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: T-start related to the initial growth of plastic zones nearby the fibers and T-end 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.