Thermal fatigue damage in cast IN-100 superalloy: Experiments and modelling

Abstract

The mechanical processes involved in thermal fatigue degradation of a cast-turbine-blade material have been studied by an induction heating procedure coupled with an advanced AC potential drop system (ACPD). In particular, three temperature histories (cycle type) were investigated using double-edge wedge specimens of cast IN-100 nickel base alloy. The respective stress-strain histories, as determined from thermo-elasto-plastic finite element analyses, are presented. Depending upon strain history, two modes of surface degradation were observed: scalloping and through-thickness cracking of a uniform oxide layer. The degree of scalloping was shown to depend on the magnitude of compressive strain at the surface. Severe scalloping was observed after 3000 thermal cycles between peak strains of -0.48% at 1050 °C and +0.08% at 400 °C. More than 3000 cycles between peak strains of -0.24% at 1050 deg;C and 0.23% at 400 °C did not produce scalloping. The number of cycles to crack initiation was found to correlate with peak compressive strain. The findings were shown to be consistent with a mechanism for scallop initiation and growth involving cyclic oxide cracking and cyclic ratchetting. Mathematical formulations to model the observed damage mechanisms are proposed. The implementation of these damage evolutive laws into design and lifing procedures is discussed.