Daewoong Suh

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.

Mechanical relaxation time scales in a Zr-Ti-Ni-Cu-Be bulk metallic glass

The relaxation time scales in a commercial-grade Zr 4 1 . 2 5 Ti 1 3 . 7 5 Ni 1 0 Cu 1 2 . 5 Be 2 2 . 5 (at.%) bulk metallic glass were examined using transient and dynamic mechanical experiments. The viscoelastic and sub-T g relaxations were well described by the Kohlrausch-Williams-Watts relaxation function. A large activation energy (4.0 eV) and small nonexponentiality parameter (approximately 0.5) were observed for viscoelastic relaxation above T g consistent with the cooperative nature of atomic movements leading ultimately to viscous flow. Conversely, a small activation energy (0. 1 eV) and large nonexponentiality parameter (approximately 0.9) were observed for the sub-T g relaxation suggesting localized atomic adjustments which may involve different structural units or mechanisms. The glass transition was manifested as a decoupling of the sub-T g and viscoelastic relaxation. The resulting transition temperature determined at a selected time scale was in agreement with the value obtained from calorimetric studies.

The effects of hydrogen on viscoelastic relaxation in Zr–Ti–Ni–Cu–Be bulk metallic glasses: implications for hydrogen embrittlement

Abstract The effects of hydrogen on the viscoelastic relaxation behavior of a Zr–Ti–Ni–Cu–Be bulk metallic glass have been investigated in an attempt to elucidate hydrogen-affected flow and fracture behavior. Dynamic mechanical testing was performed to study relaxation behavior near the glass transition temperature. Relaxation time constants were increased in the presence of hydrogen with a concomitant increase of thermal activation energy. In addition, the glass transition temperature was increased and crystallization kinetics retarded in the presence of hydrogen leading to enhanced thermal stability. Positron annihilation spectroscopy was employed to study the interaction of hydrogen and open-volume regions. While hydrogen charging was found to decrease the open-volume regions in the amorphous phase, an increase in free volume was observed in the crystalline counterpart. The amorphous phase was found to have a greater hydrogen absorption capacity compared to its crystalline counterpart. Relaxation behavior, crystallization kinetics and the interaction of hydrogen with the amorphous microstructure are discussed. Finally, the effects of retarded relaxation processes on fracture resistance are considered.

Effects of open-volume regions on relaxation time scales and fracture behavior of a Zr-Ti-Ni-Cu-Be bulk metallic glass

Abstract The plane strain fracture toughness of a Zr–Ti–Ni–Cu–Be bulk metallic glass was found to be significantly degraded after annealing at 300 °C. The fracture morphology was changed from characteristic vein patterns to cleavage-like features with little evidence of plasticity. Positron annihilation spectroscopy results revealed that atomic scale open-volume regions were reduced by annealing. A dynamic mechanical analysis showed that the atomic arrangement process for viscous flow was retarded as a result of annealing. The observed annealing embrittlement is ascribed to the loss of stress relief by viscous flow at the crack tip that results from the anneal-out of open-volume regions

Temperature dependence of positron annihilation in a Zr–Ti–Ni–Cu–Be bulk metallic glass

A strong temperature dependence of positron annihilation with low-momentum electrons is reported for a Zr-based bulk metallic glass in the temperature range 50-300 K. The observed behavior was rationalized in terms of shallow versus deep positron traps. An interpretation of the data was presented based on the idea that there were two different types of open-volume regions: Bernal interstitial sites and thermally unstable larger holes. Bernal interstitial sites, intrinsic to the glass structure, were found to be insensitive to annealing. Alternatively, the larger holes were removed by annealing. The strong correlation between these larger holes and diffusion and viscous flow processes suggests that they may act as diffusion and flow defects.

The effects of hydrogen on deformation and fracture of a Zr-Ti-Ni-Cu-Be bulk metallic glass

Abstract Effects of pre-charged hydrogen on the deformation and fracture behavior of a Zr–Ti–Ni–Cu–Be bulk metallic glass were investigated. Flow stress was increased, and fracture toughness was degraded after hydrogen charging. The glass transition and crystallization temperatures were shifted to higher values, suggesting retarded atomic relaxation processes. The atomic rearrangement process that underlies all physical properties, including deformation and fracture, is shown to be significantly affected by hydrogen.

Flow and fracture in Zr-based bulk metallic glasses

Abstract Effects of atomic-scale open-volume regions in metallic glass structure on the flow and fracture behavior of a Zr-Ti-Ni-Cu-Be bulk metallic glass were examined. Metallic glass structure was changed by annealing at 300 °C. Studies of relaxation time scales showed that atomic arrangement processes for viscous flow were significantly retarded with annealing. Plane strain fracture toughness was significantly decreased and fatigue crack-growth rates were dramatically increased, indicating degradation of resistance to crack extension as a result of annealing. Fracture morphology completely changed from vein patterns to cleavage-like features with little evidence of plasticity with annealing. The positron lifetime and Doppler broadening experiments revealed decreased open-volume regions as a result of annealing. The observed variation of the viscoelastic relaxation time scales with annealing was well described in terms of the anneal-out of open-volume regions. The current metallic glass was found to posses a low fractional free volume of the order of 0.1% indicating dense-packed structure which contributes to the excellent glass forming ability. The loss of stress relief ability by retarded crack tip viscous flow as a result of the anneal-out of open-volume regions is believed to contribute to subsequent embrittlement.

Opportunities for materials characterization using high-energy positron beams

This review will summarize current positron research at Lawrence Livermore National Laboratory(LLNL) using high-energy positron beams. We are combining positron lifetime and orbital electron momentum spectroscopic methods with theoretical simulations to provide a better understanding of positron annihilation behavior in materials. Topics covered include correlation of positron annihilation characteristics with structural and mechanical properties of bulk metallic glass and compositional studies of embrittling features in reactor pressure vessel steels.

Bulk materials analysis using high-energy positron beams

This article reviews some recent materials analysis results using high‐energy positron beams at Lawrence Livermore National Laboratory. We are combining positron lifetime and orbital electron momentum spectroscopic methods to provide electron number densities and electron momentum distributions around positron annihilation sites. Topics covered include the correlation of positron annihilation characteristics with structural and mechanical properties of bulk metallic glasses and compositional studies of embrittling features in nuclear reactor pressure vessel steel.

Short range chemical ordering in bulk metallic glasses

We provide direct experimental evidence for a non-random distribution of atomic constituents in Zr-based multi-component bulk metallic glasses using positron annihilation spectroscopy. The Ti content around the open-volume regions is significantly enhanced at the expense of Cu and Ni, indicating that Cu and Ni occupy most of the volume bounded by their neighboring atoms while Ti and Zr are less closely packed and more likely to be associated with open-volume regions. Temperature-dependent measurements indicate the presence of at least two different characteristic sizes for the open volume regions. Measurements on hydrogen-charged samples show that the larger open-volume regions can be filled by hydrogen up to a critical density. Beyond this critical density, local atomic-scale open-volume damage is created in the sample to accommodate additional hydrogen. The onset of this local damage in positron annihilation data coincides with the onset of volume expansion in X-ray diffraction data.