J. Phys. Colloques 32 (1971) C2-112-C2-112
ANELASTICITY OF Al- Si AND Al-Mg-Si ALLOYSK. M. ENTWISTLE
University of Manchester
Wires of Al-Si and Al-Mg-Si alloys that are quenched from the homogeneous alpha phase region and are aged at temperatures around 200 °C and above develop an internal friction peak at 220-230 °C for a frequency of vibration of 1 c/s. The height of the peak is very sensitive to composition, heat treatment and grain size. It is easy to obtain spurious results on the alloys containing magnesium due to rapid volatilisation of this element during solution treatment. It proved to be possible to prevent this magnesium loss completely by anodising the specimens and heat treating them in an atmosphere of CO2. The relaxation strength decreases and the peak temperature falls from the binary Al-Si values as the Mg concentration is increased and the peak height becomes very small in the pseudo-binary Al-Mg2 Si alloys. For any composition, the peak height varies during ageing at temperatures above 200 °C. For the Al-Si alloys the peak is highest in the alloy that has been quenched and immediately heated to 250 °C. When magnesium is added the peak is small under these conditions but develops as the ageing temperature is increased to around 300 °C. For a particular treatment the peak height also drops sharply as the grain size is increased. The activation energy which describes the variation of peak temperature with frequency lies between 1.5 and 1.6 eV and compares with that of 1.3 eV for the grain boundary relaxation in zone refined aluminium. Our interpretation of the peak in the alloys is that it is a grain boundary relaxation influenced by the segregation of Si to the boundaries. The concentration of Si there will depend on the concentration of Si in the alpha phase and the relative binding of Si in the precipitates and the boundary. This will cause changes in the equilibrium distribution of Si as the temperature varies. We explain the low values of the peak in the pseudo-binary Al-Mg2Si alloy by the strong binding of Si in Mg2Si and we can correlate the variations of damping with composition and temperature in the other alloys by the lower binding of Si in Si precipitates and the variation of distribution of Si with temperature deduced from the ternary equilibrium diagram. This interpretation will be outlined in more detail when the paper is presented.