CAREER: Mapping the Genome of Metallic Grain Boundaries - Structure, Thermodynamics and Kinetics

North Carolina State University, Raleigh NC

Investigators

Abstract

Non-Technical Abstract: Grain boundaries (GBs) are two-dimensional defects in engineering materials that govern a wide array of phenomena such as diffusivity, conductivity and resistance to failure under extreme environments like high temperatures and corrosive atmospheres. The lack of robust GB structure-property relationships is considered to be one of the biggest obstacles to understanding the behavior of polycrystalline materials. To analyze the properties of GBs, it is necessary to first understand the way in which atoms are arranged at these defects. The atomic level structure of GBs may range from a highly ordered packing to a completely disordered arrangement (such as those observed in metallic glasses) depending on five crystallographic degrees of freedom. To investigate such a diverse array of atomistic structures, the PI and his team will develop a novel geometrical framework to quantify the atomistic structure of GBs as three-dimensional polyhedral units. The objective is to reduce the complexity of GBs to a minimal set of fundamental structural units. Such a classification will facilitate an efficient computation of structure-property relationships. This CAREER award also supports the integration of outreach with research. To this end, the PI and his team, in collaboration with a local Early College High School, will develop demonstrations and computer simulation exercises for high-school students to explore concepts in materials science and engineering. The PI will also train the next generation of secondary school teachers in the scientific process through summer research internships in the PI's lab. The PI will also develop open-source computer modules for teaching and disseminate the simulation tools developed during the research program. Technical Summary: Interfaces play a central role in understanding the processing-structure-property relationships in polycrystalline materials. The mechanisms for the evolution of microstructural features are profoundly influenced by the grain boundary (GB) properties (energies, and mobilites) and their anisotropies. A wide array of failure phenomena such as fatigue, stress corrosion cracking, creep, failure under dynamic and shock loading etc. are controlled by the mechanical properties of GBs in structural materials. Even the design of novel damage resistant structural materials requires the quantification of the structural features and the properties of interfaces. To better understand the atomistic structure and quantify the variation in GB structure as a function of GB crystallography, a new class of fundamental grain boundary structures, which will provide the basis set for the prediction of the structure and energy of a general interface in the complete five-dimensional crystallographic parameter space, will be identified. In order to compute this minimal set of interfaces, the PI and his team will develop an efficient and automated GB structure simulator. GBs of varying crystallographic character in the structurally simple mono-atomic, face-centered cubic metallic systems (Al, Ni and Cu) will be simulated. The Voronoi network and the Delaunay tessellation of the atoms in the GB will be analyzed to compute a three-dimensional polyhedral Structural Unit Model (SUM) and a favored set of GBs will be identified. Reduced order models for thermodynamic properties of GBs will be evaluated with the zero-Kelvin GB energy as a model thermodynamic property. The kinetic properties, such as GB mobility, will be analyzed through the analysis of the soft vibrational modes of the favored set of GBs. This CAREER award also supports the integration of outreach with research. To this end, the PI and his team, in collaboration with a local Early College High School, will develop demonstrations and computer simulation exercises for high-school students to explore the concepts of defects and their role in materials science and engineering. The PI will also train the next generation of secondary school teachers in the scientific process through summer research internships in the PI's lab. The PI will also develop open-source computer modules for teaching concepts pertaining to the crystallographic theory of interfaces and for the dissemination of simulation tools developed during the research program.