Internal Friction in the Interstitial Solid Solutions of C and O in Tantalum

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Abstract
A strongly temperature-dependent type of internal friction is observed in tantalum specimens containing a small amount of carbon or oxygen. Interanl friction peaks occur, respectively, around 150°C and 170°C for carbon and oxygen when the frequency of vibration is about one cycle per second. The activation energies associated with these relaxations are found to be 25,000 calories per mole for carbon and 29,000 calories per mole for oxygen. These observations are consistent with the viewpoint that the relaxations observed are associated with the anelasticity caused by the stress-induced preferential distribution of C or O among the various interstitial positions which have tetragonal symmetry in body-centered cubic tantalum. Such a picture has been proposed by Snoek in interpreting the relaxation phenomena observed in alpha-iron (body-centered cubic) containing small amounts of carbon or nitrogen. The observations in tantalum might thus indicate that such relaxations are characteristic of all interstitial solid solutions of body-centered cubic metals. It has been further found that the carbon relaxation peak can be interpreted in terms of a relaxation process having a single relaxation time with a unique activation energy while the oxygen peak can not. This suggests that the interstitial atoms responsible for the observed relaxation peaks in tantalum are situated at the octahedral positions in the case of carbon and at both octahedral and tetrahederal positions in the case of oxygen.