THE MnSe-MnS SYSTEM by J.M. Mehta, PoG. Riewald, and LoH. Van Vlack Abstract: Manganous selenide and manganous sulfide form a continuous solid solution at all temperatures below the solidus~ The system is azeotropic with a minimum at 1510~C near 10 mole percent MnSo Although this series has a NaCl structure, the central portion has a diffraction pattern with a false appearance of being simple cubic because the average anion atomic number is 25, the same as manganese. The lattice constant has a negative deviation from ideality. The MnSe inclusion found in selenium treated steels will dissolve sulfur which is present to give Mn(Se,S). COLLEGE OF ENGINEERING Department of Chemical and Metallurgical Engineering THE UNIVERSITY OF MICHIGAN Ann Arbor, Michigan Under Contract with Selenium-Tellurium Development Association, Inco New York, N.Y. Administered through: September, 1966 Office of Research Administration 07137-3-P

INTRODUCTION Resulfurized steels have intentional additions of sulfur so that the machinability is improved0 The sulfur combines with manganese in the steel to form MnS inclusions which are dispersed throughout the steels~ The exact mechanism by which MnS improves the machinability is not fully understood; however, there is no doubt that the deformation characteristics are important. The deformation mechanism is well understood1 and is closely related to the NaCl-type crystal of the MnS. Selenium and tellurium are also added to steels for machinability enhancement. Like sulfur, selenium combines with manganese to give MnSeo Much less is known, however, about the deformation of MnSe except to surmise that it is comparable to MnS because they have related crystal structures. This fact has suggested that the two might form solid solutions with each other, particularly since some sulfur is always present in steels to which selenium is added. The purpose of this study was to determine the phase relationships in the MnSe-MnS system. This information will then be available in analyzing the selenium-sulfur relationships in free-machining steels0 RELATED SYSTEMS Both the MnO-MnS and MnSe-MnTe systems have been studied previously0 The MnTe-MnS system is currently being studied by the present authorso MnO-MnS systems. Manganous oxide (manganosite) and manganous sulfide (alabandite) both have an NaCl-type structure, Early estimates by Wentrup2 suggested a rather extensive solid solution of MnS in MnO, and MnO in MnSo This would be by anion substitution0 A eutectic was assigned at 1280~C (2345~F) and -2 -

50 w/o MnS (45 m/o MnS), Later studies by Chao et a13 revealed that the anion substitution was much more limited than previously expected, being less than two percento The maximum sulfur substitution for oxygen in MnO was almost equal to the limit of oxygen replacement of sulfur in MnS. The eutectic temperature and composition, —12320C (2252~F) and 64 w/o MnS (60 m/o MnS), — also varied somewhat from Wentrup's report. The rather limited mutual solubility of MnO and MnS could lead one to anticipate possible limited solubilities in the MnSe-MnSo MnSe-MnTe system. These two Mn-VI compounds have been studied by Panson and Johnston' Manganous telluride has a stable NiAs structure below 1040~C where MnTe transforms to the NaCl structure. The above authors show a complete series of solid solutions between MnSe and the high temperature form of MnTe. The solidus and liquidus were not determined. An expanding two-phase field drops from the MnTe transformation temperature until at 500~C a maximum of only 12% of the selenium ions can be replaced by tellurium and only 20% of the tellurium ions can be replaced by selenium~ The greater solubility of tellurium in MnSe than sulfur in MnO may be attributed in part (a) to the greater polarizability of the heavier group VI ions, and (b) to the fact that the radius of the Te2+ ion is only 10% larger than the radius of the Se2+ ion, whereas the S2+ ion is 24% larger than the 02+ ion (Goldschmidt radii) o EXPERIMENTAL MnSe-MnS compositions were prepared by mixing and compacting high purity powders of a 1/1 stoichiometric ratio of manganese and (selenium plus sulfur)~ The powders were reacted in a graphite crucible in an evacuated pyrex tube which was heated to 600~C for one houro This compact of MnSe, -3 -

and MnS was then melted under a vacuum in a graphite crucible so a non-porous solid resulted0 The success of the above technique depends upon the controlled reaction between manganese and the Se + S so that the heat of reaction does not rupture the tube. A graphite crucible within the pyrex tube proved to be an effective heat sink0 Microscopic observations and Debye-Scherrer x-ray powder diffraction data were used in the analysis of the samples. X-ray data for the MnSe and MnS end-members matched National Bureau of Standards reported by ASTMo5 RESULTS AND DISCUSSION A complete series of solid solutions forms in the MnSe-MnS system as shown in Fig. 1o There was no evidence of phase separation at intermediate temperatureso The system is azeotropic with a minimum near 10 mole percent MnS and at 15100C, which is 25~C below 'the melting point of pure MnSe. The lattice parameter of the solid solution has a negative variation from ideality as shown in Fig~ 2. The higher polarizability of the large Se2+ ion may account for this. In qualitative terms, the Mn-S interatomic distance is less variable than the Mn-Se interatomic distance, thus giving a greater than proportional reduction in the lattice constant o The sharpest diffraction lines of samples quenched from the liquid-solid range were used with the data of Fig0 2 to establish the solidus compositions of Fig. 1. Diffraction data for the 50/50 MnSe/MnS composition appeared to give a simple cubic structure with a lattice constant of 2,645A because the average atomic number of selenium and sulfur (34 and 16) is 25, —the atomic number of manganese. This supports the interpretation of anion substitution. The NaC1 lines which were extinguished because of this coincidence

are still sufficiently weak in 75/25 compositions so they may be overlooked except by careful observation. This fact should be considered in all studies of Mn(Se,S) inclusions. The solubility contrasts between MnO-MnS and MnSe-MnS is best explained in terms of ion size ratioso The Se2+ radius is only 10 percent greater than the S2+ radius (1l91A vs 1074A). As noted previously, the S2+ ion is 24% larger than the 02+ ion (174A vs 1.32A). It must be concluded that all selenide inclusions found in selenium-treated free machining steels also contain the sulfur which is presento The composition Mn(Se,S) is appropriate since it indicates anion substitution.

REFERENCES 1o H.C. Chao, et al, "Deformation of Oriented MnS Inclusions in Low-Carbon Steel," Trans. AIME, 1965, vol. 233, pp. 1227 -31, 2. H, Wentrup, "Die Bildung von Einschltssen im Stahl," Tech. Mitt, Krupp, 1937, vol. 5, p. 131. 3n H.C. Chao, et al, "The MnO-MnS Phase Diagram," Trans. AIME, 1963, vol. 227, pp. 796-7, 4o, A.J. Panson and W.Do Johnston, "The MnTe-MnSe System," J. Inorg. Nucl. Chem., 1964, vol. 26, pp. 701-3. 5e ASTM Diffraction Pattern of MnSe, Card No, 11-683 ASTM Diffraction Pattern of MnS, Card No. 6-0518 -6 -

1700 LI QU ID 1600 - ~.-.,cm aoo 1400 w IMn C*,S ) i3 80LID $OLUTION 1300 * LIQUID 1100 oo LIQUID+ SOLID O 8 0 L I D 0 SOLID X SOLIDUS SY Xs-RAtY DIFFRACTION 1000 I a I a I I I, I, MnSe 25 50 7 5 MnS MOLE PERCENT Fig. 1. MnSe-MnS system. -7 -

5.462 \\. I 5s~\. w~\ AI 5.35 5-3 ~ 5.25 5.2So2 MnSe 2 5 5 0 7 5 'n MOLE PERCENT Fig. 2. Lattice parameters of Mn(Se,S) solid solutions. -8 -