W. S. Lai

Irradiation induced amorphization in metallic multilayers and calculation of glass-forming ability from atomistic potential in the binary metal systems

Abstract This review attempts to present first a brief summary of the up-to-date experimental studies on amorphization transition, which results in the formation of amorphous alloys or metallic glasses, by ion irradiation of multiple metal layers in the binary metal systems. Secondly, based on the framework of Miedema’s theory, thermodynamic modeling of metallic glass formation is described with consideration of the significant role of interfacial free energy of the multilayers in amorphization. Thirdly, results of molecular dynamics simulations for some representative systems are presented to show the calculation of the intrinsic glass-forming ability from interatomic potential of the binary metal systems.

Lattice stability of some Ni-Ti alloy phases versus their chemical composition and disordering

A realistic n -body Ni-Ti potential is derived and applied in molecular dynamics simulation for studying the lattice stability of the terminal solid solutions and an intermetallic compound of B2 NiTi phase. It is found that when the solute contents are increased beyond two respective critical values of 15 at% of Ni and 38 at% of Ti, the crystalline lattices of the solid solutions become unstable and transform into amorphous states, suggesting that a glass-forming range of the system is from 15 to 62 at% of Ni. In the case of B2 NiTi compound, a crystalline-to-amorphous transition can result from the introduction of either a certain amount of chemical disordering or a compositional shift from its exact stoichiometry. In addition to the B2 NiTi phase, the simulation results also give some insight concerning the phase transition behaviour upon irradiation for the Ni3 Ti and NiTi2 intermetallic compounds.

Atomic structure and physical properties of Ni–Nb amorphous alloys determined by an n-body potential

Abstract An n -body Ni–Nb potential is constructed under an embedded-atom method, which can reproduce some intrinsic properties of the alloy phases of the system. Employing the potential, a molecular-dynamics simulation is performed to derive the characteristics of the atomic structure of the Ni–Nb amorphous state, such as the pair correlation functions, structure parameters and bond angle distributions. The calculated results are compatible with those obtained from experiments and other simulations. In addition, the cohesive energy, the molar volume and the heat of formation of Ni–Nb amorphous alloys were also computed, and an apparent dependence of those physical properties on the alloy compositions were observed.

Irradiation-induced growth of nanoquasicrystals from amorphous matrix in the equilibrium immiscible Fe–Cu system

A microstructure of nanosized quasicrystals embedded in an amorphous matrix was formed in the Fe70Cu30 multilayered films upon room temperature 200 keV xenon ion irradiation. The initial Fe and Cu crystalline phases in the as-deposited multilayers transformed into a unique amorphous phase at a dose of 8×1014 Xe+ cm−2 and further irradiation, i.e., up to a dose of 5×1015 Xe+ cm−2, induced the growth of the quasicrystals in some local areas in the amorphous matrix. High-resolution electron microscopy examination revealed that the compositions of the quasicrystals and amorphous matrix were close to Fe50Cu50 and Fe70Cu30, respectively. Apparently, the above microstructure was formed through a two-step phase transition along the increase of ion dose and the amorphous-to-quasicrystal transition was discussed in terms of the similarity in the atomic configuration between the icosahedral and amorphous short-range orders.

Observation of largely enhanced hardness in nanomultilayers of the Ag-Nb system with positive enthalpy of formation

Ag∕Nb nanomultilayers with different modulation wavelengths Λ were prepared on silicon wafers by electron beam evaporation. Nanoindenter measurements show that with decreasing Λ of the multilayers, the nanohardness increases up to ∼80% for Λ=4nm, whereas the modulus is almost unchanged. This unusual behavior originates from a unique microstructure where amorphous Ag–Nb alloys form at the interfaces and grain boundaries of silver nanoparticles, as observed by cross-section high resolution transmission electron microscopy. The amorphous phases favor hardness enhancement by preventing dislocation emission and movement, whereas they have a negative contribution to the modulus because of their free volume.

Icositetrahedral and icosahedral atomic configurations observed in the Nb-Ag metallic glasses synthesized by ion beam mixing

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.

Strain-induced elastic moduli softening and associated fcc↔bcc transition in iron

Using molecular dynamics calculations we demonstrate that with decreasing the thickness of ultrathin body-centered-cubic (bcc) α‐Fe film with (001) surfaces, the biaxial strain results in first bcc(001)→face-centered-cubic (fcc) (001) transition along the inverse Bain path due to softening of C33, and then fcc(001)→bcc(011) because of shear modulus vanishing along fcc ⟨110⟩ directions. For the bulk fcc γ‐Fe, the tensile biaxial strain along the Bain path transforms fcc (001) into bcc (001) with fcc⟨110⟩‖bcc⟨100⟩, while compressive strain results in shear instability, in agreement with recent ab initio calculations.

Glass-forming ability of the Ni–Zr and Ni–Ti systems determined by interatomic potentials

Employing the n-body potentials of the Ni–Zr and Ni–Ti systems, we performed molecular dynamics simulation to study the relative stability of the terminal solid solutions versus the corresponding amorphous states as a function of solute concentrations. The terminal solid solutions transformed into amorphous states spontaneously when the solute concentrations were beyond the maximum allowable values; i.e., the critical solubilities were determined to be 14 at.% Zr in Ni and 25 at.% Ni in Zr for Ni–Zr system and 38 at.% Ti in Ni and 15 at.% Ni in Ti for the Ni–Ti system. The physical implication of the critical concentrations, as well as their correlation with the glass-forming abilities of the Ni–Zr and Ni–Ti systems, is discussed.

Structural Stability and Amorphization Transition in the Ni–Ti System Studied by Molecular Dynamics Simulation with an n-Body Potential

An n-body Ni–Ti potential is derived and verified to be capable of reproducing some physical properties in the Ni–Ti system. Applying the constructed potential molecular dynamics simulations of solid-state reaction in the Ni–Ti multilayers reveal that the growth of an amorphous interlayer follows a linear t correlation at the very beginning and then shifts to exactly a t 1/2 law and that a sharp semi-coherent interface serves as a nucleation barrier, preventing the interfacial reaction at a temperature up to 873 K. Moreover, the asymmetric growth of amorphous interlayer is found to originate from the sequential disordering of constituent metals by dissolving its partner atoms beyond its maximum allowable solubility, which is named as a solubility criterion. In addition, the calculated maximum solid solubilities also predict that an intrinsic glass forming range in the Ni–Ti system is from 15 to 62 at.% of Ni, which is in good agreement with the experimental results.

Proposed interpretation for possible solid-state amorphization in some Cu-based binary metal systems

According to a recently proposed criterion for solid-state amorphization to take place in a binary metal system, it is necessary to have a negative thermodynamic factor ΔF defined as a Gibbs free energy difference between an amorphous phase and the initial energetic state of the multilayers/or bilayer together with a relevantly large kinetic factor κ correlated to a diffusivity difference between the constituent metals of the system. The criterion is employed to discuss the possibility of solid-state amorphization in five representative Cu-based binary metal systems featuring either positive or negative heat of mixing. It turns out that the prediction based on the above argument for these Cu-based systems is in good agreement with the experimental results observed so far.