Nucleation at grain boundaries

Figures 11.2-11.6 show how the room temperature microstructure of carbon steels depends on the carbon content. The limiting case of pure iron (Fig. 11.2) is straightforward when yiron cools below 914°C a grains nucleate at y grain boundaries and the microstructure transforms to a. If we cool a steel of eutectoid composition (0.80 wt% C) below 723°C pearlite nodules nucleate at grain boundaries (Fig. 11.3) and the microstructure transforms to pearlite. If the steel contains less than 0.80% C (a hypoeutectoid steel) then the ystarts to transform as soon as the alloy enters the a+ yfield (Fig. 11.4). "Primary" a nucleates at y grain boundaries and grows as the steel is cooled from A3...

Figure 6.2. Sloichiomclric CuPl, ordered at 550°C for 157 hours. Viewed under polarised light in reflection. Shows growth of ordered domains, heterogeneously nucleated at grain boundaries and surface scratches (after Irani and Cahn 197.5).

Tompkins and Young [139] concluded that the reaction of lead styphnate monohydrate (468 to 502 K) was comparable with that of barium styphnate monohydrate. The or-time curves for both salts were not identical, but again there was nucleation at grain boundaries, followed by the advance of a non-coherent reaction interface into the reactant crystallites. The activation energy (138 kJ mol" ) is slightly less than that for the barium salt, but was ascribed to the same primary process, activation of a surface styphnate ion. 

Nis] Nishizawa, T., Hasebe, M., Computer Calculation of Phase Diagrams of Iron Alloys (in Japanese), Tetsu to Hagane, 67(14), 2086-2097 (1981) (Review, Thermodyn., 110) [1981Saul] Sauthoff, G., Speller, W., Kineties of a Phase Precipitation in Fe-Cr-Si Alloys. I. Nucleation at Grain Boundaries and other Sites , Z. Metallkd., 72(7), 457-461 (1981) (Experimental,... 

A number of w-crack configurations have been experimentally observed at triple points. Figure 6.79 shows crack nucleation at grain boundaries, formed when shear stress acts along the boundaries. Here, a wedge-type cavity was formed under creep in a SiAlON-YAG ceramic in a flexural bar studied at 1170 °C. Figure 6.80 shows the cavitation strain and the total strain as functions of the normalized beam height for the applied moments of M — 0.165 N.m and M = 0.247 N.m (i.e., at the initial, maximum applied stresses of 80 and 120 MPa). Both tests were carried out at 1170 C and interrupted before failure. For M = 0.165 N.m (a = 80 MPa), the maximum cavitation strain is 20 % of the total strain, whereas for M = 0.247 N.m (b = 120 MPa), the cavitation strain... 

Nucleation at grain boundaries makes only quantitative changes to the results... 

The nucleation rate is, in fact, critically dependent on temperature, as Fig. 8.3 shows. To see why, let us look at the heterogeneous nucleation of b.c.c. crystals at grain boundaries. We have already looked at grain boundary nucleation in Problems 7.2 and 7.3. Problem 7.2 showed that the critical radius for grain boundary nucleation is given by... 

The induction process is normally dominated by the nucleation process. The acceleration step tends to be dominated by growth, while the deceleration is a consequence of the extinction of growth, because of the impingement of diverse growth regions at grain boundaries [5],... 

Clem and Fisher (1958) use a similar treatment as above to derive the solid state nucleation kinetics for new phases at grain boundaries. They neglect orientation of the critical nucleus with respect to the host, strain energy, and coherency effects. Nucleation at the grain boundary interface removes boundary energy. Their treatment yields the following critical values ... 

Figure 9. Alloy 718 deformed at 980 °C, 10 2 s 1 to e = 0.4 in uniaxial compression, exhibiting continuous nucleation (A), as well as nucleation by grain boundary bulging (B)...

Fig. 47. Schematic representation of hysteresis loops (a) the coercive force is determined by wall nucleation (b) the coercive force is determined by inhomogeneous nucleation and pinning of walls at grain boundaries (c) the coercive force is determined by magnetic inhomogeneities distributed more or...

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