Critical Pressures of Water Vapor and Hydrogen Gas Causing Intergranular Brittle Fracture of Co₃Ti at Room Temperature

抄録

Critical partial pressures of water vapor and hydrogen gas, which caused brittle fracture at room temperature, were measured for Co₃Ti intermetallic compounds. The critical partial pressures significantly depend on deformation rate and alloy composition. The critical water vapor pressure (P_H2O^*) of a Co-21mol%Ti alloy is measured to be 0.1 Pa at a deformation rate of 0.2 mm/min. With increasing deformation rate, the P_H2O^* increases. In a Co-23 mol%Ti alloy, P_H2O^* is about four order of magnitude lower than that of a Co-21 mol%Ti alloy, indicating that the former is more susceptible to the embrittlement in air than the later. The critical hydrogen gas pressures (P_H2^*) of these two alloys are lower than the P_H2O^*, which indicates that these alloys are more susceptible to the embrittlement in hydrogen gas than in air. The difference in susceptibility between these alloys is attributed to the difference in intrinsic grain boundary strength. Environmental embrittlement of Co-21 mol%Ti is completely suppressed by an addition of 3mol%Fe, and the P_H2O^* is evaluated to be larger than 1 kPa, while the P_H2^* is measured to be 2 Pa at a deformation rate of 0.2 mm/min. It is considered that decomposition rate of water vapor is reduced by the addition of iron, resulting in the suppression of the embrittlement in air. It is proposed that higher susceptibility to environmental embrittlement of Co₃Ti intermetallic compounds than that of high strength steels is due to the acceleration of decomposition of water vapor at the surface of the intermetallic compounds.