Precipitation of chromium containing dispersoids in Al–Mg–Si alloys

Abstract

The aim of the present investigation was to study precipitation of Cr in Al–Mg–Si alloys during high temperature annealing. It has been shown that Cr containing dispersoids precipitate during heating to the homogenisation temperature. During the heating small α-Al(CrMn)Si particles nucleate at the surface of the β-AlFeSi particles and after additional heating to a higher temperature the α-Al(CrFe)Si particles grow and the β-AlFeSi particles dissolve. If the content of Cr is high (∼0·3 wt-%Cr) both α-Al(CrFe)Si and α∼-AlCrSi dispersoids are present in the matrix during heating while only α-Al(CrFe)Si dispersoids are present when the content is low (∼0·15 wt-%Cr). The nucleation mechanisms for the dispersoids are not fully understood but most likely the Cr containing dispersoids nucleate by means of an intermediate phase. This intermediate phase is found to be a ‘u phase’ precipitate.