| Magnesium alloys have received an increasing interest in the past two decades for potential applications in the automotive,aircraft,aerospace,and electronic industries.The strength of these magnesium alloys was improved by an age-hardening process.Mg-RE alloys have been increasingly attracted because of their good aging strengthening effect.Other alloying element additions can improve the strength of magnesium alloy in Mg-Nd,Mg-Gd and Mg-Y alloy,but the precipitated phases during aging process have different morphology,structure and other features.Therefore,it is necessary to study the effect of alloy element additons on the precipitation behaviors of Mg-RE alloys.However,the aging precipitation sequence and early aging precipitation mechanism in Mg-RE(Nd,Gd)binary alloys is not clear,and the characteristics of precipitated phases still need more research.Therefore,in this paper,Mg-xNd(x=0.70,1.09,1.92wt.%)alloys,Mg-yGd(y=4.25,6.59,10.7wt%)alloys,Mg-zY(z=3.07,4.94,9.17wt%)alloys have been prepared by indirect hot extrusion and subsequent solid solution treatment at 540? for 16h,and aginng treatment at 180?,200?,220?.And then the aging precipitation behaviors of Mg-1.92Nd alloy aging at 200?,Mg-10.7Gd alloy aging at 180?,Mg-9.17Y alloy aging at 180? have been investigated.The results are as followed:After the Mg-xNd(x=0.70,1.09,1.92wt.%)alloys,Mg-yGd(y=4.25,6.59,10.7wt%)alloys,Mg-zY(z=3.07,4.94,9.17wt%)alloys were hot extruded at 300?,the matrix grains appear equiaxed due to the complete dynamic recrystallization.The grain sizes of Mg-xNd(x=0.70,1.09,1.92wt.%)alloys are 11 ?m,9?m 8?m,and their hardness are 43.7HV,45.8HV,51.4HV respectively.The grain sizes of Mg-yGd(y-4.25,6.59,10.7wt%)alloys are 11?m,10?m,9?m,and their hardness are 52.4HV,67.3HV,81.2HV respectively.The grain sizes of Mg-zY(z=3.07,4.94,9.17wt%)alloys are 16?m,14?m,12p.m,and their hardness are 50.7HV,60.4HV,91.3HV respectively.The optimal solid solution treatment of Mg-RE(Nd,Gd,Y)alloys is 540?/16h.The grain sizes grow significantly after solution treatment and the alloys hardness declines.The grain sizes of Mg-xNd(x=0.70,1.09,1.92wt.%)alloys,are 204?m,181 ?m,139?m,and their hardness are 31.75HV,35.85HV,41.45HV respectively.The grain sizes of Mg-yGd(y=4.25,6.59,10.7wt%)alloys are 94?m,75pm,50?m,and their hardness are 60.57HV,64.52HV,66.7HV respectively.The grain sizes of Mg-zY(z=3.07,4.94,9.17wt%)alloys are 118?m,111 ?m,91 ?m,and their hardness are 43.93HV,44.63HV,45.00HV,respectively.When Mg-1.92Nd alloy aged at 200? for 2.5h,there is a kind of needle-like phase(10-25nmx2nm)precipitated along[1102]Mg.After aging for 7h,some ?' phase with 0.3-0.5?m×50nm lenticular morphology is precipitated on Mg matrix.A few of short strip phase is precipitated at the same time.The hardness of Mg-1.92Nd alloy reaches maximum when aged at 200? for 10h.At this time,the short strip grow to 20-40nm,and there are many disc-shaped ?1 phases dispersed on Mg matrix.The ?1 phase have FCC crystal structure(a=0.74nm),and is the main strengthening phase.With the extension of aging time to 35h,the ?1 phases disappeared,and the lenticular ? phase with 100?200nm length precipitated.The ? phase have tetragonal crystal structure(?=1.03nm,c=0.59nm),and the orientation relationship of ? phase and Mg matrix is(002)?//(1010)Mg and[100]?//[0001]Mg.When the Mg-10.7Gd alloy aged at 180? for 10h,some 20nm dish-like metastable ?"phase precipitated on Mg matrix.After aging for 63h,the ?" phase grow into particle,while there are some ?' phase with short strip morphology on Mg matrix.The ?' phase is the main strengthening phase of Mg-10.7Gd alloy.The hardness of Mg-10.7Gd alloy reaches maximum when aged at 180? for 298h,at this time the P" phase almost disappeared and the content of?' phases were significantly increased.The ?' phases(Mg7Gd)have the base-centered orthorhombic(a=0.65nm,b=2.27nm,c=0.52nm),and the orientation relationship of ?' phase and Mg matrix is[001]?'//[0001]Mg and(100)?'//(2110)Mg.With the extension of aging time to 500h at 180? the ?' phase obviously grew,while the needle-like ? phases appeared on Mg matrix,and the aging strengthening effect weakened.The aging strengthening effect of Mg-9.17Y alloy is not obvious.After aging 10h at 180?,there are some irregular granular ? phase with 5-15nm size are precipitated on Mg matrix.After aging for 63h,such granular ?" phase grew to the strip-like ?' phases with 80nm length.The hardness of Mg-9.17Y alloy reaches the aging peak when aged at 180? for 100h,and the ?' phase continues to grow and precipitated uniformly along the direction of [0110]Mg. |
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