Influence of strain rate on fracture mode of a polycrystalline Ni 3Al

Abstract

Intergranular brittleness of Ni{sub 3}Al with L1{sub 2} ordered crystal structure is still an interesting topic in intermetallic research. High tensile ductility in polycrystalline Ni{sub 3}Al has been achieved at room temperature by doping with a small amount of boron. There are three schools of thought regarding the mechanism of the so-called boron effects: (a) that boron improves ductility by enhancing grain boundary cohesion, (b) boron improves ductility by facilitating slip transfer across grain boundaries, and (c) that boron can suppress embrittlement due to environmental moisture. However, when the boron concentration in a Ni{sub 3}Al alloy was reduced to 120 wt.p.p.m., the alloy becomes strongly susceptible to environmental embrittlement with elongations of 10.1% and 25.5% in air and vacuum, respectively. Through cold work and subsequent recrystallization, defect-free polycrystalline Ni{sub 3}Al without any boron was obtained by Liu and George et al. When these polycrystalline specimens were tested, it was found that their elongations increase from around 3% in air to as much as 16% in oxygen, and 23% in ultrahigh vacuum (10{sup {minus}10} torr). Therefore, it was proposed that the intrinsic ductility of Ni{sub 3}Al is quite high, and that moisture-induced intergranular embrittlement is mainly responsible for the low ductilitymore » commonly measured in air at room temperature. However, a basic understanding of the actual embrittlement mechanism remains elusive. In this study, the effects of strain rate on fracture behaviors in a polycrystalline Ni{sub 3}Al doped with 100 wt.p.p.m. boron, were investigated by the room temperature tensile test and SEM fractographic analysis.« less