Effect of temperature and strain rate on the mechanical properties of Fe–40Al–0.6C

The microstructure and mechanical properties of an Fe–40Al–0.6C alloy was characterized in the extruded and low-temperature annealed condition. The microstructure consisted of a B2 FeAl matrix with Fe3AlC0.5 perovskite-type carbides distributed within the grains and at grain boundaries. These carbides dissolve completely in the matrix at 950°C. In addition, a small amount of free carbon in the form of graphite was also present, and at temperatures in excess of 1050°C, a reaction of the type aqC m aqCqL leading to the formation of a grain boundary liquid film occurs; quenching from above this temperature produces an extremely brittle material. Tensile testing of the extruded and low-temperature annealed material revealed a positive temperature dependence of strength, a brittle to ductile transition at ∼350°C, and a strain-rate dependency of ductility in air at room temperature accompanied by fracture transition from an intergranular mode at fast strain rates to substantially transgranular cleavage at the slowest strain rate. At elevated temperatures, a strain rate dependency of strength and ductility is noted and explains the observed loss in impact toughness at these temperatures. These observations are discussed on the basis of the observed microstructures and available theories.