Effects of minor additions of Mg and Ag on precipitation phenomena in Al–4 mass% Cu

Abstract

Calorimetric measurements and transmission electron microscopy have been used to study the effects of minor additions of Mg and Ag on mechanism and kinetics of precipitation in Al–4 mass% Cu. Isothermal studies at 30°C show that the rate of Guinier–Preston (Cu) zone formation is progressively reduced by the presence of Mg, or Mg + Ag, which is attributed to preferential trapping of vacancies so that their ability to assist diffusion of Cu atoms is reduced. At elevated temperatures, Mg enhances precipitation of θ′, whereas Mg + Ag stimulates formation of the Ω phase. Precipitation of S′ follows θ′ if Mg or Mg + Ag are present and pre-ageing increases the quantity of θ′ precipitated at the expense of S′. It has been confirmed that secondary precipitation occurs at low temperatures if alloys are first aged at elevated temperatures.