# Crystallographic relationships of T-/S-phase aggregates in an Al-Cu-Mg-Ag alloy

## TL;DR

This study elucidates the crystallographic relationships among T- and S-phases and the aluminum matrix in an Al-Cu-Mg-Ag alloy, revealing orientation relationships that influence alloy properties and guiding future alloy design.

## Contribution

It provides the first identification of S-T orientation relationships and analyzes differences in S-Al relationships at T-phase interfaces using advanced electron microscopy techniques.

## Key findings

- Identified orientation relationships between T-, S-phases, and Al matrix.
- Discovered S-T orientation relationships for the first time.
- Found differences in S-Al relationships at T-phase interfaces.

## Abstract

T-(Al20Cu2Mn3) phase dispersoids are important for limiting recovery and controlling grain growth in Al-Cu alloys. However, these dispersoids can also reduce precipitation hardening by acting as heterogeneous nucleation sites and may lead to increased susceptibility towards pitting corrosion when galvanically coupled with S-(Al2CuMg) phase precipitates. The interplay between T- and S-phases is therefore important for understanding their effect on the mechanical and electrochemical properties of Al-Cu-Mg alloys. Here, the crystallographic relationships between the T-phase, S-phase, and surrounding Al matrix were investigated in an Al-1.31Cu-1.14Mg-0.13Ag-0.10Fe-0.28Mn (at.%) alloy by combining scanning precession electron diffraction with misorientation analysis in 3-dimensional axis-angle space and correlated high-resolution transmission electron microscopy. Orientation relationships are identified between all three phases, revealing S-T orientation relationships for the first time. Differences in S-Al orientation relationships for precipitates formed at T-phase interfaces compared to their non-interfacial counterparts were also identified. These insights provide a comprehensive assessment of the crystallographic relationships in T-/S-phase aggregates, which may guide future alloy design.