ABSTRACT
In the early 50s, Bagaryatsky [1) first proposed a 4 stage precipitation sequence for the ageing of Al-Cu-Mg alloys: SSS 7 GPB zone 7 S" (GPB2) 7 S' 7 S (CuMgAh). where SSS stands for supersaturated solid solution and GPB was termed as GuinierPreston-Bagaryatsky by Silcock [2]. Fifty years later, there are still disputes on the structure of all these phases, and especially on whether S" (GPB2) truly exists as a phase separate and distinct from the equilibrium S phase. According to Shchegoleva and Buinov [3], the S" is monoclinic crystal with a=88.6° which slightly distorted S structures. Various other claims for the presence and the structure of a distinct S" have been made. For example, in Silcock's X-ray work [2], she did not observe a phase resembling the S" phase reported by Bagaryatsky [1], rather she suggested the existence of a structure rich in copper, more likely to be related to the compound AlsCusM~ with cubic structure and a = 0.827 mn. Based on TEM diffraction, Cuisiat et al. [4] suggested S" as a orthorhombic structure a= 0.405 nm, b = 0.405nm and c = 0.81mn. Shih et al. [5] proposed a partially ordered socalled GPB2 zone (S") which has a tetragonal structure and lattice parameters of a = 0.58mn, c = 0.81 nm. Recently, by calculations of formation enthalpies for GPB I complex precipitates in AI alloys using first-principles, Wolverton [6] predicted a new structure of GPB2 zone as a tetragonal structure with a= 0.401mn and c = 0.8lnm. However, none of the above structures have been independently confirmed. Other researchers (e.g., Wilson and Partridge [7] and Ringer et al. [8]) were unable to confirm the presence of the S" phase.
