Alloys containing silicon

The investigated result of oxidation behavior in still air at 800 °C of BTl-0 based titanium alloys containing silicon, aluminium and zirconium shown that at oxidation the process of the scale formation and process of the gas saturation take place together. The parabolic law describes the common oxidation rate without dependence from duration exposure. The main influence on heat resistance makes silicon with content to 2 wt. %. This effect is connected with breaking the diffusion process in metal and scale. 

In other words, this means that in this special case the silicon atoms move as if they were unaware of the carbon atoms and were concerned only with equalizing their own concentration. On the other hand, a rigorous solution of the equations for the case of carbon shows that the carbon concentration is given as a function of y = xl TT by a relatively complicated expression in which the difference of two error functions erf (x/2 ]/Dnt) occurs. However, the theoretical analysis is confirmed by experiment as can be seen from the calculated curve which is shown together with the experimental points in Fig. 7-3 [23]. The physico-chemical explanation for the entire phenomenon is based on the fact that the carbon activity Qq is higher in the alloy containing silicon than in the alloy without silicon for the same carbon content. The effect of a small concentration of a third element (Si) upon the activity of a dissolved substance (C) can be described by means of the so-called interaction parameter ... 

New prospects for the development of coating materials resistant to high-temperature sulfide corrosion have been created by combined alloying of common metals with molybdenum and aluminum and in particular by the development of novel nanocrystalline aluminum based refractory metal alloys containing silicon which are highly resistant to both sulfidizing and oxidizing environments. 

Calcium—Silicon. Calcium—silicon and calcium—barium—siUcon are made in the submerged-arc electric furnace by carbon reduction of lime, sihca rock, and barites. Commercial calcium—silicon contains 28—32% calcium, 60—65% siUcon, and 3% iron (max). Barium-bearing alloys contains 16—20% calcium, 9—12% barium, and 53—59% sihcon. Calcium can also be added as an ahoy containing 10—13% calcium, 14—18% barium, 19—21% aluminum, and 38—40% shicon These ahoys are used to deoxidize and degasify steel. They produce complex calcium shicate inclusions that are minimally harm fill to physical properties and prevent the formation of alumina-type inclusions, a principal source of fatigue failure in highly stressed ahoy steels. As a sulfide former, they promote random distribution of sulfides, thereby minimizing chain-type inclusions. In cast iron, they are used as an inoculant. 

Sodium does not form alloys with aluminum but is used to modify the grain stmcture of aluminum—silicon alloys and aluminum—copper alloys for improved machinabiUty. Sodium—gold alloy is photoelectricaHy sensitive and may be used ia photoelectric cells. A sodium—2iac alloy, containing 2 wt % sodium and 98 wt % 2iac, is used to deoxidi2e other metals. 

Silicon Reduction. The preparation of ferrovanadium by the reduction of vanadium concentrates with ferrosiUcon has been used but not extensively. It involves a two-stage process in which technical-grade vanadium pentoxide, ferrosiUcon, lime, and fluorspar are heated in an electric furnace to reduce the oxide an iron alloy containing ca 30 wt % vanadium but undesirable amounts of siUcon is produced. The siUcon content of the alloy is then decreased by the addition of more V2O5 and lime to effect the extraction of most of the siUcon into the slag phase. An alternative process involves the... 

Zirconium metal is marketed in three forms zirconium-containing silicon—manganese, kon, ferrosiUcon, or magnesium master alloys commercially pure zirconium metal and hafnium-free pure zirconium metal. The use of zkcon for the production of zirconium metal of all three types is ca 5—8% of the total U.S. zkcon consumption. 

These are used as corrosion-resistant materials. Examples are given in Table 3.32. Some can be strengthened by heat treatment. The alloys containing substantial amounts of silicon have the best foundry characteristics and a high resistance to corrosion, but are not readily machined. 

The alloys containing less than 11% silicon have resistances to low-temperature corrosion not substantially different from those of low-silicon irons containing similar amounts of other alloying elements, and will not be further discussed in this section. 

Although the high-silicon irons are often used in circumstances which expose them to atmospheric, water or soil corrosion, they are rarely installed specifically to resist these agencies. Their corrosion resistance is such, however, that in fact no normally occurring environment ever causes serious attack. This is not to say that these irons can be regarded as stainless, and in fact alloys containing less than 14-7% silicon have been reported as becoming rusty in a moist atmosphere ... 

Attempts made to produce an alloy more resistant to hydrochloric acid have resulted in alloys containing 17-18% silicon or 14-5% silicon and chromium plus 3% molybdenum. The first is produced in Britain, and the second in the United States. The reason for the increase in resistance to hydrochloric acid of the Fe-18 Si alloy is thought to lie primarily in the increased density of the silica-rich film left on the metal by initial corrosion. The addition of 6% chromium with some molybdenum to Fe-14-5 Si causes the formation of extremely stable complex carbides with the consequent complete elimination of graphite plus the formation of a more penetration-resistant silica film, probably containing chromium in substantial quantity. 

Thompson and Tracy carried out tests in a moist ammoniacal atmosphere on stressed binary copper alloys containing zinc, phosphorus, arsenic, antimony, silicon, nickel or aluminium. All these elements gave alloys susceptible to stress corrosion. In the case of zinc the breaking time decreased steadily with increase of zinc content, but with most of the other elements there was a minimum in the curve of content of alloying elements against breaking time. In tests carried out at almost 70MN/m these minima occurred with about 0-2% P, 0-2% As, 1% Si, 5% Ni and 1% Al. In most cases cracks were intercrystalline. 

The addition of beryllium and silicon to nickel-palladium alloys gives very good high-temperature brazes, especially for alloys containing aluminium and titanium. 

For prestressed concrete, either high-tensile steel wires or occasionally bars of steel alloy containing manganese and silicon, can be used. Galvanised wires may also be used for prestressed concrete, but it is recommended that they be chromated before use. 

Although silicon compounds have been used since the earliest times, the element was not obtainable until advances in technology came about. Minerals containing silicon include sand and silicates that are widely distributed (silicon constitutes approximately 23% of the earth s crust), and they have been used in making glass, pottery, and mortar for many centuries. In addition to these uses, silicon is now highly purified for use in integrated circuits (chips) and as an alloy known as Duriron that has many uses. Silicon has the diamond structure with a density of 2.3 g/cm3. 

In addition to the types of compounds discussed so far, the group IVA elements also form several other interesting compounds. Silicon has enough nonmetallic character that it reacts with many metals to form binary silicides. Some of these compounds can be considered as alloys of silicon and the metal that result in formulas such as Mo3Si and TiSi2. The presence of Si22 ions is indicated by a Si-Si distance that is virtually identical to that found in the element, which has the diamond structure. Calcium carbide contains the C22-, so it is an acetylide that is analogous to the silicon compounds. 

Magnicol [Magnetic columnar] A process for making Alnico (an iron-based magnetic alloy containing Al, Ni, Co, and Cu) crystallize with a columnar grain structure in order to optimize its magnetic properties. Successive additions of silicon, carbon, and sulfur are made to the initial melt. 

The mischmetal was introduced into hypereutectic iron-carbon-silicon alloys containing less than 0.06% sulfur in the base, or untreated iron. Morrogh further restricted the phosphorus level to less than 0.1% ( ). ... 

Barium- and calcium-bearing manganese silicon is used as an inoculant in gray and ductile iron. The alloy contains 60—65% Si, 9—11% Mn, 4—6% Ba, 1—3% Ca, and 1—1.5% Al. The combination of barium, calcium, and manganese provides excellent chill reduction, improves the graphite structure, and minimizes section sensitivity in castings having thin and thick sections. 

---