Recent developments in high-strength Mg-RE-based alloys:Focusing on Mg-Gd and Mg

Jinghui Zhng,Shujun Liu,Ruizhi Wu,c,∗,Legn Hou,Milin Zhng,c

a Key Laboratory of Superlight Material and Surface Technology,Ministry of Education,College of Material Science and Chemical Engineering,Harbin Engineering University,Harbin 150001,PR China

b Department of Materials Physics and Chemistry,Harbin Institute of Technology,Harbin 150001,PR China

c College of Science,Heihe University,Heihe 164300,PR China

Abstract Higher strength is always the goal pursued by researchers for the structural materials,especially for the lightweight magnesium(Mg)alloys which generally have relatively low strength at present.From this aspect,the present paper reviews the recent reports of a kind of Mg alloys,i.e.Mg-RE(RE:rare earths,mainly Gd or Y)casting and wrought alloys,which have been able to achieve high strength compared with common or commercial Mg alloys,from the viewpoint and content of the alloy system,alloying constitution,preparation process,tensile strength and each of the main strengthening mechanisms.This review of recent research and developments in high-strength Mg-RE alloys is beneficia for the further design of Mg alloys with higher strength as well as excellent comprehensive performance.©2018 Published by Elsevier B.V.on behalf of Chongqing University.

Keyword:Mg alloys;High strength;Rare earths(RE);Strengthening mechanism.

1.Introduction

Magnesium(Mg)is the lightest among all commonly used structural metals,with a density about two thirds that of aluminum and one quarter that of steels.Furthermore,Mg is an abundant element in the world compared with other commonly used metals.As the lightweight structural metallic materials,Mg alloys are of great interest for many potential applications including automotive,aircraft,aerospace,and 3C(computer,communication and consumer electronic product)industries and so on.Therefore,Mg alloys have received considerable research over nearly past two decades[1,2].Despite the considerable efforts made so far,the engineering applications of Mg alloys remains limited compared with that of aluminum(Al)alloys.Just considering the reason from performance,there are roughly four aspects of performance,i.e.strength(absolute or specifistrength),corrosion resistance,formability and creep resistance,are usually inadequate for the common Mg alloys.Accordingly,in recent years the most of reports about research of Mg alloys are around these four topics[3–12].

Fortunately,the Mg alloys with high strength at room temperature have been reported especially in more recent years.Researchers made full use of the precipitation hardening,grain refinemen strengthening and also some new strengthening mechanisms to improve their strengths.In order to compare the strength of various Mg alloys more easily and uniformly and show a clear development level to readers,the present paper reviews the research development of highstrength Mg alloys according to tensile properties at room temperature.Due to the increasing requirements for military and civilian applications,here we defin that the“highstrength Mg casting alloys”are the alloys with ultimate tensile strength(UTS)of above 350 MPa and the“high-strength Mg wrought alloys”are the alloys with UTS of above 400 MPa and/or yield strength(YS)of above 350 MPa.According to this standard,it can be found that almost all the high-strength Mg alloys are the new designed alloys rather than the commercial alloys,and most of them are Mg alloys with rare earths(RE).In other words,so far at least the researchers are obliged to admit the fact that Mg-RE-based alloys are the most promising high-strength Mg alloys despite the rapid development of RE-free Mg alloys with relatively high strength[13–16].

The high-strength Mg-RE alloys recently developed from the viewpoint of novel alloying designs can be roughly divided into Zn-free Mg-RE alloys,Mg-RE-Zn alloys,Mg-Zn-RE alloys.Since the properties of Mg alloys are determined not only by compositions but also by processing technologies,this article provides a concise review of new high-strength Mg-RE alloys roughly according to the alloying designs in the similar process technologies.

2.High-strength Mg-RE casting alloys

At present the casting technologies of reported highstrength casting Mg alloys are mainly related to the permanent mold gravity casting(PM).Thus in the paper,all the mentioned Mg casting alloys are prepared by PM but no high pressure die casting(HPDC),low pressure casting(LPC)and sand casting(SC)and so on.

Mg casting alloys usually have the microstructure with relatively large grains.Mg-RE casting alloys achieve their high strength mainly via age hardening,which involves(a)solidsolution treatment at a relatively high temperature,(b)water quenching to obtain a supersaturated solid-solution ofα-Mg single-phase,and(c)subsequent ageing at a relatively low temperature to finall obtain the metastable or equilibrium precipitates in the Mg matrix.The age hardening effect is depended on the quality of precipitation,i.e.the size,number density,morphology,orientation as well as structure of precipitates.Both the internal and external factors,i.e.alloy constitutions and process technologies,control the microstructure and finall determine the mechanical properties of Mg alloys.Based on a large amount of literature investigations,it can be found that generally the strength of the earlier Mg-RE alloys developed is lower than that of the latest ones even if they have the similar alloy constitutions,revealing that the control of preparation processes(casting process,heat treatment process as well as deformation process)is also very important and becomes more and more perfect.The new Mg-RE casting alloys mainly include Mg-Gd,Mg-Y,Mg-Nd based alloys[3,17–22].From the relevant references,it can be found that the Mg casting alloys with UTS＞350 MPa are only the Mg-RE alloys in which the RE is mainly Gd and RE content is at a relatively high level.For the Mg-Gd based alloys,they can further be divided into Mg-Gd-Y,Mg-Gd-Nd,Mg-Gd-YNd,Mg-Gd-Sm,Mg-Gd-Dy,Mg-Gd-Er,Mg-Gd-Ho,Mg-Gd-(-Y)-Ag and Mg-Gd-Y/Dy-Zn series[3,16,17,23–29].

Table 1 Mechanical properties of the high-strength Zn-free Mg-RE(or rather Mg-Gd)casting alloys with UTS＞350 MPa(EL:Elongation).

2.1.Zn-free Mg-RE casting alloys

Table 1 lists most of,if not all,the high-strength Zn-free Mg-RE (or rather Mg-Gd)casting alloys with UTS＞350 MPa,which have been reported up to now.These Zn-free Mg-Gd alloys can be further divided into Mg-Gd-RE(RE=Y,Nd,Dy)and Mg-Gd(-Y)-Ag series.

The precipitation sequence in Mg-Gd-RE(or Mg-Gd binary)alloys during ageing process has been well recognized:SSSS(super-saturated solid solution)→β′′(Mg3Gd,hcp,D019)→β′(Mg7Gd,cbco)→β1(Mg3Gd,fcc)→β(Mg5Gd,fcc)[3,36–44].More recently,based on the HAADF-STEM observations,Zheng et al.[45]report a more detailed precipitation sequence in Mg-Gd-Y-Zr alloys:SSSS→clusters→nucleationβ’(major)/βH(minor)→precipitateβ′(major)/βM,βT′(minor)→β1→β(equilibrium),and under peak-age condition,the strengthening structure is independent defect-freeβ’with little interaction among each other,as shown in Fig.1.Among these precipitate phases,theβ′precipitates with nanoscale and dense distribution are well known as the key strengthening phase in peak-aged Mg-Gd-RE samples[28–31,37,39,42–46],and a few reports also suggest that some timesβ′+β′′orβ′+β1coexist for the peak ageing hardening[21,42,47,48].β′phase has a base-centered orthorhombic structure(a=0.65 nm,b=2.27 nm,c=0.52 nm)and an orientation relationship with respect to the Mg matrix:(100)β′//{1-210}αand[001]β′//[0001]α[3].

Fig.1.Morphology and precipitate structure from[0001]Mg of β′precipitates in a peak-aged Mg-Gd-Y-Zr alloy[45].

Fig.2.Schematic representation of the β′precipitate morphology and habit with respect to the Mg matrix:(a)ideal arrangement of{11-20}αprecipitate plates in a triangular prismatic volume of α-Mg matrix and(b)projection of the triangular prismatic plates in(0001)α[31].

Fig.3.HAADF-STEM images showing β′precipitates in(a)Mg-8Y and(b)Mg-15Gd alloys.The electron beam is parallel to[0001]α[49].

Theβ′precipitates,which form on{11-20}prismatic planes ofα-Mg phase in a dense triangular arrangement and are vertical to basal plane ofα-Mg,provide the most effective barriers to basal dislocation slip as well as twin propagation(Fig.2)[3].Furthermore,theβ′precipitates formed in the peak-aged Mg-Gd-based alloys have a plate shape(or lenticular shape),which is more effective in impeding dislocation slip than the globularβ′precipitates formed in the Mg-Y-Nd alloys such as WE54(Fig.3),which is also one of reasons why Mg-Gd-based aged alloys have the higher strength.Here it can be seen that the strengthening effect of precipitation is related to not only their number density and nanoscale,but also the habit plane and aspect ratio.Therefore,improving the Mg alloy strength requires an increase in the number density and aspect ratio of nanoscale precipitates formed on prismatic planes.

Although prismatic precipitates is the most effective in strengthening Mg alloys at room temperature,it can be seen from Table 1 that Mg-Gd(-Y)-Ag alloys exhibit higher strength than those Mg-Gd-RE alloys reinforced by prismaticβ′precipitates.Therefore,a better strengthening effect might be expected if more types of strengthening precipitates are introduced in the Mg-Gd-based alloys.It has been reported that the addition of Ag to the Mg-Gd[33–35,50]or Mg-Y[51]systems can significantl enhance the age hardening response.Nie et al.[52]also revealed that a further increase in the maximum hardness can be achieved when Ag and Zn are added together.The precipitations in Ag-containing Mg-Gd peak-aged alloys present two phenomenons.In Mg alloys with medium Gd content(such as 6 wt%),Ag additions can enhance the age-hardening response and this improvement in hardening is associated with the nanoscale precipitate plates formed on the(0001)basal plane ofα-Mg(named asγ′′,ordered hcp,P-62)which are not observed in the Ag-free Mg-Gd alloy,but unrelated withβ′prismatic precipitates[51].While in Ag-containing Mg alloys with relatively high Gd or(Gd+Y)content,co-precipitates,i.e.β′prismatic precipitates andγ′′basal precipitates,would appear in peak-aged condition(Fig.4)[33–35].The combined strengthening effect of nanoscale basal and prismatic precipitates with high number intensity and their relative perpendicular distribution contributes to the enhanced strength for the Mg-Gd(-Y)-Ag alloys with UTS＞400 MPa(Table 1).In addition,β′phase can be refine in the Ag-containing alloys,which also improves its strengthening effect[33].

2.2.Mg-RE-Zn casting alloys

By now it is well known that a special long period stacking ordered(LPSO)structure would be introduced in Mg-RE(RE=Y,Gd,Tb,Dy,Ho,Er,Tm)alloys via addition of Zn,Cu,Ni or Co[3,53–57].In addition,LPSO phase has also been found in Mg-Gd-Al alloys[58,59].LPSO structures are chemically and structurally ordered where RE and Zn atoms occupy Mg positions on neighboring{0001}planes.They share the{0001}basal plane of Mg but are stacking ordered along the c-axis,resulting in stacking periods such as 10H,14H,18R and 24R[60–64],though the detailed structures including Zn and RE atom positions remain controversial by far[65].Mg-Gd-Zn and Mg-Y-Zn series are the most popular LPSO-Mg alloys,and 18R-LPSO and 14HLPSO structures are the most commonly observed in Mg-RE-Zn alloys.Nie et al.[60–62]investigated the structure of LPSO in Mg-Y-Zn alloys in detail(Fig.5).The composition of 18R structure is suggested to be Mg10Y1Zn1.The 18R structure has an ordered base-centered monoclinic structure(a=1.112 nm,b=1.926 nm,c=4.689 nm,b=83.25°)and the stacking sequence of the closely packed plane is ABABCACACABCBCBCABA.The orientation relationship between the 18R andα-Mg matrix is(001)18R//(0001)αand[010]18R//＜11-20＞α.The composition of 14H structure is Mg12Y1Zn1.The 14H structure has an ordered hexagonal structure(a=1.112 nm,c=3.647 nm)and the stacking sequence of the closely packed planes is ABABCACACACBABA.The orientation relationship of the 14H withα-Mg matrix is(0001)14H//(0001)αand＜0-110＞14H//＜-1-120＞α.14H-LPSO phase is more stability than the 18RLPSO phase,and 18R structure can be completely replaced by the 14H structure after the appropriate heat treatment.According to the morphology difference,LPSO phases can be roughly divided into two types:block-like shape and needlelike(lamellar-likes/tripe-like)shape(Fig.6).

Fig.4.BF TEM micrograph(a)and the corresponding SAED pattern(b)of Mg-15.6Gd-1.8Ag-0.4Zr peak-aged alloy.The electron beam is parallel to[–2110]α.(c)SAED pattern with electron beam parallel to[0001]α [17,35].

Fig.5.＜11–20＞α HAADF-STEM images showing the characteristic features for the unit cell of(a)18R,(b)14H and(c)24R[61].

The strengthening and toughening mechanisms of LPSO phases involve the following aspects at least.(1)HRTEM observations showed that the LPSO phase-Mg matrix interface was coherent along both the basal and prismatic planes[67].(2)Deformation kinking of LPSO phase is important for both the strength and plasticity of Mg alloys.The kink deformation behavior and mechanism is peculiar to LPSO structure in Mg alloys(Fig.7)[67–71].(3)The formation of the LPSO phases would increase the critical resolved shear stress(CRSS)of the basal plane and the non-basal slip would be activated by the prevention of the basal slip.In other words,the former directly contributed to the strengthening and the latter promote the improvement of ductility with the increment of the number of the slip systems.Therefore,the LPSO phase may play a unique role which overcomes/relieves the conflictin properties of strength and ductility of Mg alloys[72,73].(4)The formation of LPSO phases with high density can inhibit the nucleation and growth of the deformation twinning[74].(5)LPSO phases especially with small size can effectively accelerate the refinemen of Mg recrystallized grains during deformation process,since a stress concentration frequently occurs in the vicinity of the Mg/LPSO two-phase interface owing to the strong plastic anisotropy of the LPSO phase[75].

Some researchers even consider that LPSO phases are the greatest strengthening phase in Mg alloys[76].Therefore,recent development of high performance Mg alloys involves a kind of Mg-RE-Zn alloys containing LPSO phase.However,the strengthening effect(both strength and ductility)of LPSO phase depends sensitively on size and morphology.The size of LPSO phases in as-cast Mg-RE-Zn alloys generally is too large to effectively strengthen the alloys.Therefore,all the casting Mg-RE-Zn alloys need further proper heat treatment to improve their strength via modifying the LPSO phases and increasing other fin precipitates.Table 2 lists most of,if not all,the high-strength Mg-RE-Zn(or rather Mg-Gd(-RE)-Zn)casting alloys with UTS＞350 MPa,which have been reported up to now.For these Mg-Gd(-RE)-Zn alloys,besides strengthening by the fine LPSO phases(also basal phase)due to heat treatment,the extra composite strengthening by prismatic precipitates(β′phase)and/or basal precipitates(γ′′,γ′or SFs)is very important for the high strength[77–84].

Fig.6.(a)Optical micrograph,(b)SEM image and(c,d)TEM images showing the two different morphologies of LPSO phases[66].

Table 2 Mechanical properties of the high-strength Mg-RE-Zn(or rather Mg-Gd(-RE)-Zn)casting alloys with UTS＞350 MPa.

Nie et al.[85,86]firstl reported that in Mg-6Gd-1Zn-0.6Zr with relatively low Gd content,only an precipitation sequence(i.e.γ′′andγ′)forms on(0001)basal plane ofα-Mg phase during ageing at 250°C,and both precipitate phases form as plate-shaped particles with a large aspect ratio.Later Nie et al.[65]proposed the whole precipitation sequence in a Mg-6Y-2Ag-1Zn-0.6Zr alloy during ageing at and above 200°C,and it is inferred to be:SSSS→G.P.zones→γ′′→γ′→γ(14H)+δ(δ: other intermetallic particles at grain boundaries).The firs stage of the precipitation reaction is the formation of G.P.zones(A-type plates in Fig.8a).The G.P.zones are single atomic plane disks on(0001)α(indicated by the red arrow in Fig.8b).The second stage is the formation of the metastableγ′′precipitate phase(B-type plates in Fig.8a).Eachγ′′streak is composed of three(0001)αatomic layers enriched by heavy atoms and no stacking fault is induced(Fig.8c).Given that theα-Mg matrix follows the ABA-type stacking sequence of the close-packed planes,the bright atomic columns in the middle layer ofγ′′plate occupy a position different from the “B”layer,marked by the red arrow in Fig.8(c).This new position is designated as D by Nie et al.,which is located at the intersection point of the diagonals in the red-color rectangle frame shown in Fig.8(c).Chen et al.[87]and Gu et al.[88]as well as Abe et al.[89]have also found the almost identical stucture ofγ”-precipitate plates formed in Mg-Gd-Zn alloys.The orientation relationship betweenγ′′andα-Mg phases is(0001)γ′′ //(0001)αand[10-10]γ′′ //[2-1-10]α.Theγ′′is the main strengthening precipitate in these Mg-Gd-Zn and Mg-Y-Ag-Zn alloys during the ageing hardening.γ′′phase has an ordered hexagonal structure(P6/mmm,a=0.556 nm,c=0.424 nm),and it would be gradually replaced byγ′and then 14H-LPSO during further ageing at 200°C or ageing at temperatures above 200°C.Fig.9 shows the HAADF-STEM images of precipitate plates includingγ′′,γ′,14H-LPSO phases in a over-aged(200°C/5800 h)Mg-6Y-2Ag-1Zn-0.6Zr alloy.Theγ′phase has an ABCA stacking sequence of the close-packed plane and a disordered hexagonal structure(space group P-3m1;a=0.321 nm and c=0.780 nm).The orientation relationship is(0001)γ′ //(0001)αand[2-1-10]γ′ //[2-1-10]α.Theγ′phase is also the building block that forms the basis of the unit cells of 14H phase as well as 18R phase[60,65].

Fig.7.HAADF-STEM images showing the(a)LPSO and SFs,(b)deformation kinks in the LPSO phase and SFs and(c–f)atomic-scale segregations in kink bands of the LPSO structure,Mg layer and SFs[68].

Fig.8.(a)Low-magnificatio HAADF-STEM image showing two types of precipitate plates(A and B marked in the image)in an under-aged(200°C/20 h)Mg-6Y-2Ag-1Zn-0.6Zr alloy.Fourier-filtere HAADF-STEM images of(b)an A-type plate having a monolayer of bright atomic columns(G.P.zone)and(c)a B-type plate having three bright atomic layers(γ ′′)[65].

Fig.9.(a)Low-magnificatio HAADF-STEM image showing three types of precipitate plates in over-aged(200°C/5800 h)Mg-6Y-2Ag-1Zn-0.6Zr alloy.(b and c)HAADF-STEM image showing small precipitate clusters of γ ′′phase;the insert image in(b)is from a section of a precipitate plate marked by the rectangular frame.(d)HAADF-STEM image showing a single γ′plate which is the building block of the 14H-LPSO phase;(e)HAADF-STEM image showing precipitate plates of 14H-LPSO phase[65].

Yamasaki et al.[84]revealed that the precipitation in the Mg-11.5Gd-2.4 Zn alloy with relatively high Gd content involves the formation of basal plane stacking faults(SFs,similar asγ′[68,89])and 14H-LPSO phase(also calledγphase)on(0001)αandβ′,β1andβphases.The SFs and 14H-LPSO precipitates reportedly form at intermediate and high temperatures(300–500 °C),whereas theβ′,β1andβphases form at low temperatures(200–300°C).More recently,Wu et al.[82]reported the co-precipitates,i.e.prismaticβ′precipitates and basalγ′precipitates,would be formed in peak-aged(200°C)Mg-15Gd-1Zn-0.4Zr alloy(Fig.10),and the compositeβ′andγ′precipitates provided much stronger strengthening effects than the onlyβ′precipitates.Therefore,the prismatic and basal co-precipitates should be the most effective precipitation mode by now and this is also the main reason why the strength of Mg-Gd(-RE)-Zn and Mg-Gd(-Y)-Ag casting alloys is higher than that of Mg-Gd-RE casting alloys in peak-aged condition.

3.Mg-RE wrought alloys

Generally speaking,the microstructure of wrought Mg alloys would be obviously refine by deformation process(extrusion,rolling,etc.)compared with the corresponding casting Mg alloys.The microstructure refinemen includes the

Fig.10.BF TEM micrographs and corresponding SAED patterns of the peak-aged Mg-15Gd-1Zn-0.4Zr alloy.The beams are parallel to the[0001]α direction in(a)and the[2-1-10]αdirection in(b)[82].

dynamic recrystallization(DRX)of Mg matrix and the fragmentation of relatively large secondary phase particles formed during solidificatio process.In addition,dynamic precipitation sometimes occurs in the Mg alloys[90].At last,the basal texture is easily introduced by deformation process though it is strong or weak in different Mg alloys.All above factors induce the higher strength in wrought Mg alloys compared with the initial casting Mg alloys.Furthermore,heat deformation process combined with heat treatment can further improve the strength of Mg wrought alloys via the additional ageing precipitation strengthening[90–95].In other word,the fin Mg grains combined with the dense and fin precipitates is the general reason for the ultra-high strength of wrought Mg-RE alloys.It can be found from this review that the alloy constitutions for most of high-strength wrought Mg alloys are similar to those for high-strength casting Mg alloys.

3.1.Zn-free Mg-RE wrought alloys

Table 3 lists most of,if not all,the high-strength Zn-free Mg-RE(or rather Mg-Gd)wrought alloys with UTS＞400 MPa,which have been reported up to now.The general route for preparation of these high-strength Mg alloys is casting bar→solid solution treatment→deformation process→ageing treatment.The alloy performance is depended on the controlling of both deformation and heat-treatment processes,besides alloy constitution.For example,the cooling rate of extruded rod just after extrusion would considerably affect the microstructure refinemen and finall the mechanical properties.Here some typical examples are reviewed briefl.Li et al.[91]reported that the combined processes of hot extrusion,cold rolling and ageing can significantly enhance the strength of Mg-14Gd-0.5Zr alloy.Cold rolling could introduce dislocations to provide nucleation sites for precipitates during the following ageing process.Subsequently,the much denser and fineβ′precipitates could form due to additional cold rolling process,which finall further improve the strength of Mg-14Gd alloy(Fig.11).Hong et al.[97]reported an ultra-high strength Mg-9Gd-4Y-0.5Zr alloy fabricated by one step conventional extrusion at relatively low temperature and subsequently peak ageing treatment.This al-loy has a bi-modal microstructure with two constitutions,i.e.stretched coarse-grain region with strong basal fibe texture and recrystallization fine-grai region.Ultra-high strength can be attributed to its strong extrusion texture in stretched coarse grains and dispersed nano-scale precipitates.More recently,Yu et al.[101]also reported a ultra-high strength Mg-11.7Gd-4.9Y-0.3Zr alloys by hot extrusion at relatively low temperature and ageing.The YS(500 MPa)of this alloy is even comparable to that of T6-treated 7075 Al alloys.It is considered that there is a composite strengthening in this alloy including the bimodal microstructure(Fig.12)consisting of coarse un-DRXed grains with strong basal texture and fin DRXed grains,Mg5RE particles,β′precipitates and solute-segregated SFs.The Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr[103]seems a special alloys which has obviously high strength compared with other Mg-RE wrought alloys.Jian et al.[103]suggested that ultrahigh strength of this alloy is attributed to the nano-spaced basal SFs formed during hot-rolling.

Table 3 Mechanical properties of the high-strength Zn-free Mg-RE(or rather Mg-Gd)wrought alloys with UTS＞400 MPa.

Fig.11.TEM images of β′precipitates in Mg-14Gd-0.5Zr by(a)extruded and peak-aged processes and(b)extruded,cold rolled and peak-aged processes.The electron beam is parallel to the[0001]α[91].

Fig.12.EBSD IPF maps and SEM image of hot-extruded Mg-11.7Gd-4.9Y-0.3Zr alloy showing the bimodal microstructure[101].

3.2.Mg-RE-Zn wrought alloys

Table 4 lists most of,if not all,the high-strength Mg-REZn wrought alloys with UTS＞400 MPa,which have been reported up to now.They are mainly Mg-Gd-Zn and Mg-Y-Zn systems.As mentioned above in Mg-RE-Zn casting alloys,the microstructure of these Mg-RE-Zn wrought alloys also has LPSO phases as the basic feature,and the difference is just that the LPSO phases in wrought alloys present obviously fine morphology with small size due to breaking during deformation process or precipitation in hot deformation or heat treatment.Generally,for these LPSO-containing Mg-RE-Zn alloys in as-extruded or as-rolled states,the part and basic strengthening mechanism is grain boundary strengthening of the fin Mg grains and dispersion strengthening of the fin LPSO phase particles with high volume fraction.For the LPSO-containing Mg-RE-Zn wrought alloys in peak-aged state,the part and general mechanism for high strength is related to the fin and homogeneous recrystallized grains,the numerous plate-shaped nano-scaleβ’phase precipitates and fin LPSO particles with block-like or lamellar morphology as well as other precipitates.Here some typical examples are reviewed briefl.

Xu et al.[92–95,110–112]made a series of studies on the Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr wrought alloy.An ultra highstrength Mg alloy sheet(Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr)with bimodal microstructure was successfully produced by largestrain hot rolling and subsequent ageing treatment[92].The notable improvement in strength is attributed to the deformed grains with strong basal texture and LPSO phases inside,theβ’particles at the grain boundaries of DRXed grains and especially the dense nano-scaleβ’precipitates in the DRXed grains.Another ultra-high strength and high ductility Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr Mg alloy sheet was fabricated by extrusion,large-strain hot rolling and following peak-ageing treatment[93].The ultra-high strength is obtained due to the dense distribution ofβ’precipitates within the grains,grains with strong basal texture and fin LPSO phases pinned at the grain boundaries.A high-strength and high ductility Mg extruded bar(Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr)was prepared by direct extrusion[111].The bimodal microstructure consisting of coarse unDRXed grains with strong basal texture and fin DRXed grains with relatively random orientations,and the dense fin SFs inside the original grains mainly contribute to the high strength.Based on this as-extruded alloy,further ageing treatment can improve strength again and help to obtain ultra-high strength and high ductility extruded Mg alloy bar[94].The relative strength mechanism is mainly related to strong texture strengthening of the unDRXed grains,theβ’+γ′co-precipitates within unDRX grains(Fig.13),grain boundary strengthening of the fin DRXed grains,and the denseβ’precipitates within DRXed grains.

Yu et al.[117–119,131–134]made a series of studies on the high-strength Mg-11Gd-4.5Y-1Nd-1.5Zn-0.5Zr wrought alloy.Most of the basic strength mechanism is roughly similar to that of reports by Xu et al.[92–95,110–112].Especially,they fabricated a ultra-high strength Mg-11Gd-4.5Y-1Nd-1.5Zn-0.5Zr via the combined processes of hot extrusion,cold rolling and ageing treatment[118].Cold rolling could enhance the ageing hardening effect,since the high density of dislocations facilitates the nucleation ofβ’phase during ageing treatment.Wu et al.[106,107]proposed a new extrusion method called differential-thermal extrusion(DTE)todevelop a high-strength Mg-15Gd-1Zn-0.4Zr extruded alloy.They found that DTE is a better extrusion process for producing high-strength Mg alloys than the traditional isothermal extrusion(ITE).Kawamura et al.[124]developed a Mg-Y-Zn alloy with YS above 600 MPa via rapid solidificatio powder metallurgy(RS P/M)processing.This alloy has long been a typical untra-high Mg alloy,which is attributed to its nanocrystalline microstructure with LPSO structure in fin

Table 4 Mechanical properties of the high-strength Mg-RE-Zn wrought alloys with UTS＞400 MPa.

Fig.13.TEM and STEM observations on the unDRX grains of the extruded and peak-aged Mg-8.2Gd-3.8Y-1Zn-0.4Zr alloy:(a)BF TEM image taken along[0001]α direction,(b)BF TEM and(c)STEM micrographs taken along[11-20]α direction[94].

grains with size of 50–200 nm.Chen et al.[125]reported that the equal channel angular pressing(ECAP)instead of RS/PM can also refin grain size to nanoscale(330 nm)and considerably enhance the strength of Mg-Y-Zn alloy.They also suggested that the LPSO structures contribute to the formation of ultrafin grain size of this ECAP processed alloy.More recently,Yang et al.[104]developed a special highstrength LPSO-free Mg-RE-Zn alloy with low alloying content(Mg-3.5Sm-0.6Zn-0.5Zr)via extrusion and ageing,and the alloy strength can compared with or greater than most conventional Mg-RE based alloys with high RE additions.The high strength is due to their excellent process control and finall mainly attributed to its ultrafin grains(470–500 nm)and numerous nano-scaleβ′+β′′precipitates.

3.3.Mg-Zn-RE wrought alloys

The high-strength Mg-Zn-RE wrought alloys mainly refer to the Mg-Zn-Y-based alloys.In Mg-Zn-Y casting alloy system,the main ternary intermetallic compounds vary with Zn/Y weight ratio.The dominating secondary phases change from I-phase to W-phase and to Z-phase based on the decreasing Zn/Y weight ratio(I-phase:Mg3Zn6Y,quasicrystal structure;W-phase:Mg3Zn3Y2,cubic structure;Z-phase:Mg12YZn/Mg10YZn,LPSO structure).With high Zn and low Y content(Zn/Y weight ratio≈4–6),the alloy(ZW61,ZW82,etc.)containsα-Mg and I-phase.With low Zn and high Y content(Zn/Y weight ratio≈0.35–0.55),the alloys(WZ62,WZ125,etc.)containsα-Mg and LPSO-phase.When the Zn/Y weight ratio is about 1.5–2,the alloy(ZW106,ZW53,etc.)containsα-Mg and W-phase.In other word,the Mg-Zn-Y alloys are mainly the Mg alloys containing I-phase or W-phase or I+W phases,while Mg-Y-Zn alloys are mainly the Mg alloys containing LPSO-phase or W+LPSO phases.

Table 5 lists most of,if not all,the high-strength Mg-Zn-RE(or rather Mg-Zn-Y-based)wrought alloys with UTS＞400 MPa,which have been reported up to now.It can be found that there are two typical high-strength Mg-Zn-RE alloys,i.e.W-phase strengthening alloys and I-phase strengthening alloys.Zheng et al.[137]developed a ultrahigh strength Mg-10.3Zn-6.4Y-0.4Zr-0.5Ca alloy containing W phase by conventional hot extrusion.After extrusion,the coarse W-phase in the as-cast alloy are broken into fin particles(～500 nm),and the DRXed grains are formed around these particles,which suggest that these fin broken W-phase particles promote the dynamic recrystallization and are effective in suppressing the grain growth during the hot extrusion process.The ultrahigh strength of this as-extruded alloy is mainly attributed the bimodal microstructure which consist of ultrafin DRXed grains(～500 nm)with fin broken W-phase particles at the DRXed grain boundaries and numerous nanoscale W-phase and(MgZn2,hcp)dynamically precipitated in the un-DRXed regions(Fig.14).This report certifie that the W-phase in fin morphology is also an effective strengthening phase in Mg alloys even though researchers generally do not like it compared with quasicrystal I-phase and LPSO phase previously.

Table 5 Mechanical properties of the high-strength Mg-Zn-RE wrought alloys with UTS＞400 MPa.

Fig.14.Microstructure of as-extruded Mg-10.3Zn-6.4Y-0.4Zr-0.5Ca alloy:(a)SEM image showing band structure;(b)SEM image of broken W-phase particles;(c)TEM image taken from both DRXed regions and unDRXed regions;(d)TEM image of the DRXed regions;(e)TEM image of the un-DRXed region;(f)HAADF-STEM image showing the morphology of precipitates[137].

One of the advantages of I-phase strengthening is that it just needs a little amount of Y addition compared with LPSO or W phases strengthening.Singh et al.[138]developed a high strength and high ductility Mg-8Zn-2Y extruded alloy(Fig.15).Extrusion at relatively low temperature makes the large interdendritic I-phase broken into fin particles with sizes ranging from 1μm to about 50 nm,and produces a very fine-graine microstructure with average size of recrystallized grains of about 1μm.Furthermore,numerous I-phase and(Mg4Zn7,monoclinic)co-precipitates with average size of 15 nm would be dynamically precipitated during extrusion from a supersaturated matrix.All above factors contribute to the high-strength of Mg-8Zn-2Y extruded alloy,just fabricated by a simple process using extrusion with a little addition of Y.

4.Summary and concluding remarks

We have presented a brief review of the recent development of high-strength Mg-RE casting and wrought alloys in this article.So far reportes indicate that the study of highstrength Mg-RE alloys is mainly foused on Mg-Gd and Mg-Y systems.Here,we make several conclusions and point out some areas which would be worthwhile for further scientifiinvestigation in the future.

(1)For developing high-strength Mg-RE alloys,precipitation strengthening is very crucial.The quality of precipitates is distinctly important.A higher strength can be achieved if a high density of nanoscale co-precipitates,i.e.plate-shaped precipitates with both prismatic and basal habit planes and also large aspect ratio,can be developed in the microstructure.At present,the microstructure of Mg-RE-Zn and Mg-RE-Ag systems contains both prismatic and basal plates.The basal plates in such a microstructure have a large aspect ratio(γ′′,γ′,basal SFs or 14H LPSO),and the aspect ratio as well as number density of the prismatic plates(β′,etc.)is much lower that typical precipitates formed in high-strength Al alloys.

(2)For developing high-strength Mg-RE alloys,it is equally important to develop the methods and technologies to realize refinin grain size of theα-Mg matrix down to the submicron or even nanometer scale to fulfil the grain refinemen strengthening.

(3)Realizing the full potential and making the full advantages of LPSO/SFs structure in Mg alloys would be promising in future development of high-strength Mg-RE alloys.Japanese scholars placed great emphasis on the importance of LPSO structure.

Fig.15.Microstructure of Mg-8Zn-2Y extruded alloy:(a)SEM image;(b)TEM image;(c)TEM micrograph showing precipitates[138].

(4)Other properties should be considered at the same time of high strength.For example,the strong basal texture via deformation processes is to the disadvantage of mechanical isotropy and secondary formability.

(5)In view of the value of RE resources,the development of RE-containing Mg alloys should consider how to achieve high strength on the basis of low RE content(that is also low cost)without waste of RE resources.

(6)Besides the alloying design,the process control is very important for high-strength of Mg-RE alloys,especially in low RE containing Mg alloys.

Acknowledgments

This work was supported by Natural Science Foundation of Heilongjiang Province of China(E2017030,ZD2017010),National Natural Science Foundation of China(51671063,51771060,51871069),Fundamental Research Funds for the Central Universities(HEUCFM181002)and Foundation of State Key Laboratory of Rare Earth Resources Utilization(RERU2018017).