Cooling rate controlled basal precipitates and age hardening response of solid-soluted Mg-Gd-Er-Zn-Zr alloy

Jinlong Fu ,Weno Du,* ,Linyue Ji ,Yunfeng Wng ,Xunming Zhu ,Xin Du

a Faculty of Materials and Manufacturing,Beijing University of Technology,Beijing 100124,China

b Weihai Wanfeng Magnesium S&T Development Co.LTD,264200,China

Abstract The precipitation and age hardening response of the solid-soluted Mg-10Gd-1Er-1Zn-0.6Zr (wt.%) alloy performed by water-quenching(QC),air-cooling (AC) and furnace-cooling (FC) in terms of the volume fraction of precipitates and tensile properties were investigated in present paper.Results indicated the solid-soluted alloy contained stacking faults (SFs) and long period stacking ordered (LPSO) phase on the basal planes regardless of the cooling rate,but a larger volume fraction of the LPSO phase was formed with decreasing in the cooling rate.After aging, β′ and β1 phases precipitated on the prismatic planes,and their number density decreased but mean particle size increased with decreasing in the cooling rate.The solid-soluted alloys (QC,AC and FC samples) showed no apparent difference in yield strength (YS),but their correspondent peak-aged alloys exhibited sharp difference in hardening response.The strongest hardening response took place in the QC sample and showed 82MPa enhancement in YS,which was much larger than that of AC (+26MPa) and FC samples (+5MPa).The reason lies in that the higher cooling rate promotes the precipitation and reduces the average size of β′ precipitate.A novel cooling-rate controlled precipitation model with respect to the correlation of precipitates on basal and prismatic planes was established.From this model,the basal precipitates showed a restrictive effect on the growth and/or coarsening of β′ precipitate,and composite precipitates containing the β′ phase with fin size as well as high area-number density and lower volume fraction of the LPSO phase are preferred to strengthen the Mg-10Gd-1Er-1Zn-0.6Zr alloy.

Keywords: Magnesium alloy;Long period stacking ordered (LPSO);Stacking faults (SFs);Cooling rate;Tensile properties.

1.Introduction

Magnesium (Mg) alloys,known as the lightest structural metal,have received increasing attention in aerospace and automotive industries due to their high specifi strength and low fuel consumption [1,2].However,the poor strength and ductility of Mg alloys seriously restrict their wide application,especially compared with steels and aluminum alloys.Mg-rare earth (RE) alloys,particularly Mg-Gd system,have drawn considerable scientifi interest because they have potentials of improving mechanical properties and weakening basal texture by effective precipitation [3-5].For example,Mg-Gd-Zn alloys usually precipitate a long-period stacking ordered (LPSO) phase which is beneficia for improving strength and ductility [6].Xu et al.[7] reported that the block-shaped LPSO at grain boundaries could reduce ultimate tensile strength (UTS) and ductility whereas the lamellarshaped LPSO phase inside grains could promote the yield strength (YS) of Mg-Gd-Y-Zn-Zr alloys.Besides the LPSO phase,solute segregated stacking faults (SFs),also calledγ′phase in some literatures,are another strengthening phase in Mg-RE-Zn alloys.The SFs usually precipitate on basal planes and show larger contribution on strength than the LPSO phase [8].Jian et al.[9] reported that the nano-spaced SFs in Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr (wt.%) alloy processed by conventional hot rolling showed UTS of～600MPa,YS of～575MPa,and elongation of～5.2%.These SFs were found to be tremendously effective in impeding dislocation slip and promoting strain hardening.Recently,Xu et al.[10]compared the effects of SFs and LPSO phase on mechanical properties of Mg-Gd-Y-Zn-Zr alloy.Their results showed that the alloy containing dense SFs exhibited higher strength but lower ductility.So far,LPSO phase is believed to be more effective in improving ductility while SFs contribute more to enhancing strength [11].In addition to the LPSO phase and SFs precipitated on basal planes,metastable beta series phases,mainly containingβ′andβ1on prismatic planes,are considered as dominant contributors to strengthening Mg-Gd system alloys,especially when RE content is more than 10wt.%.Xu et al.[8,10,12] made a series of researches on Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) wrought alloy on the routes of hot extrusion,hot rolling and aging treatment.They concluded that the remarkable improvement on strength was attributed to the strong basal texture and the nanoscaleβ′precipitates.Besides,Yu et al.[13,14]produced Mg-11Gd-4.5Y-1Nd-1.5Zn-0.5Zr(wt.%) wrought alloy through a combination of hot extrusion,cold rolling and aging treatment,and they found that the UTS and YS of the alloy reached 547 and 502MPa,respectively.

Generally,above basal and prismatic precipitates can be controlled by adjusting heat treatment temperature and time according to the time-temperature-transformation (TTT)diagram [15].Recent studies [16,17] have also shown that these precipitates can probably be affected by the cooling rate after heat treatment.In present paper,we performed three kinds of cooling methods,i.e.water-quenching,air-cooling and furnace-cooling for the solute-treated Mg-10Gd-1Er-1Zn-0.6Zr (wt.%) alloy,then conducted aging at 200 °C to obtain various volume fraction of the basal and prismatic precipitates.The microstructural evolution and age hardening response were systematically investigated and a cooling rate controlled precipitation strengthening model with respect to corporation of the basal and prismatic precipitates was introduced.

2.Experimental

The ingot of Mg-10Gd-1Er-1Zn-0.6Zr (wt.%,designated as GEZ1011K hereafter) alloy was fabricated by melting the high purity Mg (99.99%),Zn (99.99%),Mg-30wt.%Gd,Mg-30wt.%Er and Mg-24wt.%Zr master alloys in an electric resistance furnace under the mixed protective atmospheres of N2and SF6.The melt was poured into a permanent mould preheated to 250 °C.The ingot actual chemical composition was determined as Mg-10.23Gd-1.20Er-0.89Zn-0.67Zr(wt.%) by X-ray Fluorescence analyzer (XRF-1800).The specimens cut from the ingot were solution treated at 510 °C for 12h in an electric resistance furnace under Ar atmosphere,then quenched by water,cooled in air and in furnace,respectively.The water-quenched,air-cooled and furnace-cooled specimens are denoted as QC,AC and FC,respectively.The QC,AC and FC specimens were aged at 200 °C in an oil bath for various times up to 150h.The samples after aging treatment are denoted as QCA,ACA and FCA,respectively.

The phase constitutions were identifie by X-ray diffraction (XRD,D/MAX-3C) with copper target.Microstructural characterization was conducted by Zeiss Axio Imager A2m optical microscope (OM) and HITACHI S4500 field-emissio scanning electron microscope (SEM).The precipitation behavior was analyzed by JEOL JEM-2100F transmission electron microscope (TEM) operating at 200kV.For TEM observation,disks with diameter of 3mm and thickness of 50μm were punched from the solid-soluted and the aged specimen slices,respectively,and mechanically polished and thinned by Gatan precision ion polishing system.

Vickers hardness was measured with a load of 49N and dwelling time of 10s.The cylinder specimens with gauge length of 25mm and diameter of 5mm were machined for tensile testing,which was conducted on a UTM-4340 machine by velocity of 0.75mm/min (strain rate=5×10-4s-1)at room temperature.

3.Results

3.1.Microstructure and precipitates of as-cast GEZ1011K alloy

Fig.1 shows the SEM images of as-cast GEZ1011K alloy.It can be found that the as-cast alloy consisted ofα-Mg matrix,interdendritic lamellar phase,cuboid phase and fishbone shaped phase locating at grain boundaries.According to the EDS results shown in Table 1,the fishbone-shape compound was identifie as Mg3(Gd,Zn) eutectic phase.The interdendritic lamellar phase having an average composition of 91.9Mg-4.88Gd-3.22Zn(at.%),with a Zn/Gd atom ratio near 1,was most probably the LPSO phase (e.g.Mg12Gd1Zn1)[18].This lamellar LPSO phase is frequently reported to form during solidification and a lower cooling rate or soaking in a lower isothermal temperature usually contributes to forming it [15,17].The cuboid-shaped phase showed a chemical composition of 73.07Gd-24.16Er-2.77Zr (at.%),which was similar to that in Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy[7].This cuboid-shaped phase has afccstructure with a lattice constanta=0.52nm,which is believed to nucleate during casting and grow during high temperature solution treatment[19].However,based on the previous research[20],the chemical composition of these particles varies with the composition of the alloys and is considerably fluctuan even in a single alloy.Besides,the standard PDF cards of these compounds are not found.At present,we just estimate the cuboid phase is RE-rich compounds according to our EDS results and previous study.The comprehensive formation mechanism and its influenc on the mechanical properties of Mg alloys are still not clear.

Table 1 The EDS results of points A,B,C and D in Fig.1.

3.2.Microstructure and precipitates of solid-soluted GEZ1011K alloy

Fig.1.SEM images of the as-cast GEZ1011K alloy,(a) low-magnificatio and (b) high-magnification

Fig.2 shows the OM and SEM images of the solid-soluted GEZ1011K alloy cooled at various rates.Regardless of the cooling rate,nearly all the fishbone-shape eutectic phase as well as the interdendritic lamellar LPSO phase in the as-cast alloy were dissolved intoα-Mg matrix after solution treatment.However,an intragranular lamellar phase appeared in the solid-soluted alloy.The average chemical composition of the intragranular lamellar phase was identifie as 92.32Mg-4.60Gd-3.08Zn (at.%),which was almost the same with the interdendritic lamellar LPSO phase.Despite the similarity of morphology and chemical composition between the interdendritic and intragranular lamellar LPSO phases,they could exhibit various types such as 10H,14H,18R and 24R [21].The prominent difference among the QC,AC and FC samples was the density of the intragranular lamellar LPSO phase.It is clear that a few grey lamellas appeared in grain interiors in the AC sample (see arrows in Fig.2d),while no such lamella could be found in the QC sample (Fig.2a and b).The lamellas became much denser and grew across whole grains in the FC sample (Fig.2e and f).The amount of cuboid-shaped compounds was small in the as-cast state but became relatively abundant after the solution treatment.However,with the decrease of the cooling rates (e.g.Fig.2b vs.Fig.2f),the average particle size of cuboid-shaped compounds was significantl decreased.

Fig.3 shows the TEM bright fiel images and corresponding selected area electron diffraction (SAED) patterns of the solid-soluted alloy cooled at various rates.For the QC sample,plate-like precipitate indicated by the white arrow was observed in Fig.3a.13 extra diffraction spots between the(0000)αand (0002)αwere visible in the SAED pattern in Fig.3b,indicating that the existence of the 14H-LPSO phase.Additionally,some fin and short precipitate (as indicated by the red arrows in Fig.3a) were sparsely dispersed in the microstructure.These precipitates were typically less than 1nm in thickness in theprojected image and were probably theγ′′phase that was discussed in Mg-1Gd-0.4Zn-0.2Zr(at.%) alloy [22].No obvious correspondent diffraction contrast was observed in the SAED pattern,which may be due to their sparsely distribution in the matrix.As for the AC sample,besides the larger volume fraction of LPSO phase,shortγ′′precipitates were gradually replaced by the long precipitates with an aspect ratio over 500:1,which could be observed in the enlarged bright fiel image shown in the inset of Fig.3c.Combination with theprojection pattern shown in the inset of Fig.3d,weak streaks were visible along thec-axis and these streaks were similar to those observed in Mg-15Gd-1Zn-0.4Zr (wt.%) alloy [23].These fin and long precipitates were designatedγ′phase.Both ofγ′′andγ′are metastable phases formed on the basal plane ofα-Mg matrix and are fault features that can be generated by the partial dislocations [22].For the purpose of simplicity,both theγ′′andγ′phases are designated SFs,in line with that for Mg-8.2Gd-3.8Y-.0Zn-0.4Zr(wt.%)alloy[10]These SFs were reported to precipitate from supersaturatedα-Mg matrix and transform from eutectic compounds [24].According to the high-resolution TEM(HRTEM) images shown in Fig.4,the LPSO phase contained many building blocks with periodical arrangement along thec-axis (Fig.4a) and each individual 14H-LPSO showed a stacking sequence of ABABABACBCBCBC(Fig.4b).Fig.4c and d revealed that SFs comprised no periodical arrangement but intrinsic stacking faults on the basal plane (indicated by the black arrows).For the FC sample,the volume fraction of LPSO phase continued to increase while the amount of SFs seemed to reduce.The cooling rate could significantl affect the volume fraction of the LPSO phase,i.e.with decreasing in the cooling rate,adequate RE and Zn atoms diffusion was fulfille and led to the formation of a larger volume fraction of the LPSO phase.

Fig.2.OM (a,c,e) and SEM (b,d,f) images of the solid-soluted GEZ1011K alloy cooled at various rates,(a,b) QC sample,(c,d) AC sample,(e,f) FC sample (The inset in Fig.2f shows the magnifie image of the intragranular LPSO).

3.3.Age hardening response and microstructure of peak-aged GEZ1011K alloy

Fig.5 presents the age hardening curves of the GEZ1011K alloy cooled at various rates.The hardness increment of the QCA sample (+31 HV) was higher than those of the ACA (+22 HV) and FCA (+20 HV) samples.It is obvious that the QCA sample exhibited greater age hardening response than the ACA and FCA samples,that is,a higher cooling rate could result in a greater age hardening response.

Fig.3.TEM bright fiel images (a,c,e) and corresponding selected area electron diffraction (SAED) patterns (b,d,f) of the solid-soluted GEZ1011K alloy,(a,b) QC sample,(c,d) AC sample,(e,f) FC sample.The inset in Fig.3c and d show the image and corresponding SAED pattern of γ′ phase.Electron beam was parallel to [110]α.

Fig.6 shows the TEM bright fiel images and corresponding SAED patterns of the peak-aged QCA,ACA and FCA samples.The SFs and the LPSO phases were retained inside grains (see Fig.6a) of the QCA sample.With decreasing in the cooling rate,the thin SFs were gradually replaced by the intragranular LPSO phase with larger length and width in the FCA sample (Fig.6e),and the transformation from SFs to LPSO was verifie by Yamasaki et al.[15].Besides these SFs and LPSO precipitates,some dense and fin precipitates,which distributed on the prismatic plane withinα-Mg matrix when the incident beam was parallel with[0001]αdirection (see Fig.6b),were also observed.The high resolution images shown in Fig.6b indicates there were two types of the prismatic precipitates after aging treatment,one was the oval-shaped precipitate (as indicated by blue arrows),the other was the rhomboid-shaped precipitate (as indicated by black arrows).The former oval-shaped precipitate had acbcostructure with lattice parametersa=2aα-Mg=0.642nm,andc=cα-Mg=0.521nm [25].The SAED pattern inserted in Fig.6b indicates that this precipitate located onplanes,which was determined by the additional diffraction points locating at 1/4and 3/4These information supported that the ovalshaped precipitate wasβ′phase.The latter rhomboid-shaped precipitate connecting the twoβ′phases was considered asβ1phase withfccstructure locating onplanes.From Fig.6,it is clear that the number density of theβ′andβ1phases decreased but their mean particle size increased with decreasing in the cooling rate.Besides theβ′andβ1phases,a tail-like structure extending from the body ofβ′phase could be visible in all the as-aged samples with various cooling rates (indicated by the rectangular regions in HRTEM images),which was also reported in Mg-2.4Gd-0.4Ag-0.1Zr(at.%) alloy [26] and Mg-12Gd-0.8Zn-0.4Zr (wt.%) alloy[27].These structures were composed of linked hexagonalshaped structures and zigzag plates with the segregation of Gd atoms though the HAADF-STEM observation [27].These protrusions may be one of transitional phases of aging precipitation sequence and are not clearly revealed at present stage.

The Cat s master did not build very much upon what he said. He had often seen him play a great many cunning tricks to catch rats and mice,15 as when he used to hang by the heels, or hide himself in the meal, and make as if he were dead; so that he did not altogether despair of his affording him some help in his miserable condition. When the Cat had what he asked for he booted himself very gallantly, and putting his bag about his neck, he held the strings of it in his two forepaws and went into a warren16 where was great abundance of rabbits. He put bran and sow-thistle into his bag, and stretching out at length, as if he had been dead, he waited for some young rabbits, not yet acquainted with the deceits of the world, to come and rummage his bag for what he had put into it.

Fig.4.HRTEM images showing (a) LPSO phase and (b) the enlarged image of rectangular region in (a);(c) two SFs precipitates and (d) the enlarged image of rectangular region in (c).Electron beam was parallel to [110].

Fig.5.Age hardening curves of the GEZ1011K alloy cooled at various rates.

3.4.Tensile properties of the GEZ1011K alloy

Fig.7 presents the tensile properties of the solid-soluted and the peak-aged GEZ1011K alloys.The cooling rate is of importance in age strengthening of the GEZ1011K alloy.As indicated in Fig.7b and d,the YS of the QC alloy sample increased from 141MPa to 223MPa after aging treatment.The increment in YS between QC/QCA samples was 82MPa,which is much larger than that between AC/ACA samples(+26MPa) and FC/FCA samples (+5MPa).In addition,the elongation of the QC sample was highest and was 9.6% and 14.1% higher than those of the AC and FC samples,respectively.However,the elongation of the QCA sample dropped sharply from 15.6%to 3.6%and indicated the lowest ductility after aging treatment.

4.Discussion

4.1.Effects of the cooling rates on precipitation

Fig.8a illustrates a schematic diagram of microstructural characteristics in terms of basal and prismatic precipitates of the solid-soluted and the peak-aged GEZ1011K alloys.The cooing rate performs a pivotal role in altering the volume fraction of basal and prismatic precipitates.Firstly,the volume fraction of basal precipitates increases with decreasing in the cooling rate.According to Fig.2,3 and 6,it is evident that the lower cooling rate was,the larger volume fraction of LPSO phase precipitated inside grains.Since the SFs can be treated as the structural unit or the primary period of the LPSO phase,the LPSO phase is believed to grow from the highly dispersed SFs,and the solute segregation plays a crucial role in forming the LPSO phase in Mg-RE-Al alloys[28].Abe et al.[29] pointed out that the transformation from SFs to LPSO phase could be accomplished via two processes:1)introducing SFs on the close-packed planes for stacking order;2) supplying solute Zn and RE atoms adjacent to the faulting layers for chemical order.Since SFs are the faulted regions where high-concentration vacancies tend to cluster,the regions are energetically favorable for the solute segregation.In the present investigation,the Gd and Zn atoms tend to diffuse to the faulted layers and form the ordering structure during cooling period after solid solution,and they have sufficien time to array at the specifi layers of Mg matrix and then precipitate large amounts of LPSO phases(see Fig.3e)in the case of the lower cooling rate (e.g.FC).By contrary,only sparsely dispersed SFs and a few LPSO phases are observed(Fig.3a) in the higher cooling rate condition (e.g.QC).The cooling rate is therefore suggested as a dominant parameter to control the amount of LPSO/SFs phases in the solid-soluted GEZ1011K alloy.

Fig.6.TEM bright fiel images of the peak-aged GEZ1011K alloy,(a,b) QCA sample,(c,d) ACA sample,and (e,f) FCA sample.The upright insets in Fig.6b,d and f are the HRTEM images of the precipitates.Electron beam was parallel to [110]α in (a,c,e) and [0001]α in (b,d,f).

Secondly,the precipitation of prismatic precipitates in aging treatment is promoted by a high cooling rate.In order to elucidate the effects of the cooling rate on the number density and morphology of the prismatic precipitates,the characteristics ofβ′precipitate in different thermal conditions are shown in Table 2.According to Table 2,the area number density of theβ′precipitate of the QCA sample is 1.94×103μm-2,which is～2.1 and～3.6 times higher than those of the ACA(0.93×103μm-2) and FCA (0.54×103μm-2) samples,respectively.In fact,the LPSO/SFs phases barely dissolve during the aging treatment,the total content of the prismatic precipitate during aging treatment is thereby strongly determined by the remained RE and Zn elements soluted in Mg matrix[30].As the highest content of solution atoms was maintained in the QC sample after solid solution treatment than those of the AC and FC samples,it is reasonable to form the largest number of prismatic precipitate in the case of the QC sample.In addition,combination with the HRTEM images in Fig.4 and HAADF-STEM observation [28,31,32],Gd and Zn atoms are periodically segregated in four certain atomic layers of 14 stacking ordered layers for the 14H-LPSO phase.However,solute elements concentrate in two atom layers around each SF.Once the basal precipitates are constructed,these RE/Zn atoms can hardly be released to form theβ′precipitates.Accordingly,comparatively fin SFs are more beneficia for forming larger number density of theβ′precipitate.

Fig.7.Tensile properties of the solid-soluted (a,b) and the peak-aged GEZ1011K alloys (c,d).(a,c) are typical engineering stress-strain curves,(b,d) are average strength and elongation.

Fig.8.The schematic diagram illustrating the microstructural characteristics in terms of basal and prismatic precipitates of the solid-soluted and the peak-aged GEZ1011K alloys.

Table 2 Microstructural characteristics of the β′ and LPSO/SFs precipitates in peakaged GEZ1011K alloy.

4.2.Effects of precipitates on strengthening mechanism

Given that all the three samples in present study were performed under the same heat treatment,the strengthening effect of grain boundaries are considered as the same.Due to the great difference of the microstructure between the samples with various cooling rates,the concentration of the alloying elements in the matrix of QC sample is higher than that in FC sample and therefore,the solution strengthening effect for the QC sample should be higher than that for FC sample.However,the results show that the yield strength for FC sample is almost the same with QC sample.This phenomenon indicates that the basal precipitates (containing LPSO and SFs) may also provide some strengthening improvement on the yield strength..In other words,the larger strengthening effect of solution atoms in QC sample may be offset by the higher basal precipitation strengthening effect in FC sample.In addition,there is not much difference between the contribution of solely LPSO and SFs on the strength improvement [8],the prominent improvement of YS (+82MPa) between the QC/QCA samples stems from the aging precipitation strengthening.Due that the prismatic precipitates offer a more effective role in hindering dislocation slipping on the basal planes[33],the strengthening mechanism of the peak-aged samples is mainly attributed to theβ′precipitation strengthening,considering the low volume fraction ofβ1phase in present study.The contribution of the LPSO/SFs precipitates to strengthening is much less than that of theβ′precipitate.The decreased content of theβ′precipitate caused by a low cooling rate is the reason of weakening age hardening response of the GEZ1011K alloy.

The precipitation strengthening induced by theβ′precipitate increases linearly with its square root of number densityN(YS∝N1/2) [34].The YS enhancement of the QCA sample is therefore induced by the largest area number density of theβ′precipitate,as demonstrated in Table 2.Furthermore,the second phases with a dispersed distribution and smaller mean size in Mg-Gd alloys usually exhibit a higher impeding effect to dislocation and thus a larger strengthening effect.In the present investigation,the length and width of theβ′precipitate were measured from the TEM images and the directions of length and width were alongandrespectively.From Table 2,the aspect ratio of theβ′precipitate in the QCA sample is about 1.87(length/width),whereas the ratio decreases to 1.63 and 1.39 in the ACA and FCA samples,respectively.The largest aspect ratio of theβ′precipitate effectively reduce the inter-particle spacing and provide the strongest strengthening effect [34,35].Therefore,the highest volume fraction as well as the smallest particle size of theβ′precipitate in the QCA sample leads to the superior YS enhancement compared with those of the ACA and FCA samples.

However,previous investigations suggest that a single phase strengthening effect,whatever for LPSO/SFs orβ′precipitates,is not as large as the combination of basal and prismatic precipitates,especially for the Mg-RE-Zn alloys with high RE content [23].The distribution and interaction between the LPSO/SFs and theβ′precipitate in the GEZ1011K alloy is shown in Fig.9.In Fig.9a,when the sample is beamed along the [11ˉ20]αdirection,the parallel arrays of the LPSO/SFs basal precipitates can be seen.Between the basal precipitates,a dark contrast representing theβ′precipitate with a perpendicular relationship to the basal precipitates can also be observed.From the high magnifie images (Fig.9b and c),the expansion and growth of theβ′precipitate towards[0001]αis heavily suppressed when the front of theβ′precipitate encounters with the LPSO/SFs precipitates.As these basal precipitates have a relatively high Young’s modulus(e.g.～67GPa for LPSO) compared to the Mg matrix (～40GPa),it becomes extremely difficul for theβ′precipitate to pass through.According to Table 2,there is not much difference of the average inter-lamellae spacing between the QCA and ACA samples.However,the FCA sample shows a much larger inter-lamellae spacing than that of ACA sample.The reduced inter-lamellae spacing of basal precipitates in ACA sample is mainly attributed to the precipitation of SFs during solid solution and aging and may lead to the smaller size ofβ′phase than that in FCA sample.In comparison to the LPSO phase,the distribution of SFs with a relatively reduced inter-plate spacing usually leads to a small average size ofβ′precipitate.In this regard,the contribution of the composite SFs andβ′precipitate to the strength should be obviously higher than that of the composite LPSO andβ′.Obviously,the introduction of the intragranular lamellar LPSO shows a negative effect on the tensile properties for the solid-soluted Mg-Gd-Zn system alloys.Since that a microstructure consisting of dense SFs precipitates without any LPSO phase was not realized under the present experimental condition,a comparatively lower volume fraction of LPSO phase should be more favorable for yielding a high strength alloy.In Fig.9d and e,the precipitation strengthening models with different volume fractions of LPSO phase are presented.Obviously,the microstructure containing lower volume fraction of LPSO phase exhibits a reduced inter-plate spacing and results in a larger number density ofβ′phase with fin particle size (as shown in Fig.9e).Based on the aforementioned discussion,it is suggested that a high cooling rate of solid-soluted alloy contributes to a large number density of finβ′precipitate,and the composite phases containing comparatively lower volume fraction of the LPSO phase and theβ′precipitate with fin particle size as well as high number density are preferred to strengthen the GEZ1011K alloy.

Fig.9.(a) General distribution and morphology of SFs and β′ precipitates and the interface between the LPSO (b) or SFs (c) and the β′ precipitate.Incident beam// [110]α,(d) and (e) show the schematic diagram of the spatial relationship between the basal precipitates and prismatic precipitates.

5.Conclusions

The influenc of cooling rate on precipitation and age hardening response of the solid-soluted GEZ1011K alloy was investigated in terms of volume fraction of precipitates and tensile properties.The following conclusions can be drawn:

(2) The cooling rate shows no obvious influenc on the YS of the solid-soluted GEZ1011K alloy,but it contributes strong aging strengthening.The strongest hardening response took place in the QCA sample and showed 82MPa enhancement in YS,which is higher than that of ACA (+26MPa) and FCA samples (+5MPa).A high cooling rate promotes the precipitation and reduces the average size of theβ′precipitate,which acts as the main strengthening phase in the GEZ1011K alloy.

(3) The basal precipitates show a restrictive effect on the growth and/or coarsening of theβ′precipitate.The basal precipitates with reduced inter-plate spacing may be favorable for the precipitation of finβ′precipitate.

(4) A high cooling rate of solid-soluted alloy contributes to a large number density of finβ′precipitate.The composite phases containing larger number density ofβ′precipitate and lower volume fraction of LPSO phase,are preferred to strengthen the casting GEZ1011K alloy.

Declarations of Competing Interest

None

Acknowledgments

This work was supported by National Key Research and Development Program of China (2016YFB0301101,2016YFB0301001),Key Science and Technology Program of Beijing Municipal Commission of Education,China(KZ201810005005).