EUTECTOID TRANSFORMATIONS ACCOMPANIED BY ORDERING

Eutectoid transformations accompanied by ordering, unlike ordinary one s, proceed through non-pearlitic modes of transformations. Eutectoid i nvariants are classified into two categories in binary systems. The eu tectoid invariant of A3(alpha)-->D0(19)(alpha(2)) + L1(0)(gamma) in th e Ti-Al binary system belongs to the first category, in which one prod uct phase has an ordered structure of a parent phase. Its transformati on product exhibits a gamma/alpha(2) lamellar structure consisting of nearly perfectly aligned alternate lamellae of gamma and alpha(2), whi ch is formed by precipitation of gamma plates in either alpha or alpha (2) matrix with the Blackburn orientation relationship. The eutectoid invariant of A1(gamma)-->D0(22)(gamma '') + L1(2)(gamma') in the Ni3V- Ni3Al pseudo-binary system is an example of the second category, in wh ich both product phases have different ordered structures of a parent phase. The transformation of a 75Ni-18V-7Al alloy results in a 'checke rboard' pattern consisting of a periodic array of columns of gamma' an d two gamma '' orientation variants, which are formed by phase separat ion simultaneous with ordering. (C) 1998 Elsevier Science Limited. All rights reserved.