DAMAGE TOLERANCE OF WROUGHT ALLOY-718 NI-FE-BASE SUPERALLOY

The influence of a modified heat treatment (MHT) and the standard heat treatment (SHT) on the damage tolerance of alloy 718 turbine disk mat erial has been studied over a range of temperatures-from room temperat ure to 650 degrees C. The influence of these heat treatments on creep, low-cycle fatigue (LCF), notch sensitivity, cyclic stability, and fat igue crack growth rate (FCGR) properties has been studied, The microst ructure developed through the MHT sequence is shown to be damage toler ant over the temperature range studied, Shot peening leads to a marked improvement in the LCF crack initiation life of the MHT material rela tive to the SHT material at 650 degrees C. Serrated grain boundaries f ormed through controlled precipitation of grain-boundary delta phase a re beneficial to elevated-temperature FCGRs, The delta-phase precipita tes formed at an angle to the grain boundaries do not make the materia l notch sensitive.