Solidification direction

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

Directional solidification to produce fiber reinforced composites with high strength and elastically stiff discontinuous Ti5Si3 fibers aligned with the solidification direction. 

The orientation of the lamella tends to be along the direction of heat flow. In polycrystalline grains solidified from a melt of eutectic composition, the orientation is more or less random but the spacing of the lamella is indicative of the rate the material was solidified. However, if a eutectic composition is directionally solidified, the lamella will be aligned along the solidification direction to form what is known as an in situ composite. If the eutectic system happens to be a low volume fraction eutectic (the eutectic composition is such that A B or vice versa), the minority phase will form a rod-like structure rather than the lamella structure described above. The spacing of the rods will still be governed by the... 

An important example of an MMC in situ composite is one made by directional solidification of a eutectic alloy. The strength, (, of such an in situ metal-matrix composite is given by a relationship similar to the HaH-Petch relationship used for grain boundary strengthening of metals ... 

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

The hardness of carbides can only be deterrnined by micro methods because of britdeness, the usual macro tests caimot be used. Neither can the extremely high melting points of the carbides be readily deterrnined by the usual methods. In the so-called Priani hole method, a small hoUow rod is placed between two electrodes and heated by direct current until a Hquid drop appears in the cavity. The temperature is determined pyrometricaHy. When high temperature tungsten tube furnaces are used, the melting point can readily be estimated by the Seger-type cone method. The sample may also be fused in a KroU arc furnace and the solidification temperature determined. 

Rotating-drum-type and belt-type heat-transfer equipment forms granular products directly from fluid pastes and melts without intermediate preforms. These processes are described in Sec. 5 as examples of indirect heat transfer to and from the solid phase. When solidification results from melt freezing, the operation is known as flaking. If evaporation occurs, solidification is by diying. 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

Siow withdrawai of mould from furnace Fig. 20.5. Directional Solidification (DS) of turbine blades. 

The solid which first appears on cooling is higher in nickel. Repeated directional remelting and solidification "zones" the copper up to the end of the bar, and leaves most of the bar increasingly pure in nickel. 

Hydrogen fluoride accounts for about 907o of the gaseous fluoride emitted from the electrolytic cell. Other gaseous emissions are SO2, COj, CO, NO2, HjS, COS, CS2, SFg, and various gaseous fluorocarbons. Particulate fluoride is emitted directly from the process and is also formed from condensation and solidification of the gaseous fluorides. 

The growth of crystals—or more generally the solidification of a sohd from a fluid phase—is definitely not an equilibrium problem. Why, therefore, should we discuss here equihbrium thermodynamics, instead of treating directly, for example the coagulation of two atoms and then simply following the growth of the cluster by adding more particles with time ... 

---