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

Abstract

CobaltÀrhenium (CoÀRe) 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 KampmannÀWagner'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.

Reassessment of the Matrix Composition of Co-Re-Cr-Based Alloys for Particle Strengthening in High-Temperature Applications and Investigation of Suitable MC-Carbides

“…The precipitation of nano-sized TaC after ST, displayed in Figure 3 , matches the findings in [ 16 ] for the compositions of Co-17Re-1.2Ta-1.082C and Co-17Re-15Cr-1.2Ta-1.082C. Upon cooling, the transformation from fcc-Co to hcp-Co completes at the hcp-Co finish temperature.…”

“…Lower C/Ta-ratios also reduce the temperature range of the allotropic hcp ⇄ fcc phase transformation of the Co-Re-Cr matrix due to the fcc-stabilizing effect of free Ta [ 15 ]. Taking into consideration that the solubility of Ta drops significantly in hcp-Co compared to fcc-Co [ 16 ], the TaC precipitation during cooling from the solution heat treatment temperature starts with the phase transformation of the matrix from fcc to hcp, i.e., at lower temperatures in the case of C/Ta-ratios < 1. Therefore, less time for the nucleation and growth of TaC is available during cooling after solution treatment when the C/Ta-ratio is reduced, which results in a smaller particle size [ 15 ].…”

“…Therefore, less time for the nucleation and growth of TaC is available during cooling after solution treatment when the C/Ta-ratio is reduced, which results in a smaller particle size [ 15 ]. Vice versa, the addition of Cr as a hcp-stabilizing element leads to the rise of the hcp ⇄ fcc phase transformation temperature range and consequently produces larger particles [ 16 ]. Still, C/Ta-ratios less than 1 are not desirable in the design of TaC-strengthened Co-Re-based alloys because of the risk of retained fcc-Co after the solution heat treatment [ 15 ].…”

“…Firstly, TaC dissolves in fcc-Co and does not contribute to the creep resistance anymore. Secondly, martensitic and/or diffusion-controlled fcc → hcp transformation during service may lead to grain refinement, adversely affecting creep strength [ 16 ]. Finally, the fcc matrix is significantly softer than the hcp matrix [ 8 ], and creep processes seem to be faster for Co-Re alloys with more metastable fcc-Co.…”

Investigation of TaC and TiC for Particle Strengthening of Co-Re-Based Alloys

“…Hereby, differently oriented hcp-grains grow typically into one fcc-grain, but eventually only one dominant hcp-grain remains. As demonstrated by in-situ small angle scattering experiments [20], TaC precipitation already occurs during cooling once the fcc/hcp transformation starts. If TaC particles precipitate with an orientation relationship to a hcp-grain that is later overgrown by another one, the particles do not reorientate themselves to the final grain orientation.…”