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

这项研究项目致力于原子长度尺度上的裂纹钝化和伴随缺陷的产生，这些过程深刻地影响结构金属和合金的宏观机械响应。高强度钢、铝合金等在受到低温、应力和/或恶劣化学环境等不利条件的影响时，性能和可靠性会受到影响。这个项目的范围超越了传统的连续介质-机械处理，因为它试图在从宏观到原子的不同长度尺度上适当地模拟材料的行为，同时自我协调地连接各种理论。这种方法的一个优点是，线弹性断裂力学以及与之相关的应力奇异性和经验断裂准则被丢弃，取而代之的是施加在近原子长度尺度上的物理激励准则。具体地说，这项工作将探索：1)在存在预先存在的、明显可移动的位错的情况下，“脆性”裂纹扩展的现象；2)纳米尺度钝化尖锐裂纹的作用；以及3)孪生和复杂的堆积层错形成的机制。这项研究的具体系统包括铝和高强度钢，重点是航空航天应用(例如，老化飞机的开裂情况，以及与涡轮和火箭动力装置相关的极端温度和化学环境)。此外，将开展一项试点计划，探索使用微机械夹具(“MEMS”技术)来测试这项工作中产生的一些概念，和/或测试关键材料的性能。

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.