Thermal Stress Relaxation in Al-Al2O3 (f) Composites During Thermal Cycling

The thermal stress relaxation mechanisms in Al 99.99% reinforced with SAFFIL alumina short fibers have been investigated by mechanical spectroscopy during long period and low temperature thermal cycles (T = 2 K/min, 100 K - 450 K). During these cycles large internal stresses are built in the specimen due to the difference in thermal expansion coefficient between the matrix and the reinforcement. It has been found that the internal friction, the elastic modulus and the zero point drift behaviours are extremely sensitive to these internal stresses and specially to the relaxation mechanism operating : dislocation generation or dislocation creep. The internal friction shows a broad non anelastic maximum during cooling which is attributed to dislocation movement in a continuously developing plastic zone nearby the fibers. The elastic modulus behaviour as a function of temperature leads to define two particular temperatures : Tstart related to the initial growth of plastic zones nearby the fibers and Tend related to their later overlapping. The zero point coordinate describes during each thermal cycle a characteristic trajectory which may be analyzed in terms of a thermal fatigue loop. The study is performed in composites with different volumetric fraction of fibers and matrix strenght

The Internal Damping of Al-Al2O3 (f) Composites During Thermal Cycling : the Effect of Fibre Content and Matrix Strength

A simple model is proposed to describe the observed effect of the volume fraction of fibres on the internal friction, elastic modulus and deformation at zero applied stress during the cooling portion of a thermal cycle between 450 K and 100 K. The sample is considered as composed by three phases, each one showing different intrinsic damping: the reinforcing fibre, the aluminium matrix far from the fibre and the zone around the fibre, close to the matrix-fibre interface, called the plastic zone. It is assumed that the main contribution to the internal friction arises in these plastic zones, which grow during cooling in a non strain hardening matrix. A uniform dislocation density in the zone is considered. The internal damping of the whole sample is predicted, as a function of fibre content, for the limit case of uniform strain. The model predicts satisfactorily the characteristic temperatures defined in the elastic modulus curve if they are identified as the temperatures for the beginning of growth of the plastic zone, and the temperature for overlapping of fully developed zones, respectively