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Minor additions of various grain boundary strengthening elements to single crystal Ni-base superalloys have been shown to improve castability, but their effects on mechanical properties are not well understood. A comparative investigation of baseline CMSX-4 and variations modified with carbon, carbon and boron, and carbon and nitrogen has been performed. These modifications have been shown to form interdendritic MC-type carbides with different morphologies. Heat treatments resulted in the breakdown of some carbide structures into small, spherical carbide particles of similar composition to the as-cast features. Tensile and high cycle fatigue (HCF) testing was conducted at 850 o C. The three carbon-containing modifications showed lower tensile strength than the baseline due to cracking of carbides and greater porosity. Fatigue crack initiations for Un-HIPed specimens occurred at pores that were adjacent to carbides in the modifications, which in general had shorter lifetimes than the baseline. HIPing dramatically increased fatigue lifetimes for all variations. Removal of porosity resulted in crack initiations at carbides and highlighted differences in fatigue behavior for varying carbide morphologies. Script carbide networks observed in the carbon only modification exhibited significant cracking that initiated failure, limiting the fatigue lifetime. Blocky carbides in the carbon and nitrogen modification remained intact, which resulted in longer fatigue lifetimes. The intact carbides did experience de-cohesion from the γ/γ' matrix that showed signs of plastic deformation in the vicinity of carbides.

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