Eutectoid transformations accompanied by ordering

Eutectoid transformations accompanied by ordering, unlike ordinary ones, proceed through non-pearlitic modes of transformations. Eutectoid invariants are classified into two categories in binary systems. The eutectoid invariant of A3(α)→D019(α2) + L10(γ) in the Ti-Al binary system belongs to the first category, in which one product phase has an ordered structure of a parent phase. Its transformation product exhibits a γα2 lamellar structure consisting of nearly perfectly aligned alternate lamellae of γ and α2, which is formed by precipitation of γ plates in either α or α2 matrix with the Blackburn orientation relationship. The eutectoid invariant of A1(γ)→D022(γ″) + L12(γ′) in the Ni3V-Ni3Al pseudo-binary system is an example of the second category, in which both product phases have different ordered structures of a parent phase. The transformation of a 75Ni-18V-7Al alloy results in a ‘checkerboard’ pattern consisting of a periodic array of columns of γ′ and two γ″ orientation variants, which are formed by phase separation simultaneous with ordering.