Microstructure Evolution of Alloy 625 Foil and Sheet During Creep at 750oC

Abstract

Creep-rupture tests in air of foils and sheets of the nickel-based superalloy 625 at 750oC and 100 MPa have been conducted, and indicate the additional processing required to achieve foil form reduces creep life compared to thicker-section wrought product forms. Both scanning and transmission electron microscopy were employed to examine as-processed and creep-tested specimens to correlate observed microstructures and creep behavior. Prior to creep testing, the morphology consists of gamma phase with M6C precipitates. This morphology changes during creep to one consisting of orthorhombic delta phase extending across gamma grains, and grain boundaries dominated by the presence of rhombohedral mu phase, delta phase, and a diamond-cubic eta phase. Additionally, temperature ranges of equilibrium phase field stability were calculated using JMatPro. The phases predicted and their compositions generally agree with those observed within the superalloy after creep testing.