DSC sample preparation for Al-based alloys

Abstract

Differential Scanning Calorimetry (DSC) is a useful technique for the study of phase transformations and has been widely applied to study precipitation in aluminium alloys (e.g. Ref.1). One reason for its popularity is the perceived ease of sample preparation, which generally involves grinding to a thickness of about 1 mm followed by punching of discs. Alternatively cylindrical rods may be machined and the sample discs can be cut from these rods. It is however well known that punching, grinding, machining and cutting all introduce deformation in Al-based alloys, and this influences precipitation in most heat treatable Al-based alloys by providing sites for heterogeneous nucleation, and by annihilating quenched-in excess vacancies (2). Garcia-Cordovilla and Louis (3) have investigated the influence of specimen preparation on DSC results obtained for a commercial Al-Cu based alloy (AA2011): it was shown that in samples punched and ground after solution heat treatment, the θ’ (Al₂Cu) formation effect had shifted to lower temperatures when compared to samples punched and ground prior to heat treatment.

In the present work the effect of sample preparation on precipitation during DSC heating of a monolithic 8090 (Al-Cu-Mg-Li-Zr) alloy and an 8090 MMC is investigated. The 8090 alloy system seems especially suited for such a study since the main precipitation reactions which occur in this alloy (GPB-zone, δ’(Al₃Li) and S’(Al₂CuMg) formation) cover a wide range of different types of precipitation reactions. GPB-zone formation is thought to occur via homogeneous nucleation and is thought to depend critically on the amount of excess vacancies available (4,5). Several possible paths leading to the formation of the L12 ordered δ’ phase from the disordered Al-rich FCC phase have been proposed in the literature (see e.g. Ref. 6,7,8,9 ). Different paths are thought to operate at different temperatures, with homogeneous nucleation and growth occurring in the high temperature range. In each possible path the amount of excess vacancies is expected to influence the rate of δ’ formation by enhancing diffusivity of Li, whilst coarsening of δ’ is thought to be assisted by dislocations (10,11). Additionally heterogeneous nucleation of δ’ on β’ (Al₃(Zr,Li)) precipitates occurs (see e.g. Ref.12,13). S’ (Al₂CuMg) nucleation occurs mostly heterogeneously and several nucleation sites have been reported in the literature: dislocations (if present) and subgrain boundaries appear to be the most potent sites, followed by dislocation loops resulting from clustering of excess vacancies ( ), although fewer dislocation loops are formed in 8090 alloys as a result of the high Li and Zr vacancy binding energies. Furthermore S’ has been reported to nucleate on δ’ in 8090 alloys (14). An additional complicating factor related to sample preparation in 8090 alloys is lithium loss during solution treatment (15,16,17); Li loss will alter the composition of the surface layer and, unless removed, it will influence thermal reactions associated with precipitation and dissolution of particles.

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