High Cycle Fatigue Behaviors Of Mg-Zn-Y-Zr Alloys

As the lightest metal materials,magnesium alloys have been widely applied to transportation,aerospace equipment and so on.At present,due to the low strength,magnesium alloys are likely to manufacture components that bear relatively low load.However,important loaded parts are easy to cause fatigue failure under alternating load,it is necessary to develop a kind of magnesium alloys with high strength and high fatigue properties.The present study was aimed at Mg-2.3Zn-0.18Y alloys manufactured by our laboratory.Adding different Zr contents(0.03%,0.06%,0.13%at.%)to Mg-2.3Zn-0.18Y alloys and then extruding them were to obtain excellent properties.Firstly,study was put on fatigue behaviors of Mg-2.3Zn-0.18Y-xZr alloys.Secondly,taking the Mg-2.3Zn-0.18Y-0.13Zr alloys with the best fatigue properties for example,it was possible to improve their fatigue properties by ageing treatment and shot peening,respectively.At last,making an attempt to explain the fatigue behaviors and strengthening mechanisms of magnesium alloys after aforementioned treatment.The conclusions are the following six aspects:(1)With increasing Zr contents,the average grain sizes decreased,ultimate strength,yield strength and elongation of Mg-2.3Zn-0.18Y-xZr(x=0.03?0.06?0.13 at.%)alloys were improved a lot.Mg-2.3Zn-0.18Y-0.13Zr alloys reached up to 346MPa,292MPa and 26.7%,respectively.At the same time,its cycle fatigue strength was 150MPa,which was the best among all alloys.In addition,it was revealed that twins were formed in coarse grains from the post-fatigued microstructures of Mg-2.3Zn-0.18Y-0.03Zr alloys,which suggested twining deformation took place in the alloys.(2)During the high cycle experiments of as-extruded Mg-2.3Zn-0.18Y-xZr alloys,crack initiation region had only one site under high stress amplitude(180MPa)while had several sites under low stress amplitude(160MPa).Cracks usually initiated on surfaces of samples or secondary phases near surfaces,and propagated along cleavage planes.It is typical of transcrystalline rupture.(3)The appropriate technological parameters of ageing treatment were 180°C,12h.After ageing treatment,the ultimate strength,yield strength and elongation of Mg-2.3Zn-0.18Y-0.13Zr alloys were 353±4MPa?323±4MPa ? 20.3%±2.0%.(4)Fatigue strength of under-aged Mg-2.3Zn-0.18Y-0.13Zr alloys was 160MPa,increased by 6.7%,It was MgZn2 phases precipitated during ageing treatment that inhibited the dislocation slip in basal plane,and the critical stress of crack initiation was raised to higher extent,which extended fatigue life.(5)The appropriate air pressure was 0.4MPa.The Mg-2.3Zn-0.18Y-0.13Zr alloys were sensitive to shot peening and the range of proper parameters was narrow.(6)Compared to as-extruded alloys,the fatigue strength of Mg-2.3Zn-0.18Y-0.13Zr alloys under the appropriate shot peening conditions were 173MPa,had an improvement of 22.5MPa,increased by 15%.The crack initiation region was transferred from surface to sub-surface,and the grain sizes of deformation layers were finer.The time of crack initiation became longer,thus prolonging the fatigue life.