Development of Strong yet Ductile Chemically Complex Metallic Films Based on Structural and Chemical Design at the Nanoscale


Student thesis: Doctoral Thesis

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Award date24 Jun 2022


The human exploration of metallic materials has entered into a new epoch with the emergence of chemically complex alloys (CCAs) and high entropy alloys (HEAs). Unlike conventional alloys with one or two principal elements, CCAs or HEAs are composed of multi-principal elements in equimolar or near-equimolar concentrations. Since 2004, extensive research has already shown that this new class of metals may attain exceptional properties, such as high strength and hardness, high toughness, excellent corrosion resistance, good thermal stability, etc. While the previous studies were mainly focused on bulk metals, my current research is centered on the synthesis of chemically complex metallic films (CCMFs) with exceptional mechanical properties.

In the literature, magnetron sputtering is one of the most widely used techniques in synthesizing metallic films. Although this method was thoroughly studied for conventional metals (e.g. elemental metals and binary alloys), the physical mechanisms that govern the formation of CCMFs are not clear yet. To begin with, I will discuss the thermodynamics for the formation of CCMFs based on a model I developed. By carefully controlling the magnetron sputtering parameters, I synthesized a series of FeCoNiNb0.5 thin films with different metastable phases, including amorphous and metastable crystalline phases.

Next, I synthesized the FeCoNiCrCu CCMFs and studied their mechanical behavior. Owing to their metastable nature, the annealed thin films exhibit nano-scale chemical fluctuations without detectable phase transition. Compared to the single-phase FCC metals hitherto reported, our as-deposited and thermally annealed CCMFs attain the highest yield strength (3.4 to 4.2 GPa) under micro-compression due to the combined action of nano-grains and growth twins. Furthermore, according to the experiments and atomistic simulations, strength-plasticity synergy was achieved as the nano-scale chemical fluctuation was able to promote deformation twinning in the CCMFs.

Finally, I synthesized freestanding FeCoNiCrCu nanosheets with the polymer surface buckling enabled exfoliation method that was based on magnetron sputtering. Due to the reaction of sputtered FeCoNiCrCu nanoparticles with the molecular chains in poly-vinyl alcohol (PVA), I obtained a dual-phase nanostructured cermet that mainly consisted of amorphous metallic oxides and metallic nanocrystals. Such CCA-derived cermet nanosheets exhibit an excellent combination of strength (~3.2 GPa) and ductility (~50% strain) under atomic force microscopy (AFM) indentation at ambient temperature, which outperforms conventional cermets in terms of ductility. I also studied the physical mechanisms underpinning the mechanisms of plasticity in the CCA-derived cermet, which could be attributed to stress induced liquefaction in the amorphous metallic oxide phases.