W.J. Choyke

Ion beam modification of 6H/15R SiC crystals☆

Abstract Large single crystals of silicon carbide consisting predominantly of 6H polytype have been implanted with hydrogen, nitrogen, or aluminum ions at 300 K. Rutherford backscattering-channeling techniques have been used to characterize atomic displacement effects resulting from implantation and postimplantation annealing at 573 K. Amorphization fluences for all three ions correspond to deposition of a critical damage energy of 2 × 1021 keV cm 3 . Annealing of N+ or Al+ implanted crystals for 0.5 h at 573 K produces appreciable recovery in damaged but crystalline regions. Defect annealing is inhibited in amorphous and heavily doped areas of the crystals.

Implanted hydrogen effects at high concentrations in model low Z shielding materials

Abstract Shielding of near-plasma structural components has long been recognized as an important consideration in Tokamak operation. Various low Z materials have been proposed and tested to determine sputtering characteristics, hydrogen retention and isotope exchange kinetics. The mechanical response of these materials to high fluence atomic displacement damage and accumulation of hydrogen and helium is receiving extensive attention at the present time. We report on detailed mechanistic studies of a model shielding material. More extensive treatments are given elsewhere. 1,2

Ion irradiation effects on high strength, high conductivity copper alloys

Abstract Microstructural effects in a solid solution cold work strengthened copper alloy (Cu−0.15 wt.% Zr), a precipitation strengthened alloy (beryllium copper) and a dispersion hardened alloy (Cu−0.6 wt.% Al as Al2O3 particles) after high energy ion irradiation at fluences from 1 dpa to 20 dpa at temperatures from 523 K to 773 K have been characterized by detailed TEM investigation. Atomic displacement processes result in texture-dependent accelerated recovery by dislocation climb and glide in the Cu−0.15 wt.% Zr alloy. Coarsening (or disappearance) of GP zones and accelerated growth of CuBe precipitates are observed in the beryllium copper alloy. Microstructural evolution in the oxide dispersion strengthened alloy entails progressive amorphization and dissolution of the Al2O3 particles.

Pulsed laser damage characteristics of vapor-deposited copper mirrors on silicon carbide substrates

Pulsed 100-nsec 10.6-microm laser damage characteristics of composite bare copper mirrors were determined. The mirrors were prepared by vapor deposition of a copper film on polished silicon carbide substrates formed by chemical vapor deposition, Comparisons are made with a bulk oxygen-free high-conductivity (OFHC) copper mirror prepared by precision diamond turning. Vapor-deposited Cu on SiC was found to be inherently superior in resisting surface damage related to defects and to thermal stress. However, melt thresholds are comparatively lower. The overheating problem was analyzed by a thermal modeling of thresholds measured on mirrors having different film thicknesses. Results are consistent with reduced thermal conductivity in the SiC substrate as compared with bulk Cu. The effect of the interface was evaluated from samples that had been ion polished prior to deposition and on others where an intermediate chromium layer was deposited. The scattering and figure of the mirrors are also reported.

Ion polishing of copper: some observations.

The change in absorptivity at 10.6 microm of copper samples, at various stages of polishing including ion polishing, is reported. The ion polishing apparatus is also described; it employs a low energy Xe ion beam incident at a glancing angle to maximize the removal rate while minimizing ion penetration and damage. The sample to be polished is rotated and the beam scanned across the surface to produce uniform removal of material. Starting with single crystal copper that had been mechanically polished and by using combinations of ion polishing and vacuum annealing, the absorptivity, determined by calorimetry, was reduced from 0.92% to 0.76%. Although optical figure and scatter were not monitored, SEM pictures show that the surface features become smoother with ion polishing. A similar experiment was performed on a single crystal copper sample that had been electropolished, and the combination of ion polishing and annealing lowered the absorptivity from 0.97% to 0.89%.

Critical cavity sizes in dual-ion bombarded 304 SS part I: Experiment

Abstract A multikey computer file system and several algorithms relating measured cavity size distributions, point defect sink strengths and an equation of state for helium atoms in gas bubbles have been used to determine lower bound critical cavity sizes from dual-ion bombardment data for 304 SS. The results are discussed within the framework of the critical cavity size concept for a transition from gas-driven to bias-driven cavity growth.

Deuterium and helium trapping at TiC particles in ferritic steel

Abstract First wall and blanket materials in Tokamak machines must accommodate increasing concentrations of helium and hydrogen isotopes. Alloy design principles point to the efficacy of trapping He and hydrogen at finely dispersed precipitates to minimize their impact on mechanical properties. Titanium carbide particles are known to trap He effectively in austenitic stainless steel. Less is known about TiC as a trap for helium and hydrogen isotopes in ferritic steels. This paper demonstrates the feasibility of directly measuring the trapping of helium and deuterium at TiC-ferrite interfaces using atom probe field ion microscopy.

Evolution of cavity size distributions in dual-ion irradiated austenitic stainless steel and Fe-Cr-Ni ternary alloys☆

Abstract The first four moments of experimentally measured cavity size distributions in dual-ion irradiated 304SS, Fe-12Ni-15Cr and Fe-30Ni-15Cr alloys have been calculated for a range of fluences, helium injection rates, and irradiation temperatures. The moments are shown to correctly describe the effects of alloy composition, fluence, helium and temperature on the evolution of the cavity size distributions. Experimentally determined moments are compared with those calculated from cavity nucleation and growth theories. The moments reflect the competition between nucleation and growth processes and provide insight into the details of the transient low swelling regime.

Implantation rate effects on microstructure

Abstract We report a detailed TEM study of rate effects in a metal (304 SS) where we dope with an insoluble atom (He) and create the displacement damage with high energy Si. The rates of doping and the rates of producing lattice damage are independently varied during dual implantation. In addition to varying the doping rates of the He the magnitude of the displacement damage prior to He implantation is also varied (beam history). We find that the beam history has virtually no effect on maximum bubble size but it has a major effect on the average cavity diameter. A weak dependence of cavity number density on helium implantation rate is found. The total dislocation density is relatively independent of the doping rate and beam history at 550 and 700°C, whereas the loop fraction is sensitive to beam history at these temperatures. Acicular precipitate formation is weakly dependent on doping, doping rate and more strongly dependent on doping concentration and temperature. This form of solute segregation is very sensitive to beam history.