NEW ASPECTS OF THE SOLID-PHASE EQUILIBRIA IN THE TERNARY PB-SN-CA SYSTEM FOR X(CA)LESS-THAN-OR-EQUAL-TO-25 AT.PERCENT

New crystallographic data prove that CaPb3, and Sn3Ca-L1(2), compounds belong to the same single-phase Ca(Pb,Sn)(3) domain in the ternary Pb-Sn-Ca equilibrium phase diagram. A tentative calculation of this diagram shows that about 2 wt.% Sn, introduced in the alpha-Pb matrix, allows a decrease in the solubility of Ca at 400 K from 10(-2) to 10(-7) wt.%. Simultaneously, the L1(2), compound, in equilibrium with the alpha-Pb matrix, crosses very rapidly the chi(Ca),=0.25 line, continuously from the Pb-Ca border to near the Sn-Ca border, when increasing the Sn content in the alpha matrix from zero to 8 wt.% Sn at 400 K. A very large triangle formed by the three phases alpha-Pb (8 wt.% Sn), beta-Sn (1.7 wt.% Pb), Ca25Pb5Sn70-L1(2), appears in this diagram at 400 K. This triangle provides a ternary eutectic, calculated at 424 K. This, calculation brings to light some reasons for the process transition for the hardening of Pb-Ca grid-battery alloys, from discontinuous to continuous, when increasing the tin content.