Atomic-scale modelling of intergranular segregation: The case of alloys with strong size-effect

Number of alloys properties depends on the structure and on the chemical composition of defects. Within this context, we present a review of the results obtained on intergranular segregation for the Sigma=5 (310) <001> symmetrical tilt grain boundary and for a system presenting a strong tendency towards phase separate in the bulk (i.e. Cu - Ag). This study is performed by coupling Monte Carlo simulations taking into account atomic displacements and an effective lattice-gas model derived on a rigid lattice. Monte Carlo simulations show the existence of a 2D compound in the grain boundary plane for an alloy with a strong tendency towards bulk phase separate. The detailed matching of the Monte Carlo results on the effective lattice-gas model allows us to determine the intergranular segregation driving forces, the difference between the size of both elements being the dominant one. Moreover, the coupling of the two methods highlights a significant contribution of the vibrational entropy to intergranular segregation. A satisfactory evaluation of this entropic term must take into account the vibrational coupling between nearest neighbours and the differential thermal expansion near the interface. By using the effective lattice-gas model to determine segregation isotherms for the grain boundary, we show the existence of a first-order multilayer phase transition. A similar behaviour is obtained for the open (310) surface, indicating that this multilayer property of the phase transition is due to the open nature of the inter-face plane. Finally, when approaching the bulk solubility limit, Monte Carlo simulations show a pre-precipitation, the initial grain boundary being split into two Cu / Ag interfaces separated by a metastable monocrystalline phase.