In-situ loading neutron-diffraction studies of a cobalt-based superalloy

ULTIMET^® alloy is a cobalt-based superalloy that undergoes a deformation-induced phase transformation from a face-centered-cubic (fcc) phase to a hexagonal-close-packed (hcp) phase. The transformation was studied during monotonic loading, stress-controlled high-cycle fatigue (HCF) and strain-controlled low-cycle fatigue (LCF). The HCF was performed at a maximum stress of σ_max=840 MPa and a minimum stress of σ_min=84 MPa, while the tensile experiment was terminated after reaching σ_max=890 MPa. The LCF was performed at a maximum strain of σ_max=1.25% and a minimum strain of σ_min=-1.25%. The monotonic-loading experiment results reveal that the hcp phase forms after reaching the yield stress. During HCF, the hexagonal phase forms immediately upon the first fatigue cycle, with no further change in structure upon subsequent deformation. In contrast to HCF, the hcp phase forms gradually during LCF. In fact, fatigue cycle 12 is reached before the hcp phase is resolved by neutron diffraction. Diffraction patterns from subsequent fatigue cycles reveal that the hcp phase continues to accumulate as LCF progresses. These observations can be related to the input of mechanical energy during different loading modes.