Fe3Al-based iron aluminides have been of interest for many years because of their excellent oxidation and sulfidation resistance. However, limited room temperature ductility (<5%) and a sharp drop in strength above 600 °C have limited their consideration for use as structural materials. Recent improvements in tensile properties, especially improvements in ductility produced through control of composition and microstructure, and advances in the understanding of environmental embrittlement in intermetallics, including iron aluminides, have resulted in renewed interest in this system for structural applications. The purpose of this paper is to summarize recent developments concerning Fe3Al-based aluminides, including alloy development efforts and environmental embrittlement studies. This report will concentrate on literature published since about 1980, and will review studies of fabrication, mechanical properties, and corrosion resistance that have been conducted since that time.

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A study of the exposure of SiC at 1200°C and high water-vapor pressures (1.5 atm) has shown SiC recession rates that exceed what is predicted based on parabolic oxidation at water-vapor pressures of less than or equal to ϳ1 atm. After exposure to these conditions, distinct silica-scale structures are observed; thick, porous, nonprotective cristobalite scales form above a thin, dense silica layer. The porous cristobalite thickens with exposure time, while the thickness of the underlying dense layer remains constant. These observations suggest a movingboundary phenomenon that is controlled by the rapid conversion of dense vitreous silica to a porous, nonprotective crystalline SiO 2 .

The oxidation of SiC at 1200°C in a slowly flowing gas mixture of either air or air ؉ 15 vol% H 2 O at 10 atm (1 MPa) was studied for extended times to examine the effects of elevated water-vapor pressure on oxidation rates and microstructural development. At a water-vapor pressure of 1.5 atm (150 kPa), distinct SiO 2 scale structures were observed on the SiC; thick, porous, nonprotective cristobalite scales formed above a thin, nearly dense vitreous SiO 2 layer, which remained constant in thickness with time as the crystalline SiO 2 continued to grow. The pore morphology of the cristobalite layer differed depending on the type of SiC on which it was grown. The crystallization and growth rates of the cristobalite layer were significantly accelerated in the presence of the high water-vapor pressure and resulted in rapid rates of SiC surface recession that were on the order of what is observed when SiO 2 volatility is rate controlling at high gas-flow velocities (30 m/s). The recession process can be described by a paralinear kinetic model controlled by the conversion of dense vitreous SiO 2 to porous, nonprotective SiO 2 .