Study On The Precipitation Behavior Of Mg-Gd-Y-Zn Alloy During Aging Treatment

The EW75(Mg-7Gd-5Y-1Nd-0.5Zr)alloy designed by the research group exhib:ited excellent mechanical properties(tensile strength 450MPa,elongation 5%).In recent years,the long period stacking ordered(LPSO)structural phases have been found in the Mg-rare earth(RE)-Zn alloys,which increases the elongation of the alloy and enhances the plasticity.The Mg-7Gd-3Y-xNd-1Zn-0.5Zr(x=1,Owt.%)alloy is designed to achieve the LPSO and βthrough homogenization,single-stage and two-stage aging,so that the alloy has excellent strength and improved elongation.Although there are many studies on the microstructure and properties of Mg-RE-Zn alloys,the aging precipitation behavior is still controversial.Based on this paper,this paper uses scanning and transmission electron microscopy techniques to study the microstructure changes and precipitation phase evolution of alloys in as-cast,homogenized,single-stage and two-stage aging processes.The microstructure of the two as-cast alloys consists of a-Mg matrix,network and bulk eutectic phase,bulk and needle-like LPSO phase.The chemical composition of the eutectic 3 phase(f.c.c.,a=0.74 nm)is(Mg,Zn)3RE,and that of 14H-LPSO is Mgl2(RE,Zn).The Nd element inhibits the production of 14H-LPSO.After double-stage homogenization treatment 520℃/48h+530℃/2h,the two as-cast alloys were over-fired.After single-stage homogenization at 525℃/48h,quenching in warm water at 30℃,both the P and 14H-LPSO phases in the two as-cast alloys are dissolved back into the matrix,leaving only the dot-like Zr/Zn-rich phases.Dot-like Zr/Zn-rich phases have little effect on subsequent single-stage and double-stage aging.The precipitation behavior of the alloy during the single-stage aging process is investigated.The two alloys subjected to a single-stage homogenization treatment are aged at 240℃,and Nd increases the thermal stability of the alloy.The alloy with an Nd content of lwt.%was subjected to TEM analysis at 240℃ for aging,and the precipitates containedβ-type(ordered solute clusters,zigzag GP zones,β’,βP’,and β1)and γ-type(γ’,14H-LPSO(γ))The ordered solute clusters and zigzag GP zones form in the habit plane {1100}α.The formation of β’ is accomplished by ordered stacking of zigzag GP zones along<1010>α.Theβ’ and βp’ are the major aging strengthening precipitates.β’ is b.c.o.structure,lattice parameter is a=0.64nm,b=2.22nm,c=0.52nm,with {1120}αas the habit plane,completely coherent with the matrix,and its structural model is Mg7RE.The βp’precipitate is considered to be the precursor of the β’ precipitate,and its structure needs further study.β1,is f.c.c.structure,lattice constant is a=0.74nm,with {1010}α as the habit plane,and the matrix is not coherent,and the structural model is Mg3X,which is transformed from β’ precipitate,γ’ is a metastable LPSO structural phase that will be converted to 14H-LPSO.The γ’formed on the(0001)α is completely coherent with the matrix,and its stacking sequence is "ABCA" type.It shows the excellent thermal stability which can be attributed to the fact that the γ’ or 14H-LPSO retard the growth of β’ and β1,low diffusion rate of RE and physical character of β’and β1.The precipitation behavior of the alloy during the double-stage aging process is investigated.The two alloys subjected to single-stage homogenization treatment are aged at 500℃/2h in the first-stage aging.The microstructure of the two alloys consists of bulk βphase and acicular 14H-LPSO,and there are no other nanometer-scale precipitates.The transmission analysis of the alloy with Nd content of lwt.%in the second-stage aging of 500℃/2h+240℃,the precipitates contains β-type(β’βp’,β’F andβ1)and γ-type(γ’,14H-LPSO).The crystallographic characteristics of β’,βp’,β1,γ’and 14H-LPSO are similar to those of single-stage aging.The bridge structure β’F precipitate(b.c.o.)has a lattice constant of a=0.64nm,b=1.14nm,and c=0.52nm,which is completely coherent with the matrix.β’F is transformed from β’ coarsening and is also the site where the β1 is easily nucleated.The formation of the β1 from the β’F can be accomplished by a simple atomic replacement and structural relaxation.The two-stage long-term aging Nd and Zn began to enrich in the β1 and LPSO structures.