Intrinsic ductility and environmental embrittlement of binary Ni 3Al

Abstract

Polycrystalline, B-free Ni[sub 3]Al (23.4 at.% Al), produced by cold working and recrystallizing a single crystal, exhibits room temperature tensile ductilities of 3-5% in air and 13-16% in oxygen. These ductilities are considerably higher than anything previously reported, and demonstrate that the intrinsic' ductility of Ni[sub 3]Al is much higher than previously thought. They also show that the moisture present in ordinary ambient air can severely embrittle Ni[sub 3]Al (ductility decreasing from a high of 16% in oxygen to a low of 3% in air). Fracture is predominantly intergranular in both air and oxygen. This indicates that, while moisture can further embrittle the GBs in Ni[sub 3]Al, they persist as weak links even in the absence of environmental embrittlement. However, they are not intrinsically brittle' as once thought, since they can withstand relatively large plastic deformations prior to fracture. Because B essentially eliminates environmental embrittlement in Ni[sub 3]Al - and environmental embrittlement is a major cause of poor ductility in B-free Ni[sub 3]Al - it is concluded that a significant portion of the so-called B effect must be related to suppression of moisture-induced environmental embrittlement. However, since B-doped Ni[sub 3]Al fractures transgranularly, whereas B-free Ni[sub 3]Al fractures predominantly intergranularly,more » B must have the added effect that it strengthens the GBs. A comparison with the earlier work on Zr-doped Ni[sub 3]Al shows that Zr improves the ductility of Ni[sub 3]Al, both in air and (and even more dramatically) in oxygen. While the exact mechanism of this ductility improvement is not clear at present, Zr appears to have more of an effect on (enhancing) GB strength than on (suppressing) environmental embrittlement.« less