The present work concerns the investigation of the behaviour of helium in neutron irradiated copper-boron alloys in the temperature range 300ºC and 900ºC. It has been found in this work that bubbles nucleate preferentially upon dislocations which are created by the annealing of displacement damage. After gas precipitation has been completed, the bubbles grow by migration and coalescence at such a rate that the average bubble radius is proportional to t1/14, where t is the time. Helium causes solution hardening and the strength of an alloy containing bubbles is inversely proportional to the bubble spacing. One atomic per cent of helium causes a 3 µ ohm-cm increase in electrical resistivity. The changes which occur in the resistance during annealing are complicated and contributions arise from the concentration of helium in solution and the changing distribution of bubbles. An activation energy of 0.98 ± 0.1 eV has been obtained for the first stage of annealing during which all of the gas leaves solution to form bubbles.
The degeneration of grain boundaries during the annealing of copper-helium alloys has also been investigated. The grain boundary bubbles are stable at temperatures below 700ºC, but at higher temperatures they grow spontaneously by absorbing vacancies. Resistivity measurements are particularly sensitive to grain boundary degeneration and can be used to investigate the process in its earliest stages. Increasing the temperature after bubbles have reached an equilibrium size causes breakaway growth of grain boundary bubbles at temperatures below those at which it would normally occur. Gas concentrations as low as 46 ppm are sufficient to cause grain boundary degeneration at 800ºC. The ultimate failure of grain boundaries is caused by the growth of bubbles until they touch and the conditions which contribute to this growth in the absence of an applied stress have been examined.
A theory has been proposed in which the lenticular growth of grain boundary bubbles and the resultant force generated across the boundary by the gas within them are considered, in order to explain spontaneous cracking at high temperatures. It has been proposed that most of the vacancies which contribute to bubble growth enter the bubbles in the plane of the boundary. A non-equilibrium shape results and the forces due to gas pressure gradually exceed the restraint imposed by surface tension effects. The driving force for this mechanism of non-equilibrium growth is derived from the chemical potential gradient for vacancies between the grain boundary and the bubble surface.
The theory predicts a minimum temperature for spontaneous bubble growth which is comparable with experimental observations.
The effect of applied stress has also been considered and an equation which predicts the time to rupture as a function of stress and temperature has been derived.
The stress-rupture properties of O.F.H.C. copper, Cu-0.04 wt.% B and heat treated Cu-0.05 at.% He have been compared in terms of the theoretical model. The activation energy for the process of fracture was found to be 2.17 ± 0.2 eV in all three materials which is close to that for volume diffusion in copper. At temperatures below a critical temperature, the stress-rupture properties have been found to be independent of the presence of gas in the grain boundary voids and to depend only upon the void distribution. The only apparent effect of gas is to alter the size and distribution of voids within the grain boundaries. The distribution of helium bubbles can be controlled by heat treatment and the stress-rupture properties of Cu-B alloys can be improved at low stresses by introducing a coarse distribution of bubbles. However, the number of voids in the grain boundaries was found to increase with stress even when helium bubbles were present.
At temperatures above a critical temperature, the pressure of gas within a bubble was found to produce an internal stress, in addition to the applied stress, which decreased the time required for rupture. This additional stress arose when the stress due to pressure exceeded the restraint caused by surface tension during lenticular bubble growth.
The common assumption, that stressed grain boundaries in which void growth occurs, act as vacancy sources is not supported by the experimental evidence. Rather it appears that vacancies flow towards the boundaries from the surrounding matrix.