Analyzing dynamic strain aging and yield strength anomaly in IN740H, a nickel-based superalloy comprising low volume fraction of γ′

Abstract

IN740H, a Ni-based superalloy comprising a low volume fraction of γ′, is a promising material for superheater and reheater components in advanced ultra-supercritical (AUSC) power plants. Here, its tensile deformation behavior across a temperature range of 25–800 °C and strain rates of 10−3 to 10−5 s−1 was investigated. Distinct stress-strain curve serrations, consistent with dynamic strain aging (DSA), were predominantly observed between 400 and 600 °C. At lower temperatures and strain rates, Type E serrations appeared, which transitioned to Type B and then to Type B + C serrations with increasing temperature. At 600 °C, Type B serrations emerged at high strain rates, gradually transforming into Type B + C serrations as rates decreased before disappearing at lower rates. Notably, the alloy displayed negative strain rate sensitivity in the DSA regime. Furthermore, the alloy also showed yield strength anomaly (YSA), wherein the yield strength decreased initially with temperature, followed by an increase above a specific temperature. Fractography reveals a transition from transgranular dimple fracture behavior to intergranular fracture behavior with an increase in temperature. Post-deformation electron backscatter diffraction analysis indicated predominant grain alignment along {100} and {111} planes, such that {111}-oriented grains exhibited a lower Schmidt factor and the formation of fewer sub-grain boundaries. This work comprehensively characterizes DSA and YSA in IN740H in the temperature range relevant to AUSC power plants, thereby providing much-needed information to assess its potential suitability for applications under harsh conditions.