Microstructure And Mechanical Properties Of Mg-4Y-1Ca-xZn(x=0,1,3,5wt.%) Alloys In Different Process States

On the basis of Mg-4Y-1Ca,four experimental alloys with different Zn contents were obtained by semi-continuous casting,namely Mg-4Y-1Ca,Mg-4Y-1Ca-1Zn,Mg-4Y-1Ca-3Zn,Mg-4Y-1Ca-5Zn.After the appropriate heat treatment and hot extrusion of the as-cast alloy,the microstructure and mechanical properties of the as-cast,extruded and heat-treated alloys were investigated by methods of metallographic observation,SEM,XRD,EDS,stretching or compression.The conclusions are as follows:?1?The as-cast microstructures of the four alloys are consisted of magnesium matrix and eutectic structure at the grain boundaries.The grain sizes of the alloys are 88.1?m,64.9?m,58.1?m,and 49.9?m,respectively.The Mg₂₄Y₅ in the matrix alloy is distributed in spots.In alloys containing 1wt.%and 3wt.%Zn,the X phase is distributed in a gray network along the grain boundary and the white flake-like W phase+Mg₂₄Y₅ phase or W phase is distributed at the X phase boundary.Containing 5wt.%Zn alloy,W phase is white bright network and arachnoid distribution.The compression properties of as-cast alloys are significant better than their respective tensile properties.With the increase of Zn content,the grain refinement and the increase of X phase result that the tensile mechanical properties of the alloy are continuously improved;the compressive strength of the alloy is firstly increased and then decreased.The four as-cast alloys mainly.have brittle fracture.?2?After the solution treatment of the as-cast alloy,the second phase decomposed and the number of the second phase is significantly reduced;the grain size is not changed obviously.After the aging treatment,the four alloys have age hardening,and the second phase with the highest hardness precipitates in the alloys,meanwhile,the grains become uniformly equiaxed grains.After heat treatment,the strength and elongation of the four as-cast alloys does not improve effectively.Therefore,these alloys remain brittle.?3?The Mg₂Ca phase in the as-extruded alloy is spherical on the matrix.All of Mg₂4Y₅in the matrix is dissolved in the magnesium matrix.In extruded direction,the Mg₁₂YZn?X phase?of Mg-4Y-1Ca-xZn?1 wt.%,3.wt%?is broken and shows long strip shape,and Mg₃Y₂Zn₃?W phase?is distributed in fine particle shape.Mg₃Y₂Zn₃?W phase?in Mg-4Y-1Ca-5Zn is broken into big bulk particles and disperse distribution.Because the grains are became refinement and the second phase morphology is changed after hot extrusion treatment,so the comprehensive mechanical properties of the alloy are greatly improved.In addition,the compression properties of the alloy are significantly higher than the tensile properties.The compression in the extrusion direction increases with the addition of Zn element;the compression ratio decreases with the addition of Zn.Compressed perpendicular to the extrusion direction,the strength is significantly higher than the extrusion direction but the compression ratio is lower than the extrusion direction.