Suppression of Grain Boundary Nitrogen Degradation by Boron in Superalloys
Superalloys enable the hottest sections of jet engines to operate at high temperatures, but environmentally assisted degradation limits their lifetime. In polycrystalline alloys, nitrogen penetrates beneath the protective surface oxides, to produce embrittling grain boundary nitrides. Boron is added in trace amounts to improve grain boundary strength, yet its role, if any, in oxidation-related degradation remains unclear. Here we show that boron segregates to an internal nitride--matrix interface in a Ni/Co-base $γ$/$γ^{\prime}$ strengthened superalloy. Both B-free and B-containing alloys develop stratified chromia/alumina scales at 800~$^{\circ}$C in \SI{100}{\hour}, but the B-free variant produces larger, deeper and more elongated Ti/Nb-rich subsurface grain boundary nitrides. Correlative 4D S/TEM and atom probe tomography identify the particle as a chemically complex B1 nitride and reveal interfacial boron segregation. This shows that trace alloying as an alloy strategy can control damaging internal reaction product precipitation and growth by core-shell segregation.