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Z phase is one of the three basic units by which the Frank-Kasper (F-K) phases are generally assembled. Compared to the other two basic units, that is, A15 and C15 structures, the Z structure is rarely experimentally observed because of a relatively large volume ratio among the constituents to inhibit its formation. Moreover, the discovered Z structures are generally the three-dimensional ordered Gibbs bulk phases to conform to their thermodynamic stability. Here, we confirmed the existence of a metastable two-dimensional F-K Z phase that has only one unit-cell height in the crystallography in a model Mg-Sm-Zn system, using atomic-scale scanning transmission electron microscopy combined with the first-principles calculations. Self-adapted atomic shuffling can convert the simple hexagonal close-packed structure to the topologically close-packed F-K Z phase. This finding provides new insight into understanding the formation mechanism and clustering behavior of the F-K phases and even quasicrystals in general condensed matters.

The highly thermal stability precipitates strengthen creep resistance for alloys, which require precipitates to resist coarsen at elevated temperature. In the Mg-RE-Zn (Ag) series alloys, the key strengthening phase - γ’’ phase, remains its single-unit-cell height throughout aging process. The origin of such extremely thermal stability of γ’’ phase is still unclear. By using the first-principles calculations, it is found that the formation of γ’’ phase introduces compressive strain to its surrounding α-Mg lattice and simultaneously alter charge distribution of α-Mg lattice near the α-Mg/γ’’ hetero-interface. These two variations over α-Mg lattice lead to lower vacancy formation energies and migration energy barriers for solute atoms near the α-Mg/γ’’ hetero-interface in comparison with that of bulk condition. Consequently, the variations facilitate rapid diffusion of solute atoms near the γ’’ phase and promote nucleation rate of other γ’’ plates around it. On the basis of ledge-thickening model, the origin of nucleation/growth of γ’’ phase with single-unit-cell height was explained. Thermodynamically and kinetically, the solute atoms are not apt to migrate into the nearest basal plane adjacent to the γ’’ phase. Moreover, the lower aging temperature (∼200 °C) and almost completely coherent α-Mg/γ’’ hetero-interface lead to the ledges are extremely hard to nucleate on the hetero-interface. Therefore, γ’’ plates maintain single-unit-cell height throughout aging process.

Interfacial segregation is ubiquitous in mulit-component polycrystalline materials and plays a decisive role in material properties. So far, the discovered solute segregation patterns at special high-symmetry interfaces are usually located at the boundary lines or are distributed symmetrically at the boundaries. Here, in a model Mg-Nd-Mn alloy, we confirm that elastic strain minimization facilitated nonsymmetrical segregation of solutes in four types of linear tilt grain boundaries (TGBs) to generate ordered interfacial superstructures. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy observations indicate that the solutes selectively segregate at substitutional sites at the linear TGBs separated by periodic misfit dislocations to form such two-dimensional planar structures. These findings are totally different from the classical McLean-type segregation which has assumed the monolayer or submonolayer coverage of a grain boundary and refresh understanding on strain-driven interface segregation behaviors.

Segregation of solutes at a phase boundary or interface is ubiquitous in the multicomponent materials and has a serious effect on their mechanical properties; it is therefore becoming a more import...

Twinning is a common deformation mechanism in metals, and twin boundary (TB) segregation of impurities/solutes plays an important role in the performances of alloys such as thermostability, mobility, and even strengthening. The occurrence of such segregation phenomena is generally believed as a one-layer coverage of solutes alternately distributed at extension/compression sites, in an orderly, continuous manner. However, in the Mn-free and Mn-containing Mg-Nd model systems, we reported unexpected three- and five-layered discontinuous segregation patterns of the coherent {101̅1} TBs, and not all the extension sites occupied by solutes larger in size than Mg, and even some larger sized solutes taking the compression sites. Nd/Mn solutes selectively segregate at substitutional sites and thus to generate two new types of ordered two-dimensional TB superstructures or complexions. These findings refresh the understanding of solute segregation in the perfect coherent TBs and provide a meaningful theoretical guidance for designing materials via targeted TB segregation.

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.

The crystal structure of the single-unit-cell thickness γ″ phase, as a key strengthening phase in Mg-RE-Zn (Ag) series alloys, has been extensively studied, and several structural models have been proposed in the past two decades. However, these reported models, and even the lattice constants at the same proposed structure, are scattered severely, which has led to considerable confusion and not available for further mechanical property simulation and prediction of Mg alloys containing this phase. In this study, by using first-principles calculations, the crystal structure of γ” phase is clarified, resolving the discrepancies among different experiments, and its intrinsic mechanical properties have also been studied for the first time. It is verified that the γ″ phase contains quasi-five atomic layers, instead of the previously reported tri-layer, and surprisingly, its crystal structure has many variants, which would change with the alloy composition. Besides, with the help of the simulated selected area electron diffraction (SAED) patterns, it is found that the atoms in the central layer remain partially ordered distribution, and this ordered extent primarily depends on the atomic ratio of RE: Zn(Ag) and the solute content in an alloy. That is, the ordered extent increases with decreasing the atomic ratio of RE:Zn(Ag) and/or increasing solute content of alloy, and vice versa. Ag and Zn dissolved in the γ″ phase would produce almost opposed mechanical anisotropy for the γ″ phase under the identical crystal structure, and the addition of Ag shows more efficient on increasing the shear modulus of γ″ phase.

Z phase is one of the three basic units by which the Frank-Kasper phases are generally assembled. Compared to the other two basic units, i.e., A15 and C15 structures, the Z phase structure is rarely experimentally observed because of a relatively large volume ratio among the constituents to inhibit its formation. Moreover, the discovered Z structures are generally the three-dimensional (3D) ordered Gibbs bulk phases to conform to their thermodynamic stability. Herein, we confirmed the existence of a metastable two-dimensional (2D) Frank-Kasper Z phase with one unit-cell height in the crystallography in a model Mg-Sm-Zn system, by using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with density functional theory (DFT) calculations. This finding is important for understanding the relationship between the traditional crystal structures and the quasicrystals, and it is also expected to provide a new insight to understand the clustering and stacking behavior of atoms in condensed matters.

The β1 precipitate is a key strengthening-phase in Mg-rare-earth alloys, however, the formation mechanism concerning it has not yet been addressed. Herein, we have uncovered a diffusional-displacive dominated formation mechanism for the β1 phase, using aberration-corrected scanning transmission electron microscopy observation combined with first-principles calculations, in an aged Mg Sm model alloy system. Shear of the nucleated { 10 1 ¯ 0 }hcp zig-zag monolayers along a direction parallel to fcc with a shear angle of ~5.26°, followed by atoms shuffling on the non-close-packed { 10 1 ¯ 0 }hcp planes, can transform the hexagonal close-packed Mg3Sm structure (β” or βH’ intermediate phase) to the face-centered-cubic structure Mg3Sm (β1 phase). Besides, the formation of the β1 phase can also be realized from the other βS’ or βL’ intermediate phase (c-bco, Mg7Sm) via solute diffusion coupled with shuffle transformation manner. A new habit plane of { 10 1 ¯ 0 }hcp // { 1 1 ¯ 0 }fcc and [0001]hcp // [110]fcc on the non-closed packed plane has been identified, which is completely different from the traditional displacive transformation mechanism usually happened on the closed-packed plane. This finding enriches the diffusional-displacive transformations.

The ordered stacking of solute atoms based on elliptically shaped self-adapted clustering leads to the generation of the quasi-unit cell. The bonding of these ellipse-like quasi-unit-cell rods by the Mg atomic columns forms a two-dimensional planar structure., This paper reports a new structured prismatic platelet, self-assembled by an ellipse-like quasi-unit cell, precipitated in Mg–In–Yb and Mg–In–Ca ternary alloys and aged isothermally at 200°C using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy combined with density functional theory computations. The ordered stacking of solute atoms along the [0001]α direction based on elliptically shaped self-adapted clustering leads to the generation of the quasi-unit cell. The bonding of these ellipse-like quasi-unit-cell rods by the Mg atomic columns along the 〈〉α directions formed a two-dimensional planar structure, which has three variants with a {}α habit plane and full coherence with the α-Mg matrix. This finding is important for understanding the clustering and stacking behaviors of solute atoms in condensed matter, and is expected to guide the future design of novel high-strength Mg alloys strengthened by such high-density prismatic platelets.

Generally, the Mg–Sm system is one of the magnesium light rare-earth alloys (Mg-LRE), and its aging precipitation behavior is similar to that of the Mg–Nd or Mg–Ce alloys. For a long time, the β phase, precipitated after the β1 intermediate phase, has been considered to be Mg12Sm, although this has never been experimentally verified. In this study, however, we clearly confirm that the β phase is Mg5Sm with a face-centered cubic structure (space group: F 4 ¯ 3m; a = 22.34 A) by aberration-corrected scanning transmission electron microscopy (STEM) combined with quantitative STEM simulation. This is similar to the case of magnesium heavy rare-earth alloys (Mg-HRE). The lenticular-shaped β plates have a { 10 1 ¯ 0 }α habit plane and is fully coherent with the α-Mg matrix, with the orientation relationships among the β phase, β1 phase, and α-Mg matrix being ( 1 1 ¯ 1 )β//( 1 1 ¯ 1 )β1//( 11 2 ¯ 0 )α and [110]β//[110]β1//[0001]α. This finding improves our understanding of the precipitation/strengthening mechanism in the Mg–Sm based alloys and is expected to guide the future design of novel high-strength heat-resistant Mg-RE based alloys.