An EBSD based investigation on the deformation mechanisms under HCF-creep interaction in a Ni-based superalloy (alloy 617M)

Abstract

The study presents an insight into the deformation mechanisms under high cycle fatigue-creep interaction (HCF-creep interaction) in a Ni-based superalloy (Alloy 617 M), using electron backscatter diffraction (EBSD) as an important tool. HCF-creep interaction tests are carried out at 700 °C by imposing periodic holds at mean stress (σm) during HCF cycling. Detailed EBSD investigation using kernel average misorientation (KAM) map indicates a non-uniform distribution of local misorientation especially at the grain boundaries, with increase in σm, which is attributed to the increased creep damage therein. Creep cavitation through grain boundary sliding is found to increase with an increase in the hold temperature to 750 °C owing to a higher dislocation mobility triggered by precipitate coarsening and dislocation climb. Change in twin boundary character from Σ3 to other higher angle boundaries caused by dislocation-twin interaction is noticed under HCF-creep interaction, leading to partial detwinning. A decrease in the fraction of Σ3 annealing twin boundaries through partial detwinning is observed, which is found to be more pronounced when the creep hold is maintained at 750 °C. This caused a loss of strength at the above temperature, through coalescence of secondary intergranular cracks, in turn leading to intergranular failure. In contrast, the hold imposed at 700 °C with similar loading parameters resulted in run out.