Tailoring bimodal grain structure of Mg-9Al-1Zn alloy for strength-ductility synergy:Co-regulating effect from coarse Al2Y and submicron Mg17Al12 particles

Yong-Kang Li,Min Zh,*,Hai-Long Jia,Si-Qing Wang,Hong-Min Zhang,Xiao Ma,Teng Tian,Pin-Kui Ma,c,Hui-Yuan Wang,c,*

aState Key Laboratory of Superhard Materials,Jilin University,Changchun 130012,China 5988 Renmin Street,Changchun 130025,China

b Key Laboratory of Automobile Materials of Ministry of Education and School of Materials Science and Engineering,Nanling Campus,Jilin University,No.

cInternational Center of Future Science,Jilin University,Changchun 130012,China

Abstract Grain boundary strengthening is an effective strategy for increasing mechanical properties of Mg alloys.However,this method offers limited strengthening in bimodal grain-structured Mg alloys due to the difficultl in increasing the volume fraction of fin grains while keeping a small grain size.Herein,we show that the volume fraction of fin grains(FGs,～2.5μm)in the bimodal grain structure can be tailored from～30vol.% in Mg-9Al-1Zn(AZ91)to～52vol.% in AZ91-1Y(wt.%)processed by hard plate rolling(HPR).Moreover,a superior combination of a high ultimate tensile strength(～405MPa)and decent uniform elongation(～9%)is achieved in present AZ91-1Y alloy.It reveals that a desired bimodal grain structure can be tailored by the co-regulating effect from coarse Al2Y particles resulting in inhomogeneous recrystallization,and dispersed submicron Mg17Al12 particles depressing the growth of recrystallized grains.The finding offer a valuable insight in tailoring bimodal grain-structured Mg alloys for optimized strength and ductility.? 2021 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Magnesium alloys;Bimodal grain structure;Second-phase particles;Recrystallization;Strength;Ductility.

1.Introduction

Magnesium alloys,as an important category of low density(1.74g/cm3)engineering materials,continue to receive significan attention for weight-saving applications,particularly in the automotive,electronics and aerospace industries[1-3].Mg-Al-Zn alloys are the most commonly used and cost-effective commercial Mg alloys due to their relatively high strength,reasonable corrosion resistance,and good damping capacity[4].Among this alloy series,AZ91 is one of the most popular magnesium alloys because its relatively high strength at room temperature,good castability,and excellent corrosion resistance as well as the low-cost.However,AZ91 alloy usually exhibits poor plastic and inadequate strength in comparison to Al alloys,which severely restricts its widespread industry applications[2].

Recently,bimodal grain structures have been a subject of interest as they favor a desirable strength-ductility synergy in AZ91 alloys[5-8].For instance,Wang et al.reported that multimodal-grained AZ91 alloy possessed both high strength and high ductility in comparison to their fine-graine counterparts[6].Zha et al.investigated the influenc of deformation temperature on the formation of a bimodal grain structure in AZ91 alloy and explored the formation mechanism of bimodal grain structure by Schmidt factors analysis for grains of different sizes[7].Furthermore,based on the bimodal structures,Zhang et al.clarifie the individual roles of fin and coarse grains(CGs)as well as their synergy effect on enhanced tensile properties of a bimodal Mg-8Al-2Sn-1Zn alloy[9].Sun et al.reported that the heterogeneous grain distribution in AZ91 alloy can produce dislocation pile-up in the FG/CG interfaces and thus introduce the back-stress strengthening[10].Furthermore,Jin et al.developed a modifie Hall-Petch relationship,which depicts strengthening mechanisms of bimodal-grained AZ91 alloy consisting of both ultra-fine/fi grains and CGs[11].

On the basis of Jin et al.[11],it is believed that better performance can be obtained by optimizing the volume fraction of FGs in bimodal grain structures.Unfortunately,to the best of our knowledge,effects of the volume fraction of FGs on mechanical properties of bimodal grain-structured Mg alloys have rarely been reported,due mainly to the difficult in tailoring the volume fraction of FGs and meanwhile keeping a small grain size.It is attributed to the fact that grain growth is readily to occur in FGs region and hence diffi cult to increase the number of grain boundaries(GBs)[12].Hitherto,only Yang et al.[13]reported that the mechanical properties of a Mg-9Gd-4Y-0.5Zr alloy were successfully enhanced by increasing the volume fraction of FGs in the bimodal grain structure.Note that it remains challenging so far to manipulate the volume fraction of FGs in bulk Mg sample,owing to the inevitable grain growth during hot deformation process.

As well-known,recrystallization could be manipulated via particle stimulated nucleation(PSN)from coarse particles(<1μm)and Zener pinning effect from submicron particles(>1μm)concurrently[14].Some previous studies have reported that preexisting coarse particles can promote recrystallization by PSN[14,15],and other studies have provided evidence that fin particles can retard recrystallization[16,17].Potentially,it is believed that the bimodal grain structure can be tailored as desired,provided the PSN of recrystallization and Zener pinning of GBs during processing can be utilized properly.

Furthermore,the novel HPR technique has received considerable attention over the last years due to its favorable operability,large thickness reduction(～85%)and good consistency with regard to obtaining multimodal grain-structures in Mg alloys[6,7].The new HPR route,thus,offers opportunities for preparation of bimodal grain-structured Mg alloys and manipulating the volume fractions of FGs.Therefore,this study focuses on optimizing the volume fraction of FGs in bimodal grain-structured AZ91-Y alloys by co-regulating effect from coarse Al2Y and submicron Mg17Al12particles.In this work,we show that the volume fraction of FGs(～2.5μm)in the bimodal grain structure can be tailored from～30vol.% in AZ91 to～52vol.% in AZ91-1Y processed by HPR.Meanwhile,the CGs decrease in size and become more uniformly embedded in continuously connected fine-graine areas with increasing Y content.Therefore,a superior combination of a high ultimate tensile strength(～405MPa)and decent uniform elongation(～9%)is achieved in present optimized AZ91-1Y alloy.The present study can contribute to better understanding of the way to achieve desirable strength-ductility synergy in bimodal grain-structured Mg alloys.

2.Material and methods

The alloys with nominal compositions of Mg-9Al-1Zn-xY(AZ91-xY,x=0,0.5,1,1.5wt.%)were prepared by melting pure Mg(99.99%),pure Al(99.90%),pure Zn(99.90%),and Mg-23wt.% Y master alloy in an electric resistance furnace.The melt was flushe continuously with SF6and CO2gas(mixing ratio was 0.5:95.5)in a graphite crucible at 680 °C and then poured into a steel mold.The compositions of investigated materials were measured with an optical spectrum analyzer(ARL 4460,Switzerland),as summarized in Table 1.The as-cast ingots were solution-treated at 415°C for 20h to dissolve Mg17Al12particles and then quenched into hot water at～90°C immediately.The solutiontreated ingots were machined into plates with dimensions of 50mm×20mm×6mm.The plates were rolled to～1mm with a thickness reduction of～85% by a single pass via a novel HPR process.For the details of HPR where two hard plates(made of hardened steel～50 HRC,thickness of 1mm)were added between rollers and the sample during rolling,one can refer to[6].All HPRed sheets were annealed at 250°C for 5min for subsequent microstructure observations and tensile tests.Tensile test specimens with a dimension of 10mm×4mm×1mm in the gauge section were machined from the rolled sheets parallel to the rolling direction(RD).The tensile tests were performed on an AGS-X-100kN electric universal testing machine(SHIMADZU,Suzhou,China)at room temperature(～25°C)under a strain rate of 10?3s?1.At least 6 samples were tested for each condition to ensure the reliability of results.

Table 1The chemical compositions of AZ91-xY alloys(x=0,0.5,1,1.5wt.%)in weight percent(wt.%).

Table 2The volume fraction(fM)and average diameter(dM)of Mg17Al12 particles,as well as the volume fraction(fA)and average diameter(dA)of Al2Y particles.

X-ray diffraction(XRD,Model D/Max 2550PC Rigaku,Japan)was undertaken in Siemens D5000 at 50kV,Cu-Kαradiation,using 50 steps per degree and a count time of 1s per step.The analysis of XRD peak broadening is based on the Rietveld refinemen[18].The lattice parameters of Al2Y and microstrains of Mg matrix were estimated using XRD line broadening analysis with the Material Analysis Using Diffraction(MAUD)software[19,20].The scanning electronic microscopy(SEM)samples were prepared via grinding and polishing,followed by chemically etching in acetic picric solution(5g picric acid,5ml acetic acid,10ml distilled water,80ml ethanol).The microstructures were examined by SEM(VEGA 3 XMU,TESCAN,Czech)equipped with energy disperse spectrometer(EDS,X-MAX50).According to quantitatively stereology[21],the area fraction could represent volume fraction in the metals.Therefore,the volume fractions and diameters of precipitates were analyzed by Image J software.At least ten images were included for each condition to ensure the reliability.The specimens were treated by mechanical grinding and then electro-polished with an AC2solution at 20V for 120s for electron backscattered diffraction(EBSD).EBSD was performed at an accelerating voltage of 20kV a step size of 0.4-0.6μm.EBSD data were collected using AZtec and Channel 5 software to analyze grain orientations,grain sizes and micro-textures,etc.The nanosized particles were investigated by transmission electron microscopy(TEM,JEM-2100,JEOL,Japan).Thin foils for TEM were prepared by mechanical polishing(20-30μm)and then prepared through ion milling for～3h at 3 to 5kV.Using TEM samples,transmission Kikuchi diffraction(TKD)was performed using a JSM-7900F instrument(JEOL,Japan).

Fig.1.(a)The XRD patterns of HPRed AZ91-xY alloys(x=0,0.5,1,1.5wt.%);(b)enlarged section of the XRD patterns.

3.Results

3.1.Microstructure observations

Fig.1 illustrates XRD patterns of the HPRed AZ91-xY alloys.Note that Al2Y phase appears in the Y-containing alloys,in addition to the Mg17Al12phase and Mg matrix.Fig.2(a)-(h)illustrates SEM microstructures of HPRed AZ91-xY samples.One can see that coarse Al2Y particles are randomly distributed in the Mg matrix,as marked by yellow arrows(Fig.2(b)-(d)),as identifie by EDS in Fig.3.EDS analysis indicates that the block-shaped phase(A point)mainly consists of Al and Y elements(Fig.3).EDS analyses of a total of 6 block-shaped phase show the atomic ratio of Al:Y is,which is close to the atomic ratio of Al2Y phase[22].Moreover,the lattice parameter of the new phase was calculated to be 0.784nm from XRD data using MAUD software,in accord with the theoretical value(0.772nm)of Al2Y[23].Combined with EDS and XRD analyses,the new phase is therefore identifie as Al2Y phase.Moreover,the magnifie maps reveals that numerous Mg17Al12particles of submicron sizes(200-500nm)have precipitated along GBs in FGs region,as marked by white arrows(Fig.2(e)-(h)).TKD-EBSD experiments were performed to present the orientation relationship between Al2Y particles and Mg grains in the AZ91 sample(Fig.4).A total of ten Al2Y particles surrounding by Mg grains were analyzed by using AZtec and Channel 5 software.One can see that the Al2Y particles are nearly random-oriented,without a definit orientation relationship with respect to the surrounding Mg grains(Fig.4(b)).Although an epitaxial orientation relationship between Mg and active Al2Y inoculants in a Mg-10wt.% Y alloy,i.e.[110]Mg‖[1]Al2Y,(0002)Mg‖(22)Al2Y,was identifie using TEM[24],it is reasonable that Al2Y particles in present AZ91-Y samples are nearly random-oriented as the particles have been fragmented and redistributed in the matrix during HPR process.

Fig.2.SEM micrographs of the HPRed samples(a)AZ91,(b)AZ91-0.5Y,(c)AZ91-1Y,and(d)AZ91-1.5Y;(e-h)the FG regions corresponding to(a-d),respectively.

Fig.3.(a)SEM image and EDS mapping analysis for HPRed AZ91-0.5Y alloy and(b)the EDS composition analysis of point A.

Fig.4.(a)EDS mapping analysis of AZ91-1Y in TKD experimental sample;(b)phase mapping by TKD to show Al2Y particles and Mg grains.Orientations of Al2Y are represented by unit cells and Euler angles.

The volume fractions and average diameters of Mg17Al12and Al2Y particles in AZ91-xY alloys are summarized in Table 2 and plotted in Fig.5.Especially,the volume fraction(fA)distribution with diameter(dA)shows the escalating trend of Al2Y(Table 2 and Fig.5(b)).ThefAanddAof the Al2Y particles increase from 0.5vol.% and～2μm to 10.2vol.% and～9μm,respectively,with Y addition from 0.5 to 1.5wt.%(Fig.5).Meanwhile,the volume fraction of Mg17Al12particles continually decreases from～8.6vol.% in AZ91 to～3.3vol.% in AZ91-1.5Y(Fig.5(a)and Table 2).For comparison,the mass fractions of Al2Y and Mg17Al12particles have also been calculated by Jade 6 software.The density of Mg,Al2Y and Mg17Al12is 1.74,3.84 and 2.08g/cm3,respectively.The volume fraction of Al2Y is estimated to be～0.4,～8.9 and～11.0vol.% in AZ91-0.5Y,AZ91-1Y and AZ91-1.5Y,respectively.For Mg17Al12,the corresponding value is estimated to be～7.9,～6.5,～6.1 and～3.8vol.%,respectively.Obviously,the values calculated by XRD data are in well accordance to those derived from SEM data.

Fig.6(a)-(d)shows EBSD maps of HPRed AZ91-xY samples.Notably,all HPRed samples feature a bimodal grain structure consisting of CGs of several tens of micrometers and FGs of a few micrometers.The CGs are unrecrystallized ones containing profuse substructure,meanwhile the FGs are recrystallized ones with an average size of～2.5μm bounded mainly by high angle boundaries(HABs,>15°).The black areas in Fig.6(c)and(d)corresponding to unidentificatio points rise significantl as the added Y content increases.These regions,located along GBs and later identifie as Al2Y particles and submicron recrystallized Mg grains surrounding them,are extremely difficul to resolve by EBSD.The corresponding pole figure of FG regions in AZ91-xY samples are presented in Fig.6(e)-(h),showing that the intensity of basal texture of FGs is slightly reduced with Y added.Nevertheless,the section of the XRD patterns between 30° and 40° presented in Fig.1(b)shows that the change of Y addition induces a shift to small angle for Mg diffraction peaks.Note thatα-Mg diffraction peaks shift towards lower angles(i.e.larger lattice parameter)which does not increase with Y addition.It indicates that there is a constant amount of Y atoms in solid solution in the investigated AZ91-xY alloys,although most of the added Y react with Al to form Al2Y particles as revealed by XRD and EDS analysis.Moreover,a small amount of Y atoms in solid solution can promote the activation ofdislocations[25],which can partially accounts for the slightly weakened basal texture of FG regions with Y added as shown in Fig.6(e)-(h).Meanwhile,the constant amount of Y atoms in solid solution in Mg matrix(Fig.1(b))may explain that the texture has not changed appreciably with added Y content increasing.The grain size distributions of FGs in HPRed AZ91-xY samples are summarized in Fig.7.The volume fraction of FGs increases from～30 to～52vol.% in the AZ91-xY alloy with an increase in Y content from 0 to 1wt.%.It suggests that the volume fractions of FGs and CGs can be successfully tailored by forming different amounts of Al2Y and Mg17Al12particles and with varying Y additions.To facilitate this discussion,we added optical images of HPRed AZ91-xY in a Supplementary data(Figure S1).Obviously,the bimodal grain structures change gradually with increase of Y content.Though both the average grain size and volume fractions of CGs decrease in AZ91-1Y sample(Fig.7(c)),bimodal grain structures still existed in the AZ91-1Y sample.

Fig.6.EBSD orientation maps of the HPRed AZ91-xY alloys:(a)AZ91,(b)AZ91-0.5Y,(c)AZ91-1Y,and(d)AZ91-1.5Y;pole figure corresponding to FGs of HPRed AZ91-xY alloys:(e)AZ91,(f)AZ91-0.5Y,(g)AZ91-1Y,and(h)AZ91-1.5Y.It may be noted that the black regions in EBSD orientation maps cannot be recognized due to the presence of Al2Y particles confirme by EDS analysis.

Fig.7.The grain size distributions of HPRed AZ91-xY alloys:(a)AZ91,(b)AZ91-0.5Y,(c)AZ91-1Y,and(d)AZ91-1.5Y.

Thereafter,the impact of coarse Al2Y particles on microstructure evolution of AZ91-Y alloy during HPR is revealed by EBSD maps,which were taken in CGs regions around coarse Al2Y particles.Fig.8(a)and(b)shows EBSD mapping of the HPRed AZ91-1Y sample and band contrast image of a local region i,respectively.The presence of coarse Al2Y particles in magnifie region i is verifie by EDS analysis in Fig.8(b).Fig.8(c)shows detailed misorientation angles(in degrees)of GBs surrounded the coarse Al2Y particles.The PSN process induced by coarse Al2Y particles is analyzed by investigating regions i and ii collectively(Fig.8).According to the recrystallization theory of two-phase alloys[14,16,26],the regions surrounding coarse particles possess relatively high stored energy,which is preferential areas for recrystallization nuclei.For region i,after HPR,the misorientation angles of GBs adjacent to Al2Y particles are much higher than that in areas far away from it.A large number of FGs with misorientation angles of～20-50° have formed and even some ultrafin HAB grains can be observed around the Al2Y particle.It can also be seen that the misorientations of HABs generally decrease with increasing distance from the Al2Y particles.For region ii,the PSN process is almost complete and fin DRX grains are commonly observed.The deformation zone is consumed by the nucleation and growth of recrystallization grains.It reveals that Al2Y particles play a key role in promoting recrystallization process and hence the formation of FGs,which is effective for manipulating the volume fraction of FGs in bimodal grain structure of AZ91 alloy.

Furthermore,dislocation density(ρ)in the regions surrounding Al2Y particles can be roughly estimated byρ≈θ/(bδ),whereθis the accumulated misorientation angle in degrees within a distanceδ,andbis the Burgers vector[27,28].By assumingb=3.2×10?10m for pure Mg and deriving the values ofθandδfrom misorientation profile in CGs interiors,the dislocation density in CGs of HPRed AZ91-xY samples can be roughly estimated form Fig.6(a)-(d).The estimated local dislocation density values are 1.9-3.1×1014,4.1-5.8×1014and 12.4-13.8×1014m?2in the samples with 0,0.5,and 1wt.% Y additions,respectively.As CGs are barely visible in the HPRed AZ91-1.5Y sample,which is out of consideration here in dislocation density calculations.By comparison,the dislocation density has also been calculated by Williamson-Hall technique[19,29,30],which is～16.4×1014,～27.3×1014and～54.5×1014m?2for the HPRed AZ91,AZ91-0.5Y and AZ91-1Y alloys,respectively.Note that the values of dislocation density calculated by Williamson-Hall technique are higher than those estimated from EBSD data.It is reasonable as the latter only provides density of geometrically necessary dislocation.Nevertheless,the dislocation density values calculated from XRD and EBSD data both reflec the prominent role of Al2Y particles on high-density dislocation accumulation in the HPRed AZ91-xY alloys.

Fig.8.(a)EBSD orientation maps showing evidences of PSN induced by coarse Al2Y particles(region i)and the completed recrystallization(region ii)in the HPRed AZ91-1Y sample;(b)magnifie SEM map of region i and EDS mapping analysis;(c)misorientations(in degrees)of selected GBs around Al2Y particles in region i.

TEM micrographs of FG regions of AZ91-1Y are presented in Fig.9(a)-(f).Fig.9(a)shows that recrystallization nuclei sites are developed in areas containing high strain energy around the Al2Y particle,where the occurrence of PSN is further confirme by the relatively low dislocation contrast of the ultra-fin grains.Fig.9(b)and(c)are representative micrographs showing sharp clean GBs of the nearly formed regulated ultrafin Mg grains.Fig.9(d)-(f)shows some spherical Mg17Al12particles of 200-500nm precipitated along GBs.The corresponding diffraction pattern for the Mg17Al12precipitates indicated by white cross is shown in Fig.9(f).These fin Mg17Al12particles dispersed at GBs are effective in pinning GBs migration and restricting grain growth[14,17],(Fig.9(d)-(f)).It reflect that the volume fraction and size of FGs in the bimodal grain structure is closely related to the dispersed fin Mg17Al12particles,which is beneficia for attaining a bimodal grain structure with increased volume fraction of FGs.

3.2.Mechanical properties

The tensile engineering stress-strain curves of HPRed AZ91-xY samples are shown in Fig.10(a).The corresponding ultimate tensile strength(UTS),yield strength(YS)and uniform elongation(Eu)are summarized in Fig.10(b).The Eu,UTS and YS increase from～7.1%,～376 and～288MPa in AZ91 to～9.4%,～405 and～323MPa in AZ91-1Y,respectively.Compared to other studies regarding AZ91[1,2,6,9-11,31,32],the present optimized AZ91 alloy by Y addition possesses impressive mechanical properties with a combination of UTS being～405MPa,YS being～322MPa,and 9.4% Eu.Remarkably,the toughness,area under the stress strain curve to the point of fracture,increases significantl from～2.71×106J/m3in the AZ91 alloy to～3.78×106J/m3in the AZ91-1Y alloy(Fig.10(c)),which is competitive related to the toughness of rolled AZ91 in the range of～1.7-3.6×106J/m3reported in literature[1,2,6,11,31,32].Fig.10(d)compares the elongation and UTS values of AZ91-xY alloys and wrought AZ91 alloys reported in literature.Although the strength of AZ91 alloys can be greatly improved by SPD processes like ECAP and ARB,which is usually at the expense of uniform ductility,i.e.,mostly less than 5%(Fig.10(d)).Thereby,the present HPRed AZ91-1Y alloy possesses a superior combination of high strength and ductility in comparison to present HPRed AZ91 alloy and also the published results.Based on the modifie Hall-Petch relation of bimodal grain structured AZ91 alloy[11],the high UTS of the HPRed AZ91-1Y alloy should be attributed partially to the increased volume fractions of FGs,i.e.,from～30 to～52vol.%.

Fig.9.TEM maps of FG regions in the HPRed AZ91-1Y alloy:(a)Al2Y particles and PSN recrystallized grains;(b)-(d)bright fiel images showing ultrafin grain features,Mg17Al12 precipitates and dislocation tangle zones;(e)microstructure of the FG regions;and(f)the magnifie map of(e),showing Mg17Al12 precipitates pinning GBs and the insert is the corresponding diffraction pattern of the Mg17Al12 precipitate indicated by white cross.

However,UTS and YS in the AZ91-1.5Y alloy decline to～365 and～295MPa,respectively.Meanwhile,the Eudecreases to～7%.The decreased YS is mainly due to the weakened GBs strengthening from decreasing volume fraction of FGs(～32vol.%,Fig.7(d)).In addition,the present of numerous coarse hard Al2Y particles in AZ91-1.5Y is prone to inducing microcracks,due to particle breaking-up and void formation around them,especially within areas clustered with particles.This will normally lead to a decreased ductility and toughness[33,34](Fig.10).

4.Discussion

4.1.Effect of second-phase particles on recrystallization process and microstructure evolution

According to the PSN theory[14,15,26],DRX grains that nucleate and grow in present samples during HPR involves two competing processes,i.e.,driving pressure from stored energy surrounding coarse Al2Y particles promotes DRX(Fig.8(a)-(c)),and Zener pinning pressure from dispersed submicron Mg17Al12particles on GBs retards grain growth(Fig.9(d)-(f)).

The driving pressure(PD)for recrystallization due to PSN of coarse Al2Y particles can be calculated as[26,28]:

whereαis a constant equal to 0.5,Gis the shear modulus(1.66×104MPa),bis the Burgers vector(3.2×10?10m)andρis the dislocation density in CGs described inSection 3.1.According to calculation results,the values ofPDare～2.1,～3.9 and～11.2×105N m?2for AZ91,AZ91-0.5Y and AZ91-1Y samples,respectively.

The Zener pinning pressure(Pz)from fin dispersed Mg17Al12particles can be calculated as[28,35]:

wherefManddMare volume fraction and average diameter of Mg17Al12particles,respectively(see Table 2),andγbis the specifi energy from the curvature of GBs(0.5J m?2for Mg[28]).According to calculation results,the values ofPZare～5.2,～3.8,～3.1 and～2.0×105N m?2for AZ91,AZ91-0.5Y,AZ91-1Y and AZ91-1.5Y,respectively.The calculatedPDandPZvalues for HPRed AZ91-xY samples with varying Y contents are shown in Fig.11(a).Furthermore,with retarding pressure from boundary curvature(Pc)and grain radius(R),fina equation of driving pressure for recrystallization(P)is[36]:

Table 3The volume fraction(f)and weighted average(d)of grain size of the CGs and FGs.

Fig.10.(a)Engineering stress-strain curves of the HPRed AZ91-xY alloys;(b)evolution of mechanical properties with varying Y contents;(c)toughness histogram of the HPRed AZ91-xY alloys(x=0,0.5,1,1.5wt.%);(d)a comparison of the elongation and UTS values between the HPRed AZ91-xY alloys and previously reported AZ91 alloys(ECAP[10,48-50];rolling[4,51-53];multidirectional forging,MF[54];asymmetric hot rolling,ASR[31];accumulative roll bonding,ARB[55];differential speed rolling,DSR[42,56-58];extrusion[8]).

In particular,with taking into account the value ofPc,the different values betweenPDandPZplay a major role in retarding recrystallization process.For example,PDvalue is～11.2×105N m?2for the AZ91-1Y sample,which is much larger than thePZvalue of～3.1×105N m?2for the AZ91-1Y sample.Thus,recrystallization is more likely to occur and a high volume fraction of FGs were obtained in the HPRed AZ91-1Y sample.In contrast,PDvalue is～2.1×105N m?2for the AZ91 sample,which is smaller than thePZvalue of～5.2×105N m?2.Therefore,DRX become difficul in some grains due to insufficien driving pressure for DRX and suffi cient Zener pinning pressure in AZ91 alloy.It is believed that the collective effect of PSN induced by coarse Al2Y particles and Zener pinning by submicron Mg17Al12precipitates can be utilized to tailoring the volume fraction of FGs in the bimodal grain structured AZ91 alloy.It may be noted that there is a critical volume fraction of Al2Y particles(～8vol.%),below which a greater number of fin recrystallized grains are induced by PSN.However,a further increase in the volume fraction of hard Al2Y particles is prone to inducing microcracks upon deformation due to likely cracking in the sample.

4.2.Strengthening mechanisms

Tensile properties of present HPRed AZ91-xY samples agree well with the variation trend of volume fractions of FGs in the bimodal grain structure,and the AZ91-1Y sample shows a high strength and decent ductility.The enhanced yield strength of bimodal-grained structured materials can be estimated from various strengthening factors:GBs,precipitates,and solid solution in the matrix[32,37-39].

Recently,different approaches based on Hall-Petch relation have been suggested to evaluate strengthening mechanisms by considering different microstructural parameters including HABs,low angle grain boundaries and dislocations[40].As present AZ91-xY alloy has a bimodal grain structure,it appears more reasonable to estimate the GBs strengthening contribution from a modifie Hall-Petch relation[11]:

Fig.11.(a)Calculated values of the driving pressure from Al2Y particles and Zener pinning pressure from Mg17Al12 particles with varying volume fraction of FGs;(b)contributions of GB strengthening,precipitate strengthening and solid solution strengthening to the YS of AZ91-xY(x=0,0.5,1,1.5wt.%);and(c)comparison between the calculated and measured YS values.

whereffinandfcoarseare the volume fractions of FGs and CGs,respectively,whiledfanddcare the average grain sizes of the FGs and CGs,respectively.For a more accurate estimation,the relevant information is summarized in Table 3.According to the modifie Hall-Petch relation for the bimodal grained Mg alloys[11],σ0,f～130 andkf～290 are inputted for FGs,meanwhile,σ0,c of～78 andkc of～507 are used for CGs.To accurately calculate the strengthening contribution from GBs,the weighted average of grain size(dcfor CGs anddffor FGs)has been proposed(Table 3).Thereby,the GB-strengthening contribution is～201,～213,～262,and～241MPa for the AZ91,AZ91-0.5Y,AZ91-1Y and AZ91-1.5Y samples,respectively(Fig.11(b)).Notably,the contribution of GBs strengthening amounts to～70% of the overall strength in present HPRed AZ91-xY samples.

Orowan strengthening is another mechanism responsible for the rapid increase in the strength of HPRed AZ91-xY alloys containing dispersed articles.The relationship between strength increment and parameter of precipitates(fandd)in deformed metallic materials can be described by Eq.(5)[41,42]:

whereMis the Taylor factor,G=1.66×104MPa is the shear modulus andbis the Burgers vector,v=0.35 is the Poisson’s ratio in Mg alloys[42].TheMdepends on texture of Mg which ranges from 2.1 in a condition of very strong texture and only pyramidal slip activated,to 4.5 in a condition of random texture[43,44].In the present work,Mvalue of FGs is taken as 4.5 since DRXed regions exhibit random texture,whileMvalue of CGs is taken as 2.5 due to the strong basal texture.The volume fraction(f)and diameter(d)of Mg17Al12and Al2Y particles in HPRed AZ91-Y alloys are shown in Table 2.Accordingly,the contribution from precipitate strengthening is calculated to be～37,～28,～26,and～16MPa in the HPRed AZ91,AZ91-0.5Y,AZ91-1Y,and AZ91-1.5Y samples,respectively(Fig.11(b)).

The Al and Zn solid solutions are known to strengthen Mg-Al-Zn alloys through reducing dislocation mobility by atomdislocation interactions.In our previous work[11],strengthening contribution from solid solution was estimated to be～40MPa assuming that all Al and Zn atoms are in solid solution in HPRed AZ91 sample(Fig.11(b)).Note that it is actually overestimated as the concentration of Al solute atoms reduces with Mg17Al12and Al2Y formed.It is apparent that contributions from precipitate strengthening and solid solution strengthening are minor relative to the GBs strengthening contribution.

The total yield stress can be assumed to be the sum of various strengthening contributions at the yield point(ε0=0.2%),as follows:

whereσYSis the total yield stress.The calculated values for a range of strengthening conditions are presented in Fig.11(c).Accordingly,the total yield stress is estimated to be～277,～281,～328,and～297MPa in the HPRed AZ91,AZ91-0.5Y,AZ91-1Y,and AZ91-1.5Y samples,respectively(Fig.11(c)).These calculated values are consistent with the measured values with an average deviation of 4%.Note that the AZ91-1Y sample exhibited highest uniform elongation(～9%)among all the samples.It suggests that the volume fractions of FGs can be tailored by forming different amounts of Al2Y and Mg17Al12particles with varying Y additions in the AZ91-xY alloy,i.e.,increases from～30 to～52vol.%with an increase in Y content from 0 to 1wt.%(Fig.7).It is considered that the better ductility behavior of AZ91-1Y sample is attributed to the modifie characteristic of CGs and increased volume fraction of FGs in the bimodal grain structure.The optimized bimodal structure has been proposed as consisting of CGs uniformly embedded in continuously connected FGs.In other words,the change of grain size of CGs from about two hundred micrometers to several tens microns became more favourable for ductility.Liu and Wu et al.[45,46]have reported that the“embedded coarse grain”is more effective for the accumulation of geometrically necessary dislocations,thereby leading to enhanced ductility.Meanwhile,interface of CGs and FGs also act as obstacles to crack propagation.Consequently,the AZ91-1Y sample having the CG zones embedded in the fine-graine matrix is less prone to crack[47].

5.Conclusions

To summarize,we have obtained various volume fractions of FGs,from～30 to～52vol.%,in a bimodal grainstructured AZ91-xY alloy processed by HPR via co-regulating effect from coarse Al2Y and submicron Mg17Al12particles.An optimized volume fraction of FGs(～52vol.%)and wellmaintained FGs size have been obtained in the HPRed AZ91-1Y alloy,which exhibits a high strength and good ductility,i.e.,a YS of～323MPa,UTS of～405MPa and Euof～9%.The high strength and uniform ductility is attributed mainly to the increased volume fraction of FGs featured with a weakened texture.This study reveals that the volume fraction and size of FGs in bimodal grain structure could be manipulated by the co-regulating effect from the driving pressure induced by coarse particles and the pinning pressure of submicron particles on the grain growth offers a key insight into understanding the relationship between the mechanical properties and volume fractions of FGs in bimodal grain-structured AZ91 alloys.

Acknowledgments

This work was primarily supported by The Natural Science Foundation of China under Grant Nos.51922048,51871108,51625402 and 51671093.Partial financia support came from the Fundamental Research Funds for the Central Universities,JLU,Program for JLU Science and Technology Innovative Research Team(JLUSTIRT,2017TD-09),and The Changjiang Scholars Program(T2017035).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2021.01.008.