Dislocation dynamics and plasticity in micropillars and thin films.

摘要

Mechanical strength in bulk materials is a size-independent property of the material. However, in the micro to nanometer range, strength becomes dependent on the material size. Strain gradient plasticity theory has been used to explain the size dependent material behavior observed in micro-indentation experiments. However, recent micro-compression experiments on single crystals pillars has shown that the strength of the pillars is dependent on size even in the absence of strain gradients. This requires a new explanation for the observed size effect.;In crystalline solids, strength can be described as the materials ability to resist plastic deformation, which is related to the motion of dislocations. Dislocation dynamics simulations can be used to track the evolution of dislocation structures and hence predict strength in crystalline solids. However, to simulate dislocations in small volumes, the free surfaces must be accounted for accurately. To do this, these simulation codes must be modified to correct for the image stress, which is the difference between the bulk and microcrystal stress fields. These image stress fields are computed using a spectral method which can both quickly and accurately capture the effects of free surfaces for both thin films and cylinders. These methods are needed since dislocation dynamics codes track the evolution of complicated dislocation structures over millions of time steps. Dislocations intersecting free surfaces are accounted for by using a combination of techniques including virtual dislocations and the Yoffe correction.;The possibility of dislocation starvation in micro-pillars is investigated using a combination of dislocation dynamics and molecular dynamics In FCC gold micro-pillars, dislocations are shown to leave the pillars quickly making starvation a likely explanation for the observed size effect. However, in BCC molybdenum micro-pillars, dislocations are able to self replicate. This means that a single dislocation, once nucleated, can generate a significant amount of plastic deformation and that BCC microcrystals are unlikely to be starved. Furthermore, this provides a plausible explanation of recent micro-compression tests on molybdenum alloy pillars that collapse under high stress.;The effects of strain gradients are also investigated through molecular dynamics simulations of gold nanowires in torsion. Plasticity in these wires is observed to be strongly dependent on the orientation of the wires. The orientation specifically can affect the homogeneity of the plastic deformation. Wires oriented along the (110) axis deform homogeneously with the nucleation of coaxial dislocations, similar to Eshelby twist, while wires oriented along the (100) and (111) axis deform heterogeneously with the formation of twist boundaries.