Experimental Investigation of the Epsilon Phase in Pb–Bi System

The Pb–Bi (Lead–Bismuth) binary system has gained some prominence in recent years due to the possible use of eutectic alloys as primary-circuit coolant in generation IV nuclear reactors. Apart from the terminal solid solutions A1-Pb and A7-Bi, the system presents also one intermetallic phase with the hexagonal close-packed structure (A3), usually denoted ε phase. Due to the low temperatures involved in this system, attainment of equilibrium is difficult, and in addition, the proximity of x-ray scattering factors for both elements raises questions about a possible polymorphic transition. In the present work, three samples with compositions Pb–29 wt.%Bi, Pb–31 wt.%Bi and Pb–33 wt.%Bi were prepared and characterized by x-ray diffraction. The lattice parameters of ε are consistent with those reported in the literature, and their dependence on Bi content was determined as well. Only the hexagonal phase was identified in the 29 wt.%Bi sample, although the currently assessed Pb–Bi phase diagram predicts the presence of the Pb-rich A1 terminal solid solution in this composition. A 2-year annealing at room temperature was performed and the XRD characterization results for these samples are compared with the original data.