Abstract
The microstructural transformations of binary nanometallic multilayers (NMMs) to equiaxed nanostructured materials were explored by characterizing a variety of nanoscale multilayer films. Four material systems of multilayer films, Hf-Ti, Ta-Hf, W-Cr, and Mo-Au, were synthesized by magnetron sputtering, heat treated at 1000 °C, and subsequently characterized by transmission electron microscopy. Binary systems were selected based on thermodynamic models predicting stable nanograin formation with similar global compositions around 20–30 at.%. All NMMs maintained nanocrystalline grain sizes after evolution into an equiaxed structure, where the systems with highly mobile incoherent interfaces or higher energy interfaces showed a more significant increase in grain size. Furthermore, varying segregation behaviors were observed, including grain boundary (GB) segregation, precipitation, and intermetallic formation depending on the material system selected. The pathway to tailored microstructures was found to be governed by key mechanisms and factors as determined by a film’s initial characteristics, including global and local composition, interface energy, layer structure, and material selection. This work presents a global evaluation of NMM systems and demonstrates their utility as foundation materials to promote tailored nanomaterials.
Highlights
Nano-metallic multilayers (NMMs) provide an unparalleled nanostructured working space due to their periodic stacking structure that enables control of the layer thickness, the local composition and the density and structure of the interfaces [1,2]
NMMs can leverage the favorable characteristics of nanostructured materials that can be credited to the high density of grain boundaries (GBs) and interfaces while expanding possible microstructural arrangements
Key features such as characteristic layer thickness, coherency, lattice mismatch, initial grain size, and material selection all contribute to the initial microstructure, which in turn dictate the microstructural evolution under heat treatment
Summary
Nano-metallic multilayers (NMMs) provide an unparalleled nanostructured working space due to their periodic stacking structure that enables control of the layer thickness, the local composition and the density and structure of the interfaces [1,2]. NMMs allow for microstructural features that can be tailored while providing limitless compositional space. These characteristics provide a foundational scientific testbed to study and surmount limitations encountered in nanostructured materials including lack of thermal stability and ductility, as well as being limited to finite material systems [3,4,5,6,7]. PVD techniques, magnetron sputtering, expand the possible material systems that may be synthesized as NMM configurations while promoting control over initial features such as interfaces and grain structure. NMMs can leverage the favorable characteristics of nanostructured materials that can be credited to the high density of grain boundaries (GBs) and interfaces while expanding possible microstructural arrangements. Individual studies on different resulting microstructures are highly encouraging [22,23,24], but what remains in question is an overarching view of the mechanistic relationships between different initial multilayer structures and the final nanostructure achieved after thermally-driven microstructural transformations [25]
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