Creep Micromechanics in Meso-Length Scale Samples

Abstract

This study focuses on the creep response of meso-length scale systems, whose smallest dimensions lie in the range of a few hundred micrometers to a few millimeters. Here, uniaxial creep experiments were performed on commercially pure Al polycrystals using samples having the smallest (or, characteristic sample) dimension of 500 μm to 7 mm in the “five”-power law regime. Compared to the interior, the near-surface region developed creep compliant dislocation substructures, marked by lower dislocation density and larger subgrain size. Furthermore, the extent of this surface-affected region (SAR) inversely varied with applied stress and was limited by the grain size. As SAR became comparable to the characteristic sample dimension (CSD), the creep response of the material was significantly affected by the “weaker” surface, thereby reducing the overall creep resistance. This also resulted in a decrease in stress exponent with a decrease in CSD. The differential creep resistance of the surface and the interior leads to load-shedding between the “weak” surface and the “strong” interior. A microstructure-sensitive iso-strain composite model quantifying this load-shedding effect was formulated to explain the observed creep response across various length scales. The critical insights into the creep micro-mechanics obtained in this study, therefore, seamlessly unify the power-law creep response at large and small length scales.