The effect of martensite formation on the mechanical behaviour of NiAl

The intermetallic compound NiAl with B2 structure was studied in the composition range Ni(55)Al(45)-Ni(64)Al(36) (at.%). Ni-rich NiAl transforms to martensite by rapid cooling. B2-NiAl has a high yield stress, whereas the martensite phase deforms already at low stresses. The microstructure and mechanical behaviour was investigated for as-cast inhomogeneous polycrystalline NiAl and homogeneous polycrystalline and monocrystalline NiAl. Stress-induced martensite was observed at the crack tip in bulk monocrystalline NiAl, which did not affect the observed fracture toughness K(Ic). In contrast to this, the fracture toughness of the as-cast inhomogeneous polycrystalline NiAl was strongly influenced by the presence of thermal and stress-induced martensite. This contrasting behaviour is related to the observation that the martensite in monocrystalline NiAl is formed reversibly at the crack tip and re-transforms after crack tip advancement whereas at least some martensite in inhomogeneous as-cast NiAl forms irreversibly at crack tips without re-transformation. (C) 1999 Elsevier Science Ltd. All rights reserved.