Lamellae, eutectic

LIQUID-SOLID AND SOLID-SOLID PHASE TRANSFORMATIONS 

The transformation is only favorable if AG t 0, which establishes a minimum threshold for the lamellar spacing, A  

FIGURE 6.37 Minimum lamellar spacing versus temperature as predicted from Equation 6.58 for a eutectic solidification reaction that occurs on cooling below T. Greater undercooling favors a finer lamellar spacing. 

Question Consider the eutectic solidification of a liquid to a + where the volume fraction of the / phase is relatively low. In this case, rather than solidifying in a lamellar structure, the phase may solidify as a series of rods embedded in a matrix of the a phase. Assuming that the rods are evenly spaced in a square array in the a matrix, calculate the critical 0-phase volume fraction (0 ) below which the rodlike morphology is more energetically stable than the lamellar morphology. 

FIGURE 6.38 Schematic illustration of the lamellar-rodlike eutectic micro structures discussed in Example 6.38. 

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More recent extensions of the theory (see citations in [122]) gave indications that the orientation of the lamellae (under isotropic material parameters) is not necessarily parallel to the growth direction of the front but may be tilted so that the lamellae travel sideways at some specific angles [138]. Finally it was found that the standard model of eutectic solidification has an intrinsic scaling structure [141-147]... 

K. Kassner, A. Valance, C. Misbah, D. Temkin. New broken-parity state and a transition to anomalous lamellae in eutectic growth. Phys Rev E 4S 1091, 1993. 

Crystallization, by definition, implies that the initial structure be a glass, followed by the nucleation and growth of a crystalline phase, be it the equilibrium one or a metastable phase. The process is a first-order transformation and involves atomic diffusion, or at least atomic shuttles. Types of crystallization reactions that occur include polymorphous crystallization, which is a composition invariant transformation such as that in Fe-B, and eutectic crystallization, T, in FeNiPB glass, where line lamellae of iron-nickel austenite and mclastable (FeNiJj PB phases grow cooperatively. 

FIGURE 4.14 Typical SEM image of boundary between eutectic colonies. (Reprinted from Calderon-Moreno, J.M. and Yoshimura, M., Stabilization of zirconia lamellae in rapidly solidified alumina-zirconia eutectic composites, J. Eump. Ceram. Soc. 25 (2005) 1369-1372, with permission from Elsevier Science.)... 

Figure 8. SEM view of fracture of Ti-3Al-5Zr-4Si alloy at 400 °C. Picture shows cleavage microcracking of a -lamellas, ductile with formation of knife fracture of interlamellar 13-phase and ductile void coalescence of eutectic between dendrites (shown by arrow).

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... 

Schematic of the mechanism by which a melt of the eutectic composition solidifies into alternating a and p lamellas as described by the Jackson-Hunt theory.

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