J.E. Pawel

Irradiation performance of stainless steels for ITER application

Abstract The proposed normal operating temperature range for the ITER first wall/shield structure (100–250°C) is below the temperature regimes for void swelling (400–600°C) and for grain boundary embrittlement (500–700°C). However, the neutron doses for the basic performance phase (3–4 dpa) and the extended performance phase (20–30 dpa) are such that large changes in yield strength, deformation mode, and strain hardening capacity will be encountered which could significantly affect fracture properties. Yield strength increases rapidly with dose in the 60–300°C regime with the increase tending to saturate at 1–3 dpa. Under certain conditions, radiation hardening is accompanied by changes in the stress-strain relationship with the appearance of an initial yield drop and a significant reduction in strain hardening capacity. This paper reviews the low temperature (

Effects of low temperature neutron irradiation on deformation behavior of austenitic stainless steels

Abstract Two experiments have been conducted to quantify the effects of neutron irradiation on the deformation and fracture behavior of solution annealed austenitic stainless steels irradiated to doses ranging from 3 to 19 dpa at temperatures from 60 to 400°C. For all alloys, yield strength increases rapidly with dose in the 60–300°C regime. Radiation hardening is accompanied by changes in the flow properties with the appearance of an initial yield drop and a significant reduction in strain hardening capacity. The magnitude of the changes is dependent upon both neutron dose and irradiation temperature, with reductions in strain hardening capacity occurring most rapidly in the range 250–350°C. It is shown that for neutron doses up to about 3 dpa, although the changes in deformation mode reduce the fracture thoughness, the toughness remains satisfactorily high.

Implications of radiation-induced reductions in ductility to the design of austenitic stainless steel structures

Abstract In the dose and temperature range anticipated for ITER, austenitic stainless steels exhibit significant hardening with a concomitant loss in work hardening and uniform elongation. However, significant post-necking ductility may still be retained. When elongation ( e u ) is well defined of a plastic instability, e u is found to sustain reasonably high values out to about 7 dpa in the temperature range 250–350†C, beyond which it decreases to about 0.3% for 316LN. This loss of ductility has significant implications to fracture toughness and the onset of new failure modes associated with shear instability. However, the retention of a significant reduction in area at failure following irradiation indicates a less severe degradation of low-cycle fatigue life in agreement with a limited amount of data obtained to date. Suggestions are made for incorporating these results into design criteria and future testing programs.

Fracture toughness of candidate materials for ITER first wall, blanket, and shield structures☆

Abstract Although the design of the International Thermonuclear Experimental Reactor is still in an evolutionary phase, the most probable choice for a structural material is a 300 series austenitic stainless steel. An experiment was carried out in the High Flux Isotope Reactor to quantify the effects of neutron irradiation on the fracture toughness properties of a range of austenitic stainless steels. The alloys investigated were provided by programs in the European Community, United States, and Japan; they included 316, 316L, and titanium-stabilized steels in cold-worked, annealed, and welded conditions. Subsize disk compact tension specimens were irradiated to a dose of about 3 dpa and 50 appm helium (the expected fusion value) at temperatures of either 60 to 125°C or 200 to 300°C. With the exception of a cold-worked air-melted heat of 316, all materials retained excellent fracture toughness, with KJ values varying from 150 to 400 MPa √m over the test temperature range from 90 to 250°C.

Comparison of elastic—plastic fracture toughness of irradiated and cold-worked JPCA using miniaturized DCT specimens

Abstract J—R curves of an austenitic stainless steel in solution annealed and cold worked conditions were obtained using miniaturized fracture toughness specimens and standard compact tension specimens. Results indicate that the specimen size effect for the cold worked steel was small. JQ values of irradiated miniaturized specimens agreed well with those of cold worked specimens with similar yield stress levels. This suggests that the irradiation induced degradation of the fracture toughness is mainly dependent upon the irradiation hardening.

Fracture Toughness of Irradiated Candidate Materials for ITER First Wall/Blanket Structures

Disk compact specimens of candidate materials for first wall/blanket structures in ITER have been irradiated to damage levels of about 3 dpa at nominal irradiation temperatures of either 90 or 250{degrees}C. These specimens have been tested over a temperature range from 20 to 250{degrees}C to determine J-integral values and tearing moduli. The results show that irradiation at these temperatures reduces the fracture toughness of austenitic stainless steels, but the toughness remains quite high. The toughness decreases as the test temperature increases. Irradiation at 250{degrees}C is more damaging than at 90{degrees}C, causing larger decreases in the fracture toughness. Ferritic-martensitic steels are embrittled by the irradiation, and show the lowest toughness at room temperature.

Ion-beam-enhanced adhesion of iron films to sapphire substrates

Abstract The effect of implantation of different ion species on the adhesion of iron films to sapphire substrates was investigated. The implantation energies were adjusted to ensure that the ion concentration profiles, damage profiles, and recoil distributions were the same for each species. For all implantations, the peak ion concentration was at the film-substrate interface. The adhesion of the films was measured by a pull test and a scratch test. For a fluence of 1 × 10 15 ions cm -2 , implantation of chromium (300 keV) and iron (320 keV) increased the bond strength whereas implantation of nickel (340 keV) did not. The effect is proposed to be due to changes in the interfacial energy resulting from the presence of the ion species at the interface. Only a narrow zone is affected; the mixing at the interface is less than 10 nm.

Transmutation-induced embrittlement of vanadium and several vanadium alloys in HFIR

Abstract Vanadium, V1Ni, V10Ti and V10Ti1Ni (at%) were irradiated in HFIR to doses ranging from 18 to 30 dpa and temperatures between 300 and 600°C. Since the irradiation was conducted in a highly thermalized neutron spectrum without shielding against thermal neutrons, significant levels of chromium (15–22%) were formed by transmutation. The addition of such large chromium levels caused strong embrittlement. At higher irradiation temperatures radiation-induced segregation of transmutant Cr and solute Ti at specimen surfaces caused strong increases in the density of the alloy. The resultant shrinkage, possibly compounded by thermal cycling, led to cracks developing at all intersections of grain boundaries with the specimen surface. This caused specimens irradiated at 500°C or below to often fail during retrieval from the reactor, as well as during electropolishing and other handling operations. At 600°C, the cracking and embrittlement processes are so severe that only a fine dust, composed mostly of individual grains or chunks of grains, was found in the irradiation capsule.

Effect of boron on post irradiation tensile properties of reduced activation ferritic steel (F-82H) irradiated in HFIR

Reduced activation ferritic/martensitic steel, F-82H (Fe-8Cr-2W-V-Ta), was irradiated in the High Flux Isotope Reactor (HFIR) to doses between 11 and 34 dpa at 400 and 500 C. Post irradiation tensile tests were performed at the nominal irradiation temperature in vacuum. Some specimens included {sup 10}B or natural boron (nB) to estimate the helium effect on tensile properties. Tensile properties including the 0.2% offset yield stress, the ultimate tensile strength, the uniform elongation and the total elongation were measured. The tensile properties were not dependent on helium content in specimens irradiated to 34 dpa, however {sup 10}B-doped specimens with the highest levels of helium showed slightly higher yield strength and less ductility than boron-free specimens. Strength appears to go through a peak, and ductility through a trough at about 11 dpa. The irradiation to more than 21 dpa reduced the strength and increased the elongation to the unirradiated levels. Ferritic steels are one of the candidate alloys for nuclear fusion reactors because of their good thermophysical properties, their superior swelling resistance, and the low corrosion rate in contact with potential breeder and coolant materials.

Using Weibull Statistics to Analyze Ion Beam Enhanced Adhesion as Measured by the pull Test

The adhesion of iron films to single crystal Al 2 O 3 substrates was investigated using a pull test. Chromium (300 keV) or nickel (340 keV) ions were implanted to a fluence of 1 × 10 15 ions-cm -2 after film deposition. The adhesion test results were widely scattered due to a random distribution of interfacial flaw sizes controlling the failure nucleation. Because Weibull statistics were developed to describe the failure probability due to a population of flaw-initiated cracks, the Weibull distribution was chosen to analyze the data. Modifications in the adhesion strength due to the ion implantation were reflected in the failure distributions. It was found that the chromium ions improved the adhesion of the Fe/Al 2 O 3 system while the implantation of nickel did not.