Precipitation phenomena in Al–Ag–Zn alloys

The precipitation sequence in Al–Ag–Zn alloys with an electron to atom ratio 2.9 has been found to be GP zones → [epsilon]′(h.c.p.) → [epsilon](h.c.p.) for Ag/Zn atomic ratios from 4.0 to 0.20. The sizes of GP zones formed during the quenching, determined from the broadening of X-ray diffraction lines and by electron microscopy, are of the order of 30 to 40 Å for all alloys investigated. The growth rate of GP zones during ageing at 150 °C after quenching increases with the Zn content. GP zones remain f.c.c. during ageing, with a somewhat smaller unit-cell dimension than the matrix. This misfit increases with the Zn content. A change of GP zone shape for alloys with Ag/Zn ≤ 1 has been observed to be: spherical zones → cuboids → prolate ellipsoids elongated in 〈100〉_mtx directions. In parallel with growth and dissolution of GP zones metastable precipitates [epsilon]′ are formed, this process being more rapid for smaller Ag/Zn ratios. Unit-cell dimensions of the [epsilon]′ phase depend not only on the composition of the alloy content but probably also on the mechanism of nucleation. Heterogeneous nucleation of [epsilon]′ has been observed for all the alloys, but a direct GP zones → [epsilon]′ transformation probably takes place in the alloys with high Zn content. In all the alloys the {111 }_mtx||{0001}[epsilon]′ and [10]_mtx||[110][epsilon]′ orientation relationship has been approximately obeyed. The [epsilon]′ phase remains partially coherent with the matrix in {111} planes for Ag/Zn ≥ 1.