Abstract
Effects of grain size, γ' particle size and grain boundary configuration on high-cycle fatigue properties of a Ni-base superalloy Inconel 751 were studied over a wide range of temperature up to 900°C.
Below 750°C, fatigue life increased as grain size decreased because of the slower propagation rate of fatigue cracks which generated along coarse slip bands. When the temperature raised up to 900°C, the intergranular cracking became predominant with decreasing grain size since dislocations could be readily released from pile-ups by recovery and intergranular oxidation might be accelerated, while transgranular cracking still remained in coarse grained specimens. The propagation rate of intergranular cracks was so much higher as compared with the transgranular ones that the maximum fatigue life at 900°C was attained at the specimen of medium grain size in which the mixed trans- and inter-granular cracking occurred. Zigzag grain boundaries with coarse carbides had not a beneficial effect on the fatigue strength and fracture mode at 900°C even when the intergranular cracking took place. It was also found that effects of γ' particle size and temperature on fatigue strength were related not only to changes in dislocation-particle interaction with particle size, but also to temperature dependence of γ' strength of particle itself.
