Microstructures And Properties Of RS-Mg-Gd-Y-Zn-Zr Alloys Reinforced By LPSO Structure

RS-Mg-8Gd-4Y-xZn-0.5Zr(x=0,1,2,3,wt%)ribbons were prepared by single roller rapid solidification,the high strength and corrosion resistant extruded RS alloy bars were prepared by reciprocating extrusion,forward extrusion and aging treatment.The microstructure and LPSO structure type of rapidly solidified alloys were characterized by OM,SEM,EDS,XRD,DTA and TEM.The effects of alloy elements and preparation technology on the phase composition and LPSO structure type of rapidly solidified magnesium alloys were investigated,and the influence mechanism of phase composition and LPSO structure type on the mechanical properties and corrosion resistance properties of the alloy were discussed.The main conclusions are as follows:(1)The optical microstructures of RS-ribbons are ultrafine equiaxed,columnar and equiaxed regions from the copper roller to the free surface,respectively.The phase compositions of the RS-ribbons after the addition of Zn element are compose of a-Mg matrix,X-phases,W-phases and quasicrystal I-phases,and with the increase of Zn content,the number of the second phases increases and the size becomes larger.The phase compositions of 0%Zn RS-ribbons before and after the aging heat treatment are a-Mg matrix and a small amount of dispersion distribution rich RE particle phases.In addition,the X-phases of the 3%Zn RS-ribbons before and after the aging heat treatment are 18R-LPSO structures,and the aging treatment does not change the type of LPSO structures,but the β’ phases precipitate out from the alloy matrix.(2)The structures of the RS-ribbons are refined by reciprocating extrusion and forward extrusion deformation,and the phase compositions of 0%Zn RS alloy are not changed.The quasicrystal I-phases in RS-Mg-8Gd-4Y-xZn-0.5Zr(x=1,2,3,wt%)extruded bars disappeare and the number of LPSO structures increases.The LPSO structures of X-phases in extruded 3%Zn RS alloy bars consist of 18R and 14H.Hot extrusion deformation leads to the formation of 14H-LPSO structures in the RS alloy bars.(3)The extruded 3%Zn alloy RS bars consiste of a-Mg and X-phases after being aging treated at 175℃ for 60 h,200℃ for 36 h and 200℃ for 72 h;It consiste of a-Mg,X-phases and W-phases after being aging treated at 200℃ for 48 h and 200℃ for 60 h;and there are only α-Mg after being treated at 200℃ for 84 h and 225℃ for 60 h.In addition,there are only 18R-LPSO structures and no 14H-LPSO structure by aging heat treatment at 200 ℃ for 60 h.(4)Before and after aging treatment,the hardness of 2Zn%RS-ribbons reach the maximum value of 108.6 HV0.1 and 115.3 HV0.1,respectively.The 3%Zn RS-ribbons get the peak hardness of 110.1 HV0.1 when treate at 200℃ for 36 h,under the combined action of fine grain strengthening,aging precipitation strengthening and solid solution strengthening.(5)Before and after aging treatment,the hardness of extruded 3%Zn RS-bars reach the maximum value of 112.7 HV50 and 120.3 HV50,respectively.During the process of hot extrusion,the RS ribbons is subjected to three-dimensional mechanical rubbing and deformation,flow,inversion and crushing,and finally welded together.The microstructures of RS-ribbons are further refined by dynamic recrystallization to achieve fine grain strengthening effect.And the extruded 3%Zn RS-bars get the peak hardness of 120.3 HV50 by aging treatment at 200℃ for 60 h.(6)The fracture styles of extruded RS-bars before and after aging treatment are both ductile fracture.The tensile properties of 3%Zn are the best in four kinds extruded RS-bars,and the tensile strength and yield strength are 420.5 MPa and 418.3 MPa,respectively.The elongation of extruded 2%Zn RS-bars is the best,at 10.1%.The reduction of elongation of alloy is caused by the continuous precipitation and growth of hard brittle W-phase in alloys.The tensile properties of extruded 3%Zn RS-bars are the best by aging treatment at 200℃ for 60 h,and the tensile strength,yield strength and elongation are 452.9 MPa,450.7 MPa and 6.7%,respectively.In the process of aging heat treatment,the tensile strength of the alloys is increased by the precipitation of grain refining and strengthening phase particles significantly,which reduce the elongation.(7)Among the four kinds of extruded RS-bars,extruded 1%Zn RS-bars reach the best corrosion resistance.The aging treatment can significantly improve the corrosion resistance of extruded RS-bars,and extruded 1%Zn RS-bars retain the best corrosion resistance by aging treatment at 200℃ for 60 h.