Nanomechnics of crystalline materials : experiments and computations

Abstract

In this thesis, experimental, computational and analytical approaches are employed to examine systematically the mechanisms of deformation in crystalline materials. Such insight can be used to exploit and avoid contact in actuator and sensor applications, to derive mechanical properties for engineering of materials, and to investigate the fundamental role of defects. Here, localized mechanical contact of material surfaces is utilized to elucidate the effects of length scales on the transition from elastic (reversible) to plastic (irreversible) deformation. As the mechanical response of a material can be described by parameters which range from empirical constitutive (stress-strain) relations to fundamental descriptions of atomic interactions, the deformation response can be related to global mechanical properties such as yield strength, as well as to local phenomena such as dislocation nucleation. The concurrent design and implementation of experiments including micro- and nanoindentation and uniaxial compression, in situ experiments on a model, two-dimensional crystalline analogue, and computational modeling at the continuum (finite element) and atomistic (molecular dynamics) levels presented herein provide a unique opportunity to develop and validate hypotheses and analytical algorithms. Indeed, one of the major conclusions of this thesis is that the mechanical response observed for a specific volume of material under contact is a unique function of the deformation mechanisms described within that length scale regime.

Ultimately, the goal of this thesis is to provide a synergystic interpretation of deformation in crystalline materials by examining in detail the operative mechanisms under local, finite strain. This interpretation has been attained at the continuum level via development and experimental verification of a closed-form set of algorithms which convert an experimental indentation response into a set of elastic and plastic mechanical properties, and also predict the indentation response of a material via a corresponding set of mechanical properties. Modifications of this continuum interpretation under conditions of finite material thickness and residual stress profiles elucidate explicitly the effect of material length scales. At the atomistic level, this interpretation of deformation is framed in terms of an energetic elastic instability criterion which is validated experimentally and computationally for a particularly important instability: dislocation nucleation. Finally, the effects of material length scales such as grain size on the onset and development of dislocation-mediated deformation.