Microstructure and Small-Scale Deformation of Al₀.₇CoCrFeNi High-Entropy Alloy

Citation

Giwa, Adenike Monsurat (2019) Microstructure and Small-Scale Deformation of Al₀.₇CoCrFeNi High-Entropy Alloy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/PSWX-RY20. https://resolver.caltech.edu/CaltechTHESIS:06252018-172548450

Abstract

Novel engineering materials are continuously being designed for structural applications, particularly for improved mechanical properties such as high strength, enhanced ductility, and great thermal stability. High entropy alloys (HEAs) as an emerging material can be distinguished from other metal systems as a five-or-more-component alloy in which the constituents are in equiatomic or near equiatomic proportions, thereby maximizing the configurational entropy.

This thesis is focused on understanding the microstructure of an aluminum-containing HEA in relation to its small-scale mechanical properties. Physical phenomena such as size-effect, slip sizes, temperature effect, crystallographic orientation effect, influence of interface, and small perturbations in atom motions are studied.

Uniaxial compression experiments were conducted on nanopillars fabricated from the individual phases (i.e. Face Centered Cubic (FCC) and Body Cubic Centered (BCC) present in the Al_0.7CoCrFeNi HEA. We observed the presence of a size-effect in both phases, with smaller pillars having substantially greater strengths compared with bulk and with larger sized samples. The size-effect power law exponent m in τ_y α D^-m for the BCC phase was − 0.28, which is lower than that of most pure BCC metals, and the FCC phase had m = − 0.66, which is equivalent to most pure FCC metals. These results are discussed in the framework of nano-scale plasticity and the intrinsic lattice resistance through the interplay of the internal (microstructural) and external (dimensional) size effects.

In addition to higher stresses observed at cryogenic temperature in both phases, the microstructural analysis of the deformed pillar via Transmission Electron Microscopy (TEM) showed that FCC pillars undergo deformation by planar-slip dislocation activities even at temperatures of 40 K. Bulk FCC HEAs have been studied to deform via twinning mechanism at low temperatures. The BCC phase, however, confirms dislocation–driven plasticity and twinning at 40 K. These results are explained from the intrinsic nature of the dislocation structure of both phases at low temperatures.

The effect of an 'interphase' in micron-sized HEA pillars was studied from different orientation configurations of the BCC | FCC phases. Slip transmission across the phases was observed in high symmetry orientation combination of both phases. Configurations having a mixture of both low and high symmetry orientations vary in deformation mechanisms. We explain these findings in relation to crystal orientation effect of the combining half pillars, competing plastic mechanisms, dislocation – boundary interactions and how these findings correlate with their mechanical response.

Also, we conducted dynamic mechanical analysis on the FCC and BCC HEA nanopillars to reveal their damping properties. Higher storage modulus and damping factor values were observed in FCC and BCC the nanopillars. Storage Moduli in the nano-sized HEAs are a factor of 2 greater than both bulk BCC and FCC HEA counterparts. The difference is due to greater surface contribution of the external atoms in the small-sized HEAs.

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