Creep properties and deformation mechanisms of single-crystalline $\gamma^\prime$-strengthened superalloys in dependence of the Co/Ni ratio

TL;DR

This study investigates how varying the Co/Ni ratio in single-crystalline superalloys influences their creep properties and deformation mechanisms at high temperature, revealing that an intermediate CoNi ratio offers optimal creep strength.

Contribution

It provides new insights into the effect of Co/Ni ratio on creep behavior, microstructure, and deformation mechanisms in $ extgamma^ extprime$-strengthened superalloys, including quantification of strengthening contributions.

Findings

Creep rates vary by over an order of magnitude with Co/Ni ratio.

Intermediate CoNi alloys show the highest creep strength.

Different shearing mechanisms are observed depending on Ni or Co richness.

Abstract

Co-base superalloys are considered as promising high temperature materials besides the well-established Ni-base superalloys. However, Ni appears to be an indispensable alloying element also in Co-base superalloys. To address the influence of the base elements on the deformation behavior, high-temperature compressive creep experiments were performed on a single crystal alloy series that was designed to exhibit a varying Co/Ni ratio and a constant Al, W and Cr content. Creep tests were performed at 900 {\deg}C and 250 MPa and the resulting microstructures and defect configurations were characterized via electron microscopy. The minimum creep rates differ by more than one order of magnitude with changing Co/Ni ratio. An intermediate CoNi-base alloy exhibits the overall highest creep strength. Several strengthening contributions like solid solution strengthening of the $\gamma$ phase, effective diffusion coefficients or stacking fault energies were quantified. Precipitate shearing mechanisms differ significantly when the base element content is varied. While the Ni-rich superalloys exhibit SISF and SESF shearing, the Co-rich alloys develop extended APBs when the $\gamma^\prime$ phase is cut. This is mainly attributed to a difference in planar fault energies, caused by a changing segregation behavior. As result, it is assumed that the shearing resistivity and the occurring deformation mechanisms in the $\gamma^\prime$ phase are crucial for the creep properties of the investigated alloy series.