Development of prediction method of grain boundary and intragrain microstructures at an atomistic scale by utilizing GC Monte-Carlo methods.

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Monte Carlo simulations have been applied to examine microstructural developments of precipitation structure at and near the grain boundary, to understand the formation mechanism of precipitation-free zone and influences of vacancy and solute atom depletions on the kinetics of PFZ formation. To realize simultaneous grain boundary and intragrain precipitations, we adopted Monte-Carlo simulation with the grain boundary set as a mass reservoir. This enabled us treating the whole system on a rigid lattice system.Vacancy depletion near the grain boundary simply delayed the whole precipitation kinteics, and no clear formation of PFZ was observed. In contrast, simultaneous grain boundary and intra grain precipitation lead to a formation of well-defined precipitation free zones, whose interface is defined by an abrupt extinction of precipitates, without any smooth change in the number density or size of precipitates. On the other hand, this simultaneous process poses a rather strict limitation between the PFZ width and interparticle distance, which should lie in the same order of magnitude. This does not necessarily agree with reported experimental observations. We found that a long-range solute depletion prior to precipitaion may lead to a PFZ microstructure whose PFZ width is far more larger than the interparticle distances of the precipitates, which agreed with actual PFZ microstructures in many cases.For the mutinary calculation required to examine the effects of vacancy-solute and solute-solute interaction, the code is still not efficient enough to simulate a model system large enough to examine the size including long-range depletion area for PFZ formation. This point needs further development.