Outi Söderberg

Various magnetic domain structures in a Ni–Mn–Ga martensite exhibiting magnetic shape memory effect

Magnetic domain structures of the Ni–Mn–Ga martensite were observed by means of type I and type II magnetic contrast in scanning electron microscope. The different configuration of magnetic domain patterns coupled together with the twin structures were studied in multivariant, two-variant, and single-variant martensite. The martensitic band contains broad stripelike magnetic domains following the easy axis of magnetization, i.e., the crystallographic c axis. These stripe domains are connected by 90° domain walls creating a staircaselike structure in the adjoining bands. It is found that the internal twins, substructures of the martensite twin domains, are distorted into a zig–zag shape in order to accommodate the main band magnetization. Furthermore, the dagger-shaped stripe domains occur only when the internal twins are present. When the sample exhibits the single-variant state, the internal twins disappear totally and the stripe magnetic domains spread over the whole specimen. The configuration observed...

Crystal structure and twinning in martensite of Ni1.96Mn1.18Ga0.86 magnetic shape memory alloy

Abstract The martensite structure in the Ni 1.96 Mn 1.18 Ga 0.86 magnetic shape memory single crystal is determined as 5R monoclinic at room temperature. Twin structure of martensite is found to be a result of the three twin types coexistence. Crystallography of macro- and internal twins in studied 5R martensite was determined using X-ray diffraction methods.

Corrosion behaviour of Fe–Mn–Si based shape memory steels trained by cold rolling

Abstract Fe–Mn–Si based high nitrogen steels have been studied in recent years for potential industrial applications. These steels show good shape memory properties, high strength and excellent ductility. In the present study, the effects of training history on the corrosion properties of Fe–Mn–Si–Cr–Ni based high nitrogen steels were investigated. The corrosion behaviour of shape memory alloys was analyzed by implementing anodic polarisation measurements and immersion tests. The shape memory steels in annealed, deformed and recovered conditions were studied to examine the training effect on their corrosion behaviour. The features of the anodic polarisation curves indicated a general corrosion type of these steels. The experimental results showed that Cr and Mn had a marked influence on the corrosion behaviour of the steels, followed by Ni, N and V. It was also apparent that the deformation during the shape memory training by cold rolling decreased the corrosion stability, and the recovery heating reduced further their corrosion resistance. However, further studies are needed in order to better understand the corrosion behaviour of the investigated alloys.

Martensitic transformations and mobility of twin boundaries in Ni2MnGa alloys studied by using internal friction

Internal friction (IF) was measured in two non-stoichiometric Ni2MnGa alloys. Several IF peaks attributed to some restructuring of the martensitic lattice were observed below Ms. From the strain dependence of IF, the activation enthalpy for movement of twin boundaries between martensitic domains was estimated to be equal to 0.02–0.04 eV.

High-cycle fatigue of 10M Ni-Mn-Ga magnetic shape memory alloy in reversed mechanical loading

Application of Ni?Mn?Ga magnetic shape memory alloys in magnetic-field-induced actuation relies on their performance in long-term high-cycle fatigue. In this paper the performance and changes in the microstructure of a Ni?Mn?Ga 10M martensite single crystal material are reported in a long-term mechanically induced shape change cycling. The longest test was run for 2 ? 109 cycles at a frequency of 250?Hz and a strain amplitude of ? 1%. After the test a clear increase of the dynamic stiffness of the material was detected. Three specimens out of ten were cycled until fracture occurred and their fracture mechanism was studied. It was observed that the macroscopic crack growth took place roughly at a 45? angle with respect to the loading direction that was along the 100 crystallographic direction of the sample. The macroscopic fracture plane seemed to correspond roughly to the {111} crystal planes. On a microscopic scale the fracture propagated in a step-like manner at least partly along crystallographic planes. The steps at the fracture plane correspond to the {101} twin planes, with the height of steps along the 101 direction. The final fracture of the samples occurred in a brittle manner after the critical stress was exceeded.

Direct optical observation of magnetic domains in Ni-Mn-Ga martensite

This letter reports the direct optical observation, i.e., without polarization, of the magnetic domain structure explained by a large surface relief in Ni–Mn–Ga martensite. The authors suggest that the relief is due to the different straining of the surface and the bulk caused by the internal stresses associated with the magnetic shape memory effect. As a result of the relief the projection of the (011) twin traces upon the (010) plane creates the observed zigzag pattern. The surface tilt angle calculated from the zigzag pattern is ∼3°.

Shape Memory Alloys for Biomedical Applications

NiTi shape memory alloy (SMA) products appeared to the medical markets in 1980’s, their global market being more than US

Ferromagnetic resonance in non-stoichiometric Ni1−x−yMnxGay

130 billion in 2002. In most medical applications material must be biocompatible. NiTi offers the bodytemperature activated shape memory effect (SME), superelasticity (SE) and the damping capacity, which all can be applied in medical use. The dental arch wires and stents are benefiting from SE. The NiTi vena cava filters obtain their umbrella shaped mesh when SMEactivated. Generally the NiTi tubes and guidewires are applied in the minimally invasive medical procedures and in the interventional radiology. There are numerous steerable, hingeless, kink resistant, highly flexible clinical instruments that may provide constant force. NiTi is used for the dental implants and the attachments of the partial dentures and for the orthopaedics. In the latter one the main applications are the clamps for connecting bone fractures or parts for e.g. the spinal bentcalibration bar. Miniaturization has enabled small SMAactuators that are applicable in active endoscopes with allround bending and in actuators for kidney or heart pumps. The main risks using NiTi are the insecure fatigue life and possible cytotoxicity.

Room Temperature Synthesis of Magnetite (Fe3-δO4) Nanoparticles by a Simple Reverse Co-Precipitation Method

Abstract Non-stoichiometric alloys Ni 1− x − y Mn x Ga y characterised by different values of MSME (from 0.2% to 7.3%) were studied using ferromagnetic resonance (FMR). The angular dependence of the FMR signals was measured in the martensitic and austenitic states of the samples just before and after martensite–austenite transition. Experimental data were used for the determination of the magnetisation 4 πM s and anisotropy parameters K 1 , K 2 for the martensitic state and K 1c for the austenitic state. All studied alloys were characterised by large values of the anisotropy parameters of the first and second orders. A special feature of the alloys possessing high MSME is a larger value of the coefficient K 2 . The temperature dependence of the FMR signals was investigated in the temperature range from below M s to above T C , where FMR was replaced by conduction electron spin resonance (CESR). Magnetically induced strain in the martensitic phase was measured as a function of the applied magnetic field. The main difference between the alloys in the martensitic state revealing the large or small MSM strain is the behaviour of the electronic structure. In the alloys with the small MSM strain, all the electrons are involved in the ferromagnetic system. On the contrary, in the alloy with the large MSM strain, the narrow resonance line of one electron subsystem is present separately in the FMR spectra. An intensive signal of CESR is observed in the alloys with the large MSME, which is an evidence for a high concentration of free electrons. The suggestion made is that the high concentration of free electrons, i.e. enhanced metallic character of interatomic bonds, assists MSME.

Effect of nitrogen on damping, mechanical and corrosive properties of Fe–Mn alloys

Magnetite (Fe3-δO4) nanoparticles with the size less than 30 nm have been synthesized by using a simple reverse co-precipitation method at room temperature. During the process, ferrous sulfate (FeSO4•7H2O) powder was used as an iron precursor, and ammonium hydroxide (NH4OH) as a precipitating agent. The experiment was carried out in ambient atmosphere without any surfactant added. In this method, the base solution for the precipitation process was adjusted to have a pH value suitable for the formation of the magnetite phase. The iron salt precursor was added into the solution during the synthesis by two different synthesis protocols. The phase, morphology and magnetic characteristic of differently synthesized magnetite particles were characterized by using an X-ray diffraction (XRD), transmission electron microscope (TEM) and vibrating sample magnetometer (VSM). The morphologies of the particles were spherical or irregular in shape depending on the synthesis protocol used. Magnetic measurement shows that the particles are ferromagnetic at room temperature with relatively high saturation magnetization and low hysteresis. The saturation magnetization and magnetic hysteresis of the particles varied with preparation reaction conditions and the resulting oxidation state of the particles.