Ageing and overageing of lead-calcium alloys

Lead-calcium alloys are commonly used for their high mechanical properties, compared with pure lead. These alloys evolve quickly at room temperature and could in a few months, or years, undergo a softening (overageing). During the last decade, much research has been carried out on the subject without any unanimous results, due mainly to the diversity of the alloy's transformations and the difficulties of observation (wide range of kinetics, heterogeneity of transformations, etc.). The use of several in situ techniques developed specifically for lead alloys in our laboratories has enabled us to identify five transformations in lead-calcium alloys. The structural hardening begins by two discontinuous transformations. The first one is complete and the second one of ordering type remains incomplete. The end of the hardening is due to a thin precipitation of Pb(3)Ca, which is followed by a coarsening (softening) of macroprecipitates aligned in strings. The last softening stage comes with the formation of lamellar Pb(3)Ca precipitates by coalescence of the macroprecipitates.