The effect of concentration upon helium precipitation in irradiated copper boron alloys

The lattice parameter of copper is proportional to the concentration of dissolved interstitial helium up to approximately 0.03 at %. When the concentration of helium is increased to 0.08-0.1 at %, the gas precipitates to form bubbles during irradiation at temperatures in the range 75° C-100° C. During annealing at 500° C, the lattice parameter contracts owing to the formation of helium-vacancy complexes, at a rate which is proportional to the concentration of helium. Subsequent expansion of the lattice, caused by the precipitation of these complexes to form bubbles, also occurs at a rate which is proportional to helium concentration. The total time required for gas precipitation is inversely proportional to the helium content and this behaviour is associated with an increasingly fine scale of gas bubble nucleation. The average size and spacing of bubbles formed during the complete precipitation of helium increase with decreasing gas concentration. The bubble density is related to the number of dislocations generated by the annealing of displacement defects caused by fast neutron and alpha particle collisions. However, some homogeneous nucleation may also occur in materials with a high gas concentration. As the gas concentration of helium increases, a rapid initial contraction of the lattice develops during the first two minutes of annealing at 500° C. It is suspected that many of the helium atoms formed during irradiation are closely associated with potential vacancy sources such as jogs on dislocations, or clusters of vacancies caused by displacement collisions. At temperatures above those at which the vacancies become mobile, a proportion of these helium atoms enter substitutional solution very rapidly because the diffusion distances are short