Equilibrium Solidification

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. 

Fig. 7. Constitutional supercooling, (a) impurity concentration profile during solidification (b) actual temperature T and equilibrium freezing temperature T...

The iron-carbon solid alloy which results from the solidification of non blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequendy refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, die level depending on the application. The first solid phases to separate from liquid steel at the eutectic temperature, 1408 K, are the (f.c.c) y-phase Austenite together with cementite, Fe3C, which has an orthorhombic sttiicture, and not die dieniiodynamically stable carbon phase which is to be expected from die equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to h on and carbon from room temperature up to 1130 K... 

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 ... 

However, since the latter reaction is an equilibrium process the Claus sulfur still contains traces of H2S (200-350 ppm, mostly in the form of polysulfanes) which causes serious problems since the H2S partly escapes on cooling or solidification of the sulfur. Obviously, the H2S content needs to be controlled because of the extreme toxicity of hydrogen sulfide and its ability... 

Non-Equilibrium Solidification of Metastable Materials from Undercooled Melts... 

The equilibrium, room temperature structure of pure cobalt is hep. The fee structure is stable at high temperatures (422 °C to 1495 °C) and has been retained at room temperature by rapid solidification techniques [101], X-ray diffraction analysis was used to probe the microstructure of bulk Co-Al alloy deposits containing up to 25 a/o Al and prepared from solutions of Co(II) in the 60.0 m/o AlCfi-EtMelmCl melt. Pure Co deposits had the hep structure no fee Co was observed in any of the deposits. The addition of aluminum to the deposit caused a decrease in the deposit grain size and an increase in the hep lattice volume. A further increase in the aluminum content resulted in amorphization of the deposit [44], Because the equilibrium... 

The phase distribution observed in the alloys deposited from AlCb-NaCl is very similar to that of Mn-Al alloys electrodeposited from the same chloroaluminate melt [126 129], Such similarity may also be found between the phase structure of Cr-Al and Mn-Al alloys produced by rapid solidification from the liquid [7, 124], These observations are coincident with the resemblance of the phase diagrams for Cr-Al and Mn-Al, which contain several intermetallic compounds with narrow compositional ranges [20], inhibition of the nucleation and growth of ordered, often low symmetry, intermetallic structures is commonly observed in non-equilibrium processing. Phase evolution is the result of a balance between the interface velocity and... 

Eutectic point (Tc) A single point on a temperature concentration phase (or state) diagram for a binary solution (e.g., water and sugars or salts) where the solution can exist in equilibrium with both crystalline solute and crystalline solvent. Under equilibrium conditions, cooling at Te results in simultaneous crystallization of solvent and solute in constant proportion and at constant temperature until maximum solidification has occurred (based on Fennema, 1996). 

However, V4+ compounds still play an important role for the activity of the catalyst because an equilibrium exists between V5+ and V4+ compounds in the melt. The degree of reduction to inactive V4+ increases at low temperature and high S02 partial pressure, and it has also been found to depend on the liquid dispersion on the support [9], Furthermore, at temperatures below 500°C some V4+ compound precipitates and gradually depletes the melt of V5+ when the temperature is lowered, and this partial solidification eventually causes the activity to drop to practically zero at some minimum operating temperature of about 350°C. 

In alloys such as AA3004 some of the major issues concern solidification and therefore it is interesting to look at this in detail. However, as solidification in Al alloys rarely occurs imder equilibrium conditions, a more detailed examination of this issue will be found in the next chapter. 

Matte-slag-gas reactions in Cu-Fe-Ni sulphide ores. Sulphide ores are a major source of Cu, Ni and precious metals. A basic principle of the extraction processes is to blow air into the molten sulphide in order to oxidise (1) S, which forms a gas and (2) Fe, which forms predominantly FeO and then partitions to a slag phase which covers the matte. A key element in the recovery of the metals is the solidification of the matte which separates into a sulphur-rich matte and metal-rich liquid. This process may occur under non-equilibrium conditions with precious metals concentrating in the last metallic liquid. 

Figure 10.79 Percent liquid remaining during matte solidification under both equilibrium and Scheil--Gulliver solidification conditions (from Taylm and Dinsdale 1990).

For a number of applications, particularly those associated with conditions of continuous cooling or heating, equilibrium is clearly never approached and calculations must be modified to take kinetic factors into account. For example, solidification rarely occurs via equilibrium, amorphous phases are formed by a variety of non-equilibrium processing routes and in solid-state transformations in low-alloy steels much work is done to understand time-temperature-transformation diagrams which are non-equilibrium in nature. The next chapter shows how CALPHAD methods can be extended to such cases. 

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