Microstructure evolution of alloy 625 foil and sheet during creep at 750 °C

Foil and sheet forms of the nickel-based superalloy 625 have been examined in an 'as-processed' condition and following creep-rupture testing in air at 750 °C and 100 MPa. Both scanning and transmission electron microscopies were employed to correlate microstructures with creep behavior, and indicate the additional processing required to achieve foil form reduces creep life compared to thicker-section wrought product forms. Prior to creep testing, the microstructure consists of γ phase with M₆C precipitates. This microstructure changes during creep into one consisting of orthorhombic δ phase extending across the γ grains, and grain boundaries dominated by the presence of rhombohedral μ phase, δ phase, and a Si-rich variant of diamond-cubic M₆C (η phase). Thermodynamic modeling was also used to calculate the stable temperature ranges and compositions of equilibrium phases. The phases predicted by modeling and their compositions generally agree with those observed within alloy 625 after creep testing.