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Enhanced Stability of Nanocrystalline Metals with Amorphous Grain Boundary Complexions via Compositional Manipulation

  • Author(s): McDevitt, Charlette Grigorian
  • Advisor(s): Rupert, Timothy J
  • et al.
Abstract

The implementation of nanocrystalline metals is of growing interest in a variety of applications due to the enhancement of mechanical properties with the reduction of grain size. However, the significant grain boundary volume in these materials is associated with thermal instability and a lack of ductility. Recent studies have focused on manipulating the grain boundary structure and chemistry in an attempt to address these issues. Grain boundary complexions describe interfacial phases which exist in equilibrium only with their abutting phases. Amorphous complexions, in particular, have been previously proven to improve the thermal stability of binary nanocrystalline alloys, while simultaneously improving their ductility due to their great capacity for dislocation absorption. However, this type of complexion is only stable at high temperatures and requires very high quench rates to be retained in the microstructure during cooling, limiting the practicality of these materials for use in lower temperature applications. Investigating the formation of these interfacial phases is also of particular interest in multicomponent alloys, as most alloys used in engineering applications are composed of more than two elements.

In this thesis, we seek to improve the utility of nanocrystalline metals by manipulating the composition of the amorphous grain boundary complexion in order to maximize their thickness and stability against transformation to ordered grain boundaries upon cooling. First, the formation of thick amorphous intergranular films is demonstrated in nanocrystalline ternary alloys. Hf additions to a binary Cu-Zr alloy are shown to contribute to the thermal stability of the alloy, while also facilitating the formation of thicker amorphous complexions than the binary analogs. The concept of developing materials with high-entropy complexions is also explored, with further improvements to thermal stability and complexion thickness shown in a Cu-Zr-Hf-Nb-Ti alloy. Finally, the influence of grain boundary composition on complexion transformations is investigated. We find that ternary alloy compositions form amorphous complexions which are more stable against transformation to thinner or ordered grain boundaries than binary alloys, reducing the critical cooling rate necessary to retain the equilibrium structure at high temperatures and therefore improving the potential for their use in lower temperature applications. In summary, we find that careful alloy design can lead to the formation of thick amorphous complexions which are stable against transitioning upon cooling, which can further improve the mechanical properties and enhance the utility of nanocrystalline metals.

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