Materials Science Forum Vols. 738-739

Abstract: In this work we investigate magnetocaloric effect and heat capacity of Gd cold rolled ribbons. Such materials are easy to produce, they are flexible and convenient for using in magnetic cooling devices. It is shown that the magnetocaloric effect is strongly dependent on thickness of the ribbons. Severely rolled ribbons demonstrate rather a small magnetocaloric effect. However, a special heat treatment procedure makes it possible to enhance the effect up to the value observed in polycrystalline Gd.

Abstract: Two representatives of Ni(Co)-Mn-Al metamagnetic shape memory alloy system, Ni45Co5Mn31Al19 and Ni35Co15Mn35Al15, have been studied with respect to their magnetocaloric properties. Experimental study of the magnetocaloric effect by a direct measurement of the adiabatic temperature change ΔTad revealed that in both the samples ΔTad depends on the measurement protocol as well as on the magnetic prehistory of the samples. For the applied magnetic field µ0H = 1.93 T, the largest adiabatic temperature change, |ΔTad| ~ 0.7 K, has been observed in the Ni35Co15Mn35Al15 sample at T = 464 K.

Abstract: Ni-Mn-Ga-based magnetic shape memory materials were elaborated by spark plasma sintering method. Micron sized particles were prepared by spark-erosion method in liquid argon and liquid nitrogen from preliminary melted magnetic shape memory master alloys Ni-28.5Mn-22.5G at.%. Powders were annealed in H2 gas atmosphere before sintering. Depending on the cryogenic liquids the morphology of particles alternated between spherical solid and hollow balls. The conditions of spark eroded powder elaboration strongly effect on the morphology, martensitic behavior, and structure of sintered samples. In particularly, martensite has 5M structure in bulk material; 14M and non-modulated martensites were found in annealed powders and sintered compacts correspondingly.

Abstract: In this work we experimentally studied the MCE in the Heusler alloys Ni_2.19-xFe_xMn_0.81Ga (x = 0.01, 0.02, 0.03, 0.04) and Ni_2.16-xCo_xMn_0.84Ga (x = 0.03, 0.06, 0.09). Magnetocaloric effect was measured by the direct method using the installation from the company AMT&C. The temperature dependencies of ∆T_ad for the magnetic field change ∆H = 2 T were measured. The phase transition temperatures were determined from temperature dependencies of low field magnetization measured by original setup using Hall effect. Studies have shown that replacement of the Ni atoms with the iron atoms slightly reduces the temperature magnetostructural transition for x = 0.01,0.02, and starting with x = 0.03, magnetic and structural transitions occur separately and a further increase in iron concentration leads to the Curie temperature increase, while the temperature martensitic transformation decreases. When replacing the Ni atoms with the Co atoms of the martensitic transformation temperature and Curie temperature increase.

Abstract: In this paper we firstly propose and study a microscopic model of multiple twin boundary motion using the first principles and Monte Carlo simulations in Heusler Ni-Mn-X (for example, X = Ga) alloys on real tetragonal lattice. The two variants of the low temperature martensite which divided by twin boundary are considered. The Heisenberg model for magnetic subsystem and Blume-Emery-Griffiths (BEG) one for structural subsystem with magnetostructural interaction between these subsystems are used. The influence of external magnetic field and anisotropy on the twin boundary motion is studied. It is shown that proposed model gives the picture of twin boundary motion as in experiments.

Abstract: In this work, we analyze two Mn50Ni50-xSnx alloys with Sn content i.e., x = 5 and 7.5 respectively. These alloys are produced as ribbon-shape by melt spinning. Their structural transformation is checked by calorimetry. Martensitic transformation temperatures of these alloys strongly depend on the composition. From X-ray diffraction analysis, the 14M monoclinic phase is the main phase in both alloys, but in the alloy Sn5 appears a minor tetragonal phase too.

Abstract: In this work the magnetic properties (exchange parameters, magnetic moment of non-stoichiometric Ni–Mn–Ga Heusler alloys with structural disorder by the help of ab initio calculations have been performed. Theoretical composition dependences of the total magnetic moment are in qualitative agreement in qualitative agreement with experimental data. Calculations of exchange parameters show that interactions between the Mn atoms in regular positions and Mn atoms in Ni and Ga positions are antiferromagnetic.

Abstract: The Ti-21.8Nb-6Zr and Ti-19.7Nb-5.8Ta (at.%) shape memory alloys are thermomechanically treated by cold drawing and post-deformation annealing at 550-600°C forming a nanosubgrained structure in the β-phase. Cyclic mechanical testing using a “loading-unloading” mode with 2% tensile strain in each half-cycle reveals the non-perfect superelastic behavior of both alloys during the very first cycles of testing, which becomes perfect during further mechanocycling. The Young’s modulus of thermomechanically-treated alloys is low (about 45 GPa), and it decreases during mechanocycling (n=10 cycles) down to 25-35 GPa, approaching the Young’s modulus of cortical bone tissues. The Young’s modulus obtained in the 10th cycle is stable or changes only slightly during a further 40-day pause at room temperature and then during repeated mechanocycling. The residual strain per cycle, the transformation yield stress and the mechanical hysteresis decrease during mechanocycling. Subsequent to a 40-day pause at room temperature, they restore their initial values. Repeated mechanocycling is accompanied by a repeated decrease of these parameters.

Abstract: The article represents results of influence of different severe plastic deformation (SPD) techniques on TiNi alloys. It is demonstrated that strength and shape memory effect (SME) of TiNi can be significantly enhanced due to formation of ultrafine-grained (UFG) and nanocrystalline (NC) structures by SPD. Influence of equal channel angular pressing (ECAP), high pressure torsion (HPT), multi-step SPD deformations (ECAP plus cold rolling) on structure, mechanical and functional properties of TiNi alloys is considered. There are represented first results of influence of equal channel angular pressure-Conform (ECAP-C) on TiNi alloys, which is a perspective technology for industrial fabrication of UFG metals and alloys.

Abstract: The peculiarities of martensitic transformations and formation of nanostructured states in metastable austenitic steel (Fe-18Cr-8Ni-Ti) after severe plastic deformation by high pressure torsion are investigated. It is shown that during severe plastic deformation with increased strain rate not only direct (γ→α΄) but also reverse (α΄→γ) martensitic transformations occur, which is revealed by the changes in the volume content of α΄ - martensite during deformation. The fragments thought to be formed by direct and reverse martensitic transformations and those of dynamic recrystallization of austenite are observed.