Katharine M. Flores

Mean stress effects on flow localization and failure in a bulk metallic glass

Abstract The effect of stress state on strain localization and subsequent failure of a bulk metallic glass alloy is examined. It is shown that failure is associated with a critical tensile mean stress of 0.95 GPa. This is in contrast with previous work utilizing superimposed compressive mean stresses, which found that failure resulted at a critical effective stress. Interestingly, the critical tensile mean stress measured in this study causes the same dilatation as a 274 K temperature increase, nearly to the glass transition temperature. The effect of mean stress on elastic variation of the average free volume is added to a strain localization model. This model describes the compressive mean stress behavior very well, and predicts a strong sensitivity to tensile mean stresses.

Local heating associated with crack tip plasticity in Zr–Ti–Ni–Cu–Be bulk amorphous metals

Deformation in metallic glasses is generally considered to arise from flow in localized shear bands, where adiabatic heating is thought to reduce glass viscosity. Evidence has been inferred from the veined fracture surfaces and molten droplets reported for metallic glasses. In this work, the detailed spatially resolved surface temperature increase and subsequent dissipation associated with crack tip plasticity in a Zr-Ti-Ni-Cu-Be bulk metallic glass is characterized for the first time. Maximum temperatures of up to 54.2 K were estimated from a heat conduction model and shown to be in excellent agreement with a non-hardening plasticity model for the heat generated by a propagating crack. Local cooling was also observed and shown to be consistent with thermoelastic effects.

Enhanced Toughness Due to Stable Crack Tip Damage Zones in Bulk Metallic Glass

Bulk metallic glass alloys exhibit an impressive range of mechanical properties including large elastic strains to failure ({epsilon}{sub el} {approximately} 2{degree}) and high tensile strengths ({approximately}2 GPa). This paper describes the fracture behavior of a Zr{sub 41.25}Ti{sub 13.75}Ni{sub 10}Cu{sub 12.5}Be{sub 22.5} bulk metallic glass observed using a single edge notched fracture sample loaded in tension (SEN(T)). Fracture toughness values in excess of 130 MPa{radical}m were found. These high values are associated with significant crack tip plastic deformation and blunting. Localized shear bands and branched cracks form an energy dissipating damage zone at the crack tip. Surprisingly, the extent of the damage zone is less than that expected for a polycrystalline metallic material with similar yield strength and toughness.

Characterization of Free Volume in a Bulk Metallic Glass Using Positron Annihilation Spectroscopy

The free volume of metallic glasses has a significant effect on atomic relaxation processes, although a detailed understanding of the nature and distribution of free volume sites is currently lacking. Positron annihilation spectroscopy was employed to study free volume in a Zr-Ti-Ni-Cu-Be bulk metallic glass following plastic straining and cathodic charging with atomic hydrogen. Multiple techniques were used to show that strained samples had more open volume, while moderate hydrogen charging resulted in a free volume decrease. It was also shown that the free volume is associated with zirconium and titanium at the expense of nickel, copper, and beryllium. Plastic straining led to a slight chemical reordering.

Structural Evolution and Kinetics in Cu-Zr Metallic Liquids from Molecular Dynamics Simulations (Postprint)

Abstract : The atomic structure of the supercooled liquid has often been discussed as a key source of glass formation in metals. The presence of icosahedrally coordinated clusters and their tendency to form networks have been identified as one possible structural trait leading to glass-forming ability in the Cu-Zr binary system. In this paper, we show that this theory is insufficient to explain glass formation at all compositions in that binary system. Instead, we propose that the formation of ideally packed clusters at the expense of atomic arrangements with excess or deficient free volume can explain glass forming by a similar mechanism. We show that this behavior is reflected in the structural relaxation of a metallic glass during constant pressure cooling and the time evolution of structure at a constant volume. We then demonstrate that this theory is sufficient to explain slowed diffusivity in compositions across the range of Cu-Zr metallic glasses.

Laser deposition of a Cu-based metallic glass powder on a Zr-based glass substrate

Laser Engineered Net Shaping (LENS™) is a laser-assisted manufacturing process that offers the possibility of producing metallic coatings and components with highly nonequilibrium microstructures. In this work, the microstructure developed by LENS deposition of Cu 47 Ti 33 Zr 11 Ni 8 Si 1 powder on a bulk metallic glass substrate, with nominal composition Zr 58.5 Nb 2.8 Cu 15.6 Ni 12.8 Al 10.3 , is investigated. Single-layer deposition results in the formation of an inhomogeneous but partially amorphous layer above a crystalline heat-affected zone. Elemental analysis of the deposited layer indicates incomplete mixing of the powder with the melt pool. The as-deposited alloy exhibits a single glass transition event and its primary crystallization event is consistent with the first crystallization temperature of the Cu-based powder. Subsequent remelting of this layer results in a still partially amorphous deposit with a uniform composition of (Zr + Nb) 51.8 Cu 24.7 Ti 3.4 Ni 16.4 Al 3.7 . The remelted layer exhibits a structural rearrangement immediately prior to the primary crystallization event, possibly associated with the formation of a quasicrystalline phase.

Crack-tip plasticity in bulk metallic glasses

Abstract The fracture and plastic deformation mechanisms of a Zr–Ti–Ni–Cu–Be bulk metallic glass are reviewed. In particular, the effects of stress state and local heating on the development of shear bands are discussed. The temperature increase at the crack tip during fracture was measured and was found to be consistent with predictions of the dissipation of plastic work as heat. In order to examine the effect of stress state on failure, tensile bars with notches of varying severity were used to superimpose a tensile mean stress. A constant mean stress was found to provide an accurate correlation with the onset of strain localization and failure.

Towards an understanding of tensile deformation in Ti-based bulk metallic glass matrix composites with BCC dendrites

The microstructure and tension ductility of a series of Ti-based bulk metallic glass matrix composite (BMGMC) is investigated by changing content of the β stabilizing element vanadium while holding the volume fraction of dendritic phase constant. The ability to change only one variable in these novel composites has previously been difficult, leading to uninvestigated areas regarding how composition affects properties. It is shown that the tension ductility can range from near zero percent to over ten percent simply by changing the amount of vanadium in the dendritic phase. This approach may prove useful for the future development of these alloys, which have largely been developed experimentally using trial and error.

Chemical Reactions of Portland Cement with Aqueous CO2 and Their Impacts on Cement's Mechanical Properties under Geologic CO2 Sequestration Conditions.

To provide information on wellbore cement integrity in the application of geologic CO2 sequestration (GCS), chemical and mechanical alterations were analyzed for cement paste samples reacted for 10 days under GCS conditions. The reactions were at 95 °C and had 100 bar of either N2 (control condition) or CO2 contacting the reaction brine solution with an ionic strength of 0.5 M adjusted by NaCl. Chemical analyses showed that the 3.0 cm × 1.1 cm × 0.3 cm samples were significantly attacked by aqueous CO2 and developed layer structures with a total attacked depth of 1220 μm. Microscale mechanical property analyses showed that the hardness and indentation modulus of the carbonated layer were 2-3 times greater than for the intact cement, but those in the portlandite-dissolved region decreased by ∼50%. The strength and elastic modulus of the bulk cement samples were reduced by 93% and 84%, respectively. The properties of the microscale regions, layer structure, microcracks, and swelling of the outer layers combined to affect the overall mechanical properties. These findings improve understanding of wellbore integrity from both chemical and mechanical viewpoints and can be utilized to improve the safety and efficiency of CO2 storage.

Micropillar fabrication on bovine cortical bone by direct-write femtosecond laser ablation.

We investigated fabrication of cylindrical micropillars on bovine cortical bone using direct-write femtosecond laser ablation. The ablation threshold of the material was measured by single-pulse ablation tests, and the incubation coefficient was measured from linear scanned ablation tests. A motion system was programmed to apply multiple layers of concentric rings of pulses to machine pillars of various diameters and heights. The diameter of the top surface of the pillar was found to steadily decrease due to incubation of damage from successive layers of pulses during the machining process. Pillar top diameter was predicted based on a paraxial beam fluence approximation and single-pulse ablation threshold and incubation coefficient measurements. Pillar diameters predicted as successive layers of pulses were applied were well-matched to experiments, confirming that femtosecond laser ablation of the cortical bone was well-modeled by single-pulse ablation threshold measurements and an incubation coefficient.