In situ flow stress of pure aluminium constrained by tightly packed alumina fibres

The in situ matrix flow stress of continuous fibre-reinforced aluminium is measured in tension along the fibre axis. We use a new, tighter, estimate for the effect of differential Poisson contraction between fibres and matrix and take into consideration nonlinear elastic fibre behaviour; these improvements remove inconsistencies found in earlier work. Resulting in situ matrix flow stress curves are characterized by a substantial gain in hardness of the matrix as compared to the unreinforced alloy, and a strong Bauschinger effect. These effects are caused by dislocation emission during cooldown by matrix/fibre thermal strain mismatch. The surprising insensitivity of hardening to the prior rate of composite cooldown suggests that thermal dislocational hardening starts already at temperatures where unreinforced pure aluminium would creep rapidly. The absence of significant recovery during furnace cooldown is attributed to a small amount of iron in supersaturated solution, and/or to subgrain boundary pinning at the fibres.