Mechanical properties and thermomechanical processing of metallic glasses -- the role of elemental distributions and size-dependent properties of shear transformation zones

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Non-technical AbstractMolten metallic alloys of certain compositions can be frozen to produce metallic glasses. In contrast to conventional metals, which are crystalline, the atomic arrangements in metallic glasses exhibit significant disorder. The high 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, which can lead to catastrophic fracture. In crystalline materials, heat and deformation treatment are routinely used for property improvement, and the associated mechanisms have been understood for many years. A parallel understanding of the atomic-scale rearrangements in metallic glasses is a scientific challenge, since the baseline arrangements are not known. However, such an understanding is crucial to improving their properties. A recently developed method of characterizing small atomic clusters called shear transformation zones (STZs), undergo shape change 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 STZ dynamics will be compared between similar alloys that differ in their ductility. Atom-probe tomography, a method that can yield atomic-scale composition mapping, will be used to determine the role of potentially heterogeneous chemical composition, and its correlation with the STZ population. The expected results include realistic input into simulations of macroscopic behavior. The insights gained will help interpret a range of past experimental results, and assist future development of ductile alloys. The proposed work will promote the education and training of graduate and undergraduate students, who will learn materials fundamentals, develop the ability 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.Technical AbstractMetallic glasses exhibit high strength and stiffness, making them attractive for structural applications. However, flow localization often leads to catastrophic failure at a shear band. As in crystalline materials, thermo-mechanical treatment can improve their properties. In contrast to crystalline materials, an atomic level understanding of mechanical properties of metallic glasses and their response to thermo-mechanical treatment is still lacking. Plastic deformation of metallic glasses is accommodated by thermally activated shear of atomic clusters, known as shear transformation zones (STZs). Because there is no known mechanism for imaging STZs, their microscopic properties had been studied mostly in physical analogues and numerical models, which cover a limited dynamic range of time. Recently, the anelastic relaxation kinetics of different metallic glasses have been measured. These measurements reveal an atomically quantized hierarchy of STZs, resolved by the number of atoms they comprise. The shape of the size-density distributions of potential STZs, obtained from the spectra, varies significantly with the alloy. The proposed work is aimed at gaining an atomic scale understanding of how the size distribution of potential STZs a) correlates with ductility, stored enthalpy and chemical heterogeneity; b) varies between conventional metallic glasses and those with intense high-frequency (beta) mechanical relaxations, which have been shown to be ductile; c) is affected by thermomechanical treatment, including thermal cycling, cyclic and elastostatic deformation. Using a combination of quasi-static and cyclic experiments, more than eight orders of magnitude in relaxation time will be covered. Relaxation-time spectra will be computed from the data. The stored enthalpy will be determined by differential scanning calorimetry. Atom-probe tomography (APT) will be used to identify deviations from random elemental distributions, including size distributions of clusters enriched in an alloying element. Correlating the APT results with the size distribution of potential STZs will elucidate the role of local chemistry in shear transformations. Experimental input will be provided for mesoscopic simulations of macroscopic behavior. The proposed work will enhance the atomic-scale understanding of processing and deformation of metallic glasses, bringing the community's understanding of metallic glass properties closer to that for crystalline metals. The results will assist the future development of ductile alloys, thus impacting future technology. Graduate and undergraduate students will a) learn materials fundamentals; b) develop the ability to plan and conduct combined experiments and modeling, and to communicate their results. The PI and/or graduate students will participate in high-school visits in under-resourced parts of the state of Michigan.