Variable precipitation behaviors of Laves phases in an ultralight Mg-Li-Zn alloy

Weixin Lou ,Hongo Xie,? ,Xioo Zho ,Junyun Bi ,Hehng Zhng ,Yi Wng ,Xinze Li,Hucheng Pn,Yuping Ren,c,Gowu Qin

aKey Laboratory for Anisotropy and Texture of Materials (Ministry of Education),School of Materials Science and Engineering,Northeastern University,Shenyang 110819,China

b State Key Laboratory of Rolling and Automation,Northeastern University,Shenyang 110819,China

c Research Center for Metal Wires,Northeastern University,Shenyang 110819,China

Abstract Precipitation habits plays a decisive role in strengthening materials,especially for Mg alloys the non-basal plane precipitation is necessary but very limited.Generally,the precipitates would nucleate and grow up in a specific habit plane owing to the constraint of free-energy minimization of the system.Herein,in an aged ultralight Mg-Li-Zn alloy,we confirmed that the precipitates dominated by C15 Laves structure could form in a variety of habit planes,to generate three forms of strengthening-phases,i.e.,precipitate-rod,precipitate-lath,and precipitate-plate.Among which,the precipitate-plates are on basal plane as usually but precipitate-rods/laths are on non-basal plane,and such non-basal precipitates would transform into the basal (Mg,Li)Zn2 Laves structure with prolonged aging.These findings are interesting to understand the precipitation behaviors of multi-domain Laves structures in hexagonal close-packed crystals,and expected to provide a guidance for designing ultralight high-strength Mg-Li based alloys via precipitation hardening on the non-basal planes.

Keywords: Laves phase;Precipitation;Habit plane;Orientation;Magnesium alloys.

1.Introduction

Precipitation hardening is one of effective strategies in strengthening of materials,in which the precipitation habits play a crucial role in determining the mechanical properties[1–5].And it is generally accepted now that the precipitates nucleate and grow up in a specific habit plane,for example,typically theθseries precipitates form in the {001}Alplanes[6],theβseries precipitates generate in the {1010}Mgprismatic planes [7],or theT1phase,Ωphase,andγ’’ phase grow up in the close-packed planes(basal planes,i.e.,{111}Alor {0001}Mgplanes) [8–10].For these specific precipitates,their different precipitation behaviors along the given habit planes can minimize total free energy of a system when diffusion is enabled.

Laves phase is a class of crystalline compounds composed of the topologically close-packed atomic-layers [11,12].So far,it is reported that more than 900 kinds of Laves phases have been confirmed experimentally [13] in metallic alloys[11–14],inorganic colloids [15],and soft matters [16].In aged light alloys,the Laves phase is also considered to be a strengthening phase,such as theηphase in the Al-Zn-Mg-Cu alloy [17],theβ2’ phase in the Mg-Zn alloy [1,18],and the Al2Ca phase in the Mg-Al-Ca alloy [19,20],usually lying on the low-energy close-packed planes like a plate.In a recent study,however,rod-shaped precipitates associated with the Laves phase formed perpendicular to the basal plane has been confirmed in the Mg-Zn alloy [21].Atomic-scale high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM) observation indicated that these precipitaterods,β1’ phase,containing two-dimensional five-fold symmetry locally and short-range ordered C14 and C15 Laves structures grow up on the {0001}Mgbasal plane,while periodical arrangement along the [0001]Mgdirection [21].Confirmation of the elusiveβ1’ phase in Mg-Zn binary system means that the Laves structures can not only precipitation in the low-energy basal plane.

Mg-Li based alloys are the lightest structural metallic materials,however,their low strength and poor corrosion resistance,together with thermal instability have limited their wider applications[22–24].The addition of Zn to Mg-Li alloy can induce the precipitation ofθ-MgLiZn(fcc,a=0.744 nm)phase in theα-Mg matrix,resulting in an increase of strength[22,25,42].However,it is well known that such basal precipitation ofθ-MgLiZn has limited contribution to the improvement of strength.On the other hand,the understanding of aging precipitation behaviors of Zn-containing Mg-Li alloys are very limited up to date,and the fine structure of the precipitate,such as the so-called fccθ-MgLiZn phase,has never been confirmed by any atomic-scale HAADF-STEM observation.The lack of deep understanding of the precipitation behavior limits the further development of Mg-Li-Zn alloys.

Here,in an aged ultralight Mg-4Li-4Zn(wt.%)alloy,using aberration-corrected HAADF-STEM technique,we confirmed that (Mg,Li)Zn2C15 Laves structure dominated precipitates can form not only on the low-energy basal plane,but also on the prismatic planes,even perpendicular to the basal plane.A five-fold Laves multi-domain structure assembled by C15 Laves phases without any translational periodicity was thus been discovered.We believe that these findings are important for understanding the formation mechanism of multi-domain Laves structures,and expected to provide a meaningful guidance to design ultralight high-strength Mg-Li based alloys by tuning non-basal plane precipitation hardening.

2.Experimental procedures

The alloy used in this experiment was melted in a vacuum induction furnace to obtain a Mg-4Li-4Zn(wt.%)alloy by using pure magnesium (99.9 wt%),pure zinc (99.9 wt.%),and Mg-10 wt.% Li master alloy.The molten alloy was stirred and kept at 760 °C for 10 min and poured into a steel mold preheated to 300 °C.Then,the ingot was solution treated at 450 °C for 12 h,followed by water quenching and cut the ingot into small pieces of 10 × 10 × 10 mm.Finally,aging at 100 and 150 °C in oil-bath furnace.Disks of the TEM specimens with a diameter of 3 mm manually ground to 60 μm and prepared by twin-jet electropolished(TenuPol-5)at roomtemperature,using a phosphoric acid: ethanol=3:5 solution.Then,the TEM specimens were slightly trimmed with Gatan PIPS 695 (3.0 kV ion-gun emergy under a 3.5°milling angle).TEM and STEM observations were carried out using JEMARM200F at an accelerating voltage of 200 kV,equipped with probe Cs corrector and cold-field emission gun.The probe convergence was 25 mrad which yielded a probe size of less than 0.1 nm,and the camera length was set to 8 cm which yielded a collection semi-angle of 48–327 mrad.

The first-principles calculations were conducted using the Vienna ab-initio simulation package (VASP) [26] with projector augmented-wave (PAW) [27,28] pseudopotentials.The exchange-correlation potential was described by the generalized gradient approximation (GGA) functional of Perdew,Burke and Ernzerhof (PBE) [29].A high plane-wave cutoff energy of 400 eV and dense k-point sampling based Gammacentered Monkhorst-pack scheme were employed to guarantee high numerical accuracy.The geometry optimization process was performed using a conjugate gradient algorithm until the final force on each atom was less than 0.02 eV/?A.

The formation energy,Ef,can approximately characterize the stability of compounds in thermodynamics [30].And the formation energy in present calculation,Ef,was calculated as following equation:

Here,E(MgxLiyZnz),E(Mg),E(Li),and E(Zn)represent the total energy of the compound,the energies of Mg,Li and Zn unit cells,respectively.

3.Results and discussion

To better understand and describe the multi-domain Laves structures precipitated in the Mg-Li-Zn alloy,Fig.1 provides a detailed schematic diagram of typical Laves structures.Atomic icosahedral clusters have a high stacking density and a low free energy of system [31,32],and an atomic number of “…1–5–1–5–1…” icosahedral cluster model is exhibited in Fig.1a,suggesting that the central atom (yellow atom-sphere) is in contact with 12 atom-spheres.Generally,the Laves structures can be considered as a periodic arrangement of icosahedron columns (Fig.1b) along the pseudo fivefold axis promoting the formation of the minimum rhomboid structural unit (Fig.1c),and self-assembly of these rhomboid tiling patterns under some geometrical constraints to generate the present Laves phases.From the viewpoint of crystallography characteristics,there are only three types of Laves structures frequently observed to date[11–13]:(ⅰ)C14 Laves phase(Fig.1d,representative alloy: MgZn2),can be considered as the rhomboid struct-ural units are periodically arranged along the [0001] direction to form a hexagonal close-packed crystal (hcp,space group: P63/mmc),which has a stacking sequence of“…AB…”;(ⅱ)C15 Laves phase(Fig.1e,representative alloy: MgCu2);can be regarded as the rhomboid structural units are periodically arranged in the 〈111〉 directions to build a face-centered cubic structure (fcc,space group:Fd3m),with a stacking sequence of “…ABC…”;(ⅲ) C36 Laves phase (Fig.1f,representative alloy: MgNi2),it can be considered as double-layer rhomboid structural units are periodically arranged along the [0001] direction to generate a di-hcp crystal (space group: P63/mmc),and with a stacking sequence of “…ABA’B’…”.

Fig.1.Schematic diagram of the Laves phases.(a) Three-dimensional atomic structure model of the icosahedron.(b) Modeled atomic structure of the icosahedron column.(c) Modeled atomic arrangement of the rhomboid structural unit,which is constituting the Laves phase.(d–f) Structural models of the three Laves phases: (d) C14 type Laves phase;(e) C15 type Laves phase;(f) C36 type Laves phase.

The TEM bright-field images of the Mg-4Li-4Zn alloy after isothermally aged at 100 °C for 400 h,viewed along the [0001]α,[1100]α,and [1120]αdirections,are shown in Fig.2a–c.And the insets are the corresponding select-area electron diffraction (SAED) patterns.From these images,it can be found that there are high-density variform precipitates with different sizes distributed in theα-Mg matrix.And these precipitates can be summarized into the following three forms: (ⅰ) rod-shaped precipitates,which are perpendicular to the {0001}αbasal planes;(ⅱ) lath-shaped precipitates,they have three variants and with one of {1120}αhabit planes;(ⅲ)plate-shaped precipitates,which are parallel to the {0001}αbasal planes.The three kinds of precipitates are marked by red,blue,and green dashed circles,respectively,in Fig.2a–c.Local parts in Fig.2 are enlarged and shown in Fig.2d–f.High-resolution TEM image of a precipitate-rod is displayed in Fig.2d,under this imaging condition,the structural characteristics of the precipitate-rod would not be well represented.The corresponding SAED pattern presented in Fig.2g indicates that the precipitate-rod has sharp diffraction spots,but without any symmetry on the basal plane,which is different from theβ1’ precipitates in binary Mg-Zn alloy [21].Periodic diffraction lattice (Fig.2h) of the precipitate-lath indicates that the structure is totally different from the precipitaterod,means that the precipitate-lath has a strict orientation relationship with theα-Mg matrix,and high-resolution TEM image (Fig.2e) clearly shows that the precipitate-lath formed along the (2110)αprismatic plane.High-resolution TEM image (Fig.2f) and the corresponding SAED pattern (Fig.2i)indicate that the precipitate-plates should have an AB2type Laves phase characteristic,and similar results have been reported in other alloy systems [12,14,18].

Fig.2.TEM images and corresponding SAED patterns of the Mg-4Li-4Zn alloy after isothermally aged at 100 °C for 400 h The electron beam is parallel to the (a,d–i) [0001]α,(b) [1100]α,and (c) [1120]α directions.(a–c) Bright-field TEM images.(d–f) High-resolution TEM images.The insets in Fig.2a–c,and Figs.2g–i,are the SAED patterns.The representative precipitate-rod,precipitate-lath,and precipitate-plate in Fig.2a–c have been marked by red,blue,and green dashed circles,respectively.

To precisely reveal the structures and atomic coordinates of these precipitates,atomic imaging by HAADF-STEM was used.Fig.3a provides a[0001]αHAADF-STEM image showing a precipitate-rod with bright-contrast in theα-Mg matrix.Each bright dot inside the precipitate-rod represents a Zn-rich atomic-column because the brightness of individual atomic-columns in the HAADF-STEM image approximates the square of the average atomic numbers (the atom number is 3 for Li,12 for Mg,and 30 for Zn) [33–35].Local part in Fig.3a is further enlarged and presented in Fig.3b,the atomicscale HAADF-STEM image indicates the precipitate-rod has a Laves structure characteristic,and the “…ABC…” stacking sequence is consistent with the C15 Laves phase illustrated in Fig.1e.In the Mg-Li-Zn system,the atomic-radius of Mg,Li,and Zn is 1.60,1.54,and 1.39 ?A,respectively,in which the Li atomic-radius is very close to that of Mg,means the atomic position of Mg in the Laves structure can be substituted easily by Li atoms.Consequently,local chemical composition of the C15 Laves structure can be represented as (Mg,Li)Zn2.Fig.3c displays a minimum rhomboid structural unit model for the (Mg,Li)Zn2Laves phase,which l value of the side length is measured to be ～4.54 ?A

Fig.3.HAADF-STEM images of the precipitate-rod and the first-principles calculations of C15 Laves structure.The electron beam is parallel to the [0001]α direction.(a) Low-magnification HAADF-STEM images.(b) Local part in Fig.3a is further enlarged,atomic-scale HAADF-STEM image indicates the precipitate-rod locally with a C15 Laves structure characteristic.(c) Rhomboid structural unit model for the (Mg,Li)Zn2 Laves phase.(d) Formation energy of the C15 (Mg,Li)Zn2 Laves phase with different Li contents.(e) Local part in Fig.3a is modeled and enlarged,five orientation C15 rhombus variants bonding together to form a star pattern is presented.

In order to evaluate the thermal stability of the C15 (Mg,Li)Zn2Laves structure and search for its potential optimal composition ratio,the first-principles calculations based on density functional theory(DFT)were performed[36,37].With the random substitution of Mg and Li atoms,the Mg-Li-Zn alloy system was constructed and the formation energies of Mg(1-x)LixZn2(x=0,0.2,0.4,0.6,0.8,and 1.0)C15 Laves phases were calculated.Fig.3d provides the formation energy curve with random substitution ratio of Li atoms.The negative formation energy values indicate that all the (Mg,Li)Zn2C15 Laves phases are energetically stable [38,39].And the formation energy would gradually decreases and reach the minimum with the doping of Li atoms by 0.8 at%,which indicates that the (Mg,Li)Zn2C15 Laves structure would reach the most stable state.Therefore,it can be speculated that the chemical composition ratio of the most stable C15 Laves structured precipitate is Mg0.2Li0.8Zn2.

Local part labeled by colorful star in Fig.3a is modeled and enlarged in the Fig.3e,and five orientation C15 rhombus variants bonding together to form a multi-domain Laves superstructure is presented.Five-fold symmetry is forbidden for the periodic crystals because of no translational or rotational symmetry [40,41].Our observation here indicates that the precipitate-rod is not a single-crystal phase,but a Laves multi-domain structure on nanometer scale,which is well consistent with the weak and random diffraction spots in Fig.2d.

The structures of precipitate-rods are further characterized and provided in Fig.4a–f.These HAADF-STEM images clearly shows that these rod-shaped precipitates are neither the C14 laves phase,nor the C15 and C36 Laves structures,but multi-domain structures formed by the combination of multiple Laves crystallites.Among which,the C15 Laves structure is dominant.The stacking sequences of these multi-domain Laves structures have been depicted with cyan polylines in these precipitate-rods.Additionally,some Laves defects resulting from the bonding of multiply orientated Laves crystallites can be found in Fig.4d–f.Compared to the binary Mg-Zn nanodomains assembled by rhombus structural units and equilateral hexagon structural units [21],such multi-domain structures dominated by C15 Laves phase with a better crystallinity.

Fig.4.HAADF-STEM images showing the precipitate-rods in the aged Mg-4Li-4Zn alloy.The electron beam is parallel to the [0001]α direction.(a–d,f)Atomic-scale HAADF-STEM images.(e) Low-magnification HAADF-STEM image.

Fig.5 provides more[0001]αSAED patterns for the multidomain precipitate-rods,and the diffraction contrasts come from the precipitate-rods have been indicted by cyan arrows in the images.From Fig.5 combined with Fig.2g above,it can be concluded that a unified orientation relationship between theα-Mg matrix and precipitate-rods cannot be determined due to the diversity of the precipitate-rods.

Fig.5.SAED patterns of the precipitate-rods in the aged Mg-4Li-4Zn alloy.The electron beam is parallel to the [0001]α direction.The cyan arrows indicate the electron diffractions from the precipitate-rods.

HAADF-STEM images of the precipitate-laths,viewed along the [0001]αdirection,are displayed in the Fig.6a–c.Low-magnification image in Fig.6a corresponds to the brightfield TEM image shown in Fig.2e,and local part of the precipitate-lath is enlarged and atomic-scale HAADF-STEM image is presented in the top-right inset.It indicates clearly that the precipitate-lath has an“…ABC…”stacking sequence,which is consistent with the C15 Laves structure.However,the precipitate-laths have no unified fcc Laves structure,as shown in Fig.6b,c.This precipitate-lath is also composed of a multi-domain Laves structure with the enlarged local part in Fig.6c,and stacking sequence locally of the precipitate-lath has been identified by cyan rhombus.What is worth mentioning that these (Mg,Li)Zn2precipitate-laths have three variants of{1120}αhabit planes,which is different from the other prismatic precipitates with {1010}αhabit planes identified in the other Mg alloys [1,3,4,7].

Fig.6.HAADF-STEM and bright-field TEM images showing the precipitate-laths in the aged Mg-4Li-4Zn alloy.The electron beam is parallel to the (a–c)[0001]α and (d–f) [1100]α directions.(a–c,e,and f) HAADF-STEM images.(d) Bright-field TEM image,the inset is the corresponding SAED pattern.

Fig.7a provides a bright-field TEM showing a precipitateplate lying on the (0001)αbasal plane.The corresponding SAED pattern is shown in the top-right inset,which is highly consistent with the result presented by Yamamoto and coworkers in 2003 [42].Atomic-scale HAADF-SETM image of such precipitate-plate is presented in Fig.7b.It can be concluded that the precipitate-plate has a C15 Laves structure.However,not all of precipitate-plates are of C15 Laves structure,a precipitate-plate with both “…ABC…” and “…AB…”stacking sequences exhibited in Fig.7c indicates that the precipitate-plates can also be multi-domain Laves structures in the [0001]αdirection.And the orientation relationships among the C14 Laves phase,C15 Laves phase,andα-Mg matrix are determined to be [1120]C14// [110]C15// [1100]αand (0001)C14// (111)C15// (0001)α.Fig.7d provides a low-magnification HAADF-STEM image viewed along the[1120]αdirection showing a (Mg,Li)Zn2precipitate-plate formed on the (0001)αbasal plane.In binary Mg-Zn alloys,the structure ofβ2’ precipitate-plate is considered to be the C14 MgZn2Laves phase [1,18,21],which is different from such C15 Laves phase,or the multi-domain Laves structures dominated by C15.And this finding unambiguously verifies that the precipitate-plates in the Li-containing Mg-Zn system have a (Mg,Li)Zn2type Laves characteristic,not the previously reported MgLiZn phase only with fcc structure [42,43].

Fig.7.Bright-field TEM and HAADF-STEM images showing the precipitate-plates in the aged Mg-4Li-4Zn alloy.The electron beam is parallel to the (a-c) [1100]α and (d) [1120]α directions.(a) Bright-field TEM images,the inset is the corresponding SAED pattern.(b–d) HAADF-STEM images.

In the early-aged period,we noticed that the three types of precipitates formed in theα-Mg matrix almost simultaneously.The (Mg,Li)Zn2C15 Laves phase dominated precipitate-rods/laths would be replaced by the plate-shaped precipitates gradually with prolonged aging.As evidenced by TEM/HAADF-STEM analysis in Fig.8,it can be seen that the previously dominated precipitate-rods/laths have been replaced by the C15 (Mg,Li)Zn2Laves precipitate-plates in the over-aged sample.Therefore,it indicates that the Laves structure on the {0001}αbasal planes is the lowest energy state,and the non-basal plane phases would be gradually replaced by the basal ones with prolonged aging or increased temperatures.

Fig.8.Bright-field TEM and HAADF-STEM images of the Mg-4Li-4Zn alloy aged at 100 °C for 400 h,and then subjected to continuous isothermal aging at 150 °C for 400 h The electron beam is parallel to the [1100]α direction.(a) Bright-field TEM image,the inset is the corresponding SAED pattern.(b) Atomic-scale HAADF-STEM image.

For magnesium alloys,the introduction of nano-sized precipitates through alloying is by far the most effective way to improve resistance for dislocation motion.The stronger the resistance to dislocation motion,the higher the yield strength obtained.And there is no doubt that the increment in critical resolved shear stress (CRSS) can be used to describe the effect of precipitates on strengthening [1,44–45].For precipitation-hardening alloys,the CRSS increment can be produced by a combination of dispersion hardening (shearresistant) and shearable precipitates to impede the movement of dislocations during plastic deformation.The CRSS increment produced by prismatic nanoplates are always larger than those produced by basal nanoplates or spherical nanoparticles in the case of the same volume fraction of the second nanoprecipitate,and the difference in the CRSS increment increases significantly with increasing the aspect ratio of nanoprecipitate.And for a certain ratio,the maximum difference in the CRSS increment can even reach two orders of magnitude [1].Recently,Mg-Li based alloys containing Zn element have received widespread attention due to lightweight requirement [46,47].The average density of such Mg-4Li-4 Zn (wt.%) alloy is 1.641 g/cm3,which is lower than the density of typical AZ31 magnesium alloy in widespread usage[48].If a higher precipitation density and a finer precipitation size of the precipitate-laths/rods can be designed with reasonable alloying and aging treatment,an ultralight high-strength Mg-Li-Zn based alloy would be expected.

4.Conclusions

In summary,the variable precipitation behaviors of Laves phases in an aged hcp Mg-Li-Zn alloy has been systematically investigated using atomic-scale HAADF-STEM.The morphology,structure,and orientation of these precipitates are fully unraveled.The precipitate-rods are perpendicular to the {0001}αbasal plane,and self-assembly of the rhomboid structural units under some geometrical constraints lead to the generation of the multi-domain Laves structures without any translational symmetry.And a five-fold Laves multi-domain structure assembled by (Mg,Li)Zn2C15 Laves phases (fcc,space group: Fd3m,a=7.38 ?A) has been identified.The precipitate-laths containing C15 Laves phase and short-range ordered domain locally have a {1120}αhabit plane,with the orientation relationships between the C15 Laves precipitatelath andα-Mg matrix being [0001]α// [110]C15and (1120)α// (001)C15.The C15 Laves structure dominated precipitateplates,containing some C14 Laves domains,lying on the(0001)αbasal plane,and the orientation relationships among the C14 Laves phase,C15 Laves phase,andα-Mg matrix is [1120]C14// [110]C15// [1100]αand (0001)C14// (111)C15// (0001)α.Additionally,this work has unambiguously verified that the plate-like precipitates in theα-Mg matrix have an (Mg,Li)Zn2type Laves characteristic,not the previously reported MgLiZn phase.These results are expected to lead to new insights into the clustering and stacking behaviors of solutes,especially into the formation mechanism of the multi-domain Laves structures,and would provide a practical guidance for designing and developing novel ultralight highstrength Mg-Li based alloys.

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

The authors acknowledge the National Natural Science Foundation of China (Grant No.51525101,No.51971053,No.52101129),the Project of Promoting Talents in Liaoning Province(No.XLYC1808038),the Fundamental Research Funds for the Central Universities(Grant No.N2002018),and the Project funded by China Postdoctoral Science Foundation(2020M670774).The authors extend their gratitude to Prof.Yonghui Sun and Dr.Na Xiao (Analytical and Testing Center,Northeastern University) for their help with the Cs-corrected HAADF-STEM technique.