Microstructure And Mechanical Properties Of High Strength Mg-Zn-(Y/Gd)-Zr-(Ca) Alloys Containing W Phase

Magnesium alloys are very famous structural and functional materials recently.In addition,magnesium alloys not only have wide markets in the fields of aerospace,military products,transportation project and 3C,but are meaningful for further innovation to industry and consumption in the future.In order to developing high strength magnesium alloys,the Mg-Zn-RE system alloys containing W phases have been developed by controlling the Zn/RE ratio of 1.7 in present study.The Mg-10Zn-6(Y/Gd)-Zr alloys with different rare earth elements and alloying content have been fabricated by permanent mold direct-chill casting and traditional extrusion process.The wrought Mg-Zn-Y system alloys with W phase strengthening containing lower content of rare earth elements makes fabrication costs lower,while can obtain higher strength than some heavy rare earth wrought magnesium alloys.The effects of alloying elements,fraction of W phases,distributions and size of W phases on the microstructures and the relationship between microstructures and mechanical properties have been discussed in present study.In addition,the strengthening and toughening mechanisms have been also analysed.Thus,this present study can offer a theoretical direction for development of ultra-high strength rare earth wrought magnesium alloys.In the Mg-Zn-Y alloys,the addition of Gd element has a great effect on the microstructures.The as-extruded Mg-Zn-Y-Zr alloys are consisted of bimodal microstructure containing DRXed grains and un DRXed regions.And the broken W phase particles are distributed on the particle bands along the extrusion direction.With Gd element increasing,the microstructure turns to be fully recrystallization.And the W phase particles are dispersed in the ?-Mg matrix and some W phase particles are surrounded with the icosahadral phase layers.The strength are decreasing and the ductilities are increasing with addition of Gd.The addition of Gd does not benefit to the improvement of strength.Thus,Y element is selected as the alloying element to develop ultra-strength wrought rare earth magnesium alloys.The fraction of W phases is effective on as-extruded alloys.The eutectic lamellar structures are broken into particles after extrusion,and these particles are distributed on the bands along the extrusion direction.With fraction of W phases decreasing,the numbers and the size of particle bands are decreasing.However,the bimodal microstructures do not change and the size of W phase particles in the DXRed regions and dynamic W phase precipitates in the un DRXed regions remains unchanged with fraction of W phase changing.With fraction of W phases decreasing,the tensile yield strength is decreasing.However,when the fraction is further decreasing,the tensile yield strength is not further decreasing,and the bimodal microtructrues keep unchanged.This result indicates that the strength of the asextruded alloys will not change with such bimodal structures remaining.Thus,such special bimodal structures resulting from W phases are very effective on improvement of strength.The micron-scale W phase particles are mainly distributed in the DRXed grain boundaries.With micron-scale W phase particles are distributed more homogeneously,the alloys strength is decreasing and the ductilities are increasing.With the W phase particles distributed more densely on the particle bands,the cracks are more easily formed and proporgated through the matrix leading to failure.When the W phase particles are distributed more homogeneously,the particles bands are homogeneously distributed.Then small micro-cracks are easily formed on the particle bands,but not easily proporgated resulting in better ductility.Nano-scale W phase precipitates are distributed in the un DRXed regions.The size and distribution of nano-scale W phase precipitates have a great effect on the microstructures and mechanical properties of Mg-Zn-Y-Zr-Ca alloys.With the size of nano-scale W phase precipitates increasing,the ability of inhibition to the dislocation movement is decreasing leading to the fraction of un DRXed regions decreasing.In addition,with the size of nano-scale W phase precipitates increasing,the density of precipitations is decreasing,and the distances of precipitations are increasing leading to lower strength and higher ductility.In this study,an ultrahigh strength Mg-10.3Zn-6.4Y-0.4Zr-0.5Ca(w.t.%)alloy with medium RE content is developed by permanent mold direct-chill casting and traditional extrusion process.And the strengthening and toughning mechanisms are further investigated and discussed.The as-extruded Mg-10.3Zn-6.4Y-0.4Zr-0.5Ca(w.t.%)alloy exhibits ?b of 466 MPa,?0.2 of 447 MPa and ? of 4.7%.These properties are approach to those of the T8-treated 2024 aluminum alloy.The yield strength of the as-extruded Mg-10.3Zn-6.4Y-0.4Zr-0.5Ca(w.t.%)alloy is even higher than many as-extruded Mg-Gd-Y-Zn-Zr alloys with RE content greater than 12 w.t.%.The ultrahigh strength mainly attributed to fine DRXed grains with fine broken W phase particles at the DRXed grain boundaries and nano-scale W phase and small amount of ?(17)' phase dynamically precipitated in the un-DRXed regions.Fine W phase is an effective strengthening phase in Mg alloys.