Understanding the Deformation of Heterogeneous Nanocrystalline Metals - Integrating in situ Experiments with Stochastic Crystal Plasticity

纳米晶体金属是由尺寸为100纳米或更小的晶粒（微晶）组成的材料。与传统金属相比，小的晶粒尺寸赋予这些材料几种期望的性质，包括非常高的强度和增加的韧性。然而，许多常用的方法来生产纳米晶金属导致高度不均匀的微观结构，具有宽的晶粒尺寸和取向分布。这种异质的微观结构导致它们的机械性能的大的可变性，这限制了它们的实际应用。该奖项支持基础研究，使用实验和建模相结合的方法来预测和量化异质纳米晶金属机械性能的不确定性。可靠和准确地预测纳米晶金属行为的能力将加速其在技术和科学结构应用中的采用，这将直接有利于美国经济和社会。这项研究涉及多学科方法，包括先进的微加工，材料科学和力学，并将导致代表性不足的群体更广泛地参与研究，并对工程教育产生积极影响。具有宽或双峰晶粒尺寸分布的纳米晶金属通常表现出强度和延展性的最佳组合，因此从工程角度来看非常有吸引力。然而，为了准确地预测这些材料的力学响应，有必要考虑其内在的微观结构的异质性和纳米塑性的随机性。受此启发，一个新的随机晶体塑性框架通知原位实验将被用来模拟非均质纳米晶金属的变形。该模型将明确考虑晶粒尺寸和取向的变化，以及纳米晶金属中位错破裂的统计。新的原位透射电子显微镜实验纳米晶金属薄膜与控制的微观结构将被用来直接获得输入参数的模型，如临界分解剪切应力的分布。该模型的预测能力将与独立的原位X射线衍射实验的数据进行验证。

Nanocrystalline metals are materials composed of grains (crystallites) whose sizes are of the order of 100 nanometers or less. The small grain size endows these materials with several desirable properties including very high strength and increased toughness compared to conventional metals. However, many commonly used methods to produce nanocrystalline metals lead to highly nonuniform microstructures with a wide distribution in the size and orientation of grains. Such heterogeneous microstructures lead to a large variability in their mechanical properties, which limits their practical use. This award supports fundamental research to predict and quantify uncertainty in the mechanical properties of heterogeneous nanocrystalline metals using a combined experimental and modeling approach. The ability to reliably and accurately predict the behavior of nanocrystalline metals would hasten their adoption in technological and scientific structural applications, which will directly benefit the U.S. economy and society. This research involves a multi-disciplinary approach comprising of advanced microfabrication, materials science and mechanics and will lead to broader engagement of underrepresented groups in research and positively impact engineering education. Nanocrystalline metals with a wide or bimodal grain size distribution often exhibit the best combination of strength and ductility and hence are very attractive from an engineering perspective. However, to accurately predict the mechanical response of these materials it is necessary to consider both their intrinsic microstructural heterogeneity and the stochastic nature of nanoscale plasticity. Motivated by this, a new stochastic crystal plasticity framework informed by in situ experiments will be used to model the deformation of heterogeneous nanocrystalline metals. The model will explicitly take into account both the variation in grain sizes and orientations as well as the statistics of dislocation bursts in nanocrystalline metals. Novel in situ transmission electron microscopy experiments on nanocrystalline metal films with controlled microstructures will be used to directly obtain input parameters for the model such as the distribution of critical resolved shear stresses. The predictive capability of the model will be verified with data from independent in situ x-ray diffraction experiments.