Improving the weldability and service performance of nickel- and iron-based superalloys by grain boundary engineering

The principal limitation of today's Ni- and Fe-based superalloys continues to be their susceptibility to intergranular degradation arising from creep, hot corrosion, and fatigue. Many precipitation-strengthened superalloys ar e also difficult to weld, owing to the formation of heat-affected zone (HAZ ) cracks during postweld heat treatments (PWHTs). The present work highligh ts significant improvements in high-temperature intergranular degradation s usceptibility and weldability arising from increasing the relative proporti on of crystallographically ''special'' low-Sigma CSL grain boundaries in th e microstructure. Susceptibility to intergranular degradation phenomena is reduced by between 30 and 90 pet and is accompanied by decreases in the ext ent and length of PWHT cracking of up to 50-fold, with virtually no comprom ise in mechanical (tensile) properties upon which the functionality of thes e specialty materials depends. Collectively, the data presented suggest tha t "engineering" the crystallographic structure of grain boundaries offers t he possibility to extend superalloy lifetimes and reliability, while minimi zing the need for specialized welding techniques which can negatively impac t manufacturing costs and throughput.