Charakterisierung der Schädigungsmechanismen bei mechanischer und thermomechanischer Ermüdung einer hochfesten γ-TiAl-Legierung

Aim of this work was the investigation of the high temperature fatigue behavior of the gamma-TiAl-alloy TNB-V2. Total strain-controlled fatigue experiments under isothermal (LCF) and thermo-mechanical (TMF) conditions were performed at a strain ratio of R=-1. The alloy shows under LCF conditions a clear dependency of the fatigue life on the strain amplitude. At low temperatures and high strain amplitudes strain aging leads to a cyclic hardening until fracture. The microstructure shows mainly formation of twining and high dislocation density. Above 650°C cyclic softening takes place at high strain amplitudes. The microstructure shows above the BDTT a degradation of the lamellar morphology, where a transformation of the excess alpha2 phase into the gamma phase takes place. Under in-phase (IP)-condition the compressive mean stresses exhibit a beneficial effect on the fatigue life. This leads at low strain amplitudes to higher fatigue lives than under LCF conditions. The TMF experiments show also that a low minimum temperature of 350°C enhances the dynamic strain aging and causes together with the environment a considerable decrease of the fatigue life under out-of-phase (OP) condition. In contrast to LCF microstructure the TMF microstructure shows no dynamic recrystallisation in the gamma-grains. Under TMF condition dislocations are generated during the minimum temperature phase. The fatigue lives under IP, OP, and LCF conditions were described by means of a single model, which consists of two parts. In the first, part the TMF hysteresis loop was simulated by means of a modified multi-component model. In the second part, a damage parameter for the fatigue life description was defined. The parameters of the stress-strain response needed for this model were calculated by means of the multi-component model mentioned above. Due to the combination of these two models a microstructure reference was assured during fatigue life description.