Micro-alloying-driven transformation mechanisms of T1p to T1 in Al-Cu-Li-Mg-Ag alloys

Abstract

This study presents an atomistic experimental and theoretical investigation into the different stages of evolution from the early T_1p phase to fully matured T₁ in an Al-Cu-Li-Mg-Ag alloy, with the central aim of clarifying the role of the micro-alloying elements Mg and Ag in the T_1p → T₁ transformation. In addition to the heterogeneous nucleation of the T₁ precipitate on pre-existing dislocations, the present work provides direct experimental support for homogeneous T_1p nucleation, with transformation into T₁ proceeding via a distinct pathway involving nucleation of dislocation loops or antiparallel pairs within T_1p, independent of pre-existing dislocations. Contrasting earlier conclusions, the T_1p → T₁ transformation is found to initiate with an atomic rearrangement within the T_1p Al-Cu layers into Kagomé nets (“Al-Cu layer reorientation”), with Shockley partial dislocations (SPDs) activation and passage following only after this point. This structural transformation order is explained by the Kagomé nets introducing structural geometries promoting the SPD activation. Li and Mg at the interface are identified as primary contributors to the SPD passage-induced transformation driving force. Ag complements this process indirectly by facilitating Li incorporation. The final stages of transformation are diffusion-limited, with Cu and Li incorporation representing the slowest step. These results substantiate atomistically the role of the micro-alloying elements Mg and Ag in homogeneous T_1p nucleation and transformation.