Precipitation hardening in Pb-0.08wt.%Ca-x%Sn alloys--the role of the pre-ageing

The precipitation hardening mechanism in both supersaturated and pre-aged Pb-0.08%Ca-x%Sn alloys was investigated by means of hardness and calorimetric measurements (differential scanning calorimetry, DSC) and transmission electron microscopy observations. DSC studies have shown that a pre-ageing treatment of the ternary alloys at 363 K has a favourable effect by reducing the driving force of the overaging process. Furthermore, this preliminary treatment seems to give a finer and denser particle distribution than for only quenched alloys. This latter effect could explain the higher hardness of the pre-aged alloys. For the tin-rich alloy (∼2.0 wt.%), the pre-ageing treatment considerably reduces the micro-segregation of Sn at the grain (sub-)boundary. This phenomenon is accompanied by an increase of the precipitate tin content and also by the appearance of a lattice parameter mismatch of 2.9% between the L12 precipitate and the lead matrix. From all of these experimental results, it appears that the processes leading to the reduction in interfacial energy are not the rate determining step for the overageing