Correlating Grain Boundary Microstructure and Ductility Loss on Aging in Haynes® 25

Haynes® 25 is a cobalt-base superalloy known for its excellent high temperature strength and corrosion resistance. However, this alloy undergoes severe embrittlement on aging at elevated temperatures. Historical research associated this ductility loss with the formation of a Co₂W Laves phase, but more recent studies have challenged the existence of this phase after prolonged aging at temperatures above 600°C. In this study, scanning and transmission electron microscopy were used to characterize the chemistry and location of precipitates in the alloy after aging at temperatures of 675 and 850°C for times up to 12,000 hours. These data were compared with tensile data from the same heat of material in an attempt to develop quantitative relationships between microstructural observations and ductility loss. It was found that the change in fractional coverage of the grain boundaries by a M₆C carbide precipitate (composition W₃Co_1.5Cr_1.5C) on aging correlates well with ductility loss on aging at 850°C. The change in fractional coverage on aging at 675°C did not correlate as well, suggesting that the prevalent mechanism for ductility loss at this temperature may not be the precipitation reaction.