Aging Precipitation Behavior And Mechanical Properties Of Mg-Gd-Y-Zn-Zr Alloy

In addition to the inherent advantages of traditional magnesium alloys such as high specific strength and specific stiffness,good dimensional stability and excellent electromagnetic shielding performance,rare earth(RE)magnesium alloys also have some new characteristics such as high temperature strength and good corrosion resistance.Therefore,it is of great significance to develop high performance Mg-RE alloys.Aging precipitation strengthening is an important means to improve the mechanical properties of Mg-RE alloys.In the recent years,researchers have done a lot of research on the heat treatment process and aging precipitation behavior of Mg-RE alloys.Due to the interaction of numerous RE elements increasing the number and varieties of the second phases,there are still some controversies about the high-temperature(?250?)aging behavior of the alloy,the microstructure and transformation process of nano-scale precipitations.The aging behavior of as-cast Mg-9Gd-2Y-2Zn-0.5Zr(wt.%)alloy at different temperatures(200?,250?,300?),the characteristics(including crystal structure,lattice parameters,habitual plane,orientation relation,etc.)of the precipitated phases and the effect of precipitations on the mechanical properties of the alloy were studied by Scanning Electron Microscope(SEM),Transmission Electron Microscope(TEM),Universal Tensile Tester,Vickers and Brinell hardness tester.The experimental results are as follows:(1)Aging precipitation sequence and characteristics of precipitated phases:(1)Aging precipitation sequence:(a)200?:???????₁??;The stacking fault(SF)formed in?-Mg matrix at the early aging period did not change into Long Period Stacking Ordered(LPSO)phase with the prolongation of aging time.(b)250?:???????₁??;SF?14H-LPSO.(c)300?:The needle-like?phase which precipitated in?-Mg matrix at the early aging period remained existing in the aging process;SF?14H-LPSO.(2)Characteristics of precipitated phases(including chemical composition,lattice type,habitual plane,lattice parameters,orientation relationship with?-Mg matrix):??phase:Mg₃RE;super-lattice hexagonal(D019);precipitated from(0001)_?base plane;a=2�a_?-Mg?0.64nm,c=c_?-Mg?0.52nm;[0001]_???[0001]_?,[11(?) 0]_???[11(?) 0]_?,[10(?)0]_???[10(?)0]_?.??phase:Mg_3⁵RE;base-centered orthorhombic(cbco);precipitated from(11(?)0)_?cylinder plane;a=2�a_?-Mg?0.64nm,b=8�d_(01(?)0)?-Mg?2.22nm,c=c_?-Mg?0.52nm;(001)_???(0001)_?,(110)_???(10(?)0)_?,[100]_???[2(?)(?)0]_?.?₁phase:Mg_3⁵RE;face-centered cubic(fcc);precipitated from(10(?)0)_?cylinder plane;a=0.74nm;[110]_?1?[0001]_?,(1(?)1)_?1?(11(?)0)_?.?phase:Mg₅RE;face-centered cubic(fcc);precipitated from(101 0)_?cylinder plane;a=2.23nm;[110]_??[0001]_?,(11(?))_??(1(?)00)_?,(1(?)1)_??(11(?)0)_?.14H-LPSO:Mg₁₂REZn;close-packed hexagonal(hcp);precipitated from(0001)_?base plane;a=1.11(?)nm,c=3.647nm;(0001)_14H?(0001)_?,[01(?)0]_14H?[11(?)0]_?(3)Effect of aging time on morphology of precipitated phases:??phase:no obviously change with the prolongation of aging time.??phase:the size of??phase gradually coarsening with the prolongation of aging time,and some of coarsening??phase produced necking region.?₁and?phase:the size of these phase gradually coarsening with the prolongation of aging time.14H-LPSO:with the prolongation of aging time,the width of the strip-like 14H-LPSO phase increased and then gradually changed into bulk 14H-LPSO phase.(2)The law and mechanism of aging hardening and strengthening:(1)The law of aging hardening and strengthening:(a)The law of aging hardening:The aging hardness of the alloy increased at first and then decreased with the prolongation of aging time at 200?and 250?.With the increasing of aging temperature,the peak aging time shortened and the hardness value decreased.The peak aging time of the alloy at 200?was 104h,and the Vickers and Brinell hardness values were 120HV,121HB respectively.The peak aging time of the alloy at 250?was 62h,and the Vickers and Brinell hardness values were 101HV,95HB respectively.There was no obvious peak aging time of the alloy at 300?.Within the time range of 0?52 h,the aging hardness of the alloy fluctuated up and down.The aging hardness value of the alloy showed a decreasing trend in the time range of 52?224 h.(b)The law of aging strengthening:The tensile strength of the alloy increased at first and then decreased with the prolongation of aging time at 200?and 250?.At 300?,the tensile strength of the alloy decreased gradually with the prolongation of aging time.(2)The mechanism of aging hardening and strengthening:The characteristics of aging precipitations were closely related to the hardness and tensile strength of the alloy.The phases precipitated from(10(?)0)_?or(11(?)0)_?cylinder plane with large number of precipitates,uniform distribution,small size and large aspect ratio could effectively hinder the dislocation slip and improve the hardening performance of the alloy.Since the nano-scale size,large amount of precipitation,uniform distribution and large aspect ratio,the??phase which precipitated from(11(?)0)_?cylinder plane at the peak aging time of 200?and 250?could significantly increase the aging hardness of the alloy.For each aging period of 200?,250?and 300?,the strengthening and hardening law of the alloy was same,so the effect mechanism of aging precipitated phases on the tensile strength of the alloy was the same as hardening.