Molecular Dynamics Simulations of Dislocation Mobility in the Transonic Regime
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2023-10-19
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Abstract
Determining dislocation mobility, i.e., the relationship between dislocation velocity and shear stress, is essential for predicting material response to external loading. However, dislocation mobility is not fully understood, especially for dislocations moving near or above a material's lowest sound speed. We use molecular dynamics to investigate the effect of free surfaces on the mobility of an edge, a screw, and a twinning dislocations in single crystal copper. We find that the upper limit of dislocation velocity depends on the dislocation type. By correlating instantaneous velocities v and resolved stresses tau, we show that the mobility law of each dislocation type can be decomposed into piece-wise continuous ranges of uniform motion separated by bands of forbidden velocity. The lower limiting velocity for uniform motion of all dislocations is the Rayleigh wave speed. When forced to propagate at an average velocity greater than c^R, dislocations exhibit periodic, intermittent, discrete jumps in instantaneous velocity between c^R and higher velocity branches of the mobility law. Bands of forbidden velocity may be calculated directly from the velocity dependence of dislocation drag coefficients. These findings provide atomistic insights of plastic deformation near free surface under high strain-rate loading.
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Dislocation mobility, Molecular dynamics, Rayleigh wave, forbidden velocity