Effect of the state of aggregation of lead on the yield stress of NaCl : Pb and KCl : Pb

The effect of the state of aggregation of lead on the hardening behaviour of NaCl : Pb and KCl : Pb has been systematically investigated. It is concluded that for KCl : Pb, the yield stress is independent of lead aggregation at concentrations up to about 550 p.p.m. Furthermore, no hardening effect seems to be associated with Suzuki-phase precipitates formed by suitable thermal treatments. For NaCl : Pb, however, the aggregation of lead after fast quenching induces a marked hardening for concentrations ≳ 50 p.p.m. The ageing curves have been determined for a number of temperatures up to 350°C. In the lower-temperature range (T ≲ 100°C), there is an increase in yield stress with annealing time up to a flat saturating stage. In the higher temperature range (T > 100°C), a ‘softening’ region is observed for long annealing times after a maximum value of the yield stress. The results are qualitatively analysed in terms of precipitation-hardening mechanisms. Finally, a possible method for determining the solubility of incoherent precipitates in alkali halide crystals is proposed.