| Compared with traditional magnesium alloys,rare earth magnesium alloys are favored for their high strength and high corrosion resistance.Studies have shown that the introduction of LPSO structure into rare earth magnesium alloys could help improve the comprehensive properties of rare earth magnesium alloys,especially the plastic processing properties.Many scholars have done research on Mg-RE alloys containing LPSO structure phases,but there are still disputes about the structural evolution of LPSO phases in various states during the preparation and processing of magnesium alloys.The plastic deformation mechanism is still unclear.Based on this,this paper selected the Mg-7Gd-5Y-1Nd-xZn-0.5Zr(x=1,1.5,2)alloy to study the second phase structure after different heat treatment states and deformation,focusing on the analysis the structure of LPSO phase and its influence mechanism on plastic deformation.The microstructure distribution and second phase structure of as-cast alloys with three components were studied.The structure of 1Zn,1.5Zn and 2Zn as-cast alloys was mainly composed of ?-Mg matrix,eutectic phase on grain boundary and other fine second phases.With the increase of Zn content,the volume fraction of the second phase in the alloy gradually increases.The eutectic phase mainly includes the fcc(Mg,Zn)3RE phase and Mg5(RE,Zn)phase,as well as the block LPSO phase.The block LPSO phase was mainly 18R-LPSO with a small amount of 14H-LPSO,and the atomic layer stacking in some regions had incomplete periodicity.Lamellar LPSO phase without a complete stacking period was observed inside the grains.The changes of the alloy structure and the second phase after homogenization heat treatment were studied.With the extension of the homogenization time,the(Mg,Zn)3RE phase and Mg5(RE,Zn)phase on the grain boundary gradually re-dissolved and disappeared.After 48 h of homogenization,only the block LPSO phase remained on the grain boundary.The block LPSO phase after homogenization heat treatment was mainly 14H-LPSO.Nearly all lamellar LPSO phases re-dissolved and almost completely disappeared.Furnace cooling+pre-precipitation treatment with different durations were performed on the homogenized alloy.During the furnace cooling process,a small amount of lamellar LPSO phases precipitated inside the grains;with the extension of the pre-precipitation time,the amount of lamellar LPSO phase gradually increased.Lamellar LPSO phase had the structural characteristics of 14H-LPSO,and its stacking period was imperfect.The pre-precipitated alloy was subjected to a thermal compression test.The results showed that the deformation behavior of the lamellar LPSO phase was related to its orientation during the thermal compression deformation process.By analysing the dynamic recrystallization behavior during the deformation process,it can be seen that lower strain rate,higher deformation temperature and larger strain are more beneficial to the nucleation and growth of dynamic recrystallization of the alloy.The grain boundaries,grain boundary block LPSO phase and the intragranular lamellar LPSO phase were all able to promote the occurrence of dynamic recrystallization,but the promotion effect was gradually weakened.The priority of the nucleation position was the grain boundary,the block LPSO phase and the lamellar LPSO phase.The lamellar LPSO phase provided a nucleation fulcrum for dynamic recrystallization,and at the same time,the dynamic recrystallized grains can only grow in the direction in which the lamellar stretches,forming elongated dynamic recrystallized grains.When strain rate>0.1 s-1 and deformation temperature<450?,dynamic recrystallization only nucleates and grows at the grain boundary.When strain rate?0.1 s-1 or deformation temperature? 450?,dynamic recrystallization starts to nucleate at the position of the block LPSO phase;when strain rate ?0.01 s-1 and deformation temperature? 450?,dynamic recrystallization begins to nucleate at the position of the lamellar LPSO phase in the crystal. |
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