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High-rate dislocation motion in stable nanocrystalline metals Jeffrey T Lloyd1,a) 1

Impact Physics Branch, US Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005-5066, USA Address all correspondence to this author. e-mail: [email protected]

a)

Received: 6 December 2018; accepted: 30 January 2019

Dislocation-mediated plasticity in stable nanocrystalline metals, where grain boundary motion is suppressed, is revisited in the context of dislocation elastodynamics. The effect of transient waves that emanate from the generation and motion of dislocations is quantified for an idealized Cu–10 at.% Ta system with grain sizes on the order of 50 nanometers. Simulations indicate that for this material, as dislocation velocities approach 0.6– 0.8 times the shear wave speed, grains several grain diameters away from the initial glide event experience a large transient shear stress for a finite duration. These transient shear stresses increase with increasing glide velocity and can activate nucleation sites far from the original nucleation event. These considerations are used to explain recent experimental observations of a lack of increase in flow stress with increasing loading rate, as well as localization resistance, in this class of stable nanocrystalline metals.

Introduction The mechanical response of most engineering metals subjected to abrupt, dynamic loads is dictated by the behavior of dislocations. The strain rate is related to the density of gliding dislocations, denoted Nm, through the Orowan equation [1]. e_ p ¼

Nm bv M

;

ð1Þ

where b is the Burgers vector, v is the mean velocity of gliding dislocations, and M is the appropriate Taylor factor. Constitutive theories that connect the density of mobile dislocations and their associated mean velocity are able to reproduce most salient features of dynamic deformation in metals, despite neglecting microstructural features such as grain size. As grain sizes approach the nanocrystalline regime, approximately 10–100 nm, the volume fraction of grain boundaries and material near grain boundaries is significant. In conventional nanocrystalline materials, grain boundary motion via sliding, shuffling, and diffusion can account for a significant portion of plastic deformation. The increased role of grain boundary–mediated plasticity with decreasing grain sizes has been provided as a rationale for the higher strain rate and temperature sensitivity observed in nanocrystalline FCC metals than their coarse-grained counterparts [2, 3, 4, 5, 6]. Even at

ª Materials Research Society 2019

cryogenic temperatures, where diffusional processes are frequently suppressed, grain boundary motion is still observed in nanocrystalline Cu [7]. Recently, a new class of stable nanocrystalline alloys has been developed, wherein grain boundaries are intentionally pinned by solute clusters of large atoms [8, 9, 10]. Unlike pure nanocrystalline metals, grain boundaries in stable nanocrystalline alloys remain pinned up to a significant portion of the melting temperature [11, 12].