| As the lightest metal structural material,magnesium alloy has the advantages of low density,high specific strength and stiffness,good damping performance and easy processing,and is widely used in aerospace,transportation and other fields.The failure mode of metal components is fatigue failure under cyclic load,and the surface quality is often the main cause of fatigue failure.The surface modification can effectively improve the fatigue resistance of magnesium alloy components.Based on this,agestrengthened Mg-Y-Nd-Gd-Zr alloy was selected as the research object in this paper.Tension-pressure high-cycle fatigue tests(R=-1)were carried out on three kinds of magnesium alloys in different states: solid solution,solid solution + ultrasonic rolling(USRP)and solid solution + ultrasonic rolling and re-aging.Fatigue behavior and life extension mechanism of Mg-Y-Nd-Gd-Zr alloy with gradient structure were analyzed.The main conclusions are as follows:The surface integrity of Mg-Y-Nd-Gd-Zr alloy was best improved by ultrasonic rolling at 0.09 MPa.A gradient layer of about 818μm depth was formed on the surface,which was composed of large plastic deformation zone,transition zone and gradient twin zone.The residual stress after surface modification presented a " √ " type distribution.The maximum residual compressive stress(296MPa)occurred in the subsurface layer(96μm from the top layer).The fatigue limit of gradient Mg-Y-Nd-GdZr alloy constructed by ultrasonic surface rolling is 97.3 MPa,which is about 21.6%higher than that of solid solution magnesium alloy(80MPa).Both of them are caused by crystal slip caused by shear stress,which leads to crack nucleation.The characteristics of fatigue bands indicate that the gradient structure has no obvious effect on crack propagation.On the contrary,the main crack propagation rate is higher than that of solid-solution magnesium alloys.It is indicated that the life extension of crack initiation and microcrack propagation is the main reason for the fatigue life extension of graded magnesium alloys.After ultrasonic rolling + aging treatment,the fatigue limit of magnesium alloy increases to 135 MPa,which is 37.7MPa,about 38.7% higher than that of the sample in rolling state.During the aging process,recovery recrystallization occurs in the surface layer,which releases part of the residual stress and makes the fatigue crack initiation zone shift from the inside to the secondary surface of the sample.The strength of gradient hardening layer enhanced by precipitated phase strengthening is the main reason for the increase of fatigue limit. |
