Investigation of non-classical creep behavior of Inconel 617 alloy at 700 degrees C and 800 degrees C through interrupted tests and microstructural characterizations

Abstract

The 'non-classical creep behavior' of the Inconel 617 alloy consisting of the miniscule primary and secondary creep regimes followed by a prolonged tertiary creep regime, has been investigated through interrupted tests at 700 degrees C/275 MPa and 800 degrees C/95 MPa, having nearly similar time to rupture (-3500 h) to understand the role of microstructural evolution. A steady state tertiary creep region present at 800 degrees C/95 MPa, is found to be absent at 700 degrees C/275 MPa. The microstructural changes including formation of gamma' and fine secondary carbide precipitates, and twin boundary generation observed on interrupting the creep tests at both the aforementioned conditions after varying durations, have been investigated using optical, scanning and transmission electron microscopy, electron back scattered diffraction (EBSD) and thermal analysis. The formation of ample fine gamma' precipitates along with the fine secondary carbides in the samples subjected to creep tests at 700 degrees C with interruptions after varying fractions of time to rupture lead to increase in hardness, which is found to be lower for similar tests at 800 degrees C due to the absence of gamma' precipitates. Moreover, the EBSD analysis has confirmed a higher length fraction ratio of twin boundary (TB) to high angle grain boundary (HAGB) in the samples exposed to the steady state tertiary creep regime (almost 95% of total rupture time) at 800 degrees C. The increased TB to HAGB length fraction ratio is responsible for improving the creep resistance by delaying the damage accumulation, as the TBs are typical coincident site lattice boundaries with lower energy and greater packing density.