High temperature fatigue behaviour of TZM molybdenum alloy under mechanical and thermomechanical cyclic loads

High temperature isothermal mechanical fatigue and in-phase thermomechanica l fatigue (TMF) tests in load control were carried out on a molybdenum-base d alloy, one of the best known of the refractory alloys, TZM. The stress-st rain response and the cyclic life of the material were measured during the tests. The fatigue lives obtained in the in-phase TMF tests are lower than those obtained in the isothermal mechanical tests at the same load amplitud e. It appears that an additional damage is produced by the reaction of mech anical stress cycles and temperature cycles in TMF situation. Ratcheting ph enomenon occurred during the tests with an increasing creep rate and it was dependent on temperature and load amplitude. A model of lifetime predictio n, based on the Woehler-Miner law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature a re introduced in the model so as to evaluate TMF lives in load control. Wit h this method the lifetime prediction gives results corresponding well to e xperimental data. (C) 2000 Elsevier Science B.V. All rights reserved.