Lamellar eutectics

Table 13 is a representative Hst of nickel and cobalt-base eutectics for which mechanical properties data are available. In most eutectics the matrix phase is ductile and the reinforcement is britde or semibritde, but this is not invariably so. The strongest of the aHoys Hsted in Table 13 exhibit ultimate tensile strengths of 1300—1550 MPa. Appreciable ductiHty can be attained in many fibrous eutectics even when the fibers themselves are quite britde. However, some lamellar eutectics, notably y/y —5, reveal Htde plastic deformation prior to fracture. 

K. Kassner, C. Misbah. Spontaneous parity-breaking transition in directional growth of lamellar eutectic structures. Phys Rev A 44 6533, 1991. 

A. Valance, C. Misbah, D. Temkin, K. Kassner. Analytic theory for parity breaking in lamellar eutectic growth. Phys Rev E 48 92A, 1993. 

The exponentially decrease of the total variance with increasing sample mass is shown in Fig. 2.7. It can be seen that the uncertainty of sampling, s2samp, decreases and becomes statistically insignificant when the sample amount m exceeds the critical sample mass. Instead of mcnt the proportional critical sample volume vcrit may also be considered, represented, e.g. by a critical microprobe diameter dcnt. Results of homogeneity investigations of alloys, ores, and lamellar eutectics by EPMA (Electron Microprobe Analysis), which correspond to the curve of Fig. 2.7, have been presented by Danzer and Kuchler [1977]. 

CSEM/SEI of broken lamellar eutectic micro structure for system H20-Na2S04. In between large grains ofprimary ice are eutectic colonies comprised of lamellar mirabilite grains in an ice-I matrix. The bulk composition of this sample is 4.0 wt%Na2S04, which corresponds to that of the eutectic. 

The above classification scheme can also be applied to two-phase growth ( ). When both phases are low entropy-change the growth kinetics are unimportant and diffusion dominates. This leads to rod or lamellar eutectic growth forms, as shown in Fig. 8. Figure 9 shows the calculated shape of such an interface compared to the observed shape for various growth conditions ( ). These growth forms are common in metallic and some ionic... 

Figure 9.17 Photomicrograph showing the microstructure of a lead-tin alloy of composition 50 wt% Sn-50 wt% Pb. This microstructure is composed of a primary lead-rich a phase (large dark regions) within a lamellar eutectic structure consisting of a tin-rich phase (hght layers) and a lead-rich a phase (dark layers). 400x.

Fig. 5.2 (a) A lamellar eutectic structure in the carbon tetrahromide-hexachlor-... 

Fig. 5.4 (a) Schematic representation of the growth front of a lamellar eutectic... 

Fig. 6.7 The relative surface areas for lamellar eutectic arrangements, S i, compared with those of fibres. So, or vice versa, as functions of the volume proportions (a) is the volume fraction of the minor constituent (h)is the volume fractmt of the major constituent. Eutectic alloys having the lamellar habit are marked X and those with a fibrous habit arc marked O Hellawell ).

FIG. 1 The microstructure of eutectic Bi-Sn solder alloy rapidly solidified. The scanning electron microscope (SEM) micrograph shows an irregular lamellar eutectic microstructure, a mixture of Sn-rich (dark matrix), and Bi-rich phases (light phase). (Courtesy of W. K. Choi, IBM Corp.)... 

Two approaches have been taken to produce metal-matrix composites (qv) incorporation of fibers into a matrix by mechanical means and in situ preparation of a two-phase fibrous or lamellar material by controlled solidification or heat treatment. The principles of strengthening for alloys prepared by the former technique are well estabUshed (24), primarily because yielding and even fracture of these materials occurs while the reinforcing phase is elastically deformed. Under these conditions both strength and modulus increase linearly with volume fraction of reinforcement. However, the deformation of in situ, ie, eutectic, eutectoid, peritectic, or peritectoid, composites usually involves some plastic deformation of the reinforcing phase, and this presents many complexities in analysis and prediction of properties. 

Main uses ofNa alloys. Hypoeutectic Al-Si alloys (from 5 mass% Si to the eutectic) through the so-called modification (structural modification of the normally occurring eutectic) achieve somewhat higher tensile properties and improved ductility. Modification is obtained by the addition of elements such as Na (or Sr, Ca, Sb) and results in a finer lamellar or fibrous eutectic. Phosphorus, which reacts with sodium, interferes with the modification mechanism. Sodium can be used as the reductant of several chlorides in the preparation of metals such as Ti (Hunter process), Zr, Hf, Nb, Ta. 

Similar to Voltaren" Emulgel, oily droplets of an eutectic mixture of lidocaine and prilocaine are dispersed in a hydrogel to provide local anesthesia to the skin for injections and siugical treatment (Emla cream). A further possibility is the dermal administration of a liposome dispersion as a spray (Heparin PUR ratiopharm Spriih-gel "). After administration, water and isopropylic alcohol evaporate partially resulting in an increase of concentration and in a transition from the initial liposome dispersion into a lamellar liquid crystal [32]. The therapeutic effect appears to be influenced favorably by the presence of lecithins rather than by the degree of liposome dispersion. 

Coaxial intergrowth is a paragenetic relation that describes crystals of two different species growing with a common axis the misfit ratios between the two crystals in the direction of the common axis are small, without exception. The formation of coaxial intergrowth can be understood to be one crystal conjunct to the other in an epitaxial relation, where both continue to grow. If a liquid of eutectic A-B component is solidified from one side (unidirectional solidification), crystals of the two phases A and B precipitate in dotted, columnar or lamellar (with common axis) form, and show unique textures for unidirectional solidification. This is a well known phenomenon in metallurgy. 

Eutectic reactions, liquid —a + f), can result in several geometric configurations of a and /3. When the volumes of both phases are nearly equal, the most common morphology is lamellar. This is true of the Cu-Ag and Pb-Sn eutectics. If the amount of one phase is much less than the other, the eutectic is likely to be rods of one phase surrounded by the other phase (e.g., NiAl-Cr, TaC-Ni... 

Fig. 41. Classification of eutectic microstructures in terms of volume fractions and entropy of solution for an anomalous growth velocity of 5 X 10 4 cm/sec a, regular lamellar b, regular rods c, broken lamellar d, irregular e, complex regular f, quasiregular and g, irregular fibrous. After Elliot [23], Reproduced with permission of the ASM International, Metals Park.

The lamellar form of eutectic is only capable of resolution under high magnifications in slowly cooled alloys, while, under the same conditions of cooling, the reticular form is visible under low magnifications. ... 

Perlite is an eutectoid phase mixture composed of approximately 87 % ferrite and 13 % cementite and occurs in iron materials with a carbon content between 0.02 % and 6.67 %. The eutectic point is at 723 °C and 0.83 % carbon. At carbon contents below 2.06 %, perlite appears as individual metallographic constituent, whereas above 2.06 % carbon, it occurs in a phase mixture with cementite and ledeburite. In perlite, cementite predominantly appears in lamellar form. 

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