An atomistic study of tilt grain boundaries with substitutional impurities

Abstract

An atomistic study of symmetrical [001]tilt boundaries in f.c.c. metals with a non-equilibrium distribution of substitutional impurities in the boundaries has been carried out. The atomic interactions described in the previous paper have been used. They consist of empirically constructed pair potentials and density dependent terms. The density dependent contribution to the total energy is relaxed together with the central force contribution, which produces an overall volume change of the block. Bismuth and silver as solute atoms in copper, and silver as a solute in gold have been studied. The relaxations associated with an array of periodically and widely spaced solute atoms in otherwise ideal crystals of the two hosts are examined first. The grain boundaries studied are ∑ = 5 (210), 36.87° and bE = 17 (530), 28.07°. In both hosts the structure of the (530) boundary consists of two units of the (210) boundary and one octahedron from the ideal crystal orientation. Impurity atoms were then introduced successively at two sites in (210) boundaries and five sites in the (530) boundaries. The impurity atoms were introduced periodically with a large repeat cell so that only low concentrations were considered. For each case the energy of segregation. E γ to the sites selected, the concomitant relaxation of the host boundary structure and the change in grain boundary energy. Δγ b have been found. E s is found to be very sensitive to the local atomic environment of the solute atom in the host boundary and there is not a substantial difference in the overall segregation propensity to the two boundaries selected. Bismuth atoms cause larger distortion and expansion of the host copper boundaries than does silver. E s for the former is governed principally by the size effect while this can only partially explain the variation of E s for silver atoms at the various sites of the boundaries in copper. Segregation is energetically favorable for silver atoms at all sites considered in the boundaries in gold. It is shown that while there must be implicit dependences of E s on the physical and chemical parameters of the solvent, solute and corresponding alloy, the value of E s is dominated in any specific case by the strong effect of the local atomic environment. This leads to a strong selectivity of segregation sites. Possible implications of the results of the atomistic study for understanding of the propensity for segregation, effects of impurities on facetina and embrittlement are discussed.