X-ray evidence of intermediate stages during precipitation from solid solution

Intermediate metastable structures resulting from the partial decomposition of solid solutions have been found in the Cu-Ni-Al, Fe-Ni-Al and Fe-Cu-Ni systems. In each case a single-phase structure, stable at high temperatures, breaks up into two phases on cooling. The first stage in the decomposition is the formation of islands, in which atoms are segregated prior to precipitation. Islands of CuNi 2 Al segregate from a copper-rich solid solution. Islands of FeNiAl segregate from an iron-rich solid solutions. These islands are recognized by the formation of the superlattices characteristic of the precipitating phases. During the decomposition of FeCu 4 Ni 3 into two phases, a laminated structure is produced, in which copper-rich laminae alternate with laminae rich in iron and nickel. Each has a tetragonal structure with the same a axes as in the former solid solution; in the copper-rich laminae c > a ; in the other laminae c < a .