Surface-controlled dislocation multiplication in metal micropillars

Abstract

Understanding the plasticity and strength of crystalline materials in terms of the dynamics of microscopic defects has been a goal of materials research in the last 70 years. The size-dependent yield stress observed in recent experiments of submicrometer metallic pillars provides a unique opportunity to test our theoretical models, allowing the predictions from defect dynamics simulations to be directly compared with mechanical strength measurements. Although depletion of dislocations from submicrometer face-centered-cubic (FCC) pillars provides a plausible explanation of the observed size-effect, we predict multiplication of dislocations in body-centered-cubic (BCC) pillars through a series of molecular dynamics and dislocation dynamics simulations. Under the combined effects from the image stress and dislocation core structure, a dislocation nucleated from the surface of a BCC pillar generates one or more dislocations moving in the opposite direction before it exits from the surface. The process is repeatable so that a single nucleation event is able to produce a much larger amount of plastic deformation than that in FCC pillars. This self-multiplication mechanism suggests a need for a different explanation of the size dependence of yield stress in FCC and BCC pillars.mechanical strength ͉ plasticity

A new regime for mechanical annealing and strong sample-size strengthening in body centred cubic molybdenum

“…2d,e) and those in previous works 4,12 may result from the onset of the dislocation self-multiplication as described in ref 5. However, it is worth noting that the predicted critical stress in ref 5. for smaller pillars (with obvious mechanical annealing observed experimentally) marks the upper bound instead of lower bound of the measured strength.…”

“…Second, mechanical annealing has not been experimentally observed in Mo pillars 23 , even though the concept of mechanical annealing does not contradict Weinberger and Cai's simulation in which a single screw dislocation can be pushed out of the pillar without self-breeding at lower stresses ( < 5.5 GPa in D = 36 nm pillar) 5 . Mechanical annealing 24 refers to the reduction of dislocation density in the deforming volume, when dislocation generation and accumulation is outweighed by dislocation annihilation and escape from the surfaces.…”

“…In contrast, in Mo pillars, it has been shown that dislocation tangles are retained even after extensive plastic deformation 23 . This has been rationalized by MD simulations of Mo pillars as due to dislocation self-multiplication at higher stresses ( > 5.5 GPa in D = 36 nm pillar), arising from the cross-kinks generated along the screw dislocations that have non-coplanar core structure 5,14 . Many in the community believe that mechanical annealing is not possible for BCC metals regardless of sample size.…”

“…T he deformation behaviour of small-volume metals, in particular the effects of sample size (that is, the diameter D of micro-and nano-pillars) on the apparent strength σ, has been attracting considerable attention recently [1][2][3][4][5][6][7] . A number of studies of single-crystal micro-and nano-pillars have demonstrated the 'smaller is stronger' trend, with the flow stress increasing markedly with decreasing sample size.…”

“…Particularly interesting are the behaviours of BCC metals, which in bulk form tend to exhibit significant deviations from FCC metals. Molybdenum (Mo) is a representative and widely studied example [3][4][5][6][12][13][14] of such BCC metals.…”