Enthalpy of formation and heat capacity of Fe-Mn alloys
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KNOWLEDGE of the thermodynamic properties of liquid and solid Fe-Mn alloys is of great importance to obtain quantitative information about the driving force of technically important solid-state transformations, such as the g (austenite) → a (ferrite) transformation. In steel, the g → a transformation is a principal solid-state transformation which, in many cases, determines the microstructure and, consequently, the performance of the material. Data of the thermodynamic properties of the Fe-Mn system can be combined with experimental phase-equilibria data to evaluate, using an optimization procedure, the equilibrium-phase fields and also the thermodynamic data. Direct experimental information about the enthalpy of formation of liquid and solid Fe-Mn alloys is scarce. Only a single experimental run has been performed to determine the enthalpy of mixing of liquid Fe-Mn alloys.[1] Directsynthesis calorimetry was applied for the measurement of the enthalpy of formation of g-Fe-Mn alloys.[2] Phase-diagram optimizations were carried out several times[3–6] on the basis of experimental enthalpy[1,2] and activity[7] data and experimental phase-equilibria data.[8,9,10] In the present work, the enthalpy of mixing of liquid Fe-Mn alloys, the enthalpy of formation of g-Fe-Mn solid solutions, and the heat capacity of pure iron and g-Fe-Mn alloys were measured by applying isoperibolic calorimetry and the differential thermal analysis (DTA) technique, respectively. The enthalpy of formation of a-Fe-Mn solid solutions and the change in the magnetic contribution to the enthalpy due to the addition of Mn were deduced from the experimental data. VICTOR T. WITUSIEWICZ, Guest Scientist, and FERDINAND SOMMER, Professor and Research Associate, are with the Max Planck Institute for Metals Research, 70569 Stuttgart, Germany. Contact e-mail: [email protected] ERIC J. MITTEMEIJER, Director, Max Planck Institute, is Professor, Institute for Physical Metallurgy, University of Stuttgart, 70569 Stuttgart, Germany. Manuscript submitted October 31, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
A. Liquid Alloys The partial enthalpies of mixing of liquid Fe and liquid Mn and, thereby, the integral enthalpy of formation of liquid Fe-Mn alloys were measured as a function of the atomic fraction of iron (x) within the range of 0 ⱕ x ⱕ 0.45 at 1700 ⫾ 5 K. A high-temperature isoperibolic calorimeter operating up to 1800 K was used to measure the partial enthalpy of mixing.[11] The constant temperature is provided by the temperature-constant zone of a rhodium-wound furnace. The alumina reaction crucible is positioned in a massive molybdenum block. This block consists of several segments and is suspended in an alumina tube. A revolving mechanism on top of the calorimeter is used for the sample addition. The use of a stirrer guarantees complete and rapid dissolution of the samples. The temperature change occurring during the dissolution is measured by a thermopile made of W-5 wt pct Re/W-20 wt pct Re, directly mounted below the reaction crucible.
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