Solidification rate

The phases present in products can differ from those predicted from equilibrium diagrams. Nonequilibrium metastable phases form at solidification rates experienced in commercial ingots. Because of the low rate of diffusion of iron in alurninum, equilibrium conditions can only be established by long heat treatments and are very slowly approached at temperatures below about 550 °C. Small additions of other elements, particularly manganese, can also modify the phase relations. 

This process, originally designated as RSR (rapid solidification rate), was developed by Pratt and Whitney Aircraft Group and first operated in the late 1975 for the production of rapidly solidified nickel-base superalloy powders.[185][186] The major objective of the process is to achieve extremely high cooling rates in the atomized droplets via convective cooling in helium gas jets (dynamic helium quenching effects). Over the past decade, this technique has also been applied to the production of specialty aluminum alloy, steel, copper alloy, beryllium alloy, molybdenum, titanium alloy and sili-cide powders. The reactive metals (molybdenum and titanium) and... 

Connection between Transport Processes and Solid Microstructure. The formation of cellular and dendritic patterns in the microstructure of binary crystals grown by directional solidification results from interactions of the temperature and concentration fields with the shape of the melt-crystal interface. Tiller et al. (21) first described the mechanism for constitutional supercooling or the microscale instability of a planar melt-crystal interface toward the formation of cells and dendrites. They described a simple system with a constant-temperature gradient G (in Kelvins per centimeter) and a melt that moves only to account for the solidification rate Vg. If the bulk composition of solute is c0 and the solidification is at steady state, then the exponential diffusion layer forms in front of the interface. The elevated concentration (assuming k < 1) in this layer corresponds to the melt that solidifies at a lower temperature, which is given by the phase diagram (Figure 5) as... 

To qualify for equilibrium solidification, the solidification rate must be slower than the solute diffusivity in the solid ... 

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... 

Thomas J. D. Tzavaras A. A., Solidification and solidification rates in continuous casting of steel. Proceedings of the Continuous Casting Symposium of the 102 AIME annual meeting. Metallurgical Society of AIME, 1973, 125-140... 

In melt spinning processes a metal melt is forced through dies as a thin jet into a liquid medium so quickly that the solidification rate is faster than the rale of disintegration of the jet into droplets. 

Fig. 4.3. Heater power, fraction solidified and solidification rate as a function of time during solidification process...

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