BaiXin Liu

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.

Long-range empirical potential model: extension to hexagonal close-packed metals

An n-body potential is developed and satisfactorily applied to hcp metals, Co, Hf, Mg, Re, Ti, and Zr, in the form of long-range empirical potential. The potential can well reproduce the lattice constants, c/a ratios, cohesive energies, and the bulk modulus for their stable structures (hcp) and metastable structures (bcc or fcc). Meanwhile, the potential can correctly predict the order of structural stability and distinguish the energy differences between their stable hcp structure and other structures. The energies and forces derived by the potential can smoothly go to zero at cutoff radius, thus completely avoiding the unphysical behaviors in the simulations. The developed potential is applied to study the vacancy, surface fault, stacking fault and self-interstitial atom in the hcp metals. The calculated formation energies of vacancy and divacancy and activation energies of self-diffusion by vacancies are in good agreement with the values in experiments and in other works. The calculated surface energies and stacking fault energies are also consistent with the experimental data and those obtained in other theoretical works. The calculated formation energies generally agree with the results in other works, although the stable configurations of self-interstitial atoms predicted in this work somewhat contrast with those predicted by other methods. The proposed potential is shown to be relevant for describing the interaction of bcc, fcc and hcp metal systems, bringing great convenience for researchers in constructing potentials for metal systems constituted by any combination of bcc, fcc and hcp metals.

Reverse sequence of formation of titanium nitrides by nitrogen implantation

Room‐temperature implantation was conducted for the thin titanium films by 80‐keV nitrogen ions. It was found that TiN began to appear at a dose around 2×1017 N/cm2, and the titanium film converted entirely into TiN after 1×1018 N/cm2 implantation. Surprisingly, Ti2N, which has a lower N/Ti ratio than TiN, was only detected at an even higher implantation dose, e.g., as high as 2×1018 N/cm2. This reverse sequence of titanium nitride formation was attributed to the structural compatibility between the matrix and new phase being formed. Viewed in this light, a shearing mechanism is proposed, which can explain the titanium nitride formation, and is also applicable to other metal nitrogen systems.

Ab initio Calculation to Predict Possible Non-Equilibrium Solid Phases in an Immiscible Y–Nb System

In the equilibrium immiscible Y–Nb system, the total energies of the possible structures for Y 3 Nb and YNb 3 non-equilibrium phases were calculated as a function of their lattice constant(s), under the frame work of the Vienna ab initio simulation package (VASP) and the calculated results predicted the relative stability of the Y 3 Nb and YNb 3 phases crystallizing in four possible simple structures, i.e. A15, D0 19 , L1 2 and L6 0 structures, respectively. Experimentally, a fcc Y 3 Nb non-equilibrium phase was indeed obtained by ion beam mixing and its lattice constant determined by diffraction analysis was in agreement with the calculated value.

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.

Critical Solid Solubility of the Ni–Ti System Determined by Molecular Dynamics Simulation and Ion Mixing

From a realistic n-body potential of the Ni-Ti system, the critical concentrations of the Ni- and Ti-rich solid solutions were determined by molecular dynamics (MD) simulation to be 38 at% Ti and 15 at% Ni, respectively, beyond which a disordered atomic configuration was more stable than the respective crystalline solid solutions. It follows that the central composition range bounded by the critical solubilities, i.e. within 38-85 at% of Ti, can be considered as the glass-forming range of the system, which was confirmed by room temperature 200 keV xenon ion mixing of alternately deposited Ni-Ti multilayered films. Moreover, MD simulation of a Ni-Ti bilayer revealed that during the solid-state amorphization reaction, the growth of the amorphous interlayer followed exactly a t 1/2 law and grew faster towards the Ti lattice than to the Ni side. The physical origin of such an asymmetric behaviour was found to be due to a difference in critical solid solubilty of the constituent metals.

Formation of the Ni–Zr–Al Ternary Metallic Glasses Investigated by Interatomic Potential through Molecular Dynamic Simulation

Under the framework of second moment approximation of the tight binding theory, a realistic interatomic potential is first developed for the Ni–Zr–Al ternary metal system and then applied to predict the glass-forming ability of the system through molecular dynamics simulation. It is found that when the composition falls into the hexagonal region defined by six vertexes of Ni 20 Zr 80 Al 0 , Ni 0 Zr 65 Al 35 , Ni 0 Zr 25 Al 75 , Ni 20 Zr 0 Al 80 , Ni 40 Zr 0 Al 60 , and Ni 77 Zr 23 Al 0 , the super-saturated solid solution becomes unstable and spontaneously turns into the disorder state, i.e., the metallic glass state. The defined composition region could be considered as a quantitative glass-forming ability, within which the Ni–Zr–Al ternary metallic glass is predicted to be energetically favored to form. Interestingly, the prediction based on the interatomic potential matches well with experimental observations.

Proposed truncated Cu–Hf tight-binding potential to study the crystal-to-amorphous phase transition

Proposed truncated Cu–Hf tight-binding potential was constructed by fitting the physical properties of Cu, Hf, and their stable compounds, i.e., Cu5Hf, Cu8Hf3, Cu10Hf7, and CuHf2. Based on the constructed potentials, molecular dynamics simulations were carried out to compare the relative stability of the crystalline solid solution and the disordered state. Simulation results not only reveal that the physical origin of crystal-to-amorphous transition is the crystalline lattice collapsing when the solute atoms exceeding the critical concentration, but also predict that the glass forming range (GFR) of the Cu–Hf system is 21–77 at. % Cu, which covers the GFRs determined by various metallic glass-producing techniques. Ion beam mixing experiments of the Cu–Hf system were conducted using 200 keV xenon ions and the results show that a uniform amorphous phase can be obtained in the Cu23Hf77 sample, matching well with the GFR determined by the interatomic potential, which, in turn, provides additional evidence to ...

Structural stability of the metastable solid solution in the equilibrium immiscible Ag-Mo system predicted by an Ab initio derived potential

Based on the cohesive energies and lattice constants of a few possible non-equilibrium Ag–Mo compounds obtained by ab initio calculation, a Finnis–Sinclair (FS) potential of the equilibrium immiscible Ag–Mo system is derived. Applying the proven realistic potential, molecular dynamics simulation is carried out to study, at an atomic scale, the structural stability of the Ag-based solid solutions. The simulation results predict that the fcc crystalline structure can be preserved until the Mo concentration reaches 13 at.% and the correlation of the lattice constant of the Ag-based solid solutions vs the Mo concentration is in good agreement with Vegard’s Law. In addition, the heats of formation of the fcc Ag–Mo solid solutions calculated by the derived potential are quite agreeable with that calculated by Miedema’s theory.

Chemical and topological short-range orders in the ternary Ni-Zr-Al metallic glasses studied by Monte Carlo simulations.

Based on the recently constructed Ni-Zr-Al n-body potential, Monte Carlo simulations are performed to study the glass formation and associated structural evolutions in the system. The micro-chemical inhomogeneity (MCI) parameter and Honeycutt and Anderson (HA) pair analysis are employed to investigate both the chemical short-range orders and topological short-range orders for the ternary Ni-Zr-Al metallic glasses. Results reveal that remarkable chemical short-range orders (CSROs) exist in the ternary Ni-Zr-Al metallic glasses and are strongly influenced by the chemical interactions among the constituent elements. Moreover, topological short-range orders are clearly formed in the ternary Ni-Zr-Al metallic glasses, with the most remarkable characteristic being the icosahedral local packing. Similarly to CSRO, the extent of icosahedral short-range orders formed in the Ni-Zr-Al system varies distinctly with the chemical composition. In addition, simulation results reveal that chemical short-range orders and topological short-range orders turn out to be influenced by different factors. Unlike CSRO, both chemical interactions and geometrical constraints play important roles in forming the topological short-range orders.