The Structural Basis for Fracture Toughness and Elasticity of Metallic Glasses

Johns Hopkins University, Baltimore MD

Investigators

Abstract

TECHNICAL: Bulk metallic glasses combine some of the outstanding mechanical properties of metals, including high strength and stiffness, with the processing flexibility of glasses. They do have a significant limitation, however, in that localization of plastic deformation into shear bands leads to fracture at small plastic strains, particularly in tension. Nevertheless, some metallic glasses are quite tough (with fracture energies similar to those of high strength crystalline alloys) while others are truly brittle. The origins of this difference are not well understood. An understanding of the fundamental physical mechanisms responsible would be an important contribution to the development of new alloys with improved toughness and damage tolerance. PI seeks to address this challenge through a research program that combines experimental and computational techniques to explore fundamental aspects of the competition between plastic deformation and fracture of metallic glasses. The recently observed correlation between elastic properties and fracture energy is a potentially useful link for understanding the relationship between structure and fracture. PI will examine the competition between brittle fracture and plastic flow in detail. A centerpiece of this effort will be detailed structural characterization of the plastic zone around a crack tip using high energy x-ray scattering performed in situ during loading. This technique allows measurement of elastic strain (and thus stresses) in the metallic glass with excellent (~10 um) spatial resolution in both the crystalline and amorphous phases. This will allow PI to explore the conditions under which shear bands are initiated near crack tips. As part of this effort, PI will explore the effects of three variables known to mediate the transition between brittle and tough behavior: temperature, structural relaxation, and alloy composition. The goal is to develop a detailed, realistic model of the mechanism by which a region of distributed plastic deformation develops around a crack tip. The second major aspect of this program will be to explore how the elastic properties and the shear modulus in particular, are influenced by anelastic atomic rearrangements. PI seeks to correlate observed changes in structure under load with measured changes in the elastic constants, under the influence of temperature, relaxation, and composition. PI will also connect this behavior to structure through detailed ex situ characterization, including resonant x-ray scattering and fluctuation electron microscopy. By understanding the ways in which atomic rearrangements (which may be connected to plastic deformation) influence the shear modulus PI can begin to understand the connection between elastic properties and fracture at a fundamental level. NON-TECHNICAL: The research program will advance the education and training of graduate students, undergraduate students, and local high school students by integrating them into a research team that includes the PI and the other members of his research group. The results of the basic scientific research will be disseminated via journal publications, conference and seminar presentations (by the undergraduate and graduate students as well as the principal investigator), and the web. High school students, including some from under-represented groups, will participate through a cooperative program with Project Ingenuity at Baltimore Polytechnic High School.