Precipitation behaviors and mechanical properties of a solution-treated Mg-Gd-Nd-Zn-Zr alloy during equal-channel angular pressing process

Zhenzhen Gui, Fen Wng, Junyi Zhng, Dexin Chen, Zhixin Kng,?

a School of Mechanical and Electrical Engineering, Guangzhou University, 230 Wai Huan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, China

b Guangdong Key Laboratory for Advanced Metallic Materials Processing, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China.

c Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan University, 601 Huangpu Avenue West, Guangzhou 510632, China

Abstract A magnesium alloy processed by equal-channel angular pressing (ECAP) exhibited excellent microstructure refinement and improved strength and hardness.The comprehensive mechanical properties of magnesium alloys have supported the expansion of their applications in the automotive, aerospace, and biomedical industries.Herein, pre-treatment of a solution-treated Mg-2.9Gd-1.5Nd-0.3Zn-0.3Zr alloy was conducted to investigate the precipitate behavior and microstructure evolution during the ECAP process. β1 phase grains quickly precipitated from the solution-treated alloy, which accelerated grain refinement and enhanced the ductility after the ECAP process, as compared to the as-cast alloy reported in our previous study.Moreover, spherical precipitates (～200nm) and fine phases (～100nm) precipitated along the stripe-like Zn2Zr3 phase, which formed a kabap-like structure dispersing homogeneously in the solution-treated alloy during the ECAP process.Owing to grain refinement,dislocations,spherical β1 precipitates,and texture evolution,the solution-treated alloy after eight passes of ECAP exhibited good comprehensive mechanical properties, with the ultimate tensile strength, yield strength, and elongation values reaching 210.9 MPa, 263.9 MPa, and 27.9%, respectively.

Keywords: Mg-Gd-Nd-Zn-Zr alloy; Precipitations; Solid solution; ECAP; Mechanical properties.

1.Introduction

Magnesium and its alloys are recognized as promising structural materials owing to their low density, high specific strength, and good castability with great potential in electronic, automotive, aerospace, and biomedical applications [1-4].However, owing to their hexagonal close-packed(hcp) crystal structure and the operation of limited slip systems, magnesium and its alloys usually exhibit low mechanical properties, which limits their practical application [5,6],such as limiting their large-scale application in the fields of aerospace and transportation as structural materials [7,8].To create useful magnesium alloys, further plastic deformation is a necessary step to enhance their strength and formability [9].It is well established that severe plastic deformation (SPD) is effective in producing significant grain refinement and thus improving both strength and ductility [10-13].

Currently, equal-channel angular pressing (ECAP) is one of the most researched SPD methods for producing bulk fully dense ultrafine-grained (UFG) metallic materials[14-17], showing a stronger grain refinement effect than conventional deformation processes [18].In addition to grain refinement, solution strengthening and precipitation hardeningare common approaches for strengthening the magnesium alloys [19-21].Microstructures with fine grains and dispersed metastable phases can be obtained by heat treatment and plastic deformation [22].Several studies have attempted to explain the influence of thermal treatment on the microstructure and mechanical properties during deformation [23-27].Apps et al.[19,28]reported the effects of dispersoids and coarse second-phase particles on the rate of grain refinement during severe deformation processing of aluminum alloys.The Al-0.2Sc alloy with Al3Sc dispersoids showed a larger grain size than the single-phase Al-0.13Mg after ECAP by inhibiting the formation of micro shear bands.Yuan et al.[29]compared the microstructural evolution and mechanical properties of an ECAPed AZ91Mg alloy with and without solution treatment.Heat treatment before ECAP promoted the formation of spherical precipitates, thereby providing a high yield strength and low ductility product after four-pass ECAP.Moreover,it effectively increased the dislocation density and accelerated grain refinement.Choi et al.[30]discussed the effect of thermal treatment on the mechanical properties of ZK60 magnesium alloy processed by high-ratio differential speed rolling.Both the strength and elongation of the solutiontreated samples reduced slightly, contributing to the larger grain size compared with the rolled as-cast sample.The results seem to be conflicting, with no uniform rule, and few studies have reported alternate pretreatments of the as-cast and solution-treated magnesium rare-earth (Mg-RE) alloys after ECAP [18].

In this study, solution-treated Mg-2.9Gd-1.5Nd-0.3Zn-0.3Zr alloy (hereafter referred to as Mg-Gd-Nd-Zn-Zr alloy)was processed under continuous ECAP processing with differing numbers of passes.The solubility of Gd in the alloy is good, which is beneficial for adjusting the solid solution and precipitation in the microstructure, and the addition of Gd improves the mechanical properties according to previous studies [31].The atomic ratio of Nd is very close to that of Mg, and easily forms a substitutional solid solution.Moreover, the standard electrode potential of Nd is close to that Mg, which is advantageous for avoiding the formation of micro galvanic corrosion.Thus, Gd and Nd are the main alloying elements in this designed magnesium alloy.Zn is advantageous for improving the yield strength, and Zr has the advantage of decreasing the grain size of casting magnesium.The content of each element is determined through consideration of economic cost and comprehensive performance.With good mechanical properties, Mg-Gd-Nd-Zn-Zr alloy is considered to be a potential structural material in automotive,aircraft, aerospace industries, and biomedical applications.In this report, the precipitation behavior of the pretreated Mg-Gd-Nd-Zn-Zr alloy will be clarified and the effect of the newly generated precipitates during the ECAP process on the microstructures and mechanical properties will be shared.

2.Experimental procedure

Fig.1.ECAP die design.

A mixture of high-purity Mg (≥99.99%), Zn (≥99.99%),and a master alloy of Mg-25%Gd, Mg-30Nd, and Mg-30%Zr were melted in an electric resistance furnace at 730-780°C with shielding gas (99.5% N2and 0.5% SF6) to prepare the as-cast Mg-2.9Gd-1.5Nd-0.3Zn-0.3Zr (wt.%) alloy followed by cooling at room temperature (～27°C).The chemical composition of the alloy was analyzed by X-ray fluorescence(Axios PW4400, PANalytical B.V., Netherlands).Then, the melt was poured into a preheated steel mold at 740°C.The as-cast alloy was homogenized at 520°C for 16h and quenched to room temperature in water, which we term a solution-treated alloy.The solution-treated alloy was cut into dimensions of 12×12×80mm to prepare ECAP billets.The ECAP process was performed using a die with a channel angleΦ= 90°and fillet angleΨ=37° Subsequently, the ECAP sample was extruded with a ram speed of 0.4 mm·s-1through the pressing route Bc[32,33],which has been demonstrated as an effective way to obtain ultrafine grains[34].The billets were processed for 1,4,and 8 passes at a constant temperature of 37.5±2°C.The directions of obtained ECAPed alloys were determined according to the die.The extrusion direction (ED), transverse direction (TD) and normal direction (ND) are shown in Fig.1

All samples for testing were cut from the center of the ECAPed billets.Samples for microstructure, XRD and TEM analysis were cut perpendicular to extrusion direction (ED,as shown in Fig.1).Tensile testing samples were machined along the longitudinal direction of the billets and the tensile tests were conducted on a SANSCMT 5105 (MTS, China)universal testing machine with a strain rate of 1.0×10?3s?1at room temperature.The samples for microstructure observation were investigated using an optical microscope(DMI 5000, Leica, Germany), scanning electron microscopy(SEM, Phenom proX, Phenom-world B.V., Netherlands), and transmission electron microscopy (TEM, JEM-2010, JEOL,Japan).Before SEM observation, the samples were polished and etched with an acetic-picral solution.X-ray diffraction(XRD, D8 ADVANCE, Bruker, Germany) analyses were used to investigate the phases present in the samples.Texture evolution during ECAP passes was also analyzed on the transversal plane by investigating the pole figures of the(0002) and (1010) planes using the Schultz back reflection method.The test parameters of macroscopic texture testing are as follows: goniometer radius: 320mm; diver-gence slit: 2.00mm; generator voltage: 40kV; and generator current: 40mA.

Fig.2.Microstructure characteristics of the Mg-Gd-Nd-Zn-Zr alloys: (a) Optical microscopy (OM) image of the as-cast alloy, (b) OM image of the solutiontreated (ST) alloy, and (c) SEM image of the ST alloy.

Fig.3.SEM micrographs of Mg-Gd-Nd-Zn-Zr alloys conducted at different treatment: (a) ST+ECAP 1p, (b) ST+ECAP 4p, and (c) ST+ECAP 8p.

3.Results

3.1.Microstructure of Mg-Gd-Nd-Zn-Zr alloys before and after ECAP

Fig.2(a) and 2(b) show the optical microscope of the ascast (AC) and solution-treated (ST) Mg-Gd-Nd-Zn-Zr alloys before ECAP.As seen in Fig.2(a), the microstructure of the AC alloy is composed ofα-Mg matrix with island-like eutecticβ1compounds distributed along the grain boundaries as indicated by the yellow arrows.The initial grain size of the AC alloy was approximately 50μm, as calculated by the mean linear intercept method.The island-like eutecticβ1compounds were identified as the (Mg,Zn)3RE phase, based on previous research [35].After solution treatment of the AC alloy, a portion of the grains grew to more than 100μm, as shown in Fig.2(b) and 2(c).The amount of eutecticβ1compound in the ST sample decreased sharply compared to that in the AC sample.The disappearance ofβ1compounds was owing to elements of RE and Zn diffusing and solid solutioning into theα-Mg matrix at high temperature (520°C) over the 16h treatment time.Some isolated phases are observed inside the grains, as indicated by the red arrow in Fig.2(c), which are thought to be the ZnZrxphase [36].

SEM images of the ST Mg-Gd-Nd-Zn-Zr alloy with different numbers of ECAP passes are presented in Fig.3.Fig.3(a)displays a SEM image of the solution-treated Mg-Gd-Nd-Zn-Zr alloy after 1 ECAP pass (named as ST+ECAP 1p).It can be compared with the ST sample in Fig.2(b) and (c), except for a decrease in grain size in some local area, the grain size of most regions of the ST+ECAP 1p sample has no obvious reduction.The reason may be that the deformation degree of the 1p ECAP is not sufficient to achieve a large area of dynamic recrystallization (DR).The deformation storage energy of each individual ECAP pass contributes to only local decrease in the grain size.Also, some deformation twinning is observed in the ST+ECAP 1p alloy.

After the ST alloy received a four-pass ECAP treatment,new phase precipitates appeared in theα-Mg matrix, as shown in Fig.3(b); this new phase was not homogeneously dispersed in the samples.Part of the fine new phase was generated around the ZnZrXphase (indicated by the red ellipse),and another coarse precipitate appeared at theα-Mg matrix boundaries without the ZnZrXphase (indicated by the white ellipse).With eight ECAP passes, this precipitating phenomenon becomes more obvious such that several regions offine precipitates and comparatively coarse precipitates appear in Fig.3(c).

Fig.4.TEM images of the ST+EACP treated Mg-Gd-Nd-Zn-Zr alloys in the region with (a-c) and without (d-f) stripe-like Zn2Zr3 phase: (a, d) 1-pass, (b,e) 4-pass, (c, f) 8-pass, and the inserted image in (a-c) is the selected area electron diffraction (SAED) pattern of the pointed β1 phase.

TEM analysis was conducted to investigate the precipitations, and bright field (BF) images of ST samples with ECAP 1p, 4p, and 8p are shown in Fig.4.The regions with and without the ZnZrXphase in the SEM images are also observed in the TEM-BF images.Precipitation in the region with the ZnZrXphase (in accordance with the red ellipse area in Fig.3) is shown in Fig.4(a)-(c).It is interesting that with an increase in the number of ECAP passes, the number of precipitates formed in this region increases rapidly along the stripe-like ZnZrXphase in the form of a kabap-like structure.The stripe-like phase consisted of Zn2Zr3, as indicated by the yellow arrow in Fig.4(a)-(c), and the EDS of the stripe-like Zn2Zr3phase is displayed in Fig.A.1.The size of the precipitates remains similar regardless of the number of ECAP passes.In addition, the growth of the precipitates seems to be inhibited by the stripe-like Zn2Zr3phase.This type of microstructure has not been observed in previous research [36].In contrast, there exists a sphericalβ1phase distributed homogeneously in theα-Mg matrix without the ZnZrXphase, as shown in Fig.4(d)-(f).The inserted images in Fig.4(d)-(f) show the selected area electron diffraction(SAED) pattern of the selectedβ1phase, as indicated by the white arrow in Fig.4(d)-(f).Theβ1phase ranges from 50 to 300nm in diameter.Meanwhile, as the number of ECAP passes increased, the amount ofβ1phase precipitates at the grain boundaries also increased, as shown in Fig.4(d)-(f).These increased precipitations acted as nucleation sites to induce DR, refine grains, and reduce dislocation density.

The grain refinement resulting from DR with increasing ECAP passes can be seen in Fig.4(d)-(f).With the ST alloy after 1 pass ECAP, the stored deformation energy was not sufficient to conduct DR in large areas.In contrast, the stored deformation energy triggered dislocations in the grains and promoted dislocation movement and accumulation to form sub-grain boundaries, as indicated by the ellipse in Fig.4(d).With four-pass ECAP, the stored deformation energy was sufficient to conduct DR in large areas, so that these refined grains are presented in SEM and TEM-BF where the grain boundaries are clearly observed,as shown in Fig.4(e)pointed out by ellipses.The average grain size of the ST+ECAP 4 pass was refined to ～ 2μm.After eight pass ECAP,the grains were further refined, the numerousβ1 spherical phase precipitates were distributed at the triangle boundaries and an accumulation of new dislocations restricted further movement, as indicated by the rectangle in Fig.4(e)-(f).

3.2.XRD analysis and texture evolution during ECAP

Fig.5 depicts the XRD pattern of the ST Mg-Gd-Nd-Zn-Zr alloy with different ECAP treatment passes.The ST sample is composed of anα-Mg matrix andβ1-(Mg,Zn)3RE, which is in accordance with the SAED pattern of the TEM analysis.Compared with the AC Mg-Gd-Nd-Zn-Zr alloy in our previous study [37], the ST alloy displays the peaks of theZn2Zr3phase, and the diffraction peaks of theβ-(Mg,Zn)3RE phase decreases implying the dissolution of the eutecticβcompounds into theα-Mg matrix.Diffraction pattern of XRD of ST sample shows that main peaks existing a small left migration in 2θfrom the standard peaks of Mg (ICCD PDF 89#?5003), this phenomenon may due to the dissolution of Zn in the Mg matrix to enlarge lattice constant of Mg during solid solution process.The peak strength of Mg (002) in the studied alloys increased with the ECAP passes.This result is attributed to the basal texture of (002) caused by the ECAP process, which is accordance with the following texture analysis in Fig.6.The peaks of the (Mg,Zn)3RE phase appear again in the XRD pattern of the ST samples following the different ECAP passes, which is due to the newβ1 phase precipitate.

Fig.5.XRD patterns of ST+ECAP treated Mg-Gd-Nd-Zn-Zr alloys with different ECAP passes.

(0002) and (1010) pole figures of the ST Mg-Gd-Nd-Zn-Zr alloys processed by ECAP for 1, 4, and 8 passes were obtained using XRD to investigate the texture evolution during the ECAP process, as shown in Fig.6.With 1 pass ECAP, no significant texture component was detected with the orientation of the (0002) basal plane or (1010)cylindrical plane, Fig.6(a) and (b).The (0002) basal plane presents a slightly enhanced(Imax=54)after 4 ECAP passes,and theImax after 8 ECAP passes was weakened to 44.This was due to the cooperative effect of DR and dynamic precipitation [38].

3.3.Mechanical properties

Fig.6.(0002) and (1010) pole figures of the ST+ECAP treated Mg-Gd-Nd-Zn-Zr alloys.

The engineering stress versus engineering strain curves obtained from tensile tests of the as-cast, ST, and ECAP processed Mg-Gd-Nd-Zn-Zr alloys are presented in Fig.7(a),which are defined as AC, ST, ST+1p, ST+4p, and ST+8p,respectively.The variation in the yield stress (YS), ultimate tensile strength (UTS), and elongation with different numbers of ECAP passes are plotted in Fig.7(b).Compared with the AC Mg-Gd-Nd-Zn-Zr alloys, both the ductility and tensile strength of the ST Mg-Gd-Nd-Zn-Zr alloys enhance considerably owing to the solid solution strengthening and elimination of dendritic segregation.The YS and UTS of the ST alloy were enhanced about 20MPa and 35MPa compared with the as-cast alloy, respectively.The ductility of the ST alloy was improved obviously for the reason of changing the fracture mode.Detailed analysis please referred out previous study [37].It is apparent that the comprehensive mechanical properties of Mg-Gd-Nd-Zn-Zr alloys simultaneously increase with the number of ECAP passes as shown in Fig.7(a).Although the ductility of the ST samples (23.0%) was reduced after 1 pass (12.4%), the UTS of the ST+1p sample (235.2 MPa) increased slightly than that of the ST sample (195.5 MPa).When the number of ECAP pass increased to 8, both the YS, UTS and elongation reached the highest levels, which were 210.9 MPa, 263.9 MPa, and 27.9%,respectively.

Fig.8 shows the fracture morphology of the Mg-Gd-Nd-Zn-Zr alloy under different conditions after tensile testing.Trans-grain cleavage rupture is the main fracture characteristic of the ST alloy.Clearly visible angles existed between cleavage facets, as shown in Fig.8(a).Trans-grain cleavage and ductile tearing were the main fracture characteristics of the ST+ECAP 1p alloy.The cleavage facets displayed flat surfaces, and ductile tearing areas appeared as shallow dimples,as shown in Fig.8(b).After 4 and 8 ECAP passes, ductile tearing becomes the main fracture mechanism, and numerous deep dimples appeared at the fracture surface, as shown in Fig.8(c) and 8(d).Dimples in the 8 pass ECAP samples are much larger and deeper than that in the 4 pass ECAPsamples, which was consistent in that the former presents a longer elongation and better ductile properties.

Fig.7.Mechanical properties of the Mg-Gd-Nd-Zn-Zr alloys through casting, solution-treatment and ST+ECAP treatment: (a) engineering stress-strain curves,and (b) YS, UTS, and elongation values obtained through tensile tests.

Fig.8.Fracture morphology of the Mg-Gd-Nd-Zn-Zr alloy under different conditions after tensile testing: (a) ST, (b) ST+ECAP 1p, (c) ST+ECAP 4p, and(d) ST+ECAP 8p.

Fig.9.Quantities of the β1 phase under different conditions calculated based on XRD pattern using the WPF method.

4.Discussion

In the ST alloy for 1 pass ECAP, a large number of dislocations emerged because of the strong shear deformation and high extrusion temperature [39].Meanwhile, a new necklacelikeβ1phase began to precipitate from theα-Mg matrix,which led to precipitation strengthening.When the number of ECAP passes increased to 4, DR occurred in the ST+ECAP samples, with an average grain size of ～2.0μm.Using the XRD pattern and crudely estimated by the whole pattern fitting and Rietveld refinement method (WPF), the increased quantities of fine newβ1phase in ST samples from the solution treatment to ST+ECAP 4p was 3.4%,as shown in Fig.9.When the solution treatment was conducted before ECAP, the precipitate nucleated together under continuous intense plastic deformation, which consequently led to finer precipitates and higher dislocation density [29].This accelerated the rate of DR, which was determined by the dislocations present near the critical plastic strain regions.After the 8 pass ECAP, the increased quantity of the newly generatedβ1phase in the ST sample was 3.8%, which is consistent with the grain size of the ECAPed ST samples.Moreover, the precipitate behavior pattern of the ST sample during ECAP in Fig.3 is rarely reported.It was visible that two sizes of the sphericalβ1phase were generated in the ST samples.One is coarse with an average size of ～200nm, which precipitated in the region without the Zn2Zr3phase,and the other is the fine precipitates(～100nm)combined with the Zn2Zr3phase,forming a kabablike structure.This is due to the heterogeneous nucleation that came up preferentially at the Zn2Zr3phase boundary.

Texture evolution could be attributed to the cooperative effect of DR and dynamic precipitation during ECAP.The fine precipitates emerged during ECAP had a strong ability to prevent the refined grains from rotating and was in favor of texture weakening [38].Based on the quantity of the Mg-Gd-Nd-Zn-Zr alloys under different conditions, the quantity of fine precipitates in the ST alloy after 1 pass was approximately 2.1%.During the next 4 ECAP passed the stronger dynamic precipitation and DR (the grain was refined to ～1.5μm) of the ST sample would weaken the texture effectively.When the number of ECAP passes increased to 8,the fine precipitates generated increased to 3.8%, which led to a decrease in the texture intensity in the ST+8p samples.

The mechanical properties of the alloy were mainly influenced by the three factors discussed previously, including grain size, secondβ1phase, and texture.First, according to the Hall-Petch relationship [40], grain refinement is an effective way to enhance both strength and ductility [41,42].Second, fine precipitates dispersed from theα-Mg matrix during ECAP can be conducive to the improvement of strength by hindering the dislocation motion.Finally, for poor symmetry and deformability, texture has a significant influence on the mechanical properties of Mg alloys [43].In this study,the UTS of the ST+ECAP 1p alloy slightly increased from 195.5 to 235.2MPa compared with the ST alloy, owing to the accumulation of dislocations and precipitation strengthening of the newβ1phase.However, the elongation of the ST+1p alloy decreased from 23.0% to 12.4% owing to the increasing dislocations [44].The decrease in elongation in the ST+ECAP 1p alloy resulted from the appearance of a large number of dislocations and the sphericalβ1phase.The dislocations tangle to form walls, which restricts their movement during tensile testing.The existence of a large number of sphericalβ1phase precipitates in the alloy hinders the dislocation movement during tensile testing.The tensile elongation of the ST+ECAP alloys increased with the increase in ECAP passes from 1 to 8 passes.As mentioned above, the grain size was noticeably refined from ECAP 1p to ECAP 4p,as well as the DR which occurred in larger regions.This was the main reason for the enhancement of the elongation of the ST+ECAP 4p alloy, compared with that of the ST+ECAP 1p alloy.

Magnesium alloys for biodegradable application are always designed with micro-alloying.The total amount of alloying elements is usually not over 5%.So the mechanical properties of biodegradable magnesium alloy are not that as high as for structural application.Erinc [45]put forward criteria to evaluate the feasibility of available Mg alloys and processing techniques for manufacturing biodegradable implants: the yield strength (YS)＞200MPa and the elongation (EL)＞15% should be satisfied.Sezer [46]summarized mechanical properties of powder processed Mg-based alloys and results show that YS and EL of the most mg-based alloys are around 200MPa and 20%, respectively.Liu [47]summarized mechanical properties of binary Mg-RE alloys, and most alloys present YS less than 250MPa and EL value less than 25%.Zhao [48]found that 0.5 Sn addition results in improvement in the strength and elongation, and the YS value of extruded Mg-1Zn-0.5Sn alloy achieved 115MPa, as shown in Fig.3.Sabbaghian [49]reported that the YS values of the Mg-4Znand Mg-4Zn-0.5Mn alloys are 198.4 and 233.4MPa through extruding process, respectively.In conclusion, the YS, UTS and EL of ST+ECAP 8p alloy in our work are 210.9MPa,263.9MPa and 27.6%, respectively.Comprehensive properties of the ST+ECAP 8p alloy are at the forefront of the biodegradable magnesium alloys.

5.Conclusions

This study discussed the effect of increasing ECAP passes of solid-solution treated Mg-Gd-Nd-Zn-Zr alloy on its microstructure evolution, precipitate behavior, and mechanical properties.The following conclusions are drawn:

(1) The microstructure of the solid-ST solid-solution treated Mg-Gd-Nd-Zn-Zr alloy consisted ofα-Mg matrix and a ZnZrxphase.

(2) Two types of precipitates formed in the ST Mg-Gd-Nd-Zn-Zr alloy during the ECAP process: spherical precipitates (β1phase) found without the Zn2Zr3phase at a grain size of ～200 nm, and a finerβ1phase (～100nm)dispersed along with the stripe-like Zn2Zr3phase to form a kabap-like structure.

(3) The ECAPed 8p samples possessed improved grain refinement, new dislocations existing in refined grains,sphericalβ1 precipitates of bimodal sizes, and texture evolution, which contributed to the YS, UTS, and elongation of the ECAPed 8p alloy reaching 210.9 MPa,263.9 MPa, and 27.9%, respectively.

Acknowledgments

This work was financially supported by theRegional Joint Youth Fund Project of Guangdong Basic and Applied Basic Research(Grant No.2020A1515110619),Guangzhou Science and Technology Plan Project(Grant No.202002030356),the2019 Youth Innovative Talents Project of General Colleges and Universities in Guangdong Province(Grant No.2019KQNCX106), and theTalent Cultivation Project of Guangzhou University(Grant No.RP2020126).

Appendix A

Fig.A1.Energy spectra of stripe-like phase in TEM micrograph.