Compression creep of PM aluminum matrix composites reinforced with SiC short fibres

Abstract

The compression creep behaviour of Al–SiCfiber metal matrix composites (MMC), made by hot-pressing (HP), was evaluated at various temperatures and over several orders of magnitude of strain rates. The interpretation of metal flow-patterns during the whole deformation cycle was complex owing to the fact that the short-fibre distribution in the composites was roughly planar. However, every specimen showed a well-defined flow stress or plateau (σp true) up to the end of the tests that were associated with nearly 50% linear compression strains. Such stresses clearly increased with the volume fraction (f) of fibres and strain rates, and decreased with increasing temperatures. Cross-examination of the creep curves [log strain rate (γ˙) versus log shear stress (τ)] for both the HP Al matrix and composites show an apparent stress exponent nap = [δ(lnγ˙)/δ(ln τ)] clearly increasing while decreasing τ. This anomalous behaviour can be attributed to the existence of a finite threshold stress (τ0) for every composition. This threshold stress appears to be related to the oxide contamination (judged from TEM observations) of the matrix, as a result of the use of powder metallurgy (PM) synthesis method. Following certain approximations during deformation behaviour of PM specimens reinforced with ceramic particles, the present data, for short-fibre reinforced MMC, seems to be consistent with the mechanism of dislocation climb that is characterized by an stress exponent of around five, and an activation energy close to that for self-diffusion in pure aluminum (143.2 kJ mol−1).