Environmental embrittlement of binary and Zr-doped Ni₃Al

This paper summarizes results of our recent work on moisture-induced environmental embrittlement in Ni₃Al. We took the unconventional approach of starting with single crystals of B-free Ni₃Al, which were cold rolled and recrystallized to produce crack-free polycrystalline material. Our results show that the intrinsic ductility (approximately 16%) of Ni₃Al (23.4 at.% Al) is considerably higher than previously thought; however, it is severely embrittled by moisture in air (ductility dropping from a high of approximately 16% when tested in oxygen to a low of approximately 3% in air). Since B-doped Ni₃Al does not show such embrittlement (i.e., ductilities high in both air and oxygen), we conclude that a significant part of the beneficial effect of B must be related to suppression of this environmental effect. However, B must also improve grain boundary (GB) cohesion in Ni₃Al, since our B-free alloy fractures intergranularly whereas B-doped alloys in general fracture transgranularly. Addition of a small amount (0.26 at.%) of Zr to Ni₃Al significantly improves its ductility: to 11-13% in air, and 48-51% in oxygen. The ductilities observed in oxygen are comparable to the highest ever ductility observed in B-doped Ni₃Al, indicating that the GBs in this Zr-doped alloy are not intrinsically brittle (rather, environmental embrittlement is the main reason for its brittleness). Zr dramatically increases the resistance of Ni₃Al to GB fracture - perhaps by increasing GB cohesion. However, Auger analysis shows little or no Zr segregation on the GBs of Ni₃Al, making it unclear how it might actually affect GB cohesion. Zr does not significantly increase the resistance of Ni₃Al to environmental embrittlement; nor does it suppress intergranular fracture. In both these respects Zr behaves differently than B.