Microscale Mechanical Deformation Behaviors And Mechanisms In Bulk Metallic Glasses Investigated With Micropillar Compression Experiments

Microscale Mechanical Deformation Behaviors and Mechanisms in Bulk Metallic Glasses Investigated with Micropillar Compression Experiments

Characterizing Structure, Properties, and Deformation in Metallic Glasses and Olivine Using Instrumented Nanoindentation

Micro- and nanomechanical testing can provide significant insight about the structure, properties, and behavior of materials. These techniques are nondestructive, require only limited amounts of material, and have been known to detect a brittle-to-ductile transition in mechanical behavior due to a size effect. This work utilizes this type of testing to explore fundamental questions about the structure, properties, and behavior of two disparate material systems: metallic glasses and olivine. Metallic glasses are metallic alloys devoid of any long-range order. Their unique atomic structure imbues them with properties such as a high elastic strain limit, near-theoretical strengths, and the ability to be thermoplastically formed. Despite their high strengths, metallic glasses suffer from an intrinsic lack of tensile ductility compared to other high-performance materials. Recent studies have shown that the macroscopic deformation behavior of these materials might be controlled by structural heterogeneities, the exact nature of which remains ill-defined. To further this area of research, the heterogeneous microstructure of a Zr-based monolithic bulk metallic glass as well as the glass phase of a Ti-based bulk metallic glass matrix-crystalline composite was investigated using nanoindentation and dynamic modulus mapping. Significant spatial variations in the mechanical properties measured by both techniques suggest a hierarchical arrangement of mechanical heterogeneities in bulk metallic glasses and their composites. Moreover, a previously unobserved elastic microstructure, comprising an interconnected network of elastic features, was revealed by dynamic modulus mapping. Parameters such as aspect ratio and orientation of the microstructural features were defined here, which highlighted the presence of microstructural domains or colonies in the elastic microstructure. The effects of heat treatment and deformation on these heterogeneities were also investigated. The rheology of olivine plays an important role in the dynamics of Earth's upper mantle. At conditions of low temperature and high stress, such as in semi-brittle regions of the lithosphere, the deformation mechanism transitions into low temperature plasticity. Low temperature plasticity is difficult to study in typical laboratory conditions, requiring high confining pressures to suppress cracking in favor of dislocation glide. Low temperature plasticity of olivine was investigated using nanoindentation and micropillar compression. Nanoindentation provided a means of achieving plastic deformation in the absence of cracking, but measurements obtained via this method are notoriously difficult to translate into uniaxial properties. Using available models to obtain these properties, the data were fit to an established low temperature plasticity flow law, which predicted Peierls stresses for the olivine in the range of 5.32 - 6.45 GPa. As a complement to the nanoindentation, room temperature plasticity was also achieved using micropillar compression. While some of the pillars exhibited catastrophic shearing after a dwell time during creep testing, other pillars showed evidence of plastic deformation after creep testing that was confirmed to be dislocation slip. The data obtained from the micropillar compression was in good agreement with the flow law fits from the nanoindentation. These results provide increased confidence in the extrapolation of high-pressure and high-temperature laboratory experiments to low-temperature conditions and illustrate the applicability of micromechanical testing methods to the study of mineral rheology.

Improving Mechanical Properties of Bulk Metallic Glasses by Approaches of In-situ Composites and Thin Films

Bulk-metallic glasses (BMGs) exhibits lots of unique properties, such as, high strengths, high hardness, high specific strengths, superior elastic limits, high corrosion resistance, etc. However, the applications of BMGs are still quite limited due to their intrinsic brittleness and low ductility at room temperature. Many efforts have been conducted to improve the plasticity of BMGs, in which metallic-glass-matrix composites (MGMCs) and thin-film metallic glasses (TFMGs) are two popular and effective approaches. Nevertheless, the deformation mechanisms for the improved plasticity of MGMCs and TFMGs are still far from satisfactory understanding, which will be investigated using both experimental and simulation methods in the present work. For the MGMCs, in situ high-energy synchrotron X-ray diffraction experiments and micromechanics-based finite element simulations have been conducted to examine their lattice strain evolution. The entire lattice-strain evolution curves can be divided into elastic-elastic (denoting deformation behavior of matrix and inclusion, respectively), elastic-plastic, and plastic-plastic stages. Characteristics of these three stages are governed by the constitutive laws of the two phases (modeled by free-volume theory and crystal plasticity) and geometric information (crystalline phase morphology and distribution). The deformation behavior, especially the fatigue behavior, of TFMG materials has been investigated on the some substrates, including 316L stainless steel, BMG, etc. The results show that the four-point-bending fatigue life of the substrates is greatly improved by Zr- and Cu-based TFMGs, while Fe-based TFMG, TiN, and pure-Cu films are not so beneficial in extending the fatigue life of 316L stainless steel. However, quite limited work is reported on the fatigue behavior of TFMG coated on the BMG substrate, which can be a very interesting topic. Moreover, a synergistic experimental/theoretical study are conducted to investigate the micro-mechanisms of the fatigue behavior of TFMGs adhered to BMG substrates. Furthermore, shear-band initiation and propagation under deformation are investigated using the Rudnicki-Rice instability theory and the free-volume models employing finite-element simulations.