Factors governing strength and ductility of metallic composites

Two commercial Zn–Al alloys, constituting a natural composite consisting of ductile fibres of face-centred cubic aluminium in the less ductile close-packed hexagonal Zn-rich matrix, variously subjected to warm rolling and warm equal channel angular processing, were investigated to examine the factors governing strength and ductility. Comparison with the published behaviour of an aluminium–copper alloy subjected to similar treatment suggested that the observed behaviour and the interpretation offered were general. When strong crystallographic orientation occurred — as in rolling — strength, and to a less extent ductility, largely depended on crystallographic orientation; ductility strongly depended on the processing route (rolling or equal channel angular processing), but the diffuse orientation characteristic of equal channel angular extrusion allowed the inverse Hall–Petch parameter (as a measure of filament thickness) to display a role in that case. High ductility of the rolled material correlated with orientation of a large number of grains of the close-packed hexagonal matrix phase for easy slip. Strengthening by reduction in size of the units of structure is likely to be available in metallic composites only when dislocation hardening substantially outweighs orientation softening.