Simulations Of Short-range Orders In Solid Solutions And Amorphous Alloys

At present, the study about ordered materials has been relatively comprehensive, but the understanding of disordered materials is still in its infancy, especially the short-range order inside disordered materials. Due to the chemical short-range interaction between different components, the distribution of atoms inside materials is inhomogeneous and tends to be short-range ordered, especially in solid solutions and amorphous alloys in which the atomic configurations can be short-range ordered and long-range disordered. Thus, reasonable structure models and methods are necessary for the study of short-range order inside solid solutions and amorphous alloys, such as clusters and defect structures, since the traditional crystallography can’t describe their structures.Using first-principles calculations, we employ the cluster-plus-glue-atom model and Le Claire model to study the clusters and defect structures inside solid solutions and amorphous, and we also analyze their correlation with properties. The cluster-plus-glue-atom model was proposed by Dong et al in Dalian University of Technology, which is attempting to describe the clusters inside solid solutions and amorphous. Le Claire model can describe the interaction between solute and vacancy inside solid solutions by analyzing the diffusion of vacancy. Therefore, we studied three kinds of BCC solid solutions:low elastic β-type Ti-Mo-Nb alloys for biomedical applications, V-4Cr-4Ti alloy and Chinese low activation martensitic (CLAM) steel for fusion reactors, as well as Zr-Ni-Al amorphous with large glass forming ability.In the first section, we introduce the short-range order inside solid solutions and amorphous, including the short-range parameter for solid solutions and radial distribution functions for amorphous. Then, we focus on the description of cluster-plus-glue-model and Le Claire model, which have been used in this thesis. Finally, we introduce the properties and applications of solid solutions and amorphous. The theoretical background of first-principles calculations is introduced in the second section, as well as the calculated method of elastic properties, Nudged Elastic Band method and ab initio molecular dynamics simulation.Using cluster-plus-glue-atom model and first-principles calculations, we study the correlation between short-range order and elastic properties inside β-type Ti-Mo-Nb alloys in the third section. With random solid solution model, the calculated Young’s modulus is very sensitive to the atomic structure inside the supercell as much as dozens of GPa, even negative values. Using the cluster-plus-glue-atom model, we calculate the Young’s modulus of [Mo-Ti14](Ti, Mo, Nb) and [(Ti, Mo)-Ti14]Nb3alloys, which agrees with the experimental data. Our theoretical results can construct an Ashby map of Elp vs BIG. Our results support the description of cluster-plus-glue-atom model for the structures of solid solutions, and confirm that this model could use to design alloys.In the forth section, we investigate the diffusion mechanisms and the effects of solute atoms inside V-4Cr-4Ti alloy and CLAM steel, which are the candidate structural materials for fusion power reactors. Due to the irradiations, there are a lot of vacancies can be produced inside the materials. Thus, this kind of materials must contain lots of alloying elements. Firstly, using Nudged Elastic Band method, we discuss the interaction of solute/trace elements with vacancy inside V-4Cr-4Ti alloy, like vacancy formation energies and migration energies. We also calculate their diffusion coefficients inside vanadium using Le Claire model. Our calculations find that Si has a slow diffusion rate in vanadium through the vacancy mechanism, and vacancy shows strong interactions with Si and Ti. Thus, there are vacancy-Ti/Si complexes formed inside alloy, which is beneficial to the formation of TisSi3phase, increasing the irradiation resistance of alloy.In the forth section, we also discuss the alloying mechanism of CLAM steel, which has decreased W content and increased Ta content to improve its performance under irradiation. Using Le Claire model, we find that W has a slower diffusion rate in a-Fe, increasing the interaction between Fe and W. Thus, CLAM steel has less W to avoid the formation of Fe2W Laves phase. Then, we investigate the interaction between solute atom and C. We find that Ta can trap C, leading to the formation of TaC. Our theoretical results support the composition designs of V-4Cr-4Ti alloy and CLAM steel.In the fifth section, we perform ab initio molecular dynamics simulations to study the rapid quenching progress of Zr60Ni26.7Al13.3(with cluster formula of [Ni-Ni2Zrg](Al2Ni)) alloy and discuss the evolution of short-range clusters with temperature. We calculate the structure factor, pair-correlation function, coordination numbers and distribution of local clusters inside Zr60Ni26.7Al13.3alloy. As the temperature decreased, our theoretical results reveal that the atomic configuration around Ni is dominated by CN11Ni-AlNi3Zr8clusters. Thus, improve the cluster formula as [Ni-AlNi2AlZr8](AlNiZr) for Zr6oNi26.7Al13.3alloy, instead of the previously proposed cluster formula of [Ni-Ni2Zrg](Al2Ni).