ATOMIC-SCALE STUDIES OF SOLUTE-ATOM SEGREGATION AT GRAIN-BOUNDARIES -EXPERIMENTS AND SIMULATIONS

This paper presents two atomic scale approaches to study grain boundar y (GB) segregation phenomena. The first is an experimental one that co mbines transmission electron microscopy (TEM) with atom-probe field-io n 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 t o explore systematically GB phase space. APFIM is also used to determi ne segregation profiles with atomic resolution. An application is pres ented for this combined experimental approach for a single phase Fe(Si ) alloy. The second involves Monte Carlo simulations of solute-atom se gregation at GBs in bicrystals of single-phase f.c.c, alloys; this app roach is also used to systematically explore GB phase space. The atoms are allowed to interact via long-range continuous embedded atom metho d potentials, and so-called transmutational ensemble is employed. The results show that, unlike the previously investigated Au-Pt system, th e (002) twist boundaries are enhanced in solute atoms on both sides of the phase diagram. For low-angle (002) twist boundaries on the Pt-ric h side the atomic sites enhanced in solute concentration are arranged in hourglass-like structures centered on the square grid of primary gr ain boundary dislocations. While for the same boundaries on the Ni-ric h side the atomic sites enhanced in solute concentration are located i n bipyramidal regions based on the squares cells of the same grain bou ndary dislocations. Thus, the atomic sites that are enhanced on one si de of the phase diagram are not affected on the other side and vice ve rsa.