Microscopy Allows Near-Atomic Scale Resolution

Scientists at Berkeley Lab's National Center for Electron Microscopy (NCEM), in collaboration with colleagues from Denmark and France, have made the first direct high-resolution observations of melting and solidification in bimetallic alloys. Using nanometer-sized "ingots" placed inside the Center's powerful transmission electron microscope, the researchers were able to image and characterize the initial stages of liquid formation on a distance scale two orders of magnitude smaller than previously feasible.

Although the melting and solidification of solids are macroscopic phenomena, the early stages of both processes involve poorly understood interactions at the near-atomic scale. For this reason, a full understanding of these important phenomena has been difficult to achieve.

The NCEM team developed a new experimental technique to overcome this limitation. Using ion implantation or melt spinning of lead (Pb) and cadmium (Cd) into aluminum (Al), they were able to form isolated, 2-50nm "ingots" of a lead-cadmium alloy embedded as inclusions in a matrix (the "crucible") of the aluminum. At room temperature the ingots had distinct pure lead and pure cadmium segments each with their own, characteristic crystal structures. During in-situ heating inside NCEM's analytical electron microscope, the lead-cadmium alloy was observed to begin to melt at the triple line where the three metals are in contact (see figure).

The results were somewhat surprising. In contrast to single-phase nanoscale inclusions, which usually melt at an elevated temperature ("superheating), or isolated nanocrystals which melt at a lower temperature than the bulk, these bimetallic alloy nanoscale inclusions melted at approximately the same temperature as the bulk alloy. Examination of many such melting events also allowed the team to measure the characteristic angle of contact between the liquid Cd-Pb alloy and the solid Al matrix, which is a measure of the interfacial energy

The group performed complementary experiments using Argonnne National Laboratory's Intermediate Voltage Electron Microscope (IVEM-Tandem). Pure lead inclusions in Al were held at a temperature below their melting point and observed in-situ in the microscope. The unique capability of the IVEM-Tandem to provide simultaneous TEM observation and ion implantation was used to implant Cd ions while observing the structure of the inclusions. This work showed for the first time how melting initiates at specific crystallographic facets as the Cd concentation in the inclusions is increased.

These observations will be of great value in the continuing study of fundamental questions concerning the mechanism of phase transformations. Alloy melting and solidification are also of importance in technologically important processes such as soldering in the semiconductor industry, welding in the steel industry and directional solidification in aerospace alloys.

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