Influence of precipitate morphology on intermediate temperature creep properties of a nickel-base superalloy single crystal

The influence of γ’ precipitate morphology on the-creep behavior of the single crystal nickel-base superalloy NASAIR 100 at 760-°C was investigtated. As-heat treated crystals with cuboidal γ’ particles and crystals given an additional pre-rafting treatment to form a continuous lamellar structure were creep tested at stress levels which produced rupture lives ranging from 40 to 2500 hours. At high applied stresses, the crystals with cuboidal γ’ had both lower minimum creep rates and longer rupture lives than the crystals with lamellar γ. At lower stress levels, the initially cubic γ’ material maintained a lower crep rate, but exhibited a similar rupture life compared to the pre-rafted crystals. Examination of the microstructures which developed during creep indicated that dislocations could shear the semi-coherent γ’ rafts relatively easily compared to the coherent cuboidal γ’. In tests at lower applied stresses, slow directional coarsening of the initially cuboidal γ’ resulted in the development of a lamellar structure similar to that in the pre-rafted material, such that the rupture lives of the two materials were similar.