Internal-friction study of the interstitial-substitutional effect on the deformation behaviour of Nb-O, Nb-Ta-O and Nb-Mo-O single crystals

To investigate the interstitial-substitutional interaction in dislocations, the effect of O on the temperature, frequency and amplitude dependence of the internal friction Q^-1 in Nb-O, Nb-20 mol% Ta-O and Nb-20 mol% Mo-O single crystals has been studied (f= 1.55-8.2 Hz) in the temperature range from 298 to 1473K. In our previous study, Nb-Mo and Nb-Ta single crystals were found to be strengthened by solute O. It was also suggested that the interstitial-substitutional interaction in dislocations contributes to the increase in their critical resolved shear stress (CRSS). In this study, Snoek-type relaxation peaks due to O are observed between 500 and 700 K in all the single crystals. The Snoek peak of Nb-20 mol% Mo-O consisting of several peaks is analysed. The activation energy of the Snoek peak in Nb-20 mol% Mo-O is higher than that of Nb-O. These results are attributable to the existence of the interstitial-substitutional complexes. The amplitude dependence of Q ^-1 at intermediate and high temperatures decreases as the O content increases. Moreover, the breakaway stress of Nb-20 mol% Mo-O still has a high value at 1200 K and does not decrease much even at 1473 K. This suggests that the formation of Mo-O complexes reduces the dislocation mobility at high temperatures. From the results, the effect of the interstitial-substitutional interaction on the CRSS was discussed.