Field-Ion Microscope Studies of Ni4W Type Ordering in the Near-Surface of Ni-16.6 at%W Alloy

Abstract

Field-ion microscope observations were made on Ni-16.6 at.%W alloy quenched from the high temperature α phase region to investigate Ni4W type ordering in the near-surface during in-situ annealing. Somewhat below the peritectoid temperature, disc-shaped Ni4W domains of the particular variants were formed preferentially at some of {420}FCC planes on the surface regions with their disc faces being {110}BCT superlattice planes (at 950°C for ca. 5 min). The relaxation time of occurrence of an ordered domain at one of the {420}FCC nucleation sites and the rate of its thickness-increase were evaluated to be 520±260 s and 0.10±0.08 Å/s, respectively. Surface layers of a few to fifteen angstrom in thickness with lower degree of order were formed upon annealing for a few minutes at 920–860°C. The degree of order in the near-surface also decreased upon annealing partially ordered tips at the same temperatures.