Exploring diffusion of ultrasonically consolidated aluminum and copper films through scanning and transmission electron microscopy

Sietins, Jennifer
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University of Delaware
Ultrasonic consolidation (UC) is a promising manufacturing method for metal matrix composite pre-preg tapes or foils that utilizes a layer build-up technique. The process involves three main variables: applied load, oscillation amplitude, and rolling speed. A main advantage of this process is the ability to manufacture multi-material parts at lower processing temperatures compared to other metal matrix composites processes. A major disadvantage, however, is a lack of understanding of diffusion during the ultrasonic consolidation process, which is expected to affect the microstructure, bond quality, and strength within the interface region. The role of diffusion during the low temperature, short duration ultrasonic consolidation process was explored. First, scanning electron microscopy (SEM) x-ray energy dispersive spectroscopy (XEDS) was used to measure concentration profiles of ultrasonically consolidated high purity aluminum and copper through which the interdiffusion coefficients were calculated. It was found that the experimental accelerating voltage had a significant impact on the measurement of the concentration profiles, and associated interdiffusion coefficients, due to the interaction volume interference. The effect of the interaction volume on the concentration profiles was confirmed through Monte Carlo simulations of electron trajectories, and the error due the interaction volume was quantified. The results showed the diffusion distance was too small for accurate measurements with SEM XEDS even at low accelerating voltages. To significantly reduce the error due to the interaction volume, transmission electron microscopy (TEM) samples were prepared using a focused ion beam (FIB) to ensure a uniform thickness. The TEM XEDS concentration profile and images revealed intermetallic phase transformations that occurred during the welding process. TEM images also showed dislocation pile-up located at the subgrain/bulk aluminum interface. This microstructural feature supports continuous dynamic recrystallization of grains through the rearrangement of dislocations. The apparent interdiffusion coefficient closely matched the bulk diffusion concentration profile for the times and temperatures of the ultrasonically consolidated sample. Support for bulk diffusion was independently found through calculating the minimum critical temperature for bulk diffusion dominance over grain boundary diffusion for the grain sizes measured.