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Title: Intermetallic phase transformations in 3XXX aluminium alloys
Author: Alexander, D. T. L.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2003
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There is interest in modelling the industrially important intermetallic phase transformation of Al6(Fe,Mn) to a-Al-(Fe,Mn)-Si that occurs during homogenisation of direct-chill cast ingots of 3XXX aluminium alloy. The kinetics of this "6-to-a" transformation are studied to aid the modelling. The initial investigation uses an Al-0.5Fe-1.0Mn-0.2Si (wt.%) alloy. The morphology of the transformation and the associated local composition changes are studied using a range of electron optical techniques, including energy-filtered transmission electron microscopy (TEM), convergent-beam electron diffraction and field-emission-gun scanning electron microscopy. The transformation proceeds by eutectoid decomposition of the Al6(Fe,Mn) phase into a-phase and aluminium solid solution. The transformation requires intake of silicon from the surrounding matrix, which we believe is supplied along the migrating transformation front. This gives a eutectoid growth rate that is comparatively fast; hence nucleation is found to be the key rate-controlling factor. Intake of manganese is not required for transformation, but does occur on continued annealing. The morphology of transformed particles is not static: eutectoid aluminium coalesces and ripens into "Al-spots", and wetting of Al6(Fe,Mn)/a-phase interfaces by the matrix may ultimately lead to fragmentation of two-phase particles. Roll-bonded diffusion couples permit the study of 6-to-a nucleation by steadily increasing the silicon content in an Al-Fe-Mn alloy (initially containing only Al6(Fe,Mn) particles). Nucleation is found to be sporadic. Results also indicate that the transformation preferentially nucleates at boundaries in the matrix - this is confirmed by TEM studies. If there is insufficient eutectoid nucleation, transformation is by the dissolution of Al6(Fe,Mn) supplying solute for the growth of a-phase. However, preliminary diffusion studies using silicon-coated alloy blocks indicate that a typical ingot microstructure gives sufficient 6-to-a nucleation for the alloy to achieve equilibrium levels of a-phase eutectoidally.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available