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

Abstract

This study reassesses the optimum matrix composition of Co-Re-Cr-based alloys for strengthening by MC-type carbides. It is found that the composition of Co-15Re-5Cr is ideally suited for this purpose as it allows the solution of the carbide-forming elements such as Ta, Ti, Hf, and C within a matrix consisting entirely of fcc-phase (typically at 1450 °C), having a high solubility for these elements, while precipitation heat treatment (typically at 900–1100 °C) occurs in a hcp-Co matrix, displaying a much lower solubility. In the case of the monocarbides TiC and HfC, this was investigated and achieved for the first time in Co-Re-based alloys. TaC and TiC emerged as suitable particles in Co-Re-Cr alloys for creep applications due to a large population of nano-sized particle precipitation, which is not the case for the mainly coarse HfC. Both Co-15Re-5Cr-xTa-xC and Co-15Re-5Cr-xTi-xC exhibit a formerly unknown maximum solubility close to x = 1.8 at.%. Therefore, further research on the particle-strengthening effect and the governing creep mechanisms of carbide-strengthened Co-Re-Cr alloys should focus on alloys with the following compositions: Co-15Re-5Cr-1.8Ta-1.8C and Co-15Re-5Cr-1.8Ti-1.8C

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

“…The compositions of all alloys are given in Table 1. All alloys from Table 1 exhibit, up to 1100 • C, a complete hcp phase [9]. For alloy production, a two-step melting process was performed.…”

“…Different strengthening strategies were developed to achieve high strength and good creep resistance even at temperatures beyond the service temperatures of Ni-based superalloys. 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.…”

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

“…A prior study on the precipitation behavior of TaC and TiC in Co-Re-Cr was conducted in [11], from which the results in Figure 1 stem. For a holistic presentation of the precipitation behavior, the authors decided to provide scanning electron images of the microstructure in addition to the new images from TEM.…”

“…Recently, the research focus was set on TaC, TiC and hafnium carbide (HfC) precipitates in Co-Re-Cr, where the latter, unlike TaC and TiC, yielded a distribution of coarse HfC and intermetallic Co-Hf phases. Only an insignificant amount of HfC precipitated as nano-sized particles [11]. For this reason, this study focuses on TiC and TaC, investigating their morphology and effect on mechanical behavior via compression tests at room temperature, as well as strain-rate controlled creep tests at 800 • C and 900 • C. To ensure a single-phase fcc state during solution heat treatment and a single-phase hcp state at ambient and elevated temperatures, Co-15Re-5Cr was used as basis (all mentioned compositions throughout this work are stated in at.%).…”

“…For this reason, this study focuses on TiC and TaC, investigating their morphology and effect on mechanical behavior via compression tests at room temperature, as well as strain-rate controlled creep tests at 800 • C and 900 • C. To ensure a single-phase fcc state during solution heat treatment and a single-phase hcp state at ambient and elevated temperatures, Co-15Re-5Cr was used as basis (all mentioned compositions throughout this work are stated in at.%). For the particle strengthened materials, Co-15Re-5Cr-1.8Ta-1.8C and Co-15Re-1.8Ti-1.8C were selected as composition because prior work [11] showed that 1.8% of the MC-forming elements is close to the solubility limit at temperatures around 1450 • C. Furthermore, the orientation relationship for these MC carbides is determined with precession-enhanced selected area electron diffraction (P-SAED) and scanning precession electron diffraction (SPED). Finally, transmission electron microscopy (TEM) images of creep-deformed specimens provide clear evidence of particle-dislocation interaction for both TaC and TiC.…”