Mechanism of enhanced precipitation phenomena in Pb(Sb) alloys with trace element As
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I. I N T R O D U C T I O N
THE Pb-Sb alloy system is known as the most important material among practical lead base alloys, so many investigations have been reported. 1-5The aging process in this alloy system is simple: the rhombohedral equilibrium Sb phase, which contains a small amount of Sb atoms in solid solution, is formed directly from Pb-rich supersaturated solid solution? -3 On the other hand, the effect of various additional trace elements in aging characteristics of this alloy system have been investigated by one of the authors ~and it is shown that the effect of trace element As is remarkable: Both the distribution density of precipitates and the decomposition rate of the solid solution in Pb-Sb alloy are greatly increased with a very small arsenic addition, and the age hardening is also greatly enhanced. In order to explain the above phenomena, two models have been presented previously. Hopkins and Theweites 5 assumed a model of interstitial As atoms in lead lattice, in which the resultant stress field around the As atom would be relaxed by that around the Sb atom which is smaller than the Pb atom, when they are in the nearest neighbor position with each other. Thus the clustering tendency of solute atoms around As atoms would be expected to increase the density of Sb precipitates in aging process. The other is a model given by one of the authors? The fractions of Sb-As and As-As pairs in a Pb-Sb-As alloy were calculated at solid solution temperatures and fairly larger values than in the case of regular solution were obtained, from which some grouping of Sb atoms around As atoms and As atom clusters were anticipated. Such a grouping of Sb K. TSUMURAYA is Associate Professor, School of Engineering, Meiji University, Kawasaki 214, Japan, and S. NISHIKAWA is Professor, Institute of Industrial Science, University of Tokyo, Tokyo 115, Japan. Manuscript submitted September 18, 1979.
atoms would be effective as a nucleus for the heterogeneous nucleation of Sb precipitates. Tsumuraya and Nishikawa 6 discussed the kinetics of decomposition process based on electrical resistivity measurement in a Pb-0.038 wt pct As alloy. Although the decomposition rate was large, the possibility of interstitial solution of As atom in Pb lattice could be rejected by analyzing the activation energy in each stage of aging in this alloy. It is not plausible, therefore, that there could be heterogeneous nucleation at interstitially dissolved As atoms. As the above shows, there is no doubt that the groups of As atom are responsible for the nuclei of precipitation. In the present investigation, one more step has been made to clarify the true mechanism of heterogeneous nucleation and of enhanced precipitation phenomena in the Pb-Sb alloys with trace additional element As.
II. E X P E R I M E N T A L Four Pb-Sb alloys containing 1 and 1.7 pct Sb with and without about 0.01 pct As and four Pb-As alloys were selected as shown in Table I, in order to compare the aging process in these alloys with that in the Pb-0.038 pct As alloy. 6 They
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