Chromium – Iron – Molybdenum

Abstract

The Cr-Fe-Mo system has been studied extensively. Both Cr and Mo are important elements used in steels, in the past and in the present days. Knowledge of the phase diagram of such a system is therefore of significant interest, especially now, when the number and amount of alloying elements in steels and other alloys is increasing, and intermetallic phases are playing a more important role. One of the earliest experimental studies was carried out by [1931Wev], who examined the α/γ region in this ternary system. This information was reviewed by [1949Jae], who published the phase diagram showing the α/γ loop in the Cr-Fe-Mo system. The first information about the ternary phase, τ1, was published by [1949And, 1951And], and the structure was correctly identified. [1950Put] studied the σ phase region in the ternary system. The first study of the complete ternary system was carried out by [1951Put]. They studied about 30 ternary alloys by thermal analysis in order to determine the melting temperatures in this system and established partial isothermal sections of the phase diagram. A second experimental study of the liquidus surface of the system was published by [1957Tak], who employed a similar experimental technique to that used by [1951Put]. Further experimental studies were published by [1950Duw, 1952Bue, 1953Koh, 1954Kas, 1954McM, 1957Age, 1957Bec, 1957Gol, 1962Gri, 1964Alf, 1966Kim, 1971Yam, 1976Kie, 1981Zha]. They studied mainly phase relations and properties of solid phases in the Cr-Fe-Mo system. The only study of thermodynamic properties of this system, namely the specific heat at low temperatures, was published by [1971Bau]. Nevertheless, great inconsistencies were found in the results of various works published before 1980. The first review of this system was presented by [1971Wes], who pointed out many discrepancies in the existing data, which were discussed in extensive reviews by [1984Ray, 1988Ray]. One problem was that different versions of the Fe–Mo binary phase diagram were available at different times, resulting in misinterpretation of intermediate phases found in the ternary system. Also, the identification of the ternary phases existing in this system was complicated because of uncertainties concerning the number of such phases, their stability regions and crystal structure. The lack of information available in the reviews [1984Ray, 1988Ray] prompted more recent experimental and theoretical studies to be carried out by several research groups. A comprehensive work was published by [1988And], who experimentally and theoretically studied the complete Cr-Fe-Mo system in the temperature range 950–1200°C. Further experimental works were published by [1986Liu, 1988Liu, 1989Liu, 1990Liu], who studied partial or complete isothermal sections at 850, 1050, 1100, 1180, 1200 and 1250°C. A short review of these studies has been given by [1994Rag]. The lattice parameter of the τ1 phase was reported by [1990Ere]. Since then, no other systematic experimental study of this system has been published. Several authors have also modeled the phase equilibria in the Cr-Fe-Mo system theoretically. [1975Kau] applied the CALPHAD approach to this system. All intermetallic phases were described as stoichiometric. The most detailed theoretical modeling was carried out by [1988And], later augmented by [1992Qiu]. They also used the CALPHAD method. Currently, more attention is being paid to properties of (Cr,Mo) steels and other commercial multicomponent alloys containing Cr and Mo, however, these works do not contain new information about the phase diagram of the Cr-Fe-Mo system. Some studies concerning intermetallic phases existing in the ternary system will be discussed briefly in the Miscellaneous section. The basic studies of phase equilibria and crystal structure of the phases are listed in Table 1.