The Influence of Inclusions on Low Cycle Fatigue Life in a P/M Nickel-Base Disk Superalloy

The high alloy content of advanced nickel-base disk superalloys calls for powder metallurgy (P/M) processing to minimize segregation produced by conventional cast & wrought processing. Although the technology has developed to allow reliable application of P/M materials, of concern are ceramic inclusions which are intrinsic to the process due to the use of ceramic crucibles in producing the raw meltstock and in the powder atomization process itself. For robust disk design, the impact of ceramics on Low Cycle Fatigue (LCF) must be assessed. Actual production material can be evaluated but the low frequency of larger inclusions means that impracticably large volumes of material must be tested. To address this problem, tests were run on powder seeded with controlled distributions of ceramics. The alloy was the current GEAE disk material Rene’ 88 DT (R’X8DT). Two types of ceramic seeds were added at two size/density combinations. Small seeds (-270/+325 mesh) were added to -270 mesh powder at rates (numbers per unit volume) predicted via probabilistic calculations to cause surface initiations. Large seeds (-SO/+100 mesh) were added to the powder at rates predicted to cause internal initiations. The powder was consolidated and processed using production parameters. LCF tests were made on samples taken from fully heat treated forgings at 204’ C and 649” C at two stress levels. Roughly half of the bars were shotpeened to study the ability of this processing to suppress surface initiations. The impact of the seeds was significant and was a function of seed type and size, temperature, and bar surface condition. At 204” C life decreased with increasing seed size up to a maximum life reduction of 33-50%. Shotpeening only slightly improved the lives of small seeded bars. At 649” C, the seeds had dramatic impacts. Small seeds reduced life by 1-2 orders of magnitude and large seeds by a further l-2 orders of magnitude. Shotpeening did suppress surface initiations and significantly improved life. The relative impacts between 204” C and 649” C and the impact of shotpeening depended on failure mechanisms -- at 204” C most of the failures initiated at facets, explaining a relatively minor seeding impact at this temperature while at 649” C almost all failures initiated at the seeds (or at intrinsic inclusions in the unseeded material).