INTERNAL DAMPING EFFECTS DUE TO THE THERMAL-EXPANSION MISMATCH BETWEEN ALUMINUM AND SILICON IN AN AL 11.8-PERCENT-SI ALLOY

In order to study the behaviour of the interface between particle and matrix, low frequency Internal Damping (I.D.) has been investigated with a torsion pendulum in an Al 11.8 wt%Si alloy containing large silicon particles with sizes up to 10 mum. The I.D. spectra are different on heating and on cooling the specimen. They exhibit a broad poorly defined maximum situated around 240 K on heating and 190 K on cooling, and a high temperature background on which a peak centered around 485 K seems to be superposed. Moreover, I.D. is increased over the whole temperature range 100-550 K as the heating or cooling rate, Absolute value of T is increased, or the oscillation frequency of the pendulum is decreased. The analysis of the experimental results is focused on the temperature rate dependent contribution delta(T) that is shown to be linked with the internal stresses induced by the difference in the coefficients of thermal expansion (CTE) of the silicon particles and the aluminium matrix. A model is outlined to describe delta(T) in terms of emission or movement of dislocations that could be induced by the variation of thermal stresses during the I.D. spectrum measurement. A linear relationship between delta(T) and the temperature rate is derived from the model and this law appears to be approximately followed by the experimental data. The other contributions to I.D. are discussed in terms of grain boundary sliding, lattice dislocation movements and diffusional stress relaxation around Si particles.