Conventional Solidification

The majority of ah these classes, even noneutectic ahoys, have been processed successfuhy by rapid solidification technology. This technology provides a beneficial alternative in the form of a flexible ductile foh when materials that are inherently brittle are used. Examples are the nickel—boron—shicon ahoys and many others, when produced using conventional technology (5). 

The next four sections will discuss some of the above work in detail. The first two sections concern solid state transformations with section (11.3.1) concentrating on steels while section (11.3.2) looks in more detail at the DICTRA programme. The last two sections concern liquid- solid transformations. Section (11.3.3) deals with conventional solidification while section (11.3.4) deals with rapid solidification. 

BNL claims that polyethylene encapsulation allows for greater waste loading and has a better waste form performance than conventional cement solidification, allowing for 70% fewer drums to be processed and shipped for disposal of some government waste streams. The technology is commercially available. 

Offers cost advantages when compared to other stabilization alternatives (2 to 5 times less expensive than conventional methods of solidification or stabilization). 

In conventional aluminum-deoxidized steels, the extra-low dissolved oxygen content maintains the sulfur in solution in the liquid stoel until the end of the solidification process. Since sulfur exhibits virtually no solubility in the solid steel — less than 0,001% in the bcc structure — a > iS-Fe eutectic suddenly precipitates at the as cast grain boundaries, the so called "Type II" structure described by Sims ( ). This weakens the as cast structure and also results in elongated MnS inclusions in the hot rolled steel plates, coils, sheets, bars, wires etc, as shown in Figure 5,... 

In most Al-containing alloys, the shape of the particles was tear-drop like due to the tight surface oxide film. The typical shape was shown in Fig.l. The effect of rapid solidification on microstructures is shown in Fig. 5 for AI2CU (precursor for Raney Cu) with a small amount of Pd (11). In the case of slowly solidified (conventional) precursor, most of the added Pd was solidified as a secondary Pd rich phase shown by white dendritic structure in Fig.5 (a). On the other hand, no such secondary phase was observed in a rapidly solidified precursor as shown in Fig.5 (b). 

Although the activity obtained by the MA route was not the highest in this case, the effectiveness of MA was still much higher than that of conventional catalyst. In some cases, MA is more effective than rapid solidification for making supersaturated precursors. An example is the Al-Co-Cu ternary system (11). The ranking of effectiveness of these methods may in general depend on the alloy system. 

Similar result was obtained for skeletal Si . The solubility of Si in the a -A1 solid solution is negligibly small so that the two phases a -A1 and relatively coarse Si existed in the conventionally prepared Al-Si alloy. By hydrochloric acid leaching, only the coarse stable Si remains after leaching, so that it is difficult to form fine Si particles by leaching. In this case, the rapid solidification increases the solubility to about 12 at%. The specific surface area of skeletal Si formed from RWA was in the range of 65-75mVg and the mean particles size was about 30-50nm by direct observation of the skeletal Si. 

In a relatively new process for production and fractionation of fine particles by the use of compressible media - the PGSS process (Particles from Gas-Saturated Solutions) - the compressible medium is solubilized in the substance which has to be micronized [58-61]. Then the gas-containing solution is rapidly expanded in an expansion unit (e.g., a nozzle) and the gas is evaporated. Owing to the Joule-Thomson effect and/or the evaporation and the volume-expansion of the gas, the solution cools down below the solidification temperature of the solute, and fine particles are formed. The solute is separated and fractionated from the gas stream by a cyclone and electro-filter. The PGSS process was tested in the pilot- and technical size on various classes of substances (polymers, resins, waxes, surface-active components, and pharmaceuticals). The powders produced show narrow particle-size distributions, and have improved properties compared to the conventional produced powders. 

Practical use of the solidification and crystallization characteristics of milk lipids has been made in the manufacture of butter which is more easily spread than butter made conventionally. Based on the knowledge that temperature and mechanical manipulation can influence crystallization behavior, various methods of working butter have been devised to produce a softer product (Taylor et al. 1971 Schaap et al. 1981). Another approach has been to separate triglyceride fractions according to solidification or melting ranges and reblend fractions to achieve a softer butter (McGillivray 1972 Black 1975 Frede et al. 1980). 

The second limiting case approximates conventional metallurgical casting processes in which the cooling rate is on the order of 10-3 to 10° K/s. As a result, the solidification rate is several orders of magnitude too fast to maintain equilibrium. The most widely used classical treatment of nonequilibrium solidification is by Erich Scheil (Scheil, 1942), who was at the Max-Planck-Institute for Metals Research in Stuttgart. The model assumes negligible solute diffusion in the solid phase, complete diffusion in the liquid phase, and equilibrium at the solid-liquid interface. In this case, Eq. 4.16 can be rewritten as... 

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