• Shingledecker, John P.
• Mohale, Ninad
• White, Calvin L.
• Sanders, Paul G.
• Milligan, Walter W.

Abstract

<p>Nickel-base superalloys that are in service for long periods of time in fossil energy powerplants have microstructures that evolve with time.The strengthening gamma prime phase coarsens, and in some alloys, a new phase (eta) forms at grain boundaries, and its effect on creep performance is not fully understood. In order to study the effects of eta phase on the creep of a typical nickel-base superalloy, Alloy 263 was aged for 1,000 hours at 1123 K.This resulted in precipitation of eta phase at and near the grain boundaries, reduction of the volume fraction of gamma prime, and increased gamma prime precipitate size.The Aged 263 was creep tested at 973, 1023 and 1073 K at various stresses, along with the Standard Alloy 263, and an experimental alloy that contains only eta phase and no gamma prime, with a chemical composition close to that of Alloy 263.</p><p> The creep performance of the Standard 263 was superior at all conditions, with the exception of creep ductility, which was substantially elevated in the Aged 263 and the eta alloy.The enhanced creep ductility in the aged microstructure is proposed to be related to the grain boundary regions being denuded in gamma prime, and also the presence of rich matrix dislocation sources in the eta/gamma interfaces. Deformation mechanisms were determined via transmission electron microscopy.The dominant mechanism changed from traditional precipitate shearing at 973 K to a combination of traditional and partial-dislocation shearing at 1023 K to climb-assisted precipitate by-pass at 1073 K.</p><p> Based on these mechanisms, an existing physically-based creep model was modified to predict steady-state creep rates in both the Standard and Aged 263.The presence of the eta phase was not addressed explicitly, but indirectly through the change in the gamma prime volume fraction and size.The model was very successful at 973 K, moderately successful at 1073 K, and a complete failure at 1023 K.</p><p> The reason for the failure at 1023 K was that the physically-based creep model for shearing is based on traditional precipitate shearing, and so the onset of partial dislocation-based shearing and the accompanying stacking faults would require a completely new model formulation.</p>

Topics

• impedance spectroscopy
• grain
• nickel
• phase
• grain boundary
• chemical composition
• positron annihilation lifetime spectroscopy
• Photoacoustic spectroscopy
• transmission electron microscopy
• dislocation
• precipitate
• precipitation
• deformation mechanism
• ductility
• creep
• superalloy
• stacking fault
• phase evolution