Precipitation of dispersoids containing Mn and/or Cr in Al–Mg–Si alloys

The formation of dispersoids containing Mn and/or Cr in Al–Mg–Si alloys was studied metallographically and by means of electrical resistivity measurements. Special emphasis was placed on the nucleation mechanisms. For alloys containing Mn and/or Cr it was shown that during the heating of the as-cast alloys to 580°C (which is a typical homogenization temperature) and at a heating rate of 3 K min−1, an intermediate phase, referred to as the ‘u-phase’ nucleated on the β′-Mg2Si needles. The phase has a hexagonal unit cell with a=0.670 nm, c=0.808 nm. The precipitates were rod-shaped and were lined up in the [100] directions in the Al lattice. The phase was found to be rich in Mn and/or Cr. With continued annealing, dispersoids containing Mn and Mn+Cr nucleated heterogeneously on the ‘u-phase’ precipitates before these precipitates dissolved. The nucleation sites for the dispersoids with Cr were not conclusively identified.