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No abstract

Recent alloy development efforts have shown that Fe3Al-based alloys can have room temperature tensile ductilities of 10–20% and yield strengths of 500 MPa at temperatures to 600 °C. These property improvements are important for enabling the use of iron-aluminides for structural applications that require their excellent corrosion resistance. New data are presented here from creep-rupture studies on Fe3Al and on Fe3Al-based alloys containing molybdenum or niobium plus zirconium. Binary Fe3Al alloys have low creep resistance, but the addition of 2 at. % Mo or 1% Nb plus 0.1% Zr increases the creep life and reduces the minimum creep rate, with the niobium-containing alloy being the strongest. The improvement in creep life is the result of a combination of factors which include grain boundary strengthening, resistance to dynamic recrystallization during stressing, precipitation strengthening, and changes in the formation and mobility of the dislocation network. Correlation of optical, scanning electron, and transmission electron microscopy data suggests that the intergranular creep failure found in Fe3Al after creep testing at 550–650 °C is related to weak high-angle grain boundaries and to formation of subgrain boundary arrays, which reduce the ability of dislocations to glide or multiply to produce matrix plasticity. The addition of niobium/zirconium results in solid solution strengthening effects, as well as the formation of fine MC precipitates (a small amount of carbon is present as a contaminant from the casting process) which strengthen both the matrix and grain boundaries. The result relative to the binary alloy is increased creep-rupture strength and life coupled with a change to a ductile-dimple transgranular failure mode. This suggests that the mechanisms that cause failure during creep can be controlled by macro- and microalloying effects.

No abstract

A family of creep-resistant, Al2O3-forming austenitic (AFA) stainless steels was recently developed. The alloys exhibit excellent oxidation resistance up to ∼800°C, but are susceptible to internal attack of Al at higher temperatures. In the present work, higher levels of Ni, Cr, Al, and Nb additions were found to correlate with improved oxidation behavior at 900°C in air. The alloys generally appeared to be initially capable of external Al2O3 scale formation, with a subsequent transition to internal attack of Al (internal oxidation and internal nitridation) that is dependent on alloy composition. Compositional profiles at the alloy/scale interface suggest that the transition to internal oxidation is preceded by subsurface depletion of Al in the lower-Al compositions. In higher Al-containing compositions, NiAl second-phase precipitates act as an Al reservoir, and Al depletion may not be a key factor. Alloy design directions to increase the upper-temperature limit of protective Al2O3 scale formation in these alloys are discussed.