Lattice Thermal Conductivity of a Neutron-Irradiated Copper-Aluminum Alloy

Low-temperature thermal-conductivity results are presented on two Cu +9-at.%-Al polycrystals which show that fast-neutron irradiation has two effects on a coldworked sample of this alloy. In the well-annealed specimen, irradiation produced defects which scatter long-wavelength phonons, giving a well-defined $T^{-2\mathrm{}}$ contribution to the lattice thermal resistivity. Observations by other authors using electron microscopy suggest that the defects are planar-vacancy-generated dislocation loops 75-100 Å in diameter. The loop density is calculated from the thermal resistivity and is found to be in reasonable agreement with the electron-microscopy results. In the second sample, deformed in tension at room temperature and then annealed at 573°K prior to irradiation, the same irradiation treatment produced a contribution to the lattice resistivity which was similar in temperature dependence but larger, by a factor of 3, than that produced in the well-annealed specimen. A model for this effect is suggested, based on two distinct contributions to the radiation-produced thermal resistivity: (i) dislocation loops of similar size and concentration as in the well-annealed sample; (ii) reformation of solute atmospheres around dislocations. These atmospheres had been thermally dispersed by annealing; their reformation is due to enhanced solute diffusion resulting from a large vacancy concentration produced by irradiation. The work of Mitchell et al. has shown that solute atmospheres in Cu+10 at.% Al enhances phonon scattering by dislocations.