HIGH-TEMPERATURE ISOTHERMAL AND THERMOMECHANICAL FATIGUE ON A MOLYBDENUM-BASED ALLOY

High temperature isothermaI fatigue and in-phase thermomechanical fati gue (TMF) tests in stress control were performed on a molybdenum-based alloy, TZM. Two temperature levels of 350 and 500 degrees C were chos en for isothermal fatigue tests and a temperature interval of 350-500 degrees C was applied to thermomechanical fatigue tests.The stress-str ain response and the hfe cycle of the material were measured during th e tests. It appears that temperature variations influence the fatigue life of the alloy and the material is further damaged with combined st ress and temperature cycles. The analysis of the hysteresis loops show ed that a ratcheting phenomenon and a cyclic softening behavior of the material occurred during each kind of the fatigue tests. The observat ions of the microstructure of the damaged specimens demonstrated that the fracture of the alloy was, on the whole, a brittle process under h igher cyclic stress conditions. (C) 1998 Elsevier Science S.A. All rig hts reserved.