Environmental embrittlement and other causes of brittle grain boundary fracture in Ni3Al

In this paper we review intrinsic and extrinsic sources of grain boundary brittleness in Ni3Al. Special attention is given to the recently discovered phenomenon of environmental embrittlement in Ni3Al, and how variables such as test environment, strain rate, temperature, grain boundary character, and microalloying additions affect ductility and fracture behavior. It is shown that environmental embrittlement is a major reason for the low ductility of Ni3Al in air; consistent with this, ductility is found to increase at high strain rates, cryogenic temperatures, and in environments containing low levels of water vapor. When environmental effects are carefully suppressed, the tensile ductility of Ni3Al increases dramatically, indicating that Ni3Al is not as intrinsically brittle as once thought. It is shown that the character of grain boundaries in Ni3Al depends strongly on the processing conditions. Some of these processing-related changes appear to be associated also with changes in ductility; however, additional research is needed to understand the detailed connection between the two. The ductilizing effect of boron in Ni3Al is shown to be related principally to its role in suppressing environmental embrittlement (partly by slowing down hydrogen diffusion). In addition, B enhances grain boundary strength and suppresses intergranular fracture. Much less is known about the way in which other alloying elements improve ductility. One possibility, which needs additional research, is that they alter the grain boundary character distributions in Ni3Al.