Effect of Squareness of Initial γ ′ Precipitates on Creep-Rupture Life of a Ni-Base Single Crystal Superalloy at 760/982 °C

An approach to determination of squareness of initial γ ′ precipitates ( S 2D ) is proposed to evaluate its effect on creep-rupture life ( t r ) of nickel-base single crystal (SC) superalloys. It is found that the 760/982 °C rupture life varied with the change in regional S 2D caused by redistribution of W when 1st-step aging temperature changed in full heat treatment on superalloy DD83 investigated. The longest creep-rupture life occurred at the highest value/the lowest difference in S 2D in the interdendritic regions/between the typical dendritic regions in DD83. It is also found that S 2D is a weighted function of the area fraction ( F 2D ), spacing ( h ), and size ( d ) of γ ′ precipitates and is closely related to t r in a series of SC superalloys. In addition, the variation of S 2D with F 2D (here, thermodynamic mole fraction is approximately expressed by F 2D ) through lattice misfit ( δ ) in the SC superalloys with F 2D ranging from ~ 60 to 75 pct is well correlated. Therefore, to reveal and to better understand these relationships and correlations may help to optimize the phase variables in order to achieve a long rupture life of SC superalloys. In addition, functions to reveal the interrelationships of F 2D , volume fraction ( F 3D ), S 2D , and cuboidness ( S 3D ) of initial γ ′ precipitates are derived considering their shape changes. All of these are hoped to be helpful in practical applications and in understanding the true meaning of the related variables.