A precipitate (first designated as X) in certain Al-Zn-Mg alloys is shown to be a metastable phase formed before the equilibrium phase, T, found in these alloys, and consequently is designated T′. A similar metastable phase also occurs in an Al-Ag-Mg alloy which has an equilibrium T phase. Analysis of X -ray precession camera photographs of alloys in these two systems shows that T′ has a hexagonal unit cell with parameters a = 13.88 Å, c = 27.52 Å and a = 14.11 Å, c = 28.04 Å for the Al-Zn-Mg and Al-Ag-Mg alloys, respectively. The orientation relationship to the matrix is (0001)T′∥(111)A1, (1010)T′∥(112)A1.
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