Ab initio Modelling of the Early Stages of Precipitation in Al-6000 Alloys

TL;DR

This study uses ab initio calculations and mesoscale modeling to analyze the energetics and stability of early-stage precipitates in Al-6000 alloys, revealing insights into nucleation and growth mechanisms.

Contribution

It provides a detailed energetic analysis of nanometric precipitates and validates mesoscale models for predicting their stability and interactions.

Findings

Needle-shaped precipitates are unstable to growth at the smallest sizes.

Elastic strain energy influences precipitate stability and growth.

Early precipitation stages may be diffusion-limited.

Abstract

Age hardening induced by the formation of (semi)-coherent precipitate phases is crucial for the processing and final properties of the widely used Al-6000 alloys. Early stages of precipitation are particularly important from the fundamental and technological side, but are still far from being fully understood. Here, an analysis of the energetics of nanometric precipitates of the meta-stable $\beta''$ phases is performed, identifying the bulk, elastic strain and interface energies that contribute to the stability of a nucleating cluster. Results show that needle-shape precipitates are unstable to growth even at the smallest size $\beta''$ formula unit, i.e. there is no energy barrier to growth. The small differences between different compositions points toward the need for the study of possible precipitate/matrix interface reconstruction. A classical semi-quantitative nucleation theory approach including elastic strain energy captures the trends in precipitate energy versus size and composition. This validates the use of mesoscale models to assess stability and interactions of precipitates. Studies of smaller 3d clusters also show stability relative to the solid solution state, indicating that the early stages of precipitation may be diffusion-limited. Overall, these results demonstrate the important interplay among composition-dependent bulk, interface, and elastic strain energies in determining nanoscale precipitate stability and growth.