Homogenization Of LPSO Phase In Mg-Gd-Y-Zn-Zr Alloy And Its Effect On Microstructure And Properties Of Hot Rolled Sheet Metal

Mg-Gd-Y-Zn-Zr alloy containing long period ordered phase(LPSO phase)has been widely used in military industry,aerospace,automobile and other engineering fields due to its excellent mechanical properties.In order to further investigate the formation rule of LPSO phase in semi-continuous casting MG-9.8Gd-3.5Y-2Zn-0.5Zr(wt.%)alloy during homogenization and its mechanism on thermal deformation behavior and dynamic recrystallization behavior,In this paper,the evolution and regulation mechanism of LPSO phase during homogenization treatment were studied by means of metallography,scanning electron microscopy,energy spectrum,X-ray diffraction,electron backscatter diffraction and tensile test at room temperature,and the effects of LPSO on the microstructure,dynamic recrystallization behavior and mechanical properties of sheet metal during hot rolling were investigated.The main work and conclusions of this paper are as follows:(1)The microstructure and second phase distribution characteristics of Mg-9.8Gd-3.5Y-2Zn-0.5Zr ingot under semicontinous casting condition were studied.The second phase accounts for 19.86% of the as-cast microstructure,which is mainly composed of dendrite structure,α-Mg matrix,intercrystalline eutectic phase and massive LPSO phase.(2)The evolution of LPSO phase in Mg-9.8Gd-3.5Y-2Zn-0.5Zr alloy during homogenization was investigated.The morphology and volume fraction of LPSO phase in homogenized structure are affected by homogenization temperature and time.With the increase of temperature,the as-cast dendrite microstructure gradually transforms into intergranular LPSO phase,and the growth rate of lamellar LPSO phase increases and the number of LPSO phase increases.The volume fraction of intergranular LPSO phase tends to be stable gradually at the same temperature and the lamellar LPSO phase precipitates along the grain boundary into the matrix and begins to coarse after penetrating the grain.At 535℃,the intergranular LPSO phase was homogenized and converted into W phase.The lamellar LPSO phase was not precipitated at this temperature.(3)The effect of LPSO on the microstructure and dynamic recrystallization behavior of Mg-9.8Gd-3.5Y-2Zn-0.5Zr hot-rolled sheet was investigated.The recrystallization behavior in plate is continuous dynamic recrystallization.The intergranular LPSO phase induced recrystallization nucleation by PSN mechanism and promoted the nucleation of recrystallization near deformed grains and broken intergranular massive LPSO phase.In the process of hot rolling,the lamellar LPSO phase will be twisted and deformed harmonically,which hinders the nucleation and growth of dynamic recrystallized grains near it.The volume fraction of recrystallization is the highest(25.06%)when the initial state is homogenized at 525℃×36h.(4)The effect of LPSO on the texture morphology and mechanical properties of Mg-9.8GD-3.5Y-2Zn-0.5Zr alloy hot rolled sheet was studied.In the homogenized initial state of 525℃×36h,there are high volume fraction of intergranular massive LPSO phase and intralagellar lamellar LPSO phase in the microstructure of the sheet,the weakest texture(maximum extreme density 6.48),the best mechanical properties(yield strength 291.3MPa,tensile strength 374 MPa,elongation 9.2%).