Effect of addition of molybdenum or niobium on creep-rupture properties of Fe 3 Al
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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.
I. INTRODUCTION Binary Fe3Al is an ordered intermetallic alloy, which can exist in one of two ordered structures depending on temperature: an "imperfect" B2 ordered structure at temperatures of approximately 550-800 °C or a more perfect D0 3 ordered structure below 550 °C. Iron aluminides based on Fe3Al have excellent oxidation and corrosion resistance.1 However, until recently their potential use as structural material has been hindered by low room temperature ductility (70 at. % Zr)35 were dispersed uniformly throughout the matrix and were associated with dislocations [Fig. 12(a)]. These small precipitates were more coarsely distributed after creep than the fine precipitates observed in the as-heat-treated control specimen. The small matrix and grain boundary precipitates most likely contributed both to the lower creep rate and the better creep-rupture resistance observed in
this specimen. Precipitates having the L2X Heusler structure, which were observed by Dimiduk et al. in Fe-25Al-2Nb, 4 9 were not observed in the Fe3Al + INb alloy of this study. At a higher stress of 276 MPa at 650 °C, creep rupture occurred after only 0.5 h (28.5% elongation). TEM showed a high co
