Effect Of Solid Solution And Extrusion On Microstructure And Mechanical Properties Of Mg-9Gd-xAl Alloy

In this paper,Mg-9Gd-xAl alloy was prepared by high energy ultrasonic stirring casting method,and it was solution treated and extruded after casting.The microstructure evolution and mechanical properties of as-cast,solid solution and extruded Mg-9Gd-xAl alloys were systematically investigated by OM,SEM,XRD,TEM,Vickers hardness and room temperature tensile tests.The composition and forming conditions and mechanism of the lamellar phase formed inside the grains during the solution treatment was explored.The evolutional mechanism of the lamellar phase and other second phases during the extrusion process were revealed.The mechanisms of grain refinement and strength of the alloy matrix were explored.The main research results are as follows.The grain size of as-cast Mg-9Gd-xAl alloy decreases with the increase of Al content.At the same time,the distribution and morphology of the second phase in the alloy was changed by the Al element additions.Mg₅Gd phase changes from a short rod to a network as the Al content increases.Mg-Al-Gd phase changes from granular to fishbone,and then transforms into a petal shape.The size of granular Al₂Gd phase distributed in the central region of the matrix grains continuously increases.The alloy has the best comprehensive mechanical properties when the Al content reaches 0.8 wt.%,and its tensile strength reaches 135MPa and the elongation reaches 14.5%.During the solution treatment process of Mg-9Gd-xAl alloy,it was found that the Al₂Gd phase could not dissolve while the Mg₅Gd phase and Mg-Al-Gd phase disappeared with the increase of solution time.In addition,the lamellar phase appeared only when the Al content was less than 1 wt.%,which was resided inside the crystal grains,and the inner alignment directions of the different crystal grains were inconsistent.The lamellar phase consists of Mg,Al and Gd elements and the atomic ratio is close to 1:1:1,and it dissolves in the matrix with the prolongation of solution treatment time.It is found that the lamellar phase is transition phase of the Mg-Al-Gd phase.Since the lamellar phase provides excellent second phase strengthening effect,Mg-9Gd-0.4Al,Mg-9Gd-0.6Al,and Mg-9Gd-0.8Al alloys have higher strength.CoMPared with the as-cast strengths,solid solution treatment increase the strengths of the alloys by65 MPa,72 MPa and 60 MPa,respectively.After extrusion deformation,the grain of Mg-9Gd-xAl alloy matrix is remarkably refined and the degree of grain refinement increases as the Al content increases.The average grain size was reduced from 5.13?m of Mg-9Gd-0.4Al alloy to 2.66?m of Mg-9Gd-0.8Al alloy.In addition,the Al₂Gd phase was not fragmented during the extrusion process,the distribution was slightly improved.Mg-Al-Gd phase was significantly fragmented and distributed more homogenously,but the granular Mg-Al-Gd phase still agglomerates with Al content increases.The lamellar phase was fragmented and evenly distributed at the grain boundary.The grain size was refined by the two effect:on the on hand,it is the local deformation and strain gradient of the matrix caused by the larger Mg-Al-Gd phase and Al₂Gd phase,which promotes the recrystallization grain nucleation.On the other hand,the lamellar phase hinders the grain boundary motion during the grain growth of the recrystallization boundary.Mg-9Gd-0.8Al alloy has good mechanical properties after extrusion deformation,the yield strength reached 296.2MPa,the tensile strength reached 342 MPa,and the elongation reached 23.6%,which are determined by the synergistic effect of fine grain strengthening and the Orowan strengthening.