Phase relationships in the iron-rich Fe-Cr-Ni-C system at solidification temperatures
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I.
INTRODUCTION
THEsolute elements Cr, Ni, and C in iron constitute the compositional base of a major class of corrosion-resistant alloys. Many compositions of these alloys have two or more of these elements at high concentrations. In such cases, the phase relationships in the multi-component system cannot be known accurately, simply by extrapolation from the limiting binary systems, or by calculation employing only the binary interaction parameters in a thermodynamic model of the alloy system. The relationships have to be determined either by direct measurement in the composition polyhedron, or by calculation employing measurements of the Gibbs free energy of the participating phases in the multicomponent system. This has been demonstrated by the authors to be the case for the iron-rich ternary subsystems of the Fe-Cr-Ni-C system. ~'2 Thus, in the quaternary system, one expects that measurements are needed to depict accurately the effect of a fourth component on the phase relationships. On the other hand, as the number of components is increased, the influence of each on the behavior of the alloy system becomes less. This can be observed mathematically in a thermodynamic model by examination of the effect that each additional component exerts on the total free energy of the multi-component system. Nonetheless, measurements, albeit fewer in number, are still needed to verify where extrapolations are permissible, either directly in the composition polyhedron, or by calculations employing only the thermodynamic parameters for the subsystems. This study was part of a larger study of the iron-rich Fe-Cr-Mn-Ni-C system, in which the Fe-Cr-Ni-C, the FeMn-Ni-C, and the Fe-Cr-Mn-C quaternary subsystems were studied. This analysis includes a study of all of the iron-rich binary and ternary subsystems. Those pertinent to the D.M. KUNDRAT,formerlyResearch Fellow, Massachusetts Institute of Technology,is Senior Research Engineer, Armco Inc., Middletown, OH 45043 and Adjunct Professor, Department of Materials Science and MetallurgicalEngineering,Universityof Cincinnati, Cincinnati, OH. J. E ELLIOTTis Professorof Metallurgy, Departmentof Materials Science and Engineering,MassachusettsInstituteof Technology,Cambridge, MA 02139. Manuscript submittedJune 3, 1985. METALLURGICALTRANSACTIONSA
present study are the binary subsystems Fe-C, 1 Fe-Cr, 1 and Fe-Ni 2 and the ternary subsystems Fe-Cr-C, ~Fe-Ni-C, 2 and Fe-Cr-Ni, 2 and these have been or will be published elsewhere. Thus, the present paper is confined to an analysis of only the quaternary composition field of the Fe-Cr-Ni-C system.
II.
PREVIOUS M E A S U R E M E N T S
A. Unary Systems Fe, Cr, Ni, and C Table I lists the equilibrium transformation temperatures of the pure elements accepted in this study. The symbols and y refer to the bcc and fcc structures, respectively, and L and Gr refer to the liquid phase and graphite, respectively.
B. Binary Systems Fe-C, Fe-Cr, Fe-Ni, and Cr-Ni The quaternary system Fe-Cr-Ni-C has six binary subsystems that form the edges of the F
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