Interfacial Reactions Between BaAl2Si2O8 and Molten Al Alloy at 850°C.

A corrosion cup test was undertaken using BaAl2Si2O8 and Al4.1Zn3.2Mg alloy, heated in air for 150 h at 850°C. Electron probe microanalysis, X-ray diffraction, and scanning electron microscopy coupled with energy dispersive spectroscopy were used to identify the mineralogical and microstructural changes at the interfaces. The microstructural results revealed three microstructural areas: (1) Spinel layer with large numbers of Al alloy channels; (2) interfacial area with mainly alumina, spinel, and BaAl2Si2O8; and (3) interdiffusion zone chemically close to barium hexaaluminate. The principal observations are: BaAl2Si2O8 was highly resistant to molten Al alloy corrosion owing to sluggish kinetics, as evidenced by the observation of unreacted BaAl2Si2O8 grains in the interfacial area., The nature of the microstructure, particularly an interdiffusion zone instead of a continuous layer of precipitated alumina at the interface between the Al alloy channels and the unreacted BaAl2Si2O8 supports the conclusion that the corrosion mechanism is governed by interdiffusion (Si/Ba and Al/Mg) and substitution., The formation and limited retention of an MgO layer at the metal-ceramic interface played a critical role in alloy oxidation and the consequent interfacial phenomena. [ABSTRACT FROM AUTHOR]