Aging and overaging processes in Pb-0.08%Ca- x%Sn alloys using transmission electron microscopy and differential scanning calorimetry

The precipitation hardening and overaging mechanisms in three Pb-0.08%Ca- x%Sn alloys were investigated by means of hardness and calorimetric measurements (DSC) and transmission electron microscopy (TEM) observations. It is found that the highest tin content, close to 2 wt.%, allows delaying the overaging phenomenon in ternary Pb–Ca–Sn alloys. DSC studies show that tin has a favourable effect on the aging by providing further nucleation sites. Furthermore, the precipitate coarsening due to overaging shifts to higher temperatures upon increasing the Sn content. Using TEM to observe the microstructural transformations shows the presence of an epitaxial relationship between precipitates and the matrix during both aging and overaging. In the course of overaging, coarse precipitates adopt a lamellar morphology and their growth occurs preferentially along the [111] compact atomic plane direction. The tin content of coarse precipitates of (Pb 1− x Sn x ) 3Ca increases with rising overaging time and follows the tin content of the initial alloy.