Effects Of RE-Y On Aging Behavior Of Mg-Gd-Zn Alloy

RE（short for Rear Earth）magnesium is a research hotspot in magnesium alloys.Compared to the traditional magnesium alloys,Mg-RE is attracting more and more attention and is promising to be used during industrial development.There are rich rear earth reservations in China and RE magnesium alloy poccess good over-all properties,so it has important significance.However,the RE content of traditional Mg-RE alloy is relatively high,which increases the whole cost and density of alloy and this might be adverse.So,in this paper,a little RE-Y is added in Mg-3Gd-1Zn alloy with heat treatment,the effect of Y on RE magnesium alloys contains less RE is studied.In this paper,Mg-3Gd-1Zn-xY（x=0,0.5）alloys were prepared by induction melting at the atmosphere of argon gas.The effects of Y addition on microstructure and mechanical properties of as-cast,solution Mg-3Gd-1Zn-xY（x=0,0.5）alloy were comprehensively researched.Based on the results,the processes of aging in Mg-3Gd-1Zn-xY（x=0,0.5）alloy were studied.OM、XRD、SEM、TEM and such devices and measures are used to study two kinds of as-cast alloys,the results indicated that the as-cast GZ31 alloy and GZ31-0.5Y alloy mainly consist ofα-Mg,MgZn2 and MgGd3 phases.As for GZ31-0.5Y alloy,with the increasing of Y,MgZn2 phase increases,the amount of MgGd3 phase reduces,Y is mainly dissolved in the matrix.The strength of as-cast GZ31-0.5Y alloy is relatively higher than that of GZ31 alloy,the main strengthening phase in matrix is MgGd₃ phase.Solid solution GZ31 alloy and GZ31-0.5Y alloy mainly consist ofα-Mg,MgZn₂ and MgGd₃ phases.After 530℃,4h solid solution treatment,the amount of MgZn₂ phase decreases and MgGd3 phase increases in both alloys while MgGd3 phases are more in GZ31-0.5Y.The hardness of solid solution alloys is relatively lower than that of as-cast alloys,this is because that the precipitations are dissolved in matrix after solution treatment,which weakens the strengthening effects of precipitation hardening of precipitates,in addition,grains have grown after solution treatment.The hardness of solid solution GZ31-0.5Y alloy is relatively higher than solid solution GZ31 alloy,this is caused by the solution hardening of Y and more MgGd₃ phases in alloy.Aging GZ31 alloy consists ofα-Mg,MgGd3,MgZn2 and X phase.X phase possesses little strengthening effect on matrix.The under-aged alloy contains a little X phase,the amount of MgGd₃ and MgZn₂ is few.MgGd₃ phase increases,MgZn₂ phase and X phase decrease as aging time increases.When the alloy reaches peak aging stage,MgGd₃ phases are much precipitated,MgZn₂ phase deduces and X phase decreases.When in over-aged stage,X phase appears in alloy,MgGd3 phase decreases and MgZn2 phase increases,the micro hardness of alloy turns to become lower.When the aging time passes 50h,X phase disappears gradually,MgGd₃ phase appears more and MgZn₂ decreases.Micro hardness curves of GZ31 aging alloy is in“W”shape,the alloy is in under-aged state from 0.5h to 10h;the micro hardness of alloy increases from 1h to 10h,the hardness reaches peak point that is 68.4Hv when the aging time reaches 10h;the hardness decreases continuously after 10h when the alloy starts over-aged stage,while the micro hardness increases again when reached valley which is54.9Hv,the micro hardness recovered to 61.0Hv when the time reaches 225h.Aging GZ31-0.5Y alloy consists ofα-Mg,MgGd₃ and MgZn₂.The under-aged alloy contains a little of MgGd3 and MgZn2 phases.MgGd3 phase increases,MgZn2 phase decreases as aging time increases.When the alloy reaches peak aging stage,MgGd₃ phases are much precipitated,MgZn₂ phase deduces.When in over-aged stage,MgGd₃ phase decreases and MgZn2 phase increases.When the aging time passes 100h,MgGd3 phase becomes more and MgZn₂ phase decreases.Micro hardness curves of GZ31-0.5Y aging alloy is in“W”shape,the alloy is in under-aged state from 0.5h to 20h;the micro hardness of alloy increases from 1h to 20h,the hardness reaches peak point that is 68.4Hv when the aging time reaches 20h;the hardness decreases continuously after 20h when the alloy starts over-aged stage,while the micro hardness increases again after reaching valley which is 55.8Hv,the micro hardness recovered to 60.0Hv when the time reaches 225h.The analysation demonstrates that the addition of Y elements shows weak strengthening effects to GZ31 alloy.This is because the addition of Y results in the accumulation and coursing of MgGd3 phase which weakens its precipitation hardening effects,while the solid solution hardening effects of Y elements cannot offsets such reduction of mechanical properties.The addition of Y has a series effects on alloys,such as:After adding Y,the shapes of MgZn₂ phase in aging alloy are also be changed,MgZn₂ phase in GZ31 alloy is long rod shape,while the one in GZ31-0.5Y is bulk shape;results in the delay of GZ31-0.5Y reaches peak-aging state;results in the delay of GZ31-0.5Y reaches starting time of hardness increase again in over-aged stage.