T.S. Byun

On the stress dependence of partial dislocation separation and deformation microstructure in austenitic stainless steels

Abstract In the austenitic stainless steels the separation of Shockley partial dislocations is known to play an important role in the plastic deformation and produces a variety of deformation microstructures depending on test and material conditions. Theoretical calculations have been carried out in an attempt to explain the origin of the deformation microstructures which include large stacking faults and twins. Force balance equations for the leading and trailing partials are established by considering the Peach–Koehler force from an applied stress field, repulsive force between leading and trailing partial dislocations, attractive force due to the stacking fault energy, and resistance (or damping) force to the glide of the partial dislocations. For a simple dislocation and stress arrangement, an expression for separation distance was derived from the force balance equations. The results indicate that the separation distance varies with the directional relationship between the applied stress and the Burgers vectors of glide dislocations. Also, the separation distance increases with the applied stress and can diverge when the applied stress exceeds a critical stress. The critical stress is readily achievable in the uniform strain range by strengthening measures like irradiation, lowering test temperature, and increasing strain or strain rate. Further, using a stress-based analysis, some predictions were attempted for the influence of radiation-induced defects on deformation microstructure in austenitic stainless steels.

Dose dependence of defect accumulation in neutron irradiated copper and iron

Abstract In order to investigate the difference in defect accumulation between fcc Cu and bcc Fe, tensile specimens were neutron irradiated at ≃70 °C in the HFIR reactor at Oak Ridge National Laboratory to fluences in the range of 4.5×1020–4.7×1024 n/m2 (E>1 MeV) corresponding to displacement dose levels in the range of about 0.0001–0.8 dpa. Irradiated specimens were characterized using positron annihilation spectroscopy, transmission electron microscopy and electrical conductivity measurements. A limited number of iron specimens were also tensile tested. At 0.0001 dpa, a low density of very small vacancy clusters (1–3 vacancies) were detected in iron, while bigger three-dimensional cavities were observed at higher doses. Both their density and average size increased with increasing dose level. In contrast, no such cavities were observed in copper. Irradiation led to an increase in yield stress and a decrease in the uniform elongation for iron.

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.

Tensile properties of candidate SNS target container materials after proton and neutron irradiation in the LANSCE accelerator

Abstract Tensile specimens of five austenitic stainless steels and two ferritic/martensitic (f/m) steels were irradiated under spallation conditions at temperatures between 60°C and 164°C to doses between 0.4 and 11 dpa. The irradiations were performed at the Los Alamos Neutron Science Center (LANSCE) accelerator in a beam of 800 MeV protons and in the mixed spectrum of protons and spallation neutrons from a tungsten target. Tensile testing was done at room temperature at a crosshead speed of 0.005 mm/s, corresponding to a strain rate of 10 −3 s −1 . All materials showed considerable irradiation hardening and loss of ductility. For EC316 LN stainless steel, which is the recommended material for construction of the spallation neutron source (SNS) mercury target container and shroud, the yield strength (0.2% offset) was increased by a factor of three at 11 dpa. This steel retained a significant uniform elongation of 6%, as did the other austenitic steels. The two f/m steels entered plastic instability failures at strains less than 1% for all doses.

Tensile properties of ferritic/martensitic steels irradiated in HFIR, and comparison with spallation irradiation data

Abstract Tensile properties of four ferritic/martensitic steels, 9Cr–1MoVNb, 9Cr–1MoVNb–2Ni, 9Cr–2WV, and 9Cr–2WVTa, and two bainitic steels, 3Cr–3WV and A533B, were measured after irradiation to doses up to 1.2 dpa at temperatures in the range 60–100 °C in the High Flux Isotope Reactor and compared with two ferritic/martensitic (F/M) steels irradiated with 800 MeV protons and spallation neutrons in the LANSCE facility at 60–164 °C. Irradiation hardening in the steels was strong, and all of them displayed plastic instability shortly after yield for irradiations of 0.054 dpa and higher. Despite large loses in elongation, all failures occurred in a ductile manner. The dose dependencies of the increases in the yield strength with dpa were similar for all six steels, and contained a pronounced change at about 0.05 dpa. Below 0.05 dpa, the hardening exponent was 0.5–0.6, consistent with a barrier hardening mechanism. Above 0.05 dpa, the exponent was reduced to 0.1–0.2, which is speculated to be due to intervention by dislocation channeling. A trend curve for correlating changes in yield strengths of F/M steels with dose at irradiation temperatures below 160 °C is offered.

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.