Controlling Inter- and Intra-granular γ′ Through Heat Treatment

Wrought Ni-based superalloys with low-processing cost and temperature capabilities beyond Alloy 718 are of significant interest for disk rotors in gas turbine engines. For these applications, damage tolerance and crack growth resistance are critical design priorities, particularly under conditions that produce time-dependent deformation from dwell fatigue cycles with sustained peak stresses. It is understood that dwell crack growth rates can be reduced by increasing grain size and by coarsening grain boundary γ′ to produce serrated grain boundaries. A heat treatment involving interrupted furnace cooling following solution heat treatment and secondary aging has been developed for a new potential low-processing cost alloy. This produced a fine grain size and coarse grain boundary γ′ whilst maintaining fine intragranular precipitates. Limited serration of grain boundaries was also achieved. The ultimate tensile strength of the alloy in this heat treatment condition exceeded that of Alloy 718 and approached that of Alloy 720 at 650 °C and 700 °C, although the yield strength was similar to, or below that of Alloy 718. Fatigue crack growth from 5 Hz cycles at 650 °C was slow and largely transgranular, however, two tests with 120 seconds dwell cycles at 650 °C both led to fast, intergranular crack growth, although there was a factor of 10 difference between the measured crack growth rates. The faster dwell fatigue crack rate was comparable to previously reported data for alloy ATI 718Plus. Both alloys have similarly high Nb contents, which likely contributed to the high rates of environmentally assisted cracking. Additionally, the significant spread of grain sizes observed could have allowed the crack to largely propagate along very fine grains, further accelerating crack growth and increasing the spread in the measured dwell fatigue crack growth resistance of the alloy.