Mechanism and modeling of saw-tooth structure formation in the Ll2-Ll0 two-phase system

The strain-accommodating mechanism of formation of the saw-tooth microstructure 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, the master equation, which explicitly incorporates the effect of the transformation strain, is formulated. It is used to simulate the precipitation of the L1(0) phase from the L1(2) parent phase in the Co-Pt alloy. The computer simulation shows that the decomposition occurs heterogeneously on antiphase boundaries of the L1(2) structure if the composition is near the solubility limit of the L1(2) phase. Ir eventually produces the saw-tooth microstructure. The decomposition transforms the (010) antiphase boundary into a layer of the single-variant tetragonal L1(0) phase separating the L1(2) antiphase 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 Co38.5Pt61.5. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd AN rights reserved.