Thermo-Mechanical Fatigue Behaviour of the Gamma-Titanium Aluminide TNB-V5 with Near-Gamma Microstructure

Abstract

The efficiency of aircraft and industrial gas turbines and combustion engines depends on the maximum operation temperature and, therefore, on the properties of the commercial high temperature materials. In the temperature range 500°C to 750°C γ-titanium aluminides especially alloys of the third generation represent an attractive alternative to the established nickel-base superalloys which have the double density. Due to superimposed cyclic thermal and cyclic mechanical loadings during start-up and shut-down operations structural components in gas turbines and combustion engines may not only be exposed to isothermal but also to thermo-mechanical fatigue (TMF). In this study the cyclic deformation and fatigue behaviour under thermo-mechanical load of the γ-TiAl alloy TNB-V5 with near-gamma microstructure is evaluated. To set a fatigue-life relation strain-controlled thermo-mechanical fatigue tests were carried out with two different strain ranges, different temperature-strain cycles and different temperature ranges from 400°C to 800°C. Additional low-cycle fatigue (LCF) tests were performed at 400°C, 600°C and 800°C for comparison. Cyclic deformation curves, stress-strain hysteresis loops and fatigue lives of the tests are presented. The shortest fatigue lives are always observed in out-of phase (OP) tests, the longest in in-phase (IP) tests. Clockwise-diamond (CD) and counter-clockwise diamond (CCD) testing yield similar fatigue lives intermediate between those of OP and IP tests. For a general life prediction the double-logarithmic plot of the damage parameter by Smith, Watson and Topper vs. fatigue life is well suitable.