Phase transformation for primary particles in the surface regions of an AAl200 alloy

In the present investigation the particle structure in an AAl200 sheet ingot used for litho applications has been studied. Caustic etching of the as-cast material was seen to result in a zone close to the surface with a different etching response. This zone was identified as what is known as a fir-tree zone or an Altenpohl zone [1,2,3,4]. A variation in particle type over the cross section of the as-cast ingot was seen to follow the differences in etching response. After heat treatment of the material, the fir-tree zones were no longer visible, and the accompanying change in particle structure was studied. Samples from the subsurface regions and from a distance of -20 cm from the surface has been investigated before and after heat treatment. In the as-cast material, the sample from the surface was dominated by featherlike particles with long strings of particles, identified as Al.Fe. While closer to the centre A13Fe and Al6Fe were seen to be the main phases, however, some Alf'Fe and probably some a-AIFeSi was also found in this sample. After heat treatment, the particle structure was seen to change, and the surface sample contained mainly Al3Fe in addition to a small amount of Al.Fe. The change in particle structure during heat treatment is discussed with reference to the change in etching response.