Solidification

Many solids, especially metals, are produced from liquid precursors, and control of solidification is important in the development of the appropriate microstracture. Rapid solidification can lead to amorphous or poorly crystalline products. Slow cooling can lead to the formation of large crystals or single crystals. As these observations indicate. 

There are two important steps involved. Nuclea-tion is the initial formation of tiny crystallites. As a liquid cools, small volumes tend to take on a stmcture similar to that of crystals, which will ultimately form. This occurs especially at mould edges, on dust particles and so on, which act as sites for nucleation. Nucleating agents can be added deliberately to cause this to happen. The formation of nuclei is suppressed during glass formation. If only one nucleus forms, a single crystal is produced. If many nuclei form, a polycrystaUine soUd results. 

The ultimate microstructure of a solid will depend on how quickly different crystal faces develop. This controls the overall shape of the crystallites, which may be needle-like, blocky or one of many other shapes. The shapes will also be subject to the constraint of other nearby crystals. The product will be a solid consisting of a set of interlocking grains. The size distribution of the crystallites will reflect the rate of cooling of the solid. Liquid in contact with the cold outer wall of a mould may cool quickly and give rise to many small crystals. Liquid within the centre of the sample may crystallise slowly and produce large crystals. Finally, it is important to mention that the microstructure will depend sensitively on impurities present. This aspect is discussed in Chapters 4 and 8. 

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According to the testing problems referring the analysis of casting solidification and... 

In the converse situation free of gravity, a drop assumes a perfectly spherical shape. At one point, the U.S. Space program tested this idea with the solidification of ball bearings from molten metal drops in microgravity conditions. 

Mix I ml. of benzaldehyde and i ml. of aniline in a small evaporating-basin, place the latter on a boiling water-bath and stir the mixture gently with a glass rod. Globules of water soon appear on the oily layer. After about 20 minutes place the basin in ice-water, and stir the contents well, whereupon solidification should rapidly occur. (If the material does not solidify, replace the basin on the boiling water-bath for a further 10 minutes.) Break up the solid material in the basin, transfer to a conical flask, and recrystallise from rectified spirit. The benzylidene-aniline is obtained as colourless crystals, m.p, 52° yield, o-8 g. 

The use of impure reactants may give a deep red syrup and delay considerably tbe final solidification they should, therefore, be redistilled in a vacuum before use. 

The first part of this preparation (as far as the solidification of the sodium cyanoacetate) must be carried out in the fume-cupboard. Add 30 g. of monochloroacetic acid to 60 ml. of water contained in a wide evaporating-basin (about 12-15 diameter)... 

Aqueous solutions do not usually ignite even though the solute is highly inflammable, e.g., an aqueous solution of ethanol containing less than 50% of the latter. When aqueous solutions of solid substances are heated on a crucible lid, they usually "spit" vigorously immediately before solidification. 

Some less reactive tertiary amines can be mixed with an excess of methyl toluene-/)-sulphonate, m.p. 28 , and the mixture (without a solvent) heated to a much higher temperature. The mixture is allowed to cool, but before solidification occurs, it is thoroughly stirred with ether to extract unused sulphonate, and the insoluble quaternary metho-toluene-/)-sulphonate may then crystallise. If ciystallisation does not occur, dissolve this residue in ethanol and treat one portion with ethanolic picric acid (to precipitate the methopicrate) and another portion with cold concentrated ethanolic sodium iodide (to precipitate the methiodide). (M.ps. of the siilphon.ates, pp. 553 -554.)... 

The composition of the liquid will therefore pass along bg the composition of the solid will at the same time follow the curve fc. At the point c, the last traces of liquid of composition gfare just disappearing and solidification... 

Place an intimate mixture of 125 g. of powdered, anhydrous zinc chloride and 26-5 g. of acetophenonephenylhydrazone in a tall 500 ml. beaker in an oil bath at 170°. Stir the mixture vigorously by hand. After 3-4 minutes the mass becomes hquid and evolution of white fumes commences. Remove the beaker from the bath and stir the mixture for 5 minutes. Then stir in 100 g. of clean, white sand in order to prevent solidification to a hard mass. Digest the mixture for 12-16 hours on a water bath with 400 ml. of water and 12 ml. of concentrated hydrochloric acid in order to dissolve the zinc chloride. Filter off the sand and the crude 2-phenylindole, and boil the solids with 300 ml. of rectified spirit. Treat the hot mixture with a little decolourising carbon and filter through a pre-heated Buchner funnel wash the residue with 40 ml. of hot rectified spirit. Cool the combined filtrates to room temperature, filter off the 2-phenylindole and wash it three times with 10 ml. portions of cold alcohol. Dry in a vacuum desiccator over anhydrous calcium chloride. The yield of pure 2-phenylindole, m.p. 188-189°, is 16 g. 

There is a strong tendency for gallium to supercool below its freezing point. Therefore, seeding may be necessary to initiate solidification. 

Note 2. Further cooling may result in partial solidification. 

Fat or oil Solidification point, °C Specific gravity (15°C/15°C) Refractive index Acid value Saponification value Iodine value... 

Solidification gravity Refractive Acid Saponification Iodine... 

When we speak of the solidification of the extruded polymer, we use the term in the broadest sense It includes crystallization, vitrification, or both. The extent of the drawing of the fibers and the rate and temperature of the drawing affect the mechanical properties of the fiber produced. This conclusion should be evident from a variety of ideas presented in the last three chapters ... 

Solidification. The heat of the electric arc melts a portion of the base metal and any added filler metal. The force of the arc produces localized flows within the weld pools, thus providing a stirring effect, which mixes the filler metal and that portion of the melted base metal into a fairly homogeneous weld metal. There is a very rapid transfer of heat away from the weld to the adjacent, low temperature base metal, and solidification begins nearly instantaneously as the welding heat source moves past a given location. 

Fig. 6. Weld pool shape and resultant weld—metal solidification direction, (a) Slow welding speed, (b) Rapid welding speed.

Fig. 7. (a) Impurity elements are rejected into the Hquid between the dendritic solidification fronts, (b) Corresponding impurity concentration profiles. Cq, weld metal composition k, impurity partitioning coefficient in the Hquid maximum impurity soHd solubiHty eutectic composition at grain... 

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