Fatigue and creep of single crystal blade alloys

The fatigue behaviour of CMSX4 has been investigated at high and low frequencies under both load and strain control. Low frequency fatigue tests (0.25 Hz) show a decrease in life with temperature whereas high frequency tests (100 Hz) show an increase in life with temperature. These trends are similar to the well documented effect of temperature and strain rate on the yield strength of Ni-superalloys. The yield stress, taken as a function of temperature and strain rate can be used to normalise fatigue data at different temperatures and frequencies. Testing at higher temperatures or lower frequencies results in planar crack growth whilst at lower temperatures or higher frequencies crack growth is crystallographic. In both situations life is controlled by initiation. Initiation of a planar crack requires time and temperature and occurs when creep damage is present. The difficulty in initiating a crystallographic fatigue crack is thought to be related to the yield strength of the material at the relevant testing conditions.