Effect Of Heat Treatment Process On Microstructure And Properties Of Mg-Y-Sm-Zn-Zr Alloy

As the lightest engineering structural materials,magnesium alloys are known as green materials of the 21st century because of their low density,high specific strength,and good vibration damping properties.The strength and creep resistance of magnesium alloys will be greatly reduced with the increases of temperature.This limits the widely application in national defense and automobile industry.Rare earth?RE?is one of the most effective elements for improving the heat resistance of magnesium alloys.Without increasing the rare earth content,heat treatment strengthening is an effective way to further improve the mechanical properties of rare earth magnesium alloys.The development of high-strength and heat-resistant RE-containing magnesium alloys is a research hotspot in recent years.On the basis of the Mg-Y-Sm-Zn-Zr alloy with independent intellectual property,the Mg-3.4Y-3.6Sm-2.6Zn-0.8Zr alloy was prepared and the influence of heat treatment on microstructures and properties were studied.Optical microscopy?OM?,X-ray diffraction?XRD?,scanning electron microscopy?SEM?and room-temperature tensile tests were used to examine the microstructure and mechanical properties of the as-cast and heat-treated alloys.The microstructure evolution,fracture mechanism and strengthening mechanism of the alloys were investigated.And the optimal heat treatment condition for the alloy was determined.The main conclusions are as follows.For as-cast alloy,the grain of?-Mg is fine and equiaxed,and the average grain size is 21?m.The main phases in as-cast alloy are?-Mg,?Mg,Zn?₃?Y,Sm?,Mg₁₂?Y,Sm?Zn and lamellar long period stacking ordered?LPSO?structure.By DSC thermal analysis,the melting point of the eutectic phase?Mg,Zn?₃?Y,Sm?and?-Mg are 502.7? and615.5?.The yield strength,tensile strength and elongation of the as-cast alloy at room temperature are 122.3MPa,191.5MPa and 5.7%,respectively and the fracture behavior is intergranular quasi-cleavage brittle fracture.After solution treatment at 500?×15h?hot water quenching?,the grain size does not grow significantly.The phase composition remains unchanged,while the morphology of second phase shows some differences.The?Mg,Zn?₃?Y,Sm?phase in eutectic transforms to spherical structure,and Mg₁₂?Y,Sm?Zn phase transforms to long strip.Moreover,lamellar LPSO structure is precipitated within the grain as the cooling rate slows down.The optimum yield strength,tensile strength and elongation of hot water quenching?80??are 119.4MPa,245.3MPa and 21.4%,respectively.The fracture behavior of the solid solution alloy is diverted from intergranular brittle fracture to transgranular quasi-cleavage fracture.The main strengthening method is phase transformation strengthening.After aging at 225? for 40 h?air cooling?,the alloy reaches a peak age of 78.55 HV,and the yield strength,tensile strength,and elongation at room temperature reach 170.0MPa,260.8MPa,and 14.1%,respectively and the main strengthening method is precipitation strengthening.The optimum heat treatment process of Mg-3.4Y-3.6Sm-2.6Zn-0.8Zr alloy is500?×15h?hot water quenching?+225?×40h?air cooling?.The solution and aging treatment significantly improve the mechanical properties of the alloy.Comparing with as-cast alloy,the yield strength and tensile strength of the heat-treated alloys increase by39%and 36%,respectively.