Investigation of TaC precipitation hardening in Co-Re-based alloys
Mechanical Properties of Metallic Materials and their Microstructural Origins
Final Report Abstract
Co-Re alloys are a candidate material for high-temperature applications [2, 3] and can be strengthened by TaC precipitates. In the frame of this project, microstructural analysis by scanning electron microscopy was used in conjunction with neutron and X-ray scattering techniques to study the process of TaC precipitation in these alloys. In-situ small-angle neutron scattering (SANS) was used to comprehensively study the precipitate nucleation and coarsening kinetics at high temperatures. Thereby, the activation energies for nucleation, growth and coarsening of the precipitates could be determined. Further, neutron diffraction experiments revealed that the Co-Re matrix undergoes an allotropic phase transformation from 𝜖-Co (hcp) to -Co (fcc) at around 𝑇 ≈ 1500 K that can be used to dissolve and precipitate the TaC particles in a controlled fashion. Cr and B are important for the alloy performance at high temperatures and their influence on the Co-Re phase transformation and precipitate stability was also studied. It is shown that Cr stabilizes 𝜖-Co, while B lowers the matrix transformation temperature, i.e. stabilizes 𝛾-Co. In addition, the creep behavior of the Co-Re alloy was investigated in laboratory tests and during in-situ diffraction experiment with synchrotron radiation at 1373 K. X-ray diffraction and microscopy yielded supplementary results on the high-temperature behavior of the Co-Re alloys. In the following, the main results of the project are summarized: 1. The monocarbide TaC was found to be the only stable tantalum carbide phase in the investigated Co-Re-Ta-C-(Cr)-(B) alloys. 2. It was found that precipitation of TaC is directly coupled to the allotropic phase transformation between the “low temperature” -phase and the high temperature -phase. As the solubility of Ta and C is significantly higher in than in , the transformation leads to TaC dissolution while the transformation causes precipitation. From this finding, a number of important conclusions can be drawn: a. An -matrix is required to ensure stability of fine TaC precipitates during elevated temperature service. b. The / transformation can be purposefully used for precipitation strengthening, enabling dissolution of the TaC phase during solution heat treatment, followed by controlled reprecipitation. 3. The allotropic phase transformation temperature is strongly dependent on alloy composition. Ta, dissolved in the matrix, was identified to stabilize the -phase, thus reducing the transformation temperature. The same holds true for B, albeit to a lesser extent, while the opposite holds true for Cr. The amount of freely dissolved Ta is controlled by the C/Ta ratio. It increases with decreasing C/Ta ratio. 4. The available C content influences the maximum volume fraction and the size distribution of TaC precipitates at high temperatures. A higher C content results in a higher volume fraction of the TaC precipitates and a smaller inter-particle distance. 5. A semi-coherence orientation relationship between TaC precipitates and the 𝜖-Co matrix was deduced from diffraction experiments. A lattice misfit of < 2.73 % was found. Thereby, the occurrence of particles with elongated morphology and an orientation relationship to the Co-Re matrix could be explained. 6. The TaC precipitation during cooling from solution heat treatment was characterized by in-situ SANS. A kinetic model was created and fitted to the data, allowing to evaluate the activation energies for nucleation and growth of the TaC particles. 7. Creep tests revealed a lack of strengthening by the TaC precipitates. Further research is necessary to elucidate the reason for this result. Creep experiments during in-situ synchrotron diffraction showed furthermore that the phase equilibrium of the 𝛾-Co and 𝜖-Co phases is changed in the Co- Re-Ta-C system under load. This causes a phase transformation and grain refinement of the alloy matrix. 8. A software package with a large variety for fitting of 3D models to 2D SANS data was developed.
Publications
- Current status of Co-Re-based alloys being developed to supplement Ni-based superalloys for ultra-high temperature applications in gas turbines, Kovove Materialy-Metallic Materials 53(4) (2015) 287-294
D. Mukherji, P. Strunz, R. Gilles, L. Karge, J. Rösler
- In Situ Neutron Diffraction Characterization of Phases in Co-Re-Based Alloys at High Temperatures, Acta Physica Polonica A 128(4) (2015) 684-688
P. Strunz, D. Mukherji, R. Gilles, U. Gasser, P. Beran, G. Farkas, M. Hofmann, L. Karge, J. Rösler
(See online at https://dx.doi.org/10.12693/APhysPolA.128.684) - Effect of composition on the matrix transformation of the Co-Re-Cr-Ta-C alloys, Metals and Materials International 22(4) (2016) 562-571
P. Beran, D. Mukherji, P. Strunz, R. Gilles, M. Hofmann, L. Karge, O. Dolotko, J. Rösler
(See online at https://doi.org/10.1007/s12540-016-5697-2) - Stability of TaC precipitates in a Co–Re-based alloy being developed for ultra-high-temperature applications, Journal of Applied Crystallography 49(4) (2016) 1253–1265
R. Gilles, D. Mukherji, L. Karge, P. Strunz, P. Beran, B. Barbier, A. Kriele, M. Hofmann, H. Eckerlebe, J. Rösler
(See online at https://doi.org/10.1107/S1600576716009006) - The influence of C/Ta ratio on TaC precipitates in Co-Re base alloys investigated by small-angle neutron scattering, Acta Materialia 132 (2017) 354-366
L. Karge, R. Gilles, D. Mukherji, P. Strunz, P. Beran, M. Hofmann, J. Gavilano, U. Keiderling, O. Dolotko, A. Kriele, A. Neubert, J. Rösler, W. Petry
(See online at https://doi.org/10.1016/j.actamat.2017.04.029)
