DISCONTINUOUS AND CONTINUOUS HARDENING PROCESSES IN CALCIUM AND CALCIUM TIN MICRO-ALLOYED LEAD - INFLUENCE OF SECONDARY-LEAD IMPURITIES

Different transformations in lead-calcium and lead-calcium-tin alloys are observed with various complementary techniques such as anisothermal microcalorimetry, optical and electronic microscopy, hardness measurements. Three alloy states are studied: as-cast, rehomogenised/water-quenched, rehomogenised/air-cooled. With binary lead-calcium alloys, three successive discontinuous transformations are observed, namely: an initial and complete discontinuous transformation with regular moving of the front of reaction; a second and incomplete discontinuous transformation (puzzle-shaped); a third and incomplete discontinuous transformation with precipitation of Pb3Ca. The role of secondary-lead impurities is complex: Ag reduces and Bi accelerates the rate of the discontinuous reaction, while Al refines the grain size. Lead-calcium-tin are characterised by the Sn/Ca ratio. For very small values of this ratio, the hardening is similar to that of lead-calcium alloys. For high ratio values, the hardening takes place after an incubation period and proceeds via a continuous micro-precipitation of the (PbSn)3Ca compound. For intermediate ratios, the different processes are able to operate separately in sequence. Ag increases the rate of the continuous precipitation and reduces the incubation time. No significant effects are observed with Bi or Al. The kinetic laws of the different transformations are presented and values for the energy of activation are determined.