THRESHOLD CREEP-BEHAVIOR OF DISCONTINUOUS ALUMINUM AND ALUMINUM-ALLOY MATRIX COMPOSITES - AN OVERVIEW

Several sets of creep data for aluminium and aluminium alloy matrix composites reinforced by silicon carbide particulates, silicon carbide whiskers or alumina short fibres are analysed. It is shown that for this class of discontinuous composites the threshold creep behaviour is inherent. Applying the concept of threshold stress, the true stress exponent of minimum creep strain rate of approximately 5 follows from the analysis even when the matrix solid solution alloy exhibits Alloy Class creep behaviour, for which the value of 3 for the true stress exponent is typical. The creep strain rate in the discontinuous aluminium and aluminium alloy matrix composites is shown to be matrix lattice diffusion controlled. The usually observed high values of the apparent stress exponent of creep strain rate and the high values of the apparent activation energy of creep are then rationalized in terms of the threshold creep behaviour. However, the origin of the threshold stress decreasing with increasing temperature but not proportional to the shear modulus in creep of discontinuous aluminium and aluminium alloy matrix composites is still awaiting identification. The creep-strengthening effect of silicon carbide particulates, silicon carbide whiskers and alumina short fibres is shown to be significant, although the particulates, whiskers and short fibres do not represent effective obstacles to dislocation motion.