Thermomechanical Fatigue Behavior of the High-Temperature Titanium Alloy IMI 834

摘要

The isothermal and thermomechanical fatigue(TMF) behavior of the titanium alloy IMI 834 was studiedbetween 350℃ and 650℃ in air and vacuum ,respectively. Transmission electronmicroscopy (TEM) observations revealed that themicrostructure established in the TMF tests wasgoverned by the maximum temperature within the cycle.However, if the maximum temperature does not exceed600℃, planar dislocation slip prevails and similarmicrostructures are formed regardless of the testtemperature and the testing mode (TMF and isothermal,respectively). As a result, the stress-strainresponse in TMF tests can be assessed from thecorresponding isothermal data. Wavy dislocation slipwas found to determine the stress-strain behavior ifthe maximum test temperature exceeded 600℃.Moreover ,in TMF tests with a maximum test temperatureof 650℃, the dislocation arrangement formed din thehigh-temperature part of the hysteresis loop was foundto e stable throughout the cycle and to affectsignificantly the stress-strain response at the lowtemperatures. Although in-phase (IP) and out-of-phase(OP) TMF tests led to an almost identicalmicrostructure, OP loading was always found to be mostdetrimental. The interaction between the embrittledsubsurface layer, caused by oxygen uptake, and thehigh tensile stresses developing in the low-temperature part of the hysteresis loop in OP testseases crack initiation and initial crack propagationand results in reduced fatigue life.