Effect Of Gd Addition And Extrusion Process On Microstructure And Mechanical Properties Of Mg-Al-Si-Gd Alloy

In this work,Mg-4Al-1Si-xGd?x=0,1and 2?alloys were prepared.The effect of different Gd contents on the microstructure and mechanical properties of Mg-4Al-1Si-xGd?x=0,1and 2?alloy was studied.The refinement effect and mechanism of rare earth Gd on Mg₂Si phase were further explored.Hot extrusion of solid solution Mg-4Al-1Si-xGd?x=0,1and 2?alloys was carried out to study the relationship between the microstructure evolution and mechanical properties of alloy and the content of Gd.The alloy with homogeneous microstructure and highest plasticity after extrusion was selected for two passes of equal channel angular pressing.The microstructural evolution of Mg-Al-Si-Gd alloy during combined deformation was investigated,and the mechanical properties of Mg-Al-Si-Gd alloy were analyzed by quantitative calculation.The effect of the combined deformation on the size and volume fraction of precipitated phase was clarified.The strengthening mechanisms of the precipitates with various size and broken Mg₂Si in extruded magnesium alloys were revealed.The results show that a new granular Si₃Gd₅ phase can be formed by introducing rare earth Gd,which effectively modifies the size,shape and distribution of the second phase.The morphology of Mg₂Si phase was changed from large fish bone to short rod,and a small amount of island-like Mg₁₇Al₁₂phases were uniformly distributed in the matrix.However,there was not new phase containing Al and Gd formed,and the addition of Gd did not change the volume fraction of Mg₁₇Al₁₂ phase in the alloy according to the measurement results.With the increase of Gd content,larger Si₃Gd₅ phases were observed in the alloy.Compared with Mg-4Al-1Si alloy,adding a certain amount of Gd can effectively improve the ultimate tensile strength and elongation of the alloy,but the yield strength can be decreased.With 1 wt%Gd,the comprehensive mechanical properties of the alloy can be the best.The ultimate tensile strength,the yield strength and the elongation were 119.7 MPa,62.3 MPa and 5.5%,respectively.The extruded Mg-4Al-1Si-xGd?x=0,1and 2?alloy underwent complete dynamic recrystallization and finer grains were obtained.At the same time,Mg₂Si particles were broken and distributed in the form of strips along the extrusion direction,but no large amount of Mg₁₇Al₁₂ precipitates were observed.With the increase of Gd content,the average grain size increased and the volume fraction of Mg₂Si phase was reduced.The ultimate tensile strength and elongation of extruded Mg-4Al-1Si-xGd alloy were improved first and then decreased,while the yield strength decreased continuously.With 1 wt%Gd,the extruded Mg-4Al-1Si-1Gd alloy exhibited the best comprehensive mechanical properties.The ultimate tensile strength,the yield strength and the elongation were 263.1 MPa,180.6 MPa and 25.1%,respectively.According to the fracture analysis,the size,distribution and volume fraction of the second phase had a great influence on the strength and plasticity of the alloy.The relatively poor plasticity of extruded Mg-4Al-1Si alloy can be due to the local agglomeration of fragmented Mg₂Si phases,which facilitated the initiation and propagation of cracks.The combinational deformation including hot extrusion and equal channel angular pressing can further refine the grains of extruded Mg-4Al-1Si-1Gd alloy and weakened the banded distribution of the second phase by the combination of equal channel angular pressing.With decreasing the ECAP temperature,the grain size of the alloy was further refined,and the average grain size was reduced from4.68?m to 2.20?m.In addition,a lot of Mg₁₇Al₁₂ particles were precipitated in the matrix after two ECAP passes at 200 C.The average size of Mg₁₇Al₁₂ particles was only 0.43?m and the volume fraction was 2.33%.The optimum mechanical properties were obtained.The tensile strength was 293.8 MPa,the yield strength was 241.5 MPa,and the elongation reached 30.1%.Compared with extruded Mg-4Al-1Si-1Gd alloy,the alloy subjected to combined deformation exhibited higher strength,which can be attributed to grain boundary strengthening caused by finer grains and Orowan strengthening induced by fine precipitates.Microscale voids near Mg₂Si phase were observed in all alloys.To some extent,it was proved that the fracture of the alloys was mainly due to the difference of deformation ability between Mg₂Si phase and matrix,which resulted in stress concentration and crack formation,leading to the fracture of the alloys.And the microcracks in incompletely broken Mg₂Si phase were also one of the factors leading to the fracture of the alloy.