Magnetic and magneto-mechanical properties of Ni-Mn-Ga magnetic shape memory alloys

Ni-Mn-Ga alloys close to stoichiometric Ni50Mn25Ga25 (at. %) composition have recently gained considerable interest due to the possibility of rearrangement of their martensite microstructure in magnetic field. The rearrangement is accompanied by large strains of up to 10%. This effect is different from ordinary magnetostriction and it is referred to as magnetic shape memory effect (MSME). The Thesis presents the first attempt to study the temperature limits of irreversible and reversible MSME by exploiting a theoretical model and experimentally determined temperature dependences of magnetic and other material properties governing the existence of MSME. The obtained predictions are compared with direct observations of MSME. Extraordinary magneto-mechanical effects in Ni-Mn-Ga alloys, not discussed previously, are investigated and compared with theoretical models in this Thesis. These effects include reversible MSME with strain close to 6%, magnetic field controlled superelasticity with strain close to 6%, and up to 30% changes of magnetization during loading in static magnetic field. Unique simultaneous measurements of strain and magnetization on Ni-Mn-Ga alloys are presented for various experiments such as, e.g., during MSME and reversible MSME. The broad spectrum of experiments presented in the Thesis corroborates the important role of magnetic anisotropy, twinning stress and temperature for existence and reversibility of MSME. The simultaneous measurement of strain and magnetization in various experiments confirms experimentally the close relation between martensite microstructure and its magnetic properties and demonstrates the interplay between martensite microstructure and magnetic field. Good agreement of all presented experimental results with the used theoretical model supports validity of the model and shows that the model is suitabile for predicting temperature and stress limits of MSME or reversible MSME, and for modelling of magnetic-field induced superelasticity. Some of the presented experiments can additionally be considered as application examples. The original findings presented in this Thesis broaden the general scientific understanding of MSME and can serve as informative source when considering possible engineering usage of Ni-Mn-Ga alloys as actuators, sensors, or intelligent material.reviewe

Transformation Paths from Cubic to Low-Symmetry Structures in Heusler Ni2MnGa Compound

In order to explain the formation of low-temperature phases in stoichiometric Ni2MnGa magnetic shape memory alloy, we investigate the phase transformation paths from cubic austenite with Heusler structure to low-symmetry martensitic structures. We used ab initio calculations combined with the generalized solid state nudged elastic band method to determine the minimum energy path and corresponding changes in crystal lattice. The four-, five-, and seven-layered modulated phases of martensite (4O, 10M, and 14M) are built as the relaxed nanotwinned non-modulated (NM) phase. Despite having a total energy larger than the other martensitic phases, the 10M phase will spontaneously form at 0 K, because there is no energy barrier on the path and the energy decreases with a large negative slope. Moreover, a similar negative slope in the beginning of path is found also for the transformation to the 6M premartensite, which appears as a local minimum on the path leading further to 10M martensite. Transformation paths to other structures exhibit more or less significant barriers in the beginning hindering such a transformation from austenite. These findings correspond to experiment and demonstrates that the kinetics of the transformation is decisive for the selection of the particular low-symmetry structure

Kelainan Peak Expiratory Flow Rate Dibandingkan dengan Keluhan Sistim Pernapasan – studi pada 75 anggota pasukan kuning, Surabaya

Telah dilakukan kajian untuk membandingkan hasil pemeriksaan faal paru Peak Expiratory Flow Rate (PEFR) dalam mendeteksi kelainan paru obstruktif dengan anamnesis adanya keluhan batuk, pengeluaran dahak, sesak nafas dan nafas berbunyi. Sebagai subyek penelitian adalah anggota pasukan kuning Kotamadya Surabaya yang bertugas di jalan raya sebanyak 75 orang. Dipilih anggota pasukan kuning karena dalam melaksanakan tugas sehari-hari diduga mendapat pemaparan polusi debu dan gas buang kendaraan bermotor. Hasil yang didapat adalah proporsi kelainan PEFR secara keseluruhan sebesar 42,67%, di antara yang mengeluh keluhan batuk sebesar 24,00%, di antara yang mengeluarkan dahak sebesar 14,67%, di antara yang sesak nafas sebesar 22,67% dan di antara yang nafasnya berbunyi sebesar 10,67%. Ternyata didapatkan hubungan antara kelainan faal paru PEFR dengan adanya keluhan batuk (p = 0,0216), dengan keluhan pengeluaran dahak (p = 0,0005), dengan keluhan sesak nafas (p = 0,018) dan dengan keluhan nafas berbunyi (p = 0,0000). Subyek dengan kelainan faal paru PEFR tanpa keluhan berkisar antara 30,67 –36% dari seluruh subyek yang diperiksa

Low temperature a/b nanotwins in Ni50Mn25+xGa25x Heusler alloys

We have found low temperature a/b nanotwins having (110) twinning plane in a five-layered modulated martensite phase of Ni50Mn25+xGa25x (at. %) Heusler alloys and identified the particular region in phase diagram where the nanotwinning occurs. Evolution of the structure with decreasing temperature was studied by X-ray diffraction using single crystals exhibiting magnetic shape memory effect. The merging of (400) and (040) lines upon cooling for 2.6 < x < 3.5 indicated a/b nanotwinning originating from the refinement of initially coarse a/b twins. Refinement of the twins with decreasing temperature was observed directly using scanning electron microscopy. The prerequisite for nanotwinning is an extremely low twin boundary energy, which we estimated using first-principles calculations to be 0.16 meV/2. As the nanotwinning distorts the relation between the crystal lattice and the X-ray diffraction pattern, it should be taken into consideration in structural studies of Ni-Mn-Ga Heusler alloys

Effect of electron localization in theoretical design of Ni-Mn-Ga based magnetic shape memory alloys

The precise determination of the stability of different martensitic phases is an essential task in the successful design of (magnetic) shape memory alloys. We evaluate the effect of electron delocalization correction on the predictive power of density functional theory for Ni-Mn-Ga, the prototype magnetic shape memory compound. Using the corrected Hubbard-model-based generalized gradient approximation (GGA+U), we varied the Coulomb repulsion parameter U from 0 eV to 3 eV to reveal the evolution of predicted material parameters. The increasing localization on Mn sites results in the increasing stabilization of 10M modulated structure in stoichiometric Ni2MnGa in agreement with experiment whereas uncorrected GGA and meta-GGA functional provide the lowest energy for 4O modulated structure and nonmodulated structure, respectively. GGA+U calculations indicate that 10M structure is more stable than other martensitic structures for U > 1.2 eV. The key features of density of states (DOS) responsible for the stabilization or destabilization of particular martensitic phases calculated with GGA+U are found also in DOS calculated with advanced quasi-particle self-consistent GW (QSGW) method. It supports the physical background of Hubbard correction. Moreover, the calculations with U = 1.8 eV provide the best agreement with experimental data for lattice parameters of stoichiometric and off-stoichiometric alloys. (C) 2021 The Authors. Published by Elsevier Ltd

Ab initio study of Ni2MnGa under shear deformation

VK: T20309Peer reviewe

Nanotwinned (inter)martensite transformation interfaces in Ni50Mn25Ga20Fe5 magnetic shape memory single crystal foil

Publisher Copyright: © 2022 Elsevier Inc.Using common and high resolution transmission electron microscopy (TEM), we study the magnetic and crystal phase structures as well as their evolution in Ni50Mn25Ga20Fe5 magnetic shape memory material with high Curie point. The particular alloying by Fe and changing thickness of the TEM foil enable us to observe all martensitic phases known in Ni-Mn-Ga Heusler alloy system and their respective transitions simultaneously. Starting from cubic austenite at about 10 nm foil thickness, the structure evolves via peculiar interleaved stripes of austenite and five-layered modulated 10M martensite to pure 10M phase with a low density of stacking faults at 40 nm thickness. With further increasing thickness, the 10M phase transforms gradually to seven-layered modulated 14M martensite with an increased density of stacking faults. Finally, the non-modulated tetragonal NM phase appears within the 14M phase by detwinning of nanotwins forming the modulated phases. High resolution TEM further confirms that nanotwinning and stacking faults are inherent structure features tightly connected with the lattice modulation and intermartensite transformations. Overall, the evolution of average lattice shows the same general trend, an increasing simple shear with (11¯0) shuffling plane, across the whole phase sequence. Additionally, we found large local variation of lattice parameters in all phases, which is is ascribed to strong lattice softening in the vicinity of the martensitic transformation and high density of stacking faults in 14M martensite lattice.Peer reviewe