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The internal friction of cold-worked Cu-Al alloys is measured with an inverted torsion pendulum in the temperature range between - 180°C and + 100°C. Among 10 specimens with different contents of aluminium ranging from 0·01 to 2 at.%, distinct peaks are observed for the two compositions, 0·13 and 0·18 at. % Al, the peak height being larger for the former; detailed studies are made on 0·13 at. % Al specimens.The peak is introduced by plastic deformation and disappears on annealing at temperatures around + 150°C. For a specimen annealed in an oxidizing atmosphere prior to the final deformation, the peak is no longer observed. The peak is considered to be a combined effect of dislocations and aluminium atom-vacancy complexes.

Precipitation processes in solid solutions have been studied by assuming the existence of a criticalsize for clusters. The kinetics have been followed by solving simultaneous differential equations for two types of the cluster size dependence of reaction constants: (1) size-independent and (2) size-dependent (proportional to the number of solute atoms in the respective cluster), the former case having been discussed by Damask, Danielson and Dienes (1965). Most of the calculations are made with an 'equilibrium approximation'; the validity of the approximation has been examined in detail.It has been found that the shape of reaction curves and the resultant particle size distributions are very different for the two cases. Although Damask et al. claimed that the critical size of clusters could be estimated from experimental data, this is valid only for a special case of size-independent reaction constants, which would never be realized in actual precipitation processes.It is pointed out that the critical size in the sense of Damask et al. is not the same as that in the thermodynamical theory of nucleation.

The theory of stochastic processes is applied to annihilation processes of lattice defects in the presence of traps. If the transport equations for a diffusing particle are given, the analytical expression for the mean first passage time for a given site (mean life time) can be derived by inverting the corresponding jump probability matrix.As an immediate application of the theory, diffusion of dislocation kinks in the presence of a trap or a high potential barrier is considered. The method is extended to three-dimensional diffusion problems. Decomposition and formation of impurity-vacancy complexes are considered for an f.c.c. lattice. The effect of the change in the saddle point energy for vacancy jumps near a substitutional solute atom is examined in detail to find significant influences on the rates of both formation and decomposition processes.

The damping spectrum of the Snoek effect is calculated for ternary b.c.c. alloys in which substitutional atoms exist both as isolated and as paired atoms. The calculation predicts the appearance of two peaks, P1 and P2, in addition to the ordinary Snoek peak. The peak P2 is associated with interstitial atoms in the vicinity of pairs, while the peak P1 consists of contributions from interstitial atoms near pairs as well as those near isolated substitutional atoms.The present theory also predicts that the relaxation of interstitial atoms around pairs (P2) may occur through by-pass diffusion if the activation energy for direct jumps is very high.

The Snoek effect of interstitial atoms in the ordered, body-centred cubic lattice (the CsCl type lattice) is considered. In this structure an exact solution of the problem is possible in contrast to the case of dilute alloys discussed in previous papers. The rate equations and expressions for the internal friction are derived for three kinds of applied stress: pure shear stress, and 〈111〉 or 〈100〉 tensile stress.The relationship between the probability of occupation of various sites by interstitial atoms and the relaxational normal modes is discussed for the shear stress case. It is concluded that decomposition of the damping spectrum into Debye peaks has little physical significance.