ＴｉＡｌ基金属間化合物の低温および高温環境脆化

Abstract

TiAl-based intermetallic alloys with various alloy compositions and microstructures were tensile tested in various environmental media as functions of temperature and strain rate. Environmental media used in this study were vacuum, air, water vapor, a mixture gas of 5 vol%H2+Ar, O2 gas, N2 gas and Ar gas. All the TiAl-based intermetallic alloys showed low tensile strength or tensile elongation in air, water vapor and a mixture gas of 5 vol%H2+Ar compared to those in vacuum. The reduction of tensile strength or tensile elongation (i.e. environmental embrittlement) was observed not only in low temperature range mostly reaching 600 K but also in high temperature range mostly from 600 K to 1000 K (sometimes temperatures higher than 1000 K). The low- and high-temperature environmental embrittlement depended on the alloy composition (or microstructure). The low-temperature environmental embrittlement diminished at higher strain rates. The high-temperature environmental embrittlement diminished not only at higher strain rates but also at lower strain rates. The possible species causing the high-temperature environmental embrittlement is hydrogen atoms decomposed from water vapor (H2O) or hydrogen gas (H2), similar to those causing the low-temperature environmental embrittlement. Also, it is suggested that the oxidized scale is effective in reducing the high-temperature environmental embrittlement.