V.K. Lindroos

Composition and temperature dependence of the crystal structure of Ni–Mn–Ga alloys

The crystal structure of ferromagnetic near-stoichiometric Ni2MnGa alloys with different compositions has been studied at ambient temperature. The studied alloys, with five-layered (5M) and seven-layered (7M) martensitic phases, exhibit the martensitic transformation temperature (TM) up to 353 K. Alloys with these crystal structures are the best candidates for magnetic-field-induced strain applications. The range of the average number of valence electrons per atom (e/a) was determined for phases 5M, 7M, and nonmodulated martensite. Furthermore, a correlation between the martensitic crystal structure, TM and e/a has been established. The lattice parameters ratio (c/a) as a function of e/a or TM has been obtained at ambient temperature for all martensitic phases. That the paramagnetic-ferromagnetic transition influences the structural phase transformation in the Ni–Mn–Ga system has been confirmed.

Interactions between tungsten carbide (WC) particulates and metal matrix in WC-reinforced composites

Abstract A variety of experimental techniques have been used to investigate the interactions between tungsten carbide (WC–Co 88/12) particulates and the matrix in some new wear resistant cobalt-based superalloy and steel matrix composites produced by hot isostatic pressing. The results show that the chemical composition of the matrix has a strong influence on the interface reaction between WC and matrix and the structural stability of the WC particulates in the composite. Some characteristics of the interaction between matrix and reinforcement are explained by the calculation of diffusion kinetics. The three-body abrasion wear resistance of the composites has been examined based on the ASTM G65-91 standard procedure. The wear behavior of the best composites of this study shows great potential for wear protection applications.

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...

Giant Magnetostrictive Materials

The magnetostrictive materials exhibit a strain caused by the orientation of the magnetic moment when exposed to a magnetic field. A particular class of magnetostrictive materials is called magnetic shape memory (MSM) alloys or ferromagnetic shape memory alloy (FSMA) materials, which can change their shape remarkably when subjected to magnetic field. In the following, the MSM materials are introduced and their extraordinary structure, properties, and performance are described.

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.

Effect of crystal structure on magnetic-field-induced strain in Ni-Mn-Ga

Magnetic shape memory materials are expected to have a high potential in practical applications. Several ferromagnetic materials exhibiting the large magnetic-field-induced strain have been found in recent years. The largest field-induced strain is observed in Ni-Mn-Ga system. The most important experimental results on crystal structure, magnetic anisotropy and twinning stress of martensitic phases in Ni-Mn-Ga having tetragonal five-layered, orthorhombic seven-layered and tetragonal non-layered crystal structures are reported. Depending on the martensite crystal structure Ni-Mn-Ga alloys are able to show a really giant strain response (approximately 6% in tetragonal five-layered or 10% in orthorhombic seven-layered martensitic phase) in a magnetic field less than 1 T. Contrary to these two phases, a detectable field-induced strain is not observed in non-layered tetragonal martensitic phase in Ni-Mn-Ga system. Effect of crystal structure is in a good agreement with calculation of the magnetic-field-induced strain based on the model developed by authors. The effect of composition on appearance of undesirable non-layered tetragonal martensitic phase in Ni-Mn-Ga system is discussed based on the new experimental results.

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

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

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.