Composition: Cu-8.4%P (eutectic composition).

Processing: Sand casting.

Etching: 30 s immersion in FeCl₃ aqueous solution.

After polishing at etching, the specimen shows iridescent surface with a radial structure. Columnar grains are poorly defined. As shown in the micrographs, the liquid has transformed into a eutectic aggregate with a lamellar arrangement of α phase (solid solution of P in Cu) and a compound with a well-defined stoichiometry (Cu₃P). The presence of dendrites of primary Cu₃P phase indicates that the alloy is not exactly eutectic. These dendrites tend to nucleate ate the interfaces between pores and the liquid.

The two phases that are present in this system are easy to discern.  After etching, the Cu-rich α phase darkens and acquires a red tone, while copper phosphide remains white. Another interesting feature is the the coarse and irregular distribution of the phases at the locations where eutectic colonies meet. 

Cu-P diagram

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