Discontinuous and continuous precipitation in Cu-13 wt.% Sn and Al-20 wt.% Ag alloys

Abstract

The relation between the continuous and discontinuous precipitations, and the discontinous precipitation kinetics and mechanisms in Cu-13 wt.% Sn and Al-20 wt.% Ag alloys, have been investigated by metallographic observations, differential scanning calorimetry, X-ray diffraction and microhardness measurements. During ageing of the supersaturated solid solution of Al-Ag, after the preprecipitation of the Guinier Preston zones and the continuous precipitation of the plates of the transition phase γ′, the cellular precipitation of the equilibrium phase γ occurs and grows from the grain boundaries but lamellae are not perpendicular to the reaction front. At high temperature intergranular precipitation occurs without the reaction front or cell. Both types of precipitation occur in the Cu-13 wt.% Sn alloy but very slowly. However, continuous precipitation predominates after a high deformation amount and reduces the amount of chemical driving available for both the initiation and propagation of discontinuous precipitation.