| High-cycle fatigue, involving the premature initiation and/or rapid propagation of small cracks to failure due to high-frequency (vibratory) loading, remains the principal cause of failures in military gas-turbine propulsion systems. The objective of this study is to examine whether the resistance to high-cycle fatigue failures can be enhanced by grain-boundary engineering, i.e., through the modification of the spatial distribution and topology of the grain boundaries in the microstructure. While grain boundary engineering has been used to obtain significant improvements in intergranular corrosion and cracking, creep and cavitation behavior, toughness and plasticity, cold-work embrittlement, and weldability, only very limited, but positive, results exist for fatigue. Accordingly, using a commercial polycrystalline nickel base gamma/gamma' superalloy, ME3, as a typical engine disk material, sequential thermomechanical processing, involving alternate cycles of strain and annealing, is used to (i) modify the proportion of special grain boundaries, and (ii) interrupt the connectivity of the random boundaries in the grain boundary network.; The processed microstructures are then subjected to fracture-mechanics based high cycle fatigue testing to evaluate how the crack initiation and small- and large-crack growth properties are affected and to examine how the altered grain boundary population and connectivity can influence growth rates and overall lifetimes.; The effect of such grain-boundary engineering on the fatigue-crack-propagation behavior of large (∼8 to 20 mm), through-thickness cracks at 25, 700, and 800°C was examined. Although there was little influence of an increased special boundary fraction at ambient temperatures, the resistance to near-threshold crack growth was definitively improved at elevated temperatures, with fatigue threshold-stress intensities some 10 to 20% higher than at 25°C, concomitant with a lower proportion (∼20%) of intergranular cracking. This work demonstrated that for cracks large compared to the scale of the microstructure, the principal role of an increased fraction of "special" grain boundaries is to enhance resistance only to intergranular cracking.; Microstructurally small fatigue cracks exhibit considerably scattered growth rates at ambient temperatures and there is little discernible overall effect of an increased fraction of special boundaries on the growth rates of small cracks due to scattering. Crystallographic cracking shows deflection at grain boundaries, preferably along {lcub}111{rcub}. The analysis on the crack growth perturbation and crack deflection indicates that grain boundaries with higher misorientation angles, particularly twin boundaries (Sigma3), may be more effective in locally retarding small crack propagation. |
