Activity of manganese in liquid Ni-Mn alloys
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-AI-,O~cement Cr+Cr
Ni-Mn
AI203
reaction tube
-AI203/Y203
[1]
The oxygen potential at the alloy electrode is established by equilibrium between three condensed phases: Mn(/) + 89 + (1 + x)A1203(ct)----~ MnA12+2~O4+3x(sp)
[2]
Three phase equilibria of this type have not been utilized previously in the study of alloy thermodynamics using the emf method. The normal practice is to saturate the alloy with the oxide of reactive metal. The difficulty in finding a container material resistant to MnO and the availability of accurate thermodynamic data on manganese aluminate 2'3 are the main reasons for the choice of three phase equilibria in this study. A schematic diagram of the apparatus is shown in Figure 1. The alumina crucible containing the alloy is supported on an alumina sheath inside a vertical reaction tube. Synthetic MnA1204 is added to the melt and the crucible is covered with an alumina lid to minimize the loss of manganese by vaporization. Two holes in the lid permit the insertion of the solid electrolyte tube and a tungsten lead. The junction between the tungsten lead and the platinum wire is maintained in the constant temperature zone of a molybdenum furnace, thus eliminating thermoelectric force between the two leads. The solid electrolyte tube and the conducting lead are usually suspended above the melt. A mixture of Cr and Cr203 is placed inside the solid electrolyte tube, with a platinum lead embedded in the mixture. The molar ratio of Cr to Cr203 in the reference electrode was 1:2 for alloy compositions producing positive emfs (XMn > 0.18), and 2:1 for compositions giving negative emfs. In each case, by taking an excess of the component that is consumed by the flux of oxygen through the K. T. JACOB is Associate Professor with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, Canada, M5S 1A4. Manuscript submitted May 28, 1981. METALLURGICAL TRANSACTIONS B
~Ai203
Fig. 1 --Schematic diagram of the apparatus.
electrolyte, polarization at the reference electrode is eliminated. The electrolyte tube is then filled tightly with thoria powder to prevent manganese vapor from reaching the reference electrode. Prepurified argon gas is passed through the reaction tube at a rate of 300 ml min -1. After the cell is assembled, the furnace is heated to 1683 K. Preliminary results indicated that the equilibrium between the alloy, manganese aluminate, and alumina is established in approximately four hours. After this time interval, the solid electrolyte tube and the tungsten lead are lowered into the melt and the emf is monitored as a function of time using a high impedance digital voltmeter (> 10121~). For alloys containing less than 40 at. pct Mn, the emf remains constant for approximately two minutes, after which it begins to decrease slowly. The signal decay can be arrested and reversed by vibrating the solid electrolyte tube. This indicates that the decrease in emf is caused by concentration polarization at the alloy electrode due to the flux of oxygen through the electroly
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