Influence of grain size and heat treatment on the microstructure and mechanical properties of the nickel-base superalloy U 720 LI

The purpose of the current investigation was to develop an optimized balance between creep resistance and LCF-strength of U 720 LI, a high-strength nickel-base superalloy. While a fine grain size is beneficial for improved LCF-strength, it is detrimental to high-temperature creep resistance. Therefore, microstructures with medium grain sizes in the range of 50–150 μm were produced by appropriate recrystallization treatments. The coarse grain size showed improved creep resistance and lower yield stress as well as a tendency for lower LCF-strength. By increasing the amount of fine γ′-precipitates by modified aging treatments, it was possible to increase the yield stress considerably with a concomitant increase in creep strength.