Plasticity at the micron scale

Abstract

Over a scale which extends from about a fraction of a micron to tens of microns, metals display a strong sizedependence when deformed non uniformly into the plastic range: smaller is stronger. This eect has important implications for an increasing number of applications in electronics, structural materials and MEMS. Plastic behavior at this scale cannot be characterized by conventional plasticity theories because they incorporate no material length scale and predict no size eect. While micron sized solid objects are too small to be characterized by conventional theory, they are usually too large to be amenable to analysis using approaches presently available based on discrete dislocation mechanics. The relatively large numbers of dislocations governing plastic deformation at the micron scale motivate the development of a continuum theory of plasticity incorporating size-dependence. Strain gradient theories of plasticity have been developed for this purpose. The motivation and potential for such theories will be discussed. Important open issues surrounding the foundations of strain gradient plasticity will also be addressed and a few critical experiments identi®ed. #

Incorporating strain gradient effects in a multiscale constitutive framework for nickel-base superalloys

“…At these length scales, the material strength is observed [1] to be size dependent, with an increase of strength at decreasing dimensions, i.e. smaller is stronger.…”

Nanoscale friction and wear maps

“…Many studies have shown a strong size or scale dependence for mechanical properties such as indentation hardness Bhushan 1998Bhushan , 1999aBhushan -c, 2007aNix & Gao 1998;Hutchinson 2000), tensile strength (Hutchinson 2000) and bending strength (Sundararajan & Bhushan 2002), indicating that the bulk properties of many materials differ from those on the micro/nanoscale. The scale invariance of the theory of linear elasticity and the conventional plasticity theories has lead to the formulation of the strain-gradient plasticity theory (Fleck et al 1994;Nix & Gao 1998;Gao et al 1999;Huang et al 2000;Hutchinson 2000). The theory, developed for microscale deformation, predicts a dependence of mechanical properties on the strain gradient, which is scale dependent.…”

An Overview of Innovative Strategies for Fracture Mechanics at NASA Langley Research Center

“…At the microscale, near-crack-tip plasticity is dominated by the presence of large plastic strain gradients and the corresponding geometrically necessary dislocations (GNDs). The effect of GNDs on conventional plasticity formulations is overviewed by Hutchinson 16 . In this case, a critical issue is the underestimated work hardening during plastic deformation within the strain gradient dominated field.…”