An Electron Microscopic Investigation of Phenomena Associated with Solid Solution of Oxygen in Niobium

A transformation process in the solid solution period of the niobium oxygen reaction was investigated by electron microscopy and diffraction. In thin foils submitted to a heat treatment under reduced air pressure ordering of the introduced oxygen interstitials took place. This process could be followed in transmission. The transformation starts with a nucleation mechanism where by small nuclei composed of small parallel domains in crystallographic directions are formed. These nuclei grow together to form a cross hatch pattern. Selected area diffraction reveals superlattice reflections of an ordered phase. The formation of irregular shaped superlattice nuclei and an epitaxial grown suboxide phase is also described. It is supposed that all these phenomena represent a transient stage in the transformation to more stable suboxyde and oxyde phases of niobium.