High frequency fatigue crack propagation behavior of a nickel-base turbine disk alloy

Fatigue crack propagation tests were conducted on the powder metallurgy nickel-base superalloy KM4 at room temperature. Two different heat treatments were investigated, one which produced a relatively coarse grain size around 55 μm, and another which produced a very fine grain size around 6 μm. Tests were conducted at 50 Hz and 1000 Hz in an advanced servohydraulic testing machine at R-ratios between 0.4 and 0.7. There was no effect of frequency on the fatigue behavior at room temperature, which is expected in this type of alloy, and this result yields confidence in the reliability of the servohydraulic fatigue testing system. The threshold stress intensity for fatigue crack propagation decreased with decreasing grain size and with increasing R-ratio, again as expected. With increasing grain size, the crack path tortuosity and the crystallographic facet size on the fracture surface both increased substantially, leading to increases in roughness-induced closure and a higher apparent threshold. The threshold ΔK values measured at 10^-10 m/cycle corresponded to essentially infinite lifetimes, as very small decreases in ΔK from the threshold values resulted in complete crack arrest, and led to difficulty in restarting the crack growth at higher ΔK levels. Finally, comparisons of the observed thresholds with existing models revealed significant discrepancies between the predicted and measured values.