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

Abstract

A corrosion cup test was undertaken using BaAl 2 Si 2 O 8 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 BaAl 2 Si 2 O 8 ; and (3) interdiffusion zone chemically close to barium hexaaluminate. The principal observations are:1. BaAl 2 Si 2 O 8 was highly resistant to molten Al alloy corrosion owing to sluggish kinetics, as evidenced by the observation of unreacted BaAl 2 Si 2 O 8 grains in the interfacial area. 2. 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 BaAl 2 Si 2 O 8 supports the conclusion that the corrosion mechanism is governed by interdiffusion (Si/Ba and Al/Mg) and substitution. 3. 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.

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

“…Sintering was done at 1350°C for 5 h. A more detailed description of these procedures is given elsewhere (E. Adabifiroozjaei, P. Koshy, R. Pardehkhorram, E. Rastkerdar, J. Hart, and C.C. Sorrell, unpublished data) . Figure (a) and (b) show the X‐ray diffraction (XRD) pattern and microstructure, respectively, of the synthesized anorthite.…”

“…The assembly was heated in air to 850°C or 1150°C and soaked for 150 h or 24 h, respectively. More details for these procedures are given elsewhere (E. Adabifiroozjaei, P. Koshy, R. Pardehkhorram, E. Rastkerdar, J. Hart, and C. C. Sorrell, unpublished data) …”

“…Prior work reporting the investigation of the Al 7075 alloy/BaAl 2 Si 2 O 8 system showed that the corrosion mechanism varies significantly with the reaction temperature (E. Adabifiroozjaei, P. Koshy, R. Pardehkhorram, E. Rastkerdar, J. Hart, and C.C. Sorrell, unpublished data) . At low temperature (850°C), the formation of a diffused region between the Al alloy and BaAl 2 Si 2 O 8 was observed to control the corrosion process; at high temperatures (1150°C and 1250°C), increased vaporization of Mg and Zn from the Al alloy and their subsequent oxidation and reaction with BaAl 2 Si 2 O 8 considerably altered the microstructure of the metal–ceramic interface through the formation of complex silicates and aluminates (E. Adabifiroozjaei, P. Koshy, R. Pardehkhorram, E. Rastkerdar, J. Hart, and C.C.…”

“…In order to hinder or prevent these reactions, anti‐wetting additives, such as CaF 2 or BaSO 4 , have been introduced (E. Adabifiroozjaei, P. Koshy, R. Pardehkhorram, E. Rastkerdar, J. Hart, and C.C. Sorrell, unpublished data) . Ceramics containing these additives are heat treated at temperatures typically in the range 900°C–1200°C during which secondary phases, such as anorthite (CaAl 2 Si 2 O 8 ; with CaF 2 ) or celsian (BaAl 2 Si 2 O 8 ; with BaSO 4 ) are formed.…”

“…At low temperature (850°C), the formation of a diffused region between the Al alloy and BaAl 2 Si 2 O 8 was observed to control the corrosion process; at high temperatures (1150°C and 1250°C), increased vaporization of Mg and Zn from the Al alloy and their subsequent oxidation and reaction with BaAl 2 Si 2 O 8 considerably altered the microstructure of the metal–ceramic interface through the formation of complex silicates and aluminates (E. Adabifiroozjaei, P. Koshy, R. Pardehkhorram, E. Rastkerdar, J. Hart, and C.C. Sorrell, unpublished data) …”