Ternary Eutectic Phase Research Articles

Cooling schedule effects on the microsegregation in AlMgSi alloys

By means of a numerical simulation technique the extent of microsegregation during dendritic solidification for given cooling conditions can be predicted. A model including dendrite arm coarsening and solid state back diffusion has been developed in which a thermodynamic formulation of the ternary phase diagram is used. Following up the results for binary alloys, the influence of the shape of the cooling curve on the extent of microsegregation and the solidification path is investigated for aluminium alloys containing 0.8 to 1.2 wt.% Mg and 2.8 to 3.2 wt.% Si. The varying amounts of binary and ternary eutectic phases, dendrite arm spacings and the exact position of the solidification path on the liquidus surface are calculated for solidification times ranging from 0.1 to 10 000 seconds with three types of cooling curves (slow/fast, linear, fast/slow). Extreme changes of the cooling curve shape for a given solidification time affect the microstructural parameters as much as changes in the solidification time by several orders of magnitude for a given cooling curve shape. Therefore, predictions of microsegregation and microstructure from solidification time are valid only for a specified shape of the cooling curve.

Geel Dikkop

The onset and initial progress of atmospheric corrosion of ternary alloyed Zn–Mg–Al coatings induced by an aqueous sodium chloride environment was studied in detail by means of surface sensitive techniques, X-ray photoelectron spectroscopy and Auger electron spectroscopy. Two fundamental processes were found to be involved in the initial corrosion attack: (i) dissolution of the surface layer consisting of MgO and (ii) preferential anodic dissolution of both, binary and ternary eutectic phases constituting the coating, which led to the formation of characteristic cracks/holes and was followed by a predominant deposition of zinc corrosion products around anodic regions.