Temperature recalescence

The specific heats of solids at low temperatures are appreciably less than at higher temperatures. A maximum specific heat has been observed in the case of iron at 740° and nickel at 320° (Lecher, 1908). Since these are the temperatures at which recalescence and loss of magnetic properties occur, the close relation of specific heat to molecular structure is evident. 

For an alloy droplet, the post-recalescence solidification involves segregated solidification and eutectic solidification. 619 Droplet cooling in the region (1),(2) and (6) can be calculated directly with the above-described heat transfer model. The nucleation temperature (the achievable undercooling) and the solid fraction evolution during recalescence and post-recalescence solidification need to be determined additionally on the basis of the rapid solidification kinetics. 154 156 ... 

Allotropy of Iron.—When a bar of pure iron is allowed to cool from its melting-point to 0° C., its time-temperature cooling curve exhibits three breaks,4 or arrests, designated by the symbols Ar4, Ar3, and Ar2 respectively.5 These arrests are due to evolution of a small amount of heat consequent upon some internal alteration in the metal, whereby the rate of cooling is retarded. Indeed, the evolution of heat at the Ars point is sufficient to raise the temperature of the iron by a very appreciable amount. The phenomenon is termed recalescence.6 Similarly, on reheating the metal, three arrests, due to heat absorption, are... 

Figure 8.6 shows a typical cooling curve during recalescence for the sample crystallized at AT (=Tm — Tn) = 242 K, where Tn is the nucleation temperature. The release of the latent heats of nucleation and crystallization increased the sample temperature up to Tm, and the solid/liquid coexistence state followed at Tm- If this temperature profile is rotated clockwise by 90°, it can be seen that the shape of Fig. 8.6 is similar to a part of growth path (2). 

The recalescence stage, during which the supercooling-induced, rapid crystal growth releases latent heat, which results in a temperature increase until the droplet has reached its equilibrium freezing temperature, Tf. 

As indicated in Fig. 16.1, a droplet that exhibits supercooling, once the nucleation starts, experiences a rapid temperature increase until it reaches the equilibrium freezing temperature, Tf. The portion of the droplet volume that is solidified during this recalescence stage, V, can be estimated firom the heat balance equation... 

Typical cooling curve during solidification of a metal showing the recalescence as the release of latent heat causes a brief increase in temperature. 

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