A novel insight into the primary creep regeneration behaviour of a polycrystalline material at high-temperature using in-situ neutron diffraction

Primary creep regeneration (PCR) is observed during cyclic creep deformation in many engineering alloys. PCR is a phenomenon in which reverse inelastic strain fully, or partially, resets the early creep strain hardening memory of the material. The current understanding regarding the origin of the PCR behaviour in engineering alloys is limited to the phenomenological observations from the changes in dislocation structures whereas a good mechanistic understanding of the PCR behaviour is crucial for developing robust plasticity-creep predictive models. In this study, we investigated the micromechanical origin of PCR behaviour in type 316H stainless steel at 650 °C using high-temperature mechanical testing and neutron diffraction. A cyclic creep experiment was conducted in-situ at a neutron diffraction beamline, during which various degrees of unloading and reverse loading were applied to the specimen, followed by creep deformation under a load above the material's yield strength. Partial PCR was observed after reverse plastic loading for all the creep dwells, which is contrary to current high-temperature lifetime assessment's procedures advice which is to account for full recovery of primary creep after any reverse plastic loading. The extent of PCR is observed to be proportional to the magnitude of reverse plastic strain up to a level of 0.5% reverse plastic strain. From the measured neutron diffraction data, a strong correlation was observed between the changes in magnitude of the accumulated intergranular residual lattice strains and the macroscopic primary creep strain. Moreover, the increases of lattice strain to saturation and transition from primary creep to secondary regime occur at the same time. Based on these correlations it can be postulated that the PCR behaviour observed due to cyclic loading in type 316H stainless steel at elevated temperature originates from the accumulation of intergranular residual lattice strains during the reverse plastic loading and those time-dependent changes during the creep dwells.