Mechanism of Reversion in Pb-Sb Alloys with Trace Element As

Abstract

The thermal stability of Sb-rich precipitates is investigated by means of the calorimetric method and electrical resistivity measurement in order to make clear the anomalous enhancement effect of a trace additional element As (0.01 wt%).
Electrical resistivity is decreased with ageing time monotonously. With small arsenic additions, the precipitation of Pb-Sb alloys is greatly accelerated, and remarkable reversion phenomena are observed at the lower temperature side than that of the heat absorption due to the resolution of equilibrium precipitates in their specific heat vs temperature curves. In the case of Pb-Sb alloys free of arsenic, only a faint reversion reaction is observed, but a considerably longer time is necessary for the completion of reaction. It is shown that rather complicated reversion phenomena in Pb-Sb alloy system are fully interpreted by the critical size theory considering the change in size of precipitates, the degree of supersaturation and the relation between ageing and reversion temperature.
The resolution mechanism of the precipitates is as follows: Since the coarsening process of the fine precipitates proceeds at the state of nonisotherm and nonequilibrium, the system dissolves the fine precipitates smaller than the critical size at the reversion temperature, so as to cancel the free energy increase of the system due to the interfacial energy between precipitates and matrix.
The reversible precipitates are considered as fine equilibrium phase containing As atoms considering from its reversion behavior. Anomalous enhancement effect in Pb-Sb alloys with small arsenic addition may be attributed to the refinement and increase in distribution density of Sb-rich precipitates by As atoms or its clusters as “catalyzer” in the nucleation of the precipitates.