Interfacial Reactions Between Anorthite (CaAl2Si2O8) and Al 7075 Alloy at 850°C and 1150°C

The present work reports an investigation of the interactions of Al 7075 alloy and anorthite at 850 degrees C (150h) and 1150 degrees C (24h). Transmission electron microscopy, 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 metal-ceramic interface. At 850 degrees C, the phase formation mechanisms were (a) Si4+-Al3+ interdiffusion between the Al alloy and anorthite to form calcium dialuminate (CA(2)) and Ca2+-Mg2+ interdiffusion between the Al alloy and calcium dialuminate to form spinel. At 1150 degrees C, spinel + Al2O3 and calcium hexaluminate (CA(6)) + CA(2) were the major and minor phase mixtures, respectively in the corroded area. A thin layer of calcium monoaluminate (CA), gehlenite, and Si was present in the immediate vicinity of anorthite. The early stages of corrosion at 1150 degrees C and 850 degrees C were identical. However, due to thickening of the corroded region (viz., spinel formation) and enhanced evaporation of Mg at the higher temperature, the interdiffusion path evolves from Si4+-Al3+ + Ca2+-Mg2+ to Si4+-Al3+ + Ca2+-Al3+, thus establishing the following phase evolution path at the interface: AnorthiteGehleniteCACA2CA6Al2O3