Field-ion investigations of tantalum-molybdenum solid solutions

Abstract

A field-ion microscope study of the tantalum-molybdenum system has been carried out at liquid hydrogen temperatures. The purpose of this investigation was twofold; the first being to increase knowledge on the field-ion microscopy of alloys, and the second being to categorize the segregation effects which, at high concentration levels, embrittle the alloys. Micrographs have been obtained from three alloys of composition Mo-8%Ta, Mo-5o%Ta and Ta-10%Mo respectively, and also from the two pure metals. The Ta-Mo alloy micrographs obtained show far more irregularity than those from W-Mo alloys. The greater regularity of the Mo-8%Ta micrographs over those from Ta-Mo has been shown to be in accord with predictions based on field evaporation from dilute alloys, and the unexpected large difference in the regularity has been accounted for by considering the polarization and heat of solution terms in the field evaporation equation. Computer simulation of images from clustered alloys has been performed on the assumption that solute atoms do not participate in image formation. A band structure results when deviations from randomness become appreciable in the clustered alloy. The contrast to be expected from a demixed alloy in the Ta-Mo system has been discussed, but the available experimental evidence did not reveal any such contrast effects. It has been concluded that the width of such demixed zones probably exceeds 1000 Å. However, a tendency for local clustering on the [200] planes was observed in the Ta-5o%Mo alloy.