Routes To Development Of Near-Surface Alloy Composition Anomaly

Thermophysical data in the literature for metallic alloys, including singleelement specimens, exhibit a high degree of variability. Of iInterest 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