Intermittent plasticity in the presence of complex microstructures

Rizzardi, Quentin Pierre Gabriel

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https://hdl.handle.net/2142/115467 Copy

Description

Title

Intermittent plasticity in the presence of complex microstructures

Author(s)

Rizzardi, Quentin Pierre Gabriel

Issue Date

2022-04-15

Director of Research (if dissertation) or Advisor (if thesis)

Maass, Robert

Doctoral Committee Chair(s)

• Maass, Robert
• Bellon, Pascal

Committee Member(s)

• Sehitoglu, Huseyin
• Shoemaker, Daniel

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Materials Science
• Plasticity
• Metallurgy
• Mechanics

Abstract

The main tenets of dislocation theory assume that dislocation behavior in the crystal structure is statistically Gaussian and can be accurately determined by taking the average behavior over a sufficiently large scale. Since discontinuous strain bursts had already been observed at the time, the theory was known to be incomplete from the outset, but it took the advent of more advanced micro-scale experimental setups to study these bursts in detail, and finally understand the core dislocation mechanism behind this intermittent plasticity: critical dislocation networks that move as a collective in a stochastic fashion and are devoid of scale and therefore a deviation from Gaussian statistics. Here, early results demonstrated the applicability of mean-field models, which in turn suggested universal scale-free intermittent flow characterized with well-defined scaling exponents of the underlying avalanche statistics. More recent experiments, however, nurture the opposite view: the behavior of dislocation dynamics is indeed materials-dependent and suspected to be influenced by microstructural changes. On the basis of these results, a logical follow-up is to further detail the nature of intermittent slip and use that knowledge to incorporate defining aspects of microstructure in our description of intermittent plasticity. To this end, we conduct in-situ compression of micro-scale crystals for all experiments. Our first goal is to track slip formation as discontinuous mechanical events are recorded, and to determine the potential existence of a “unit event” with larger events being the result of simultaneous activation of separate unit events. While no unit event was found, we find correlating trends between the localization of slip at the surface and the dynamics of its mechanical trace: the creation of new slip lines tends to result in events with singular, sharp velocity profiles, while reactivation of existing lines shows a more sluggish behavior. We then consider the introduction of microstructural complexity in the context of intermittent plasticity. First, we study the impact of temperature in a BCC crystal structure in which smooth dislocation glide is known to be thermally activated. Our experiments show some key differences between Gaussian and intermittent non-Gaussian dislocation behavior: while dislocation avalanches show temperature dependence in their size statistics, they are otherwise dynamically athermal. Our second microstructural consideration is the presence of innate lattice distortion due to chemical and topological disorder. We trace the spatio-temporal dislocation avalanche dynamics in a prototypical high-entropy alloy (HEA). All parameters describing the collective correlated dislocation dynamics reveal no distinguishable differences to a pure FCC reference crystal, questioning the belief that dislocation motion is generally impeded by such lattice distortions. Finally, we also consider the introduction of precipitates with the study of an AlCu alloy with a well-described precipitation sequence. We find that solid solution, short-range ordering and precipitates do have an impact on event statistics, although that impact cannot be directly linked to precipitate size or distribution. Rather, our main finding is that shearability, as well as the ratio of precipitate-to-sample size, are the main driving force behind changes in dislocation avalanche behavior and may indeed almost completely negate them. This dissertation presents these results as proofs of the importance of microstructure in materials design and in our fundamental understanding of plasticity. We would expect future work to focus on other potential microstructural factors and their impact on deformation and/or failure behavior, for instance grain size or orientation in polycrystalline materials. We also believe that introducing these material-dependent findings into plasticity models is an important step to achieve truly accurate simulation models of plastic behavior.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Quentin Rizzardi

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