Nucleation and growth of the \gamma' (AlAg2) precipitate in Al-Ag(-Cu) alloys

TL;DR

This study investigates the nucleation and growth mechanisms of the b3' (AlAg2) precipitate in Al-Ag(-Cu) alloys using advanced electron microscopy techniques, revealing detailed atomic processes and growth barriers.

Contribution

It provides new insights into the atomic-scale nucleation and growth processes of b3' precipitates, highlighting the role of ledge addition and strain energy barriers.

Findings

Precipitation begins with silver segregation to stacking faults.

b3' plates are typically 2 or 3 times the lattice parameter thick.

Growth by ledge addition faces a shear strain energy barrier.

Abstract

Precipitation of the \gamma' (AlAg2) phase was investigated in Al-Ag(-Cu) alloys using high resolution transmission electron microscopy and scanning transmission electron microscopy. Precipitation commenced with segregation of silver to stacking faults, followed by thickening in steps corresponding to single unit cell ledges. In conjunction with gradual segregation of silver and aluminium into ordered layers this yielded \gamma' phase platelets with a thickness of either 2 or 3x the AlAg2 c-lattice parameter. Plates with a thickness of 2 c(AlAg2) would not achieve self-accommodation of the shape strain for transformation. Further thickening of the precipitates was slow, despite considerable silver segregation around the precipitates. Growth by the addition of single unit cell height ledges is expected to lead to an additional shear strain energy barrier to ledge nucleation and this may contribute to a process of nucleation-limited growth.