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The Influence of Texture on the Magnetoelastic Properties of Polycrystalline TbDy Alloys

Citation

Good, Nathan R. (2001) The Influence of Texture on the Magnetoelastic Properties of Polycrystalline TbDy Alloys. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:10152010-111044693

Abstract

An investigation into the influence of texture on the magneto elastic properties of cold-rolled polycrystalline terbium-dysprosium alloys has been performed. Significant influence of grain orientations on the thermal expansion, magnetostrictive and magnetomechanical damping properties of TbDy were observed. Drop cast ingots of TbDy alloys were deformed by cold-rolling and annealing with the aim of reorienting grains for large magnetostrictive strains and damping capacities. Thermal expansion coefficients of single crystal and polycrystalline samples of TbDy were measured from 77 - 570 K. These measurements confirmed the expected strong reorientation of the c-axes of grains toward the direction of applied force during deformation. The anisotropy of thermal expansion between the rolling and applied stress directions provided a measure of the effectiveness of various sample preparations for maximizing magnetocrystalline anisotropy. Furthermore, the effects of magnetic phase transitions on thermal expansion through the Curie and N eel points of TbDy revealed thermal expansion anisotropy between grain orientations in the rolling and transverse directions of deformation. Magnetostrictive strains along the rolling direction of polycrystalline TbDy alloys were measured at 77 K. Saturation magnetostriction of up to 55% of previous single crystal results were observed. Minimal applied stress was required to obtain near maximum magnetostrictions for all of the samples tested. This suggests a preloading mechanism within the grain structure of polycrystalline TbDy not present in single crystals. Also in contrast with single crystal measurements, the performance of more economical commercial purity (99.7%) samples was seen to be somewhat higher than similarly prepared high purity (99.94%) samples. Resistance to deterioration of performance over multiple cycles was observed, as changes in magnetostriction over 150 cycles at 0.1 Hz were within measurement errors. By comparing thermal expansion anisotropy of TbDy samples with peak magnetostrictive strain, a clear proportionality between texture and magnetostrictive performance was established. Deviations from this pattern by samples with more deformation and annealing suggest microstructural mechanisms beyond average grain orientation impacting magnetostriction. Magnetomechanical damping effects were observed for polycrystalline TbDy samples through compression stress-strain curves. Elastic moduli at 77 K was measured to be up to 80% less than at 300 K, with a large hysteresis present in the stress-strain curves of all samples tested below the Curie point. Damping capacity was measured as the stress-strain hysteresis loop divided by the total area under the stress-strain curve. Damping capacities up to 23% were measured for polycrystalline TbDy alloys. Larger magnetomechanical damping was observed at lower strains, with higher strains corresponded to saturation of magnetic domain realignment, smaller damping capacities and larger elastic moduli. Samples with larger magnetostrictions displayed larger damping capacities over a wide range of applied stresses, and also had consistently lower elastic moduli at 77K. Mechanisms of damping were investigated by fitting magnetostriction, elastic modulus and estimated strain of damping saturation of TbDy alloys to a model of magnetomechanical energy dissipation. This model relates magnetomechanical damping to magnetic hysteresis, neglecting microstructural influences not present in magnetostriction data. The damping capacities predicted by this model were approximately an order of magnitude higher than experimental results. This result suggests a prominent role of microstructural interactions in the domain realignments responsible for magnetoelastic damping.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Applied Physics
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Applied Physics
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Fultz, Brent T.
Thesis Committee:
  • Unknown, Unknown
Defense Date:24 May 2001
Record Number:CaltechTHESIS:10152010-111044693
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:10152010-111044693
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6145
Collection:CaltechTHESIS
Deposited By: Rita Suarez
Deposited On:15 Oct 2010 19:46
Last Modified:26 Dec 2012 04:31

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