An in situ TEM study of nucleation and growth of the Ω phase in an Al-4.0Cu-0.5Mg-0.5Ag alloy

Abstract

Addition of small amounts of Ag to Al-Cu-Mg alloy with high Cu:Mg ratios stimulates precipitation of a new phase, designated Ω, which has an orthorhombic structure and forms as thin coherent hexagonal plates on {lll} α planes. This phase significantly increases the age hardening characteristics of the alloy, particularly at elevated temperatures. The mechanism of nucleation of Ω phase is not known but is believed to be fundamentally different from that of θ’ and S’ phases which also form in these alloys. The presence of Ag and Mg only in trace quantities makes the problem of finding the actual mechanism of nucleation of Ω phase difficult.An in situ hot stage TEM study was conducted on an Al-4.0Cu-0.5Mg-0.5Ag(wt.%) alloy in order to understand the nucleation and growth behavior of Ω phase. Two types of initial microstructures were used: 1) homogenized, solution-treated and cold water-quenched, 2) as-quenched and aged for 1 hr. at 190°C. The as-quenched structure shown in Fig. 1(a) consists of a high density of small dislocation loops, some helical dislocations and a few straight dislocations. When this foil was heated in the microscope, observable precipitation occured at about 205°C.