A STUDY OF THE HIGH-TEMPERATURE AGING OF AL-CU-MG-AG ALLOY-201

The microstructural evolution of Al-Cu-Mg-Ag alloy 201 during ageing a t 190-degrees-C after solution treatment, quenching and room-temperatu re annealing has been investigated by a joint differential scanning ca lorimetry and transmission electron microscopy (TEM) analysis, with th e aim of clarifying the early stages of OMEGA-CuAl2 precipitation. Aft er room-temperature ageing, the TEM observations show the presence of both copper Guinier-Preston zones (GPZs) and a phase exhibiting a hexa gonal structure with lattice parameters a = 0.507 nm and c = 0.692 nm, here called OMEGA'. At this stage, the [111]Al zone axis electron dif fraction pattern exhibits a structure completly different from those r eported in literature when the OMEGA phase is present. This pattern ca n be interpreted by taking into account the GPZ and OMEGA' structures. During ageing at 190-degrees-C the copper GPZ formed at room temperat ure immediately disappear while the well known precipitation sequence THETA'' --> THETA' occurs. At the same time, OMEGA' slowly disappears while OMEGA precipitates appear on {111}Al planes. After 60 min at 190 -degrees-C, OMEGA particles are clearly visible but OMEGA' is still pr esent. S' (CuMgAl2) precipitates appear after only 60 min at 190-degre es-C. They are heterogeneously nucleated on defects and a copious prec ipitation occurs after only ageing for 10 h. The present results sugge st that the early phase, here named OMEGA', is the precursor of the OM EGA-CuAl2 phase typical of this family of alloys. The whole precipitat ion sequence at 190-degrees-C is therefore alpha(sss) --> GPZ + OMEGA' --> THETA'' + OMEGA' --> THETA' + OMEGA + S'.