Eutectic structure

Carbon steels as received "off the shelf" have been worked at high temperature (usually by rolling) and have then been cooled slowly to room temperature ("normalised"). The room-temperature microstructure should then be close to equilibrium and can be inferred from the Fe-C phase diagram (Fig. 11.1) which we have already come across in the Phase Diagrams course (p. 342). Table 11.1 lists the phases in the Fe-FejC system and Table 11.2 gives details of the composite eutectoid and eutectic structures that occur during slow cooling. 

Ledeburite The composite eutectic structure of alternating plates of y and FejC produced when... 

At 577°C the eutectic reaction takes place the liquid decomposes into solid (Al) mixed with solid Si, but on a finer scale than before (bottom of Fig. A1.32). This intimate mixture of secondary (Al) with secondary Si is the eutectic structure. 

Eutectoid structures are like eutectic structures, but much finer in scale. The original solid decomposes into two others, both with compositions which differ from the original, and in the form (usually) of fine, parallel plates. To allow this, atoms of B must diffuse away from the A-rich plates and A atoms must diffuse in the opposite direction, as shown in Fig. A1.40. Taking the eutectoid decomposition of iron as an example, carbon must diffuse to the carbon-rich FejC plates, and away from the (carbon-poor) a-plates, just ahead of the interface. The colony of plates then grows to the right, consuming the austenite (y). The eutectoid structure in iron has a special name it is called pearlite (because it has a pearly look). The micrograph (Fig. A1.41) shows pearlite. 

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

Since 1980, the zinc-5-aluminium (notably Galfan which has a mischmetal addition) alloys, which are essentially based on the eutectic structure, have been developed commercially. They give 30-200% increase in corrosion resistance in the atmosphere and are extremely flexible. They can be used for sheet, wire and some types of tube galvanising whereas the zinc-55%-aluminium alloy is restricted to sheet. 

As a eutectic front advances during solidification, the solutes must partition between the two phases, as suggested in Figure 10.19. With increased rates of solidification, there is less time for diffusion so the eutectic structures are finer. 

On further cooling to just below the eutectic temperature, the remaining liquid which has the eutectic composition will freeze immediately according to the eutectic reaction. The solid structure will thus be the mixture of the primary phase of A and the eutectic structure which is the fine mixture of A and B. 

The liquid fraction just above the eutectic temperature is converted into die eutectic structure when the temperature is lowered below the eutectic. 

The final product will consist of large crystals of A and B which have crystallised before reaching the point e, and small crystals of eutectic structure of A, B and C which have crystallised at the point e. 

At point c, both A and B co-crystallise forming eutectic structure until all liquid is consumed. The temperature remains constant during this eutectic reaction ... 

At g, a eutectic reaction takes place until the last portion of liquid is completely consumed three solid phases AC, BC and B crystallise out together to form a eutectic structure. 

Bartenev, G. M. The Quasi-Eutectic Structure of a liquid Eutectic. Izv. Akad. Nauk SSSR, Otd. Tekhn. Nauk, Met. i Toplivo 3, 138—140 (1961). 

Figure 1 shows phase diagram of indium and antimony. Indium and antimony formed indium antimonide alloy at compositional ratio of 50 at.% 50 at.%, and formed eutectic structure of Indium antimonide and antimony at compositional ratio of 70 at.% 30 at.%. 

In the ribbon with x=5 the amorphous structural component was absent, X-ray study registered the existence of a-Al together with Al4Ce and Al3Sc crystalline intermetallics (Fig. 4c). However, ribbon hardness remained at almost the same level (Fig.7). Evidently, it can be explained by the formation of a very fine-scale structure of a eutectic type with a relatively small amount of a-Al grains (Fig. 6b). Small plates of both intermetallics of 20-60 nm in thickness were found in the eutectic by dark-field TEM investigation. A further increase of x caused a drop of hardness (Fig. 7), and in the ribbon Al91Ce2Sc7 no areas with eutectic structure were found (Fig. 

While the bond coat composition corresponds to a eutectic ratio, the extremely short dwell time of the only mechanically mixed powder particles in the hot core of the plasma jet is not sufficient to produce features akin to a eutectic structure. Instead, the bond coat consists of seemingly unrelated streaks of intertwined... 

Figure 7 shows the joint interfaces in ground C-SiC/Cu-clad-Mo joints made using Ticusil. There is evidence of good braze/composite interaction (Figs. 7a b), and relatively large quantities of Ti (18,6 atom%). Mo (36.4 at%) and Ag (45 at%) are detected within the C-SiC composite (point I, Fig. 7b). The SiC coating on the composite surface has been removed by grinding and an intimate composite-to-braze contact established. Silicon is detected at -15-20 pm distance within the braze region near the interface (point 4, Fig. 7b). As before, the braze matrix displays the Ag-rich and Cu-rich two-phase eutectic structure with the Ag-rich phase preferentially segregating at the C-SiC surface (Fig. 7b). The Ag-rich phase has also preferentially deposited at the interface on the Cu-clad-Mo side (Fig. 7c). Interestingly, there is some carbon dissolution and diffusion in braze (points 1 2, Fig. 7c) and also in Mo (point 5, Fig. 7c) to a depth of -30 pm. Additionally, some Cu (10.6 at%) from the clad layer was detected within the Mo substrate (point 5, Fig. 7c).

Is has been found that the process of nitride formation begins with nitriding of a finely differentiated eutectic structure resulting in the formation of titanium nitride. Due to increase in volume caused by nitrogen absorption, dispersion of large particles of ferrotitanium along interphase boundaries occurs which leads to increase in speed of the process. 

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