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Medha Veligatla

September 27 @ 2:00 pm - 4:00 pm MDT

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Dissertation Information

Title: Effects of magnetic domain and twin boundary interactions on magneto-mechanical properties of magnetic shape memory alloys

Program: Doctor of Philosophy in Materials Science and Engineering

Advisor: Dr. Peter Mullner, Materials Science and Engineering

Committee Members: Dr. Paul Simmonds, Materials Science and Engineering, Dr. Eric Jankowski, Materials Science and Engineering, Dr. Carlos J. Garcia-Cervera, Materials Science and Engineering

Abstract

Magnetic shape memory (MSM) alloys deform substantially when exposed to a magnetic field. This recoverable plastic deformation occurs through crystallographic twinning. Thereby the internal magnetic domain structure modulates the deformation mechanisms through the interaction of magnetic domains with twin boundaries. We study the meso scale magneto-structural interactions that affect the macroscopic material properties of MSM alloys. The study at the meso length scale is most effective as it allows for resolving interactions at magnetic domain wall width resolution with reasonable computing cost. We apply micromagnetics simulations to evaluate the evolution of magnetic domains, their interaction with twin boundaries, the distribution of magnetic energies, and semi-quantitatively assess the magneto-mechanical properties of MSM alloys.

This dissertation addresses the following phenomena demonstrated by experimental findings: 1. The sample shape dependence of twin boundary propogation. The results are useful to design actuators. Due to the sample shape, the demagnetization factor varies with the direction of the external magnetic field. Especially when the magnetic field is perpendicular to the long edge of the sample (high demagnetizing field), the magnetic energy intermetently increases with deformation (at low fields) which hinders twin boundary motion and results in gradual actuation. Whereas when the applied magnetic field is parallel to the long edge of the sample (lowest demagnetizing field), the energy decreases with deformation and the twin boundary moves instantaneously resulting in abrupt actuation. 2. Magnetic domain and twin boundary interactions that result in work hardening. This study addresses the monotonically increasing stress with ongoing deformation in fine twinned MSM alloys. Additional “vertical” magnetic domains form in densely twinned MSM alloys. The interaction of twin boundaries with these vertical magnetic domains results in magneto-elastic defects, which generate high local magneto-stresses. These interaction sites act as obstacles for twinning disconnections similar to coherent particles in precipitation-hardened aluminum alloys. Whereas in a low twin density MSM alloy, these magneto-stress concentrations are dilute and their effectiveness is reduced by the synergistic action of many twinning disconnections. 3. Effect of magnetic field inclination on mechano-electrical energy conversion. This study aids in evaluating the power harvesting capacity of MSM alloys. Using the concept of inverse magneto-plasticity (i.e. deformation-induced change of magnetization), experiments were performed to convert mechanical energy to electrical energy under a non-perpendicular bias magnetic field. The highest power output was obtained when the biased magnetic field was inclined with respect to the loading direction. The inclined magnetic field biases the magnetic domain structure such as to increase the magnetization component in loading direction. This increases the conversion rate from mechanical to electrical energy.

When the MSM material accommodates the meso scale interactions between the magnetic and crystallographic structures, magnetic structures evolve with global and local spatial energy gradients and concentrations of magnetostress. These modulations hinder twin boundary mobility and determine the macroscopic magneto-mechanical properties of MSM alloys.