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

纳米晶金属是由尺寸在100纳米或更小的颗粒(微晶)组成的材料。细小的颗粒尺寸赋予了这些材料一些理想的性能，包括与传统金属相比非常高的强度和更高的韧性。然而，许多常用的制备纳米晶金属的方法导致了高度不均匀的微观结构，其颗粒的尺寸和取向分布很广。这种非均质微结构导致其力学性能变化很大，从而限制了其实际应用。该奖项支持使用实验和建模相结合的方法来预测和量化非均质纳米晶金属机械性能的不确定性的基础研究。能够可靠和准确地预测纳米晶金属的行为将加速它们在技术和科学结构应用中的采用，这将直接造福美国经济和社会。这项研究涉及多学科方法，包括先进的微制造、材料科学和力学，并将导致代表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。具有宽或双峰晶粒度分布的纳米晶金属通常表现出最好的强度和塑性组合，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，有必要同时考虑它们固有的微观结构异质性和纳米尺度塑性的随机性质。受此启发，一种新的随机晶体塑性框架将被用来模拟非均质纳米晶金属的变形。该模型将明确考虑纳米晶金属中晶粒度和取向的变化以及位错猝发的统计。利用具有可控微结构的纳米金属薄膜的新型原位电子显微镜实验，可以直接获得模型的输入参数，如临界分辨剪应力的分布。模型的预测能力将用独立的原位X射线衍射实验数据进行验证。

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.