THE DENSITY DISCONTINUITY ASSOCIATED WITH THE PRECIPITATION OF THE 2ND-PHASE IN PB+7.45 AT.PERCENT SN ALLOY

Associated with the precipitation of an Sn-rich phase from a Pb + 7.45 at.% Sn alloy during cooling at a constant rate from an elevated temp erature well above the solvus temperature, there is a measurable and p recipitous decline in density which begins at a temperature slightly b elow the solvus temperature for this alloy. This decline takes place o ver a Limited temperature range and is attributed to the formation of excess vacancies created during the nucleation of the precipitation re action. A similar formation of excess vacancies has been observed usin g positron annihilation spectrometry in a similar alloy during cooling through the solvus temperature, During this decline in density, the c oncentration of these vacancies much exceeds the equilibrium vacancy c oncentration for this alloy. The excess vacancies are unstable and, wi th continued cooling, quickly anneal out over a narrow temperature ran ge allowing the versus temperature curve to regain linearity. (C) 1997 Elsevier Science S.A.