Mechanical behavior of an Fe-40Al-0.6C alloy

The mechanical properties of Fe-40 at.% Al containing 0.6 at.% C have been evaluated in the extruded and low-temperature annealed condition as a function of temperature and strain rate. In the as-extruded condition, the yield strength at room temperature is similar to 510 MPa with an associated plastic tensile elongation of 2.3%. With increasing test temperature, the strength decreases steadily to similar to 350 MPa at 600 degrees C; a brittle-to-ductile transition is recognized in the 300-400 degrees C regime. Upon subjecting the extruded material to a two-step low-temperature anneal, the room temperature strength decreases to similar to 310 MPa and ductility increases to 4.4%. When the strength of this material was measured as a function of temperature, a positive temperature dependence of strength was observed in the 400-600 degrees C range while the brittle-to-ductile transition temperature range decreased by similar to 75 degrees C. At a fairly constant yield stress, ductility steadily decreased with decreasing strain rate at room temperature in air from similar to 12% at 4 x 10(-1)/s to 1.7% at 4 x 10(-7)/s. At -30 degrees C, the ductility for comparable strain rates, was significantly higher than that at room temperature. These observations, when compared to data in the literature, verify that while fracture occurs by environmental embrittlement, it is delayed in this carbide-containing alloy. The carbides are thought to act as hydrogen traps and delay the arrival of critical amounts of hydrogen to the crack lip thereby delaying embrittlement. The role of carbide size and distribution on tensile properties was examined by developing isochronal and isothermal aging curves for this alloy. (C) 1998 Acta Metallurgica Inc.