Thermomechanical Nanomolding: Characterization And Fabrication
Date of Award
Spring 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering & Materials Science (ENAS)
First Advisor
Schroers, Jan
Abstract
In this thesis, I present my work on a nanofabrication technique, thermomechanical nanomolding(TMNM) which is explored and then further developed as a materials characterization tool. It was initially utilized as a nanofabrication technique using crystalline materials as feedstock, enabling formation of the nanorods molded out of the feedstock of interest. It is shown to be quite versatile for a variety of crystalline phases. I present results whereby TMNM enables nanofabrication of a wide variety of materials including pure metals, alloys, and intermetallics nanorods, of heterostructures combining different phases, and of hierarchical structures combining patterns at micro- and nano-scale. Upon further inquiries into its mechanism, it became apparent that this technique could also inform us about the feedstock’s microstructural features. In particular, diffusion-based mechanisms are discussed and used for characterization. It is observed that different features of the microstructure may have a strong effect on the local diffusion characteristics in their vicinity. And during the nanomolding process, the diffusion flux ‘flowing’ through the microstructure due to the imposed stress gradient, gets accumulated into the nanomold. This accumulated flux shows up as nanorods on top of the feedstock. Thus, in presence of any microstructural features enabling significantly fast/slow diffusion, long/short nanorods are observed which in turn can be mapped back onto the microstructural feature. Further, since the diffusive flux is spatially separated from the microstructure, this procedure can yield composition and magnitude of the diffusive flux directly. Length and composition analysis of the formed nanorods allows for the determination of the rate, composition, and constituents’ diffusivities of the diffusion flux moving through the alloy microstructure. After verifying this technique on pure metals and simple alloys, I use this technique to reveal diffusive flux in general alloys. From the different alloys investigated, I could observe that in solid solutions, the diffusive flux is dominated by the fastest diffusing constituent. For stoichiometric intermetallics, their composition is maintained in the nanorods, as the constituents tend to diffuse, maintaining the same stoichiometry as that of the feedstock. For eutectic alloys, the flux’s overall diffusivity is greatly enhanced over that expected from the average diffusivities of the constituent elements. I discuss this thus far unknown revealed eutectic mechanism, which is present in the majority of multicomponent alloys, and it can help explain performance differences among alloys in diffusion-controlled processes and allows for designing alloys with especially high or low diffusivities. I further discuss that TMNM can be utilized to locally probe the diffusive behavior of different types of microstructures and propose it as a local deformation mapping (LDM) technique. LDM is presented as a tool to probe the multi-scale nature of microstructure-property relations, through characterizing microstructures at high spatial resolution (~10 nm) over a large area (~cm2).
Recommended Citation
Raj, Arindam, "Thermomechanical Nanomolding: Characterization And Fabrication" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1425.
https://elischolar.library.yale.edu/gsas_dissertations/1425
