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

点击翻译按钮获取中文摘要

0000142This research project addresses crack blunting andconcomitant defect generation at the atomic length scale,processes that profoundly impact the macroscopicmechanical response of structural metals and alloys. Theperformance and reliability of high strength steels,aluminum alloys, etc., are compromised when subject toadverse conditions such as low temperature, stress, and/orharsh chemical environments. The scope of this projectgoes beyond traditional continuum-mechanical treatments,in that it attempts to appropriately model material behaviorat the various length scales from macroscopic to atomistic,while self-consistently bridging the various theories. Oneadvantage of the approach taken is that linear elasticfracture mechanics, as well as the stress singularities andempirical fracture criteria associated therewith, arediscarded in favor of physically-motivated criteria imposedat the near-atomic length scale. Specifically, this effort willprobe: 1) the phenomenon of "brittle" crack growth in thepresence of pre-existing, apparently mobile, dislocations; 2)the role of nanoscale blunting of a sharp crack propagatingthrough a dislocation-free zone, embedded in a plasticallydeforming medium; and 3) the mechanics of twinning andcomplex stacking fault formation. The results are expectedto improve our understanding of the brittle-to-ductiletransition and to yield practical methods for reducing thelikelihood of brittle failure of structural metallic alloys.The specific systems to be considered in this researchinclude aluminum and high strength steels, with a strongfocus on aerospace applications (e.g., cracking scenarios inaging aircraft, and the extreme temperature and chemicalenvironments associated with turbine and rocketpowerplants). In addition, a pilot program to explore theuse of micromachined fixtures ("MEMS" technology) totest some of the concepts arising from this work, and/or tomeasure key materials properties, will be undertaken.