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

Abstract

Cobalt-Rhenium (Co-Re)-based alloys are currently investigated as potential high-temperature materials with melting temperatures beyond those of nickel-based superalloys. Their attraction stems from the binary Co-Re phase diagram, exhibiting complete miscibility between Co and Re, whereby the melting temperature steadily increases with the Re-content. Thus, depending on the Re-content, one can tune the melting temperature between that of pure Co (1495 °C) and that of pure Re (3186 °C). Current investigations focus on Re-contents of about 15 at.%, which makes melting with standard equipment still feasible. In addition to solid solution strengthening due to the mixture of Co- and Re-atoms, particle strengthening by tantalum carbide (TaC) and titanium carbide (TiC) precipitates turned out to be promising in recent studies. Yet, it is currently unclear which of the two particle types is the best choice for high temperature applications nor has the strengthening mechanism associated with the monocarbide (MC)-precipitates been elucidated. To address these issues, we perform compression tests at ambient and elevated temperatures on the particle-free base material containing 15 at.% of rhenium (Re), 5 at.% of chromium (Cr) and cobalt (Co) as balance (Co-15Re-5Cr), as well as on TaC- and TiC-containing variants. Additionally, transmission electron microscopy is used to analyze the shape of the precipitates and their orientation relationship to the matrix. Based on these investigations, we show that TiC and TaC are equally suited for precipitation strengthening of Co-Re-based alloys and identify climb over the elongated particles as a rate controlling particle strengthening mechanism at elevated temperatures. Furthermore, we show that the Re-atoms are remarkably strong obstacles to dislocation motion, which are overcome by thermal activation at elevated temperatures.

Temperature-Dependent Young’s Modulus of TaC- and TiC-Strengthened Co-Re-Based Alloys

“…For high-temperature applications, TaC and TiC turned out to be particularly attractive as they can be precipitated on the nanoscale, and furthermore, excellent high-temperature stability was demonstrated in the case of TaC [12]. A recent study on the creep behavior of particlefree as well as TaC-and TiC-strengthened Co-15Re-5Cr alloys at 800 • C and 900 • C (all mentioned compositions throughout this work are stated in at.%) highlighted significant strengthening contributions stemming from the essentially immobile solute Re atoms in the matrix and, in particular, from the particle-dislocation interaction, which was measured to be in the same order of magnitude as the (room temperature) Orowan stress [13]. These promising results are an important step toward Co-Re-based high-temperature alloys and demand further research for a better understanding of the underlying strengthening mechanisms.…”

“…They are based, on the one hand, on solid solution strengthening using Re atoms, which in current alloys make up from 15 at.% [9] to 31 at.% [7]. On the other hand, particle strengthening has been investigated, with Cr 2 Re 3 [10], Cr 23 C 6 [11], TaC [12], TiC, and HfC [13] being considered so far. For high-temperature applications, TaC and TiC turned out to be particularly attractive as they can be precipitated on the nanoscale, and furthermore, excellent high-temperature stability was demonstrated in the case of TaC [12].…”

“…Moreover, its temperature dependency has to be known to account for temperature effects on creep strength. So far, the elastic properties of Co-Re-based alloys have only been assessed in [13]. However, it turned out that the compression tests that were conducted were not able to reasonably reveal Young s modulus due to limitations in the test setup.…”

The Effect of Replacing Ni with Mn on the Microstructure and Properties of Al2O3-Forming Austenitic Stainless Steels: A Review

“…The negative effect of a high Mn content on the creep properties of AFA steel should be due to the inhibitory effect of Mn on the precipitation of the nano-sized B2–NiAl phase and secondary NbC phase, as shown in Figure 2 and Figure 3 [ 36 , 39 ]. These nano-sized precipitates play a major role in the creep resistance process [ 33 ].…”

“…In order to analyze the mechanism of Mn participating on oxidation, the oxidation rate constant and free energy of several typical oxides were collected as shown in Figure 6 a,b. The oxidation rate constant of Al 2 O 3 oxide layer is more than 2.5 g 2 cm −4 s −1 lower than Cr 2 O 3 oxide layer and the free energy is nearly 300 kJ/mol lower than Cr 2 O 3 oxide layer, which indicates that the Al 2 O 3 oxide film will be formed more slowly as well as be denser and more stable [ 2 , 35 , 39 , 43 , 44 , 45 ]. Figure 6 c analyzed the mechanism of the effect of Mn addition on the oxide film of AFA steel.…”