Evolution of elemental composition profile and its effects on thermal diffusivity

Full Text: In a recent experiment, we examined the relationship between the surface elemental composition profile of a two-element metallic alloy specimen and its gray-body emission and thermal diffusivity properties. The heated surface of an 80-20 nickel-chromium alloy ribbon was imaged onto a spectroscopic grade 2-D array detector for measurement of its thermal radiation intensity, while the depth-resolved elemental composition profile was measured intermittently by means of time-resolved spectroscopy of laser-produced plasma (LPP) plumes from the surface. The results showed a depth-dependent composition profile, and its detail affected the surface emissivity. The composition profile evolved in time at rates dependent on the temperature. For instance, the composition of a surface area, freshly exposed by LPP excitation, moved toward another that was enriched in chromium due to preferential diffusion of chromium. Existence of depth-dependent surface composition profiles have also been documented in studies of different alloy systems by the method of LPP plume spectroscopy. We have now initiated a new series of investigation into a number of competing processes that contribute to the formation of elemental composition profiles. We report on a study in which the effect of gravity on the composition profile formation is examined for the four-element system of Wood's alloy. A sample of the alloy is melted and re-solidified under controlled conditions. The top and bottom surfaces are analyzed for composition while the thermal diffusivity is measured at different depth as new sub-micron layers are removed by successive laser ablation. The top and bottom surfaces have been found to have two different composition profiles as a result of elemental mass density disparity manifesting in element-specific buoyancy during solidification. The non-contact spectroscopic method has been used for thermal diffusivity measurement, which has demonstrated a clear causal relationship between thermal diffusivity and elemental composition. (author)