Ilkka Aaltio

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.

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.

ESOMAT 2009 - 8th European Symposium on Martensitic Transformations

The high temperature transformations (e.g. liquidus, ordering temperature) of the alloys Ni47.7Mn31.2Ga21.1, Ni49.7Mn28.7Ga21.6, and Ni49.6Mn24.0Ga26.6, Ni47.3Mn30.3Ga20.3Fe2.1, Ni49.9Mn28.3Ga20.1Fe1.7, Ni51.3Mn14.4Ga26.3Fe8.0, Ni47.3Mn25.5Ga24.5Cu2.7, Ni48.3Mn29.7Ga21.1Cu0.9, and Ni49.4Mn23.3Ga25.6Cu1.7 were studied for the practical melting and annealing purposes. At first the chemical compositions (SEM-EDS) and the martensitic and magnetic transition temperatures (DSC, ac magnetic susceptibility) of the alloys were determined. High temperature DSC measurements were made in argon with 10 K/min. Two first measurements were carried out in the solid state (301 1273 K) and in the third measurement the material was melted (max meas. temp. 1573 K). The ordering temperature was obtained from the measurements in the solid state. As the e/a ratio was above 7.53 the ordering temperature was in the range of 1019-1039 K, otherwise a clear change was observed. The variation in heating and cooling was less than 5 K with small quaternary additions, but alloying of 8% Fe increased this difference to 18 K. Alloys with close Ni/Mn/Ga-ratio showed only minor differences in solidus and liquidus temperatures, but if there was a clear change in the Ni/Mn-ratio even those alloys having close e/a ratios showed a clear difference in melting behavior. When Ni/Mn is 1.5with higher values not clear region could be determined.

Comparison between nitrogen alloyed Fe - Mn-based damping steels and ferritic damping steels

A variety of damping steels have been developed in recent years for construction purposes. Ferritic and martensitic high-damping steels are the most important groups of damping steels. In the former case, damping is based on magnetoelasticity and in the latter case damping is mainly based on dissipative motion of the interfaces between martensite and austenite phases. Damping properties of two steels of both groups were compared in this report. Compositions of the ferritic steels were Fe-12Cr-3Mo and Fe-2.5Al-0.5Si, and compositions of the martensitic steels were Fe-17.5Mn and Fe-13.7Mn-0.2N. Damping properties of all of those steels were good at room temperature. The highest damping capacity was in alloy Fe-13.7Mn-0.2N. This alloy is the most suitable of these steels to engineering applications because its mechanical properties are better than those of the other steels studied.

Recent developments of magnetic SMA

In the shape memory alloys (SMAs) the thermal triggering induces reversible dimensional change by the phase transformation – these materials may also be ferrior ferromagnetic, however, here only the ferromagnetic SMAs are discussed. In certain SMAs the austenitemartensite phase transformation is influenced by the magnetic field as either austenite or martensite is promoted by the field and this is exploited for the dimensional changes. However, in the magnetic shape memory (MSM) alloys no phase transformation occurs as the remarkable dimensional changes take place by the twin variant changes in the martensitic phase activated by the external magnetic field at constant temperature. In addition to the phase transformation or magnetic shape memory effect, the applied magnetic field may also result in the conventional magnetostriction (MS), enhance the superelasticity (magneticfieldassisted superelasticity MFAS) or induce the giant magnetocaloric effect (GMCE). Certain alloys such as NiMnGa may even be multifunctional showing more than one of these effects. The present paper gives an overview of the different types of the magnetically activated SMA alloys, their properties as well as their potentials for applications in the frameworks of the recent studies.

Properties of Fe-Al-Si high-damping steel

Several high-damping materials have been developed in recent years to decrease vibration and noise levels of machines and structures. The most important damping steels have been Fe- 12Cr-based alloys. These steels exhibit high damping capacity combined with rather good mechanical and corrosion properties. A new vibration damping Fe-2.5Al-0.5Si steel has been developed by NKK-Corporation in Japan, and it is produced under a trade name of NKK- SERENA. This steel is a potential multi-purpose damping steel, because it is more economical than the previous steels. Damping capacity of NKK-SERENA is very high in wide temperature and frequency ranges, and its mechanical properties are similar to those of common structural steels. In this study, mechanical, welding and corrosion properties, and the results of the microstructural characterization are presented.

Long-Term Cyclic Loading of 10M Ni-Mn-Ga Alloys

Martensitic 10M Ni-Mn-Ga single crystals are usually applied in the magnetomechanical actuators. Their behavior in the long-term cycling and the factors influencing upon are thus important to know. There are several publications concerning this. However, consistent statistical data is still missing to large extent. In this report we review and analyze the data available in the literature. In conclusion it can be stated that 10M Ni-Mn-Ga single crystals survive well in long-term actuation (millions of cycles) when the frequency of twin variant reorientation is not exceeding 250 Hz, and the strain used in actuation is below 3 %. There are several factors influencing the long-term behavior and these are discussed in more details in this work.

X-ray Diffraction Reciprocal Space Mapping Study of Modulated Crystal Structures in 10M Ni-Mn-Ga Martensitic Phase

The 10M modulated crystal structure in Ni-Mn-Ga martensitic phase with about 0.5 MPa twinning stress, was studied by X-ray diffraction reciprocal space mapping (RSM). The experimental procedure is established for collecting large range of RSM with scattering planes inclined to the surface of specimen. The investigation focused on the superlattice reflections caused by the modulation, which always appeared in two <110> directions in bulk material. The distribution of two modulation domains varies with scattering locations.

Magnetic Shape Memory - Polymer Hybrids

Martensitic Ni-Mn-Ga based alloys are known for the Magnetic Shape Memory (MSM) effect, which upon application of an external magnetic field can generate a strain up to 12 % depending on the microstructure of the martensite. The MSM effect occurs by rearrangement of the martensite variants, which is most advantageous in single crystals. Single crystals are, however, rather tedious to produce and there has been attempts to achieve MSM effect in polycrystals. However, in polycrystals the magnetic field induced shape change remains low as compared to single crystals. As an alternative to the former, hybrid MSM materials offer several advantages. When compared to single crystals, hybrids have extended freedom of shaping, lower raw material price, relatively large MSM strain and easier manufacturability. Embedding MSM particles into a suitable polymer matrix results in actuation function or good vibration damping performance. In the present study we report on the mechanical, structural and magnetic properties of MSM polymer hybrids, which are prepared by mixing gas-atomized Ni-Mn-Ga MSM powder into epoxy matrix and aligning the magnetic particles in a magnetic field.

Neutron Diffraction Study of a Non-Stoichiometric Ni-Mn-Ga MSM Alloy

Abstract. The structure and chemical order of a Heusler alloy of non-stoichiometric composition Ni-Mn-Ga were studied using constant-wavelength (1.538 Å) neutron diffraction at 363K and the diffraction pattern was refined using the FullProf software. At this temperature the structure is austenite (cubic) with Fm space group and lattice constant of a = 5.83913(4) [Å]. The chemical order is of critical importance in these alloys, as Mn becomes antiferromagnetic when the atoms are closer than the radius of the 3d shell. In the studied alloy the refinement of the site occupancy showed that the 4b (Ga site) contained as much as 22% Mn; that significantly alters the distances between the Mn atoms in the crystal and, as a result, also the exchange energy between some of the Mn atoms. Based on the refinement, the composition was determined to be Ni1.91Mn1.29Ga0.8