Search For The Microscopic Origin Of Defects And Shear Localization In Metallic Glasses

This proposed research addresses one of the long outstanding fundamental problems in materials science, the mechanisms of deformation in amorphous metals. Due to the lack of long-range translational order, details of structural defects and their behaviors in metallic glasses have not been accessible in experiments. In addition, the small dimensions of the amorphous alloys made early by rapid quenching impose severe limit on many standard mechanical and microscopy testing. As a result, the microscopic mechanism of deformation in the amorphous materials has not been established. The recent success in synthesis of bulk metallic glass overcomes the difficulty in standard testing; but the barrier for understanding the defect process and microscopic mechanisms of deformation still remains. Amorphous metals deform in a unique way by shear banding. As a result, there is no work hardening, little macroscopic plasticity, and catastrophic failure. To retain and improve the inherent high strength, large elastic strain, and high toughness in amorphous metals, a variety of synthesis activities are currently underway including making metallic glass matrix composites. These new explorations call for a quantitative understanding of deformation mechanisms in both the monolithic metallic glasses as well as their composites. The knowledge is expected to give insight and guide to design, processing and applications of this new generation of engineering materials. This DOE funded research takes the approach of computer simulation and modeling to tackle this problem. It is expected that with the increasing power of computers, the numerical modeling could provide the answers that are difficult or impossible to get from experiments. Three parallel research tasks were planned in this work. One is on search of atomic structural defects and other microscopic mechanisms underlying the deformation process. The second is the formulate a general model to describe shear localization, shear band formation and propagation on mesoscopic scale. The last is to determine the constitutive behaviors of the amorphous metals from the knowledge gathered from the atomistic and mesoscopic modeling, as well as experiments. The continuum description of deformation and fracture in metallic glass is expected for predicting and analyzing mechanical performance of bulk metallic glass products and components in real applications. With the support of the DOE grant, several major breakthroughs have been made. Among the highlights are (1) quantitative characterization of free volumes, (2) dynamic modeling of breakdown process in disordered Ising models, and (3) development of a novel mesoscopic modeling method using phase field, or Ginzburg-Landau Theory. These progresses laid a firm foundation for the future advance in comprehensive understanding of deformation mechanisms in amorphous metals. The future works are laid out that address not only the remaining or unfinished tasks and topics, but also the further extension and development from the knowledge learned from the current research. Among these topics are (1) micromechanics of defects, (2) composite modeling, (3) a theory of shear localization by combining microscopic defect properties with mechanics, and (4) continuum modeling of glassy metal composites and products in services.