Study of Chromium Activity in the Cr-Fe-N System by Galvanic Cell Method
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TRODUCTION
THE role of nitrogen in stainless steels and other alloy steels containing Cr is an important subject in view of the fact that the addition of a small amount of nitrogen can improve the hardness of the alloys. The knowledge of the thermodynamics of the Cr-Fe-N system is essential for the understanding, as well as the predicting the chemical and physical behaviors of these alloys in various environments. The Cr-Fe-N system has been investigated a number of times,[1–4] with Adcock[5] and Blix[6] among the first to study this system. Hertzman and Jarl[7] analyzed the available experimental information of the Cr-Fe-N system by the use of thermodynamic models with which isothermal sections of phase diagram were computed between 1173 and 1573 K. It should be pointed out that the liquid phase has not been involved in Herzman and Jarl’s assessment. By considering the new experimental information carried out by Feichtinger et al.,[8] a reassessment including liquid phase has been carried out by Frisk,[3] and the assessment results are now available in the TCFE6[9] database of the Thermo-Calc software.[10] As reported in Frisk’s work, there are some differences in the experimental results on locations of phase boundaries between different works. Reliable and consistent JIAYING XIE, Research Scholar, is with the Department of Wide and Heavy Plate Research, Shougang Technology Institute, Beijing 1000 43, P.R. China. Contact e-mail: jyxie2005@hotmail. com LIDONG TENG, Senior Researcher, and SESHADRI SEETHARAMAN, Professor, are with the Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. NANXIAN CHEN, Professor, is with the Department of Physics, Tsinghua University, Beijing, 1000 84, P.R. China. Manuscript submitted June 5, 2009. Article published online October 30, 2009 172—VOLUME 41A, JANUARY 2010
experimental information is significantly essential to clarify the phase relations in the Cr-Fe-N system. To the knowledge of the present authors, there is no experimental data of the chromium activity in the Cr-Fe-N system available in the literature. The purpose of the present work is to investigate the activities of chromium and the phase relations in the Cr-Fe-N alloys. In this work, an electromotive force (EMF) method involving CaF2 solid electrolyte was employed in order to measure the chromium activity in Cr-Fe-N alloys in the temperature range 973 to 1123 K. It is generally known that the CaF2 galvanic cell technique is reliable for the generation of thermodynamic activity data in high-temperature systems,[11,12] except at high PO2 ; and has recently been used to investigate the thermodynamic properties of the Mn-Ni-C-N system.[13] The nitrogen content in the Cr-Fe alloys equilibrated at 1173 K in pure N2 gas (101,325 Pa) was measured using the inert-gas fusion thermal conductivity (IGFTC) method.
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EXPERIMENTAL
In the present work, EMF measurements with CaF2 as the solid electrolyte were performed in order to investigate the chromium activities in the Fe-C
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