The effect of Al2Cu precipitates on the microstructural evolution, tensile strength, ductility and work-hardening behaviour of a Al–4 wt.% Cu alloy processed by equal-channel angular pressing

Abstract

The tensile strength of metals and alloys can be considerably increased by severe plastic deformation, a consequence of the extreme grain refinement thus achieved. However, ductility is generally reduced and therefore a number of strategies have been implemented to minimize this drawback. The present paper examines the effect of precipitation on a cast and homogenized Al–4wt.% Cu alloy processed by equal-channel angular pressing (ECAP). Two initial conditions were studied: (i) solid solution obtained by fast cooling after homogenization; and (ii) coarse Al2Cu precipitates obtained by a very slow cooling from the homogenization temperature. After ECAP processing these groups were submitted to heat treatments performed at 100 and 170°C. The Al-α grain size was reduced to 200–300nm and the tensile strength was substantially increased. As for the work-hardening capacity, reinterpretation of the stress–strain data was carried out in terms of Kocks–Mecking plots. The experimental work was devised so as to produce different precipitate dispersions, whose effect on the work-hardening rate was correlated with the quantitative metallographic parameters of such dispersions. The beneficial effect of a very fine Al2Cu dispersion was thus observed, and this behaviour was attributed to dislocation pinning by the precipitates, an occurrence whose main effect is a decrease in the dynamic recovery rate. Implementation of this strategy will have positive technological effects since, besides the improvement in strength, a definite correlation between uniform elongation and work-hardening capacity was clearly demonstrated.