FIELD KINETIC-MODEL AND COMPUTER-SIMULATION OF PRECIPITATION OF LL(2)ORDERED INTERMETALLICS FROM FCC SOLID-SOLUTION

A phenomenological stochastic field kinetic model of coherent precipit ation of Ll(2) ordered intermetallics From a disordered f.c.c. solid s olution has been developed. It explicitly takes into account both the lattice misfit strain and the four types of antiphase domains formed a s a result of the Ll(2) ordering. Based on the concentration wave repr esentation of the ordered state, a coarse grain approximation of the n on-equilibrium free energy functional of concentration and long-range order parameters has been formulated. Its relation to the microscopic free energy model has been discussed. Precipitation kinetics and micro structural development of gamma' phase in Ni-Al have been investigated by two-dimensional computer simulations based on this model. The resu lts reveal new features which are attributed to the Ll(2) ordering. Th e ordered nature of the precipitates changes the coarsening mechanisms and kinetics and greatly affect the morphology of the mesoscopic micr ostructure. (C) 1998 Acta Metallurgica Inc.