Brittle-to-ductile transition in polycrystalline NiAl

The brittle-to-ductile transition (BDT) of stoichiometric NiAl and Ni-49Al-1Ti alloys was investigated using tensile and fracture toughness testing methods. The BDT in these alloys was found to be associated with a sharp drop in strain hardening rate, which causes the general yielding and plastic instability (necking) to precede unstable fracture in notched and tensile specimens, respectively. The addition of Ti increases the strength, low temperature toughness, and BDT temperature of NiAl significantly. The occurrence of intergranular fracture in NiAl alloys has been correlated with the development of localized internal stresses near grain boundaries owing to a lack of sufficient slip systems. The results of this study suggest that the vacancy concentration in NiAl depends significantly on the stress state and the amount of deformation. This phenomenon may be responsible for the microvoid coalescence fracture mechanism observed in tensile specimens, and the enhancement of dislocation climb at moderate temperatures. (C) 1997 Acta Metallurgica Inc.