Influence Of Rare Earth Elements On The Precipitate-free Zone Development And Creep Behavior Of Mg Alloy

Magnesium（Mg）alloys are more and more attractive in aerospace and automobile industries due to their low density and high specific strength.However,the low creep resistance at medium to high temperatures limits its application range greatly.The addition of rare earth elements can effectively improve the high temperature properties and the creep resistance of Mg alloys.For these reasons,this subject studied the tensile creep behavior for Mg-13Gd,Mg-9Gd-4Y,Mg-9Gd-3Nd and Mg-10.5Y（wt.%）alloys with and without aging treatment under same temperatures and various stresses.Microstructural evolutions during creep were observed by optical microscopy（OM）,scanning electron microscopy（SEM）,electron backscatting diffraction technology（EBSD）and transmission electron microscopy（TEM）.The main conclusions are obtained as follows:1.For Mg-13Gd alloy,the creep resistance of AA sample is higher than that of SS sample.Cross-slip and twinning controlled the creep behavior of the SS sample,while the creep behavior of the AA sample was controlled by cross-slip and the pyramidal<c+a>slip.Although the final PFZ width in AA sample was still wider than the PFZ width in SS sample,the large decreased ratio of PFZ width helps enhance the creep resistance of AA sample.The large-scale’phases retarded cross-slip and<c>dislocations bypassing precipitate tips strengthened the creep resistance of AA sample,while the low density of dynamic precipitated’phases cannot retard dislocation movement and twin growth effectively.2.For Mg-9Gd-4Y alloy,the creep resistance of SS sample is higher than that of AA sample.Cross-slip was the main creep mechanism for both SS and AA samples,and the precipitates of them were’phases.The PFZ width in AA sample was wider than the PFZ width in SS sample,which was the main factor causing the difference in their creep resistance.3.For Mg-9Gd-3Nd alloy,the creep resistance of AA sample is better than that of SS samples.It is found that there was only’phase dynamic precipitated in SS sample during creep,the dominant creep mechanism of SS sample was cross-slip,there were chain-like arranged’phase and large-scale lath-like₁phase in AA sample,the dominant creep mechanism of AA sample was cross-slip and pyramidal<c+a>slip.Although the final PFZ width in AA sample was still wider than the PFZ width in SS sample,the negative effect of the PFZ width was smaller than the strengthening effect caused by the’phase and₁phase hindering the dislocation movement.4.For Mg-10.5Y alloy,the creep resistance of AA sample is higher than that of SS sample.Twinning and dislocation climbing controlled the creep behavior of the SS sample,while the creep behavior of the AA sample was controlled by cross-slip and the pyramidal<c+a>slip.The precipitates of them were’phases.The twinning deteriorated the creep resistance of the SS sample,while the large-scale and high-density precipitates in the AA sample hindering the<c>dislocation movement played a strengthening role.Within 36 h,the rate of PFZ widening of the AA sample was significantly lower than that of the SS sample,which was also an important reason for improving the creep resistance of AA sample.5.The order of the creep resistance of SS samples from strong to weak is:Mg-9Gd-3Nd>Mg-9Gd-4Y>Mg-10.5Y>Mg-13Gd.And the order of the creep resistance of AA samples from strong to weak is:Mg-10.5Y>Mg-9Gd-3Nd>Mg-9Gd-4Y>Mg-13Gd.The role of Gd element:by increasing the density of the precipitation phase and hindering the movement of dislocations,the threshold stress can be increased;the narrowing of the PFZ can be achieved by the re-dissolution and re-precipitation of the’phase.The role of Y element:replacing part of Gd atoms with Y atoms,the SS sample by increasing the asymmetric lattice distortion and the AA sample by reducing the diffusion rate improves the creep resistance;increasing the content of Y element can improve the creep resistance by reducing the diffusion rate and increasing the threshold stress;the width of PFZ is also reduced due to the lower diffusion rate of the Y atoms.The role of Nd element:replacing part of Gd atoms with Nd atoms,for SS sample,since the atomic radius of Nd atoms is larger than that of Gd atoms,the asymmetry of lattice distortion is further increased;for AA sample,the addition of Nd element promotes the precipitation and improves the threshold stress,which is beneficial to improve the creep resistance.Besides,the addition of Nd element increases the density of the precipitate,hinders the movement of dislocations,and reduces the rate of PFZ widening.