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.
The three‐dimensional morphology of topologically close packed phases (TCP) in an experimental 6 wt% rhenium‐containing superalloy is examined with focused ion beam tomography and compared to the existing knowledge gained with classical microscopy of two‐dimensional cross‐sections. The results show completely new insights compared to investigation of cross‐sections. With the new technique, three different types of TCP‐phase morphologies and the orientation of their growth planes could be identified: complex shaped plates, needles, and laths. In the classical cross‐section, they all appear to be needles. A part of the needles is penetrating the plate‐like precipitates. Possible conclusions on the growth mechanism are discussed.
Three experimental Ni-base superalloys of the second, third and fourth generation are studied in this work to clarify the effect of Re and Ru on the precipitation of TCP phases. Thermodynamic calculations using the CALPHAD method are performed based on the local chemical composition obtained with the use of electron probe microanalysis. The effects of microsegregation and g 0 -precipitation are taken into account. The results, complemented with scanning electron microscopy observations, show that Re increases, while Ru decreases the inclination of the material to precipitate TCP phases. The effect of both elements is assigned to thermodynamic rather than kinetic reasons. The CALPHAD-calculations do predict the effect of Re on the TCP phase formation in the investigated alloys correctly, but not the effect of Ru.
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