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Research and Publications - Department of Metallurgical and Materials Engineering

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Browsing Research and Publications - Department of Metallurgical and Materials Engineering by Subject "AB-INITIO INVESTIGATIONS"

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    A computational investigation into the microstructures and stability of the zeta phase in transition metal carbides and nitrides
    (Taylor & Francis, 2018) Weinberger, Christopher R.; Yu, Hang; Wang, Billie; Thompson, Gregory B.; Colorado State University; Drexel University; University of Alabama Tuscaloosa
    A high-volume fraction of the zeta phase in multiphase group VB transition metal tantalum carbides has been shown to dramatically increase fracture toughness. This has been attributed to its unique nanoscale lath-based microstructure. However, what governs the microstructure and how it forms is still not well understood. In this paper, we propose a precipitation model for the formation of these phases and demonstrate that the anisotropic surface energies govern the observed zeta-phase morphology. The energetics and zeta-phase microstructure for other group VB carbides were found to be similar. In contrast, multiphase hafnium nitrides can form both thin-lath-based microstructure as well as large, single zeta-phase grains. The difference between hafnium nitride and the group VB carbides is attributed to the relative bulk free energies and low-temperature stability between the phases.
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    Plasticity mechanisms in HfN at elevated and room temperature
    (Nature Portfolio, 2016-10-06) Vinson, Katherine; Yu, Xiao-Xiang; De Leon, Nicholas; Weinberger, Christopher R.; Thompson, Gregory B.; University of Alabama Tuscaloosa; Drexel University
    HfN specimens deformed via four-point bend tests at room temperature and at 2300 degrees C (similar to 0.7 T-m) showed increased plasticity response with temperature. Dynamic diffraction via transmission electron microscopy (TEM) revealed < 110 > {111} as the primary slip system in both temperature regimes and < 110 > {110} to be a secondary slip system activated at elevated temperature. Dislocation line lengths changed from a primarily linear to a curved morphology with increasing temperature suggestive of increased dislocation mobility being responsible for the brittle to ductile temperature transition. First principle generalized stacking fault energy calculations revealed an intrinsic stacking fault (ISF) along < 112 > {111}, which is the partial dislocation direction for slip on these close packed planes. Though B1 structures, such as NaCl and HfC predominately slip on < 110 > {110}, the ISF here is believed to facilitate slip on the {111} planes for this B1 HfN phase.