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

Creep crack growth rates originally gathered for the gamma' 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 degrees C. However, whereas (a) over dot(C)(K-a) rates at 750 degrees C are significantly lower than those at 850 degrees C, equivalent (a) over dot(C) (C*) rates are relatively insensitive to temperature, comparing well with the plane strain (a) over dot(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 degrees C, this appears not to be possible at 750 degrees 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 degrees C (from 750 degrees C), or at 750 degrees C (from 850 degrees C) on (a) over dot(C)(K-a) creep crack growth rates and t(R)(sigma(ref)) overall lives.