Nucleus Morphology and Energy

The simplest case occurs when the a-phase and /3-phase crystals have different compositions but still match almost exactly in all three dimensions. The critical nucleus can then form with a coherent interface and is therefore of relatively low energy.1 Also, any strain energy will be small. This condition is met during the precipitation of Ag-rich precipitates in a A1 + 4 at. % Ag matrix [8] and Co-rich precipitates in a Cu + 1 at. % Co matrix [9] where the precipitates are coherent and essentially spherical in shape. 

1 As discussed in Section B.6, coherency must always be specified relative to a reference structure. In this case, it can be either the a or /3 crystal. 

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