TaC Precipitation Kinetics During Cooling of Co−Re‐Based Alloys

Lukas Karge,Ralph Gilles,Dirk Honecker,Debashis Mukherji,Norbert Schell,Pavel Strunz,Přemysl Beran,Michael Hofmann,Sebastian Busch,Joachim Rösler

doi:10.1002/adem.202100129

Abstract

Cobalt−rhenium (Co−Re) alloys are developed for high‐temperature applications at ≈1200 °C and are strengthened by dispersion of nanosized tantalum carbide (TaC) precipitates. Herein, the precipitation behavior during cooling from supersolution depending on the cooling rate and the addition of chromium is presented. The phase composition (matrix phases and TaC) is analyzed from the wide‐angle neutron diffraction patterns measured in situ during temperature cycling. The precipitation of nanosized TaC particles is measured by in situ and ex situ neutron and X‐ray small‐angle scattering. The in situ measurements are used to extract the temperature‐dependent volume fraction of the precipitates; the final size distribution after cooling is extracted from the ex situ measurements. A Kampmann−Wagner's numerical (KWN) model is adapted to isochronal cooling processes. The in situ measurements give the unique possibility to calibrate the model parameters, whereas the ex situ measurements are used to assess the model predictions.

Highlights

The upper efficiency limit η of a constant-pressure heat engine is given by the one of the ideal Brayton cycle, η ≤ 1 À Tmin=Tmax
By identifying the feature originating from TaC, the amount of crystallized TaC precipitates was extracted from these measurements and, in reverse conclusion, the concentration of Ta that is dissolved in the matrix at any temperature
The precipitation of nanosized TaC particles in two CoÀRe-based alloys was studied with neutron diffraction (ND) and Small-angle scattering (SAS)

Summary

Introduction

The upper efficiency limit η of a constant-pressure heat engine is given by the one of the ideal Brayton cycle, η ≤ 1 À Tmin=Tmax. While the methods of power generation are currently shifting. Precipitated metal carbides, e.g., the monocarbide tantalum carbide TaC, strengthen the CoÀRe alloy.[5,6,7] Tantalum carbides are attractive because of their stability at elevated temperatures. Transmission electron microscopy (TEM) investigations after creep deformation showed pinning of dislocations at toward methods that do not rely on converting heat to the TaC particles due to an attractive particle dislocation. Hofmann Heinz Maier-Leibnitz Zentrum Technische Universität München Lichtenbergstr. Hofmann Heinz Maier-Leibnitz Zentrum Technische Universität München Lichtenbergstr. 1, Garching bei München 85747, Germany

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