Step Energy Analysis of Liquid Inclusions of Pb with Cd, In and Sn in Solid Al

Abstract

The thermal behavior of liquid multicomponent inclusions consisting of Pb with Cd, In and Sn, embedded in a solid Al matrix has been investigated using in-situ transmission electron microscopy. By investigating the evolution of liquid Pb or Pb-alloy inclusions in-situ over a range of sizes and temperatures we are able to determine the factors that control the kinetics of shape equilibration. The most critical factor in this process is the step energy of {111} facets in the interface between solid Al and liquid Pb or Pb alloy. Changing the composition of the liquid inclusions makes it possible to study the effect of alloy composition on the step energy.The samples were produced by rapid solidification and contain a dense distribution of nanoscale inclusions, with sizes in the range of approximately 10-200 nm. The in-situ microscopy observations of the shape of the liquid Pb-Sn, Pb-In and Pb-Cd inclusions were made at temperatures between 200-650°C. Careful measurements of inclusion shapes were performed using the program described in [1].Similar to the results reported by Gabrisch et al.[2], a clear hysteresis in the inclusion shape is observed during heating and cooling. All particles are found to round off at a critical rounding temperature T r which increases non-linearly with inclusion size. This kinetic limitation to shape equilibration results from the need for ledge nucleation due to cusps at {111} orientations in the solid-liquid interface energy. The nucleation barrier disappears at the critical temperature for interface roughening, which was found to be about 550°C for Pb in Al [1]. At this temperature particles can reach their equilibrium shape independent of size.Interestingly, we found that Pb-Sn particles attain equilibrium at significantly lower temperatures than pure Pb or other Pb-alloy inclusions, indicating that the interface roughening temperature is indeed decreased by the presence of Sn. The micrographs in Figure 1 show a Pb-Sn particle of 107 nm in size completely melted at 253°C that only rounds off at 350°C, about 100°C below the rounding temperature for a pure Pb inclusion of the same size. In contrast, additions of Cd or In to the liquid Pb have no major effect on the step energy. This is illustrated in Figure 2, which shows a heating sequence of an Al matrix containing Pb-In inclusions. It is apparent that, similar to pure Pb inclusions, the large particle retains its faceted shape even at 425°C. Alloy additions may act as a surfactant, significantly affecting the kinetics of particle motion, coalescence and shape equilibration. In this study we explore the effect of alloying on the step nucleation as a function of temperature. Further investigations on the surfactant concentration and its distribution within the Pb inclusions are in progress [3]. References[1] H. Gabrisch et al., Microsc. Microanal. 90 (2002) 13.