Strain-induced phase transformation in a cobalt-based superalloy during different loading modes

The strain-induced face-centered cubic (FCC) → hexagonal-close packed (HCP) phase transformation in a cobalt-based superalloy was investigated with four in situ loading neutron-diffraction experiments: monotonic tension, monotonic compression, high-cycle fatigue, and low-cycle fatigue. The transformation onsets for the four respective cases were 685 MPa, 698 MPa, 1 cycle, and 3 cycles, respectively. The HCP phase accumulated at rates of 0.1 wt.%-MPa−1 and 0.05 wt.%-MPa−1 for the tension and compression cases, respectively. For the cyclic-loading cases, the accumulation rates were found to be inversely proportional to the number of fatigue cycles. The results under the different loading modes suggest that the phase transformation occurs according to a tensile plastic-work criterion.