Effect of a large population of seeded alumina inclusions on crack initiation and small crack fatigue crack growth in Udimet 720 nickel-base disk superalloy

•Crack initiation behavior of a large population of alumina inclusions was quantified.•Four inclusion crack initiation and crack growth regimes identified and characterized.•Majority of inclusions initiate very early even under long LCF life conditions.•Presence of “Iceberg” effect with apparent small inclusions leading to early failures.•Very shallow machining induced residual stresses have significant effect on LCF lives. Crack initiation, crack coalescence and small crack growth behavior were monitored for over 400 seeded inclusions during interrupted low cycle fatigue testing conducted on the P/M Udimet 720 nickel disk alloy at 650 °C. Two types of seeded alumina inclusions with average sizes of 54 µm and 122 µm were used in the study performed at varying loading conditions resulting in LCF lives ranging from 2,000 cycles to over 1,000,000 cycles. The fatigue behavior was sub-categorized into four groups. Visual maps detailing inclusion size/cycle history were developed. The effect of surface residual stresses on the fatigue life was also investigated.