Microstructures at 1250°C of Chromia–Forming Carbides– Reinforced Nickel–based Alloys after Addition of Tantalum; Effects of the Cr and Co Contents

Université de Lorraine, CNRS, IJL, F-54000 Nancy, France

Corresponding author: Patrice Berthod, Université de Lorraine, CNRS, IJL, F-54000 Nancy, France, E-mail: [email protected]
Received: September 07, 2020
Published: October 12, 2020

ABSTRACT

Cast nickel–based alloys can access to good mechanical properties at elevated temperatures even in total absence of gamma prime precipitates. Versions in which TaC carbides are present in significant quantities have recently emerged at laboratory scale but further optimization is required prior to possible industrial use. The main remaining problem is an insufficient stability of the TaC phase due to competition with chromium. This latter element is an essential element for the hot corrosion resistance and it cannot be removed. However, a better rating in Cr and a limited introduction of cobalt, element known to be more favorable for TaC stability, may lead to TaC–reinforced nickel–based superalloys. In this work thermal tests and thermodynamic calculations contributed to a better knowledge of the effect of added cobalt onthe nature of the carbides population in chromium–rich alloys based on nickel. On the one hand three alloys with different amounts in added cobalt were synthesized and exposed to 1250°C isothermally on long time. On the other hand thermodynamic calculations were carried out using a database especially designed for this used but not yet perfect. Adding Co to a {Ni, 25 wt.%Cr}–base containing carbon and tantalum obviously allows favoring predominant TaC at the expense of chromium carbides. The TaC fraction was accurately determined for each Co addition and verified by image analysis. Thermodynamical calculations showed the same trends but with a different origin: chromium carbides exist alone in the alloy when Co starts to be added while TaC are already present in the real alloy in absence of cobalt. Improvements need to be brought to the used database to better predictions and to take benefit of it to explore the possible high temperature microstructures when the contents in Co, Ta, Cr and C vary over more extended intervals than tested in this work.

KEYWORDS: Cast nickel alloys; Tantalum; Elevated temperature microstructures; Thermodynamic calculations