First-principle Studies Of Structures And Physical Properties Of Binary Amorphous Alloys

Composition design is considered to be one of basic issues in the study of bulk metallic glasses (BMGs)."Cluster-plus-glue atom" model, as one of the composition design methods, has been successfully applied in several metallic glass systems. Recently, the BMGs can be obtained in several binary alloy systems by copper mold casting, such as Cu-Zr, Cu-Hf, Ni-Nb and Ni-Ta. Because of the simple components, these inary systems became good subjects to understand the fundamental theoretical problems for he formation of BMG alloys. In term of "cluster-plus-glue atom" model, we have performed b initio molecular dynamics (AIMD) simulation of Cu-Zr, Ni-Nb and Ni-Ta alloys at d(?)scending temperatures, and discuss their structures and properties. Fe-B alloy is a typical prototype of the transition-metal-metalloid glasses, and they have intriguing magnetic behavior, and wildly used as a type of soft magnetic materials. We conducted a comprehensive study on the structural, electronic, and magnetic properties as well as the relationship with each other for amorphous Fe-B alloys using first-principles simulations.As the first study, we have performed AIMD simulation of rapid quenching process for Cu-Zr (Cu64Zr36) alloy at descending temperatures (from2000K to400K), and discussed the evolution of short-range order with temperature. The pair correlation functions, coordination polyhedra (clusters), and chemical compositions of the most abundant local clusters have been analyzed. It is found that icosahedral short-range orders exist in the liquid and glass states, and it becomes dominant in the glass states. Moreover, we demonstrated the existence of Cu-centered Cu8Zr5icosahedral clusters as the major local structural unit in the Cu64Zr36amorphous alloy. This finding agrees well with "cluster-plus-glue atom" model of Cu-Zr-based BMG.Secondly, we employed AIMD method to simulate the rapi quenehing proeess of Ni-Nb (Ni62.5Nb37.5) alloy at descending temperatures (from1800K lo300K), and discussed the relationship between short-range order and temperature. We found that icosahedral short-range order exists in the liquid and glass states, and it becomes dominant in the amorphous states. Moreover, we demonstrated the existence of Ni-centered Ni7Nb6icosahedral clusters as the major local structural unit in the Ni62.5Nb37.5amorphous alloy. This finding agrees well with "clustcr-plus-gluc-atom" model for the Ni-Nb bulk metallic glasses. In Ni62.5Nb37.5glass, the first Peak Position of Ni-Nb Pair correlation function is smaller than the sum of Ni and Nb atomic radii, meaning a stronger interaction between Ni and Nb atoms than metallic bonding. The point is clarified by electronic structure analysis which shows a strong interaction between d electrons of Ni and d electrons of Nb, developing a metallic bonding with weak covalence between Ni and Nb atoms.Ni-Ta BMG with compositions around Ni60Ta40is a newly found binary BMG with high glass forming ability and extraordinary mechanical strength. Then using AIMD method, the local atomic structure, elastic properties and electronic structures of Ni60Ta40glass have been explored. We demonstrated the existence of icosahedral Ni7Ta6clusters as the major Ni-centered clusters. This finding agrees with "cluster-plus-glue-atom" for Ni-Ta binary BMG. The elastic moduli of Ni6oTa4o glass were also computed and the experimental Young’s modulus is well reproduced. Analysis of electronic structures further revealed that the interaction between d electrons of Ni and Ta atoms are responsible for the experimentally observed ultrahigh mechanical strength for the Ni-Ta BMGs.In the last part of this paper, the structural, electronic, and magnetic properties of serials of amorphous Fe100-xBx are investigated using first-principles calculations. In these amorphous alloys, the short-range order is manifested as a series of Fc-or B-centered polyhcdra such as tricapped trigonal prism, icosahedron, and bcc-like structural unit. The clectron densities of states of the amorphous alloys resemble those of crystalline Fe borides, which further confirm the similarity of the local order in the amorphous and crystalline phases. All B atoms carry small negative moments of about-0.1μB, while small negative moments are also found on very few Fe sites for the Fe-rich compositions (x=9,17), which can account for the average magnetic moment per Fe atom decreases nonlincarly with increasing B composition.