Enhancement of high temperature strength and room temperature ductility of iron aluminides by alloying

The chemical ordering in intermetallics results in reduced atomic mobility and therefore increased resistance to plastic deformation at elevated temperatures. This intrinsic source of high temperature strength leads to the inherent brittleness of polycrystalline ordered intermetallics at room temperature. The requirements for optimum high temperature strength and ductility at ambient temperature are often incompatible. Iron aluminides possess high strength up to 873 K. There is an anomalous (positive) temperature dependence of yield and flow strengths. Iron aluminides have yet to achieve satisfactory elevated load bearing capability. Alloy additions have the potential for improving elevated temperature strength and room temperature ductility; whichever is more critical for the application. Elements such as Cr, Ti, Mn, Co, and Mo produce higher flow stress due to solid solution strengthening. Elements such as Zr, Ta, Nb, Re, and Hf go into solution partly, reprecipitate, effectively pin dislocations and thereby cause strengthening. Mo, Zr, and Hf produce good tensile strength at elevated temperatures but ductility decreases. Element B strengthens by grain boundary cohesion. The improvement in room temperature ductility can be achieved through modification of the crystal structure by changes in stoichiometry, macroalloying, microalloying, and control of the environment. B, TiB2, and Cr are notable for enhancing ductility. The paper is an overview of the present status of iron aluminides in this respect.