Factors influencing the analytical representation of creep crack development in Alloy 939

Creep crack growth rates originally gathered for the 𝛾′ strengthened cast nickel based superalloy Alloy-939 in terms of apparent stress intensity factor are re-evaluated as a function of the time dependent C* parameter, with crack propagation rates being insensitive to grain size at 750 and 850 °C. However, whereas a˙C(Ka) rates at 750 °C are significantly lower than those at 850 °C, equivalent a˙C(C*) rates are relatively insensitive to temperature, comparing well with the plane strain a˙C(C*) rates predicted using an approximate reference stress based C* model. While overall CT-specimen creep crack growth times for Alloy-939 can be predicted using uniaxial creep-rupture strength data when expressed in terms of reference stress at 850 °C, this appears not to be possible at 750 °C. The observation is explained with respect to evidence provided by a modified time dependent failure assessment diagram. The influence is examined of thermal transient durations at 850 °C (from 750 °C), or at 750 °C (from 850 °C) on a˙C(Ka) creep crack growth rates and tR(𝜎ref) overall lives.