Microstructural Evolution And Atomic-level Segregation In Metallic Alloy Films: An Approach Towards Grain Boundary Engineering

Date

2019

Authors

Parajuli, Prakash

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Abstract

Grain boundaries (GBs) have long been recognized as ubiquitous planar defects in polycrystalline materials. They control microstructural evolution and significantly alter the mechanical, electrical, and other important structural and functional properties of the materials. Controlling and understanding the microstructural evolution (texture and GB character) and GB segregation in atomic-scale could allow us to further explore the possibility and extent of grain boundary engineering to design materials of desired properties. In this dissertation, pure (Au and Al) and alloyed (Au/Pd and Al alloy 7075) polycrystalline metallic thin films were deposited using physical vapor deposition. Starting from the structural analysis of nanoparticles, microstructural features (texture and GB character) and their evolution due to annealing (up to 12 hours) and alloying, and GB segregation were studied using advanced electron microscopy techniques. Annealing caused an increase in the content of <111> orientated grains and low CSL GBs. We demonstrated the formation of five-fold annealing twin in Au/Pd thin films for the first time. GB segregation in Al alloy GBs were analyzed in detail. We established Cu as a principal GB segregating element of this alloy and presented the atomic-scale structure of Cu segregation-induced GB complexions on various <111> tilt GBs of the films. Finally, a detailed analysis of misorientation dependence GB segregation behavior displayed two distinct types of segregation patterns, point (GBs misorientated by less than 28 degrees and parallel array (GBs misorientated by more than 28 degrees). This is the first time, we demonstrated experimental observation of parallel array GB segregation behavior. These type of analysis will be an important key in successful GBE and potentially contribute for the efficient design of alloys for particular applications.

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Keywords

Al Alloy, Electron Microscopy, Grain Boundary, Grain Boundary Segregation, Micro-structural Evolution, Thin Films

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Department

Physics and Astronomy