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

Abstract

An approach to determination of squareness of initial γ′ precipitates (S2D) is proposed to evaluate its effect on creep-rupture life (tr) of nickel-base single crystal (SC) superalloys. It is found that the 760/982 °C rupture life varied with the change in regional S2D 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 S2D in the interdendritic regions/between the typical dendritic regions in DD83. It is also found that S2D is a weighted function of the area fraction (F2D), spacing (h), and size (d) of γ′ precipitates and is closely related to tr in a series of SC superalloys. In addition, the variation of S2D with F2D (here, thermodynamic mole fraction is approximately expressed by F2D) through lattice misfit (δ) in the SC superalloys with F2D 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 F2D, volume fraction (F3D), S2D, and cuboidness (S3D) 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.