Year of Publication

2015

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Mechanical Engineering

First Advisor

Dr. Haluk E. Karaca

Abstract

Magnetic Shape Memory Alloys (MSMAs) have the unique ability to change their shape within a magnetic field, or in the presence of stress and a change in temperature. MSMAs have been widely investigated in the past decade due to their ability to demonstrate large magnetic field induced strain and higher frequency response than conventional shape memory alloys (SMAs). NiMn-based alloys are the workhorse of metamagnetic shape memory alloys since they are able to exhibit magnetic field induced phase transformation. In these alloys, martensite and austenite phases have different magnetization behavior, such as the parent phase can be ferromagnetic and martensite phase can be weakly magnetic. The magnetization difference between the transforming phases creates Zeeman energy, which is the main source for magnetic field induced phase transformation, is unlimited with applied field and orientation independent. Thus, metamagnetic shape memory alloys can be employed in polycrystalline form and provide higher actuation stress than conventional MSMAs. High actuation stress levels and frequencies in metamagnetic shape memory alloys are promising for magnetic actuation applications.

Effects of heat treatments and cooling rates on the transformation temperatures, magnetization response and shape memory behavior under compressive stress were explored in Ni45Mn36.5Co5In13.5 [100] oriented single crystalline alloys to obtain high transformation temperatures, large magnetization difference, and low hysteresis behavior. It was found that transformation temperatures increase with higher heat treatment temperatures and decrease drastically at lower cooling rates. Temperature hysteresis decreased with increasing heat treatment temperatures. It was revealed that transformation temperatures, hysteresis, and magnetization response can be tailored by heat treatments via modifying interatomic order.

Magnetic and mechanical results of NiMn-based metamagnetic alloys in single and polycrystalline forms as functions of composition, stress, temperature and magnetic field (up to 9 Tesla) were revealed through thermal-cycling under stress and magnetic field; stress-cycling as functions of temperature and magnetic field; and magnetic-field-cycling under stress at several temperatures experiments. Single crystalline samples of NiMnCoIn showed recoverable strain of 1.5 % due to magnetic field induced reversible phase transformation under constant stress and strain of 3.7 % by magnetic field induced recovery after variant reorientation of martensite. The magnetic field effect on the superelasticity and shape memory effects were also explored in selected orientations of [100], [110] and [111].

Fe-based ferromagnetic shape memory alloys have received considerable attention due to their better workability, strength, and lower cost compared with commercial NiTi based SMAs. The shape memory properties of a ferrous single crystalline alloy, FeNiCoAlNb, were investigated along the [100] orientation by thermal cycling under constant stress and superelasticity tests in both tension and compression. Aging was used to form nano-size precipitates to demonstrate shape memory behavior and tailor the shape memory properties. It was found that after proper heat treatments, [001] oriented FeNiCoAlNb showed a compressive strain of 15%, low temperature dependent superelastic behavior, high compression-tension asymmetry, and high compressive strength (~3GPa). The orientation dependence of the mechanical properties of FeNiCoAlNb single crystals were investigated along the [100], [110], [012] and [113] orientations. In addition, martensite phase showed higher magnetization than austenite phase as opposed to NiMn-based metamagnetic shape memory alloys. This magnetization difference is promising because it can allow magnetic field induced forward transformation. Ferrous alloys have great potential for high strength, temperature independent, and large scale actuator applications.

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