Structural and transient internal friction due to thermal expansion mismatch between matrix and reinforcement in Al-SiC particulate composite

Microyield phenomena during thermocycling of Al 7075 alloy/15 vol.% SiC metal-matrix composite due to mismatch of the thermal expansion coefficients of the matrix and the SiC particles are studied over the temperature range from similar to 50 to 300 K by means of interval friction techniques. The transient component of the internal friction is measured in the infrasonic frequency range, whereas the structural strain amplitude-independent and strain amplitude-dependent internal friction is investigated at ultrasonic frequencies. Combining the two techniques enables us to follow the variation of the defect structure during thermocycling as well as to evaluate basic contributions to the microyield phenomenon. The following conclusions are drawn: (1) thermal stresses are responsible for creation of fresh, i.e. mobile dislocations, whose density does not differ significantly, for the same temperature, during heating and cooling or in consecutive thermocycles; (2) yielding is much more pronounced during the first cooling from a stress-free state than in the consecutive thermocycles; this pronounced yielding is due to long-range motion of approximately the same amount of fresh dislocations as in consecutive thermocycles; and (3) during thermocycling, the matrix undergoes dynamic strain ageing; its intensity depends strongly on temperature and therefore varies continuously in a thermocycle. (C) 2001 Elsevier Science B.V. All rights reserved.