Multiplex intermetallic phases in a gravity die-cast Mg?6.0Zn?1.5Nd?0.5Zr (wt%) alloy

Fnzhi Meng, Shuhui Lv, Qing Yng, Xin Qiu, Zixing Yn, Qin Dun, Jin Meng

aState Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,PR China

b School of Materials Science and Engineering, Changchun University of Science and Technology, 130022, PR China

Abstract Components and crystal structures of the intermetallic phases in a gravity die-cast Mg?6.0Zn?1.5Nd?0.5Zr(wt%)alloy were investigated using transmission electron microscopy.The results indicate that this alloy has multiple intermetallic phases and various inner faults.Totally,six eutectoid intermetallic phases, namely W (MgNdZn3), T (Mg39Zn55Nd6), (Mg,Zr)Zn2, Z (Mg28Zn65Nd7), H2 (Mg15Zn70Nd15), and H1(Mg24Zn64Nd11), were simultaneously observed at grain boundaries, and six precipitates (Z, Mg7Zn3, T, Mg4Zn7, β1＇-MgZn2 and β2＇-MgZn2)were found in α-Mg grains.Furthermore, faults like sub-grain boundaries, orientation domains (coherent with the same matching plane but with different orientations), stacking faults and twins were observed in the eutectoid intermetallic phases.Finally, some new orientation relationships between the known intermetallic phases were revealed.This paper can provide new insight into alloy design for Mg?Zn?RE(RE: rare earth) based alloys.

Keywords: Magnesium alloys; Intermetallic phase; Structure; Transmission electron microscopy (TEM); Orientation relationship.

1.Introduction

Adding Zn and rare earth (RE) into Mg has been widely accepted as an effective method aimed to improve the mechanical properties, corrosion resistance and thermal conductivity of Mg alloys [1,2].Therefore, Mg?Zn?RE system has recently received actively attentions in automotive, aerospace, biomedical and electronic industries [3,4].Since the intermetallic phases play a key role on alloys’properties, many investigators devoted their efforts to reveal the intermetallic phases of Mg?Zn?RE based alloys [5-25].To date, more than ten Mg-Zn-RE ternary compounds were revealed, such as Mg3Zn6RE (I) icosahedral phase [5,6],Mg12ZnRE 14H/18R long-period stacking ordered (LPSO)phase [7-10], MgZn2RE (W) phase [11,12], Mg19Zn81RE20(or Mg21Zn62RE17) (W’ or F) phase [12-14], Mg28Zn65RE7(Z) phase [15-17], Mg24Zn64RE11(or Mg22Zn64RE14) (H1)phase [13], Mg15Zn70RE15(H2 or H) phase [5,15,18],Mg26Zn52RE22(T) phase [19-22], Mg45Zn53RE2(B) phase[23], Mg9Zn72RE19phase [13,24], Mg17.8Zn65.6RE16.6phase[13,24], hexagonal Mg3Zn6RE phase [13,25], etc.Among these phases, the LPSO, I and W phases were frequently observed in traditional Mg?Zn?RE based alloys, and the LPSO phase was generally believed as having satisfactory strengthening effects on alloys’ strength [3,26,27]while the latter two phases with poor strengthening effects on strength and even being adverse on the ductility [28,29].Since different intermetallic phases resulted in significantly different influences particularly on alloys’ mechanical properties,it is therefore of overwhelming importance to reveal the detailed components of the intermetallic phases for a given alloy.

Mg?6Zn?0.5Zr (ZK60) alloy is one of the most widely used commercial alloys because its remarkable age hardening response [1,30,31].To modify its microstructures aimed at improving the mechanical properties, further alloying has become the primary method.Among the adopted alloying additives,RE elements,i.e.La[21,32],Ce[20,23,33],Nd[34,35],Sm [36,37], Gd [38-40], Y [41,42], and MM (misch metal)[43], have been used most, and the widely accepted addition content is in the range of 1-2wt%.However, there are some inconsistencies with regard to the intermetallic phases in RE-containing ZK60 alloys.For example, the dominant intermetallic phase in the Ce-modified ZK60 alloys was reported to be a ternary Mg-Zn-Ce phase, namely T phase, which has a C-centered orthorhombic structure [20-22].However,several researchers believed the ternary Mg-Zn-Ce phase as W phase that has a face-centered cubic structure [33].In addition, Qiu et al.[34]reported two ternary Mg-Zn-Nd intermetallic phases in the Nd-modified ZK60 alloy, but Zhou et al.[35]reported that there is no ternary phase in the Nd modified ZK60 alloy whose eutectoid phases were identified as Mg41Nd5and MgZn2.With respect to the Gd or Y modified ZK60 alloys, ternary intermetallic phases were widely reported as W, W’ or I-phase [38-40].Moreover, other ternary phases such as H1, H2 B and Z were not observed in the ZK60 alloy with 1-2wt% RE.Thus, it is necessary to characterize the detailed components of intermetallic phases in the RE modified ZK60 alloys.

Inner faults in intermetallic phases are important for understanding phase transformation and their influence on alloys’hot deformation.Although Kim et al.[44]found a number of faults such as antiphase boundaries(APBs)and stacking faults(SFs) in the MgZn2precipitates, almost no investigators have paid attention to the detailed inner faults of the ternary intermetallic phases in the RE modified ZK60 alloys.Finally,Mg-Zn precipitates were frequently observed in the RE modified ZK60 alloys, but there are also inconsistencies with respect to their types.For instance, Jeong et al.[20]reported that the fine Mg-Zn precipitates in the RE modified ZK60 alloys are MgZn and MgZn2, while Silva et al.[33]believed them as Mg7Zn3, He et al.[22]as only MgZn2, and Wei et al.[19]as Mg4Zn7and MgZn2.As is known, amounts of fine precipitates always appeared in the Mg matrix of the RE modified ZK60 alloys after hot-deformation.Nevertheless,whether they precipitated during hot-deformation or come from the as-cast sample is still unrevealed.Thus, answers to the above queries are urgent and significant for the further development of the RE modified ZK60 alloys.

Among the reported RE modified ZK60 alloys, the Mg?6.0Zn?1.5Nd?0.5Zr (ZEK620) alloy is the representative one with excellent mechanical properties at both room and high temperatures [34].Therefore, the ZEK620 alloy was selected in this work to investigate the intermetallic phase components formed during solidification.Simultaneously, the relationships between the aggregated intermetallic phases and their inner faults were thoroughly studied using transmission electron microscopy (TEM).

2.Experimental procedures

The ZEK620 ingot was fabricated by electric melting of 9.78kg high-pure Mg (99.99wt%) and 0.72kg Zn(99.99wt%), 0.9kg Mg?20wt% Nd (99.85wt%) and 0.6kg Mg?30wt% Zr (99.64wt%) master alloys in a steel crucible whose inner wall was covered by ZnO.The melt was protected by the mixture air atmosphere (CO2+1vol% SF6) during whole process.Firstly, all raw materials were preheated to 350°C in an electric resistance furnace, and then pure Mg and Zn, and Mg?20wt% Nd mater alloy were melted in the steel crucible.Until the melt temperature was approximately 760°C, the Mg?30wt% Zr master alloy was added.Afterwards, the melt was fully stirred for 8 min, followed by keeping static for 40 min.Finally, the melt was poured into a water-cooling iron mold with the diameter and length of ～92mm and ～800mm, respectively, when its temperature was ～710°C.The mold was previously heated to ～200°C before casting.The cooling temperatures were recorded by artificially reading the display of a thermocouple which was directly contacted with the ingot.Finally, the actual chemical composition of the produced alloy was determined through an inductivity coupled plasma atomic emission spectroscopy(ICP-AES).

Samples for microstructural characterizations were cut from the center region with a radius of ～35mm, of the obtained ingot.Microstructural characterizations were performed using optical microscopy (OM), X-ray diffractometer (XRD)at 40kV and 40mA with CuKαradiation (λ=0.15406nm),backscatter scanning electron microscopy (SEM), and FEI Tecnai G2F20 TEM equipped with energy dispersive X-ray spectroscopy (EDS) operated at 200kV.OM and SEM specimens were polished with Al2O3suspension after grinded using various SiC papers.Then, they were slightly etched by a mixture of 5ml acetic acid, 5g picric acid, 10ml H2O, and 100ml ethanol.Grain size was measured using Nano Measurer software.ThinΦ3mm discs for TEM observations were cut with a thickness of 0.5mm using wire cutting machine,then ground to a thickness of 20±5 μm, and finally ionmilled using a Gatan Precision Ion Polishing System (PIPS)in the Ar ion-beam thinning process and cooled by liquid nitrogen.

3.Results

The composition of ZEK620 was measured as Mg?6.12Zn?1.54Nd?0.49Zr (impurities: 0.0036 Si, 0.0003 Fe, 0.0012 Mn, and＜0.0001 Ni) in wt%.Fig.1 shows the cooling curve of the studied alloy, which illustrates a typical cooling curve compared with those of the traditional magnesium alloys, with the main cooling rate of 1?3.6 °C/s.Fig.2a shows its representative OM image, wherein approximately equiaxedα-Mg grains along with semi-continuous reticular grain boundaries can be clearly observed.The average grain size was measured to be 45±7 μm.From the corresponding backscatter SEM image (Fig.2b), it can beseen that most grain boundaries are occupied by intermetallic particles.Qiu et al.[34]reported two intermetallic phases in an alloy with highly similar composition to the studied alloy.However, at least three kinds of intermetallic phases were observed in the studied alloy with different morphologies and bright contrast: the network phase with relatively lower brightness, the blocky phase with relatively higher brightness,and the granular phase, which were labeled as A-C, respectively, in Fig.2c and d.Furthermore, there are numerous fine precipitates locally distributed in a small region, or lined up, or scattered inα-Mg matrix near grain boundaries (highlighted by green dotted lines, circles, or arrows, respectively,in Fig.2d).Fig.3 presents the XRD pattern of the ZEK620 alloy.Several intermetallic phases, namely W, T, MgZn2, Z and H2, were deduced to be possibly existed in the studied alloy.In the following, the crystal structures of intermetallicphases in the ZEK620 alloy will be investigated in detail using TEM.

Fig.1.The measured cooling cure of the gravity die-cast ZEK620 alloy.

Fig.2.(a) OM image and (b-d) backscatter SEM images of the gravity die-cast ZEK620 alloy.

Fig.3.XRD pattern of the gravity die-cast ZEK620 alloy.

Fig.4.(a-c) BF-TEM images, (d-i) the corresponding SAED patterns and (j-l) the EDS spectra along with the analysis results for the eutectoid intermetallic phases in the studied alloy.

3.1.W and T phases

Fig.4a-c shows the bright-field TEM(BF-TEM)images of the blocky, the network and the granular phases, respectively.According to the corresponding selected area electron diffraction (SAED) patterns (Fig.4d-i), both blocky and granular phases are W (face-centered cubic structure,Fmm) while the network phase is T (C-centered orthorhombic structure,Cmcm).Measurements illustrate that the lattice parameter of W is 0.697nm, which is larger than that of the W phase in the Mg?Y?Zn (a=0.683nm) [6]and the Mg?Gd?Zn(a=0.690nm) [13]systems.Additionally, more than ten EDS measurements (Fig.4j and l) suggest a composition of Mg44?52Zn37?43Nd10-13for the W phase.In previous work,several formulae were reported for the W phase as Mg3Y2Zn3,MgYZn3, MgCeZn2, and Mg(Mg,Zn)2Gd [6,13,45].Considered the influence of Mg matrix on EDS detection and the Zn/Nd ratio of ～3, the formula of the studied W phase was reasonably marked as MgNdZn3.Since different RE atoms have different radii, both chemical composition and RE type managed the lattice parameters of W phase in various Mg?Zn?RE systems.With the T phase, measurements demonstrate its lattice parameters as:a=0.954nm,b=1.128nm andc=0.932nm, which are well in line with those reported in the Mg?Zn?La/Ce/MM (MM=misch metal) systems (a=0.96nm,b=1.12nm andc=0.94nm)[19-22].Wei et al.[19]reported that the lattice parameters of T phase were various in different alloys, such asa=0.96nm,b=1.12nm andc=0.94nm in the Mg?8Zn?1.5MM alloy whilea=1.01nm,b=1.16nm andc=0.99nm in the Mg?5Zn?10MM alloy.And they identified that the T phase with larger lattice parameters contains relatively lower Zn concentration but higher Mg concentration.In this work, EDS measurements (Fig.4k) illustrate the composition of T phase as Mg39±1.8Zn55±1.1Nd6±0.7, which is different from those reported in the Mg?6Zn?1La (Mg48Zn42Nd10) [21]and Mg?8Zn?1.5MM (Mg25.8±2.1Zn51.7±1.9Nd22.5±2.6) [19]alloys.However, their lattice parameters are approximately equal.Therefore, the lattice parameters of T phase may tolerate a relatively wide composition range.

Fig.5.(a) BF-TEM image and (b, c) HRTEM images of the W phase.The insert in (a) is the corresponding SAED pattern and that in (c) is the corresponding FFT pattern.The blue and green dotted lines in (b) are the traces of (111) plane in the left and right part, respectively.

Fig.6.(a) BF-TEM image, (b) the corresponding SAED pattern and (c, d) the HRTEM images for the T phase, (e, f) the inversed Fourier transferred images of figures (c) and (d), respectively.The thin green dotted lines highlighted the planar faults, the thick ones labeled the {100}T plane traces, the short yellow lines marked the mismatches, and the blue boxes schematically shows the unit cells of the T phase.

Fig.5a shows a BF-TEM image of the W phase.It is obvious that it contains many planar faults.According to the high-resolution TEM (HRTEM) images, some faults are sub-grain boundaries (SGBs, highlighted by a green arrow in Fig.5b), and the others marked by orange arrows cannot be identified as SFs although steaks along [111]Wdirection can be clearly observed in the corresponding fast Fourier transferred (FFT) pattern (Fig.5c).It seemed to be consisted of many planar faults.Fig.6a presents a BF-TEM image of the lamellae particle.Indeed, the network phase is consisted of numerous convex segments which follow random orientation relationships (ORs).It is clear that there are also many planar faults in the convex sections, mainly along two directions.The corresponding SAED pattern (Fig.6b) demonstrates that the convex segment is T, and the corresponding HRTEM images (Fig.6c and d) and inversed Fourier transferred images (Fig.6e and f, respectively) indicate that the faults are SFs lying on the (010)T(Fig.6c and e) and (100)Tplanes(Fig.6d and f).

In the ZEK620 alloy, coexistence of W and T was observed such as shown in Fig.7a.The corresponding EDS mappings of Mg, Zn, Nd and Zr present that the relatively small phase is enriched with more Zn but less Nd.Their SAED patterns (Fig.7b and c) confirm that the relatively coarse phase is W while the smaller one is T.Furthermore,the T phase is semi-coherent with the W phase (Fig.7d),following an OR as: (110)W//(110)Tand [001]W//[13]T(thecorresponding analysis of the stereographic projection was shown in Supplementary Fig.S1).Fig.8a shows another lamellae particle containing two phases.Some planar faults were also observed.Fig.8b and c gives the SAED patterns from the region marked by orange and pink circles, respectively.They reveal that the lamellae particle simultaneously contains W and T, and some (023) twins existed in the T phase which was confirmed by the corresponding HRTEM image (Fig.8d).Fig.8e shows the enlarged BF-TEM image of Fig.8a.Some other ultra-fine planar faults (marked by green arrows) can be observed.The corresponding SAED pattern(Fig.8f)and HRTEM image(Fig.8g)confirm them as SFs in the T phase lying the (100)Tplane.Moreover, W and T are semi-coherent (Fig.8h), following a different OR from the above reported one,as(011)W//(010)Tand[41]W//[302]T.

Fig.7.(a) HAADF-STEM image along with the EDS mappings, (b, c) SAED patterns and (d) HRTEM image along with the corresponding FFT patterns for the W phase attached with a small T phase.

3.2.H2 and H1 phases aggregated with the T phase

Fig.9a shows a BF-TEM image of the boundary between two small convex segments in a lamellae particle.Interestingly, there are two new intermetallic phases except the T phase (Fig.9b, d and f), namely H2 (hexagonal structure,P63/mmc,a=0.90962nm andc=0.94112nm[13]), and H1 (hexagonal structure,P63/mmc,a=3.353nm andc=0.895nm [13]).The H2 phase is also widely reported as the H phase [6,13,18].Fig.9c, e, and g presents the HRTEM images of the phase boundaries among T, H2 and H1, and the FFT patterns from local regions were also present.Combined with the stereographic projection analysis,these three phases follow certain ORs as: (010)T//(010)H2and[10]T//[01]H2, (112)H2//(121)H1and [01]H2//[53]H1, and(052)H1//(010)Tand [95]H1//[10]T.Furthermore, orientation domains (coherent with the same matching plane but with different orientations [46,47]) were observed in the H1 phase(Fig.9h and i).As mentioned above, H2 and H1 are simultaneously semi-coherent with T, and H1 is also semi-coherent with H2.Thus, the formation of orientation domains in the H1 phase might be to simultaneously satisfy matching with both H1 and T, thus decreasing the total system entropy.

3.3.Z phase coexisted with T phase

Fig.10a shows the BF-TEM image of a lamellae particle, wherein some faults (marked by orange arrows) can be observed.Fig.10b and c gives the corresponding SAED pattern and HRTEM image, respectively, which suggest that the lamellae particle contains both Z (hexagonal structure,P63/mmc,a=1.4633nm andc=0.8761nm [17]) and T.A certain OR between them was revealed as: (001)T//(120)Zand[10]T//[27]Z.Furthermore,the faults are the narrow T zones inlaid in the Z phase.Then, it could be deduced that the metastable Z phase will transform to the T phase.

3.4.Zr-enriched MgZn2 phase

Zn-Zr intermetallic phases were frequently observed in the Mg?Zn?Zr system, particularly after solution treatment[1,48,49].However, it is unclear whether they formed during casting processes.Fig.11a shows the HAADF-STEM image of a lamellae intermetallic phase, wherein some small surficial particles can be clearly observed, with sizes in the range of 100?500nm.And these small particles areenriched with Zn and Zr (Fig.11b).However, their SAED patterns such as shown in Fig.11c cannot be reasonably indexed by the traditional Zn-Zr compounds such as Zn2Zr3,Zn2Zr and ZnZr [48], but well by the well-known MgZn2phase (hexagonal structure,P63/mmm,a=0.5253nm andc=0.8568nm [51,52]).Bhattacharjee et al.[50]studied the precipitates in an Mg?6.2Zn?0.6Zr alloy and found that Zr prefers to segregate in some Mg-Zn phases.Therefore,the small particles are the Zr-enriched (Mg,Zr)Zn2phase with the MgZn2-type structure, which was further confirmed by the corresponding point EDS analysis result (Fig.11d)of Mg15.40±0.23Zn65.16±0.74Zr19.23±0.73Nd0.21±0.15.Nie et al.[3]investigated the Zn-Zr phases in a Mg?5Zn?2Gd?0.4Zr alloy and reported that they formed during solidification and survived during solution treatment.In the present work, no traditional Zn-Zr phases were detected.Thus, the formation mechanisms of the Zn-Zr phases existed in the heat-treated samples needs additional elaborate investigations to be revealed.Many TEM observations indicate that the small(Mg,Zr)Zn2phase is generally coexisted with the T phase,such as shown in Fig.12a.The corresponding SAED patterns(Fig.12b and c) reveal an OR between (Mg,Zr)Zn2and T as: (001)(Mg,Zr)Zn2//(100)Tand [210](Mg,Zr)Zn2//[001]T(The corresponding stereographic projection analysis was shown in Supplementary Fig.S2).

Fig.8.(a) BF-TEM image, (b, c) the corresponding SAED patterns obtained from the regions highlighted by the orange and the pink circles, respectively,and (d) HRTEM images along with the corresponding FFT patterns of the twin boundary in T phase, (e) the magnified BF-TEM image and (f) the SAED pattern form the region highlighted by a blue circle, (g, h) HRTEM images of the SF in T phase and the W/T phase boundary, respectively.

3.5.Fine precipitates in α-Mg grains

As indicated in Fig.2d, there are a mass of fine precipitates in theα-Mg grains.It is reported that some fine precipitates could form before nucleation of Mg, such as Al8Mn5in the Mn-modified AZ31 alloys, resulted in some precipitates inα-Mg grains [52].On the other hand,some precipitates can also precipitate in theα-Mg grains because of the slow cooling rate, such as the 14H-type LPSO precipitate in the Mg?Zn?Y/Gd systems [53,54].A representative HAADF-STEM image shown in Fig.13a demonstrates many fine precipitates discretely distributed in theα-Mg grains of the studied alloy.The enlarged image(Fig.13b) and EDS measurements (Fig.13c) manifest at least two types of precipitates: the polygonal phase with relatively higher brightness and containing Nd, and the other one being flaky and with almost free Nd.According to HRTEM images (Fig.13d-f) along with the corresponding FFT patterns (Fig.13g-i), the polygonal precipitate is Z while the flaky one is Mg7Zn3(body-centered orthorhombic structure,Immm,a=1.408nm,b=1.449nm, andc=1.403nm[22]).Both Z and Mg7Zn3have certain ORs with Mg matrix as: (100)Z//(100)α?Mgand [0001]Z//[110]α?Mg,(12)Mg7Zn3//(0001)α?Mgand [20]Mg7Zn3//[110]α?Mg,respectively.Additionally, the Mg7Zn3phase in some cases aggregates with the Z phase, following an OR as:(010)Mg7Zn3//(010)Zand [20]Mg7Zn3//[0001]Z.

Fig.9.(a) BF-TEM image, (b, d, f, h) the SAED patterns and (c, e, g, i) the HRTEM images along with the corresponding FFT patterns from local regions labeled by 1-8 for the H2 or H1 phase coexisted with the T phase.

Fig.10.(a) BF-TEM image, (b) SAED pattern, and (c-f) HRTEM image along with the corresponding FFT patterns for the particle simultaneously containing Z and T.

As mentioned in the above, the Z phase might transfer to the T phase.Fig.14a shows the microstructure of anα-Mg grain.There are two types of precipitates: the coarse one with sizes being over 100nm and the fine one only several nanometers.From the corresponding HRTEM image(Fig.14b), the coarse precipitate simultaneously contains Z and T.The stereographic projection analysis illustrates a same OR with that revealed in the above.In Mg?Zn?RE?Zr system alloys, fine precipitates were frequently observed after aging or hot-extrusion [30-33,36].Generally, the fine precipitates were identified as MgZn2while the relatively coarser ones were identified as Z [36,55]or I-phase (quasi-crystalline icosahedral structure,Fm,ar=0.519nm) [39].However,it is unclear whether they have yet preexisted in the samples before solution treatment or hot-extrusion.In this work,the results suggest that the Z phase in the extruded samples might be from the as-cast ingot.Additionally, numerous much fine precipitates were also existed in theα-Mg matrix (Fig.14c).Based on their sizes and morphologies,they can be grouped into three types: the spherical precipitate (～25nm, marked by pink arrows), the rod-like precipitate (marked by green arrows) and the ultra-fine spherical precipitate (～5nm, marked by blue arrows).It is known that theβ1＇precipitate is rod-like and lies in the (110)Mgplane,with the growth direction being [0001]Mg[56].Thus, the rodlike precipitate in the studied alloy isβ1＇.Fig.14d shows the HRTEM image of the spherical precipitate.Its FFT pattern illustrates that it is I-phase and follows an OR with the Mg matrix as: (5-fold)I-phase//(0001)Mg, and [2-fold]I-phase//[110]Mg.Therefore, the I-phase in the extruded Mg?Zn?RE alloys may also be from the as-cast ingots.Additionally, coexistence of both I-phase and Mg4Zn7(based-centered monoclinic structure,B2/m,a=2.596nm,b=1.428nm,c=0.524nm,andγ=102.5° [23]) was also observed (Fig.14e), following an OR as (5-fold)I-phase//(100)Mg4Zn7, [2-fold]I//[021]Mg4Zn7.Fig.14f shows the HRTEM image of the ultra-fine spherical precipitate.Its FFT pattern is fully consistent with the calculated diffraction pattern based on the OR of (0001)β2＇//(110)α, and [110]β2＇//[0001]αbetweenβ2＇-MgZn2andα-Mg reported in Ref.[57].Thus, the ultrafine precipitate in the studied alloy isβ2＇-MgZn2and follows the same OR with the Mg matrix as reported in Ref.[57].To further confirm whether all ultra-fine precipitates are MgZn2, we randomly examined more than 50 ultra-fine precipitates, and we confirm that all ultra-fine precipitates in the studied alloyare MgZn2and follow an identical OR with the Mg matrix.Therefore,the ultra-fine spherical precipitate is theβ2＇-MgZn2phase [55].Similar precipitates were also observed in the hot-extruded Mg?6Zn?1.5Sm?0.5Zr alloy[36].Thereby,the ultra-fineβ2＇-MgZn2precipitate in as-extruded Mg?Zn?RE alloys may also be from the as-cast ingots.

Fig.11.(a) HAADF-STEM image, (b) the corresponding EDS mappings for the region highlighted by a yellow dotted box, (c) the SAED pattern and (d) the point EDS spectrum along with the analysis result of the small phase inlaid in the coarse eutectoid phase.

3.6.Mechanical properties

Fig.15 illustrates the representative compressive and tensile engineering stress-strain curve of the as-cast ZEK620 alloy at room temperature, whose ultimate strength and yield strength were tabulated and compared with other casting Mg alloys [29,58-65]in Table 1.It is clear that the studied alloy owns high strength.For example, the values of ultimate strength and yield strength of the studied alloy are higher by 147% and 68%, respectively, in compression and by 80% and 98%, respectively, in tension than those of pure Mg [58], and by 102% and 35%, respectively, in compression than those of traditional AZ91 alloy [59].In addition, the yield strength of the studied alloy is comparable with that of the peak-aged WE43/54 alloys [63-65].Therefore, the multiplex intermetallic phases both at grain boundaries and inα-Mg grains have satisfactory strengthening effects on alloy’s strength.This may provide new insight in alloy design principle,and the alloy developed herein would lead to an early acceptance by the user due to its much higher strength and relatively lower cost compared with the AZ-system alloys and the casting WE43/54 alloys, respectively.

Table 1Compressive and tensile strength of various casting alloys.

4.Discussion

4.1.Aggregation of eutectic intermetallic phases

In the studied alloy, both W and T approximately equally appeared.From previous work, it is get that the formation ofvarious intermetallic phases in the as-cast RE-modified ZK60 alloys is closely related to the RE type [21,32-38,41,66,67].For example, the T phase was only reported in the alloys with RE=La, Ce and misch metal [21,32,33,66-68]while the W phase in the alloys with RE=Sm, Gd and Y [37,38,41].It is noted that Sm, Gd and Y have relatively much higher solid solubility in Mg than La and Ce.Thus, the solid solubility of RE may manage the formation of intermetallic phases.As the solid solubility in Mg of RE increases, the dominant intermetallic phases in the RE-modified ZK60 alloy gradually become the W phase from the T phase.Since the solid solubility of Nd just locates at the middle level, both W and T could simultaneously formed during solidification.On the other hand, the W phase might own relatively higher stability than the T phase, which resulted in the transformation from the T phase to the W phase.This is the underlying causes for the appearance of two ORs between W and T.Moreover, H2,H1 and Z phases were observed in the eutectoid region in the studied alloy.Singh et al.[18]pointed out that the H2 phase is similar in structure to a hexagonal Zn5Y phase and coherent with Mg matrix, and I-phase could form on the H2/Mg interface during heat treatment.Furthermore,a pseudo-quasicrystal phase named H’ (hexagonal structure,P63/mmc,a=0.91nm andc=1.97nm) was reported to aggregate with the H2 phase in Ref.[69].In this work, both coherent H2/Mg interface and H’phase were not found.Grobner et al.[13]studied the phase equilibria in the Mg?Gd?Zn alloys and found that the H2 phase is stable and forms from the liquid phase.Also, they pointed out that the H1 phase is metastable and was formed in the as-cast or quenched samples.During heat treatment,it will transform to H2 or I-phase.In this work, both H2 and H1 are semi-coherent with the T phase while no certain ORs between H2/H1 and W were revealed.In addition, H2 and H1 were generally observed in Zn?Mg?RE systems or Mg?Zn?RE systems which own much high Zn concentration.Therefore,it could be deduced that both H2 and H1 could formed at some occasional regions where Zn-segregation is very serious during solidification.During solidification, the minor H2phase was remained while a part of H1 phases transferred to the T phase due to its metastability.With respect to the Z phase, it can be described with three variants of icosahedral units and has almost identical chemical compositions with the corresponding I-phase [70].Ordinarily, transformation between Z and I-phase was reported in the Mg?Zn?RE and Zn?Mg?RE systems [17,70].Recently, Grobner et al.[13]reported that the Z phase would transform to the H2 phase, but no certain ORs between them were revealed.On the contrary, a certain OR between Z and T were revealed in this work.Therefore, the metastable Z phase would transfer to the T phase.

Fig.12.(a) BF-TEM image, the corresponding SAED patterns for (b) the small phase labeled by an orange arrow and (c) both T and (Mg,Zr)Zn2, (d) the HRTEM image along with the FFT patterns of the T/(Mg,Zr)Zn2 phase boundary.

Fig.13.(a, b) HAADF-STEM images, (c) the point EDS spectra along with the analysis results, (d-f) HRTEM images along with (g-i) the FFT patterns for the relatively coarse precipitate in the α-Mg grains.

4.2.Coexistence of various precipitates

In theα-Mg grains of the studied alloy, Mg7Zn3, Z, Iphase, Mg4Zn7,β1＇-MgZn2andβ2＇-MgZn2precipitates were simultaneously observed.β1＇andβ2＇precipitates were widely reported in Mg?Zn-based alloys after aging at a wide temperature below 320 °C [55,71-73].From Fig.1, it is noted that the period in which the temperature of the ingot located below 320 °C is over than 10min.Therefore, bothβ1＇andβ2＇in the studied alloy were formed during the later casting period.With respect to Z and Mg7Zn3, both were assessed to be metastable.It is reported that the Mg7Zn3phase is metastable and will transform to the MgZn phase during heat treatment [22,56].Also, some researchers pointed out that the Mg7Zn3phase is closely related to the I-phase.However, neither MgZn, nor singe Mg7Zn3precipitate, nor coexistence of Mg7Zn3and I was observed based on numerous TEM observations, except that Mg7Zn3followed a certain OR with the Z phase.Therefore, the Mg7Zn3phase was formed according to nucleation on the Z phase.But the Z phase is also reported as a metastable phase [13].In this work, the Z phase wasdeduced to transfer to the T phase.However, Grobner et al.[6]reported that the Z phase can be formed from H and liquid at ～607 °C [6].Thus, the Z phase might be formed during the early stage of solidification, although this needs much more elaborate work to be proved.For the nano-scale I precipitate, Huang et al.[74]studied its formation mechanism in the Mg?1.50Zn?0.25Gd alloy during hot compression.They reported that the needle-like Mg4Zn7phase previously precipitated in the Mg matrix, and then the I-phase nucleates on it following an OR as:[110]Mg//[07]Mg4Zn7//[2-fold]I.However, a different OR among Mg, I and Mg4Zn7was revealed: (5-fold)I-phase//(100)Mg4Zn7//(0001)Mg,[2-fold]I//[021]Mg4Zn7//[110]Mg, in this work.Furthermore,most I precipitates were solely located inα-Mg grains, with no attendant Mg4Zn7precipitate.Recently, Yang et al.reported that binary quasi-crystals could form inα-Mg matrix of Mg-Zn alloys during heat treatment [75].Therefore, the I phase was formed by precipitation from solid solution or by nucleation on the previously formed Mg4Zn7particles.

Fig.14.(a,c) BF-TEM images and (b,d-f) HR-TEM image along with the FFT patterns for the various precipitates in the α-Mg grains.

Fig.15.Representative compressive and tensile engineering stress-strain curve at room temperature of the gravity die-cast ZEK620 alloy.

4.3.Strengthening mechanisms of intermetallic phases

During deformation, dislocations and/or twins will have to be activated in the grains to satisfy von Mises’s criterion [76].Thus, intermetallic phases play a key role in coordinating the deformation among adjacent grains, because stresses and strains have to be transferred from one grain to other grains through intermetallic phases.It is well known that different structures have different toughness, thus leading to various influences on impeding or bypassing dislocation motion [29].Also, this work reveals that the intermetallic skeleton of the studied alloy is consisted of many structures, which results inmany phase boundaries, and many of them are coherent.Generally, coherent phase boundary due to its higher decohesion energy is stronger than incoherent phase boundary, such as grain boundaries betweenα-Mg grains or intermetallic phase(i.e.Mg17Al12) grains [77,78].Moreover, most intermetallic phases contain many faults.Therefore, the high volume of intermetallic phases as well as the dense phase boundaries and faults can efficiently impede dislocation motion and simultaneously can well coordinate the deformation between adjacent grains.These are the underlying causes for the high strength of the studied alloy.

5.Conclusion

Intermetallic phases in the gravity die-cast ZEK620 alloy were thoroughly investigated using TEM.The results indicate that the eutectic intermetallic phases are mainly consisted of W and T while several minor phases (Z, H2, H1 and(Mg,Zr)Zn2) were also observed.Interestingly, many planar faults such as SGBs,SFs and twins were found in both W and T phases, and two ORs were revealed between them in this work.The minor Z, H2 and H1 phases generally aggregated together and follow identical ORs with the T phase.Simultaneously, a great many of fine precipitates were observed inα-Mg grains with various sizes.The relatively coarse precipitate(＞100nm)are consisted of Z,Mg7Zn3or T,the relatively fine precipitate (～25nm) is with spherical morphology and was identified as I, and the ultra-fine precipitates (＜10nm) were confirmed asβ1＇-MgZn2and Mg4Zn7with rod-like shape and lying on the (110)Mgand (0001)Mgplanes, respectively, or asβ2＇-MgZn2with spherical morphology.Therefore, both eutectoid intermetallic phases and precipitates are significantly complex in the gravity die-cast ZEK620 alloy.

Acknowledgments

This work was supported by the National Natural Science Foundation of China under grants no.51701200 and 11804030, the Scientific and Technological Developing Scheme of Jilin Province under grants no.20200801048GH,and the Project for Jilin Provincial Department of Education under grant no.JJKH20190583KJ.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jma.2020.10.005.