Origin of θ′ Relaxation in Reinforced and Unreinforced AlCu Alloys

Abstract

When AlCu alloys are in the 8' precipitation state, they exhibit a peak (Pz) in their internal friction spectrum as a function of the temperature. In this connection, an unreinforced A1-4% Cu alloy and some A1-4% Cu based metal matrix composites (MMC) reinforced with lo%, 20%, and 30% A1203 fibers were characterized by internal friction measurements (IF or Q-'). The microstructure of the AlCu based composites has been found particularly favorable for the appearance of the Pz peak, and enhances 8' relaxation. This kind of result has led us to a new interpretation of 8' relaxation in AlCu alloys. 8' relaxation is important in AlCu based composites because the expected industrial utilization temperature of these materials is around 250 "C, and this implies a 8' precipitation state for their working conditions.