Anomalous internal-friction peaks observed in very-high-purity aluminium

Polycrystalline aluminium of very high purity (99.9999%) was tested by isothermal mechanical spectroscopy. Experimental results confirm the existence of a low-temperature effect and of a relaxation peak at an intermediate temperature, described in the literature as P'1 and P1. However, isothermal experiments as a function of frequency allowed us, firstly, to clarify the evolution of these relaxation effects with measurement and annealing temperatures and then to provide evidence of the unusual behaviour of P1; in fact, this peak is not associated with the classical, thermally activated relaxation peak. These results are discussed in terms of dislocation motion in a tangle microstructure.