The Micromechanics of Creep Behavior in Metallic Glasses

金屬玻璃蠕變行為微力學的研究

Student thesis: Doctoral Thesis

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Award date21 Aug 2018

Abstract

Metallic glass (MG) is a promising structural material due to its high strength, high hardness and large elastic limit. However, brittleness of MG in a bulk form at room temperature severely hinders its applications. Unlike crystalline solids, MGs lack well-defined structural defects that can easily “migrate” to initiate plasticity. Therefore, understanding plasticity initiation in MGs has been a topic of active research for decades, among which the creep study of MGs can contribute to revealing the micromechanical mechanism of plasticity initiation in MGs because of the viscoelastic and/or viscoplastic deformation at the early stage of creep. At the fundamental level, this is related to “flow defect” activation and accumulation.

Recent experiments showed that irreversible structural change or plasticity could occur in MGs even within the apparent elastic limit after a sufficiently long waiting time. To elucidate this delayed yielding phenomenon during creep, a stochastic shear transformation (SST) model is first developed in this thesis based on a unified rate theory. To validate this model, we carried out extensive atomistic simulations on different MG systems. On a fundamental level, an analytic framework is established, which links time, stress and temperature altogether into a general yielding criterion for MGs.

Since the delayed yielding is likely to correlate to the accumulative activation of “flow defects”, it is significant to investigate the atomic motion during plastic flow initiation based on the trajectories of the individual atoms. Here, we apply the molecular dynamics (MD) simulations of creep in a model MG to obtain plenty of dynamic information. Taking count of the mean squared displacement (MSD) of atoms, we are able to discover three distinctive stages of atomic motion which connects viscoelasticity and viscoplasticity, namely, ballistic, caging and diffusive motion. Interestingly, we found that a significant portion of atoms undergo anharmonic and quasi-reversible back-and-forth motion during the stage of caging motion. This quasi-reversible anharmonic motion comes along with the collective rotation of atoms, which resembles local “vortex” as in a sheared liquid and is characterized by the “curl” of the displacement field. More importantly, we demonstrated that the local atom rotation as characterized by the curl shows increasing correlation with local shear strain but is about one order of magnitude larger in intensity. Based on these interesting findings, we proposed a micromechanical mechanism of yielding in MGs which is facilitated by vortex-like atomic motion.

As deformation continues beyond the overall yielding point, the viscoelastic deformation transit to the viscoplastic mode. At room temperature, shear banding is the dominant deformation manner of creep in MGs, which is known to cause local softening in MGs. Different from the prior studies on the shear localization under the unconstrained or slightly constrained condition, we focused on the excessive shear banding caused by deep indentation, which can result in significant creep and abnormal elastic softening during operation, thereby leading to a mechanically induced liquefaction behavior similar to the regular thermally induced one. Interestingly, after the cessation of the shear banding, we observed both submicron-scale softening and hardening regions that alternate in the proximity of the shear bands.

    Research areas

  • Creep, Metallic Glasses, Micromechanics