Tensile ductility, slow crack growth, and fracture mode of ternary B2 iron aluminides at room temperature

B2 iron aluminides containing 45 at% Al and microalloying additions of 0.2 at% B and 0.1 at% Zr were alloyed with 5 at% of the 3d transition elements Ti, V, Cr, Mn, Co, Ni, and Cu. The extruded and annealed alloys were essentially single phase except for isolated stringers which appeared to be borides. Their strength, tensile ductility in air and dry oxygen, and slow crack growth behavior in air were determined at room temperature. Consistent with water vapor-induced embrittlement, the tensile ductility depended on the environment, and the crack growth resistance on the crack velocity. The binary iron aluminide Fe-45 at% Al exhibited ductilities of 8 and 19% in air and dry oxygen, respectively. Ternary alloying increased in most cases the yield strength, and always reduced the ductility. Different alloying additions influenced the slow crack growth behavior in different ways. For example, the crack growth resistance of Fe-45Al-5Cr (at%) did not depend significantly on the crack velocity, whereas that of Fe-45Al-5Mn (at%) decreased strongly as the crack velocity decreased. Alloying additions which are to the left of iron in the periodic system, such as Ti, Cr, and Mn, were associated with intergranular fracture, whereas Co, Ni, and Cu additions, to the right of iron, were associated with substantial amounts of transgranular fracture. For a given alloy, the fraction of intergranular fracture increased with decreasing crack velocity. The experimental results are rationalized in terms of the environmental sensitivity of the grain boundaries, the increase in yield strength upon alloying, and the site occupation of the ternary alloying additions. This does, however, not preclude the influence of additional factors such as differences in the chemical composition and strength of the grain boundaries in the different alloys. (C) 1997 Elsevier Science Ltd.