Diffusion braze homogenisation and contraction during re-repair heat treatments of a single crystal nickel-based superalloy

Highlights: • Eutectic borides form in 1 mm wide braze superalloy turbine components. • Re-repair heat treatment of these causes remelting leading to surface depressions. • The number of heat cycles results in equivalent number of boride bands in substrate. • Discrete borides in diffusion zone coarsen during subsequent heat treatments. • Three controlling factors are identified for boron removal from braze. -- Abstract: In this work, nickel-based braze reservoirs embedded in single crystal nickel-based superalloy substrates were cycled between one and four braze diffusion heat treatments to analyse the effect on microstructural transformation of the braze and substrate materials. Nickel-based superalloy turbine components can undergo several braze repair cycles during service life, and little has been reported on how previously repaired braze joints are affected in additional heat treatments. 1 mm wide braze joints with an initial composition of 6.8 at.% boron embedded in a single crystal substrate do not homogenise when brazed for 2.5 h above 1150 °C, resulting in the formation of eutectic boride phases. Subsequent heat treatments caused braze remelting and contraction leading to surface depressions with the potential to act as a stress raiser for crack nucleation, possibly requiring reworking which impacts on both profits and turnaround times. After four braze diffusion heat treatments, the braze had still not fully homogenised, even though four discrete bands of boride precipitates, separated by boride-free zones, could be observed in γ channels, indicating boron removal from the braze reservoir. Analysis of the metallurgy at the braze-substrate interface and the diffusion-affected zones showed that the most probable reason for controlling boron removal from the braze were: 1) decreasing boron concentration gradient across a thickening braze-substrate interface; 2) restricted boron transport across boride precipitates in the diffusion-affected zones; 3) slower transport of boron in γ', the area fraction of which changes over the braze cycle temperature range.