Creep of direct-energy-deposited Inconel 625 with evolving microstructure

Microstructure and compressive creep properties at 750–850˚C are investigated for an Inconel 625 superalloy fabricated via direct energy deposition of powders. The effects of solution treatment, test temperature, and loading direction on secondary creep rate are studied using conventional stress-jump tests, with additional stress-drop tests conducted to shed light on the effect of stress history on primary creep. After creep, as-fabricated samples maintain their columnar grains, whereas hot isostatic pressed (HIPed) samples undergo dynamic recrystallization, resulting in coarser equiaxed grains containing annealing twins. Despite the difference in microstructure, as-fabricated and HIPed samples have comparable creep resistance with a stress exponent of 6 and 5, respectively. Within HIPed samples, the stress exponent increases from 5 to 8 as the temperature decreases from 850 to 750˚C due to the denser distribution of fine δ phases (rods with lengths of a few hundred nm) formed within grains at lower temperatures (800 and 750˚C). Regardless of the creep temperature, duration, and stress history (stress-jump/ -drop), a fully recrystallized grain structure is observed in all HIPed samples. In addition, the strain accumulated during primary creep is higher for the first loading than the later stress jumps/drops, indicative of dynamic recrystallization prolonging the primary creep stage. Samples undergoing stress-drop tests show a shorter primary creep regime with less strain accumulated. Lastly, Andrade law applies for the primary creep strain at later stress jumps/drops, with a stress-dependent prefactor.