Thermodynamics of the Cr-Mn system using an isopiestic technique
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I.
INTRODUCTION
THEREhas been growing interest in the determination of thermodynamic properties and phase equilibria of binary systems of manganese with transition metals. Chromium and manganese are important components of high temperature alloys. Manganese promotes the kinetics of spinel formation on the surface of the alloys which provides oxidation resistance. The phase relation studies in the Cr-Mn-O system have recently been carried out. [~j This is important in assessing the applicability of these alloys as high temperature materials. Besides, the thermodynamic properties of the alloys are important for reliable definition of equilibrium properties and subsequent calculation of phase boundaries in multicomponent alloys. Thermodynamic properties of the Cr-Mn system have been determined in the solid state by Eremenko e t a l . [2j using a fused salt emf technique. Their results indicate positive deviations from ideality at 1023 K. Kaufman[3] has independently estimated negative enthalpy and excess entropy for the Cr-Mn bcc solid solution, such that at high temperatures the entropy term predominates over the enthalpy term, giving positive deviations from Raoult's law. Recently the thermodynamic proper9 ties of the alloys have been measured by Jacob, [4] using a Knudsen cell technique in the temperature range of 1200 to 1500 K. The results indicate mild negative deviations from ideality over the entire composition range. In view of this inconsistency in the reported results and Mn being a volatile component in the alloys which could lead to surface depletion under a dynamic set-up, an isopiestic technique has been used to measure the properties of the alloys. II.
An alloy of a given composition was crushed and ground repeatedly in an agate mortar to produce fine powders. The alloy powder so produced was then pelletized using a steel die to a pellet of 5 mm diameter and 2 mm thickness. A quadrant of such a disc was used in the actual experiments. Such a small disc was required to reduce the time taken to reach equilibrium. In these experiments, two alloys of different compositions were introduced in the upper part of a transparent quartz capsule which was then evacuated and filled with purified argon. A schematic sketch of the arrangement is shown in Figure 1. The lower part of the capsule contained a pellet of pure Mn. The capsule was then reevacuated and sealed. A chromel-alumel thermocouple attached close to the Mn pellet measured the temperature of the pellet. The capsule was then held vertically in a furnace
__.....Alloy pellets
EXPERIMENTAL ASPECTS
The metals chromium and manganese were 99.99 pct pure and supplied by Johnson and Matthey Chemicals, United Kingdom and Alfa products of Thiokol Corporation, respectively. The alloys were prepared by melting weighed quantities of the metals in a recrystallized alumina crucible using a high frequency generator under an atmosphere of pure argon. The gas was purified by passing it over calcium granules heated to 873 K. The alloys were analyzed using a Shimadzu
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