Formation and mechanical properties of bulk Cu-Ti-Zr-Ni metallic glasses with high glass forming ability

Bulk amorphous Cu52.5Ti30Zr11.5Ni6 and Cu53.1Ti31.4Zr9.5Ni6 alloys with a high glass forming ability can be quenched into single amorphous rods with a diameter of 5 mm, and exhibit a high fracture strength of 2 212 MPa and 2 184 MPa under compressive condition, respectively. The stress—strain curves show nearly 2% elastic strain limit, yet display no appreciable macroscopic plastic deformation prior to the catastrophic fracture due to highly localized shear bands. The present work shows clearly evidence of molten droplets besides well-developed vein patterns typical of bulk metallic glasses on the fracture surface, suggesting that localized melting induced by adiabatic heating may occur during the final failure event.     

Remarkable effect of Ce base element purity upon glass forming ability in Ce–Ga–Cu bulk metallic glasses

Raw Ce element materials of eleven different purities are used to prepare bulk metallic glasses with the same nominal composition of Ce₇₀Ga₈Cu₂₂ (at.%). In the high-purity regime of Ce (98.13–99.87wt.%), three distinct peaks are observed in the curve plotting the purity vs. the glassy rod critical diameter (D_c); and with ∼0.11 wt.% decrease in purity, the D_c can increase sharply from 1 to 10 mm. In the relatively low-purity regime of 96.15–98.13 wt.%, the low material purity is found to be beneficial for glass formation; and with a ∼0.61 wt.% decrease in purity, the D_c increases dramatically from 1.5 mm to at least 20 mm. Such a sensitive and systematic purity-dependent glass-forming ability has rarely been reported before in metallic glasses. It is also suggested that the high stability of the competing crystalline phases results from the mixture effect via addition of multiple impurity elements into the matrix glass-forming alloys, and that this addition of impurity elements may be the dominant factor responsible for their intrinsic glass-forming ability of these alloys. The results provide systematic evidence for the strong purity and composition effects that are present in glass formation, and can be used to shed light on scientific research and industrial applications in the field of metallic glasses.     

Serrated flow and stick–slip deformation dynamics in the presence of shear-band interactions for a Zr-based metallic glass

In this paper, shear-band interactions (SBIs) were introduced by a simple method and their effect on the dynamics of shear bands and serrated flow was studied for a Zr-based metallic glass. Statistical analysis on serrations shows that the stick–slip dynamics of interacting shear bands is a complex, scale-free process, in which shear bands are highly correlated. Both the stress drop magnitude and the incubation time for serrations follow a power-law distribution, presenting a sharp contrast to the randomly generated, uncorrelated serrated flow events in the absence of SBIs. Observations on the fracture morphologies provide further evidence and insights into the deformation dynamics dominated by SBIs. A stick–slip model for multiple shear bands with interactions is also proposed and numerically calculated. The results, in good agreement with the experimental results, quantitatively show how multiple shear bands operate and correlate, especially for those with large serrated flow events. Our studies suggest that one serration in the stress–strain curve may correspond to collective stick–slip motions of multiple shear bands for those ductile bulk metallic glasses where a large number of shear bands are observed during deformation.     

The oxidation behavior of a novel Ni-free Zr–Cu-based bulk metallic glass composite in the supercooled liquid and crystallization states

The oxidation behavior of a novel Ni-free (Zr₄₈Cu₃₂Al₈Ag₈,Ta₄)Si_0.75 bulk metallic glass composite (BMG-C) in dry air in the supercooled liquid state (SLS at 430 °C and 480 °C) and the crystallization state (CS at 520 °C and 560 °C) for 100 h were studied herein.Test results showed that the oxidation kinetics of the BMG-C in the SLS and CS followed a multi-stage oxidation rate law. The scales forming in both the SLS and CS consist of t-ZrO₂, and m-ZrO₂, CuO and Ag. In the CS, additional Al₂O₃ was observed. In the substrate area, Cu₁₀Zr₇ crystalline likely formed in the amorphous substrate in the SLS. In the CS, more crystallization phases were found in the substrate, including additional CuZr₂ observed at temperatures ≥520 °C; AlCu₂Zr was observed at 560 °C.The Ta-reinforced phase in the BMG-C was more likely to react with Si in the scales forming Ta₂Si at temperatures ≥480 °C, which resulted in cracks in the Ta. Furthermore, channels between Ta precipitates and the matrix might exist, facilitating oxygen diffusivity. As the oxidation temperature (e.g., CS) and test duration were increased, the effects of the cracks and the channels became more significant and were responsible for the fast-growth oxidation in the final stage of the test.     

Mechanical behavior of a bulk Cu–Ti–Zr–Ni–Si–Sn metallic glass forming nano-crystal aggregate bands during deformation in the supercooled liquid region

The deformation behavior of a bulk Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ metallic glass, fabricated by injection casting, has been characterized in the supercooled liquid region. The alloy deforms homogeneously and exhibits large elongation above the glass transition temperature at constant true strain rate below 1×10⁻²s⁻¹, but it shows a variation of the flow stress during deformation. The flow stress reaches a peak just after yielding and then decreases significantly with increasing strain. After the plateau level of remarkably low flow stress, it rises again and then the alloy finally fails in a brittle manner. DSC data and TEM observations for the tested alloy reveal that the alloy evolves to being crystallized during deformation. Nano-crystals are aggregated and the aggregates are aligned along the load direction. When the volume fraction of the crystalline phase is in the range up to 0.5, the nano-crystal aggregates effectively slide over each other, lowering the apparent stress level. However, as the amount of the crystalline phase further increases, the flow stress continuously increases. This behavior can be explained based on the volume-fraction rule between the crystalline phase and the amorphous phase.