Mg-9Gd-8Y-2Nd-1.2Zr Magnesium Alloy Extrusion Process And Research On Microstructure And Properities

Current study has found that the addition of rare earth elements is one of theimportant ways to strengthen the traditional magnesium. However, the study about theimpact of the combination of rare earth elements on microstructure and mechanicalproperties of the magnesium alloy is still incomplete. The light and heavy RE arecombined to improve the effect of aging precipitation. However, few studies on the heatprocess of this complex Mg-RE alloy system and how to optimize the thermal processparameters to obtain the excellent microstructure and mechanical property are found. Inaddition, the assessment about heat and creep resistance in high temperatureenvironment of the Mg-RE alloy is relatively less.The cast microstructure of the Mg-9Gd-8Y-2Nd-1.2Zr (referred GWN982K) alloyis indicated in this paper firstly. Then the homogenization treatment of GWN982K alloyis studied and the optimal process parameters are obtained. The effects of differentextrusion temperature and extrusion ratio on the microstructure evolution are discussedin detail and the relationship of microstructure and the mechanical properties at roomand high temperature is established. Then the effects of microstructure evolution andprecipitation on the mechanical properties during aging process are studied. Finally,creep tests are conducted on aged GWN982K alloy with different stress at250℃. Withthe steady-state creep rate and stress exponent the creep mechanism is determined.Results showed that: the yield strength, tensile strength and elongation of castGWN982K alloy are112MPa,149.31MPa and1.56%, which is ascribed to the coarseeutectic phase. The best homogenization process is a two-stage520℃*8h+560℃*6h.The tensile strength is increased by23.57MPa compared to the cast GWN982K alloy,the elongation rate is increased to2.02%.The GWN982K alloy can obtain a highermechanical properties than the room temperature at250℃, due to the rapid precipitationof β’ phase in250℃tensile process and the obstacle on dislocation motion.The extruded GWN982K alloy is mainly consist of the elongated deformed grainsand tiny recrystallized grains (1-3um) as well as crushed secondary phases. Thesecondary phases in extruded alloy is tiny β’ phase and RE-rich cubical phase. β’ phasesare mainly distributed in the deformed grains or fine grain zone, withMg7(Gd0.43Y0.28Nd0.29).<10-10> direction in the deformed grains is parallel to theextrusion direction(ED), showing the typical orientation feature of basal texture. Theextruded microstructure conducted with the extrusion ratio of9and350℃shows a newrecrystallization texture with c-axis parallel to ED. When the extrusion deformation issmall(extrusion ratio9), full dynamic recrystallization can occur in the extrudedmicrostructure and plasticity can be improved with the extrusion temperature of400℃which is in favor of subsequent deformation process.The yield strength, tensile strength and elongation are189.57MPa,283.74MPa and16.02%, respectively. When theextrusion temperature is400℃, the amount of fine grains and precipitates between thelong strip deformation grains in the microstructure added,with the increase of theextrusion ratio.The alloy with extrusion ratio of25obtained the highest yield and tensilestrength of205.24MPa and303.81MPa and the elongation of12.80%.Furthermore, itcan maintain at the temperature range of150-250℃, with the strength above275.09Mpa.The highest tensile strength is310.37Mpa at200℃,which is attributed tothe increasing precipitation of β’ phase and unchanged grain size.Fine grainstrengthening and the strengthening of β’ phase to basal slip are the main contribution.Lamellar β’ phases in the aged (200℃-115h) alloy coarsened, and distributed withthe orientation of60°angle or parallel to each other, indicating that β’phases have threevariants.The fullest static recrystallization occurs in the peak-aged (225℃-29h)GWN982K alloy, with the average grain size of8um, and β’ phases also grow accordingto certain orientation, but the microstructure is uniformed with the highest mechanicalproperties at RT: yield strength, tensile strength and elongation were261.62MPa,360.65MPa and7.59%. Creep at the temperature250℃and the stress ranging from40MPa to100MPa, the steady-state creep rate increases slowly from3.92E-09to2.27E-08, insensitive to stress, with excellent creep resistance. And the creep stressexponent is1.92, with grain boundary sliding as the main creep mechanism at250℃.