Nucleation of the Al6(Fe, Mn)-to-α-Al–(Fe, Mn)–Si transformation in 3XXX aluminium alloys. I. Roll-bonded diffusion couples

Abstract Roll-bonded diffusion couples are used to investigate a transformation of intermetallic particles from Al6(Fe, Mn) to α-Al–(Fe, Mn)–Si that occurs upon homogenization of 3XXX aluminium alloys. By diffusing silicon into an Al–Fe–Mn alloy, the couples permit a progressive increase in the driving force for this 6-to-α transformation, thus allowing study of the nucleation of the transformation. Initially, the aluminium matrix is highly defected from rolling. This microstructure gives frequent (yet stochastic) nucleation of a eutectoid 6-to-α transformation expected from study of direct-chill-cast 3XXX alloys. However, once the matrix has recrystallized, nucleation is restricted to particles that lie on the matrix grain boundaries. The remaining particles, unable to transform eutectoidally, dissolve and supply growth of these α-phase particles, producing marked coarsening. 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 D. Vowles and D.A. Nicol at Cambridge and S. Justice at Alcan for help with specimen preparation and microscopy. Also at Alcan, they thank R.G. Hamerton and H. Cama for many useful discussions, and K. Davis for preparing the roll-bonded diffusion couples. Notes Cross-sectional area is quoted because it has more physical significance than particle diameter for the 'Chinese-script' zone 2 α particles.