Cellular precipitation and overaging in an As-cast PbSnSrAl alloy

Abstract

The aging behavior of an as-cast lead alloy containing tin, strontium and aluminum has been investigated over the temperature range 298–423 K. After casting, the lead-rich dendritic grains of the alloy age harden to a peak Meyer hardness of 19.5 kgf mm −2 at room temperature (298 K) in about 50 h. Hardening presumably results from the general precipitation of coherent (Pb, Sn) 3Sr. The general precipitate responsible for age hardening the grains appears to be stable at temperatures ranging from 298 to 373 K; however, at these temperatures the grains are gradually decomposed by cellular precipitation into a lamellar structure of depleted solid solution and a precipitate having about half the hardness of the age-hardened grains. This behavior results in the gradual overaging of the alloy and has been observed in other lead-base alloys such as PbSnCa and PbNa. The growth rate and lamellar spacing of the cells have been determined by quantitative metallography and analyzed according to existing thepry. The analysis suggests that the cell growth is controlled by the grain boundary diffusion of strontium in the advancing cell interface. At temperatures above 373 K the alloy overages primarily by the overaging of the general precipitate structure in the dendritic grains although some cellular precipitation is observed.