EAGER: Controlling Microstructure for Strong and Damage Tolerant Nanocrystalline Metals

纳米晶金属和合金(晶粒度小于~100 nm的多晶)为结构应用提供了一系列诱人的机械性能，包括高强度和高硬度、增强的抗疲劳性能和摩擦学稳定性。这些优点来自于存在于相邻晶体之间的界面上的大部分材料，即所谓的晶界。对于这些材料，这种高比例的界面体积可能会导致诸如热不稳定和相对较差的损伤容忍度等有害影响。纳米晶金属的许多现在和未来的应用，如坚固的涂层、电互连、微纳机电系统和软磁体，使这些材料受到极端的机械压力，可以激活微观结构转变，改变有益材料的性能。这一早期概念探索性研究(AGER)奖支持围绕以下概念的研究：纳米晶合金中的晶界化学控制可用于定制纳米晶材料的热稳定性和机械稳定性，以防止在极端服务环境中的晶界迁移。对这一行为的控制可以实现前所未有的损伤容限控制，从而实现一种新颖而廉价的结构材料设计策略。在本研究计划中，研究人员的目标是控制纳米晶合金中的晶界化学，作为一种手段来编码在使用条件下热和机械驱动的晶界迁移的开始。在遇到极端机械环境的情况下(例如，在应力集中的情况下，如裂纹尖端)，可以触发应力驱动的晶界迁移以响应损伤，从而赋予材料损伤容限。这种动态的材料响应是基于局部应力触发的，该局部应力触发驱动微结构转变并耗散能量以减少灾难性失效，从而兼顾强度和韧性。这项研究将通过下列科学和技术目标完成：(A)确定和表征导致机械诱导晶界迁移和晶界生长的机制，(B)确定和表征这些机制受晶界化学影响的方式，(C)确定将偏析到晶界并调节热和应力驱动的晶界迁移的元素，(D)利用定制的晶界化学合成纳米晶合金，以及(E)进行材料表征和定量的原位力学测试。

Nanocrystalline metals and alloys (polycrystals with grain sizes less than ~100 nm) offer a suite of appealing mechanical properties for structural applications, including high strength and hardness, enhanced fatigue resistance, and tribological robustness. These virtues derive from the large fraction of material that resides at the interfaces between neighboring crystals, known as grain boundaries. For these materials, this high fraction of interfacial volume can cause deleterious effects such as thermal instability and relatively poor damage tolerance. Many present and future applications of nanocrystalline metals such as robust coatings, electrical interconnects, micro- and nano-electro-mechanical systems, and soft magnets subject these materials to extreme mechanical duress, which can activate microstructural transformation and alter the beneficial materials properties. This EArly-concept Grant for Exploratory Research (EAGER) award supports research centered on the concept that control of grain boundary chemistry in nanocrystalline alloys can be used to tailor the thermal and mechanical stability of nanocrystalline materials against grain boundary migration in extreme service environments. Control over this behavior can allow for unprecedented control of damage tolerance, thus enabling a novel and inexpensive structural materials design strategy.In this research program, the investigators aim to control grain boundary chemistry in nanocrystalline alloys as a means to encode the onset of thermally- and mechanically-driven grain boundary migration under service conditions. In cases where extreme mechanical environments are encountered (e.g. at stress concentrations such as crack tips), stress-driven grain boundary migration can be triggered to respond to damage, endowing the material with damage tolerance. This dynamic material response is predicated on local stress triggers that drive microstructure transition and dissipate energy to mitigate catastrophic failure, allowing for both strength and toughness. The research will be accomplished via the following scientific and technical goals: (a) identify and characterize the mechanisms that lead to mechanically-induced grain boundary migration and grain growth, (b) identify and characterize the manner in which these mechanisms are influenced by grain boundary chemistry, (c) identify elements that will segregate to grain boundary and modulate thermal and stress-driven grain boundary migration, (d) synthesize nanocrystalline alloys with tailored grain boundary chemistry, and (e) perform material characterization and quantitative in situ mechanical testing.