טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentGanor Yaniv
SubjectMagneto-Mechanical Characteristics of Ferromagnetic Shape
Memory Alloys
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Doron Shilo
Full Thesis textFull thesis text - English Version


Abstract

A fundamental and applied research on a new class of materials, ferromagnetic shape memory (FSM) alloys, is presented. FSM alloys exhibit a unique combination of large strains (6.5%) and high operating frequencies (5kHz) when exposed to magnetic field. These materials have significant potential in motion generation applications such as micro actuators. Despite of the large strains, the work output density of FSM alloys is limited by their relatively low blocking stress (5MPa), above which magnetic field induced strain vanishes.  This level of stress impedes the utilization of these alloys in applications where high work output by small components is required.  Substantial efforts have been directed at understanding the phenomena and determining the magneto-mechanical properties of Ni2MnGa alloys, and numerous actuation experiments and commercial cm-scale devices were realized. However, the magneto-mechanical behavior of micro scale FSM specimens has not been studied, and the full potential of these materials has yet to be revealed.


In this research, we developed novel experimental methods and predictive theoretical models for the study of the physical processes that dictate the magneto-mechanical response of FSM alloys. A multi-scale research is presented; (1) Macro-scale characterization of the magnetic field induced strain in micro actuators by means of a novel experimental testing system, (2) Micro-scale magneto-mechanical processes that link between microstructure and macroscopic properties, (3) Nano-scale characterization of twin boundaries by means of a new method for quantitative nano-scale mapping, and (4) Modeling of the magneto-mechanical response of FSM materials based on the observed mechanisms.


Fine near-surface microstructure of closure variants with spacing of 100nm were observed, and low twin boundary energies of 3erg/cm2 were calculated by energy minimization method. This small value of surface energy suggests the ability of Ni2MnGa alloy to form fine microstructures of magnetic twin variants. We have indicated that the fineness of the microstructure significantly influence the variant switching process, which is the basic mechanism for actuation, and is affected by the specimen dimensions, with smaller specimens favoring finer twins. This gave rise to the hypothesis that actuators of different scale exhibit different macroscopic behavior. Accordingly, series of tests were conducted with down-scaled specimens, and demonstrated an increase of the blocking stress by more than 100%. These findings show a fundamental relationship between the specimen size, its microstructure and its physical properties, and lend a significant contribution to the widespread search for active materials that provide both large strain and high work output density.