Segregation and Phase Transformations Along Superlattice Intrinsic Stacking Faults in Ni-Based Superalloys
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In this study, local chemical and structural changes along superlattice intrinsic stacking faults combine to represent an atomic-scale phase transformation. In order to elicit stacking fault shear, creep tests of two different single crystal Ni-based superalloys, ME501 and CMSX-4, were performed near 750 °C using stresses of 552 and 750 MPa, respectively. Through high-resolution scanning transmission electron microscopy (STEM) and state-of-the-art energy dispersive X-ray spectroscopy, ordered compositional changes were measured along SISFs in both alloys. For both instances, the elemental segregation and local crystal structure present along the SISFs are consistent with a nanoscale γ′ to D019 phase transformation. Other notable observations are prominent γ-rich Cottrell atmospheres and new evidence of more complex reordering processes responsible for the formation of these faults. These findings are further supported using density functional theory calculations and high-angle annular dark-field (HAADF)-STEM image simulations.
Funding for this study was provided by NASA’s Aeronautics Research Mission Directorate (ARMD) – Convergent Aeronautics Solutions Project and NASA’s Advanced Air Transport Technology (AATT) Project Office (ARMD). Further funding was provided by the National Science Foundation and the DMREF program under Grant #1534826.
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