Crystallization studies of the β (Mg2Pb) phase and its phase boundaries in the Pb-Mg-Bi system

Abstract

A peritectic reaction between Mg bismuthide and Mg plumbide changes from an even to an odd reaction at 310 °C (0.14 wt pct Bi and 4 wt pct Mg) owing to the incorporation of 1.6 wt pct Bi into Mg plumbide; this value was calculated from material balances of equilibrium studies of this reaction and confirmed by direct analysis of crystals using a “Cameca Microbeam” electroprobe microanalyzer. A model is presented in which an atom of Pb in the unit cell, 46 Mg2Pb Mg2Pb5, is replaced by an atom of Bi which gives a concentration of 1.63 to 1.71 wt pct Bi depending on the actual species of Mg plumbide present. The phase boundary for double saturation with Pb and Mg plumbide, established from the data of equilibrium tests, shows a minimum temperature of 251.8 °C at 0.008 wt pct Bi and 2.2 wt pct Mg. Alloys in the primary Pb phase field adjacent to this boundary show undercooling to less than 248.5 °C followed by one or two sharp temperature increases to 250.5 °C, with the initiation of the removal of Bi when double saturation occurs giving a final liquid phase containing less than 0.001 wt pct Bi. Crystallization paths for alloys in the Mg plumbide phase field show a catatectic reaction and polymorphic transformations in the intermetallic compound. The removal of Bi is dependent on the concentration of Bi and Mg in the initial alloy. In systems containing sufficient Mg, a final alloy containing less than 0.001 wt pct Bi can be produced, and these conditions have been used as the starting point for the development of a process for the removal of Bi from Pb. Finally, the crystallization paths show there is a change in the thermal properties of the liquid alloy at 0.008 wt pct Bi which is independent of the temperature and concentration of Mg, and further work is required to resolve this finding.