Sequences of precipitate nucleation

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Solid-solid nucleation theory, as recently extended to faceted nuclei, is applied to several problems in understanding experimentally observed sequences of precipitate nucleation involving transtion phases during isothermal ageing. Transition precipitates form due to a low interfacial energy resulting from a greater structural similarity to the matrix than the equilibrium phase or succeeding transition phases and in spite of a usually smaller driving force, |ΔGv|, for nucleation than any of these competing phases. Transition phases formed after the first one are predicted to nucleate preferentially at the interphase boundaries of their predecessors, also on the basis of interfacial energy, rather than ΔGv considerations. Most examples of precipitation sequences cited from the literature are in agreement with this prediction; the few exceptions found are noted and discussed. The preference of equilibrium phases for nucleating at high-energy grain boundaries and of transition precipitates for nucleation at dislocations and other lower energy sites is explained on the following basis. The high-energy boundaries reduce the net interfacial free energy needed for nucleation of either phase so much that the higher |ΔGv| for the equilibrium phase becomes dominant. At the lower energy sites, the smaller reduction in ΔG*, the free energy of activation for nucleation, provided makes the lower interfacial energy of the transition phases the dominant factor in causing such phases to nucleate more rapidly. © 1975 Chapman and Hall Ltd.

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Journal of Materials Science