B <sub> 2 </ Sub> The Type Mota Alloy Embedded Atom Method

A mass of point defects in material can be induced by high temperature quencher, plastic deformation and high energy particle irradiation. The existence and diffusion of the point defects affect the physical and mechanical properties of the material seriously. The energetic and kinetic properties of point defects determine the temporal defect-population evolution and consequent changes in the material microstructure.Therefore, researches on point defects in material have been hot issues in solid physics and material science. The study on point defects based on atomistic simulation can be taken as reference for material design and improvement, and can also be used to verify and enrich theoretical models.So, this work has important significance theoretically and practically. In the present work, the characteristic properties of B2-type MoTa alloy, including equilibrium lattice constant, formation energy, cohesive energy, surface energy and surface structure, and the properties of point defects, have been investigated by combining molecular dynamics (MD) simulation with modified analytic embedded-atom method (MAEAM).The emphases of the present work are the formation and diffusion mechanisms of the point defects in the bulk and surface of MoTa alloy. The formation and migration energies of the defects were calculated and from the energy minimization, the favorable diffusion mechanisms of the mono-vacancy, interstitial atom and di-vacancy in the bulk and the mono-vacancy in the surface were discussed detailedly. Following results are obtained:(1)Calculated lattice constant and formation energy of 3.235 A and-0.151eV are in good agreement with the, experimental and ab initio data. The fact proved that the present model is efficacious for the MoTa alloy. The cohesion energies of the alloy, Mo and Ta components are 7.611,6.821 and 8.400eV, respectively. There is no available data to compare with.(2) The calculated formation energies of Mo and Ta mono-vacancy are 3.233 and 3.218eV,anti-site defect are-0.062 and 0.485eV,and interstitial atom are 7.263 and 7.759eV, respectively. The principle of energy minimization shows that the point defect is the anti-site defect in the nonstoichiometric case. In six migration mechanisms of Mo and Ta mono-vacancies,1NNJ (e.g."INN" means "first nearest neighbour jump" etc) is the most favorable due to its lowest activation and migration energies, but it will result in an anti-site so that a disorder in the order alloy. One 1NNNJ and onel TNNJ can maintain the ordered property of the alloy but require higher activation and migration energies, so the 1NNNJ and 1TNNJ should be replaced by S[100]6NNCJ or B[100]6NNCJ (straight or bent[100] direction six nearest neighbor cyclic jumps) and [110]6NNCJ, respectively. Although the migrations of Mo and Ta interstitial atoms need much lower energy than Mo and Ta mono-vacancies, they are not main migration mechanisms due to difficult to form in the alloy.(3)For six type di-vacancies in the bulk of MoTa, the stability of the di-vacancy configurations decreases in the direction INN Mo-Ta,2NN Ta-Ta and 2NN Mo-Mo, whereas the 4NN Mo-Ta,3NN Ta-Ta and 3NN Mo-Mo configurations are unstable and tend to congregate to the former three stable configurations. Taking into account the minimization of the migration or activation energies, for all six types of the di-vacancy configurations, the multi-jumps involving a series of INN-jumps are energetically more favorable than either one-jump or two-jump diffusion mechanisms.Furthermore, in the three stable configurations of the INN Mo-Ta di-vacancy is not only easy to form, but also easy to migrate especially by the four-jump mechanism.(4) The surface energies and the surface relaxations of Mo and Ta termination (001) surfaces have been calculated. The results show that the Ta termination is more stable and the most surface layer relaxes inward for both terminations.The calculated formation energy of the mono-vacancy in the uppermost seven layers shows that the effect of the surface on the vacancy is only down to the sixth layer. It is easier for the vacancy to form in the first layer. Comparing the migration energy of the vacancy migrating in the intra-and inter-layer via INN and 2NN jump, we find that the vacancies in the surface layers is preferred to migrate to the up-layer via INN jump. Thus increase the concentration of vacancy and consequently changes the surface smoothness.