College of Engineering
Materials Engineering Department
BS in Materials Engineering
Dr. Linda Vanasupa
Scanning electron microscopy (SEM) video recording is used to characterize the solidification of small volumes of 99.999% pure gallium (Ga) and eutectic gallium-indium (eGaIn) under a high vacuum environment. Specimen are superheated to 55℃ using a hot plate, cast into spherical droplets, and cooled in situ by means of a Peltier cooling stage. Special attention is given to the preparation of the specimen prior to viewing because of gallium and its alloys’ nature to form an oxide layer when melted and air cooled. The oxide acts as a skin that inhibits the observation of microstructural features during solidification. Heated samples are etched using a 3% HCl solution to yield an oxide-free mirror finish prior to imaging. A series of observations are performed to determine whether the use of the etchant is effective in removing the oxide layer thereby enhancing viewing capabilities of the solidification process. SEM video recordings and micrographs show that the application of etchant changes the surface chemistry such that the oxide layer is reduced to a metallic chloride and causes the cast samples to bead up into spheres showing a decrease in surface tension. Cavities are observed at the surface of samples caused by gas bubbles that diffuse into the surface of the sample as the SEM chamber reaches a high vacuum. The cavities act as windows for viewing the underlying liquid and the microstructural evolution of those regions upon cooling. The technique developed and employed in the preset study can be replicated and performed for other liquid metals however should be made more robust for future application.