Investigation of the Ni-effect in Co-Re-Cr high temperature alloys
Final Report Abstract
The research conducted led to a deep understanding on the role of Ni regarding oxidation behavior, phase evolution and mechanical properties of Co-Re-Cr high-temperature alloys. Essentially all scientific questions posed initially could be answered. These answers are summarized in the following: Q1: Increasing Ni concentration enhances the diffusion coefficient of Cr which leads to a faster formation of a protective Cr2O3 layer and improves the oxidation behavior. This enhanced Cr diffusivity due to Ni addition goes along with a reduced activation energy for diffusion. Q2: It was proven that the positive effect of Ni can be pushed by an additional Si alloying as the Si addition facilitates the formation of chromia and diminishes the formation of non-protective oxides. However, the addition of 2 at.% Si simultaneously influences the alloy microstructure by promoting the σ-phase formation which damps the positive effect on the Cr activity. Q3: Only the alloys with 23 at.% Cr show a oxidation behavior with positive oxidation kinetics indicating that severe evaporation of Re oxides is prevented. Therefore, the Cr content should not be lowered down to 18 at.% as these alloys exhibit substantial mass loss during oxidation. Q4: Sufficient Ni addition changes the type of the discontinuous precipitation reaction from hcp → fcc + σ to fcc-1 → fcc-2 + σ. This increases the driving force for the reaction as the fcc-phase exhibits a lower solubility for Re, being enriched in the σ-phase, compared to the hcp-phase. Consequently, a more complete reaction with refined microstructure (due to faster reaction) is observed when Ni is added. Q5: At high temperature close to the solvus temperature of the σ-phase, the matrix is fcc irrespectively of the Ni-content. Consequently, the solvus temperature of the σ-phase of about 1450°C is essentially unaffected by the Ni-content. Q6: If the intent is to precipitate finely distributed σ-phase via the discontinuous reaction, the temperature and Ni-content should be selected such that the reaction is of the fcc-1 → fcc-2 + σ type. The alloys CoRe-23-15 and CoRe-23-25 at temperatures between 800°C and 1050°C are well suited for that purpose. Q7,8: The fcc / hcp transformation occurs by diffusion or martensitically. Ni reduces these transformation temperatures. This allows to create four fundamentally different microstructures of the matrix: (i) equiaxed grains consisting entirely of hcp-phase (0% Ni), (ii) equiaxed grains consisting entirely of fcc-phase (25% Ni), (iii) lamellar microstructures after heat treatment in the fcc / hcp two-phase field, (iv) lath-type microstructures via martensitic transformation, requiring a sufficient Ni-content to suppress diffusion controlled transformation. Q9: The hcp / fcc phase boundaries do not lead to significant strengthening. Q10: The temperature interval where the fcc / hcp two-phase field is stable is narrow. Therefore, it is not possible to design alloys with stable two-phase microstructure over a wide range of application temperatures. Q11: In terms of oxidation resistance a Ni concentration of 15 at.% and a Cr content between 18 and 23 at.% is favored to get positive oxidation kinetics but diminished/no σ formation. For hightemperature applications, the Co-Re-Cr-Ni alloy should exhibit a single-phase hcp matrix even during long-term high-temperature exposure. To meet this demand of a stable one-phase hcp microstructure, higher Re concentrations should be applied to bring the single-phase hcp matrix in accordance with increased Ni concentrations.
Publications
- Coexistence of two cubic-lattice Co matrices at high temperatures in Co-Re-Cr-Ni alloys studied by neutron diffraction, Advances in Materials Science and Engineering, 2018
P. Beran, D. Mukherji, P. Strunz, R. Gilles, M. Hölzel, and J. Rösler
(See online at https://doi.org/10.1155/2018/5410871) - Influence of σ phase on the allotropic transformation of the matrix in Co-Re-Cr based alloys with Ni addition, Metals, 8, 706 (2018)
K. Dörries, D. Mukherji, J. Rösler, K. Esleben, B. Gorr, and H.-J. Christ
(See online at https://doi.org/10.3390/met8090706) - The effect of Ni additions on the oxidation behaviour of Co–Re–Cr high-temperature alloys, Materials at High Temperatures, 35, 177-186 (2018)
K. Esleben, B. Gorr, H.-J. Christ, D. Mukherji, and J. Rösler
(See online at https://doi.org/10.1080/09603409.2017.1392111) - Effect of Cr and Ni on the microstructural evolution in Co-Re-Cr-Ni alloys International, Journal of Materials Research, 110, 1092-1104, (2019)
K. Esleben, B. Gorr, H.-J. Christ, C. Pritzel, D. Mukherji, J. Rösler, P. Beran, P. Strunz, M. Hölzel, R. Gilles
(See online at https://doi.org/10.3139/146.111855) - Effect of Different Cr and Ni Additions on Oxidation Behavior of Co-Re-Based Alloys, JOM (2019)
K. Esleben, B. Gorr, H.-J. Christ, D. Mukherji, J. Rösler
(See online at https://doi.org/10.1007/s11837-019-03868-z) - The effect of Ni and Si additions on the oxidation behavior of Co-17Re-18Cr alloys, Corrosion Science, 159, Article 108135 (2019)
K. Esleben, B. Gorr, H.-J. Christ, D. Mukherji, and J. Rösler
(See online at https://doi.org/10.1016/j.corsci.2019.108135)
