Experimental determination of the Ni–Cr–Ru phase diagram and thermodynamic reassessments of the Cr–Ru and Ni–Cr–Ru systems

•The Ni-Cr-Ru phase diagram is essential to understand the Ru effect on superalloys.•Diffusion multiples are employed to obtain four Ni-Cr-Ru isothermal sections.•A new Cr-Ru binary phase diagram is suggested based on new experimental data.•CALPHAD assessments are performed to obtain the thermodynamic parameters.

Ruthenium addition was found to be beneficial in inhibiting the formation of the detrimental topologically closed-packed (TCP) phases in Ni-based single crystal superalloys; thus, the Ni–Cr–Ru phase stability is very valuable for better understanding of the constitutional effects of Ru in multicomponent Ni-base superalloys. Four isothermal sections of the Ni–Cr–Ru system at 800 °C, 900 °C, 1000 °C and 1200 °C were constructed from results obtained from diffusion multiples using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The measured phase equilibria together with prior experimental results were used to optimize a CALPHAD thermodynamic description of the Ni–Cr–Ru ternary system using thermodynamic parameters of the Ni–Cr and Ni–Ru systems published in the literature and modified Cr–Ru parameters from the current study. The intermetallic compound sigma phase in the Cr–Ru system was described with a simplified two-sublattice model, (Cr,Ru)20(Cr,Ru)10, by taking into account future extension of the model into higher order systems. Comparison between the calculated results and the experimental measurements was made to demonstrate the reliability of the current thermodynamic parameters for the Ni–Cr–Ru system. For easy implementation of the thermodynamic parameters into some existing thermodynamic databases, a version of the thermodynamic description of the Ni–Cr–Ru system with an older sublattice model, (Cr,Ni,Ru)10(Cr)4(Cr,Ni,Ru)16, for the sigma phase was also developed.

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