Strain ratio effect on microstructural evolution during low cycle fatigue exploitation of nickel base superalloy IN 740H

• Published in: The International journal of pressure vessels and piping 2021-12, Vol.194, p.104551, Article 104551
• Main Authors: Kumar, C., Jena, P.S.M., Singh, R.K., Harmain, G.A., Sahu, J.K.
• Format: Article
• Language: English
• Subjects: Dislocation; Fatigue life; Low-cycle fatigue; Ni-base superalloy
• ISSN: 0308-0161; 1879-3541
• DOI: 10.1016/j.ijpvp.2021.104551

Strain ratio (Rε) is varied during isothermal strain-controlled fatigue tests of nickel-base superalloy IN740H at 760 °C; Rε = −1, 0, 0.5. Unlike most alloys, highest fatigue life is observed under Rε = 0 test condition compare to Rε = 0.5 and −1. From extensive electron microscopy, i.e. energy-filtered TEM (EF-TEM), WDS and EDS studies, it is confirmed that the creep effect due to tensile mean stress under Rε = 0 condition was offset due to segregation of Nb at dislocation and formation of Nb(CN) at grain boundaries, which proved to be fatigue strengthening. Debonding of Nb(CN) and γ matrix provided preferential fatigue crack initiation sites and resulted in lowering the fatigue life for Rε = 0.5 test condition. •Isothermal low cycle fatigue (LCF) test was conducted in IN 740H alloy at 760 °C by employing different strain ratio (Rε) and different strain amplitude.•Unlike most alloys, the highest fatigue life is observed under Rε = 0 test conditions compare to Rε = 0.5 and −1.•From extensive electron microscopy analysis it was found that the LCF life is improved by retardation of fatigue crack growth caused by Nb(CN) precipitated at grain boundaries in Rε = 0 test condition.