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

0000142本研究项目在原子长度尺度上研究裂纹钝化和伴随的缺陷产生，这一过程深刻地影响了结构金属和合金的宏观力学响应。高强度钢、铝合金等的性能和可靠性在低温、应力和/或恶劣的化学环境等不利条件下会受到损害。这个项目的范围超越了传统的连续力学处理，因为它试图在从宏观到原子的各种长度尺度上适当地模拟材料的行为，同时自我一致地连接各种理论。该方法的一个优点是摈弃了线弹性断裂力学，以及与之相关的应力奇点和经验断裂准则，而采用了近原子长度尺度的物理驱动准则。具体来说，这项工作将探讨：1)“脆性”裂纹在已有的、明显可移动的位错存在下扩展的现象；2)在塑性变形介质中嵌入无位错区域的尖锐裂纹的纳米钝化作用；3)孪晶和复杂层错形成机理。这些结果有望提高我们对脆性到韧性转变的理解，并为减少结构金属合金脆性破坏的可能性提供实用的方法。本研究中要考虑的具体系统包括铝和高强度钢，重点关注航空航天应用（例如，飞机成像的开裂场景，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）来测试这项工作产生的一些概念，和/或测量关键材料特性。

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.