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.
Bulk Metallic Glasses (BMGs) are disordered solids composed with metallic elements like zirconium, titanium, copper, aluminum, etc. Structurally they are distinct from typical metals and alloys; rather, they are similar to common glasses, like window glass. Their structure is random and resembles frozen metallic liquid. Since they are metallic, however, they differ from window glasses. They are not transparent, they conduct electricity, and they can be magnetic. Because bulk metallic glasses are extremely tough materials, a very large amount of energy is needed to break them. This resistance to failure is a useful mechanical property. BMGs become soft when heated and can be molded easily into shapes. The combination of superior mechanical properties and shape formability makes bulk metallic glasses promising new structural materials. However, like window glass, they fracture in a brittle manner; this lack of ductility is detrimental for engineering applications. Crystalline metals—like copper or iron—exhibit ductility, meaning that they can deform in a plastic manner before the fracture. This behavior provides a safety margin in engineering applications. The ductility in metals is controlled mainly by extended structural defects called dislocations. Atomic planes can "roll" on dislocations, resulting in plastic strains. On the other hand, the structure of bulk metallic glasses is disordered, and structural defects are not well defined or characterized. The main focus of this NSF supported project was to examine BMGs using high energy X-ray diffraction to characterize structural "defects" and their role in mechanical deformation. The second goal was to use this knowledge to design ways to improve the plasticity of BMGs. In the project, we carried out structural studies on BMG materials using synchrotron radiation (high energy X-rays) during the process of mechanical deformation and on samples that were deformed beforehand. BMG samples were deformed mechanically in tension and compression using load frame. Several studies were also performed at elevated temperatures. These diffraction studies resulted in the characterization of glass structure subjected to various modes of deformation. Plasticity of BMGs can be improved by a prior severe plastic deformation. We utilized the ECAP (equal channel pressing) procedure with different feeding rates. From our studies, we were able to explain the mechanical behavior of samples after ECAP. Before ECAP, when the sample yields due to external stress, one major shear band is formed and the BMG shears in two parts, resulting in catastrophic failure. By examining the distribution of internal strains using X-ray diffraction, we found that after ECAP the BMG sample is partitioned into many domains separated by a mesh of shear bands. The subsequent mesh of shear bands provides resistance to shearing by introducing many paths through which deformation can proceed, stop and branch—resulting in improved plasticity. We discovered the major effect of thermo-mechanical creep (TMC) on the plasticity of BMGs. TMC is a deformation of a sample that is subjected to stress at elevated temperature. We found that TMC has a remarkable effect on plasticity. TMC can improve the room temperature plasticity of brittle BMGs. Moreover, applying TMC can recover plasticity lost due to high temperature ageing. Using structural studies of samples subjected to creep by high energy X-ray diffraction, we were able to elucidate the atomistic mechanism. We found that creep promotes structural rejuvenation by local anelastic relaxation of small groups of atoms. This process is intrinsic and occurs in the entire volume of the sample. Rejuvenation is opposite to aging (aging results in brittle behavior) and thus is a desirable structural change. The graduate student contributing to the publication on creep behavior won a prestigious Acta Student Award. The key outcomes from this research include the elucidation of structural motifs in bulk metallic glasses that influence mechanical deformation and the design of effective means to improve room temperature plasticity. The project supported one graduate student. Three undergraduate students were involved in research during summer months, during which they were introduced to research and encouraged to pursue graduate studies.
