Fatigue Crack Propagation of Nickel-Base Superalloys at 650°C

The 650°C fatigue crack propagation behavior of two nickel-base superalloys, René 95 and Waspaloy, were studied with particular emphasis placed on understanding the roles of creep, environment, and two key grain boundary alloying additions, boron and zirconium. Comparison of air and vacuum data showed the air environment to be detrimental over a wide range of frequencies for both alloys. In-depth analysis of René 95 showed that at lower frequencies, such as 0.02 Hz, failure in air occurred by intergranular, environmentally assisted creep crack growth, while at higher frequencies, up to 5.0 Hz, environmental interactions were still evident but creep effects were minimized. The effect of boron and zirconium in Waspaloy was found to be important where environmental and/or creep interactions were present. In those instances, removal of boron and zirconium dramatically increased crack growth. It is therefore plausible that effective dilution of these elements may explain, in part, a previously observed trend in which crack growth rates increased with decreasing grain size.