Influence of a microalloying addition of Ag on the precipitation kinetics of an Al?Cu?Mg alloy with high Mg:Cu ratio

C. MACCHI; A. TOLLEY; R. GIOVACHINI; I. J. POLMEAR; A. SOMOZA

ACTA MATERIALIA

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2015 vol. 98 p. 275 - 287

1359-6454

Effects of microalloying with Ag on the precipitation process in an Al?Cu?Mg alloy with high Mg:Cu ratio have been investigated during artificial ageing of the ternary and quaternary compositions Al?1.5 wt.%Cu?4 wt.%Mg(?0.5 wt.%Ag) at 175 C, combining transmission electron microscopy and positron lifetime spectroscopy. In addition, coincidence Doppler broadening of the positron?electron annihilation radiation was used to provide information about the chemical environment surrounding vacancy-like defects. The results obtained for the silver-free alloy indicate that, immediately after quenching from the solution treatment temperature, vacancy v-Cu?Mg clusters are formed in the supersaturated solid solution. During the early stages of ageing these clusters become richer in Cu and Mg and laths of the S phase (Al2CuMg) nucleate preferentially on dislocation lines. Continued ageing led to gradualprecipitation of an equi-axed phase in the matrix between the S phase laths which is believed to be the cubic Z phase thought previously to form only if silver is present. For the silver-containing Al?Cu?Mg alloy, v-Cu?Mg?Ag aggregates also form immediately after quenching and the solute transport mechanisms are the same. However, precipitation of the S phase is suppressed during ageing and the silver addition promotes accelerated formation of a finer dispersion of smaller equi-axed Z phase precipitates.For both alloys, no evidence was found that the Z phase is preceded by any pre-precipitate or GP zones.Combination of coincidence Doppler broadening and microanalysis results revealed the Z phase to have a Mg:Cu ratio of 2 which indicates that it differs from the well known T phase (Al6CuMg4) which the equilibrium phase diagram indicates should form in Al?Cu?Mg alloys with high Mg:Cu ratios. Additional ageingexperiments at 240 C showed that neither the Z phase or T phase are formed in the ternary alloy, but only precipitates of the S phase, while in the quaternary alloy the S phase is strongly suppressed and mainly Z phase precipitates are formed.