FATIGUE CRACK PROPAGATION OF Ni-BASE SUPERAIJ..OYS

Time-deperu1ent Fatigue Crack Propagation (FCP) behaviors offive Ni-base superalloys were investigated at various temperatures under fatigue with various holding times and sustained loading conditions. The new concept of damage zone is defined and employed to evaluate the alloys' resistance to hold-time FCP. A special testing procedure is designed to get the maximum damage zone of the alloys. Udimet 720 and Waspaloy show shorter damage zones than alloys 706 and 718. The Jractographical analyses show that the fracture surfaces of the specimens under hold-time fatigue conditions are mixtures with intergranuJar and transgranular modes. As the extension of holding time per cycle, the portion of intergranuJar fracture increases. The effects of loading stress intensity, temperature, holding time. alloy chemistry, and alloy microstructure on damage zone and the crack growth behaviors are studied. Hold-time usually increases the alloy's FCP rate. but there are few exemptions. For instance, the steady state hold-time FCP rate of Waspaioy at 76frC is lower than that without hold-time. The beneficial effect of hold-time was attributed to the creep caused stress relaxation during the hold-time.