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The tensile ductility of three ,y'-strengthened, Ni-base superalloys is found to be reduced by high-temperature exposure. The cause of tensile embrittlement has been investigated by characterizing the effects of exposure temperature, time and atmosphere (air vs. vacuum), testing atmosphere (air vs. vacuum), strain rate and protective coatings. Results are presented to show that tensile embrittlement is environment-related, caused by interactions of nitrogen and probably also oxygen with the grain-boundary ' network which forms upon exposure as a result of the MC +J' -'MzSC6 + "v' reaction. Two modes of interactions are identified. Static interactions result from contamination of the '$f network by inward diffusion of the interstitials, associated with an apparent activation energy of 67 kcal/mole. Dynamic interactions occur instantaneously during testing in air when the i' network is exposed by surface cracking. The dissolution and/or adsorption of the interstitials is believed to lower the surface energy and fracture strength of the i" phase. In the absence of environmental interactions, the intrinsic ductility of all three alloys is shown conclusively to be enhanced, rather than impaired, by high-temperature exposure. The effects of strain rate and protective coatings are studied. High strain rates aggravate tensile embrittlement by altering the sites of crack initiation and propagation. The efficacy of three types of coating in mitigating environmental interactions is illustrated and discussed in terms of coating composition, microstructure, thickness, and ductility,

Tensile Embrittlement of Turbine Blade Alloys After High-Temperature Exposure