Metallic glass structure and structural relaxation: Molecular dynamics computer simulations and x-ray anomalous scattering measurements.

摘要

The goal of this work was to study the structure and structural relaxation of metallic glasses by combining computer simulations and direct structural measurements. Molecular dynamics computer simulations were performed on a variety of two-component systems to compare the technique with two other computer techniques, to study the glass transition of a model metal-metalloid system, and to model the structure of amorphous Cu-Zr. The structural environment around the Cu and the Zr atoms in two amorphous Cu-Zr alloys was determined from x-ray anomalous scattering measurements.; A simple two-component system quenched from the liquid to the glassy state by using the constant pressure molecular dynamics method exhibited many features of glass-forming materials. These include a quench rate dependence of properties and hysteresis of properties on reheating the glass. In addition, the Moynihan phenomenological description of low temperature relaxation fits much, but not all, of our cooling and heating data. We conclude that the transition that we have observed on the computer at a quench rate of {dollar}approx{dollar}10{dollar}sp{lcub}10{rcub}{dollar} K/second has the same characteristics as the glass transition observed in the laboratory for a wide range of materials.; We have successfully combined molecular dynamics computer simulations with x-ray anomalous scattering measurements to study the structure of amorphous Cu{dollar}sb{lcub}77{rcub}{dollar}Zr{dollar}sb{lcub}23{rcub}{dollar} and Cu{dollar}sb{lcub}33{rcub}{dollar}Zr{dollar}sb{lcub}67{rcub}{dollar}. Experiments were carried out at the Stanford Synchrotron Radiation Laboratory to determine the differential distribution functions for the two amorphous alloys. Interatomic potentials were developed for the Cu-Zr system that reproduce the structure factor for liquid Cu and liquid Zr, and that give good agreement with the general features of the differential distribution functions obtained for the two amorphous alloys. Further detailed structural information was obtained from the molecular dynamics results, including the change in the coordination number as the concentration is varied over a wide range.; We find that the molecular dynamics technique will prove extremely useful in the study of amorphous materials. Future applications will be to understand in more detail the structural relaxation processes occurring in supercooled liquids and to refine our ideas about the structure of the low temperature amorphous solids which are produced.