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.
One way that materials engineers design metals that optimize strength and toughness is by varying metal processing to alter the degree to which the arrangement of atoms in the metal is ordered or disordered. At the extremely ordered end of the spectrum one can create a single crystal, which can be very tough but susceptible to deformation. At the other end of the spectrum is the metallic glass in which atoms are arranged in an apparently random configuration. Metallic glasses are typically very strong, which is to say that they can support large forces before deforming, but they also suffer from failure mechanisms that can make them behave in a brittle manner. In this project Falk and collaborators investigated the ways in which materials that span the range from crystal to glass resist deformation under high loading and withstand failure when pushed to the brink by external forces. In one study computer simulations were used to subject a metallic glass to conditions that prevail at the tip of the crack, where stresses are most extreme. At the site where fracture begins, a vacant space—a bubble—is left behind, a process known as cavitation. The simulations revealed that these bubbles emerge in a way that can be well predicted by classical theories, but that the bubble formation also competes with attempts by the glass to reshuffle its atoms to relieve the stress. That second process is known as shear transformation. As the glass responds to pressure, which of the two processes has the upper hand—bubble formation or shear transformation—varies, and which process dominates appears to determine the toughness of the glass. This basic understanding is helpful for scientists who are developing new metallic glass alloys for products that can take advantage of the material’s high strength and elasticity, along with its tendency not to shrink when it is molded to a particular shape. The ultimate aim of these investigations is to incorporate such findings into predictive models of failure for these materials so that they can be optimized and used in applications that require materials that are both strong and fracture-resistant. Further investigations undertaken as part of this project considered how the size and density of crystals embedded within a disordered glass matrix alter the strength of the material. Computer simulations were performed in which an initially fully crystalline sample with nanometer-sized crystal grains was gradually melted. Since melting proceeds from the boundaries the grains gradually shrank. By arresting the melting suddenly and cooling the system back to room temperature the investigators produced computer representations of many different microstructures. These were subjected to shear loading in the simulations and the resulting strength was compared. In addition the role that the evolution of the embedded crystals played in the ultimate failure of the material, typically by formation of a shear band, was examined in detail. These investigations are being used to evaluate the predictive capabilities of theories that attempt to relate the response of the glass to extreme forces that result in plastic deformation, microstructural change and, eventually, failure. Beyond these simulation studies the investigators collaborated with colleagues at Korea University to understand experimental investigations carried out there. Investigators in Korea carried out loading of metallic glasses under different levels of compressive force at different temperatures. The microstructural changes were measured by performing differential scanning calorimetry. Theories developed to predict these changes were compared directly to the experimental data, demonstrating that the evolution is consistent with theories that propose that the disorder in such a glass can be treated as a system with a well-defined temperature that is out of equilibrium with the ambient temperature. Additional studies were carried out in collaboration with undergraduate researchers from Johns Hopkins University and Howard University. In order to disseminate the scientific goods of these endeavors beyond the laboratory the investigator also engaged in significant educational and outreach activities. These included a thorough revision of the way undergraduate materials scientists are educated at Johns Hopkins University to incorporate computation and simulation more completely into the educational process. A new first-year computing class embedded within the Materials Science and Engineering discipline was developed and proved highly effective. This enabled the investigator to acquire independent funding to undertake educational research that showed higher effectiveness of this approach as measured by students' relative perception of their capacity and willingness to adopt computation as part of their professional and educational lives. In addition, the investigator developed a close relationship with the Office of Teaching and Learning within the Baltimore City Public Schools that enabled the creation of a partnership between Johns Hopkins University's schools of Engineering, Education and Arts & Sciences and the school system. This partnership is now independently funded to undertake a 5-year outreach program to improve STEM outcomes in grades 3-5 in 3 distinct Baltimore city neighborhoods.
