Atomic scale studies of solute-atom segregation at grain boundaries: Experiments and simulations

Abstract

This paper presents two atomic scale approaches to study grain boundary (GB) segregation phenomena. The first is an experimental one that combines transmission electron microscopy (TEM) with atom-probe field-ion microscopy (APFIM)—APFIM/TEM—to measure quantitatively the Gibbsian interfacial excess of solute at GBs whose five macroscopic degrees of freedom are first measured by TEM; with this approach it is possible to explore systematically GB phase space. APFIM is also used to determine segregation profiles with atomic resolution. An application is presented for this combined experimental approach for a single phase Fe(Si) alloy. The second involves Monte Carlo simulations of solute-atom segregation at GBs in bicrystals of single-phase f.c.c. alloys; this approach is also used to systematically explore GB phase space. The atoms are allowed to interact via long-range continuous embedded atom method potentials, and so-called transmutational ensemble is employed. The results show that, unlike the previously investigated Au-Pt system, the (002) twist boundaries are enhanced in solute atoms on both sides of the phase diagram. For low-angle (002) twist boundaries on the Pt-rich side the atomic sites enhanced in solute concentration are arranged in hourglass-like structures centered on the square grid of primary grain boundary dislocations. While for the same boundaries on the Ni-rich side the atomic sites enhanced in solute concentration are located in bipyramidal regions based on the squares cells of the same grain boundary dislocations. Thus, the atomic sites that are enhanced on one side of the phase diagram are not affected on the other side and vice versa.