STRUCTURAL AND MARTENSITIC TRANSFORMATION OF BULK, DISORDERED AND NANOCRYSTALLINE Ni₂MnGa ALLOYS

Abstract

Ferromagnetic Ni 2 MnGa nanoparticles were prepared by post milling and annealing method. Differential scanning calorimetry studies confirm absence of martensitic and Curie transformation in the disordered state. It was found that the bulk alloy was annealed into a well-defined 7 M structure at around 1073 K. The residual stress of nanoparticles induced the disordered phase to transform into a NM tetragonal phase through atomic ordering. Thermo hysteresis curve of the \disordered" powder was linear and revealed very low moment indicative of paramagnetic nature. It is confirmed that the martensitic transformation in nanosized particles is predominantly dependent on the strain and the particles size.Since the discovery of the ferromagnetic shape memory behavior and large strain 10% of the Heusler Ni 2 MnGa alloy, 1 ferromagnetic shape-memory alloys (FSMAs) have created enormous interest due to their potential applications such as magnetic actuators and sensors. Magnetic field induced strain in these alloys is due to rearrangement of martensite variants driven by an external magnetic field. The martensitic transformation (MT) in FSMAs can be triggered not only by changes in temperature and stress, but also by changes in the applied magnetic field. This enables the devices to operate at high frequencies and facilitates their remote control. 2 A necessary condition for the shape memory effect is that the so-called MT of the crystal lattice takes place. In this phase transformation, the crystal structure changes from the parent, usually cubic austenitic phase to a lower-symmetry, often tetragonal or orthorhombic martensitic phase. The MT temperatures in Ni 2 MnGa are controlled by adjusting their chemical composition and imposing mechanical stress. 3 In shape memory alloys, MT plays an important role, and has so far been the primary object of attention. In recent years, stress induced structural transformations are studied especially in Ni 2 MnGa nanoparticles. 4,5 According to the MT temperature, there are modulated (7 M or 5 M) orthorhombic or monoclinic structure and non modulated (NM) face centered tetragonal (fct) structure in grained Ni 2 MnGa alloy during the martensitic phase transformation. 6 Generally, disordered/amorphous particles can be induced by applying various solid state processes such as particle irradiation, 7 mechanical alloying 8 and high pressure torsion. 9 On the other hand, severe plastic deformation (SPD) of NiÀMnÀGa bulk alloy by applying mechanical milling was used to achieve disordered particles. A nanocrystalline (NC) phase can be obtained by devitrification of the disordered phase followed by heat treatment. The physical properties of the Ni 2 MnGa alloys are interesting, particularly in view of creating new materials with the best useful properties. Exploring nanostructured high-surface-area materials for this purpose has recently attracted much interest. The structural and physical properties of various forms of NiÀMnÀGa alloys have been studied to date. 10À12 However, there are no syst...

119Sn Mössbauer spectroscopy for assessing the local stress and defect state towards the tuning of Ni-Mn-Sn alloys

“…[24][25][26][27] In order to enhance the multifunctional properties of the micro-particles and films, the control of defects and the local stress-state becomes fundamental as they influence directly the MT. [28][29][30][31] The presence of residual stress increases the thermal hysteresis and the irreversibility of the MT. 32 Residual stress is also retained when the samples are cycled through the phase transformation, 33 being one of the main causes of the degradation of the MT, the deformation behavior, and the shape memory effect.…”

III. Ikergazte. Nazioarteko ikerketa euskaraz. Kongresuko artikulu bilduma. Zientziak eta Natura Zientziak

“…Izatez, mikropartikulak eta nanopartikulak elementu aktibo gisa erabiltzearen planteamendua konponbide eraginkor bezala finkatzen ari da azken urteotan (Kumar et al, 2016). Hala ere, nahiz eta Ni-oinarridun Heusler alezioetan oinarritutako hauts-laginen jokaerak abantaila aipagarriak erakusten dituzten, eskala hauetan, akatsen, dislokazioen, hutsuneen, tokiko esfortzuen eta tentsioen kontrola ezinbestekoa bilakatzen da, hauek TM-an zuzenean eragiten bait dute (Peruman et al, 2011). Horrela, mikroegituraren gainerako kontrolak oraindik eta garrantzi handiagoa hartzen du hauts-laginekin lotutako propietate multifuntzionalak doitzerako orduan.…”

Effect of Annealing on the Structural and Magnetic Properties of CoNiAl FSMA

“…Magnetic controls of shape memory behavior in FSMAs have much shorter response time than that of thermal controls in SMAs. So far, many FSMAs have been reported, in particular, Ni 2 MnGa alloys have been studied extensively. In general, diffusionless structural transformation in FSMAs alters the high‐temperature cubic phase (austenite) to a lower symmetric phase (martensite), like orthorhombic and monoclinic, at lower temperatures .…”