Construction of free-energy diagrams of amorphous alloys

Topologically close-packed characteristic of amorphous tantalum

The structural evolution of tantalum (Ta) during rapid cooling was investigated by molecular dynamics simulation, in terms of the system energy, the pair distribution function and the largest standard cluster analysis. It was found that the critical cooling rate for vitrification was about R ≥ 0.25 K ps-1, two orders lower than other metals (such as Au, Ag, Al, Zr and Zn) and that the meta-stable σ phase (β-Ta) not only appears on the pathway from liquid to the stable body-centred cubic crystal, but is also easily obtained at room temperature as a long-lived metastable phase with some probability. The most interesting point is that the liquid, amorphous and β-Ta phases share a nontrivial structural homology; the intrinsic topologically close-packed (TCP) structures in liquids are inherited and developed in different ways, resulting in amorphous or crystalline solids, respectively. With highly local packing fractions and geometrical incompatibility with the global close-packed (such as hcp, fcc and bcc) crystals, TCP structures inevitably result in structural heterogeneity and favour vitrification. As a superset of icosahedrons, TCP structures are ubiquitous in metallic melts, and just before the onset of crystallization reach their maximal number, which is much bigger in Ta than in other poor-GFA metals; so we argue that the strong forming ability of TCP local structures significantly enhances the glass forming ability of pure metals. These findings open up a new perspective that could have a profound impact on the research into metallic glasses.

Metallic glass formation in Cu–Ni–Ti (Zr, Hf) systems studied by thermodynamic calculations

The glass formation regions for the family of Cu–Ni–Ti (Zr, Hf) systems have been calculated. The composition dependence of the glass forming ability was predicted and discussed in terms of the mixing enthalpy and atomic size effect.

The optimized composition of Mg–Al–Cu metallic glass investigated by thermodynamic calculations and an atomistic approach

Issues related to the glass formation of ternary Mg–Al–Cu metallic glass are investigated by thermodynamic calculations and an atomistic approach.

Atomistic approach to design favored compositions for the ternary Al–Mg–Ca metallic glass formation

An interatomic potential was constructed and applied to design favoured compositions for the ternary Al–Mg–Ca metallic glasses formation.

Formation and structural anomaly of the metastable phases in an immiscible Ag-Mo system studied by ion beam mixing and molecular dynamics simulation

For the equilibrium immiscible Ag-Mo system characterized by a large positive heat of formation, the nanosized Ag-Mo multilayered samples are designed and prepared to include sufficient interfacial free energy to elevate their initial energetic states to be higher than that of either the amorphous phase or solid solution and then subject to 200 keV xenon ion irradiation. The results show that a uniform amorphous alloy can be obtained within a composition range, at least, from 25 to 88 atom % of Mo. Interestingly, in the intermediate stage of ion irradiation, a bcc phase, an amorphous phase, and an order (bcc)-disorder coexisting state appear simultaneously in the Ag12Mo88 multilayered sample and extend over the entire bright field image with unanimously homogeneous composition. In thermodynamic modeling, a Gibbs free energy diagram of the Ag-Mo system is constructed, based on Miedema's model, and suggests that within a narrow composition regime of 85-90 atom % of Mo, the energy difference between the bcc and the amorphous phases is extremely small, which is probably the very reason for the order-disorder coexisting state to appear. In atomistic modeling, an ab initio derived Ag-Mo potential is applied to perform molecular dynamics simulations. The simulations not only determine an intrinsic glass-forming ability/range (GFA/GFR) of the Ag-Mo system to be from 10 to 88 atom % of Mo but also reveal the possibility of the formation/appearance of a crystalline and amorphous mixture in a narrow composition regime of 88-92 atom % of Mo. Apparently, the theoretical results are in excellent agreement and/or compatible with the experimental observations in ion beam mixing.

Formation of amorphous alloys by ion beam mixing and its multiscale theoretical modeling in the equilibrium immiscible Sc-W system

Unique amorphous alloys are synthesized at the compositions of 25 and 40 atom % of W by ion beam mixing in the equilibrium immiscible Sc-W system characterized by a positive heat of formation of +14 kJ/mol. In thermodynamic modeling, a Gibbs free energy diagram is constructed based on Miedema's theory, and the diagram predicts a glass-forming range of the Sc-W system to be within 12-58 atom % of W. To develop an atomistic model, ab initio calculations are first conducted to assist the construction of an n-body Sc-W potential under the embedded atom method. The proven realistic potential is applied in molecular dynamic simulations to study the crystal-to-amorphous transition in the Sc-W solid solutions, thus determining the glass-forming ability of the system to be within 15-50 atom % of W. Apparently, both theoretical predicted glass-forming ranges cover the experimentally measured one, showing an excellent agreement. We report, in this paper, the experimental results from ion beam mixing and the multiscale theoretical modeling concerning the amorphous alloy formation in the Sc-W system together with a brief discussion of the structural transition mechanism.