Don M. Parkin

Mechanical properties and deformation behavior of materials having ultra-fine microstructures

Fundamental concepts structure and physical properties mechanical response superplasticity nanoindentation synthesis and processing characterization.

Displacement threshold energy for type IIa diamond

A type IIa natural diamond was irradiated at room temperature with energetic electrons. The threshold energy for displacement of atoms from their lattice sites was determined for three principal crystallographic directions by observing the formation of defect clusters during irradiation in a transmission electron microscope. The displacement‐threshold energies were found to be 37.5±1.2 eV for the electron incident in the [100] direction, 45.0±1.3 eV in the [111] direction, and 47.6±1.3 eV in the [110] direction.

Crystal-to-amorphous transformation of NiTi induced by cold rolling

An NiTi intermetallic compound was cold rolled at room temperature by 30% and 60% thickness reductions, and microstructures were studied by means of transmission electron microscopy (TEM). In the cold-rolled samples we observed both a phase of nanometer-sized crystals and an amorphous phase. A substantially high dislocation density, 10{sup 13} to 10{sup 14}/cm{sup 2}, was evident in the transition region between crystalline and amorphous phases. A simple estimate of the elastic energy arising from this dislocation density is of the same order as the crystallization energy, suggesting that dislocation accumulation is a major driving force for amorphization in cold-rolled NiTi.

Electron irradiation induced amorphization in YBa2Cu3O7 and GdBa2Cu3O7 superconductors

Transmission electron microscope specimens of the oxide superconductors YBa2Cu3O7 and GdBa2Cu3O7 have been irradiated at 35 K with 1 MeV electrons and doses up to 1.5×1023 e/cm2. Electron diffraction data show the two different superconductors do not respond similarly with GdBa2Cu3O7 being more resistant to amorphization than YBa2Cu3O7. Amorphization appears to be dependent on displacements occurring at the Y/Gd lattice site. In addition, it appears that within each material, the electron dose required to initiate amorphization is lower for grain boundary irradiations relative to large single grain irradiations.

Radiation effects on materials in high-radiation environments: A workshop summary

Abstract A workshop on Radiation Effects on Materials in High-Radiation Environments was held in Salt Lake City, Utah (USA) from August 13 to 15, 1990 under the auspices of the Division of Materials Sciences, Office of Basic Energy Sciences, US Department of Energy. The workshop focused on ceramics, alloys, and intermetallics and covered research needs and capabilities, recent experimental data, theory, and computer simulations. It was concluded that there is clearly a continuing scientific and technological need for fundamental knowledge on the underlying causes of radiation-induced property changes in materials. Furthermore, the success of many current and emerging nuclear-related technologies critically depend on renewed support for basic radiation-effects research, irradiation facilities, and training of scientists. The highlights of the workshop are reviewed and specific recommendations are made regarding research needs.

Calculation of radiation damage effects of 800 MeV protons in a “thin” copper target☆

Abstract Radiation damage effects of 800 MeV protons incident on a 1 cm thick copper target have been calculated by using the nucleon-meson transport code to determine the nuclear reactions produced by the protons, and the theory of Lindhard et al. to evaluate the resultant damage energy deposited in the target. Damage effects were found to be nearly uniform across the 1 cm target thickness, and the results obtained can be expressed in cross section form applicable to target thicknesses from about 0.01 cm to 2 cm. The calculation yielded a damage energy cross section (depending slightly on target geometry) of about 350 barn-keV, a nuclear transmutation cross section of 1.0 barn, and indicated copious proton, neutron, and helium production. Comparison of the cross sections obtained with those of several neutron spectra of reactor interest suggests that medium energy proton bombardment may be a useful method for simulating neutron-induced radiation damage effects.

Calculation of radiation effects as a function of incident neutron spectrum

Abstract A method for calculating radiation effects parameters as a function of neutron energy is described. The computer code DON is used to generate damage parameters from ENDF/B nuclear data. Both integral data such as neutron spectrum averaged cross-sections and recoil energy spectra are generated. These data were used to characterize initial defect production in Al, Cu, Nb and Au in selected fission reactor, fusion reactor and fusion simulation neutron spectra. The principal differences between fission and fusion damage are the spatial characteristics of defect production and the importance of non-elastic scattering (e.g. He production). The comparison of the damage produced in one spectrum with that produced in another is dependent on the model or parameter chosen to represent the damage. This dependency is important in analyzing simulation spectra for fission and fusion reactor radiation damage problems.

Damage energy functions in polyatomic materials

Abstract The polyatomic form of the integrodifferential equation for the damage energy of Lindhard et al. was numerically integrated using parameters appropriate for Al 2 O 3 , CaO, MgO, NbTi, Si 3 N 4 , UC, UO 2 , Y 2 O 3 , TaO, MgAl 2 O 4 and U x Zr 1√-x C. In each material the damage efficiency of the light atom in the polyatomic material is lower at low energies and higher at high energies than it is in a monatomic material of its self atoms, while the damage efficiency of the heavy atom in the polyatomic material is lower at all energies than it is in a monatomic material of self atoms. As expected, the alteration of damage efficiencies from self-atom values becomes greater as the ratio of heavy to light atom masses increases. These effects are understood by considering the way damage efficiency, electronic energy loss, and kinetic energy transfer cross section in a collision of two atoms depend on the relative charges and masses of the two atoms. Analytic fits of the Robinson type are given for most of the calculated polyatomic damage energies. The special case of monatomic materials is also treated. The results obtained for monatomic materials are about 6% lower than those of Lindhard et al. due to a difference in the low energy behavior of the scattering cross section used. A universal formula of the Robinson type is given for monatomic materials.

High-energy-neutron damage in NB3SN: Changes in critical properties, and damage-energy analysis

Abstract Filamentary wires of Nb3Sn have been irradiated with fission-reactor, 14.8-MeV, and d-Be neutrons and the changes in critical properties measured. The changes observed scale reasonably well with the calculated damage energies for the irradiations. A critical dose for operation of these conductors in fusion-magnet applications is determined to be 0.19 eV/atom damage energy or 0.0019 dpa.

Measurement of the depolarization rate of positive muons in copper and aluminum

Abstract Positive muon spin rotation experiments for polycrystalline Cu and Al from 19K to temperatures near the melting points are reported. At low temperatures, the depolarization associated with localization of the muons at octahedral interstitial sites is seen in Cu, while in Al only slight depolarization is observed below 250K. At high temperatures, no evidence for trapping of positive muons at vacancies in thermal equilibrium is found for either metal. It is concluded that the muons either diffuse too slowly to find vacancies or, if they do find vacancies, are bound too weakly to remain trapped.