EMBRITTLING EFFECT OF IRON AND NICKEL BASE SUPERALLOYS IN HIGH TEMPERATURE LONG TIME EXPOSURES

摘要

Iron and nickel base superalloys for gas turbine applications require stable mechanical properties. However, changes in mechanical properties can occur due to microstructural instability. The most common structure change in our investigated superalloys is the coagulation of strengthening phases such as γ' and γ" and the meta-stable phase change to a stable phase, such as γ"→δ in alloy 718 and γ'→η in alloy 740, which will result in alloy softening. Embrittling effects can also occur due to these instabilities. Five representative superalloys, 15Cr/25Ni type A-286, 15Cr/40Ni type GH302 iron-base alloys, INCONEL alloy 718, INCONEL alloy 625 and INCONEL alloy 740 nickel-base alloys have been chosen for this study. The embrittling effects are related to carbide precipitation and intermetallic or TCP phases such as a, u. and a-Cr phase forming at grain boundaries. An obvious embrittling effect is a significant reduction in impact toughness. In combination with the softening and embrittling effects, severe materials performance degradation can happen and could influence the service life of gas turbine components.