Eutectoid decomposition

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

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. 

Figure 5.16. Two versions of the Pu-Ga phase diagram as reported, according to Hecker and Timofeeva (2000), by Peterson and Kassner (1988) and by Chebotarev etal. (1975). According to the first version the fee 6-Pu phase is retained at room temperature for Ga concentrations included between approximately 2 at.% and 9 at.%. According to the second version the S-Pu phase undergoes (below 100°C) a eutectoid decomposition to the a-Pu phase plus Pu3Ga. Both diagrams were extrapolations to equilibrium.

Pearlite The two-phase structure (aFe,C + Fe3C) originating from the eutectoidal decomposition of austenite (that is the C solid solution in Fe) and having the overall composition of 0.76 mass % C (3.46 at.%C). The stable Fe-graphite eutectoid has a composition of 2.97 at.% C. 

Spinodal decompositions, often observed in binary solid solutions of metals and in glasses, on the other hand, arise from thermodynamic instabilities caused by composition (Cahn, 1968). A special feature of this type of solid state transformation is the absence of any nucleation barrier. There is a class of transformation called eutectoid decomposition in which a single phase decomposes into two coupled phases of different compositions, the morphology generally consisting of parallel lamellae or of rods of one phase in the matrix of the other. 

The importance of metastable phases which persist at ambient pressure and temperature need not be emphasized. Control of the eutectoidal decomposition of the metastable BCC iron-carbon phase, austenite, below 996 K is essential to the steel... 

Similar to the 6 phase in the Nb-N system, the mononitride b-TaNj is also a high-temperature phase, but the nitrogen equilibrium pressure is even higher than that for 6-NbN. The main purpose of the work performed here was to determine the eutectoid decomposition temperature... 

Plate 4.4 Two Ta-N diffusion couples above (top) and below (bottom) the eutectoid decomposition temperature of (5-TaN, v Upon cooling b-TaN, x decomposes into c-TaN and / -Ta2N. Polarized light. 

In pure titanium, the crystal structure is dose-packed hexagonal (a) up to 882°C and body-centered cubic (p) to the melting point. The addition of alloying dements alters the a—p transformation temperature. Elements that raise the transformation temperature are called a-stabilizers those that depress the transformation temperature, p-stabilizers the latter are divided into p-isomorphous and p-eutectoid types. The p-isomorphous elements have limited a-solubility and increasing additions of these dements progressively depresses the transformation temperature. The p-eutectoid elements have restricted p-solubility and form intermetallic compounds by eutectoid decomposition of the p-phase. The binary phase diagram illustrating these three types of alloy... 

Figure 3.21 shows the change of the product composition with carburization time in hydrogen for a tungsten-carbon black mixture at 1119°C. After only 10 minutes at this temperature most of the metal was transformed to W2C, which was ffien only slowly converted to WC. This result is characteristic for the temperature range of 1050 to 1850 °C and W particle sizes of 1.3 to 20 pm [3.71]. It is remarkable that W2C forms during carburization even at 900 °C, which is well below its eutectoid decomposition teniperature of 1250°C. 

Cubic high-temperature modification it undergoes a eutectoid decomposition at 2530-2535 °C into P(W2C) + 8(WC). At room temperature it can only be obtained by rapid quenching in liquid tin. 

Fig. 5. Stability ranges for the RCo5 phases showing solidus temperature, Ts, and eutectoid decomposition temperature, Td. (After Buschow 1974). Note the pronounced effect on the phase stability of small third-element substitutions for Co ( = 3% Fe or Al).

Yam] Yamaguchi, T., Shiraishi, T., Eutectoid Decomposition of CuFc204-Fe304 Spinel Solid Solution Including CuFesOg , J. Am. Ceram. Soc., 52(7), 401 (1969) (Crys. Stmcture, Phase Relations, Kinetics, 6)... 

A microanalysis study of the eutectoid decomposition of austenite into ferrite and M2C (to bainite) at the bay in Fe-0.24C-4Mo is reported by [2003Hacl]. It was concluded that alloy element partition between ferrite and alloy carbides at the reaction front is largely responsible for the slow kinetics in this and related alloys. A thermodynamic analysis showed that ferrite-carbide interfacial energy and nonequilibrium carbide compositions reduce the thermodynamic driving force for diffusion processes (Mo partition) by up to 20% further slowing the kinetics. 

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