Metallic glasses, or amorphous alloys, are structurally disordered solids without the long-range translational order commonly seen in crystals. The structural disorder originates at the atomic scale from randomly packed atoms in proximity to each other. This disorder leads to some of the most remarkable mechanical properties that their crystalline counterparts can only envy: high strength close to the theoretical value, large elastic strain, and high toughness. Metallic glass covers a wide range of systems including transition metals, refractory metals, rare earth metals, and their alloys. These unique and superb properties make them the perfect candidates for many applications including structural components, durable and high performance equipment, coatings, and miniature devices subject to large loading, wear and corrosive protection, and precision shaping. In the past decade, extensive research and development have been done to utilize metallic glasses, especially their mechanical properties. This collaborative research is focused on probing atomic scale deformation processes and atomic structures. It combines experimental approaches using synchrotron X-ray scattering and neutron scattering, and atomistic simulations using molecular dynamics and first-principle calculations. Specifically, it addresses the following issues: (1) Atomic structures, including short- and medium-range order and their changes caused by deformation; (2) Mechanical responses and their differences for systems with different atomic structures; (3) Atomic scale characterizations of structure-mechanical responses such as free volume, local shear transformation, and local atomic bond changes that cannot be easily captured directly by experimental measurements.
NON-TECHNICAL SUMMARY:
The ultimate goal of this research is to establish the constitutive relations among stress, strain, strain rate, temperature and various physical, structural properties and compositional changes. Due to the difficulties posed by the structural disorder in metallic glasses, reliable constitutive relations must be built on detailed and accurate understanding of atomic scale processes and mechanisms. This effort contributes critically to the advancement of knowledge in this area. We also expect this effort to contribute a positive step in widening the applications of this marvelous material, thus gaining an edge for US industries in the highly competitive world market. Another integral part of this proposed work is the education and outreach program. The project follows two tracks in this regard: (1) participation in outreach education program for local minority engineering undergraduates and K-12 program for students interested in engineering careers through demonstrations, workshops, and hands-on learning experiences, (2) establishment of a close collaboration and exchange program between experimental and computational work for graduate students in Georgia Tech and University of Tennessee, contributing to a rich education experience for both undergraduates and graduate students.
Metallic glass is made metallic elements with atomic structure very different from the commonly seen crystalline metals. Their structure does not have periodic packing, that is, they arrange in space randomly. Atomic structure of materials is detected via the Bragg which is a unique outcome of the periodic structure. The lack of orderly packing in metallic glass makes the detection of the structure nearly impossible. This NSF supported research is to search for possible atomic structure of metallic glasses and its relation to mechanical properties. Therefore, the major goal of this proposed research is to relate the properties, in particular, the mechanical property to atomic motion of metallic glasses. As mentioned above, this comes from the special nature of the material, that is, the lack of long-range order, that makes impossible to "see" the atomic structure and motion in metallic glasses. Under this NSF support, we followed two tracks to explore the structure-property relation. One is starting from the properties and tries to infer the underlying structural reasons; the second is starting directly from the structure to see the responses. In the first approach, we took mechanical properties as a focus and examined the possible underlying structures. This is reflected in our case study of mechanical anisotropy and theoretical strength of metallic glasses. We collected experimental results in structural anisotropy and strength and from the results, we built theoretical models. The theoretical models provide powerful inference to the structural information. In the second approach, we obtained atomic structures from synchrotron x-ray scattering and build atomic structures. These structures, although still confined in the statistical sense, are used in various modeling of mechanical responses. The specific objectives of the above two approaches are to find how the atomic structure, or motion if the sample is under external agitation, is related to the mechanical properties. The significant results obtained so far from this projects can be summarized in the following two categories: (1) From the mechanical responses, specifically, the structure anisotropy, we recognized that the mechanical anisotropy is a critical aspect of metallic glasses. This is an important discovery since by far, most attention in the field of metallic glasses has been focused on linear elasticity and plasticity. Although found experimentally 25 years, the structural anisotropy and its related mechanical anisotropy are not related. (2) From the theoretical analysis of the strength of metallic glasses, we found that the strength depends on certain size of correlated atoms. This size is closely related to the atomic motion merging from the external applied agitation. This size shows a direct relation with the strength: the larger the size, the higher the strength. The key outcomes out of this research are that (1) we begin to realize the importance of nonlinear relations between the structure and the properties. This relation is reflected vividly in the structure-mechanical anisotropy; and (2) The existence of correlated atomic motion in mechanical strength. The research over the past 5 years produced two generations of graduate students. They learned from the research not only how to carry out scientific research but also the skills of high performance computing. Problem solving is another area this research has provided for them.
