[en] In the equilibrium immiscible Hf-Nb system characterized by a positive heat of formation, five Hf-Nb metallic glasses with overall compositions of Hf₈₄Nb₁₆, Hf₆₅Nb₃₅, Hf₄₅Nb₅₅, Hf₃₈Nb₆₂, and Hf₂₀Nb₈₀ are obtained by ion beam mixing with properly designed Hf-Nb multilayered films, suggesting a glass-forming composition range of 16-80 at. % of Nb. For the special case of Hf₄₅Nb₅₅ located at the ridge point on the convex free energy curve, dual-glass phases are formed at a dose of 2x10¹⁵ Xe⁺/cm², which results from a spinodal decomposition of the expected Hf₄₅Nb₅₅ amorphous phase. With increasing irradiation dose, fractal growth of nanocrystals (around 20 nm) appears in the major glass phase and the dimension is determined to be from 1.70 to 1.84 within a dose range of (4-7)x10¹⁵ Xe⁺/cm². In atomistic modeling, a n-body Hf-Nb potential is first constructed with the aid of ab initio calculations. Applying the constructed potential, molecular dynamics simulations using the hcp and bcc solid solution models, reveals an intrinsic glass-forming range to be within 15-83 at. % of Nb, which is compatible with the ion beam mixing experiments. Moreover, the formation of the metallic glasses and the fractal growth in association with the amorphous spinodal decomposition are also discussed in terms of the atomic collision theory and cluster-diffusion-limited-aggregation model

[en] With the aid of ab initio calculations, an n-body Fe-Nb embedded-atom potential is first constructed and then applied to study the crystal-to-amorphous phase transition through molecular dynamic simulations. The simulations determine that the glass-forming range of the Fe-Nb system is 18-83 at. % of Nb. In ion beam mixing experiments, five Fe-Nb multilayered films with overall compositions of Fe₈₅Nb₁₅, Fe₇₅Nb₂₅, Fe₅₅Nb₄₅, Fe₂₅Nb₇₅, and Fe₁₅Nb₈₅, respectively, are irradiated by 200 keV xenon ions to doses in the range of (1-7)x10¹⁵Xe⁺/cm². The result shows that the Fe-Nb metallic glasses can be synthesized within a composition range of 25-75 at. % of Nb, matching reasonably well the theoretical prediction. Moreover, in the Fe₅₅Nb₄₅ sample, a fcc-structured alloy phase with a large lattice constant of a≅0.408 nm was obtained at a dose of 3x10¹⁵ Xe⁺/cm² and the associated magnetic moment per Fe atom was measured to be 2.41μ_B. The observed magnetic moment is much greater than the initial value of 1.42μ_B in the bcc-Fe lattice and can thus serve as evidence confirming the high-spin ferromagnetic state of fcc Fe predicted by ab initio calculations. Interestingly, further irradiation induced phase separation in the Fe₅₅Nb₄₅ sample, i.e., irradiation to a dose of 5x10¹⁵ Xe⁺/cm² results in the growth of a fractal pattern consisting of Fe₇₂Nb₂₈ nanoclusters embedded in Fe₃₅Nb₆₅ matrix. The formation mechanism of the metastable phases as well as that of the fractal pattern observed in the Fe-Nb system was discussed in terms of the atomic collision theory and the well-known cluster-diffusion-limited-aggregation model

[en] An fcc-structured Fe₅₅Nb₄₅ alloy with a large lattice constant of a≅0.408 nm was synthesized by 200 keV xenon ion irradiation at a dose of 3x10¹⁵ Xe⁺/cm². The Fe atom in Fe₅₅Nb₄₅ alloy presents a distinct magnetic moment as high as 2.41μ_B, confirming a high-spin ferromagnetic state of fcc Fe predicted by ab initio calculation. Further irradiation, i.e., at a dose of 5x10¹⁵ Xe⁺/cm², induced phase separation, resulting in fractal growth consisting of Fe₇₂Nb₂₈ nanoclusters embedded in Fe₃₅Nb₆₅ matrix. The formation mechanism of alloy phases as well as fractal pattern was discussed in terms of the atomic collision theory

[en] Four Ni-Nb metallic glasses are obtained by ion beam mixing and their compositions are measured to be Ni₇₇Nb₂₃, Ni₅₅Nb₄₅, Ni₃₁Nb₆₉, and Ni₁₅Nb₈₅, respectively, suggesting that a composition range of 23-85 at. % of Nb is favored for metallic glass formation in the Ni-Nb system. Interestingly, diffraction analyses show that the structure of the Nb-based Ni₃₁Nb₆₉ metallic glass is distinctly different from the structure of the Nb-based Ni₁₅Nb₈₅ metallic glass, as the respective amorphous halos are located at 2θ≅38 and 39 deg. To explore an atomic scale description of the Ni-Nb metallic glasses, an n-body Ni-Nb potential is first constructed with an aid of the ab initio calculations and then applied to perform the molecular dynamics simulation. Simulation results determine not only the intrinsic glass forming range of the Ni-Nb system to be within 20-85 at. % of Nb, but also the exact atomic positions in the Ni-Nb metallic glasses. Through a statistical analysis of the determined atomic positions, a new dominant local packing unit is found in the Ni₁₅Nb₈₅ metallic glass, i.e., an icositetrahedron with a coordination number to be around 14, while in Ni₃₁Nb₆₉ metallic glasses, the dominant local packing unit is an icosahedron with a coordination number to be around 12, which has been reported for the other metallic glasses. In fact, with increasing the irradiation dose, the Ni₃₁Nb₆₉ metallic glasses are formed through an intermediate state of face-centered-cubic-solid solution, whereas the Ni₁₅Nb₈₅ metallic glass is through an intermediate state of body-centered-cubic-solid solution, suggesting that the structures of the constituent metals play an important role in governing the structural characteristics of the resultant metallic glasses

[en] The structural transformation and disordered atomic packing of metallic glasses in a selected immiscible system at equilibrium, i.e. the Cu-Nb system characterized by a positive heat of formation, are studied using ion beam mixing far from equilibrium. The experimental results indicate that the Cu-Nb metallic glasses could be formed in a composition range from 30 to 85 at.% of Nb and that the Cu-Nb metallic glasses are formed through two different structural phase transition routes, i.e. from the Nb-based body centred cubic and face centred cubic solid solutions, in which the two distinct predominant atomic packings have icosahedral and icositetrahedral orderings, respectively, revealed by the respective diffraction patterns. These observations not only help in formulating a general atomic structural spectrum for the binary metallic glasses, but also suggest an important concept of structural heredity: that the crystalline structure of the constituent metals plays a decisive role in determining the atomic structure of the resultant metallic glasses. (letter to the editor)

[en] Metallic glasses are obtained in an immiscible Ag-Nb system with overall composition ranging from 25 to 90 at. % of Nb by ion beam mixing. Interestingly, the diffraction analysis shows that the formed Nb-rich metallic glass features are two distinct atomic configurations. In atomistic modeling, an n-body Ag-Nb potential is derived, under the assistance of ab initio calculation, and then applied in molecular dynamics simulations. An atomic configuration is discovered, i.e., an icositetrahedral ordering, and as well as an icosahedral ordering observed in the Ag-Nb metallic glasses and in some previously reported systems. Simulations confirm that the two dominate local atomic packing units are formed through a structural phase transition from the Nb-based bcc and fcc solid solutions, respectively, suggesting a concept of structural heredity that the crystalline structure of the constituent metals play a decisive role in determining the atomic structure of the resultant metallic glasses

[en] Metallic glasses are obtained in an immiscible Nb-Ag system by ion beam mixing and an atomic configuration in the amorphous structure is discovered, i.e., an icositetrahedral ordering, which, together with an icosahedral ordering also observed in the Nb-Ag metallic glasses and in some previously reported systems, helps in formulating a structural spectrum of the amorphous solids. The experimental characterization and atomistic modeling with an ab initio derived Nb-Ag potential demonstrate the significance of structural heredity, i.e., the crystalline structures of the constituent metals play a decisive role in determining the atomic structure of the metallic glasses in the system

[en] For the immiscible Zr-Nb system characterized by a positive heat of formation (+6 kJ mol^-1), thermodynamic calculation showed that the Gibbs free energy of the properly designed Zr-Nb multilayered films could be elevated to a higher level than that of the corresponding amorphous phase as well as the supersaturated solid solutions. Accordingly, nano-sized Zr-Nb multilayered films were prepared and then irradiated by 200 keV xenon ions. It was found that amorphous phases could be obtained within a composition range 12-92 at% of Nb. Also, two metastable crystalline phases of fcc structures with different lattice parameters were also obtained. Molecular dynamic simulation was carried out, based on a proven realistic Zr-Nb potential, to reveal the atomistic mechanism of the solid-state crystal-to-amorphous transition. A brief discussion on the formation of the two metastable crystalline phases is presented