M.L. Morrison

Dynamic evolution of nanoscale shear bands in a bulk-metallic glass

Dynamic shear-band-evolution processes in a bulk-metallic glass (BMG), an emerging class of materials, were captured by a state-of-the-art, high-speed, infrared camera. Many shear bands initiated, propagated, and arrested before the final fracture in tension, each with decreasing temperature, and shear-strain profiles. A free-volume-exhaustion mechanism was proposed to explain the phenomena. The results contribute to understanding and improving the limited ductility of BMGs, which otherwise have superior mechanical properties.

Localized Corrosion Behavior of a Zirconium-Based Bulk Metallic Glass Relative to Its Crystalline State

Abstract To date, few detailed corrosion studies of the new bulk metallic glasses (BMGs) have been presented. In the present work, the aqueous electrochemical corrosion properties of BMG-11, 52.5Zr–17.9Cu–14.6Ni–5.0Ti–10.0Al (atomic percent), were investigated. Cyclic-anodic-polarization tests were conducted on amorphous and crystalline specimens in a 0.6 M NaCl solution (simulated seawater) and on amorphous specimens in a 0.05 M Na 2 SO 4 solution (simulated moisture condensation, as related to ongoing fatigue experiments in humid air), all at room temperature. In the NaCl solution, both amorphous and crystalline materials were found to exhibit passive behavior with low corrosion rates (15 μm/year or less). However, susceptibilities to pitting corrosion were observed. The amorphous material was found to be more resistant to the onset of pitting corrosion under natural corrosion conditions. In the 0.05 M Na 2 SO 4 solution, the amorphous BMG-11 was found to exhibit passive behavior with a very low corrosion rate (0.4 μm/year), and to be immune to pitting corrosion. Furthermore, when the protective passive film was removed by scratching with a diamond stylus, it was found to quickly reform. This result suggested that a corrosion influence on the fatigue properties of BMG-11 in humid air would be minimal.

Variability in Fatigue Behavior of a Zr-Based Bulk Metallic Glass (BMG) as a Result of Average Surface Roughness and Pronounced Surface Defects

Recent interest in bulk-metallic glasses (BMGs) has led to the development of amorphous alloys designed for structural applications in various fields as aircraft frames, rotating equipment, automobiles, and medical implants. Although the mechanical behavior of BMGs is being studied extensively, little attention has been paid to their fatigue behavior. Moreover, early fatigue characteristics have exhibited contradictory results. In the current research, uniaxial tension-tension fatigue experiments were performed on notched Zr52.5Cu17.9Al10Ni14.6Ti5 button-head fatigue specimens with various surface finishes. The fatigue studies were designed to better understand the influence of the average surface roughness and/or critical surface defects on the fatigue behavior of glassy alloys. It was hypothesized that geometric surface flaws would lower the observed life of a BMG sample by shortening the crack initiation phase and providing local stress concentrators. The current studies of surface conditions indicate that fatigue-endurance limits are greatly impacted by the average surface roughness with possible reductions of greater than fifty percent.

Pair distribution function analyses of structural relaxation in a Zr-based bulk metallic glass

Zr-based alloy ingots with nominal compositions of Zr 52.5 Cu 17.9 Ni 14.6 Al 10.0 Ti 5.0 (at.%), Vitre-loy 105, were isothermally annealed below the glass-transition temperature at 630 K for 10, 20, 30, 40, and 60 minutes in vacuum to obtain samples with various states of structural relaxation and compared to the as-cast state. Structural studies were performed using time-of-flight neutron diffraction followed by pair distribution function (PDF) analyses. Differential scanning calo-rimetry (DSC) was conducted to examine changes in the specific heat, which were correlated to the amount of structural relaxation in the various samples. These samples exhibited increasing structural relaxation with longer annealing times, which was evidenced in the atomic PDF. Relaxation related to the exothermic peak results in changes in the PDF that are consistent with the elimination of short and long inter-atomic distances. Further annealing led to rearrangements in the second atomic shell that may be related to local phase separation.