Evolution of Near-Surface Elemental Composition Profile and Its Effect on Thermal Diffusivity

In a series of recent experiments, utilizing the method of time resolved spectroscopy of laser-produced plasma (LPP) plumes from specimen surfaces, the nearsurface elemental composition profiles were observed to be nonuniform and significantly different from the respective bulk composition. A new study of three alloy systems is reported, with a view toward establishing the causal relationship between the near-surface elemental composition profile of a specimen and its thermophysical properties in general and thermal diffusivity in particular. The systems in question are as follows: two-element Nichrome ribbon; fourelement magnetic Mumetal foil; and four-element Wood’s alloy. The method of LPP plume spectroscopy has been used throughout to successively expose new surface layers and measure the composition and thermal diffusivity. With two of the systems, modification of the near-surface elemental composition profiles has been forced. Sustained electrical heating of a Nichrome ribbon specimen revealed preferential diffusion of chromium to the surface, affecting the spectral emissivity and thermal diffusivity as well as depth-dependent local heating rates. In the case of Wood’s alloy a sample is melted and re-solidified under a protocol that highlights gravitational forcing. The noncontact spectroscopic method has been used to discover that the top and bottom surfaces acquire two different composition profiles and exhibit commensurate disparity in the measured thermal diffusivity profiles.