Development of Near-Surface Composition Anomaly

Many multi-element alloy specimens have been shown to possess a wide variety of near-surface elemental composition profiles, which are significantly different from the bulk composition. Such composition nonuniformity adversely affects the measurement of basic thermophysical properties in alloys. In this paper is presented a new investigation into the mechanisms by which such depth-dependent near-surface elemental composition develops. Specifically, specimens of a low melting-point metallic alloy, Wood's alloy, as a model system are examined under varying thermal cycling conditions within a chamber of controlled gaseous atmosphere. The near-surface composition and thermal diffusivity are measured as a function of depth. The method of time-resolved spectroscopy of laser-produced plasma plumes emanating from the specimen surface is used. Different surface composition profiles emerge depending on the dynamic range of the thermal cycling forced on a specimen.