Iron – Molybdenum – Silicon

Abstract

The constitution of the Fe-Mo-Si system has been of interest since 1955, owing to the presence of the MoSi2 compound in the Mo-Si binary system. This compound is known to have high melting point and excellent oxidation resistance, serving as a basis for high temperature structural applications. The Fe rich ternary Fe-Mo-Si alloys were considered by the researches in the following decades as prospective for the development of materials with soft magnetic characteristics, greater hardenability, better mechanical properties. In order to investigate the solubility of iron in MoSi2, [1956Fit] studied the phase relationships in the FeMo-Si ternary system in the region of up to about 40 at.% Mo. The results were presented as an isothermal section at 1400°C. From this section, it can be seen that up to 10 at.% Fe can be dissolved in αMoSi2. Two ternary phases were also presented in this section, one of which having close to equiatomic composition. A number of the binary phases shown in the section, such as β, η, α and ζα, should not be observed at 1400°C if the Fe-Si phase diagram as presented by [Mas2] is taken into account. [1961Vog] studied the constitution of the Fe-Mo-Si system in the Fe-Mo-MoSi2-FeSi region at the temperature of the liquid-solid transformation. Six vertical sections were constructed from which [1961Vog] derived the liquidus surface projection. A ternary phase of the MoFe2Si2 composition was found. The microhardness of some alloys was also measured. [1985Ray, 1988Ray], in their thorough assessment of the work of [1961Vog], removed some of the discrepancies in the liquidus projection that arose through conflicts with more recent data relating to the binary systems and proposed hypothetical reactions for the uninvestigated region of the system. Along with the work of [1956Fit] and [1961Vog], the assessment also reviewed the studies reported by [1960Gla, 1962Gla, 1963Bar, 1965Sko, 1966Sko, 1968Gla, 1969Kar]. All of these publications concern the phase relationships in the solid state of the ternary system. [1960Gla, 1962Gla, 1963Bar] studied alloys with compositions lying along the 33 at.% Mo section to investigate the possibility of the formation of a ternary Laves phase. The existence of this phase, with a crystal structure of the MgZn2 type, was shown at an equiatomic composition as single phase; it being found as a second phase in other alloys along the 33 at.% Mo section up to the composition MoFeSi. The phase was interpreted by [1963Bar] as a solid solution of Si in the binary MoFe2 phase. [1969Tes] considered the electronic and structural factors leading to the formation of a Laves phase of this type. [1965Sko] investigated whether a phase with a crystal structure similar to the R phase was present in the Fe-Mo-Si system and reported that the Mo5Fe3Si2 ternary phase, having the hexagonal crystal structure attributed to R type phases, was observed in as cast alloys. The intermediate ternary phases forming in the Fe-Mo-Si system and their relationship with other phases of the system were studied at 800°C by [1966Sko, 1968Gla]. The existence of the Mo5Fe3Si2 [1965Sko] and MoFe2Si2 [1961Vog] ternary phases was confirmed. The Mo3FeSi and (Mo0.17Fe0.33)5Si3 ternary phases were found for the first time. An isothermal section for the Fe-Mo-Si system at 800°C was constructed. [1998Yi] examined the possibility of modifying the crystal structure and microstructure of MoSi2 by alloying with transition elements, including iron, in an attempt to improve mechanical properties. Compositions of the Fe-Mo-Si system that are enriched in iron are also of practical interest [1969Kar, 1988Nak, 1988Kum, 2002You, 1998Mie]. [1969Kar] reported on the substantial hardening observed after ageing alloys in the Fe corner of the system at 550–750°C. [1988Nak] studied the effects of rapid quenching on the suppression of ordering of the α phase. Taking into account the importance of information regarding the influence of alloying element on the stability of the γ phase during heat treatment, [1988Kum] used a subregular solution model with a contribution to the Gibbs energy from magnetic ordering to calculate the effect of Mo and Si on the stability of the γ phase in the ternary Fe-Mo-Si system at various temperatures. [2002You] studied the effect of Si additions on sintering and mechanical properties, as well as the effect of composition and cooling rate on sinter hardening characteristics of Fe-1.5 mass% Mo powders. Fe–Mo–Si 1