The initial stages of cluster nucleation for Pd or Ir adatoms on a W (1 1 0) surface and in the vicinity of surface steps are directly imaged by a field ion microscope (FIM). Three types of structures are observed. One is a one-dimensional linear chain, which is parallel to the nearest neighbor-stacking directions of the substrate. Another is a two-dimensional compact island, which is a pseudomorphic structure like the substrate. The other is a three-dimensional cluster, which shows a structural transition from bcc (1 1 0) to fcc (1 1 1). Factors affecting the structural transformation include coverage of atoms or atom chains, temperature of heat treatments and boundary of the substrate terrace. (C) 2009 Elsevier B.V. All rights reserved.