The interactions between fatigue, creep and environmental damage in Ti 6246 and Udimet 720Li

The paper reports the results of a comprehensive research programme on two different compressor disc alloys: titanium alloy Ti 6246 and the nickel based superalloy Udimet 720Li. Both alloys are used for disc applications in gas turbine engines under conditions where the rims are exposed to fatigue, creep and environmental damage. The titanium alloy was investigated at temperatures of 80, 450, 500 and 550 °C, whilst the nickel at 650 and 700 °C. The paper presents the strain-life response of plain specimens and relates these data to notch fatigue behaviour. It also explores fatigue crack propagation behaviour in air and under hard vacuum (10−6 Torr), in order to characterise the creep/environment/fatigue interactions that occur at the crack tip. The growth data encompass R values of −3 to 0.5 for cyclic and dwell waveforms. The information obtained allows the effects of environment and creep at these temperatures to be partitioned. The observed R value dependence is related to both empirical models and closure measurements. The experimental data are supported by detailed metallographic and fractographic studies using optical and SEM techniques.