Abstract
Plasticity is often controlled by dislocation motion, which was first measured for low pressure, low strain rate conditions decades ago. However, many applications require knowledge of dislocation motion at high stress conditions where the data are sparse, and come from indirect measurements dominated by the effect of dislocation density rather than velocity. Here we make predictions based on atomistic simulations that form the basis for a new approach to measure dislocation velocities directly at extreme conditions using three steps: create prismatic dislocation loops in a near-surface region using nanoindentation, drive the dislocations with a shockwave, and use electron microscopy to determine how far the dislocations moved and thus their velocity at extreme stress and strain rate conditions. We report on atomistic simulations of tantalum that make detailed predictions of dislocation flow, and find that the approach is feasible and can uncover an exciting range of phenomena, such as transonic dislocations and a novel form of loop stretching. The simulated configuration enables a new class of experiments to probe average dislocation velocity at very high applied shear stress.
Highlights
Plasticity is often controlled by dislocation motion, which was first measured for low pressure, low strain rate conditions decades ago
Molecular dynamics (MD) simulations have already been used to simulate the motion of a single “straight” dislocation at high stresses[9,10], but here we study dislocation-loop motion at high stress, and qualitatively new mechanisms arise
Controlled introduction of dislocation loops could be achieved in several ways, and here we focus on nanoindentation, where dislocation density and spatial extent can be measured from transmission electron microscopy (TEM) imaging or could be estimated indirectly by the size of pile-ups or the number of pop-ins in the loading curve up to modest penetrations
Summary
Plasticity is often controlled by dislocation motion, which was first measured for low pressure, low strain rate conditions decades ago. Many applications require knowledge of dislocation motion at high stress conditions where the data are sparse, and come from indirect measurements dominated by the effect of dislocation density rather than velocity. We make predictions based on atomistic simulations that form the basis for a new approach to measure dislocation velocities directly at extreme conditions using three steps: create prismatic dislocation loops in a near-surface region using nanoindentation, drive the dislocations with a shockwave, and use electron microscopy to determine how far the dislocations moved and their velocity at extreme stress and strain rate conditions. The collected information is sparse, and from indirect measurements like perturbation of growth in Rayleigh-Taylor experiments[5], or lateral lattice relaxation measured with x-ray diffraction[6,7,8] These experiments are dominated by the effect of dislocation density rather than velocity. Since the seminal work by Frenkel and Kontorova in 193813, Frank and van der Merwe in 194914 and Weertman in 196715, the issue of transonic and supersonic www.nature.com/scientificreports/
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