Nucleation of the Al₆(Fe, Mn)-to-α-Al–(Fe, Mn)–Si transformation in 3XXX aluminium alloys. II. Transformation in cast aluminium alloys

Abstract

The nucleation behaviour of the homogenization-induced Al₆(Fe, Mn)-to-α-Al–(Fe, Mn)–Si transformation is investigated in a companion paper to part I (a study with roll-bonded diffusion couples). Diffusion experiments using silicon-coated Al–0.53 wt% Fe–1.02 wt% Mn alloy blocks allow control of the thermodynamic driving force for transformation within a microstructure typical of a cast ingot. As expected, this microstructure appears to give ready and yet stochastic nucleation as silicon diffuses into the alloy sections. In addition, transmission electron microscopy is used to analyse partially transformed particles in heat-treated alloy samples of fixed silicon content. This confirms the suggestion made in part I that the transformation preferentially nucleates at matrix grain/cell boundaries. Nucleation theory suggests this results from the ability of the boundaries to relieve volume changes associated with the nucleation event.

Acknowledgements

During this work, D.T.L.A. was supported by an Engineering and Physical Sciences Research Council CASE studentship, in conjunction with Alcan International Ltd. The authors thank Z.H. Barber for much help with the sputter deposition, and R.J. Taylor, D. Vowles and D.A. Nicol for help with microscopy. At Alcan, they thank R.G. Hamerton and H. Cama for many useful discussions, and J. Worth and K. Davis for preparing the alloy samples.

Notes

Considering that silicon substitutes on aluminium sites in the α phase and that the α phase has also been observed with variable ratios of aluminium to silicon (Cooper and Robinson 1966, Tibballs et al. 1989), this suggestion is reasonable.