Study On Fracture Behavior Of Sand-cast Mg–6Gd–3Y Magnesium Alloys

As a promising structural material with excellent properties,such as high specific strength,stiffness and good machinability,Mg-Gd-Y-Zr magnesium alloys have been applied widely in aerospace industry in recent years.The aerospace casting usually experience complicated load in service environment,such as tensile,impact and cyclic loading.It is generally believed that safety and reliability are basic requirements for structural application.Therefore,current work aims to optimize firstly the heat treatment（solid solution and aging treatment）of sand-cast Mg-6Gd-3Y-0.5Zr magnesium alloy,and then investigate the influence of heat treatment on impact toughness,high cycle fatigue and plane-strain fracture toughness of the tested alloy.The results showed that optimum heat treatment is solutionized at 490°C for 12 h,and then aged at 212°C for 100 h.Tensile tests exhibited that as-quenched GW63 alloy shows high elongation but low tensile strength,nevertheless,aged alloy shows higher strength but worse ductility.Sand-cast GW63 alloy exhibited quasi-cleavage fracture during tensile testing in as-cast,as-quenched and peak-aging state,which were controlled by twinning during tensile deformation.Room temperature impact toughness of sand-cast GW63 alloy were 34.6,50.9 and 20.3J/cm^-2 in the as-cast,as-quenched and aged states,respectively.The impact toughness increased from 17.1 J·cm^-2 to 58 J·cm^-2 with increasing temperature between 200 and 250°C.Impact toughness was more closely related to material ductility than strength for the studied alloy.As-cast and as-quenched alloy demonstrated quasi-cleavage fracture pattern.In comparison,the aged fracture surface displayed a typical of cleavage failure pattern.GW63alloy deformed by twinning during impact testing in all thermal conditions.The high cycle fatigue strength of sand-cast GW63 alloy were 105 and 120MPa in the as-cast and aged states,respectively.For smooth specimen,fatigue crack propagated by brittle fatigue striation in the as-cast alloy,however,small fatigue arc and ductile fatigue striation were fractured characteristics of crack propagation region in the aged alloy.The small ductile deformation were yielded by twinning and dislocation slip in the as-cast and aged states,respectively.The notched fatigue strength were 25 and 55MPa in the as-cast and aged states when notched factor was 2.3,but 35 and 60Mpa when notched factor was2.7.There were loop propagation region and round collapse fracture region on the notched fatigue specimens.The fatigue propagated by cleavage pattern in as-cast notched specimen.There were two propagation stages in as-aged notched specimen,i.e.first stage was cleavage pattern and second stage was fatigue striation.The plane-strain fracture toughness values of the as-cast,as-quenched and aged alloy were 16.2,17.7 and 19.5 MPa m?,respectively,i.e.,the improvement of 20.4%has been achieved by aging precipitation strengthening in contrast with slight improvement of 9.3%by solid solution strengthening.Moreover,fracture toughness specimens exhibited transgranular fracture pattern with the help of existing fatigue crack in as-cast,as-quenched and aged states.