Deoxidation Equilibrium of Niobium in the Iron-Nickel Melts
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alloys are widely used in modern technology. The presence of oxygen in these alloys degrades their service characteristics. The thermodynamic properties of oxygen solutions in iron and nickel have been studied previously. The thermodynamic parameters of these solutions have been published.[1,2] However, since no additivity of properties of oxygen solutions in the Fe-Ni melts with respect to pure iron and nickel is observed, it is necessary to study the thermodynamic parameters of oxygen solutions in the iron-nickel alloys to optimize the production of these alloys. Upon production of iron-nickel alloys, niobium is an alloying element. This element has the higher affinity to oxygen in comparison with iron and nickel. When niobium is added to the nondeoxidized melt, its substantial portion can be oxidized and lost. In the nickel-niobium and iron-nickel alloys with niobium, the oxygen solubility was not studied earlier. For this reason, the investigation of thermodynamics of oxygen solutions in the iron-nickel alloys with niobium is of scientific and commercial interest.
II.
THERMODYNAMIC CONSIDERATION
In the iron-nickel melts, the product of interaction of niobium and oxygen dissolved in melt is NbO2 oxide. The reaction NbO2 ðsÞ ¼ ½Nb þ 2½O;
Kð1Þ ¼
½1
ð½pctNb fNb Þ ð ½pctO fO Þ2 ; aNbO2
where fNb and fO are the activity coefficients of Nb and O, respectively, can be represented as a sum of reactions NbO2 ðsÞ ¼ NbðsÞ þ O2 ðgÞ: DGð2Þ
½2
[3]
¼ 773; 302 160:15 T; J/mol ; NbðsÞ ¼ NbðlÞ:
½3
DGð3Þ ¼ 26; 921 9:8 T; J/mol[4]; NbðlÞ ¼ ½Nb1pctðFeNiÞ ; DGð4Þ ¼ RT ln
cNbðFeNiÞ MFeNi MNb 100
½4 ;
O2 ðgÞ ¼ 2½O1pctðFeNiÞ ; VIKTOR DASHEVSKII, Professor, Head of Laboratory, ALEKSANDR ALEKSANDROV and AKIM KANEVSKII, Senior Researchers, and LEOPOLD LEONT’EV, Professor, Chief Researcher, Academician of RAS, are with the Baikov Institute of Metallurgy and Materials Science (IMET), Russian Academy of Sciences (RAS), Leninskii Prosp., 49, Moscow 119991, Russia. Contact e-mail: a.a.aleksandrov@ gmail.com Manuscript submitted July 10, 2014. Article published online October 12, 2014. 220—VOLUME 46B, FEBRUARY 2015
½1a
DGð5Þ ¼ 2RT ln
½5
cOðFeNiÞ MFeNi ; MO 100
where MNb and MO, and cNbðFeNiÞ and cOðFeNiÞ are atomic masses and activity coefficients of Nb and O, respectively. METALLURGICAL AND MATERIALS TRANSACTIONS B
For Reaction [1], the oxygen concentration in melt being in equilibrium with a given niobium content can be calculated by the following equation; assuming 1n log Kð1Þ þ log aNbO2 log½pctNb log½pctOFeNi ¼ 2h i Nb eNb þ 2e NbðFeNiÞ OðFeNiÞ ½pctNb: h i o O 2eO OðFeNiÞ þ eNbðFeNiÞ ½pctO ; ½6a NbO2 oxide [Tm = 2270 K[5] (1997 °C)] is solid at 1823 K (1550 °C), therefore aNbO2 ¼ 1. Nb O O eNb NbðFeNiÞ ; eOðFeNiÞ ; eOðFeNiÞ ; and eNbðFeNiÞ are the interaction coefficients. The [pctO] term in the right side of Eq. [6a] because of its smallness can be written using the ratio 1=2 if is assumed in Eq. [1a] that fNb 1 Kð1Þ =½pctNb and fO 1. Such a substituti
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