INTERNAL FRICTION IN DILUTE SOLID SOLUTIONS

Abstract
The frequency and amplitude independent internal friction, Q^-1, of high-purity brasses of 45 µm grain size, containing 0.03 to 1.0 at.% Zn, was studied at 12 kHz at 120 and 290 K. Specimens were prestrained, after annealing, 1.5% in tension at room temperature before use ; at that level of deformation a peak occurred in the strain dependence of Q^-1 in all cases, suggesting similarity of structures. This peak value was used as the concentration dependent variable. As in the cas of concentrated alloys (Phil. Mag.A, 1984, 50, L.35), the loss was ascribed to two interdependent sources : dissipation of line energy, as bowing-out dislocations return quasi-statically to their zero-stress configuration on completion of each half-cycle, and a simultaneous loss of "elastic" energy by the alloy-atom/dislocation-core configurations. As in the case of concentrated alloys, the relation Q^-1 = αNL²[(1 - C/C_∞)²(1 + Ψ/8&Piα C/C_∞) + β]/(1 + β) accounted well for the observations. Here N is the density of vibrating dislocations, L the mean spacing between hard pinning-points, α an orientation factor, β/(1 + β) the fraction of the vibrating dislocations interacting comparatively weakly with alloys atoms (e.g. screws), and Ψ/8&Piα is a constant which, as with concentrated alloys, equals about 0.5 in the amplitude-independent region ; c_∞, which depends on L, represents the c-level (≈ 1 at.% Zn) beyond which Q^-1 changes comparatively little with zinc content. The observation that Q^-1 (c, T₁)/Q^-1(c, T₂) ≈ T₁/T₂, suggests that N increases approximately linearly with temperature in the range used.