Microscale mechanical deformation behaviors and mechanisms in bulk metallic glasses investigated with micropillar compression experiments

Abstract

Over the past years of my PhD study, the focused-ion-beam (FIB) based microcompression experiment has been thoroughly investigated with respect to the small-scale deformation in metallic glasses. It was then utilized to explore the elastic and plastic deformation mechanisms in metallic glasses. To this end, micropillars with varying sample sizes and aspect ratios were fabricated by the focused ion beam technique, which were subsequently compressed on a modified nanoindentation system. In order to obtain the mechanical properties of metallic-glass micropillars accurately, a variety of effects, such as pillar's geometry and base compliance, have been accounted for. An improved formula for the measurement of the Young's modulus was derived by adding a geometrical prefactor to the Sneddon's solution. Through the formula, geometry-independent Young's moduli were extracted from microcompression experiments, which are consistent with nanoindentation results. Furthermore, cyclic microcompression was developed in my PhD study, which revealed reversible inelastic deformation in the apparent elastic regime through high-frequency cyclic loading. The reversible inelastic deformation manifests as hysteric loops in cyclic microcompression and can be captured by the Kelvin-type viscoelastic model. Through these experimental findings, an important implication can be drawn that in metallic glasses, free-volume zones should be surrounded by tightly bonded elastic atomic clusters. The experimental results further indicate that the free-volume zones behave essentially like supercooled liquids with an effective viscosity on the order of 1 × 10⁸ Pa s. Apart from the micro-scale elastic or anelastic deformation in metallic glasses, their yielding and post-yielding behaviors were also investigated through microcompression. Their microscopic yield strengths were first extracted with a formula derived based on the Mohr-Coulomb law to account for the geometrical effects from the tapered micropillar and the results showed a weak size effect on the yield strengths of a variety of metallic-glass alloys, which can be attributed to Weibull statistics. In the static microcompression, the size effect on the yielding stress for a given stress rate can be investigated. In comparison, the nature of the yielding phenomenon can be explored with the cyclic micro-compression approach. While it is generally perceived that the yielding point in metallic glasses is strain controlled, however, direct experimental evidence is still lacking up to the moment. Through cyclic microcompression of a Zr-based metallic glass, it can be demonstrated that its yielding stress increases at higher applied stress rate but its yielding strain is kept at a constant of ~ 2%. The room-temperature post-yielding deformation behavior of metallic glasses is characterized by flow serrations, which were found sample geometry (shape + size) dependent. The geometry effect is implicative of an intrinsic ductile to brittle transition mechanism in metallic glasses, which can be formulated using the principle of energy balance. As consistent with the findings from solid micropillars, a microscale hollow pillar experiment was devised to further understand the geometry dependence of shear banding, which showed enhanced shear-band stability with the decreasing thickness of the hollow micropillars.