Multi-Axial Thermo-Mechanical Fatigue of a Near-Gamma TiAl-Alloy

Abstract

A material family to replace the current superalloys in aeronautical gas turbine engines is considered to be that of gamma Titanium Aluminide (-TiAl) alloys. Structural components in aeronautical gas turbine engines typically experience large variations in temperatures and multiaxial states of stress under non-isothermal conditions. The uniaxial, torsional and bi-axial thermo-mechanical fatigue (TMF) behaviour of this -TiAl alloy have been examined at 400 – 800oC with strain amplitudes from 0.15% to 0.7%. The tests were conducted at both in-phase (IP) and out-of-phase (OP). The effects of TMF on the microstructure were also investigated. For the same equivalent mechanical strain amplitude uniaxial IP tests showed significantly longer lifetimes than pure torsional TMF tests. The non-proportional multiaxial OP test showed the lowest lifetimes at the same equivalent mechanical strain amplitude compared to the other types of tests.

Isothermal and thermo-mechanical fatigue behavior of the nickel base superalloy Waspaloy™ under uniaxial and biaxial-planar loading

“…Results of Brookes et al [14][15][16], Meersmann et al [17,18] and Ogata [19] show that the fatigue lives for the proportional loading coincide with the uniaxial data. In the case of non-proportional loading lower number of cycles to failure was measured due to stronger hardening by slip system interactions and the associated fatigue damage.…”

“…Literature data of thermo-mechanical fatigue tests at a the tension-torsion testing rig according to Brookes et al [14][15][16], Ogata [19] and Meersmann et al [17,18] confirm that the distortion energy hypothesis according to von Mises is suitable for proportional thermo-mechanical in-phase loading to give a fatigue life correlation. However, a multiaxial test corresponding to the biaxial-planar equibiaxial IP TMF test could not be realized so far by the tension-torsion testing rig because of necessary internal and external pressure on the cylindrical specimen.…”

“…At the biaxial-planar testing system, isothermal and thermo-mechanical fatigue tests at different stress states were successfully carried out. Besides tensiontorsion testing devices [14][15][16][17][18][19], the presented biaxial-planar testing facility is a further test rig, which is able to perform a biaxial thermo-mechanical fatigue test. To our knowledge this is the first presentation of a suitable biaxial-planar testing facility for thermo-mechanical loading which met the code of practice for TMF.…”

“…However, for the non-proportional loading case it can be assumed that the distortion energy hypothesis according to von Mises does not successfully correlate the fatigue life [14][15][16][21][22][23]60,80].…”