The Influence of Microstructure on the Deformation and Failure of Ultrafine-Grained Aluminum.

Abstract

Ultrafine-grained (UFG) aluminum produced by equal channel angular pressing (ECAP) is considerably stronger than its coarse grained counterpart, while maintaining significant ductility. There have been a number of prior investigations into the material’s unique properties, yet the relationships between the heterogeneous microstructure and the microstructure-scale strain localization and active deformation mechanisms are not understood, motivating this work. This research investigates the effect of the material’s heterogeneous microstructure on strain accommodation and clarifies the relationship between macroscopic strain rate sensitivity and the ECAP processed microstructure.

To carry out this research, a new experimental methodology combining scanning electron microscopy (SEM) and digital image correlation (DIC) was developed and utilized. We devised new nano-scale surface patterning techniques, improved SEM micrograph image distortion correction methodologies, and provided a better understanding of the effects of SEM imaging parameters on noise in DIC data. This work significantly improved this methodology and enabled highly accurate displacement measurements.

Application of this experimental methodology to UFG 99.99% pure aluminum was used to characterize the relationships between the microstructure and active deformation mechanisms. Analysis of room temperature in-situ tension tests revealed that dislocation slip was the primary deformation mechanism in large grains and in grains separated by low angle grain boundaries. In regions of microstructure possessing ultrafine grains separated by high angle grain boundaries, strain localized primarily at grain boundaries. Grain boundary sliding was active at high angle grain boundaries separating distinct banded microstructure features.

Variable strain rate experiments carried out at 200°C revealed the microstructural features responsible for the UFG 99.99% pure aluminum’s enhanced strain rate sensitivity. Dislocation slip, active in large grains and grains with similarly oriented slip systems, limited strain rate sensitivity. High angle grain boundaries, particularly those separating banded microstructure features, showed the greatest strain rate sensitivity.