| Mg-9Gd-3Y alloy is the most deeply studied among rare earth magnesium alloys in our research group.Due to the characteristics of high strength,high temperature resistance,corrosion resistance and low density.It has huge advantages and potentials in the automotive industry,communications electronics industry,aerospace and military industries.It is well known that the properties of any material depend on its microstructure and structure.At present,Mg-9Gd-3Y alloy generally have poor casting performance during the casting process.The results show that the melt ultrasonic treatment can improve the second morphology,grain refinement and degassing and slag removal.With the inevitable trend of lightweight,energy saving and environmental protection development of the aerospace field and automotive industries,more and more rare earth magnesium alloy components are being applied,and some of these components are subjected to cyclic loading at high stress levels during work.Therefore,it is not only of theoretical significance to study the effect of melt ultrasonic treatment on the microstructure and low-cycle fatigue behavior of rare earth magnesium alloys,but also has certain practical value.Mg-9Gd-3Y（GW93）alloy is studied in this paper.GW93 bulk samples（ultrasonic power0W,700W,1000W,1500W,ultrasonic time 60s and ultrasonic time 0s,30s,60s,90s,ultrasonic power 1000W）were prepared by ultrasonic melt treatment and the solution casting method.The microstructure and phase composition were analyzed by metalloscope（OM）,X-ray diffractometer（XRD）,energy depressive spectroscopy（EDS）,transmission electron microscopy（TEM）and diffraction pattern,and the brinell hardness and tensile properties were tested by electronic brinell hardness machine and electronic universal testing machine.At the some time,the effect of mechanism of fatigue fracture was discussed.The main conclusions are summarized as follows:The grain of GW93 alloy after melt ultrasonic treatment is obviously refined,and the structure distribution is more uniform.The as-casting GW93 alloy and GW93-T6 alloy with ultrasonic power of 1000W,ultrasonic time of 60s have the greatest degree of grain refinement,and their average grain size is respectively 14μm and 15μm,which significantly decreases78.46%and 87.07%than that without ultrasonic treatment.Ultrasonic treatment does not change the phase composition of GW93 magnesium alloy.The as-casting GW93 alloy is composed of matrixα-Mg,Mg5（Gd,Y,Zn）,Mg24Y5,18R-LPSO structure and rare earth-rich phase.GW93-T6 magnesium alloy is composed of matrixα-Mg,β’（Mg,Zn）5Gd,Mg24Y5and a small amount of rare earth-rich block phase.The thermal stable phase 18R-LPSO structure will decompose when the holding time is more than 12 hours at 500℃.The mechanical properties of GW93 alloy are significantly improved after melt ultrasonic treatment;As the ultrasonic power or ultrasonic time increases,the hardness and tensile properties of GW93 alloy were increased significantly and then decreased.The mechanical properties of GW93-T6 alloy with ultrasonic power 1000W and ultrasonic time 60s are the best.Its hardness,tensile strength,yield strength,and elongation are 106.74HBW,360MPa,266MPa,and 5.07%,which are increased by 17.59%,18.03%,18.22%,262.14%than the without ultrasonic treatment.The GW93-T6 alloy with ultrasonic power 1500W and ultrasonic time 60s tends to decrease in hardness and room temperature tensile properties.Its hardness,tensile strength,yield strength and elongation are 104.55HBW,355MPa,260Mpa,3.21%.And the GW93-T6 alloy with ultrasonic time 90s and ultrasonic power 1000W also tended to decrease in hardness and tensile properties at room temperature.Its hardness,tensile strength,yield strength and elongation were 97.8HBW,335MPa,231MPa,2.98%,respectively.The GW93-T6 alloy with ultrasonic power 1000W and ultrasonic time 60s show obvious cyclic hardening in different applied total strain amplitudes.Plastic deformation is not observed during low-cycle fatigue deformation with the total strain amplitudes less than 03%.With the increase of ultrasonic power,the cycle life of GW93-T6 alloy first increased and then decreased,the quasicleavage on the fatigue fracture was gradually changed into dimples and fatigue bands.The GW93-T6 alloy with ultrasonic power 1000W and ultrasonic time 60s shows high cycle life under different conditions of total applied strain amplitude,and have in a stable state for a long time.The fatigue crack initiates of GW93 alloy from the surface of the specimen.The optimum ultrasonic parameters of GW93 alloy is ultrasonic power 1000W and ultrasonic time 60s. |
