metastable, compositionally invariant massive transformation in two-phase fields in phase diagrams was reexamined, and the current status of various distinguishing features of the massive transformation mode was also reviewed. References cited in this section 1. T.B. Massalski, A.J. Perkins, and J. Jaklovsky, Extension of Solid Solubility During Massive Transformations, Met. Trans., Vol 3, 1972, p 687 2. T.B. Massalski, Massive Transformations, in Phase Transformations, American Society for Metals, 1970 3. E.B. Hawbolt and T.B. Massalski, Observations Concerning the β αm Massive Transformation in Cu-Zn Alloys, Met. Trans., Vol 1, 1970, p 2315 4. T.B. Massalski, The Mode and Morphology of Massive Transformations in Cu-Ga, Cu-Zn, Cu-Zn-Ga and Cu-Ga-Ge Alloys, Acta Metall., Vol 6, 1958, p 243 5. G.A. Sargent, L. Delaey, and T.B. Massalski, Formation of "Feathery" Structures During Massive Transformation in Cu-Ga Alloys, Acta Metall., Vol 16, 1968, p 723 6. H. Gleiter and T.B. Massalski, Atomistic Model for the Growth of Feathery Structures in Duplex Massive Transformations, Acta Metall., Vol 18, 1970, p 649 7. A.J. Perkins and T.B. Massalski, Observations on the β m Massive Transformation in Two-Phase (β + ) Alloys of the Cu-Ga-Ge System, Met. Trans., Vol 2, 1971, p 2701 8. "The Massive Transformation," American Society of Metallurgical, Mining, and Petroleum Engineers Symposium, published in Met. Trans. A, Vol 15, 1984, p 410 Eutectoid Structures A.R. Marder, Senior Scientist, Technology Department, Bethlehem Steel Corporation; J.A. Kowalik, Graduate Student, Department of Metallurgy and Materials Engineering, Lehigh University Introduction EUTECTOID REACTIONS are defined as the transformation of a metastable solid phase to a mixture of two other solid phases. This transformation involves long-range diffusion due to the compositional invariance between the two products and the parent phase. The most common microstructure resulting from eutectoid transformation is a lamellar structure, for example, pearlite (Fig. 1) in the Fe-Fe3C system (Fig. 2). In this system, the austenite phase, which is face-centered cubic (fcc), decomposes at 727 °C (1341 °F) and 0.77% C to the ferrite phase, which is body-centered cubic (bcc), and the cementite phase, which is orthorhombic. Eutectoid reactions producing pearlitic structures are not limited only to ferrous alloys; Table 1 lists some ferrous and nonferrous alloy systems that contain eutectoid transformations that produce lamellar structures as well as other morphologies. Figure 3 shows a typical example of nonlamellar eutectoid decomposition in the Cu-Sn system. This article, however, will review the Fe-Fe3C pearlite eutectoid structure because of its commercial importance and the extensive body of knowledge available (Ref 2). Table 1 Eutectoid transformations in nonferrous and ferrous alloys Alloy CuAl Eutectoid composition, Wt% 11.8 Al Eutectoid temperature °C °F 565 1049 High-temperature phase and crystal structure Low-temperature phases and crystal structures Reactions observed β-bcc α-fcc γ2 (gamma brass) Lamellar pearlite; granular pearlite CuBe 6 Be 605 1121 β-bcc α-fcc β'-bcc (CsCl) Lamellar pearlite Cu-In 31.4 In 574 1065 β-bcc α-fcc δ(deformed gamma brass) Lamellar pearlite; granular pearlite Cu-Si 5.2 Si 555 1031 κ-hcp α-fcc γ-cubic (β-Mn) Granular pearlite CuSn 27.0 Sn 520 968 γ-bcc α-fcc δ(gamma brass) Lamellar pearlite; needles of αabout which δprecipitates CuSn 32.5 Sn 350 662 δ(gamma brass) α-fcc ε-orthorhombic Lamellar pearlite Fe-C 0.80 C 723 1333 γ-fcc(interstitial C) α-bcc Fe3C-orthorhombic Lamellar pearlite Fe-N 2.35 N 590 1094 γ-fcc (interstitial N) α-bcc γ'-fcc (interstitial N) Lamellar pearlite; granular pearlite Fe-O 23.3 O 560 1040 Wüstite cubic (NaCl) α-bcc Fe3O4cubic (spinel) Lamellar pearlite; granular pearlite NiZn 56 Zn 675 1247 β-cubic (CsCl) β1-tetragonal (CuAu) γ(gamma brass) Lamellar pearlite Ti-Cr 15 Cr 680 1256 β-bcc α-hcp TiCr2-fcc (MgCu2) Lamellar pearlite; granular pearlite Source: Ref 1 Fig. 1 Typical pearlite structure of alternate layers of ferrite and cementite in an Fe-0.8C alloy. Picral. 500× Fig. 2 Eutectoid region of the Fe-Fe3C phase diagram Fig. 3 Nonlamellar eutectoid structure in a Cu-27Sn alloy. Electrolytic etchant: 1% CrO3. 150× References 1. C.W. Spencer and D.J. Mack, Decomposition of Austenite by Diffusional Processes, V.F. Zackay and H.I. Aaronson, Ed., Interscience--John Wiley & Sons, 1962, p 549-606 2. N. Ridley, in Phase Transformations in Ferrous Alloys, A.R. Marder and J.I. Goldstein, Ed., TMS/AIME, Warrendale, PA, 1984, p 201-236 Pearlite Microstructures Figure 4 depicts the individual constituents of pearlite. A nodule nucleates at a grain boundary, triple point, grain corner, or surface and grows radially until impingement occurs with surrounding nodules (Ref 4). Individual colonies are present inside the nodule, each nodule having an orientation relationship with the parent austenite grain. Inside the colonies a complex microstructure forms, consisting of alternating parallel lamellae of the two product phases (ferrite and cementite). Figure 5 illustrates the typical microstructure of a partially transformed eutectoid steel in which the morphology of the nodules is apparent. Fig. 4 Microstructural features of the pearlite eutectoid transformation. (Ref 3) Fig. 5 Cross-sectional views of the microstructure of pearlite nodules in partially transformed hot-stage Fe-0.8C specimens showing nodule formation. Picral. 220×. (Ref 3) Figure 6 shows the colonies of pearlite with faults and imperfections present in the lamellar structure. Changes in orientation are observed at the boundaries between the colonies. These changes occur because of faults in the structure. Figure 7, a transmission electron micrograph, shows the cementite lamellae (dark rods) stopping at a cell boundary. Also noted is the occasional bending of the lamellae achieved by a series of growth steps. The growth of the cementite-ferrite interface, which is very sensitive to crystallographic relationships, occurs by a ledge-type mechanism (Ref 5). Figure 8 illustrates the ledges in cementite lamellae. Fig. 6 Faults found in the colonies of pearlite in an Fe-0.8C specimen. Picral. 200× Fig. 7 A transmission electron micrograph of a ferrite cell interrupting the growth or bending the cementite in a thin-foil Fe-0.8C specimen. 17,250×. (Ref 6) Fig. 8 A surface replica transmission electron micrograph showing growth steps on the cementite lamellae in an Fe-0.8C specimen. Picral. 8000×. (Ref 6) References cited in this section 3. A.R. Marder, in Phase Transformations in Ferrous Alloys, A.R. Marder and J.I. Goldstein, Ed., TMS/AIME, Warrendale, PA, 1984, p 201-236 4. B.L. Bramfitt and A.R. Marder, Met. Trans., Vol 4, 1973, p 2291-2301 5. S.A. Hackney and G.J. Shiflet, in Phase Transformations in Ferrous Alloys, A.R. Marder and J.I. Goldstein, Ed., TMS/AIME, Warrendale, PA, 1984, p 237-242 6. B.L. Bramfitt and A.R. Marder, Metallography, Vol 6, 1974, p 483-495 Crystal Orientation Pearlite colonies grow as if they were two interpenetrating single crystals. Measurements of the orientation relationships of the phases of ferrous pearlite colonies have been determined through transmission electron microscopy: Pitsch-Petch [100]c 2-3° from [13 1 ]α [010]c 2-3° from [113]α (001)c (5 2 1 )α Bagaryatski [100]c [0 1 1]α [010]c [1 1 1 ]α (001)c (211)α c, cementite; a, ferrite In ferrous alloys, the Pitsch-Petch relationship is dominant at eutectoid compositions. However, the Bagaryatski relationship is increasingly apparent in hyper- and hypoeutectoid compositions, in which the nucleation of a colony forms not on a prior-austenite grain, but on the proeutectoid ferrite or cementite. Pearlite Nucleation Pearlite nucleation is heterogeneous and is generally restricted to the austenite grain boundaries and surface sites. Saturation of these sites generally occurs within 20 to 25% of the total transformation time, followed by growth of the nodules until impingement. Pearlite nucleates in a eutectoid alloy by the cementite or the ferrite nucleating on the austenite grain boundary (Fig. 9a). This nucleus will form an orientation relationship with the prior-austenite grain (γ1) to lower the energy barrier. If the first nucleus to form is cementite, the area surrounding this nucleus will be depleted of carbon, which enhances the formation of ferrite. As the ferrite forms, the carbon is rejected into the surrounding matrix, further encouraging cementite to form. Fig. 9 Nucleation and growth of pearlite. (a) On a "clean" austenite grain boundary. (1) Cementite nucleates on grain boundary with coherent interface and orientation relationship with γ2. (2) α nucleates adjacent to cementite with a coherent interface and orientation relationship with γ1. In addition, this produces an orientation relationship between the cementite and ferrite. (3) The nucleation process repeats sideways while incoherent interfaces grow into γ2. (4) New plates can also form by a branching mechanism. (b) When a proeutectoid phase (cementite or ferrite) already exists on an austenite boundary, pearlite will nucleate and grow on the incoherent side, resulting in a different orientation relationship between the cementite and ferrite. (Ref 7) Hyper- and hypoeutectoid alloys decompose similarly. The hyper- or hypoeutectoid composition causes proeutectoid ferrite (in a hypoeutectoid composition) or cementite (in a hypereutectoid composition) to form before the pearlite transformation. In a hypereutectoid steel, for example, ferrite nucleates on the proeutectoid cementite (Fig. 9b) and forms an orientation relationship with the cementite. A similar growth process occurs for the hypoeutectoid steel, with proeutectoid ferrite forming initially at prior-austenite grain boundaries. Reference cited in this section 7. D.A. Porter and K.E. Easterling, Phase Transformations in Metals and Alloys, Van Nostrand Reinhold, 1981, p 331 Pearlite Growth The growth rate of pearlite changes as a function of the time, transformation temperature, and prior-austenite grain size. For a given temperature and austenite grain size, the transformation rate occurs in three stages. As shown in Fig. 10, at any given temperature the volume fraction of pearlite at any given time, f (t), fits an S-shaped or sigmoidal curve. Initially, the transformation rate is quite low and depends on site saturation. As more nodules develop, the rate of transformation increases. Finally, the nodules impinge, and the rate of transformation again slows as the microstructure gradually approaches complete transformation. Fig. 10 Calculated fraction austenite transformed to pearlite as a function of time for the parameters shown f(t) = 1 - exp [- πNG3t4/3], where f(t) is the volume fraction pearlite formed at any given time, t, at a given temperature, N is the nucleation rate of the pearlite colonies, and G is the rate at which the colonies grow into the austenite. (Ref 8) The temperature at which the austenite is transformed also affects the pearlite growth rate. Lowering the temperature increases the driving force for nucleation, which increases the transformation rate. Figure 11 illustrates a C-curve relationship between transformation temperature and growth rate. Finally, decreasing the austenite grain size will increase the number of nucleation sites. More nuclei growing into the austenite decrease the time for transformation and increase the transformation rate. Fig. 11 Variation in pearlite growth rate with transformation temperature. (Ref 9) References cited in this section 8. G. Krauss, Principles of Heat Treatment of Steel, American Society for Metals, 1980 9. A.R. Marder and B.L. Bramfitt, Met. Trans. A, Vol 6, 1975, p 2009-2014 Interlamellar Spacing Interlamellar spacing is a strong function of the transformation temperature. Lower temperatures will result in a finer lamellar structure. Figure 12 illustrates the relationship between the reciprocal of the interlamellar spacing and transformation temperature. Fig. 12 Effect of transformation temperature on the reciprocal of interlamellar spacing. (Ref 9) Reference cited in this section 9. A.R. Marder and B.L. Bramfitt, Met. Trans. A, Vol 6, 1975, p 2009-2014 Alloying Effects Substitutional alloying elements added to the Fe-C system affect all the transformation parameters. The transformation temperature (Fig. 13) and the eutectoid carbon content (Fig. 14) are significantly altered. Furthermore, alloying additions can significantly decrease the pearlite growth rate because of the partitioning of these elements between the ferrite and cementite (Fig. 15). Fig. 13 Effect of percentage of substitutional alloying elements on the temperature of the eutectoid transformation point in steel. See also Fig. 14. (Ref 10) Fig. 14 Effect of percentage of substitutional alloying elements on the carbon content of the eutectoid transformation point in steel. See also Fig. 13. (Ref 10) Fig. 15 Pearlite growth rate of Fe-C-X alloys. (Ref 11) In an equilibrium alloy Fe-C-X, in which X is a substitutional element, decomposition of austenite to pearlite will occur in two ways. First, substitutional elements will diffuse more slowly than carbon, which decreases the transformation rate. This type of reaction partitions the substitutional element X between the two phases. The carbide-forming elements, such as chromium and molybdenum, will concentrate in the carbide. Ferrite-stabilizing elements--silicon, for example--will concentrate in the ferrite. Figure 16 depicts the concentration of chromium, manganese, and silicon in the ferrite and cementite. The second type of decomposition in an Fe-C-X alloy occurs when the X alloy does not undergo any longrange diffusion. The rate of the reaction, therefore, is controlled solely by the diffusion of carbon. For example, nickel additions will stabilize the austenite to lower temperatures, causing high undercoolings; and preventing partitioning. Fig. 16 Schematic distribution of the alloy elements across the microstructure. (Ref 12) References cited in this section 10. E.C. Bain and H.W. Paxton, Alloying Elements in Steel, American Society for Metals, 1962, p 112 11. A.R. Marder and B.L. Bramfitt, Met. Trans. A, Vol 7, 1976, p 902-906 12. P.R. Williams, M.K. Miller, P.A. Beavan, and G.D.W. Smith, in Phase Transformations, Vol 2, The institution of Metallurgists, London, 1979, p 11.98-11. 11.100 Bainitic Structures D.V. Edmonds, Lecturer, Department of Metallurgy and Science of Materials, University of Oxford Introduction BAINITE refers to the microstructure resulting from the decomposition of austenite in steels at temperatures above the martensitic transformation and below the pearlite reaction. In plain-carbon steels, however, the bainitic temperature range overlaps the pearlite reaction range considerably. The transformation can occur isothermally, or during continuous cooling. In many alloy steels, the bainitic temperature range is separate from the pearlite range, and a bay occurs in the isothermal time-temperature transformation (TTT) diagram between the two reactions. Below the Bf (bainite finish) temperature, fully bainitic structures can be achieved, but at higher temperatures isothermal bainitic transformation may stop before complete decomposition of the austenite--a phenomenon known as incomplete reaction. The extent of decomposition is a
