THRESHOLD CREEP-BEHAVIOR OF DISCONTINUOUS ALUMINUM AND ALUMINUM-ALLOYMATRIX COMPOSITES - AN OVERVIEW

Several sets of creep data for aluminium and aluminium alloy matrix co mposites reinforced by silicon carbide particulates, silicon carbide w hiskers or alumina short fibres are analysed. It is shown that for thi s class of discontinuous composites the threshold creep behaviour is i nherent. Applying the concept of threshold stress, the true stress exp onent of minimum creep strain rate of approximately 5 follows from the analysis even when the matrix solid solution alloy exhibits Alloy Cla ss creep behaviour, for which the value of 3 for the true stress expon ent is typical. The creep strain rate in the discontinuous aluminium a nd aluminium alloy matrix composites is shown to be matrix lattice dif fusion controlled. The usually observed high values of the apparent st ress exponent of creep strain rate and the high values of the apparent activation energy of creep are then rationalized in terms of the thre shold creep behaviour. However, the origin of the threshold stress dec reasing 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 a lumina short fibres is shown to be significant, although the particula tes, whiskers and short fibres do not represent effective obstacles to dislocation motion.