Silicon nickel-base alloys

Most structural PMCs consist of a relatively soft matrix, such as a thermosetting plastic of polyester, phenolic, or epoxy, sometimes referred to as resin-matrix composites. Some typical polymers used as matrices in PMCs are listed in Table 1.28. The list of metals used in MMCs is much shorter. Aluminum, magnesium, titanium, and iron- and nickel-based alloys are the most common (see Table 1.29). These metals are typically utilized due to their combination of low density and good mechanical properties. Matrix materials for CMCs generally fall into fonr categories glass ceramics like lithium aluminosilicate oxide ceramics like aluminnm oxide (alnmina) and mullite nitride ceramics such as silicon nitride and carbide ceramics such as silicon carbide. 

Chromizing and Related Diffusion Processes. Chromizing is similar to aluminizing. A thin corrosion and wear resistant coaling is applied to low cost steels such as mild steel, or to a nickel-based alloy. In the related boroni/ing process, a thin boron alloy is produced for extreme hardness, wear, and corrosion resistance. Siliconizing is yet another process used especially lor coaling of the refractory metals Ti. Nb. Ta. Cr. Mo. and W. 

Based on the chemical composition, corrosion-resistant nickel-based alloys consist of commercially pure nickel. Ni-Cu alloys, Ni-Mo alloys, Ni-Cr-Mo alloys, and Ni-Cr-Fe alloys.63 The cast versions of the nickel-based alloys do not have the same corrosion resistance as the corresponding wrought products, mainly due to the higher carbon and silicon contents and the anisotropic microstructure of the cast products. (Rebak)5... 

Because of the broad variation in composition and response to thermal treatment of the nickel-base alloys, it is not possible to generalize mechanisms responsible for developing susceptibility to intergranular corrosion. Therefore, the following discussion of the behavior of a Ni-Mo-Cr alloy is used to illustrate the complexity of an interrelationship between alloy composition, heat treatment, corrosion environment and corrosion rate. The alloy has the nominal composition in weight percent of 14.5 to 16.5 Cr, 15 to 17 Mo, 3 to 4.5 W, and 4 to 7 Fe with maximum limits on carbon and silicon. The alloys for which the corrosion data are shown in Fig. 7.59 contained 0.045 to 0.06 wt% carbon and 0.53 to 0.80 wt% silicon and were initially quenched from 1225 15 °C (2235 25 °F), which produced a dispersion of M6C type carbides (M = Mo, W, Si) in austenite (Ref 94). These carbides were not involved in the subsequent corrosion behavior or heat treatments. Heat... 

Ceramics, particularly new ceramic composites, are widely used in the cutting-tool industry. For example, alumina reinforced with silicon carbide whiskers (extremely fine fibers) is used to cut and machine cast iron and harder nickel-based alloys. Ceramic materials are also used in grinding wheels and as abrasives because of tiieir exceptional hardness (Table 12.4). Silicon carbide is the most widely used abrasive. 

Impurities, such as phosphorus, sulfur, carbon and silicon, segregate at the grain boundary and cracking contribute to the SCC steels and nickel base alloys. 

Silicon. Silicon is typically present only in minor amounts in most nickel-base alloys. In alloys containing significant amounts of iron, cobalt, molybdenum, tungsten, or other refractory elements, the level of silicon must be carefully controlled because it can stabilize carbides and harmful intermetallic phases. However, the use of silicon as a major alloying element has been found to greatly improve the... 

Soft magnetic materials are characterized by high permeabiUty and low coercivity. There are sis principal groups of commercially important soft magnetic materials iron and low carbon steels, iron—siUcon alloys, iron—aluminum and iron—aluminum—silicon alloys, nickel—iron alloys, iron-cobalt alloys, and ferrites. In addition, iron-boron-based amorphous soft magnetic alloys are commercially available. Some have properties similar to the best grades of the permalloys whereas others exhibit core losses substantially below those of the oriented siUcon steels. Table 1 summarizes the properties of some of these materials. 

Discaloy [Westinghouse], TM for an austenitic iron-base alloy containing nickel, chromium, and relatively small proportions of molybdenum, titanium, silicon, and manganese. This alloy is precipitation-hardened and was developed primarily to meet the need for improved gas-turbine disks, one of the most critical components of jet engines. 

The capacity of the 18,650 cell appears to have reached its practical limit of 2.9 Ah based on the present graphite and planar nickel-based cathode in 2007. Further improvement in capacity is expected to be realized from the development of a silicon alloy type anode with a capacity of 700 mAh/g or more and the planar lithium-nickel-cobalt-aluminum and nickel-manganese-cobalt cathode materials with capacities approaching 250 mAh/g. New electrolytes and/or additives also are under development. 

The cell producers accomplished the performance improvements through engineering improvements in cell design, new electrode materials, and automated high-speed production to reduce the cost. The capacity of the 18650 cell had reached 2.9 Ah in 2007 based on treated graphite anode and planar-nickel-based cathode and with several kinds of electrolyte additives [12]. With further continuous improvements in all the cell components that includes silicon alloy-type anode materials, lithium-nickel-cobalt-aluminum and nickel-manganese-cobalt cathode materials, novel electrolyte and/or additives, some cell manufacturers are currently able to achieve a maximum capacity of up to 3.4 Ah for the same 18650 cell design. 

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