Environmental embrittlement: The major cause of room-temperature brittleness in polycrystalline Ni 3Al

Abstract

Single crystals of Ni{sub 3}Al have long been known to be ductile, whereas polycrystalline Ni{sub 3}Al is usually found to be brittle. Early Auger work indicated that grain boundary (GB) fracture in Ni{sub 3}Al is related to sulfur segregation at GBs, but later work showed that brittle intergranular fracture persists even when the GBs are relatively free of embrittling impurities. This led many investigators (including the authors) to conclude that the GBs in Ni{sub 3}Al are intrinsically brittle. However, harmful impurities constitute just one possible source of extrinsic brittleness. Another potential embrittler is a hostile environment. This paper reports that the main experimental hurdle in investigating environmental embrittlement in B-free Ni{sub 3}Al is the fact that, in the as-cast form, it is brittle at both high and low temperatures. So it is extremely difficult to process a cast ingot of Ni{sub 3}Al down to sheet stock without introducing cracks-as opposed to, say, FeAl or Ni{sub 3}Si, both of which can be hot rolled quite easily at elevated temperatures. When the cold-forged and recrystallized Ni{sub 3}Al alloys were subsequently tensile tested, it was found that their room-temperature ductility increased from around 2.5% in air to as much as 8.2% in oxygen,more » indicating that moisture induced environmental embrittlement has a strong effect on ductility.« less