| Rare earth magnesium alloy,as a kind of heat-resistant magnesium alloy,generally needs to be served under high temperature and load for a long time.Thus,it tends to creep,once the creep occurs,the dimensional stability of the alloy in service will decrease,eventually influencing the service effect and causing incalculable losses.Therefore,it is imperative to improve the creep resistance of magnesium alloy.In this paper,the content of Gd,aging precipitation,grain size,deformation texture,and different solute elements for Mg-Gd-MM-Zr alloy creep properties and mechanism of the influence are investigated with Mg-Gd-MM-Zr alloy as the main research object,so as to reveal the aging precipitation,grain size,texture,and the influence law of different solute elements on the creep mechanism and establish a high creep magnesium alloy composition design and preparation process optimization principle.The as-cast Mg-xGd-0.2MM-0.1Zr(x=1,2,3 at.%)alloy microstructure is composed of ?-Mg matrix and eutectic compounds,in which the eutectic compounds of lGd alloy consist of Mg5Gd and Mgi2MM phases,and the eutectic compounds of 2Gd and 3Gd alloys are Mg5(Gd0.8MM0.2)phases.After homogenization treatment(535?×24 h),the Mgi2MM phase,which is insoluble in the matrix,remains in the alloy.Its structure is consistent with that of Mg12Ce,which is tetragonal,with lattice constants of a=b=1.033 nm and c=0.596 nm.Dynamic recrystallization appears during the extrusion process of the alloy,and the fine grains are equiaxed.The Mgi2MM particles form extrusion streamlines along the extrusion direction,and the Mg5Gd phase precipitates during the deformation of 3Gd alloy.The results demonstrated that 2Gd alloy has the best creep resistance,followed by 1Gd alloy,and 3Gd alloy with the smallest grain has the worst creep resistance.Based on the previous study,2Gd alloy was selected as the alloy for subsequent experiments,and the optimal aging system was observed to be held at 225? for 7 h.The peak-aged precipitates Mg7Gd of 2Gd alloy have an orthogonal structure with the base center,with lattice constants of a=0.64 nm,b=2.22 nm,and c=0.52 nm.The phase is symmetrically distributed along with the {1120}? plane at 120°.Then,the influence of the aging precipitation on the performance of 2Gd alloy creep is studied under the creep temperature of 200?.It was discovered that when the creep properties of the aging state are higher than that of alloy extrusion state and the creep temperature is higher than or equal to 225?,the creep properties of the aging state is below the squeezed state alloy,and squeezed state alloy and aging state of creep performance difference decreases with the increase in the creep stress.The creep mechanism of the extruded and aged alloys at 200? is the dislocation slip mechanism,and the creep mechanism at 250? is the dislocation climb mechanism.As revealed from the investigation on the influence of grain size on the creep properties of 2Gd alloy,at 300?,a "deceleration-acceleration-deceleration" creep phenomenon appears in the creep curve at the initial stage.This is quite different from the traditional creep curve.The microstructure analysis is caused by the transformation of the precipitated phase during the creep process at 300?.Besides,with the increase in average grain size,the grain boundary area of the alloy decreases,and the contribution of grain boundary sliding to creep deformation decreases,making the creep resistance of the alloy continuously improve.Multi-direction forging was conducted under the guidance of a thermal working chart to explore the effect of texture on creep resistance of 2Gd alloy.The results reveal that the grain of the alloy becomes finer and the degree of texture dispersion increases with the increase in deformation passes.With the two-pass deformed alloy as an example,the creep resistance of the alloy in the compression direction is higher than that in the tensile direction,suggesting that the presence of texture leads to the anisotropy of the creep resistance of the alloy.The creep resistance of Mg-2Y-0.2MM-0.1Zr(at.%)and Mg-2Gd-0.2MM-0.1Zr(at.%)alloys with the same atomic ratio was investigated.As demonstrated by the results,the creep resistance of the 2Y alloy is higher than that of the 2Gd alloy under the creep temperature of 200?,and the creep resistance of the 2Gd alloy is higher than that of the 2Y alloy when the creep temperature is higher than 200?.The creep mechanism of 2Y and 2Gd alloys at 200? is dislocation slip.In conclusion,the creep resistance of the alloy is the best when the content of rare earth is 2 at%.If the service temperature is less than or equal to 200?,Y element can be selected as the main addition element,and T5 treatment can be carried out after deformation.If the alloy is served above 200? for a long time,Gd element can be selected as the main addition element,and proper T6 treatment can be carried out after deformation.If the service environment requires high isotropy,the alloy texture can be weakened by more than three passes of multi-direction forging. |
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