The effect of Ru on precipitation of topologically close packed phases in Re – containing Ni base superalloys: Quantitative FIB-SEM investigation and 3D image modeling

Abstract

A new approach to clarify the ruthenium effect on the precipitation of topologically close packed (TCP) phases is described in the paper. It is based on focused ion beam-scanning electron microscopy (FIB-SEM) dual beam methodology as well as three-dimensional imaging. The high-temperature capabilities of nickel base superalloys can be improved by alloying with refractory elements. With excessive refractory element content or excessive exposure to high temperature, brittle TCP phases precipitate resulting in a drop of strength. The undesirable phase transformation can be suppressed by addition of ruthenium. Although the effect is well known, its real mechanism remains open. In the present paper, the volume fraction and particle density, as well as the exact three-dimensional morphology of TCP phases as measured by FIB-SEM will be presented. The effect of ruthenium content and time of exposure is studied quantitatively. The results show that increased Ru additions slow down all stages of phase transformation and also reduce the equilibrium TCP volume fraction. The Ru effect might be due to either reduced driving force for precipitation or reduced interfacial energy.

Effect of ruthenium on the precipitation of topologically close packed phases in Ni-based superalloys of 3rd and 4th generation

“…The plotted lines represent the data fitting by use of the Johnson-Mehl-Avrami equation. For comparison, the data published earlier in [16] obtained at 1050°C are also plotted on the graph. Additionally, the equilibrium content of TCP phases calculated with ThermoCalc (version 3.1, ThermoCalc, Stockholm) using the commercially available databases TTNi8 phases are imaged as white particles of needle-or block-like morphology, being in fact plate-and lath-like particles, resp.…”

“…This leads to decreased creep resistance of the material [10,14]. Further, TCP phases are brittle and exhibit a complex lath-or plate-like morphology [15], which evolves with time [16]. Thus, cracks are easily nucleated.…”

“…Recently the present authors [16] used the Johnson-Mehl-Avr ami equation to describe the precipitation of TCP phases [23]. It was found that ruthenium influences both nucleation and growth rate of TCP phases.…”

“…This observation indicates that Ru affects the interface energy or the chemical driving force for TCP phase http formation. The TCP phases undergo a precipitation sequence [9,11,16] and thus it is most relevant to understand the Ru influence on the nucleation of the first TCP particles. Nevertheless, the mechanism of TCP phase growth also needs to be studied.…”

The Compositional Dependence of the Microstructure and Properties of CMSX-4 Superalloys

“…[44,45] Ru can effectively stabilize the microstructure by suppressing the precipitation of TCP phases. [34,46] We now attempt to predict the effect of the concentration of specific alloying elements on the microstructure and rafting kinetics, taking the base composition of CMSX-4 superalloy as the starting point. The change in alloy composition is obtained at the expense of varying Ni concentration.…”