ENVIRONMENTAL EMBRITTLEMENT OF NI3(SI, TI) SINGLE-CRYSTALS

The environmental embrittlement of Ni3(Si, Ti) single crystals is inve stigated as functions of environmental media, strain rate, temperature and crystal orientation. The environmental embrittlement is shown to occur in air, water and hydrogen gas, and to be limited to ambient tem peratures. Also, it is found that the environmental embrittlement is v ery sensitive to strain rate in decreasing the elongation with decreas ing strain rate, and is little affected by boron doping. In response t o embrittlement from environment, tensile elongation and fracture stre ss are reduced and fracture mode primarily changes from {111} cracking with ridge patterns (or dimple patterns) to {001} cracking with a riv er pattern. It is demonstrated that principal cause of the environment al embrittlement of Ni3(Si, Ti) single crystals is due to an extremely high reaction of water (i.e. H2O) in environment with reactive elemen ts of Si (and / or Ti) and of H-2 gas in the environment with the surf ace reactive element of Ni. Based on the measurement of fracture stres ses, it is proposed that the brittle mechanism, i.e. the decohesion me chanism is applicable to the environmental embrittlement of Ni3(Si, Ti ) single crystals.