Abstract
The specific heat vs. temperature curves were obtained during the re-heating of the lead-0.4 wt%Mg alloys aged for various times at the ageing temperature (room temperature, 50° and 100°C). The shapes of the specific heat vs. temperature curves obtained in this experiment were discussed, and the amounts of heat evolution and heat absorption observed on the specific heat vs. temperature curves were plotted as a function of the ageing time prior to specific heat measurements. (1) The heat evolution and the heat absorption, which were observed at 80°∼180°C and 180°∼225°C respectively on the specific heat vs. temperature curve of the alloy aged at 100°C (a high ageing temperature), were interpreted respectively as accompanying the formation of the stable precipitate Mg2Pb and the re-dissolution of its precipitate into matrix lead. (2) An additional heat absorption followed by the heat evolution accompanying the formation of the stable precipitate was observed at about 110C° on the specific heat vs. temperature curves of the alloy aged at low ageing temperatures (below 50°C). This additional heat absorption could be explained due to the “reversion phenomena” (the dispersion of G-P zone into matrix lead). (3) An irregularity in the valley of the heat evolution accompanying the formation of the stable precipitate was observed at about 110°C on the specific heat vs. temperature curve of the alloy aged 10∼70 hr at room temperature. This irregularity was also considered due to the reversion phenomena. (4) The fractional change (f) of precipitation heat with ageing time (t) at 100°C seemed to be represented Johnson-Mehl’s equation: f=1−exp(−btn), where b and n are the constants. The value of n was about 2.5.
