A Study on the Role of Grain-Boundary Engineering in Promoting High-Cycle Fatigue Resistance and Improving Reliability in Metallic Alloys for Propulsion Systems

Abstract : 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 Ni-base y/y' superalloy, Rene 104 (also referred to as ME3), as a typical engine disk material, sequential thermomechanical (cyclic strain and annealing) processing 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 high-cycle fatigue testing, first to assess the crack-propagation properties of long and small cracks to examine how the altered grain-boundary population and connectivity can influence growth rates and overall lifetimes.