John D. Hunn

Defect and void evolution in oxide dispersion strengthened ferritic steels under 3.2 MeV Fe+ ion irradiation with simultaneous helium injection

Abstract In an attempt to explore the potential of oxide dispersion strengthened (ODS) ferritic steels for fission and fusion structural materials applications, a set of ODS steels with varying oxide particle dispersion were irradiated at 650°C, using 3.2 MeV Fe + and 330 keV He + ions simultaneously. The void formation mechanisms in these ODS steels were studied by juxtaposing the response of a 9Cr–2WVTa ferritic/martensitic steel and solution annealed AISI 316LN austenitic stainless steel under the same irradiation conditions. The results showed that void formation was suppressed progressively by introducing and retaining a higher dislocation density and finer precipitate particles. Theoretical analyses suggest that the delayed onset of void formation in ODS steels stems from the enhanced point defect recombination in the high density dislocation microstructure, lower dislocation bias due to oxide particle pinning, and a very fine dispersion of helium bubbles caused by trapping helium atoms at the particle–matrix interfaces.

On the origin of deformation microstructures in austenitic stainless steel: part I—microstructures

Abstract A comprehensive characterization of room temperature deformation microstructures was carried out by transmission electron microscopy for ion irradiated and deformed AISI 316LN austenitic stainless steel. Deformation microstructures were produced by a recently developed disk-bend test method and also by a uniaxial tensile test. Cross-slip was dramatically suppressed by the radiation-induced defects and slip occurred predominantly by planar glide of Shockley partial dislocations. Deformed microstructures consisted of piled-up dislocations, nanotwin layers, stacking faults, and defect-reduced dislocation channel bands. Analyses revealed that all these features were different manifestations of the same type of deformation band, namely a composite of overlapping faulted layers produced by Shockley partial dislocations.

Plastic deformation in 316LN stainless steel – characterization of deformation microstructures

The effects of irradiation, test temperature, and strain on the deformation microstructures of a 316LN stainless steel have been investigated using a disk-bend method and transmission electron microscopy. Deformation microstructure changed progressively from a dislocation network dominant to a large stacking fault/twin band dominant microstructure with increasing radiation dose and with decreasing test temperature. Also, an increased strain level enhanced the propensity of deformation twinning. Since the stress was considered to be a key external parameter controlling deformation mechanism in 316LN austenitic stainless steel, the equivalent stress level was estimated for the examined surface of the disk sample. It was possible to categorize the deformation microstructures in terms of the equivalent stress range. A key conclusion is that the austenitic material will deform by forming bands of large stacking faults and twins when the stress exceeds a critical equivalent stress level of about 600 MPa by any of several possible strengthening measures: irradiation, increasing strain level, and decreasing test temperature.

On the origin of deformation microstructures in austenitic stainless steel: Part II—Mechanisms

Abstract Deformation microstructures of austenitic stainless steels consist of profuse pile-up dislocations, stacking faults, nanotwins, and defect-reduced channels as demonstrated in the Part I companion paper of this title [ Acta mater. , 2001, 49 (16), 3269–3276]. Yet the mechanisms of such microstructural evolution are poorly understood. Thus, a comprehensive study was conducted to understand the underlying physics of deformation in metals using radiation damage as a tool. It was found that, for energetic reasons, glide dislocations dissociated into Shockley partials during glide. Consequently, the interaction between a glide dislocation and radiation-induced defects occurs by a two-step reaction, first with the leading partial and then with the trailing partial. With this insight, the origin of deformation microstructures was explained by analyzing Shockley partial dislocations and their interactions with radiation-induced Frank loops.

Effects of helium on radiation-induced defect microstructure in austenitic stainless steel

Abstract In the construction materials surrounding the spallation neutron source (SNS) mercury target, considerable quantities of transmutation products, particularly hydrogen and helium, will be generated due to the exposure to a high flux of 1 GeV protons and associated neutrons. In an effort to investigate the effects of high helium, therefore, bubble formation and defect clustering processes in AISI 316 LN austenitic steel were studied as a function of helium concentration and displacement damage dose with 360 keV He + and 3500 keV Fe + ion beams at 200°C. Helium irradiation was less effective in producing defects such as black dots and dislocation loops than Fe + ion irradiation at equivalent displacement dose. On the other hand, the formation of helium bubbles produced a strong depressive effect on the growth of loops and the evolution of line dislocations. The results indicated that the effect of helium bubbles was augmented as the bubble number density and size increased with increasing helium beyond 1 atomic percent (at.%). In such a case, the effect of helium bubbles can be more important than that of radiation-induced defects on the evolution of microstructure and the change in mechanical properties.

Strain hardening and plastic instability properties of austenitic stainless steels after proton and neutron irradiation

Abstract Strain hardening and plastic instability properties were analyzed for EC316LN, HTUPS316, and AL6XN austenitic stainless steels after combined 800 MeV proton and spallation neutron irradiation to doses up to 10.7 dpa. The steels retained good strain-hardening rates after irradiation, which resulted in significant uniform strains. It was found that the instability stress, the stress at the onset of necking, had little dependence on the irradiation dose. Tensile fracture stress and strain were calculated from the stress–strain curve data and were used to estimate fracture toughness using an existing model. The doses to plastic instability and fracture, the accumulated doses at which the yield stress reaches instability stress or fracture stress, were predicted by extrapolation of the yield stress, instability stress, and fracture stress to higher dose. The EC316LN alloy required the highest doses for plastic instability and fracture. Plastic deformation mechanisms are discussed in relation to the strain-hardening properties of the austenitic stainless steels.

Ion-irradiation-induced hardening in Inconel 718

Abstract Inconel 718 is a material under consideration for areas in the target region of the spallation neutron source (SNS), now under construction at Oak Ridge National Laboratory (ORNL) in the US. In these positions, displacement damage from protons and neutrons will affect the mechanical properties. In addition, significant amounts of helium and hydrogen will build up in the material due to transmutation reactions. Nanoindentation measurements of solution-annealed (SA) Inconel 718 specimens, implanted with Fe-, He-, and H-ions to simulate SNS target radiation conditions, have shown that hardening occurs due to ion-induced displacement damage as well as due to the build-up of helium bubbles in the irradiated layer. Precipitation-hardened (PH) Inconel 718 also exhibited hardening by helium build-up but showed softening as a function of displacement damage due to dissolution of the γ′ and γ″ precipitates.

Origin of hardening and deformation mechanisms in irradiated 316 LN austenitic stainless steel

The effects of displacement damage and trapped helium on deformation microstructures in AISI 316 LN austenitic stainless steel were studied by applying a newly developed disk bend method to specimens irradiated with 360 keV He ions at 200°C. Radiation damage microstructures consisted of an intimate mix of black dots, dislocation loops, and very small helium filled cavities. In the unirradiated specimens, the deformation mode upon straining was planar glide with cross-slip. With increasing dose, cross-slip was progressively restricted. Correspondingly, deformation microstructure changed from dislocation network dominant to channeling dominant. The channel bands were composed of piled-up dislocations, stacking faults, and twinned layers.

Fabrication of single-crystal diamond microcomponents

We have combined a technique for the lift‐off of thin diamond films from a bulk diamond with a technique for engraving diamond with a focused excimer laser to produce free‐standing single‐crystal diamond microstructures. One microcomponent that has been produced is a 12 tooth gear ∼400 μm in diameter and ∼13 μm thick. Other microstructures have also been demonstrated, showing the versatility of this method. This process should be applicable to producing diamond microcomponents down to spatial dimensions (width and thickness) of a few micrometers.

Ion implantation of epitaxial GaN films: damage, doping and activation

Single-crystal GaN films grown on AlN buffer layers previously deposited on 6H-SiC(0001) were studied for radiation damage and its recovery using Rutherford backscattering/channeling, photoluminescence, and cross-sectional TEM. The highest fluence of (1e15 cm{sup -2}) 110 keV Mg and 160 keV Si produced little damage at implantation temperature 550 C. RT damage was higher for same fluences compared to 550 C implantation. The damage was partially annealed by RTA at 1000 C, however, this was not enough to recover the PL signal even for the lowest fluence (1e14 cm{sup -2}). XTEM of as-implanted samples revealed small clusters of defects extended beyond the projected ion range. To recover damage completely, perhaps one needs to go either much higher RTA temperature and/or implant samples in a smaller fluence increment and anneal in between implants to recover the damage.