The alpha and beta Peaks in Cold-Worked Niobium

Internal-friction measurements (at 1 c/sec) have been carried out over the temperature range from 90 to 290 degrees K, on niobium specimens deformed at room temperature. In the as-cold-worked material, a broad peak at about 115 degrees K (alpha peak) is observed. The alpha peak increases with the amount of deformation and decreases with increasing interstitial impurity content. On subsequent annealing, the height of the peak decreases by about 50% over the temperature range from 90 to 140 degrees C, and to negligible values from 250 to 340 degrees C. As a result of annealing for 2 h at 70 degrees C, a group of peaks (beta peaks) occurred at about 200 degrees K. The beta peaks are independent of the amount of deformation prior to annealing and the interstitial impurity content. On further annealing, the relaxation strength of the peaks increases with temperature up to about 100 degrees C, remains constant between 100 and 240 degrees C, and subsequently gradually decreases to negligibly low values at about 340 degrees C. The alpha peak, and its variation with deformation, impurity content, and annealing, can be accounted for in terms of relaxation mechanisms involving dislocations (i.e. a Bordoni- or Hasiguti-type process observed in fcc metals). This is a generally accepted concept at present. The beta peaks, on the other hand, could only be adequately accounted for by relaxation processes involving complexes of deformation-created point defects and interstitial impurities.