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A conceptual design of a 4 K irradiation test facility has been conducted in support of the International Thermonuclear Experimental Reactor (ITER) magnet development program. A new research reactor designated as HANARO at the Korea Atomic Energy Research Institute has a Cold Neutron Source (CNS) port that is identified to be suitable for the fast neutron irradiation of metals and insulation materials for superconducting magnets at 4 K. A 40 hours of irradiation at full power will produce 2.5×1017 n/cm2 of the ITER magnet design neutron fluence with energy above 0.1 MeV. A material testing laboratory of Irradiated Materials Evaluation Facility (IMEF) that is located next to HANARO has been equipped with 77 K test machines and fracture analysis microscopes for radioactive specimens which can be upgraded for 4 K test without any intermediate warming. CNS radiation spectrum determined by Monte Carlo method is found to be more favorable for metal irradiation than for insulation materials with absorbed gamma dose that is 7–10 times the fast neutron dose. A lead-shielded irradiation capsule design with a 1 cm diameter specimen in 3 cm cold-bore diameter and 18 cm height will require about 120 watt cooling capacity at 4.6 K.

Incoloy® alloy 908+ is a candidate conduit material for the large-scale Nb3Sn superconducting magnets of the International Thermonuclear Experimental Reactor (ITER).1 It is a nickel-iron base precipitation-hardening superalloy with a chemical composition that has been optimized for a low coefficient of thermal expansion, superior cryogenic structural properties, and phase stability during the Nb3Sn reaction heat treatment.2 The alloy precipitates γ′, Ni3(A1,Ti,Nb), as the primary strengthening phase and has demonstrated excellent mechanical properties at both 298 and 4 K.3,4 However, welds of the alloy have shown reduced fracture toughness. Since fabrication of the cablein-conduit conductors for ITER will require welding, the motive of the work has been to improve fracture toughness while maintaining adequate weld strength.

Short crack simulation (SCS) tests were used to characterize the fatigue crack growth rates da/dN and thresholds ΔKTh of a Ni-Fe superalloy proposed for conduit sheath applications in superconducting magnets at 4 K. Experiments described demonstrate the reproducibility of the SCS data and confirm that this alloy has a relatively high fatigue crack growth rate threshold at 4 K. The effects of plate thickness and thermal treatment (650°C, 180 h) on near-threshold fatigue properties are minor compared with a strong cryogenic temperature effect that increases the fatigue threshold from about 2 MPa·m1/2 at room temperature to 9 or 10 MPa·m1/2 at 4 K.

Heat treatments of magnets utilizing INCOLOY® alloy 908* as a conduit have been successfully performed in vacuum. Similar experience with large scale heat treatment in an inert gas environment such as argon is lacking. Prior studies on other nickel-iron base superalloys that are susceptible to intergranular oxygen embrittlement and cracking emphasize the importance of establishing an allowable oxygen impurity level in argon for alloy 908. Initial screening using C-ring tests have shown that cracking can occur in an argon atmosphere if proper control over the oxygen impurity level is not maintained. Stress-rupture tests performed in air show that this material is susceptible to intergranular cracking in notched sections when subjected to stresses in excess of 300 MPa for a stress-concentration factor (Kt) of 4.1 at the notch. A series of stress-rupture tests are now underway on alloy 908 base metal in oxygen containing argon atmospheres. A double-edged notched test specimen design is used to determine the rupture time as functions of applied stress, temperature and oxygen concentration. The oxygen concentration at the specimen notches is continuously measured using an electrochemical sensor. Initial results suggest that an argon atmosphere does yield an improved stress-rupture life over air at low oxygen concentrations. Results are discussed to establish whether the possibility for heat treatments in argon exists and if so what guidelines must be used for successful heat treatment.

The relationship between thermomechanical processing, hardness, grain size and mechanical properties has been studied for INCOLOY® alloy 908, a nickel-iron base superalloy developed for use as a conduit material in Nb3Sn cable-in-conduit conductors (CICC).1 The alloy has thermal coefficient of expansion properties tailored for Nb3Sn, minimizing compressive strain in the superconductor due to cooldown and the associated reduction in critical properties (upper critical field, critical temperature, and critical current). Alloy 908 also has mechanical properties at cryogenic temperatures that compare favorably with other materials. It is therefore one of the candidate materials for use in the magnets of the International Thermonuclear Experimental Reactor (ITER).2,3

This paper describes nonstandard methods of testing Charpy specimens at 4 K. We show that the initial temperature can be achieved using a helium flow method with U-type as well as C-type machines. Unfortunately, heating during impact loading weakens the correlation with quasistatic fracture toughness parameters. Alternative test procedures using fatigue precracked specimens were not fully satisfactory, either: (1) Precracking reduces the impact energy by 35–54%, but the specimen heating is still significant; (2) slow bending curtails the heating most effectively, but then the desired simplicity of a screening test is lost.

Incoloy 908 is a nickel-iron base superalloy with its coefficient of thermal expansion and mechanical properties optimized for use in Nb3Sn superconducting magnets. Thermoelastic, tensile, fatigue crack growth, fracture toughness,and Charpy impact properties, and the results of conduit pressurization tests are summarized for base and weld metal.A limited number of stress rupture tests were also performed in air. The average yield strength (0.2% offset) for the solution annealed and aged base metal is 1200 MPa at 4.2 K. The fracture toughness,KIC, is greater than 230 MPa√m at 4.2 K. The fatigue behavior at 4.2K is comparable to austenitic stainless steels. Fatigue crack growth rates are a factor of three lower at 4.2 K than 298 K and are independent of heat treatment. At 4.2 K, the 20% cold-work-then-aged material has a 20% higher yield strength and a 10% higher ultimate tensile strength. Gas tungsten arc weld (GTAW) metal with or without Incoloy 908 filler metal exhibited comparable yield and about 10% lower tensile strength when compared with that of the base metal after a 200 hour age at 650°C. Fracture toughness, tensile elongation and Charpy absorbed energy were about 40% of those of the base metal. Leak-before-break behavior was observed in an internal pressurization test at room temperature for a geometry identical to that of the US-Demonstration Poloidal Coil conduit. The stress rupture performance is better than other low COE alloys of a similar type to that of Incoloy 908.

The fundamental advantages of soft X rays over electrons in the examination of fine structures have been described in detail by various authors.(1−5) The oretically, x-ray microscopy provides higher resolution than light microscopy, higher penetration ability than electron microscopy, and, most importantly, x-ray microscopy promises the potential for imaging hydrated specimens. Therefore, x-ray microscopy could occupy a niche in biological research in three-dimensional imaging of samples in the resolution range of the electron microscopy but of substantially greater thickness. This would greatly simplify the observation and interpretation of three-dimensional ultrastructures of living specimens beyond the resolution limit of the light microscope.