Determination of Activities of Niobium in Cu-Nb Melts Containing Dilute Nb
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The partial molar excess Gibbs energy change of niobium in Cu-Nb melts can be expressed as follows: ex
DGNb ¼ 63; 280 þ 13:2TðJ/molÞ
DOI: 10.1007/s11663-015-0312-3 Ó The Minerals, Metals & Materials Society and ASM International 2015
Niobium is one of the most important microalloying elements in high-strength low-alloy (HSLA) steels.[1,2] The formation of niobium carbide and niobium nitride in the steel can improve the grain refining and retardation of recrystallization. The precipitation of these phases can increase the hardness, toughness, strength, formability, and weldability of the steel. It has also been reported that the transverse surface cracking in continuously cast products is strongly influenced by the content of niobium. To obtain the best performance of niobium-containing steel demands a precise control of the content of niobium.[3–5] In order to achieve this goal, the thermodynamic properties of niobium oxide in slags are necessary. To determine the activities of components in slags, liquid copper has usually been used as a reference metal to equilibrate with slags.[6–9] For example, by equilibrating the slag with Cu, Morita et al.[6] measured the
DAYA WANG, Doctoral Student, and BAIJUN YAN, Professor, are with the State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China. DU SICHEN, Professor is with the Department of Materials Science and Engineering, Royal Institute of Technology, 10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted October 16, 2014. Article published online February 25, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
activity of chromium oxide, Kang et al.[7] measured the activity of SiO2, and Suito[8,9] measured the solubility of TiN in BaO-TiOx and CaO-TiO1.5-TiO2. As a pre-study of the activities of niobium oxide in slag, the aim of the present work is to measure the activity coefficients of niobium in liquid copper in the temperature range between 1773 K and 1898 K (1500 °C and 1625 °C). The experimental apparatus is schematically shown in Figure 1. A resistance furnace using MoSi2 heating element was employed. An alumina reaction tube (70mm OD, 61-mm ID, and 1000-mm length) was mounted in the furnace, which was sealed with viton O-rings on its ends by water-cooled quenching chamber on the top, and a water-cooled cap on the bottom. The temperature of the furnace was controlled by a PID controller (SHIMADEN FP21) with an accuracy of ±1 K. A type C thermocouple (W-Re5/26) was placed just beneath the bottom of the sample holder to measure the sample temperature. An alumina crucible was used to hold the sample. The alumina crucible was hung by a piece of Mo wire from the shaft of a lifting device. Oring sealing between the shaft (made of stainless steel) of the lifting system and the top of quenching chamber ensured the reaction chamber vacuum tight. The sample could be lifted from the reaction zone to the quenching chamber
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