J. Hartley

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.

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.

Positron annihilation studies of fatigue in 304 stainless steel

Positron annihilation lifetime measurements were made on well calibrated fatigue samples of SS-304. Measurements were made on a high and low carbon alloy. Two separate lifetimes, indicating two defect site types were resolved in each sample. Significant lifetime changes are observed to occur early in the fatigue cycles.

High energy beam lifetime analysis

Abstract We have developed a positron lifetime defect analysis capability based on a 3 MeV electrostatic accelerator. The high energy beam lifetime spectrometer is operational with a 100 mCi 22 Na source providing a current of 5.5×105 positrons per second. Lifetime data are derived from a thin plastic transmission detector providing an implantation time and a BaF2 detector to determine the annihilation time. Positron lifetime analysis is performed with a 3 MeV positron beam on thick sample specimens at counting rates in excess of 2000 per second. The instrument is being used for bulk sample analysis and analysis of samples encapsulated in controlled environments for in situ measurements.

Positron beam lifetime spectroscopy of atomic scale defect distributions in bulk and microscopic volumes

We are developing a defect analysis capability based on two positron beam lifetime spectrometers: the first is based on a 3 MeV electrostatic accelerator and the second on our high current linac beam. The high energy beam lifetime spectrometer is operational and positron lifetime analysis is performed with a 3 MeV positron beam on thick sample specimens. It is being used for bulk sample analysis and analysis of samples encapsulated in controlled environments for in situ measurements. A second, low energy, microscopically focused, pulsed positron beam for defect analysis by positron lifetime spectroscopy is under development at the LLNL high current positron source. This beam will enable defect-specific, 3-dimensional maps of defect concentration with sub-micron location resolution. When coupled with first principles calculations of defect specific positron lifetimes it will enable new levels of defect concentration mapping and defect identification.

Positron beam lifetime spectroscopy at Lawrence Livermore National Laboratory

Defect analysis is needed for samples ranging in thickness from thin films to large engineering parts. We are meeting that need with two positron beam lifetime spectrometers: on on a 3 MeV electrostatic accelerator and the second on our high current linac beam. The high energy beam spectrometer performs positron lifetime analysis on thick samples which can be encapsulated for containment or for in situ measurements in controlled environments. At our high current beam, we are developing a low energy, microscopically focused, pulsed positron beam to enable positron annihilation lifetime spectroscopy for defect specific, 3-D maps with sub-micron location resolution. The data from these instruments with the aid of first principles calculations of defect specific positron lifetimes.

Nuclear fusion driven by Coulomb explosions of deuterium clusters

We have examined the interaction of deuterium clusters with high intensity, ultrafast laser radiation. Upon irradiation a hot plasma is created with a sufficient temperature to produce nuclear fusion. We have seen that larger clusters produce more fusion neutrons than small er clusters, consistent with a Coulomb explosion model. Fusion yields is currently limited by propagation effects. Using interferometric imaging we have examined the laser propagation and found that the laser energy is absorbed before it penetrates to the center of the gas jet.

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.

Bulk defect analysis with a high-energy positron beam

A program using a positron beam to probe defects in bulk materials has been developed at Lawrence Livermore National Laboratory. Positron annihilation lifetime spectroscopy (PALS) provides non-destructive analysis of average defect size and concentration. A 3 MeV positron beam is supplied by Sodium-22 at the terminal of a Pelletron accelerator. The high-energy beam allows large (⩾1 cm2) engineering samples to be measured in air or even sealed in an independent environment. A description of the beam-PALS system will be presented along with a summary of recent measurements.