Phase-field modeling of bimodal particle size distributions during continuous cooling

Microstructures in Nickel-base alloys typically contain a two-phase mixture of gamma/gamma'. The microstructure having a bimodal size distribution of gamma' is of particular interest because it has important property consequences [1]. In this paper, the phase-field method with an explicit nucleation algorithm is employed to investigate the microstructural development during a continuous cooling with various cooling rates. It is demonstrated that bimodal particle size distributions can be achieved at an intermediate cooling rate due to a coupling between diffusion and undercooling, in which the system experiences two peaks of well-isolated nucleation events. It is suggested that this is caused by soft impingement, followed by a renewal of driving force for nucleation, followed by a subsequent soft impingement. Under very high cooling rates, the microstructure becomes unimodal, because undercooling always outruns diffusion and the microstructure never reaches soft impingement. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.