Oral Presentation 26th ACMM “2020 Visions in Microscopy”

In situ transformations of precipitate phases in aluminium alloys (#4)

Laure Bourgeois 1 2 , Zezhong Zhang 2 3 4 , Yong Zhang 2 , Yiqiang Chen 2 5 , Nikhil Medhekar 2
  1. Monash Centre for Electron Microscopy, Monash University, Clayton, Victoria, Australia
  2. Department of Materials Science and Engineering, Monash University, Clayton Campus, Victoria, Australia
  3. Department of Materials, University of Oxford, Oxford, United Kingdom
  4. Electron Microscopy for Materials Research (EMAT), University of Antwerp, Antwerp, Belgium
  5. Thermofisher Scientific, ThermoFisher Scientific, Eindhoven, The Netherlands

Many phase transformations associated with solid-state precipitation look structurally simple yet take place with great difficulty. Classic cases of surprisingly difficult phase transformations can be found in alloy systems forming the basis of a broad range of high-strength lightweight aluminium alloys. In these systems, the difficult nucleation of strengthening phases, which are usually semi-coherent, is often preceded by the easy nucleation of another phase with strong structural similarities, typically a coherent precipitate. It is therefore of interest to investigate the reasons behind the difficult transformation from coherent to semi-coherent precipitate phases.

Using a range of scanning transmission electron microscopy (STEM) imaging and spectroscopic techniques (annular dark field STEM, bright field STEM, electron energy loss spectroscopy) both ex situ and in situ, combined with atomic scale simulations (density functional theory and semi-empirical potentials) we examine several phase transformations in the model alloy systems Al-Cu and Al-Ag. We show that certain microalloying additions, or different processing conditions applied to samples in bulk or nanoscale form, result in previously unreported precipitate phases [1-2] or promote the nucleation of existing phases [3-4]. The nucleation mechanisms of these phases involve structural templates provided by coherent precipitates [1-3] and depend critically on the availability of vacancies [1-2,4]. Based on our observations atomic-scale mechanisms are proposed for the coherent to semi-coherent precipitate phase transformation pathways and the associated energy barriers estimated. These findings suggest several approaches to stimulate these phase transformations.

  1. Z. Zhang, L. Bourgeois, J.M. Rosalie and N.V. Medhekar, Acta Mater. 132 (2017) 525.
  2. L. Bourgeois, Y. Zhang, Z. Zhang, Y. Chen and N.V. Medhekar, under review.
  3. Y. Chen et al. Acta Mater. 125 (2017) 340.
  4. Y. Zhang, Z. Zhang, N.V. Medhekar and L. Bourgeois, Acta Mater. 141 (2017) 341.
  5. The authors acknowledge funding from the Australian Research Council, the use of facilities within the Monash Centre for Electron Microscopy and computational support from the Monash Sun Grid cluster, the National Computing Infrastructure and Pawsey Supercomputing Centre funded by the Australian Government.