Transforming solid-state precipitates via excess vacancies

Laure Bourgeois,Yiqiang Chen,Nikhil V Medhekar,Zezhong Zhang,Yong Zhang

doi:10.1038/s41467-020-15087-1

Abstract

Many phase transformations associated with solid-state precipitation look structurally simple, yet, inexplicably, take place with great difficulty. A classic case of difficult phase transformations is the nucleation of strengthening precipitates in high-strength lightweight aluminium alloys. Here, using a combination of atomic-scale imaging, simulations and classical nucleation theory calculations, we investigate the nucleation of the strengthening phase θ′ onto a template structure in the aluminium-copper alloy system. We show that this transformation can be promoted in samples exhibiting at least one nanoscale dimension, with extremely high nucleation rates for the strengthening phase as well as for an unexpected phase. This template-directed solid-state nucleation pathway is enabled by the large influx of surface vacancies that results from heating a nanoscale solid. Template-directed nucleation is replicated in a bulk alloy as well as under electron irradiation, implying that this difficult transformation can be facilitated under the general condition of sustained excess vacancy concentrations.

Highlights

Many phase transformations associated with solid-state precipitation look structurally simple, yet, inexplicably, take place with great difficulty
We report direct and rapid nucleation of the θ′ phase as well as of a precipitate phase which we denote η′, on pre-existing θ′′ precipitates. We describe this nucleation pathway as templatedirected, as it involves a precursor phase (θ′′) that serves as a structural template for the nucleated phases
In an attempt to quantify why template-directed nucleation (TDN) is promoted in a nanoscale specimen but not in bulk samples subjected to conventional heat treatments, we calculated the energy barrier of nucleation for different situations using classical nucleation theory (CNT)12—see Supplementary Note 3

Summary

Introduction

Many phase transformations associated with solid-state precipitation look structurally simple, yet, inexplicably, take place with great difficulty. The transformations occur after only 10 min at 160 °C when heating a thin TEM specimen (i.e. sample with one nanoscale dimension), and are observed at temperatures as low as 120 °C (see Supplementary Fig. 1).

Results

Conclusion