Mechanism and modeling of saw-tooth structure formation in the Ll(2)-Ll(0)two-phase system

The strain-accommodating mechanism of formation of the saw-tooth microstruc ture in the Ll(2) + L1(0) two-phase ordered system is proposed. To describe the atomic scale kinetics of ordering and decomposition in this system, th e master equation, which explicitly incorporates the effect of the transfor mation strain, is formulated. It is used to simulate the precipitation of t he L1(0) phase from the L1(2) parent phase in the Co-Pt alloy. The computer simulation shows that the decomposition occurs heterogeneously on antiphas e boundaries of the L1(2) structure if the composition is near the solubili ty limit of the L1(2) phase. Ir eventually produces the saw-tooth microstru cture. The decomposition transforms the (010) antiphase boundary into a lay er of the single-variant tetragonal L1(0) phase separating the L1(2) antiph ase domains. Later, the strain accommodation transforms this single-variant L1(0) phase layer into the sawtooth pattern. The simulation results are in excellent qualitative and quantitative agreement with our TEM images of Co 38.5Pt61.5. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science L td AN rights reserved.