phase and intragranular lamellar 14H-LPSO phase and small amount of??-Mg+?-?Mg,Zn?₃Gd?eutectic structure.With the addition of Ca,the network ?-?Mg,Zn?₃Gd eutectic phase in the grain boundary of the alloy tends to be transformed to the bulky LPSO phase.The solution?500??treatment can promote the precipitation of a large number of dispersed 14H-LPSO phase from the ?-Mg matrix.?3?Compared with Ca element,the Ca/Al coaddition can promote the transformation of eutectic ?-?Mg,Zn?₃Gd phase in the grain boundary of the as-cast Mg-Gd-Zn-?Ca/Al?alloy to LPSO phase.The area fraction of LPSO phase in the alloy reaches a maximum value of 32.2% at the level of 0.65% of Ca/Al coadditon.The microstructures of as-cast Mg-Gd-Zn-?Ca/Al?alloy are mainly composed of ?-Mg matrix,block-like 18R-LPSO phase distributed in the gain boundary and intragranular 14H-LPSO phase and few?Mg,Zn?₃Gdphase.When the?Ca/Al?concentration reaches 1.30%,Al₂ Gd particles are formed which inhibits the precipitation of LPSO phase.The formation of LPSO phase in the Mg-Gd-Zn-?Ca/Al?alloy are related to the cooling rate.The slower the cooling rate,the more easily LPSO precipitates.?4?The blocky 18R-LPSO phase in the grain boundary of Mg-Gd-Zn-?Ca/Al?alloy after solid solution heat treatment for various duration time is dissolved gradually and transformed to thin strip 14H-LPSO phase partly.?5?After the hot extrusion deformation,block 18R-LPSO phase in the grain boundary was kinked and broken where dynamic recrystallization with various degrees are accompanied,and a large number of lamellar 14H-LPSO phase precipitate from the ?-Mg matrix.At the same time,the eutectic structure and the Al₂ Gd phase are crushed into small particles and distributed dispersedly.These small dispersed particles,the fine dynamic recrystallized grains and the small LPSO phase with strong and tough structure contribute to the improved strength of the alloy.The tensile strength,yield strength and elongation of the solid solutioned Mg₈3.15 Gd₁4.02 Zn₂.33 Ca₀.5 alloy after extrusion were 380 Mpa,345MPa and 11.0% respectively,and the comprehensive mechanical properties were improved obviously.The mechanical properties of as-cast Mg₈3.00Gd₁4.02Zn₂.33?Ca/Al?₀.65 alloy were also improved after extrusion.The tensile strength was 360 MPa,the yield strength 324 MPa and the elongation7.1%.