Other phases

Phase of osumilite type - MgO x Al2O3 x SiO2
This phase is metastable but can be formed in the temperature range 1050-12500C. It has not been observed in non-metallic inclusions (Kiessling and Lange 1964-1968).

Phase of petalite type - MgO x Al2O3 x SiO3
This phase is metastable but can be formed in the temperature range 900-1000 0C. It has not been observed in non-metallic inclusions.

Cubic. The lattice parameter varies due to a varying oxygen content and due to a wide range of solid solubility between TiO, TiC and TiN. Isostructural with manganosite, wustite, periclase and calcia. It is questionable whether TiO appears as deoxidation product, however thsi oxide is of metallurgical importance since oxygen, nitrogen an carbon all are soluble in the fcc metal lattice of TiX. TiN appears as a precipitate in several steels containing Ti, where it forms idiomorphic, regular, yellowish crystals usually in the grain boundaries. These precipitates often hold both carbon and oxygen in solid solution.

A high and low-temperature modification are known. Only the low-temperature form, alpha-Ti2O3 is of metallurgical interest. It is hexagonal and isostructural with corundum, eskolaite and hematite.

Ti3O5 - Anosovite
The phase is monoclinic and only stable above 1200oC, but it may be supercooled down to room temperature, however could not identify in in situ in the same material adn the nature of appearance in inclusion is unknown.

TiO2 - Rutile
Tetragonal, cassiterite type. Two other modifications of TiO2 are known, brookite and anatase. TiO2 as rutile is a component of several refractories and also of different types of welding electrodes. The metals Cr, V and Nb are soluble in the rutile phase but not Fe, Al and Mn.

FeO x TiO2 - Ilmenite
The phase is hexagonal and structurally similar to haematite. At higher temperatures a complete range of solid solubility exists, but and immiscibility gaps forms at lower temperatures. The same type of substitution is also possible between ilmenite and corundum and eskolaite, a factor of importance for inclusion formation.

Cubic, inverted spinel type. Within inclusions the phase may therefore have other double oxides of spinel type in solid solution and converselt, tiatnium is often to be found as a solid solution in the spinel inclusion phases. Several other phases with titanium are of interest as possible constituents of inclusions. For example MnO x TiO2 and MgO x TiO2. Altrough CaO x TiO2 is also known its structure is different. Some ternary phases, like CaO x SiO2 x TiO2 also are known. Many of the Ti-O phases are therefore possible as oxide component of inclusions. Ti is usually found in solid solution. Zirconium is closely related to titanium both from a crystallographic and a metallurgical point of view. Hafnium is also very similar to titanium, but little is known about its oxide compunds (Kiessling and Lange, 1964-1968).

Is isostructural with the other ternary inclusion oxides Al2O3, Cr2O3, Fe2O3 and alpha-Ti2O3. Double oxides of the spinel type and with extended homogeneity ranges are known, such as FeO x V2O3, MnO x V2O3 and MgO x V2O3. A compund with calcia, CaO x V2O5, has also be found to exist.

Is monoclinic. Double oxides of the inverse spinel type are known, of which are of interest as possible inclusion phases.

Is orthorombic. Calcium vanadates and magnesium-vanadates with V5+ are known. Nb and Ta usually have the valency 5, and several oxide compounds of these metals with Mn, Fe, Mg and Ca are known. No pure vanadium phases have been observed in oxide inclusions during the present investigarion (Kiessling and Lange, 1964-1968), but in several inclusions vanadium was found in solid solution in the spinel phase. These inclusions were all present in steel high in vanadium (high speed steels, tool steels with higher vanadium).

Sulfide inclusions, (Mn, Me)S type and double sulfides of the MnS x B2S3 type
Alpha-MnS can take considerable ammounts of other transition metals as well as Ca and Mg, into substitutional solid solution (studies of Kiessling and Lange 1964-1968). A systematic variation was found for the solid solubility limit, with maximum solubility range of as much as 60-70 wt. % for Cr and Fe in the alpha-MnS lattice. The microhardness of the solid solution is reported to be considerably higher than for pure MnS. There are several of inclusion of this type, such as: (Mn,Ti)S, (Mn,V)S, (Mn,Cr)S, (Mn, Fe)S, (Mn,Co)S, (Mn, Ni)S. Solid solution of MnS with non-transitional metals have also been observed, for instance (Mn,Ca)S and (Mn,Mg)S. Double sulfides of the AS x B2S3 type are also known (Kiessling and Lange, 1964-1968). The metal A may be Mn, Fe, Co or Ni and the metal B may be Cr.

Sulfide inclusions with transition metals of group III (the lanthanides)
Only the lanthanides (La, Ce....Lu) are interest in steelmaking. They all have a high affinity for oxygen and sulphur. Many intermediate phases have been identified in different Me-S systems of the rare earth metals. In general, the different systems have intermediate phases with the following ideal composition: MeS, Me5S4, Me3S4, MesS3, MeS2, MesO2S. The phases of great interest for inclusions in steel seem to be MeS, Me2S3, and Me2O2S. The phases of MeS-type usually have an extended homogeneity range with metal vacancies, and their formula should therefore be given as Me(1-x)S. The Me2S3 phases exist in several modifications, for example as La2S3, Ce2S3, Pr2S2, Nd2S3, Sm2S3, Gd2S3, Dy2S3, Y2S3. Several of the sulfides of the rare earth metals are very stable compunds with higher melting point, for instance CeS melts at about 2450oC. There are sveral sulfides and oxysulfides of interest in steelmaking, such as YS, LaS, CeS, PrS, NdS, Y2S3, La2S3, Ce2S2, Pr2S3, Nd2S3, Y2O2S, La2O2S, Ce2O2S, Pr2O2S, Nd2O2S.

Sulfide inclusions with transition metals of group IV (Ti, Zr, Hf)
The transition metals of group IV are all strong sulfide formers. Titanium: They are as follows: Ti6S, TiS(1-x), Ti(1-x)S, tau-phase. Several titanium sulfides were found as inclusions in steels, and the sulfide formula usually being given as TiS. Ziconium: The sulfide with higher Zr content is ZrS. A Zr sulfide with metal vacancies may exist, wiyj the formula Zr(1-x)S. The colour of the phase in the optical microscope is to be dependent of Zr content of the sulfide, changing from light yellow for high Zr contents to blue-grey for lower. For low Zr contents the sulfide is similar to MnS and it may be difficult to distiguish between the two phase. Zr sulfide is harder, however, and will not deform during deformation of the steel matrix. Hafnium: Little is known about the system Hf-S (Kiessling and Lange, 1964-1968).

Sulfide inclusions with transition metals of group V (V, Nb, Ta)
The transition metals of group IV are moderate sulfide formers in steels. V and Nb may substitute for part of the manganese in MnS. Vandium: The sulfide with higher vanadium content is V3S, but sulfides of tye type VS(1-x) and V(1-x)S are also known. Niobium: The sulfide pahse, which is richest in niobium, seems to be NbS(1-x). In the few references, where niobium sulfide inclusions are described, the phase has been called NbS. It is reported to be hard, anisotropic phase. Tantalium: No inclusions in steel with tantalium sulfides have been reported (Kiessling and Lange, 1964-1968).

Sulfide inclusions with transition metals of group VI (Cr, Mo, W)
The transition metals of group VI are weak sulfide formers in steel. Chromium: The main occurrence of chromium in sulfide inclisions therefore seems to be as (Mn,Cr)S phase. The sulfide with the higher chromium content has the formula CrS. Chromium sulfide inclusions in both steel and in cast iron have been mentioned by several authors. No reliable determination of the composition of the chromium sulfide inclusions has been reported , the formulae CrS and Cr2S3 are given but should be used with care.Molybdenum: Five intermediate phases have been reported in the system Mo-S, Mo2S3 being the phase with the highest molybdenum content. Mo sulfides have not been found as steel inclusions (Kiessling and Lange, 1964-1968). Tungsten: Tungsten sulfide inclusions in steel have not been reported.

Sulfide inclusions with transition metals of group VII (Co, Ni)
Sulfide inclusions with cobalt and nickel sulfides should therefore only be present in steel if they have very high cobalt or nickel contents and only if they are at the same time low in manganese. MnS has a range of solid solubility for both Co and Ni. It should also be noted that cubic chromium double sulfides of the spinel type are formed both by CoS and NiS. The resulting sulfides CoS x Cr2S3 and NiS x Cr2S3, are isostructural with the corresponding MnS x Cr2S3 phase.

Sulfide inclusions with non-transition metals of group I (Cu, Ag, Au)
Cooper is the only member of this group of interest as a possible sulfide former in steel, but very few references to copper sulfide inclusions (Cu2S) in steel have been reported.

Sulfide inclusions with non-transition metals of group II (Mg, Zn, Cd, Hg, Ca, Ba, Sr)
The most important sulfides of this group in steel practice are MgS and CaS. Both these sulfides have a low free energy of formation. CaS is a common inclusion phase. It often appears as a scale round oxide inclusion with CaO. CaS inclusions in steel become more frequent if the steel is vacuum-treated. No pure MgS inclusions were found in steel, but inclusions (Ca, Mg)S type are known. No information about steel inclusions with sulfides of other member of group II has been found (ZnS, CdS, HgS, BaS, SrS).

Sulfide inclusions with non-transition metals of group III (Al)
Only Al is of interest in this group, and this metal is comparatively weak sulfide former in steel practice. The most important sulfide of aluminium is the phase Al2S2. This phase is not very coomon.

Sulfide inclusions with non-transition metals of group IV (Sn, Pb)
Two intermediate phases are reported in the Sn-S system, SnS and SnS2. In the Pb-S system a cubic, intermediate phase has been found, PbS. These intermediate sulfides are, however, not been found in steelmaking where the tendency of these elements to form sufides are to small compared with the other metals present. Thus metallic lead has been observed in the form of tails around MnS inclusions where lead has been added in order to increase the machinability of steel.

Sulfide inclusions with S partly substituted by the non-metals C, O and N
Carbosulfides of interest in steelmaking have been observed with evidence of the phase (Ti,Fe)4C2S2 and Zr4C2S. These phases are sometimes reported as formed in high-carbon, sulphur-rich steels with Ti and Zr. Oxysulfides of interest in steelmaking are formed by lanthanides. The most common type has the composiyion Me2O2S with hexagonal simmetry. Nitrosulfides of interest in steelmaking have not been described (Kiessling and Lange, 1964-1968).

Inclusions with selenium (Se) and tellurium (Te)
Selenium is sometimes added to steels in order to incerase th machinability. Mn-Se system is very similar to the MnS system. A monoselenide MnSe, as well as diselenide MnS2 are known. Since there is a wide range of solid solubility bwtween alpha-MnS and alpha-MnSe, it is conceivable that MnS inclusion phase in steels containing selenium is a solid solution of the type (Mn,Me)(S,Se). Tellurium belongs to the same group as sulphur and selenium. Tellurium additions to steel have been tried (Kiessling and Lange, 1964-1968) in order to increase its machinability. Two intermediate phases are known, MnTe and MnTe2. In free cutting steels with bith Te and S, multiphase inclusions with MnS and MnTe are often formed. MnS phase often has Fe and Cr in solid solution, as well as a small ammounts of Te. MnTe often has small amounts of S and Cr in solid solution. MnTe has a low melting point, about 1150oC, and in steel deformed above this temeprature, this telluride is therefore molten.

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