Beyond Serrated Flow in Bulk Metallic Glasses: What Comes Next?
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Beyond Serrated Flow in Bulk Metallic Glasses: What Comes Next? R. MAAß This manuscript is based on an oral contribution to the TMS 2020 annual meeting and is dedicated to Prof. Peter Liaw, who for decades has shown great interest in serrated plastic flow. Here we will focus on the case of bulk metallic glasses, and begin with briefly summarizing some aspects of serrated and non-serrated inhomogeneous flow—a phenomenon that has perplexed materials scientists for decades. Four directions of research are identified that emerged out of the desire to fundamentally understand the intermittent inhomogeneous flow response. These research directions gear away from the phenomenological stress–strain behavior but put the underlying shear defect into focus. Unsolved problems and future research topics are discussed. https://doi.org/10.1007/s11661-020-05985-w The Author(s) 2020
I.
INTRODUCTION
IN the early two thousands, the topic of serrated versus non-serrated flow in bulk metallic glasses (BMGs) experienced a second wave of interest following the remarkable initial work by Kimura and Masumoto presented more than 20 years earlier.[1–3] Kimura and Masumoto recognized that the appearance of plastic flow in BMGs at low homologous temperatures (the inhomogeneous deformation regime) can be either smooth or intermittent, depending on the applied deformation rate and/or deformation temperature. In essence, this is phenomenologically very similar to dynamic strain aging of, for example, steels[4] or Al-Mg alloys,[5] where the interplay between depinning of dislocations and the re-trapping of them by solutes gives rise to the so-called jerky flow curves for particular combinations of the deformation rate and testing temperature. In fact, our recent efforts suggest that intermittent single-crystal microplasticity is another manifestation of a rather generic coupling between the far-field rate and some underlying deformation kinetics.[6] In this latter case, crystallographic slip is governed by the collective dislocation velocity during the dynamic (also referred to as an avalanche) deformation phase.
R. MAAß is with the Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, and also with the Federal Institute of Materials Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany. Contact e-mail: [email protected]; [email protected] Manuscript submitted May 29, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Whilst the macroscopic emergence of the stress–strain response has some similarities in all these cases (metallic glasses, dynamic strain aging, single crystal microplasticity), the underlying physics is clearly different. Kimura and Masumoto established via tearing, bending, and compression testing of BMGs across different temperatures that two regimes appear in an extension rate vs. 1=T representation. At constant applied deformation rate, serrated flow was observed at higher temperatures, and at lower temperatur
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