EFFECT OF BORON ADDITIONS ON THE DUCTILITY AND FRACTURE-BEHAVIOR OF NI3AL SINGLE-CRYSTALS

The effect of B additions on the ductility and fracture behavior of Ni3Al single crystals has been investigated. Tensile tests at low and high temperatures were carried out in air at constant strain rate for two selected orientations. The results show a positive effect of B additions on both the ductility and the fracture stress at room temperature, but not at high temperatures. The largest improvement in ductility over that found for the pure material was found for additions of 0.2 at.% B. Scanning electron microscopy analysis of the fracture surfaces shows a combination of massive slip on {111} planes, some regions with cleavage-like appearance, and heavily dimpled areas. The improvement in ductility at room temperature has been explained in terms of the increased interfacial strength between the metal matrix and Ca-rich non-metallic inclusions on which the microvoids nucleate. The observations presented show that B additions improve the ductility of both polycrystalline and single crystalline Ni3Al. It is suggested that a "bulk effect" should be considered in addition to the grain boundary strengthening effect of B when explaining the improvement in ductility of polycrystalline Ni3Al due to B additions.