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Weldalite 049 is a weldable, high strength Al-Cu-Li based alloy with trace amount of Mg and Ag [1, 2]. Major phases observed in the peak hardness condition are spherical, semicoherent {001} plate of and coherent {111} plate of T1 phase [3, 4]. Langan and Pickens [5] attributed the predominant strengthening phase in Weldalite 049 to the T1 phase that precipitate uniformly in the matrix. While T1 phase is known to precipitate heterogeneously on dislocations and grain boundaries in Al-Cu-Li based 2090 alloys [6], addition of Mg and Ag leads uniform dispersion of the T1 plates in the matrix [7, 8]. Because Weldalite-type alloys show the highest strength of wrought aluminum alloys, the mechanism by which the T1 plates are uniformly dispersed has been a subject of many investigations [7–10]. The role of trace additions of Mg and Ag on the nucleation of plates on the {111} planes in Al-Cu-Mg-Ag alloy has been …