Microstructure And Creep Resistance Of High Strength Mg-Gd-Y-Zr Alloy

Mg-Gd magnesium alloys have been a hot research topic at home and abroad due to their excellent mechanical properties at room and high temperature.In recent years,our research group has carried out a lot of research on Mg-Gd system multi-element rare earth magnesium alloy.It is found that there is abnormal temperature phenomenon of tensile strength in Mg-Gd-Y(Sm)-Zr alloys,and its generation law and mechanism are systematically and deeply studied.On this basis,this paper takes aged Mg-xGd-2Y-0.5Zr(x=9,11,13,15,wt.%)magnesium alloys with abnormal temperature effect on tensile strength as the research object.The microstructure,tensile mechanical properties and tensile creep behavior of the alloy with different Gd content were systematically studied.The creep behavior of the alloys was mainly studied.The evolution of matrix precipitates,grain boundary precipitates and grain boundary morphology of the representative alloy with 13%Gd content in the whole process of high temperature,high stress and long-time creep(200 h)was observed and analyzed in detail.The microstructure evolution process and creep mechanism of the alloy with abnormal temperature effect of tensile strength were clarified.The microstructure of as-cast and aged Mg-xGd-2Y-0.5Zr alloys is mainly composed of?-Mg matrix,Mg₅Gd and Mg₂₄Y₅phases.With the increase of Gd content,the grain is refined.When the Gd content is 13%,the grain is the smallest and the second phase is the most uniform.The tensile mechanical properties of aged Mg-xGd-2Y-0.5Zr alloys from room temperature to 300?were studied.The results show that the tensile strength first increases and then decreases with the increase of tensile temperature for the same alloy,and reaches the maximum value at 250?.The four alloys have obvious abnormal temperature effect of tensile strength.At the same tensile temperature,with the increase of Gd content,the tensile strength first increases and then decreases.When the content of Gd is 13%,the tensile strength reaches the maximum.The 13%Gd alloy has excellent mechanical properties at room temperature and high temperature.The main strengthening mechanisms of the alloy are fine grain strengthening,coherent strengthening of nano petal like?'phase and matrix,and grain boundary strengthening of nano elliptical?'phase.The results of the creep study show that the creep life of aged alloy with 13%Gd content under the conditions of 200-250?/110-150 MPa and 270?/110 MPa is more than 100 h,and the maximum steady-state creep rate is only 3.28×10^-9s^-1.It still shows excellent creep resistance at high temperature and high stress.The continuous dynamic observation of precipitates in the creep process of the alloy with13%Gd content during creep at 250?/150 MPa for 200h shows that the evolution sequence of precipitates in the matrix is mainly?'phase??'+?₁??'+?₁+?+18R-LPSO.With the increase of creep time,the nano petal like?'phase grows slowly(within 75 h),and then the?'phase shrinks in the long axis direction and necks in the middle,and transforms into small sheet?₁phase,and?'phase transforms into ellipsoid shape,and the two phases alternately appear in a string-like of uniform distribution.The string-like precipitates are connected to form a network.When the creep time is 200 h,the string-like precipitates still exist and are the main morphology.In addition,there are a small amount of needle like 18R-LPSO phase and flake?transformed from?₁phase,and the size is mainly at the nanometer level.At this time,the alloy is still in the steady-state creep stage.The dense grid of small modules formed by the interconnection of the string-like precipitates will have a huge hindrance to the movement of dislocations.At the same time,?'phase and 18R-LPSO phase have excellent high-temperature thermal stability and are coherent with the matrix,which is conducive to improve the high-temperature strength and creep resistance of the alloy.Therefore,in a certain temperature range,the alloy not only has higher tensile strength,but also shows excellent resistance creep properties.The dynamic observation and analysis of grain boundary morphology and grain boundary precipitates of the alloy with 13%Gd content during creep at 250?/150 MPa for 200 h show that the morphology and width of the grain boundary remain relatively stable within 100 h.With the increase of creep time,the straight grain boundaries bend and widen.However,within 25 h,the precipitated phase of the grain boundary has rapidly changed from the?'phase before the creep to the?phase.With the increase of the creep time,the?phase gradually grows up,and promotes the formation of precipitation free zone on the two sides of the grain boundary precipitate.The research on the creep fracture behavior of the alloy with 13%Gd content at 270?shows that the creep fracture of the alloy is mainly brittle fracture and intergranular fracture.Creep cavities are preferentially formed at the interface between grain boundary precipitates and precipitation free zone,which is the main reason for the creep fracture failure of the alloy at 270?.