An atomic scale structural investigation of nanometre-sized η precipitates in the 7050 aluminium alloy

Using high-angle-annular-dark-field (HAADF) scanning-transmission-electron microscopy (STEM), we have investigated eta-precipitates in the Al-Zn-Mg-Cu (AA7050) aluminium alloy. The HAADF STEM images taken along the zone axes of [10 (1) over bar0]eta, [1 (2) over bar 10]eta, and [0001]eta illustrated the projected atomic-scale configurations of eta-MgZn2 crystal, The precipitates developed in layer-by-layer growth, supplied with precursors such as Zn, Cu, and Mg, which were solute atoms segregated around the eta/Al interfaces due to the higher lattice strain energy. Stacking faults and defect layers composed of flattened hexagons were frequently observed along the zone axes of [1 (2) over bar 10]eta and [10 (1) over bar0]eta, respectively, and their formation was elucidated, similarly taking into account the layer-by-layer growth. Occasional coalescence between two precipitates yielded a complicated boundary or a twin-like boundary. Based on the differences in orientation relationships between eta-types and the Al matrix reported to date, two new types of eta precipitates have been recognized and named eta(4)' and eta(12). (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.