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

Research project


Overview: Most continuum models of plasticity in nanocrystalline (NC) metals incorporate size eects primarily through the mean grain size (d). However, microstructural heterogeneity (varia- tion in size and orientation of grains) is an intrinsic feature of NC metals. We hypothesize, based on preliminaryndings, that microstructural heterogeneity, and not merely d, dictates the plastic behavior of NC metals. The proposed work will test this hypothesis by combining in situ TEM and XRD experiments on NC metallms (d = 50 nm, thickness 0.1-1 m) with a microstruc- turally informed crystal plasticity model to elucidate the eect of heterogeneity. The distinctive feature of the model will be the use of grain clusters (large grains surrounded by smaller grains) as microstructural units representative of the bulk. Specically, we will 1) perform quantitative in situ TEM and XRD straining experiments, us- ing novel MEMS devices, on NC Al and Culms with dierent textures (bicrystalline, random) synthesized using sputtering and electrodeposition. The experiments will provide microstructural information and stress distribution at the local (TEM) as well as global scale (XRD). 2) Develop a cluster based crystal plasticity model that incorporates actual grain size and orientation distribu- tions. The model will explicitly calculate both the average stress-strain response and the internal stress and strain distribution within grain clusters that mimic real microstructures. The interaction between clusters will determine the macroscopic ow stress, elastic-plastic transition, residual stress and plastic recovery of the NC samples. The research will lead to fundamental understanding of plastic ow and stress distribution in realistic NC structures and provide critical microstructural data and mechanistic parameters for the development and validation of computational models. Intellectual Merit: The eect of microstructural heterogeneity on the plastic behavior of NC metals has been largely unexplored, both experimentally and computationally. This proposal will address this fundamental knowledge gap by 1. Directly measuring the critical strengths and stress distribution as a function of grain size and orientation using in situ TEM and XRD experiments on metallms with carefully controlled microstructures. Since the overall stress-strain response will be monitored simultaneously, the experiments will also allow us to correlate macroscopic behavior with microscale mechanisms. 2. Developing a crystal plasticity model based on grain clusters that mimic experimental microstruc- tures. The key input parameters for the model such as the distribution of critically resolved shear stress and grain level stress will come directly from the in situ experiments. This close coupling with experiments will ensure a rigorous validation of the model and allow us to quantify and predict accurately the eects of microstructural heterogeneity. Broader Impact: NC metals, in general, exhibit limited ductility. An exception to this limitation are materials with a bimodal distribution of ultrane (100-500 nm) and NC (<100 nm) grains. Thus, these materials are intrinsically heterogeneous. However, a consistent framework to model and optimize their microstructures is currently lacking. The proposed crystal plasticity framework, by explicitly incorporating microstructural heterogeneity, will critically advance the analysis and design of heterogeneous NC metals with both high strength and ductility. Research will be inte- grated with outreach to increase participation of high school students in science and engineering elds. Using existing initiatives like the Mathematics, Engineering, Science Achievement (MESA) program at ASU, students from underrepresented groups will be recruited each year to attend a short (1-2 day) course that will cover the basics of mechanics and materials characterization. The students will also perform simple mechanical tests both at the microscale (using MEMS devices) as well as the macroscale in the PI's and co-PI's labs. The graduate students working on this project will be exposed to unique interdisciplinary research comprising of MEMS, in situ material charac- terization/testing and computational modeling. The results from the project will be disseminated through peer-reviewed publications, conference presentations and online databases like Matforge. 1
Effective start/end date6/1/146/30/17


in situ
transmission electron microscopy
X-ray diffraction
grain size
residual stress
shear stress