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I.

INTRODUCTION

THE Co-Nb-Ni system has aroused interest with applications in superalloys, high-entropy alloys, etc. For superalloys, cobalt, niobium, and nickel are the most important added elements[1] and niobium can significantly increase the c¢-solvus temperature and improve the mechanical properties.[2–4] In addition, the high-entropy alloys containing these three and other elements are promising candidates for use as high-entropy superalloys due to their higher overall strength and high-temperature performance.[5,6] The CALPHAD method, used as a tool to develop such novel materials, requires accurate phase equilibria information. Recently, the Co-Nb-Ni isothermal sections at 1102 C, 927 C, 900 C, and 800 C[7,8] have been determined (Figure 1); however, some problems remain to be addressed: (1) No ternary compounds were found at 1102 C and 927 C,[7] while one ternary compound s with an MgNi2-type structure was found at 900 C and 800 C[8]; (2) One stable compound with an

CHENYANG ZHOU, CUIPING GUO, CHANGRONG LI, and ZHENMIN DU are with the School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected], [email protected] JINGBO LI is with the School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P.R. China. Manuscript submitted on February 3, 2020.

METALLURGICAL AND MATERIALS TRANSACTIONS A

Mg3Cd-type structure was reported at 927 C on the basis of the XRD data and was considered to be the solid solution of Ni in Co7Nb2,[7] but the crystal structure of Co7Nb2 should be of the Ni7Zr2 type according to synchrotron radiation XRD data[9]; (3) It was still questionable that the compound k had a polytypic mixture of the MgCu2/MgZn2-type structures.[7] Therefore, the present study aimed to determine the phase equilibria at 1100 C and 900 C and establish a reliable thermodynamic model of the Co-Nb-Ni system, which are very useful to design composition and heat treatment process for high-performance superalloys.

II.

LITERATURE REVIEW

A. Co-Nb System A review of work published before 1990 has been reported by Massalski et al.[17] based on early experimental phase diagram information.[10–16] Sprengel et al.[18] used several diffusion couples to measure phase relationships from 837 to 1195 C, but compound k3 was not found. Stein et al.[19] investigated the Co-Nb phase diagram over the whole composition range using a series of equilibrated alloys and diffusion couples by SEM, XRD, EPMA, and DTA. The reported two-phase regions k3 + k2 and k2 + k1 were very narrow. Up to 1086 C, the compound Co7Nb2 remained stable, but its crystal structure remained unknown. Recently, Leineweber et al.[9] adopted synchrotron radiation XRD to confirm the crystal structure of the compound Co7Nb2 relating to the Ni7Zr2 type. Shaipov et al.[7] measured the phase boundaries at 1102 C and 927 C, but the

Fig. 1—The reported Co-Nb-Ni isothermal sections: (a) at 1102 C[7]; (b) at 927 C[7]; (c) at 900 C