Theory and Simulation of the Transition from Amorphous to Nanocrystalline Mechanical Response

Johns Hopkins University, Baltimore MD

Investigators

Abstract

TECHNICAL SUMMARY This award supports theoretical and computational research on plastic deformation, an inherently non-equilibrium process. The PI intends to advance understanding of plastic deformation in non-crystalline solids to materials with increasing degrees of structural order. Non-crystalline solids find industrial application as metals, ceramics, semiconductors and polymers, but the fact of their disorder has discouraged the development of adequate theories for their deformation. Recently large-scale atomistic simulation has allowed dramatic progress in analyzing the structural disorder in these solids and in relating their mechanical properties to their structural evolution. In the process constitutive laws have been developed that use concepts from non-equilibrium statistical physics to connect macroscale behavior to atomic scale structure. These constitutive laws differ from existing relations insofar as they make reference to certain temperature-like intensive variables that quantify the structure. These structural parameters can be independently measured in simulation to validate the relations. Evidence of the success of these methods has been established by predictions of the mechanical response of metallic glass, an emerging structural material, both during homogeneous flow near the glass temperature and during the development of plastic localization at low temperatures. This research project will extend these investigations to include partially crystalline and nanocrystalline solids. Examining a continuum of structures over this range will test the generality of these theories of deformation for predicting plastic behavior in partially ordered solids. This will lead to a greatly increased understanding of deformation and failure in materials with varying degrees of disorder. This computational and theoretical research program will be integrated with an educational program at Johns Hopkins University (JHU) that addresses a critical need to integrate computational methods into the Materials Science and Engineering core curriculum. This will be done in the context of courses on kinetics, phase transformations, mechanics of materials and physical properties of materials. The PI is continuing to develop a course on the graduate level covering computational materials science methods for molecular simulation. In addition the PI has a history of involving undergraduates in research. This project will involve both JHU undergrads and undergraduates recruited through the NSF MRSEC and PREM programs at JHU that bring in students from around the U.S. and majority-minority institutions. NON-TECHNICAL SUMMARY This award supports research that combines simulation and theoretical statistical physics to investigate how materials deform when stressed and to develop a framework to predict plastic behavior. When a small force is applied to a material, a material will bend or deform in such a way that the material will spring back to its original size and shape when the force is removed. As the force is increased, a point is reached where the material deforms and no longer springs back to original size and shape when the force is removed. The PI aims to understand how this plastic deformation occurs in a range of materials from metals that are a mosaic of tiny crystals the size of a few nanometers to amorphous metals where the atoms are not arranged in any apparent pattern. The PI aims to directly address issues critical to the development of emerging new materials with potential applications due to their high strength and hardness. By making a strong connection between the structure of the material and the resulting mechanical properties, these investigations will provide predictive theories that can be used to analyze the connection between processing, structure and properties and the onset of precursors to materials failure. These investigations will increase understanding beyond subject metals to other materials including glassy polymers, granular media, colloids and the processes that accompany friction. This computational and theoretical research program will be integrated with an educational program at Johns Hopkins University (JHU) that addresses a critical need to integrate computational methods into the Materials Science and Engineering core curriculum. This will be done in the context of more traditional courses on materials. The PI is continuing to develop a course on the graduate level covering computational materials science methods for molecular simulation. In addition the PI has a history of involving undergraduates in research. This project will involve both JHU undergrads and undergraduates recruited through the NSF MRSEC and PREM programs at JHU that bring in students from around the U.S. and majority-minority institutions.