Field-Ion Microscope Observation of Thermal Faceting in Ni4Mo Emitter

Abstract

Modes of thermal faceting in emitters of Ni4Mo alloy quenched from the α region (FCC structure) were investigated by field-ion microscopy, using in-situ annealing without an applied field. Faceting with FCC configuration occurred, not only at 1110–1300 K or about 770 K where ordering does not practically occur in bulk, but even at 1100–990 K where nucleation of the β phase (ordered BCT structure) is very active in bulk. At 940–870 K, however, a particular type of faceting occurred at the {751}FCC poles. An interpretation of this faceting was successfully made in terms of crystallography of the ordered structure. Those facets were superlattice {211}BCT facets of the six orientation variants of the β phase. All the variants had equivalently developed. The ordered regions associated with the facets were found to be disc-shaped, a few tens of nanometers in diameter and less than about 2 nm in thickness.