Fatigue crack propagation behaviors of new developed Allvac® 718Plus™ superalloy

In this paper, the mechanical properties, especially fatigue crack propagation behaviors of a newly developed Ni-base superalloy Allvac((R)) 718Plus (TM) Alloy were investigated, as compared with those of Alloy 718 and Waspaloy. It is indicated that 718Plus (TM) alloy shows better performance on tensile, creep and fatigue crack growth tests than Alloy 718 at room temperature, 650 degrees C and 704 degrees C. In the final section of the paper, the mechanism of fatigue crack propagation behavior of superalloys and hold-time effect are discussed. It is clear that yield and tensile strengths of 718Plus (TM) alloy are higher than those of Alloy 718 and Waspaloy at temperatures up to 704 degrees C. Stress rupture life of 718PIus alloy is about the same as Waspaloy and higher than that of Alloy 718 at 704 degrees C. Regarding fatigue crack propagation properties, alloys 718Plus (TM), 718 and Waspaloy have similar fatigue crack growth rates under 3 seconds triangle loading at 650 degrees C. In comparison, 718PIus (TM) shows the somewhat better resistance to fatigue crack growth without hold-time at 650 degrees C than Alloy 718 and Waspaloy. As a matter of fact, the fatigue crack growth rate of 718Plus (TM) at 704 degrees C is slower than that of Alloy 718 and Waspaloy at 650 degrees C. During the fatigue crack growth under the trapezoid loading with 100 seconds hold-time at maximum stress, 718PLUS (TM) alloy shows the comparable resistance to hold-time FCG as that of Waspaloy alloy, while both alloys have better resistance than that of Alloy 718. Examination of the fatigue fracture surfaces by scanning electronic microscope (SEM) revealed transgranular crack propagation with striations for 718Plus (TM) at room temperature. There is a clear border between the fracture surface of the room temperature pre-crack and 0.33Hz fatigue at 650 degrees C. The fracture mode at 650 degrees C is the mixture of intergranular and transgranular modes. When the test temperature is up to 704 degrees C, intergranular is the predominant mode and the whole surface covered by a layer of oxide film, indicating that severe oxidation happens during the crack growth. The crack path of 718Plus (TM) under 3+100 seconds' hold-time conditions at 650 degrees C and 704 degrees C are predominantly intergranular, with a little transgranular cleavage fraction. The fracture was rough and covered by lots of oxide products. These oxides are believed to be from oxidation asperities of 6-phase precipitates. Fatigue crack growth of most superalloys is predominantly a cycle-dependent damage process with little frequency effect at very high frequencies. At relatively low frequencies or the test with hold-time, crack growth includes time dependent processes, which were generally ascribed to phenomena involving creep and/or environmental degradation processes. The combined mechanical and SEM fractographical analysis show the time-dependent FCG of 718Plus (TM) is rather an environmental effect than creep effect.