Publication Date

12-2014

Date of Final Oral Examination (Defense)

10-23-2014

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Supervisory Committee Chair

Peter Müllner, Ph.D.

Supervisory Committee Member

Paul Lindquist, Ph.D.

Supervisory Committee Member

Trevor Lujan, Ph.D.

Abstract

Ni-Mn-Ga is a magnetic shape memory alloy (MSMA) that exhibits large recoverable strains. The magnetic field induced strain (MFIS) occurs through twin boundary motion resulting in reorientation of the crystal structure, which is coupled to the magnetic moment of the material. The shape memory capabilities of the material make it promising for applications including microactuators, sensors, microfluid pumps, refrigeration, energy harvesting, and data storage.

In order for Ni-Mn-Ga to become a viable option for use in commercial applications the performance and reliability must be on par with industry standards. Ni‑Mn-Ga has been shown to achieve high cycles at low strains, but there is little data on the fatigue life of samples that are actuated near the theoretical strain limit. 10M Ni51.2Mn26.6Ga22.2 single crystal samples were magneto-mechanically cycled in a rotating magnetic field under minimal mechanical constraints to study the effects of crystal imperfections on fatigue life and fracture mechanics with a consideration of surface roughness. High surface roughness and areas of high local stress concentration lead to crack initiation. Cracks tended to propagate in <110> directions at an angle of 45° to the axis about which the magnetic field rotated. Crystal defects, surface imperfections, and total strain should be minimized to achieve long operational life.

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