NMR Study of Local Magnetism, Anelastic Deformation, and Liquid Behavior of Bulk Metallic Glasses

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  • March 19, 2019
Creator
  • Sandor, Magdalena
    • Affiliation: College of Arts and Sciences, Department of Physics and Astronomy
Abstract
  • In this dissertation, 27Al NMR studies of CuZr- and La-based metallic glasses were carried out to investigate local magnetic properties, the mechanism of anelastic deformation, and liquid state behavior. The first part of this dissertation will cover magnetization properties of (CuZr)93-xAl7Gdx bulk metallic glasses (BMGs) from temperature dependent 27Al NMR spectroscopy and magnetic susceptibility measurements. Significant nonlinear line broadening of 27Al spectra commencing at high temperatures is attributed to a local magnetic susceptibility distribution. Magnetization measurements confirm linewidth enhancement caused by the strong frustrated development of antiferromagnetic magnetic short-range order. The next section of this dissertation concerns constant-compression experiments that were carried out at room temperature to study the anelastic response of La50Ni15Al35 bulk metallic glasses with varying compression time and stress. 27Al NMR nutation experiments were used to measure changes of the quadrupolar interaction in order to probe local structural changes induced by anelastic recovery. It was observed that anelastic deformation results in enhanced local symmetry at Al sites and that the changes of local structure are proportional to the magnitude of stress and compression time. Modulated differential calorimetry studies were also performed to understand the correspondence between local structure changes observed and changes in free volume. These results can be further reconciled within the theory of atomic level stresses. The last topic addressed in this dissertation is temperature-dependent 27Al nuclear magnetic resonance (NMR) experiments above the liquid temperature in La-based metallic glasses. The coexistence of two liquid states was observed from phase separation driven by differences of density or chemical potential. In addition, nonlinear liquid behavior also emerged and indicates a first-order-like phase transition. The nonlinear behavior was verified in the spectroscopy and spectral linewidths. Annealing tests show that features of phase separation can be controlled, which may be significant for processing applications of metallic glasses. These NMR results provide important thermodynamic insight for the understanding different mechanisms that drive phase separation and the underlying relevance of potential energy landscape theory.
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  • In Copyright
Advisor
  • Wu, Yue
  • Sandor, Magdalena
  • Qin, Lu-Chang
  • Sen, Pabitra
  • Clegg, Thomas
  • McNeil, Laurie
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2012
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  • This item is restricted from public view for 1 year after publication.
Date uploaded
  • April 26, 2017

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