Strain Ageing Phenomena in Niobium.

Strain ageing effects in arc melted commercial niobium have been studied using the yield point return and internal friction technique, and the results have been discussed with reference to current theories of strain ageing. A comparison of both techniques has been carried out which shows that the internal friction technique frequently yields more direct information about the strain ageing process than the yield point return method. It has been shown that commercial niobium, when tested at room temperature, exhibits a yield point phenomenon and that strain ageing effects become measureable at ageing temperatures greater than 75 C. The activation energy of the strain ageing process has been found to be 25, 800 +- 1,000 cal/mole which is of the order of the activation energy for the diffusion of oxygen in niobium (26,600 cal/mole). The magnitude of the strain ageing effects have been shown to be dependent on the concentration of oxygen, and this appears to be due to the variation in the density of dislocations, as calculated from the Cottrell-Bilby equation, with oxygen content rather than the density of Cottrell atmospheres. It is believed that oxygen is responsible for the locking of free dislocations during the early stages of strain ageing. Given sufficient time and/or sufficiently high temperature the oxygen is then replaced by carbon and/or nitrogen atoms at the dislocations and returns into free solution