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Martensite Variants Rearrangement And Multi-Field Coupling Behavior Of Ferromagnetic Shape Memory Alloys

Posted on:2012-01-04Degree:DoctorType:Dissertation
Country:ChinaCandidate:F LiFull Text:PDF
GTID:1111330368493851Subject:Solid mechanics
Abstract/Summary:PDF Full Text Request
As a new kind of magnetic smart material, ferromagnetic shape memory alloys (FSMAs) have attracted more researchers'interests in these recent years. FSMAs can be controlled not only by a thermal field and/or a stress field, but also by a magnetic field. The FSMAs, having both functions of actuator and sensor, are expected to be a kind of potential smart materials of the new generation with a huge output strains and high response frequencies. The strain induced by magnetic field in FSMAs is from the transition or reorientation between different martensite variants but not the phase transformation from austentine to martensite. To the aim of describing the process of the martensite variants rearrangement (MVR) and giant magnetic field-induce strains of FSMAs, this thesis develops a macroscopic thermodynamic model of the martensite variants rearrangement and a micromechanics-base thermodynamic model. The analyses and numerical simulations on the magneto-mechanical, magneto-thermo- mechanical coupling behaviors of FSMAs are conducted.Firstly, a kinetics model describing macroscopic behavior of martensitic variants rearrangement in FSMAs is proposed, which is based on a tensor description of thermodynamic continuum mechanics taking into account magneto-thermo-mechanical coupling on the MVR process. A generalized thermodynamic driving force which rearranges the martensitic variants is derived. The thermodynamic evolution equations of the kinetics are obtained by means of the dissipation, which is induced by the twin boundary motion during rearrangement process, reaching a maximum. The ability of the model to characterize the macroscopic stress-strain behavior and magnetic field-induced strain of a ferromagnetic shape memory alloy rod under both magnetic field and compression stress field is demonstrated. And the temperature dependence of martensite variant rearrangement is described by considering the temperature dependence of the magnetization, maximum strain and switching field.Secondly, there shows that the abundant magnetic micro-structure and complicated variation of magnetic domain during the MVR process of FSMAs. The lately experimental observations show that, in the martensite variants of FSMAs, the magnetization rotation is always detected in the variants with the magnetization easy-axes perpendicular to the applied magnetic field direction, while the domain wall motion exists only in the variants with the magnetization easy-axes aligned along the field. However there are no existing theoretical models can reveal the complex magnetic microstructure behaviors currently. Aim to this purpose, an analytical model based on the statistical method and energy minimization theory of micro-magnetics is presented in the present work to capture the magnetic domain behavior of ferromagnetic shape memory alloys during the MVR. The difference between the Gibbs free energy of magnetic domain in each martensite variant is defined as the general driving force for the domain wall motion. Based on the proposed micro-mangetic statistical model, the volume fractions of magnetic domains of the different martensite variants are obtained and the mechanism of the variation of magnetic domain is revealed which further explains the experimental observations successfully.Thirdly, a micromechanical model is proposed to describe the magneto-mechanical behaviors of FSMAs during the MVR process. We treat the MVR of FSMAs as one phase of martensite variant being a inclusion inside of another phase of martensite variant. Following the Eshelby equivalent inclusion method and the Mori-Tanaka averaged scheme, we obtain the strains and stresses of FSMAs during the MVR process. Based on the micromechanics and thermodynamic theory, the total potential energy of FSMAs system and the general driving force for the twin boundary motion are derived. A thermodynamic balance equation is also obtained for volume fraction of the inclusion. By the proposed model, the critical value for the start of the martensite variant rearrangement can be predicted. And the model also explains the phenomenon why the rearrangement could not happen ever or only happen partly based on the relationship between the threshold magnetic field when the magnetization rotation finished and the critical fields when the rearrangement starts or finishes. The predictions of magneto-mechanical behavior of FSMAs are in good agreements with the experimental data.Lastly, by combining the thermodynamic statistical model of micromagnetics for describing the evolution of the magnetic domains with the micromechanics theory, a generalized micromechanical-base thermodynamic model is proposed. The model taking into account the magnetic micro-structure and variation of magnetic domain gives the volume fraction of magnetic domain and magnetization rotation angle during MVR process of FSMAs, as well as the corresponding thermodynamic balance equation related to the fraction of martensite variant. The simulation results from the generalized model show quite good agreements with experimental data. It indicates that the characterisitic and evolution of the magnetic domain inside of martensite variants play an important role in the process of MVR for FSMAs.
Keywords/Search Tags:Ferromagnetic shape memory alloys, Magnetic field-induced strain, Martensite variants rearrangement, Multi-field coupling behavior, Magnetic domain and microstructure
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