Routes To Development Of Near-Surface Alloy Composition Anomaly

Abstract

Thermophysical data in the literature for metallic alloys, including singleelement specimens, exhibit a high degree of variability. Of interest is if the reasons are intrinsic. That there exists as a rule a surface layer whose elemental composition differs from that of the bulk has been demonstrated. Such a near-surface composition anomaly can, in principle, account for the significant part of the variability. The method of time-resolved spectroscopy of emissions from laser-produced plasma (LPP) plumes has been the key in this new development because both the elemental composition and thermal diffusivity can be measured simultaneously. Multiple application of LPP analysis leads to depth-resolved measurements. A variety of alloy specimens have been subjected to different scenarios of thermal cycling to find that thermal cycling modifies the near-surface composition profile of a given specimen. A new study has been carried out with Wood's alloy as a model system by forcing the specimen into melting at increasing temperature. This investigation has revealed that the near-surface composition undergoes sharp irreversible changes when the highest elemental melting point has been exceeded before the specimen is cooled to resolidification. A possible mechanism is suggested.

Surface Position-Resolved Thermophysical Properties for Metallic Alloys

“…A Q-switched laser pulse is directed at the specimen directly from above through a focusing lens that is an integral part of the vacuum chamber [2][3][4]. The experimental setup has two additional capabilities implemented: (a) one-to-one spectroscopic imaging of the laser focal area of the specimen's surface onto the entrance slit of a spectrograph and (b) a capability to translate the specimen stage within the vacuum chamber by an x-y translation stage that is positioned outside the vacuum chamber.…”

“…The Q-switched Nd:YAG laser, vacuum pumps, gas supply handling manifold, and digital timing and recording electronics are not shown. appears to be a threshold temperature for a significant modification of Wood's alloy composition at the surface [2][3][4]. The full round of heating may be repeated up to three times with each specimen in order to accentuate the elemental composition anomaly.…”

“…Our studies with the model system of Wood's alloy have shown that it is possible to drive the elemental composition of a surface layer away from the composition of the bulk by thermal cycling of a specimen between heating to 393 • C and resolidification [4]. It is quite possible to speculate that the depth dependence is manifest due to a distribution of patches of segregated elements at the surface, whose feature size and shape are irregular and depth dependent due to thermo-fluid dynamic instability during melting and solidification.…”

“…It is quite possible to speculate that the depth dependence is manifest due to a distribution of patches of segregated elements at the surface, whose feature size and shape are irregular and depth dependent due to thermo-fluid dynamic instability during melting and solidification. In previous studies [2][3][4], composition variations were examined as a function of depth; element specific emission intensities were integrated over the focal area of the laser beam that excites the material surface. The focal area is oblong in shape, 2.3 mm long and 1.3 mm wide.…”

Benard–Marangoni Instability as Possible Modifier of Surface Alloy Composition

“…This is plainly evident in the wide variability 1 Department of Physics, Lewis Laboratory 16, Lehigh University, Bethlehem, Pennsylvania 18015, U.S.A. E-mail: ywk0@lehigh.edu seen in the thermophysical property literature. Metallic alloys, including single-element specimens, exhibit high variability, for example, in spectral emissivity and thermal diffusivity.…”