Doctoral Dissertations

Date of Award

8-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Haixuan Xu

Committee Members

Haixuan Xu, Yanfei Gao, Valentino Cooper, David Keffer

Abstract

The Nickel-based Cantor-type concentrated alloys have drawn tremendous research interest as they exhibit great potential for advanced structural materials. The origins of their mechanical performances have been largely associated with planar fault structures, such as stacking faults and deformation twins, as well as their interplay with other defects. Therefore, it is essential to investigate the energetics and the evolution of the planar faults during the deformation processes of the materials.

In the first part of this dissertation, we compute the stacking fault energies (SFEs) in the Ni-based concentrated binary and ternary alloys at both density functional theory (DFT) and atomistic levels. Distinctive compositional dependences of SFE in the three selected binary alloys (Ni-Cu, Ni-Co, and Ni-Fe) are identified with DFT calculations, among which the non-linear dependences can be attributed to magnetism of the alloys. The DFT data are used as benchmark to evaluate the fidelity of interatomic potentials designed for related systems. Using the potentials with the overall highest fidelity, the SFE calculations are extended to ternary Ni-Co-Cr and Ni-Co-Fe alloys. Moreover, considerable fluctuations of the SFEs are observed for the concentrated compositions and redeemed a unique feature of the concentrated alloys.

In the second part, we investigate the grain boundary (GB)-assisted deformation twinning and planar faulting in the Cantor-type alloys using classical molecular dynamics (MD) techniques. We reveal different modes of planar faulting processes during the simulated mechanical tensile testing in columnar nano polycrystals. The thickness of the generated twins is rather limited except when GB triple junctions are involved. The corresponding twinning stress is extracted and its relationship with temperature/general planar fault energies studied. Further simulations on the symmetric tilt GBs help clarify the dislocation reactions underlying the faulting process and establish a relationship between the twinning stress and the characteristic of the boundaries.

This research advances the understanding of planar faults and related deformation mechanisms in fcc concentrated alloys at electronic/atomistic level. Furthermore, it offers direction for tailoring the energies/evolution behaviors of planar faults by tuning the composition/GB characteristics, respectively, which can be utilized in the design and manufacture of advanced structural materials within concentrated alloy concept.

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