Strain ageing of copper-tin alpha solid solutions

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Abstract
The strain ageing of copper—tin alpha solid solutions under the influence of an applied load has been studied. It has been found that strain ageing is due to the elastic interaction of tin atoms with dislocations. The diffusion of tin to the dislocations during ageing is shown to be vacancy-controlled, with an activation energy of 0•75 ± 0•05 ev. The equation relating vacancy concentration Cv to plastic strain ε is found to be C v = Bε2•2, where B is a constant. Initially, the material contains a concentration of vacancies caused by straining, as well as a thermal equilibrium concentration. As ageing proceeds, the concentration of vacancies decreases exponentially towards the thermal equilibrium concentration, with a consequent decrease in the vacancy diffusion coefficient. This change affects the rate of strain ageing during the initial stages. An equation is developed which takes into account the effect of excess vacancies.