Amorphous Transition In Metal Systems Simulated By N-body Potential Based Molecular Dynamics

A brief review is presented concerning the progress of the metastable alloys and its theoretical studies. Meanwhile, the main theoretical methods used in this thesis are introduced, i.e. ab initio assisted construction of interatomic potentials and molecular dynamic (MD) simulation.Based on the traditional TB-SMA potential, a long-range smoothed TB-SMA potential is proposed and it overcomes the structural stability problem and cutoff problem appearing in the original TB-SMA potential. Applied the method of ab initio assisted construction of interatomic potentials, the TB-SMA potentials of the Ni-Sc and Cu-Zr systems and the long-range smoothed TB-SMA potentials of the Nb-Zr and Cu-Hf-Ti systems are constructed.Applied the proven relevant Ni-Sc, Cu-Zr and Nb-Zr potentials, MD simulations are carried out by using sandwich model and solid solution model. The main results are as follows. (1) For the metal-metal multilayers, amorphous layer at the interface grows in a layer-by-layer mode and its growth can be divided into fast nucleating stage, diffusion–growing stage and steady finishing stage. (2) Thermodynamic factor and kinetic factor are computed from the simulations and can be used to predict whether the so-called solid-state amorphization can take place in a binary metal system or not. (3) There exist two critical solid solubilities of a binary metal system, meaning that the crystalline solid solution becomes unstable and turns into a disordered state, while the solute atoms are exceeding the critical solid solubility, resulting in a crystal-to-amorphous transition. The critical solid solubilities of the Ni-Sc system are determined to be ( 1 8±2) a tom Ni% in Sc and ( 2 0±2) a tom Sc%in Ni. The critical solid solubility of the Cu-Zr system is (8±2) a tom Cu%in Zr and (1 0±2) a tom Zr% in Cu. The critical solid solubility of the Nb-Zr system is (8±2) a tom Nb% in Zr and (2 0±2) a tom Zr%in Nb. Consequently, within the composition range bounded by the two determined critical solid solubilities of a system, metallic glass is energetically favored to be formed in the system.Applying the proven relevant Cu-Hf-Ti potential, molecular dynamic simulations are carried out using solid solution model to compare the relative stability of the crystalline solid solution versus its disordered counterpart as a function of solute concentration. The simulation results not only reveal that the physical origin of the crystal-to-amorphous transition is the crystalline lattice collapsing while the solute atoms exceeding the critical value, but also predict a region in the composition triangle energetically favored for the Cu-Hf-Ti ternary metallic glass formation. The prediction directly from the n-body potential is supported by the experimental observations and is in accordance with the so-called empirical structural difference rule.