Oxidation assisted thermomechanical fatigue failure of polycrystalline superalloys

Abstract

Thermomechnical fatigue (TMF) failure of Nimonic 90, a polycrystalline superalloy, has been investigated under out of phase (OP), in phase (IP) and counterclockwise (CCD) tests. The tests have been carried out in a temperature range of 400–850°C and mechanical strain range of ±0·4%. One IP test has been carried out at a lower strain range of ±0·3%. The TMF lives are greatly affected by the TMF cycle profiles. The shortest TMF life was recorded for the IP test, whereas the longest was recorded for the OP test at the same strain level. Detailed investigation of the micromechanisms of deformation highlighted that γ′ coarsening rate during the IP test was much higher than the other tests and that this was consistent with a faster stress drop during the course of the TMF test. In addition the γ′ coarsening rate was greater for both tests than that occurring during simple thermal cycling of the alloy over the same temperature range but without load. However the more important factor determining the shorter life of the IP test compared with the OP test is related to the initiation and propagation of the failure crack in the two samples. The IP samples showed both initiation and propagation at grain boundaries while the OP test revealed grain boundary initiation followed by an intragranular propagation mode. Measurements conducted on the microcrack size revealed a much higher microcrack growth rate during the IP test. It is likely that the tensile strain during the high temperature part of the IP cycle accelerates oxygen diffusion and thus increases grain boundary oxidation at the tips of microcracks, shortening IP life.