Rare-earth silicides

Rare-Earth Silicides. Rare-earth sihcides, in the form of a ferroalloy that contain up to 33% rare earths, are used increasingly by the iron and steel industries. Whereas the term sihcides is no longer used for alloys of this type, it is stih in common usage for these materials. Eor nodular iron, addition... 

Figure 6. Photographs of CRT output from characteristic x-ray scans for the various elements listed using electron microprobe flOj. Particle scanned was a graphite nodule. Scans verify presence of a heterogeneous inclusion in the nodule. Iron was treated with magnesium and a rare earth silicide. The element distribution pictures were taken at ISOOX-...

The various rare earths are used in the foundry industry as rare earth silicides, in which the rare earth content is about 30%. Other alloys are used in which the level of rare earths is about 10% (10% cerium, 2% other rare earths) with silicon and iron comprising the bulk of the remaining elements. In the magnesium-ferrosilicon alloys, the rare earths are present in amounts from about 0.1% to 1.0%. These alloys are used differently by the various consumers. However, the effects of the rare earth elements, introduced by whatever means, are the same. 

Figure 16. Shallow surface crack at the outer radius of a sharp V2T bend on 80,000 psi steel sheet (top). Corresponding subsurface concentration of REM oxysulfides and sulfides in a slab cross section near the surface. The parent ingot was treated with 5 lbs of rare earth silicide per ton of ingot steel (bottom). The bottom picture is from a Baumann print or sulfur print, not sensitive to the oxides and thus eliminating the argument of reoxidation as main cause of surface defects in REM treated steels. Magnification, 2.5X-...

All rare earth metals can be characterised as being electropositive with respect to most other metals this fact, coupled with their large atomic radius and high reactivity towards non-metals, points the way to their widespread use as alloying constituents. However, in the manufacture of ductile iron and in steelmaking it is practice to use mischmetal or mixed rare earth silicides as the rare earth additive and from purely cost considerations this situation is unlikely to change significantly. 

The above process is applicable in all those cases in which both the metal and its oxide (which is reduced) have low vapor pressures at the reaction temperature (this is true of transition metals of Groups III to VIII). For instance, the process yields pure rare earth silicides, which are otherwise difficult to obtain. In this case the optimiun reaction pressure is approximately 0.1 mm. 

Chlorine solubility is essentially restricted by the formation of silicon tetrachloride and, in the presence of hydrogen, of trichlorosilane. The suppression of these reactions, according to our calculations, must lead to the metastable ternary compounds nickel silicide chlorides . This thesis is supported by earlier theoretical calculations [11], which indicated a possible existence of calcium silicide bromides [12], and furthermore by the preparation of rare earth silicide iodides , rare earth chlorid silicides and rare earth bromids silicides [13]. 

Refractory carbides of silicon, boron, and group 4-6 elements Borides of group 4-6 transition elements and rare earths Silicides of group 4-6 metals, in particular Ti, Mo, W Simple oxides of Al, Si, Sn, and group 4-6 metals Mixed oxides such as perovskites... 

Rare-Earth Silicides Surface and Interface Structure... 

Parth6, E. and Chabot, B. (1984) Crystal structures and crystal chemistry of ternary rare earth-transition metal borides, silicides and homologues. In Handbook on the Physics and Chemistry of Rare Earths, ed. Gschneidner Jr., K.A. and Eyring, L. (North-Holland, Amsterdam), Vol. 6, p. 113. 

Metallurgy. The strong affinity for oxygen and sulfur makes the rare-earth metals useful in metallurgy (qv). Mischmetal acts as a trap for these Group 16 (VIA) elements, which are usually detrimental to the properties of steel (qv) or cast iron (qv). Resistance to high temperature oxidation and thermomechanical properties of several metals and alloys are thus significandy improved by the addition of small amounts of mischmetal or its silicide... 

E. Parthe and B. Chabot, Crystal structures and crystal chemistry of ternary rare earth-transition metal borides, silicides and homologues 113... 

Meanwhile some rare earth metal nitridosilicates LnsSisNn (Ln = La, Ce, Pr, Nd, Sm) and LnSisNs (Ln = La, Ce, Pr, Nd) have been synthesized by the reaction of the respective silicides LnSi2 and LnSis with N2. However, a problem of this procedure is the avoidance of metallic impurities of the products [13]. 

The enthalpies of formation of binary manganese-silicon, rare earth-tin, and rare earth-lead °" ° intermetallics have been measured by independent workers. Muradov has calculated the enthalpies of formation of manganese silicides (Table 26), on the basis of reaction (43), from the results of a... 

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