Ni-Mn-Ga alloys close to the stoichiometric composition Ni2MnGa belong to the quite new family of ferromagnetic shape memory alloys. These alloys are characterized by the magnetic field induced strain (MFIS) based on the comparably easy motion of twin boundaries under a magnetic field. They are mostly chosen as a potential candidate for practical application especially promising for actuators and sensors because they are showing the largest MFIS so far. Depending on the chemical composition and heat treatment, at least three martensitic structures can be distinguished in the Ni-Mn-Ga system. However, the effect mentioned above only exists in two modulated structures. Since for the intended application of MFIS in technology polycrystalline materials seem to be more appropriate in contrast to single crystals, the specific polycrystalline aspects are considered. Factors important for decreasing the twinning stress and increasing the twinning strain of polycrystalline Ni-Mn-Ga alloys are texturing, adjusting the structure by annealing and training by thermomechanical treatments. To achieve pronounced MFIS in polycrystals, fabrication processes are needed to produce specific strong textures. The material texturing has been obtained by directional solidification and plastic deformation by hot rolling and hot extrusion as well as high pressure torsion (HPT). To examine the texture of coarse-grained Ni-Mn-Ga alloys (due to a solidification process or dynamic recrystallization), diffraction of synchrotron radiation and neutrons was applied. The texture results show that the texture of Ni-Mn-Ga subjected to directional solidification, hot rolling and hot extrusion is a fibre or weak biaxial texture. However, local synchrotron measurements reveal that the global fibre texture of the hot extruded sample is a ”cyclic” fibre texture, i.e. it is composed of components related to the radial direction rotating around the extrusion axis. This allows finding regions with a strong texture component. The texture after HPT is characterized by a strong cube with the cube favourably oriented. The initial microstructure of the Ni-Mn-Ga alloys is a typical self-accommodated microstructure of martensite. High resolution EBSD mappings show macro, micro twins and two types of microstructure. The twin plane is determined to be {110). In a typical martensitic transformation the high-temperature phase has a higher crystallographic symmetry than the low-temperature phase. Consequently, austenite may transform to several martensitic variants, the number of which depends on the change of symmetry during transformation. Generally, in a cubic-to-tetragonal transformation (5M case) three variants can form with the c-axis oriented close to the three main cubic axes of austenite. However, close examination of the high resolution EBSD mapping reveals that more than just three orientations, as expected from the Bain model, exist in Ni50Mn29Ga21. Each of three Bain variants may be split in some twin relations in different regions of the sample which differ from each other by about few degrees creating a much higher number of variants. The training process, as the last step in the preparation procedure of Ni-Mn-Ga alloys, consists of multi-axis compression finally leading to a single-variant state. Compression of polycrystalline samples leads to motion of those twin boundaries changing the volume fraction of particular martensitic variants in such a way that the shortest axis (c-axis) becomes preferentially aligned parallel to the compression axis. It allows reducing the twinning stress and maximizing the twinning strain. To understand the training process in more detail, the interaction of the twin variants with the neighbourhood of parent austenite grains was investigated.

Ni-Mn-Ga alloys close to the stoichiometric composition Ni2MnGa belong to the quite new family of ferromagnetic shape memory alloys. These alloys are characterized by the magnetic field induced strain (MFIS) based on the comparably easy motion of twin boundaries under a magnetic field. They are mostly chosen as a potential candidate for practical application especially promising for actuators and sensors because they are showing the largest MFIS so far. Depending on the chemical composition and heat treatment, at least three martensitic structures can be distinguished in the Ni-Mn-Ga system. However, the effect mentioned above only exists in two modulated structures. Since for the intended application of MFIS in technology polycrystalline materials seem to be more appropriate in contrast to single crystals, the specific polycrystalline aspects are considered. Factors important for decreasing the twinning stress and increasing the twinning strain of polycrystalline Ni-Mn-Ga alloys are texturing, adjusting the structure by annealing and training by thermomechanical treatments. To achieve pronounced MFIS in polycrystals, fabrication processes are needed to produce specific strong textures. The material texturing has been obtained by directional solidification and plastic deformation by hot rolling and hot extrusion as well as high pressure torsion (HPT). To examine the texture of coarse-grained Ni-Mn-Ga alloys (due to a solidification process or dynamic recrystallization), diffraction of synchrotron radiation and neutrons was applied. The texture results show that the texture of Ni-Mn-Ga subjected to directional solidification, hot rolling and hot extrusion is a fibre or weak biaxial texture. However, local synchrotron measurements reveal that the global fibre texture of the hot extruded sample is a ”cyclic” fibre texture, i.e. it is composed of components related to the radial direction rotating around the extrusion axis. This allows finding regions with a strong texture component. The texture after HPT is characterized by a strong cube with the cube favourably oriented. The initial microstructure of the Ni-Mn-Ga alloys is a typical self-accommodated microstructure of martensite. High resolution EBSD mappings show macro, micro twins and two types of microstructure. The twin plane is determined to be {110). In a typical martensitic transformation the high-temperature phase has a higher crystallographic symmetry than the low-temperature phase. Consequently, austenite may transform to several martensitic variants, the number of which depends on the change of symmetry during transformation. Generally, in a cubic-to-tetragonal transformation (5M case) three variants can form with the c-axis oriented close to the three main cubic axes of austenite. However, close examination of the high resolution EBSD mapping reveals that more than just three orientations, as expected from the Bain model, exist in Ni50Mn29Ga21. Each of three Bain variants may be split in some twin relations in different regions of the sample which differ from each other by about few degrees creating a much higher number of variants. The training process, as the last step in the preparation procedure of Ni-Mn-Ga alloys, consists of multi-axis compression finally leading to a single-variant state. Compression of polycrystalline samples leads to motion of those twin boundaries changing the volume fraction of particular martensitic variants in such a way that the shortest axis (c-axis) becomes preferentially aligned parallel to the compression axis. It allows reducing the twinning stress and maximizing the twinning strain. To understand the training process in more detail, the interaction of the twin variants with the neighbourhood of parent austenite grains was investigated.

Ni-Mn-Ga is a ferromagnetic shape memory alloy that can be used for future sensors and actuators. It has been shown that magnetic field can induce phase transformation and consequently large strain in stoichiometric Ni2MnGa. Since then considerable progress has been made in understanding the underlying science of shape memory and ferromagnetic shape memory in bulk materials. Ni-Mn-Ga thin films, however is a relatively under explored area. Ferromagnetic shape memory alloy thin films are conceived as the future MEMS sensor and actuator materials. With a 9.5 percent strain rate reported from magnetic reorientation, Ni-Mn-Ga thin films hold great promise as actuator materials. Thin films come with a number of advantages and challenges as compared to their bulk counterparts. While properties like mechanical strength, uniformity are much better in thin film form, high stress and constraint from the substrate pose a significant challenge for reorientation and shape memory behavior. In either case, it is very important to understand their behavior and examine their properties. This thesis is an effort to contribute to the literature of Ni-Mn-Ga thin films as ferromagnetic shape memory alloys. The focus of this project is to develop a recipe for fabricating NiMnGa thin films with desired composition and microstructure and hence unique properties for future MEMS actuator materials and characterize their properties to aid better understanding of their behavior. In this project NiMnGa thin films have been fabricated using magnetron sputtering on a variety of substrates. Magnetron sputtering technique allows us to tailor the composition of films which is crucial for controlling the phase transformation properties of NiMnGa films. The composition is tailored by varying several deposition parameters. Microstructure of the films has been investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Mechanical properties of as-deposited films have been probed using nano-indentation technique. The chemistry of sputtered films is determined quantitatively by wavelength dispersive X-ray spectroscopy (WDS). Phase transformation is studied by using a combination of differential scanning calorimetry (DSC), in-situ heating in TEM and in-situ XRD instruments. Magnetic properties of films are examined using superconducting quantum interface device (SQUID).

In this study, the shape memory response of Ni2MnGa and NiMnCoIn magnetic shape memory alloys was observed under compressive stresses. Ni2MnGa is a magnetic shape memory alloy (MSMA) that has been shown to exhibit fully reversible, stressassisted magnetic field induced phase transformation (MFIPT) in the I X-phase transformation because of a large magnetostress of 7 MPa and small stress hysteresis. The X-phase is a recently discovered phase that is mechanically induced, however, the crystal structure is unknown. To better understand the transformation behavior of Ni2MnGa single crystal with [100] orientation, thermal cycling and pseudoelasticity tests were conducted with the goal of determining the Clausius-Clapeyron relationships for the various phase transformations. This information was then used to construct a stresstemperature phase diagram that illustrates the stress and temperature ranges where MFIPT is possible, as well as where the X-phase may be found. NiMnCoIn is a recently discovered meta-magnetic shape memory alloy (MMSMA) that exhibits unique magnetic properties. The ferromagnetic parent phase and the paramagnetic martensite phase allow the exploitation of the Zeeman energy. To gain a better understanding of the transformation behavior of NiMnCoIn, thermal cycling and pseudoelasticity tests were conducted on single crystals from two different batches with crystallographic orientations along the [100](011), [087], and [25 7 15] directions. A stress-temperature phase diagram was created that illustrates the Clausius- Clapeyron relationships for each orientation and batch. SQUID tests revealed the magnetic response of the alloy as well as the suppression of the martensite start temperature with increasing magnetic field. Pseudoelasticity experiments with and without magnetic field were conducted to experimentally quantify the magnetostress as a function of magnetic field. For the first time, it has been shown that NiMnCoIn is capable of exhibiting magnetostress levels of 18-36 MPa depending upon orientation, as well as nearly 6.5% transformation strain in the [100] direction. The results of this study reveal increased actuation stress levels in NiMnCoIn, which is the main limitation in most MSMAs. With this increased blocking stress, NiMnCoIn is a strong candidate for MFIPT.