Properties of Atomic-Scale Flow Defects in Metallic Glasses

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TECHNICAL SUMMARY:This project seeks to explore fundamental properties of the defects that accommodate flow in metallic glasses below the glass transition temperature, a necessary step toward fully explaining and predicting macroscopic deformation behaviors. Plastic deformation of metallic glasses is accommodated by thermally activated shear of atomic clusters, known as shear transformation zones (STZs), which are also responsible for anelasticity. Research results will include details of non-Arrhenian behaviors, guide future atomistic simulations and provide realistic input into mesoscopic simulations for practical applications. The effect of structural relaxation state, i.e., of processing, on STZ properties will be determined. To cover a wide range of time constants, quasi-static and dynamic anelastic relaxation measurements will be conducted using bend stress relaxation, nanoindenter cantilever bending and tensile measurements in a dynamic mechanical analyzer. Data will be analyzed numerically within the framework of a standard linear solid model developed by the PI. By exploring the size-density distribution of potential STZs as a function of processing history, proposed predictions for the potential role of the free volume, or a different structural variable, will be tested. Temperature-dependent measurements will provide direct information on size-resolved activation barriers. Further explorations will address the collective behavior of STZs and the effect of sub-yield, local, irreversible atomic displacements on STZ properties. In addition to new scientific knowledge, the work will provide opportunities for alloy design and will enhance graduate and undergraduate education. It will help inspire high-school students to pursue a career in science.NON-TECHNICAL SUMMARY:The freezing of a molten metal can produce a metallic glass. The strength and elasticity of metallic glasses make them attractive for applications in medical and other mechanical devices, as well as sporting goods. However, they exhibit limited ductility and toughness. Understanding the atomic-scale rearrangements associated with permanent deformation in these materials is a scientific challenge, but it is crucial to developing metallic glasses with improved ductility. In contrast to crystalline materials, in which the position of every atom is known, atomic positions in glasses are far less well understood. Thus, the nature of atomic rearrangements is a subject of active research. A recently developed method of characterizing shear transformation zones (STZs), small atomic clusters that undergo permanent shear as a result of deformation in a metallic glass, is used in this research. Analysis of time-dependent shape recovery reveals distinct signatures of STZs, resolved by the number of atoms they comprise. Critical experiments will be performed to explain previous experimental observations. The expected results include realistic input into simulations of macroscopic behavior, benefitting future development of ductile alloys. Increased understanding will be sought for the role of slow, reversible, deformation to benefit both manufacturing and the interpretation of experiments. The proposed work will promote the education and training of graduate and undergraduate students, who will learn materials fundamentals, develop abilities to plan and conduct experiments and modeling and learn to communicate their results. The PI and graduate students will participate in high-school visits in under-resourced parts of the state of Michigan.