High temperature low cycle fatigue properties of alloy 625

The impact of a thermal overaging at 650 °C on Alloy 625 high temperature low cycle fatigue (LCF) and tensile properties was investigated. This prior thermal aging in the γ“ precipitation domain improve both the tensile properties in the 20-750 °C temperature range (Tensile resistance and Yield stress) and the LCF lifetime at 650 °C. The improvements in monotonic and LCF properties compared to the samples without any prior aging are provided by the γ” structural hardening which limits the cyclic ratcheting during LCF tests. In addition, using a combination of strain-controlled LCF experiments, interrupted stress-controlled LCF experiments followed by a tensile tests and microstructure inspections, it is shown that the microstructure of Alloy 625 without any prior aging is unstable during LCF solicitation and that the γ″ precipitation as well as the γ″→δ transformation are occurring during mechanical cycling, affecting the mechanical behavior of the alloy.