Influence of secondary precipitates and crystallographic orientation on the strength of single crystals of a Ni-based superalloy

Abstract

The effect of crystallographic orientation and aging heat treatment at 850 °C on the creep rupture strength of single crystals of a nickel-based superalloy was examined at 700 °C in detail. Initial tensile orientations were selected over a wide range on the standard stereographic triangle. The {111}〈112〉-type slip systems were found to be operative during the creep deformation. The creep behavior was found to be greatly influenced by the additional aging at 850 °C for 20 hours. It was found that the effect of the aging at 850 °C was quite different between orientations favored for the $$(\bar 111)[1\bar 12]$$ slip system and those favored for the (111) $$[\bar 211]$$ slip system and that the creep deformation mechanisms of these two slip systems were different. In the orientations favored for $$(\bar 111)[1\bar 12]$$ slip systems, in the single-aged specimens, a small mean surface-to-surface spacing due to hyperfine γ′ precipitates in the matrix channel promoted the $$(\bar 111)[1\bar 12]$$ slip and the primary creep. As a result of the additional aging at 850 °C, the hyperfine γ′ precipitates were dissolved into the matrix, and the resultant large mean surface-to-surface spacing between the cuboidal precipitates inhibited extensive shearing of the γ-γ′ structure by the $$(\bar 111)[1\bar 12]$$ slip system. As a result, the creep strengths of these orientations were increased in double-aged specimens; however, the low ductility associated with the difficulty of secondary noncoplanar slip did not enlarge rupture lifetime in the double-aged [001] specimen. In the orientations favored for the (111) $$[\bar 211]$$ slip system, creep deformation occurred by twinning shear through γ and γ′ precipitates, and a distinct effect of the aging at 850 °C was not observed. In the multiple orientation of the {111} $$\left\langle {\bar 211} \right\rangle $$ -type slip systems, i.e., the $$[\bar 112]$$ and $$[\bar 111]$$ orientations, hyperfine precipitates improved creep strength because they prevented dislocations from gliding in the matrix channel in the single-aged specimens.