Solution and Precipitation Hardening in (Ca, Mn) Sulfides and Selenides
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I. I N T R O D U C T I O N
THE compounds CaS, MnS, MnSe, and CaSe are mutually soluble at elevated temperatures. L2As a result, sulfide and selenide inclusions of calcium and manganese are single-phase at the rolling temperatures of calcium-killed steels. All of the above compounds possess the NaCl-type structure. Those pairs with anion substitutions, Ca(S,Se) and Mn(S,Se), retain their mutual solubility at lower temperatures. Those pairs with cation substitutions, (Ca,Mn)S and (Ca,Mn)Se, separate into immiscible phases at lower temperatures; however, the precipitation rates are sufficiently slow so that the solution hardening may be measured at subsolvus temperatures. Furthermore, as we shall see, the delayed immiscibility produces precipitation hardening and eventual overaging. Thus, this quaternary system provides a simple example of hardening processes in nonmetallic, inorganic materials. II. PREVIOUS W O R K The extent of solid solution between the manganese chalcogenides and the related compounds of other transition elements has been reported in several studies? -6 In addition, specific attention has been given to the (Ca,Mn) chalcogenides by Leung and Van Vlack) They observed that solid solubilities progressively decrease in the systems CaS-MnS, CaSe-MnSe, and CaTe-MnTe. In CaS-MnS, full solid miscibility extends from 1150 to 1500 ~ the liquidus minimum. With CaSe-MnSe, this range is 1070 to 1340 ~ The solid miscibility gap between CaTe and cubic MnTe intercepts the liquidus as a eutectic at 1050 ~ C-H. LEUNG is Senior Project Engineer, GTE Products Corporation, Gibson Electric Division, Delmont, PA 15626. L. H. VAN VLACK is Professor of Materials Engineering, Department of Materials and Metallurgical Engineering,The University of Michigan, Ann Arbor, MI 48109. Manuscript submitted February 18, 1980. METALLURGICALTRANSACTIONSA
The solubility of calcium in MnS and its effect on the hardness of the latter was first observed by Chao. 7 The presence of 1 pct Ca increased the diamond pyramid hardness by approximately 25 pct at 25 ~ The addition of calcium to steel as a deoxidizer introduces calcium into the sulfides that are present as nonmetallic inclusions, s Such inclusions deformed less than calcium-free steel, and, therefore, affected the machinability process. 9 Hilty and FarrelP ~ observed that calcium additions to manganese sulfide altered the deformation characteristics of the latter significantly, so as to affect the behavior of steels with MnS inclusions. Most important were the hot-workability of the steel and the reduced anisotropy of mechanical properties, each of which may be assigned to the increased hardness of the sulfide. Thus, there is an established interest in the role of the hardening mechanisms in these binary compounds. Previous work by Riewald and Van Vlack" showed the hardness effects of anion substitution in Mn(S,Se) solid solutions. Similar work has not been performed in Ca(S,Se) solutions. The above manganese compounds revealed solution hardening with a maximum at approximately 70
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