Experimental Investigation of Diagram Equilibria in the Co-Nb-Re Ternary System Xingjun Liu 1,2,3 • Dan Wu 1 • Lingling Li 1 • Mujin Yang 1 • Jinbin Zhang 1 • Jiahua Zhu 1 • Yuechao Chen 1 • Shuiyuan Yang 1 • Jiajia Han 1 • Yong Lu 1 • Cuiping Wang 1 Submitted: 18 June 2019 / in revised form: 12 November 2019 / Published online: 25 November 2019 Ó ASM International 2019 Abstract In this study, the isothermal sections of the Co- Nb-Re ternary system at 1200, and 1300 °C have been experimentally determined combining the means of elec- tron probe microanalysis (EPMA) and x-ray diffraction (XRD). The obtained experimental results showed that: (1) The Laves phase of k 3 -Co 2 Nb (C36) was stable at 1300 °C. The temperature was beyond its stability limit in Co-Nb binary system. (2) The solubility of Re in the k 3 phase was so large that the nearest k 2 -Co 2 Nb (C15) phase was essentially surrounded. (3) The solubility of Re in the l- Co 7 Nb 6 phase was 34.0 at.% at 1200 °C and 35.2 at.% at 1300 °C, respectively. (4) The liquid phase existed at 1300 °C dissolving about 4.0 at.% Re, but it was absent at 1200 °C. (5) The solid solution phase of (eCo, Re) exten- ded from Re-rich to Co-rich side. Keywords Co-Nb-Re ternary system Á electron microprobe Á high-temperature alloys Á phase equilibria 1 Introduction Co-based superalloys that have better resistance of oxida- tion and hot corrosion than Ni-based superalloys are regarded as competitive high-temperature materials. [1,2] In order to obtain better high temperature performances to meet the increasing requirements of the aerospace field, One of the most effective methods to improve the elevated temperature property is adding refractory elements such as Mo, Re, Nb, W, Ta. [3-12] For example, the melting point of elemental Re (rhenium) is 3186 °C. As an additive ele- ment, elemental Re can not only improve the strength and creep resistance of Co-based superalloys, but also refine the morphology of the alloys. [6-9] Doping refractory element of Nb can improve high-temperature strength, creep resis- tance, oxidation resistance and corrosion resistance. [10-12] However, in Co-based superalloys, the brittle and detri- mental TCP (topologically close packed) phases easily form under high temperature and pressure if the element additions are excessive. Therefore, the amounts of the Re and Nb are under precise control and the knowledge of phase equilibria in the Co-Nb-Re system is needed. How- ever, the experimental information and phase diagram of this ternary system are not established. Hence, investigat- ing the phase equilibria of Co-Nb-Re system is necessary. The three binary systems of Co-Nb, Co-Re and Nb-Re constitute Co-Nb-Re ternary system, as shown in Fig. 1. For the Co-Re binary system, it was ever published by Elliott. [13] Later, Predel [14] also reported the results for the Co-Re binary system. In 2014, Liu et al. [5] estimated the Co-Re system and the phase equilibria were consistent with the experimental data. Recently, Guo et al. [15] reassessed the Co-Re system with the new thermodynamic parameters of pure Re. The newly assessed Co-Re phase diagram by Guo et al. [15] was applied in this work. The Co-Re & Cuiping Wang wangcp@xmu.edu.cn 1 College of Materials and Fujian Provincial Key Laboratory of Materials Genome, Xiamen University, Xiamen 361005, People’s Republic of China 2 State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People’s Republic of China 3 Institute of Materials Genome and Big Data, Harbin Institute of Technology, Shenzhen 518055, People’s Republic of China 123 J. Phase Equilib. Diffus. (2019) 40:820–829 https://doi.org/10.1007/s11669-019-00772-6