A detailed theoretical analysis was made of an internal-friction relaxation peak, at about 1Hz, which occurred at about 210K in concentrated alloys. A decrease in the peak during annealing indicated that the dislocation density was largely responsible for the effect. It was shown that the observed peak was a superposition of 2 components, which were caused by the overall motion of extended dislocations (acoustic mode) and the relative motion between 2 partials (optical mode). The ratio of the relaxation strengths of the optical and acoustic modes ranged from about 0.1, in the case of a high stacking-fault energy, to about unity for a low stacking-fault energy. By assuming that solute-atom plus vacancy pairs on dislocation lines acted as mobile pinning agents which damped the motion of the dislocations, a predicted peak temperature was deduced which was close to the observed one. It was concluded that the experimentally observed increase in peak height with increasing solute concentration could be attributed only very partially to the effect of stacking-fault energy upon the optical-mode contribution. A possibly greater cause was a difference in effective dislocation density, due to a change in the dislocation arrangement with solute concentration. This might arise due to a transition from a cell structure to a planar structure.

The Mechanism of Low Frequency Internal Friction in Concentrated Cu-Al and Cu-Zn Alloys. Q.P.Kong, K.Lücke: Philosophical Magazine A, 1999, 79[9], 2185-93