The hidden order of atomic packing in amorphous structures and how this may provide the origin of plastic events have long been a goal in the understanding of plastic deformation in metallic glasses. To pursue this issue, we employ here molecular dynamic simulations to create three-dimensional models for a few metallic glasses where, based on the geometrical frustration of the coordination polyhedra, we classify the atoms in the amorphous structure into six distinct species, where “gradient atomic packing structure” exists. The local structure in the amorphous state can display a gradual transition from loose stacking to dense stacking of atoms, followed by a gradient evolution of atomic performance. As such, the amorphous alloy specifically comprises three discernible regions: solid-like, transition, and liquid-like regions, each one possessing different types of atoms. We also demonstrate that the liquid-like atoms correlate most strongly with fertile sites for shear transformation, the transition atoms take second place, whereas the solid-like atoms contribute the least because of their lowest correlation level with the liquid-like atoms. Unlike the “geometrically unfavored motifs” model which fails to consider the role of medium-range order, our model gives a definite structure for the so-called “soft spots”, that is, a combination of liquid-like atoms and their neighbors, in favor of quantifying and comparing their number between different metallic glasses, which can provide a rational explanation for the unique mechanical behavior of metallic glasses.

Microstructural improvement of Al-Cu-Li alloys with high Li content plays a critical role for the acquisition of excellent mechanical properties and ultra-low density. In this regard, the Al-Cu-Li alloy castings with high Li content from 1.5wt.% to 4.5wt.% were prepared by near-rapid solidification, followed by two-stage homogenization treatment (490 °C/16 h and 530 °C/16 h). The microstructural evolution and solidification behavior of the as-cast and homogenized alloys with different Li contents were systematically studied by combining experiments with calculations by Pandat software. The results indicate that with the increase of Li content, the grain sizes decrease, the solution ability of Cu in the matrix α-Al phase increases, while the content of secondary dendrites increases and the precipitated phases change from low melting point phases to high melting point phases under the near-rapid solidification. Additionally, by the coupling of near-rapid solidification and two-stage homogenization, the metastable precipitated phases (Al₇Cu₄Li and AlCu₃) can be dissolved effectively in the alloys with Li content of 1.5wt.%–2.5wt.%; moreover, the stable precipitated phases (Al₆CuLi₃ and Al₂CuLi) uniformly distribute at the grain boundaries in the alloys with Li content of 3.5wt.%–4.5wt.%. As a result, the refined and homogenized microstructure can be obtained.

A high Fe content easily produces Fe-rich phases with a harmful morphology, resulting in a huge detrimental effect on the properties and recycling ability of Al-Si alloys. Therefore, finding ways to effectively transform Fe-rich phases to form a beneficial phase or shape is of great significance. Accordingly, Al-Si-based alloys with Fe contents ranging from 0.1 wt.% to 2.0 wt.% were modified by different Mn additions. Moreover, experiments combined with simulations were utilized to comprehensively analyze the mechanism of Mn on the morphology and microstructural evolution of Fe-rich phases from different perspectives. The current findings determine that adding different Fe contents changes the phase-transition reactions in alloys. Without Mn, and by increasing the Fe content from 0.1 wt.% to 2.0 wt.%, the Fe-rich phases gradually convert from a skeleton-shaped α-Al₈Fe₂Si (<0.25 wt.%) to β-Al₉Fe₂Si₂ with a fibrous (0.5 wt.%), needle-like (1.0 wt.%) and plate-like shape without curvatures (2.0 wt.%). The maximum length and mean aspect ratio increase from 12.01 μm to 655.66 μm and from 1.96 to 84.05, while the mean curvature decreases from 8.66 × 10⁻² μm⁻¹ to 8.25 × 10⁻⁴ μm⁻¹. The addition of 0.35 wt.% Mn promotes a new Chinese-character and petal-shaped α-Al₁₅(FeMn)₃Si₂, with an atomic ratio of Fe and Mn of 1:1 when the Fe content is lower than 0.5 wt.%, while it transforms to β-Al₁₅(FeMn)₃Si₂ with an atomic ratio of 5:1, presenting as a refined plate-like shape with a certain curvature, as the Fe content increases to 2.0 wt.%. Mn alters the phase reactions and increases the threshold of the Fe content required for β-Al₁₅(FeMn)₃Si₂, limiting the formation and growth of them simultaneously in time and space. The enrichment of Mn atoms and solute diffusion at the growth front of β-Al₁₅(FeMn)₃Si₂, as well as the strong atomic-binding ability, can deflect the growth direction of β-Al₁₅(FeMn)₃Si₂ for it to have a certain curvature. Additionally, the enriched Mn atoms easily form α-Al₁₅(FeMn)₃Si₂ and cause the long β-Al₁₅(FeMn)₃Si₂ to be broken and refined to further reduce the damages caused to the alloy’s performance. Ultimately, the maximum length and mean aspect ratio can be effectively reduced to 46.2% and 42.0%, respectively, while the mean curvature can be noticeably increased by 3.27 times with the addition of Mn.

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb (at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated. The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 µm∙s^-1 under the temperature gradients of 15-20 K∙mm^-1, and the primary phase is transformed into an α phase at relatively higher growth rates (V >20 µm∙s^-1). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 µm∙s^-1 under a temperature gradient of 15 K∙mm^-1. The increase of temperature gradient to 20 K∙mm^-1 can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as \(L\xrightarrow{{L \to \beta }}L + \beta \xrightarrow{{L + \beta \to \alpha }}\alpha + {\beta _R}\xrightarrow{{\beta \to \alpha }}\alpha \xrightarrow{{\alpha \to \gamma }}\alpha + \gamma \xrightarrow{{\alpha \to {\alpha _2} + \gamma }}{\gamma _R} + \left( {{\alpha _2} + \gamma } \right)\), in which β_R and γ_R mean the residual β and γ