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Dennis Dimiduk1 2 Michael Uchic3 Paul Shade3

1, The Ohio State University, Columbus, Ohio, United States
2, BlueQuartz Software, Springboro, Ohio, United States
3, Air Force Research Laboratory (AFRL), Wright-Patterson AFB, Ohio, United States

About 15 years ago, plasticity strain bursts were first observed during microcrystal compression deformation experiments. Soon, nano-scale crystals we also explored and these also exhibited strain bursts. Subsequently, the nano- micro-compression technique was applied to a wide variety of both crystalline and glassy materials. New insights into deformation and dislocation ensemble behavior emerged, suggesting that aspects of the kinematics of deformation common to representations of crystalline flow in simulations, are invalid when “size effects” begin to dominate physically small samples. These occur when certain mean-field conditions no longer hold. For example, the forest-hardening model for slip resistance fails as the dislocation ensemble deviates from mean-field multiplication and storage rates. Likewise, the deformation velocity gradient cannot be described as a homogeneous sum of slips since dislocation sources are not uniformly available at small scales and heterogeneous slip localization dominates. For these conditions, the smooth, power-law like strain-rate dependence on stress, gives way to discrete dislocation avalanches. However, aspects of the experiments and inconsistencies between experimental results, simulations, and theory leave lingering open questions. One may even argue that these inconsistencies are leading to a somewhat confusing body of literature that attempts too many generalizations from far too simplified simulations, or from theory that is only reconciled within a narrow set of experiments while neglecting other results that fall outside of the range of study. This presentation focuses on dislocation behavior in crystalline materials tested using the microcrystal compression method. After a brief synopsis of prior results, numerous aspects of the state of the art are considered including strain burst behavior over the loading stage and Stages I-III of FCC metal deformation; maximum avalanche sizes; effects of loading rate; effects of sample size; aspects of materials types; experimental conditions and settings; and the space-time coupling and other aspects of plasticity avalanche phenomena. The work emphasizes well-supported observations versus those aspects needing further investigation.

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