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INTRODUCTION

IN recent years, demand has increased for a broad range of structural materials that are environmentally benign, energy efficient, light weight, and reliable in design.[1–3] The next generation of high-performance structural materials, such as ultrastrong steel, must have high strength/weight ratios, good fracture toughness, and good joining capabilities.[3] The addition of molybdenum and niobium to steel can effectively improve its strength and toughness.[4,5] Mo and Nb are strong carbide-forming elements in steel. The carbides thus produced can obstruct the motion of dislocations and prevent austenite grain growth during austenization.[4–10] However, the addition of high concentrations of Mo can stabilize the r phase.[11,12] This phase is extremely brittle, which leads to the formation of cracks and causes the toughness of the steel to deteriorate. Phase diagrams provide the basic data needed when designing material compositions and heat treatment processes. Thus, it is important to obtain the ternary

LEI ZOU, 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-mails: [email protected], [email protected] Manuscript submitted November 20, 2018. Article published online May 3, 2019 3358—VOLUME 50A, JULY 2019

phase diagram for the Fe-Mo-Nb system to ensure the correct development and design of new generations of high-performance steels.

II.

LITERATURE INFORMATION

A. Binary Systems Based on experimental data,[13–20] the Fe-Mo system has been thermodynamically optimized by several researchers.[12,21–25] The system has been subsequently reassessed by Rajkumar and Kumar[26] based on their experimental results and ab initio calculations. Therefore, the updated thermodynamic description of the Fe-Mo system provided by Rajkumar and Kumar[26] is the one used in the present work. The Fe-Nb system has also been optimized by many researchers[27–33] based on the available experimental investigations.[34–40] The optimization results of Jacob et al.[31] were adopted by Guo et al.[41] and Stein et al.,[42] and the thermodynamic parameters from Reference 31 are also adopted in this work. The Mo-Nb system has been optimized by Xiong et al.[43] based on the experimental data obtained by Kocherzhinskii and Vasilenko,[44] and their results are the one adopted in this work. The calculated Fe-Mo,[26] Fe-Nb,[31] and Mo-Nb[43] phase diagrams are shown in Figure 1. METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 1—Calculated binary phase diagrams: (a) Fe–Mo[26]; (b) Fe–Nb[31]; (c) Mo–Nb[43]

B. Fe-Mo-Nb Ternary System The first complete isothermal section at 900 C was determined by Smirnova et al.,[45] and two three-phase regions were constructed. The solubilities of Mo in l-Fe7Nb6 and Nb in l-Fe7Mo6 at 900 C were found to be about 5 and 10 at. pct, respectively. Later on, the Fe-rich (> 96 at. pct Fe) corners of the isothermal sections at 950 C, 1050 C, 1150 C, and 1250 C in the Fe-Mo-Nb s