Grain nucleation

When metals are deformed plastically at room temperature the dislocation density goes up enormously (to =10 m ). Each dislocation has a strain energy of about Gb /2 per unit length and the total dislocation strain energy in a cubic metre of deformed metal is about 2 MJ, equiva-lent to 15 J mol k When cold worked metals are heated to about 0.6T new strain-free grains nucleate and grow to consume all the cold-worked metal. This is called - for obvious reasons - recrystallisation. Metals are much softer when they have been recrystallised (or "annealed"). And provided metals are annealed often enough they can be deformed almost indefinitely. 

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...

In Ref. [4], such a circumstance is explained by the fact that the size of the grains in the crystallized layer depends not only on the cooling rate (value of supercooling), but also on the number of active centers of grain nucleation in the melt. 

There are two ways to form a PSF in GaN one by a shift of one part of a GaN crystal over the other part by R = 1/2[1101] or, alternatively, by removing a (1210) plane and then applying a 1/6[1210] shift. In our samples, GaN is grown on a thin AlN buffer ( 200 nm), which undergoes the Stransld-Krastanov [34] growth mode. Therefore, GaN grains nucleate at steps or pits in the AlN buffer... 

Heteroepitaxy can be disturbed by the formation of disordered carbon on the surface, creating nucleation sites for diamond. Diamond grains nucleated on carbon have a random orientation. 

These equations are accompanied by relation [10.26] given in Table A.3.1 of Appendix 3, which connects the fractional extent at time 0 for a grain nucleated at time tj ... 

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