R.R. Coltman

Low‐Temperature Deformation of Copper Single Crystals

Single crystals of copper were deformed at 4.2°K and 77.3°K. At 4.2°K, after a large strain produced by normal slip, jerky flow (discontinuous slip) occurs. From the study of reactor irradiated crystals, it was deduced that a packet of 30 slip lines, each containing 104 dislocations, was released to form each jerk of the discontinuous flow. For samples of certain orientation the region of discontinuous flow was followed by a region of deformation twinning. The fact that deformation twinning was taking place was verified by x‐ray methods. The twinning elements were determined to be the (111) plane and the [112] direction. At 77.3°K, discontinuous slip was not observed. In a limited range of orientations, twinning has been observed at 77.3°K. The effects of reactor irradiations on discontinuous slip and on twinning were also studied and it was determined that the occurrence of twinning was unaffected by the irradiations.

Radiation hardening of copper single crystals

Abstract The effect of nuclear radiation on the mechanical properties of copper has been studied. It has been found that the yield stress, which is substantially increased by the radiation, increases as the cube root of the flux. A strong temperature dependence of the yield stress of irradiated copper is observed with the yield stress being given by a function similar to σ = A — BT 1 2 above 40° K. A Luders band with slip lines of very large step height is associated with the enhanced yield stress at small strains. At large strains the phenomenon of overshoot is observed. The annealing kinetics of the radiation hardness have also been studied in the temperature range from 25° K to 700° K. Little or no annealing is observed in the region below 80° K. In the region from 80° K to 300° K approximately 20 % of the yield stress is recovered, with the remainder annealing in the range from 600° K to 700° K. These results have been discussed in terms of the possible mechanisms by which the hardening can occur. While by no means conclusive, these data support a dislocation locking mechanism. On the other hand a very close analogy exists between radiation hardening and the hardening which arises from the addition of impurities, e.g. the hardening in α brass. The correlation between work hardening and radiation hardening appears to be quite small.

Low‐Temperature Reactor Irradiation Effects in Metals

The effects of structural and chemical defects on the low‐temperature (30–50°K) annealing peak in low‐temperature reactor‐irradiated aluminum and copper were studied. From the fact that the density of reactor‐induced defects did not affect the annealing kinetics, it was possible to conclude that the low‐temperature annealing process was of the first order without a unique activation energy. The fact fact that both oversized and undersized atoms could suppress this annealing peak led to the conclusion that the radiation‐induced defects were more complicated than simple point defects. The suggestion is made that a defect similar to a crowdion must be created by low‐temperature neutron irradiation. This data also supports to some degree the viewpoint that a radiation‐induced defect, possibly a crowdion, has sufficient knock‐on energy to migrate several hundred atomic distances. The experiments also contain evidence which rule out all forms of vacancy‐interstitial annihilation.

Reactor Damage in Pure Metals

By relocating the fuel in the vicinity of a liquid helium cryostat located in the Oak Ridge Graphite Reactor it has been possible to separate the thermal and fast components of the reactor neutron flux. Studies of the radiation damage effects arising from each type of flux have been made. It has been found that an appreciable fraction of the reactor damage in several metals arises from thermal neutrons. The effect results from the recoil of an atom from the (n,γ) reaction at the time of thermal neutron capture. The low temperature recovery of thermal neutron damage is greater and shows more annealing peak structure than the recovery of fast neutron damage. Thermal neutron damage concentration studies have been made on cadmium, and pronounced suppression of the annealing is found as the concentration is increased. The mean primary recoil energy from a thermal neutron capture event has been calculated for several elements. Values range from about 50 eV for the heavier elements to several hundred eV for the ...

Mechanical strength of low-tempepvature-irradiated polyimides: A five-to-tenfold improvement in dose-resistance over epoxies

Neutronics calculations by Engholm show that without additional shielding even the first fusion test reactors such as the Fusion Engineering Device will produce lifetime doses at magnet insulator locations that exceed the radiation tolerance of glass-fabric-filled (gff) epoxies now used. To explore the possible use of an alternative insulator, the mechanical strength of pure and recently available gff polyimides was studied as a function of gamma-ray irradiation at 4.9 K to 100 MGy (10/sup 10/ rads). After a postirradiation anneal at 307/sup 0/K the flexure and compressive strengths of the gff materials measured at 77/sup 0/K were reduced by up to 40% for 100 MGy while the pure material changed little. Testing done at 300/sup 0/K gave similar results, but all stress values were about 40% less. Compared to earlier epoxy studies we find that, overall, the gff polyimides are 5 to 10 times more radiation resistant than comparably prepared gff epoxies.

Thermal Neutron damage in cadmium

Abstract The isochronal annealing of the damage produced by thermal neutron irradiation of cadmium at 3.6 °K has been studied for several initial doses which vary by a factor of 1000. The recovery results show a strong dependence upon initial dose. This effect, which is not seen to this extent in the fcc metals, cannot be accounted for by an irradiation annealing mechanism. In contrast to the observation of two processes involving long range defect migration for several fcc metals only one process, at high temperatures, is discernable from isochronal annealing of Cd. The presence of another process at low temperatures is clearly established by other means. Irradiation annealing effects observed during the production of damage at high defect concentrations indicate that the spontaneous annihilation volume between the defects of a new capture event and the defects from an earlier event is 80 atomic volumes. Other results suggest that damage production and recovery mechanisms may be associated with the aniso...

Debonding of epoxy from glass in irradiated laminates

Abstract Glass fabric filled epoxies irradiated at 4.7 K and examined at room temperature by 20 × stereomicroscopy showed an internal flaw structure which increasingly filled the sample as the γ dose was increased. These flaws were determined to be areas where the plastic had debonded from the glass fibers. The extent of this process correlated well with the dose-dependent loss of mechanical strength. Evidence is reported for a similar mechanism in polyimides although visible flaws have not yet been produced. Possible mechanisms for debonding are suggested. New experiments are also suggested to further clarify the failure mechanism.

Organic insulators and the copper stabilizer for fusion reactor magnets

Abstract The materials which compose the large composite superconducting fusion reactor magnets are subjected to mechanical stress, neutron and gamma-ray radiation with broad energy spectra, high magnetic fields, and thermal cycling from 4 to 300 K. Of the materials now considered for use in the magnets, results show that the organic insulators and the Cu stabilizer are the most sensitive to this environment. In response to the need for stabilizer data, magnetoresistivity changes were studied in eight variously prepared specimens of Cu throughout five cycles of an alternate neutron irradiation (4.0 K) and annealing (14 h at 307 K) program. The results were combined with those on the radiation behaviour of epoxy and polyimide organic insulators to provide a preliminary assessment of their comparative radiation resistance in a typical magnet location of the Experimental Power Reactor (EPR). It was found that insofar as the chosen conditions typify the mix of gamma-ray and neutron dose rates in fusion reactor magnet locations, the radiation resistance of the stabilizer is at best equal to that of the epoxies and much less than that of the polyimides. The poor performance of the Cu stabilizer suggests that additional shielding for magnet protection or serious magnet design changes are needed. Several needs for further organic insulator and stabilizer research are described.

Fission neutron damage rates and efficiencies in several metals

Abstract Initial rates of resistivity-measured low-temperature damage production by fission-spectrum fast neutrons have been determined for 14 metals in the same very well characterized irradiation facility. Six of these metals were fcc, five were bcc, and three were hcp. Most were of quite high purity. Observed damage rates, after correction for all known extraneous resistivity-producing effects, were compared with rates predicted by the damage calculation code “RECOIL”, using parameters chosen from the literature. These parameters, effective displacement threshold energy, Ed, and Frenkel-pair resistivity, ρF, were in many cases only best estimates, the further refinement of which may be aided by the present results. Damage efficiencies (measured/predicted rates) follow the same trends by crystal classes as seen in other fast-neutron studies.

Rates of defect production by fission neutrons in metals at 4.7 K

Abstract As part of an interlaboratory program, we have measured the resistivity-damage rates at 4.7 K for the dilute alloys, V-300 ppm Zr, Nb-300 ppm Zr, and Mo-300 ppm Zr, irradiated by virtually unmoderated fission neutrons. In addition, Al, Ni, Cu, and stainless steel have also been measured to provide a broader data base for comparison with other experimental work using a variety of neutron spectra and with defect-production theory. A broad view of the results shows that the ratio of experimentally to theoretically determined production rates (damage efficiencies) for various fast-neutron spectra ranges from about 0.25 to 0.50. On the other hand, for a given element various neutron-energy spectra peaked from 1 to 15 MeV give variations in damage-efficiency values of only 7–30%.