Formation of β＇ phase in LPSO structures in an Mg88Co5Y7 alloy

Q.Q.Jin, X.H.Sho, Y.T.Zhou, B.Zhng, S.J.Zheng, X.L.M

a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, 110016 Shenyang, China

b Materials Science and Engineering Research Center, Guangxi University of Science and Technology, Liuzhou 545006, China

c School of Materials Science and Engineering, Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin 300130, China

Abstract Formation of β＇ phase in long-period stacking ordered (LPSO) structures in an Mg88Co5Y7 (at.%) alloy after aging at 200°C for 24h or electron beam (EB) irradiation has been studied by high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM). β＇ phase was precipitated only in the Mg matrix but not in LPSO structures after aging at 200°C for 24h.LPSO structure containing stacking defects transforms into the β＇-long phase during EB irradiation, which plays a key role in accelerating solute atoms’ diffusion.New complex β＇(LPSO) structures formed in the alloy after EB irradiation, such as β＇(12H) structure with an orthorhombic lattice (Mg7Y, Cmcm,a=2a0=0.642nm, b=4a0=2.27nm, c=6c0=3.12nm).? 2020 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; Long-period stacking ordered (LPSO) structure; HAADF-STEM; β＇ phase.

1.Introduction

Mg-based alloys containing rare-earth elements have received considerable attention as promising candidates for new lightweight structural materials due to their excellent mechanical properties [1-5].Specifically, Mg-Zn-RE (RE=rare earth) based alloys are documented to be strengthened via various types of long-period stacking ordered (LPSO) structure [6-14], showing better mechanical performance at high temperatures than those without LPSO structures owing to the good thermal stability [15].Recently, intermetallicβ＇phase was detected to precipitate between LPSO structures containing AB＇C＇A building blocks, where B＇and C＇layers are enriched with Zn and RE atoms, in an Mg-7Y-4Gd-1.5Zn-0.4Zr (wt.%) alloy during aging [16].Theβ＇metastable phase has been documented to be a key strengthening precipitate phase in Mg-RE (RE=Nd, Sm, Y, Gd, Dy)alloys[17-24],which contains double RE-enriched layers parallel to (10)Mgplane.Theβ＇-short precipitate in Mg-Nd systems is Mg7Nd (orthorhombic,a=0.65nm,b=1.14nm,c=0.52nm).Theβ＇-long precipitates in Mg-RE (Y, Gd, and Dy) and Mg-Y-Nd/Gd systems are Mg7RE [18,19,22](orthorhombic,a=0.65nm,b=2.27nm,c=0.52nm), and the orientation relationship betweenβ＇and Mg matrix phase is [100]β＇//[20]αand [001]β＇//[0001]α.The coexistence of LPSO structure andβ＇precipitates in Mg alloys containing LPSO structures is expected to largely affect the mechanical behavior [16].

Recently, it was reported that an as-extruded Mg93Co1Y6(at.%) alloy strengthened by LPSO structure reached a tensile strength of 369MPa [5].There is little information related to the aging of Mg alloys strengthened by LPSO structures containing AB＇C building blocks (like 21R and 12H)so far, which is a very important aspect for the Mg-Co-Y alloys in service.In this study, we investigate the microstructural response of the typical LPSO phases in Mg-Co-Y alloys featuring AB＇C building blocks, where Co and Y atoms are enriched in B＇layers [8].Their microstructural evolution is expected to be different from those containing AB＇C＇A building blocks during aging.We detectedβ＇phase precipitated in both Mg matrix and in LPSO structures to form a newstructure in an Mg88Co5Y7alloy during aging or via electron beam (EB) irradiation.Our results provide microscopic insights into the transformation mechanism during aging or via EB irradiation and show how to tune the properties of Mg alloys strengthened by LPSO structure andβ＇phases.

2.Experiments

Nominal composition of Mg88Co5Y7(at.%) alloy was prepared in a high-frequency induction melting furnace by melting the high purity Mg, Co, and Mg-30wt.% Y master ingots under an Ar atmosphere in a graphite crucible.The precipitation ofβ＇phase in the alloy aging at 200°C for 24h was studied via high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).The microstructure of LPSO in an as-cast alloy before and after EB irradiation in TEM mode was also studied via HAADFSTEM to investigate the formation ofβ＇phase.EB irradiation experiments were carried out on an aberration-corrected Titan3TMG260-300 operated at 300kV.The alloy specimen was cut into pieces with a thickness of 500μm using high speed saw cutting machine cooled by recycle-water, and then ground to 50μm and die-cut into disks with a diameter of 3mm.TEM samples were firstly prepared by the twinjet polishing facility in a solution of 1% HClO4in ethanol at -20°C.They were then ion-milled in Gatan PIPS 691 to remove the oxide layer on sample surfaces before TEM observations.HAADF-STEM imaging was then performed with an aberration-corrected Titan3TMG260-300, which was equipped with a high-brightness field-emission gun (X-FEG)and double Cs correctors from CEOS and operated at 300kV.The probe convergence semi-angle was approximate 25 mrad and the collection semi-angle of the HAADF detector ranged from 50 to 250 mrad.Chemical compositions were analyzed by energy dispersive X-ray spectroscopy (EDS) in STEM mode.

We introduce new notations to easily describe the stacking sequence and determine the Bravais lattices and space groups of LPSO structures in Mg-M-RE alloys.Rules are as follow, AB＇C＇A (and/or AC＇B＇A) building blocks are defined as F-blocks and denoted as F (and/or ˉF); while AB＇C (and/or AC＇B) building blocks are defined as T-blocks and denoted as T (and/or ˉT).Then(n=integer) means the number of Mg layers sandwiched between the F and/or T-blocks, written asn-Mg.The subscript number refers to the number of repeated(sub) unit cells.

3.Results

3.1.Formation of β＇ phase in Mg matrix via aging at 200°C for 24h

Fig.1 presents the microstructure of LPSO structure and precipitates in the Mg matrix in an Mg88Co5Y7alloy aged at 200°C/24h.Fig.1a is a low-magnification HAADF-STEM image obtained along [20]αzone axis, showing intergrowth of LPSO structure exhibiting brighter contrast and Mg matrix.Besides, nano-size precipitate in brighter contrast was observed to embed in Mg matrix.Based on atomic-resolution HAADF-STEM images obtained along [20]αzone axis in Fig.1b, the LPSO structure can be easily determined to be 12H [8], and the precipitate can be determined to beβ＇phase[18,19,22].Interestingly, no interaction betweenβ＇phase and 12H LPSO structure was observed, and straight gaps exist betweenβ＇phase and LPSO structures, denoted by the parallel dashed white lines.We further investigated the LPSO structure and Mg matrix in an Mg88Co5Y7alloy aged at 200°C/24h via HAADF-STEM imaging along [0001]αzone axis.The precipitate in Fig.1c can be determined to beβ＇-long phase [18,19,22].Based on our previous work [8],the stacking sequence of 12H is AB＇CBCBCB＇ABAB.Thus,more layers at B positions in a unit cell and enrichment of Co/Y elements at B positions lead to brighter contrast of B positions than A and C position in the HAADF-STEM image along [0001]αdirection.Such analysis is consistent with the image in Fig.1d, which also suggests that noβ＇phase was observed to form in LPSO structure in the Mg88Co5Y7alloy aged at 200°C/24h.

3.2.Formation of β＇ phase in LPSO structure after EB irradiation

Fig.2a presents a low-magnification HAADF-STEM image obtained along [20]αzone axis in an Mg88Co5Y7as-cast alloy, showing an LPSO structure containing stacking defect structures.The LPSO structure was determined to be 15R, and both growth twin boundaries and stacking faults(SFs) were observed in 15R via atomic-resolution HAADFSTEM images.Noβ＇phase was detected in the LPSO structure in the Mg88Co5Y7as-cast alloy.However, after electron beam irradiation in the TEM model for one hour,phase transformation occurred in 15R LPSO structure.Lathshape precipitate in brighter contrast was observed in LPSO structure in an HAADF-STEM image in Fig.2b, viewed along [20]αdirection.The detailed structure of precipitate in the LPSO structure was determined by atomic-resolution HAADF-STEM images in Fig.2c-d viewed along [20]αand [100]αdirection, respectively.The parallel zigzag atom column in bright contrast in Fig.2c suggests that the precipitate isβ＇phase.Thin layers free of RE atoms exist betweenβ＇phase and LPSO structures, which is denoted by the parallel dashed white lines.We denote them RE-free gaps hereafter.Bothβ＇-long andβ＇-short phases were reported in previous work [17-24], which can be distinguished via atomicresolution HAADF-STEM imaging along [100]αzone axis.Their structure models viewed along [100]αdirection was proposed in Fig.4a-b.The atom columns containing Y elements inβ＇-long phase arrange every other raw.Fig.2d is consistent with the structure model ofβ＇-long phase in Fig.4a, which suggests that the LPSO structure transforms into aβ＇-long phase during EB irradiation.

The LPSO structures andβ＇phase with dozens of nanometers coexist in the Mg88Co5Y7as-cast alloy after EB irradiation, as shown in Fig.3a, obtained along [20]αdirection.The width of theβ＇phases is generally in the range of 5 to 100nm.Inside eachβ＇phase are substructures separated by interfaces with dark contrast, indicated by white arrows.Fig.3b presents the atomic-resolution HAADF-STEM image of theβ＇phase.Except for one SF containing AB＇C stacking sequence labeled by an arrow, all the structures in the image have ABAB stacking sequence.However, parallel zigzag arrays of bright dots distribute along the (100)αplane in Region I and II, which are equally separated from each other by 1.11nm, namely 4d.Such structural character is similar toβ＇-long (Mg7RE) in Mg-Y, Mg-Gd, and Mg-Dy alloys [18,19,22]orβ＇-short (Mg7RE) in Mg-Nd alloys.EDS measurement indicates the chemical composition of the phases in Regions I and II is Mg91Y9(at.%), free of Co element.Based on the structural character and chemical composition, the unidentified phases can be determined asβ＇-long with chemical formula denoted as Mg7Y [19].Theβ＇phase exhibits space group ofCmcmand lattice parametera=2a0=0.642nm,b=4a0=2.27nm,c=c0=0.521nm.Interestingly, no super-lattice can be observed in region III.Nevertheless, the columns in region III show slightly brighter contrast than the Mg column inβ＇phase.The intensity profiles in Fig.3c further indicates that the columns in region III contain Y atoms, which was obtained from rectangle c in Fig.3b by integrating the image intensity along the closely packed planes.Interestingly, intensity profiles in Fig.3d obtained from rectangle d in Fig.3b demonstrate that the chemical composition in Region III is approximately equal toβ＇phase in Region II, suggesting that the phase in Region III might also beβ＇phase viewed along with other directions or related variants.

Fig.1.(a) Low and (b) high magnification HAADF-STEM images of 12H LPSO structure and β＇ phase precipitated in Mg matrix in an Mg88Co5Y7 alloy aged at 200°C/24h, obtained along [20]α zone axis.Atomic-resolution HAADF-STEM images of (c) β＇ phase precipitated in Mg matrix and (d) 12H LPSO structure viewed along [0001]α zone axis.

To clarify the structures in Region III, we will analyze the orientation relationship betweenβ＇phase andα-Mg matrix in the following.According to previously reported work [18,19,22]and Fig.3b, their orientation relationship is [20]α//[100]β＇, [0001]α//[001]β＇, which can also be described as [100]α//[100]β＇, [001]α//[001]β＇.Based on the lattice parameter ofα-Mg andβ＇phase, their transfor-mation matrixes between crystalline directions and planes are calculated to beandThus, their orientation relationship can be described by their crystalline planesto be:Another parallel crystalline plane can be calculated to be (00)α//(00)β＇, as [100]α×[001]α=(00)αand[100]β＇×[001]β＇=(00)β＇.The transformation matrix of parallel crystalline directions betweenα-Mg andβ＇can be calculated to beB[25].where,D1=(h,k, l) and (h＇,k＇,l＇) are crystalline planes ofα-Mg andβ＇phase, while [u, v, w]and [u＇,v＇,w＇]are crystalline directions ofα-Mg andβ＇phase.For Mg matrix, the [010]αand[0]αare also prone to＜20＞α.Thus, the orientation relationship can be further calculated to be [010]α//[10]β＇,[0]α//[0]β＇, viawhich can also be described as [20]α//[10]β＇, [20]α//[0]β＇.

Fig.2.(a) An HAADF-STEM image showing 15R LPSO structure containing stacking defects in an Mg88Co5Y7 as-cast alloy.(b) An HAADF-STEM image showing coexistence of LPSO structure and β＇ phase obtained by EB irradiation of 15R containing stacking defects viewed along [20]α direction.Atomic-resolution HAADF-STEM images of β＇ phase in LPSO structure obtained by EB irradiation viewed along (c) [20]α and (d) [100]α direction.

Fig.3.(a) The microstructure presenting the coexistence of lath-shaped 15R, 12H, and β＇ phases.(b) Atomic-resolution HAADF-STEM image of coexistence of β＇ substructure viewed along [10]β＇ and [100]β＇ directions.These images are obtained with the EB parallel to [20]α zone axis.(c-d) Intensity profiles of the structures in rectangle c and d, suggesting that the structures in Region II and Region III are both β＇ phase viewed along different directions.

Fig.4c-d displays 3-D atomic structure model ofβ＇phase[19]viewed along the [100]β＇and [10]β＇directions, where Mg and Y atoms are presented by green and red spheres,respectively.Clearly, one Y atom and one Mg atom arrange alternately in each Y enriched zigzag column viewed along[100]β＇in Fig.4c, and their coordinate along x-axis is eitherx=0 orx=1/2.In one zigzag array, the coordinate of Y atoms in neighboring layers along x-axis isx=0 andx=1/2,alternately.One Y atom and seven Mg atoms arrange alternately in all columns viewed along [10]β＇in Fig.4d.Thus the model shown in Fig.4c corresponds to the orderedβ＇phase in Regions I and II of Fig.3b,while that in Fig.4d represents the structures with homogenous contrast in Region III of Fig.3b.Meanwhile, the ABAB… layers with brighter contrast in Fig.3 should also beβ＇phase viewed along [10]β＇(or [0]β＇) direction.

Fig.4.3-D atomic structure model of (a) β＇-long and (b) β＇-short viewed along [010]β＇ // [100]α directions.3-D atomic structure model of β＇ viewed along(c) [100]β＇ and (d) [10]β＇ directions.

3.3.Formation of β＇(LPSO) via EB irradiation

Fig.5a is an HAADF-STEM image of a typical LPSO phase in the Mg88Co5Y7as-cast alloy after EB irradiation,showing 12H-LPSO containing various stacking fault (SF)structures, obtained along [20]αdirection.12H was reported to contain AB＇C building blocks with 3 Mg layers sandwiched between them and can be denoted as T3ˉT3 by our previously introduced notations [11].The stacking sequences of SFs in Fig.5a were checked to be T2 [26], T4T4,and T7 structures.Interestingly, some SFs exhibits brighter contrast, like T7, than LPSO structures, different from the SFs with dark contrast due to the lower content of solute atoms in as-cast Mg-Zn-Y alloys [10, 11].Fig.5b-c shows the atomic configuration of T7 and T4T4, respectively.The ordered bright dots clearly arrayed zigzag along the (100)αplane, forming similar structures withβ＇phase precipitated in Mg matrix [18,19,22].Hereafter, these ordered zigzag arrays coexisting with LPSO structures are denoted asβ＇(LPSO).Our experiments suggest that most ofβ＇(LPSO) phases are related to the SFs inside LPSO structures.For example, the SF structure changes from T5ˉT3 at the top to T4T4 at the bottom in Fig.5c, with a Shockley partial dislocation (PD)located between them.

To further determine the detailed structure ofβ＇(LPSO)precipitate in 12H, an enlarged area marked with a rectangle in Fig.5b is shown in Fig.6a.Obviously, one bright dot and three dark dots arrange alternately along the basal plane, which is illustrated in line profile in Fig.6b obtained by integrating the image intensity in rectangle b.The intensity of each prismatic plane is approximately the same, as demonstrated by the line profile in Fig.6c obtained by integrating the image intensity in rectangle c, suggesting the chemical composition between these layers seems to be the same with each other.This is different from the 12H structure in the Mg-Co-Y alloy [8], where Co/Y elements are mainly enriched in B＇layers randomly within AB＇C building blocks.The structural differences betweenβ＇andβ＇(12H)result from their various stacking sequences.Based on the structure model (Fig.4c) ofβ＇, we proposed the structure model ofβ＇(12H) to determine its crystal structure.As the model illustrated in Fig.6d, the coordinate of Y atoms along x-axis in one zigzag array in AB＇C building blocks is in the range ofz=0,z=1/2,z=0 (orz=1/2,z=0,z=1/2).Neighboring zigzag arrays are separated from each other by 4d, viewed along x-axis.In single closely packed layer,the coordinate of Y atoms along x-axis in neighbor zigzag arrays isx=0 andx=1/2 (orx=1/2 andx=0) alternately.Thus, the structure model ofβ＇(12H) can be constructed in Fig.6d, without considering the size effect of solute atoms.Based on its atomic structure model,β＇(12H) can be determined to be a ort horhombic lattice (Mg7Y,Cmcm,a=2a0=0.642nm,b=4a0=2.27nm,c=6c0=3.12nm).The atomic coordination can also be determined, as is listedin Table 1.The orientation relationship between Mg matrix andβ＇(12H) phase is such that [20]α//[100]β＇(12H),[0001]α//[001]β＇(12H).The simulated electron diffraction patterns are presented in Fig.6e-g, but we could not obtain its EDPs so far since the dimension ofβ＇(12H) is very limited in this work.

Table 1The Atomic coordinates of the structure model of β＇(12H) with chemical formula Mg7Y.

Fig.5.(a) A typical HAADF-STEM image of 12H LPSO structure containing various stacking faults (SFs) in an Mg88Co5Y7 alloy after EB irradiation, in which T7 SF exhibits brighter contrast than AB＇C building blocks.(b-c) Atomic-resolution HAADF-STEM images of β＇(LPSO) precipitated in 12H LPSO structure.Images were obtained along [20]α zone axis.

Fig.6.(a) The zoom-in image of the framed area in Fig.5b.(b-c) Intensity profiles of the structures in rectangle b and c, showing the Y element redistribution after the formation of β＇(12H).(d) 3-D atomic structure model of β＇(12H).(e-g) Stimulated SAED pattern obtained from β＇(12H) along [100], [010], and[001]direction.

4.Discussion

In addition to the precipitation ofβ＇phases in Mg matrix during aging at 200°C,β＇phases orβ＇(LPSO) phases can be transformed from the LPSO structure after EB irradiation.Bin Chen et al.investigated the interactions between theβ＇phases and LPSO structures containing AB＇C＇A building blocks in Mg-10Gd-5Y-2Zn-0.5Zr (wt.%) [27]and Mg-5Y-2.5Ni-0.5Zr (at.%) alloys [28]after aging at 225°C for 48h and 90h.Theβ＇phases nucleated at the Mg nano-layers sandwiched in LPSO structures and grew along the [100]Mgand [0001]Mgdirection, respectively.Our results show similar results in Mg88Co5Y7alloys both after aging at 200°C for 24h and after EB irradiation: RE-absent gaps existing betweenβ＇phase and LPSO structure, and allβ＇phases are theβ＇-long structure, although the LPSO structures possess AB＇C building blocks.It suggests that the effects of aging at 200°C for 24h and EB irradiation are similar for Mg88Co5Y7alloys.

The EB irradiation-induced the transformation from LPSO structures toβ＇phases orβ＇(LPSO) phases will be addressed in the following.On the one hand, EB irradiation should play an important role in the transformation.EB irradiation from TEM tends to result in phase transformation in various materials [29], since it triggers the atoms’ displacement.Here,the accelerated solute atoms diffuse in the LPSO structures,and then rearrange to form new phases, likeβ＇orβ＇(LPSO)phases.On the other hand, annealing temperature should be higher than 200°C or even 225°C but lower than 400°C.We previously demonstrated that theβ-Mg24Y5stable phases with plate-like morphology distribute both in the LPSO structures containing AB＇C building blocks and Mg matrix in an Mg88Co5Y7alloy after annealing at 400°C for 16h [30].In this work,β＇phases formed in the Mg matrix but not in the LPSO structures during aging at 200°C for 24h.Theβ＇phases can transform intoβphases upon long time aging or high aging temperature [31].Thus, it seems thatβ＇(LPSO)might occur at a higher temperature between 200°C and 400°C or longer aging temperature (more than 24h).

5.Summary

Based on Cs-corrected HAADF-STEM imaging at the atomic level, we investigated the formation ofβ＇in ternary Mg88Co5Y7(at.%) as-cast alloys after aging at 200°C for 24h or after EB irradiation.Theβ＇phase precipitated only in the Mg matrix and not in LPSO structures after aging at 200°C for 24h.By contrast, LPSO structure transforms intoβ＇phases or new complexβ＇(LPSO) structures during EB irradiation, when the EB irradiation-induced displacement of solute atoms should play an important role.All theβ＇phases belong toβ＇-long phases.The new complexβ＇(12H) structure was determined to be an orthorhombic lattice (Mg7Y,Cmcm,a=2a0=0.642nm,b=4a0=2.27nm,c=6c0=3.12nm).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (Grant No.51801214 and 51871222).We are also grateful to Mr.B.Wu and Mr.L.X.Yang of this laboratory for their technical support on the Titan platform of the Titan3TMG260-300kV aberration-corrected scanning transmission electron microscope.