Role of Atomic-Scale Crack Blunting on the Ductile Versus Brittle Response of Metals

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

0000142 This research project addresses crack blunting and concomitant defect generation at the atomic length scale, processes that profoundly impact the macroscopic mechanical response of structural metals and alloys. The performance and reliability of high strength steels, aluminum alloys, etc., are compromised when subject to adverse conditions such as low temperature, stress, and/or harsh chemical environments. The scope of this project goes beyond traditional continuum-mechanical treatments, in that it attempts to appropriately model material behavior at the various length scales from macroscopic to atomistic, while self-consistently bridging the various theories. One advantage of the approach taken is that linear elastic fracture mechanics, as well as the stress singularities and empirical fracture criteria associated therewith, are discarded in favor of physically-motivated criteria imposed at the near-atomic length scale. Specifically, this effort will probe: 1) the phenomenon of "brittle" crack growth in the presence of pre-existing, apparently mobile, dislocations; 2) the role of nanoscale blunting of a sharp crack propagating through a dislocation-free zone, embedded in a plastically deforming medium; and 3) the mechanics of twinning and complex stacking fault formation. The results are expected to improve our understanding of the brittle-to-ductile transition and to yield practical methods for reducing the likelihood of brittle failure of structural metallic alloys. The specific systems to be considered in this research include aluminum and high strength steels, with a strong focus on aerospace applications (e.g., cracking scenarios in aging aircraft, and the extreme temperature and chemical environments associated with turbine and rocket powerplants). In addition, a pilot program to explore the use of micromachined fixtures ("MEMS" technology) to test some of the concepts arising from this work, and/or to measure key materials properties, will be undertaken.