P. Asoka-Kumar

Electric current enhanced defect mobility in Ni3Ti intermetallics

The effect of the application of a dc current on the annealing of point defects in Ni3Ti was investigated by positron annihilation spectroscopy. An increased defect annealing rate was observed under the influence of the current and was attributed to a 24% decrease in the activation energy of mobility. The results are interpreted in terms of the electron wind effect and the complex nature of diffusion in ordered intermetallic phases. They provide direct evidence for an increase in defect mobility in ordered intermetallics under the influence of a current.The effect of the application of a dc current on the annealing of point defects in Ni3Ti was investigated by positron annihilation spectroscopy. An increased defect annealing rate was observed under the influence of the current and was attributed to a 24% decrease in the activation energy of mobility. The results are interpreted in terms of the electron wind effect and the complex nature of diffusion in ordered intermetallic phases. They provide direct evidence for an increase in defect mobility in ordered intermetallics under the influence of a current.

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.

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.

Chemical ordering around open-volume regions in bulk metallic glass Zr52.5Ti5Al10Cu17.9Ni14.6

We provide direct experimental evidence for a nonrandom distribution of atomic constituents in Zr52.5Ti5Al10Cu17.9Ni14.6 bulk metallic glass using positron annihilation spectroscopy. The Ti content around the open-volume regions is significantly enhanced at the expense of Ni and Cu. Our results indicate that Ni and Cu atoms closely occupy the volume bounded by their neighboring atoms while Al, Ti, and Zr are less closely packed, and more likely to be associated with the open-volume regions. The overall distribution of elements seen by the positron is not significantly altered by annealing or by crystallization. Theoretical calculations indicate that the observed elemental distribution is not consistent with the known crystalline phases Zr2Cu and NiZr2, while Al3Zr4 shows some of the characteristics seen in the experiment.

Observation of fluorine-vacancy complexes in silicon

We show direct evidence, obtained by positron annihilation spectroscopy, for the complexing of fluorine with vacancies in silicon. Both float zone and Czochralski silicon wafers were implanted with ...

Positron annihilation spectroscopy and small-angle neutron scattering characterization of the effect of Mn on the nanostructural features formed in irradiated Fe–Cu–Mn alloys

The size, number density, and composition of the nanometre-sized defects responsible for the hardening and embrittlement in irradiated Fe–0.9 wt% Cu and Fe–0.9 wt% Cu–1.0 wt% Mn model reactor pressure vessel alloys were measured using small-angle neutron scattering and positron annihilation spectroscopy. These alloys were irradiated at 290°C to relatively low neutron fluences (E > 1 MeV, 6.0 × 1020 to 4.0 × 1021 n m−2) in order to study the effect of manganese on the nucleation and growth of copper-rich precipitates and secondary defect features. Copper-rich precipitates were present in both alloys following irradiation. The effect of Mn was to reduce the size and increase the number density of precipitates in the Fe–Cu–Mn alloy relative to the Fe–Cu alloy. Vacancy clusters were observed in the Fe–Cu alloy, but not in the Fe–Cu–Mn alloy. These results suggest a strong effect of Mn on vacancy diffusion and clustering.

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.

Direct observation of carbon-decorated defects in fatigued type 304 stainless steel using positron annihilation spectroscopy

We have directly observed carbon decoration of defects in fatigued 304 stainless steel using positron annihilation spectroscopy. The formation and evolution of defects during fatigue was determined by positron annihilation lifetimes and electron momentum distributions in a series of samples. We find an initial rapid change in the defect concentrations that saturates around 10% of the cycles to failure into two distinct open-volume defect populations that both trap the positrons. Analysis of the momentum distributions of atomically bound electrons demonstrates that one of the defects has high levels of carbon decoration. Electron momentum distributions also show evolution in the carbon decoration of the defects with increasing fatigue all the way to failure.

The positron microprobe at LLNL

The electron linac based positron source at Lawrence Livermore National Laboratory (LLNL) provides the worlds highest current beam of keV positrons. We are building a positron microprobe that will produce a pulsed, focused positron beam for 3-dimensional scans of defect size and concentration with sub-micron resolution. The widely spaced and intense positron packets from the tungsten moderator at the end of the 100 MeV LLNL linac are captured and trapped in a magnetic bottle. The positrons are then released in 1 ns bunches at a 20 MHz repetition rate. With a three-stage re-moderation we will compress the cm-sized original beam to a 1 micro-meter diameter final spot on the target. The buncher will compress the arrival time of positrons on the target to less than 100 ps. A detector array with up to 60 BaF2 crystals in paired coincidence will measure the annihilation radiation with high efficiency and low background. The energy of the positrons can be varied from less than 1 keV up to 50 keV.

Shock Hugoniot of single crystal copper

The shock Hugoniot of single crystal copper is reported for stresses below 66 GPa. Symmetric impact experiments were used to measure the Hugoniots of three different crystal orientations of copper, [100], [110], and [111]. The photonic doppler velocimetry (PDV) diagnostic was adapted into a very high precision time of arrival detector for these experiments. The measured Hugoniots along all three crystal directions were nearly identical to the experimental Hugoniot for polycrystalline Cu. The predicted orientation dependence of the Hugoniot from molecular dynamics calculations was not observed. At the lowest stresses, the sound speed in Cu was extracted from the PDV data. The measured sound speeds are in agreement with values calculated from the elastic constants for Cu.