Coarsening Mechanism of Ordered Precipitates in the Concentrated Al-Li Alloys and their Properties

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Decomposition phenomena of supersaturated solid solution and coarsening behavior of the delta' phase were experimentally investigated for four Al-Li alloys containing 1.80 mass%Li (6.65 mol%Li) up to 3.33 mass%Li (11.8mol%Li). The decomposition of the Al-Li binary alloy was found to take place as follows :alpha -> homogeneous ordering -> ordered domain -> delta' deltaBoth the homogeneous ordering structures and ordered domains were precursors to the metastable delta' phase and contained less Li than that of the stoichiometric composition, 25mol%Li. The delta' phase was most effective to increase the mechanical strength of Al-Li alloys. The coarsening behavior of the delta' phase was also quantitatively analyzed. The delta' precipitates were clearly observed in the TEM dark field images. The mean radius, particle size distribution, standard deviation and skewness were determined using an image analysis system. The coarsening of the delta' phase took place in accordance with the well-kno … More wn r^3 - t law (r : mean radius, t : aging time) for all alloys investigated. The coarsening rate constant, k, increased with increasing delta' volume fraction and temperature. The derived value of the interfacial energy for the delta'/matrix interface was - 0.2 J/m^2.The delta' particle size distribution (PSD) was found to have no strong dependence on aging time within the present investigation. The PSD, however, became more symmetrical with increasing volume fraction of the delta' phase. Although any of the particle-coarsening theories did not fully explain the experimentally obtained results for the delta' phase, the LSEM theory taking into account the encounter effect between particles explains weel the coarsening rate constant and PSDs. The small additional element Zr was found to produce fine composite particles and was effective to improve the mechanical properties. The obtained results are applicable to control and predict the precipitate microstructures and improve the properties of Al-Li alloys. Less