The Study of Grain-Boundary Segregation by Field-Ion Microscopy

Abstract

The field-ion microscope (FIM) allows the study of the atomic structure of a grain boundary and the adjacent regions. Certain impurities, such as oxygen, can be detected because they are associated with characteristic additional very bright image spots. The FIM has been used to investigate the distribution of oxygen in and around grain boundaries in tungsten. The oxygen was incorporated by a heat-treatment of specimen blanks ∼ 100 μm thick, for ∼ 8 h in an atmosphere of 133 × 10⁻³N/m² (10⁻⁵ torr) oxygen at 1400° C (1673 K). The oxygen-associated bright spots are thought not to be the oxygen itself but to be solvent atoms stabilized in protruding positions adjacent to the solute. The spatial distribution of bright spots in and perpendicular to the grain-boundary plane has been investigated for doped and undoped material. In each case fine-scale field-evaporation sequences were carried out so that each oxygen-associated bright spot was recorded; counting was confined to the {110} region and the decorated zones were avoided.