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Hardening on annealing (HOA) has been frequently observed in nanostructured metals and alloys. For nanostructured materials obtained by severe plastic deformation (SPD), HOA has been attributed to the reduction of dislocation sources within grains and grain boundary relaxation during annealing. In the present work, it is shown that when a bimodal grain structured (a mixture of micronsized and ultrafine grains) Al-5Cu alloy prepared by equal channel angular pressing (ECAP) was subjected to a post-ECAP natural and artificial aging treatments, the alloy shows a completely different precipitation behaviour with an accelerated precipitation kinetics. No coherent θꞌꞌ or semicoherent θꞌ precipitates form in the bulk of grains, while a large fraction of stable incoherent θ precipitates form along high angle boundaries. After artificial aging at low temperatures for a short time, a significant improvement of both ultimate tensile strength and uniform elongation was achieved without sacrificing the yield strength. A systematic microstructure characterization by EBSD, TEM and APT has been carried out to investigate the evolution of grain size, dislocation density and solid solution level of Cu as well as the precipitation of Al-Cu precipitates during natural and artificial aging treatments. A quantitative evaluation of different supposed strengthening mechanisms revealed that the segregation of Cu elements at grain boundaries plays a more important role than grain boundary relaxation and the dislocation source-limited strengthening to compensate the yield strength reduction caused by the decrease in dislocation density and solute content of Cu in solid solution.

In the structures of all metastable precipitates in Al-Mg-Cu and Al-Mg-Si alloys, we find that column surrounding of an element column in the needle/lath direction order according to simple principles. Advanced transmission electron microscopy and DFT calculations support the principles originate with a line defect, which is a segment of a <100>Al column shifted to interstitial positions. We propose the defect aids solute decomposition by partitioning the FCC matrix locally into columns of fewer and higher number of nearest neighbours, which suit smaller and larger size solute atoms, respectively. The defect explains how <100> directionality of the precipitates can arise in a cluster. Ordering of a few defects leads naturally to GPB zones in Al-Mg-Cu and to β'' in Al-Mg-Si.

The Zn-containing β" phase in Al-Mg-Si alloys has been investigated by aberration corrected high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), combined with density functional theory (DFT) calculations. The mean intensity of one Si site of the β" phase is higher than the other Si sites, suggesting partial Zn occupancy. DFT studies support that this Si site is competitive for Zn incorporation. While HAADF-STEM image simulations show an influence of the Zn distribution along the β" main growth direction, total energy calculations predict a weak Zn-Zn interaction. This suggests that Zn atoms are not clustering, but uniformly distributed along the atomic columns. The Zn incorporation has a weak influence on the β" phase where Zn is admitted as a "defect" according to the DFT studies.

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The Cu interactions with the Al-Mg-Si alloy main hardening phase β" are investigated in atomic scale, by using experimental and simulated high angle annular dark-field scanning transmission electron microscopy techniques and density functional theory calculations. Cu is located at or near the β"/Al interface, with the misfit dislocations normally observed for a precipitate of this size being absent. It is proposed that the small Cu volume is crucial to this mechanism. Present supercell based calculations cannot fully model these interactions. Main TextHardness evolution under heat treatment is a topic of central interest for age-hardenable Al alloy development. An intimate connection exists between strength and solute atom nanostructures [1]. For Al-Mg-Si alloys, maximum hardness is reached through formation and growth of semi-coherent needle-shaped (β") precipitates [2,3] possessing significant misfits with the Al matrix in the needle cross-section. For large precipitates, the misfit triggers the introduction of dislocations at the β"/Al interface. This directly influences precipitate growth, and in turn hardness evolution.The monoclinic β" phase (Figure 1

An Al-5 wt.% Cu alloy supersaturated with Cu in solid solution was subjected to equal channel angular pressing (ECAP). The microstructural evolution was systematically investigated by backscattered electron (BSE) imaging, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). It is revealed that a bimodal grain structure composed of coarse micron-sized grains and submicron-sized grains was developed after four passes of ECAP. Tensile testing showed that a high ultimate tensile strength of ~500 MPa, a high elongation to failure of ~28% and a uniform elongation of ~5% were achieved simultaneously. The deformation behaviour and the grain structure evolution during ECAP, the strengthening mechanisms of the as-deformed material, and especially, the role played by a high content of Cu have been discussed.

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