The early stage of phase decomposition of an Al-7.8at.% Li alloy was studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The atom probe concentration depht profile clearly that the δ′ with the stoichiometric Li concentration were already precipitated in the as-quenched stage. The high resolution TEM image also showed that small isolated ordered particles were present in the disordered matrix. These two results unambiguously show that the first product which we can observe experimentally is the δ′ precipitates. Therefore, it does not seem possible to obtain convincing evidence to prove that the congruent ordering precedes spinodal decomposition, because such a transformation path would have already been completed during quenching.
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