The early nucleation of a eutectic alloy during high-energy milling

Abstract

The nucleation of a eutectic alloy –at an early stage– and the factors that determine whether rod or hexagonal growth occurs were evaluated as a function of the process control agent (PCA)—or the lack of it. The study is subdivided into three separated experiments; in all cases, the samples (elemental powders of Pb and Sn) were processed by high-energy milling under composition, pressure and temperature (c-P-T) vial conditions. In the first case, it was detected an excessive mechanical kneading when the powders were processed without PCA and a monolith –as long as 5 mm in length– was obtained. However, this route should not be underestimated because the presence of α−, β− and eutectic-phases –at the surface of the monolith– were traced as nanoparticles (<100 nm). In the second and third cases, to prevent the excessive mechanical kneading and interparticle bond of the powders during milling, two different mechanochemical routes with PCA were considered: (i) tributyl phosphate (TBP) and (ii) ethanol (ETOH). Both of them were proposed to modify the viscoelastic behaviour of the powders. Nanorods resulted when TBP was used. Although structural disorder –at nanometric scale– was not traced, other effects associated with TBP like organic debris and the presence of by-products were detected. Based on the results with the ETOH option, it was shown that the precursors store energy as structural defects during milling. This suggests that the early eutectic nucleation (hexagonal crystal structure) occurred due to structural defects, crystals interfaces and phase boundaries. These acted as preferential sites for nucleation. Moreover, a typical eutectic phase distribution was detected. Finally, it was demonstrated that even after prolonged milling time and nonetheless the kind of PCA –or the lack of it, only the eutectic formation was scarcely traced; that is to say, a partial eutectic formation took place.