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The entropy changes of successive martensitic phase transformations in Ni-Mn-Ga Heusler alloys can be utilized to realize enhanced magnetocaloric properties. A detailed study of phase transformations of one such alloy, Ni2.15Mn0.85Ga, (ΔQ = 4900 J/kg at 343 K, under 140 kOe), is reported here. Upon cooling, the paramagnetic cubic (L21) austenitic phase transforms into a ferromagnetic 7M modulated monoclinic martensitic phase. This phase is stable in a narrow temperature range, and upon further cooling, transforms into a non-modulated ferromagnetic tetragonal (L10) phase. The separation between the equilibrium temperatures of the austenitic and tetragonal martensitic phases is only ~50 K. The alloy undergoes reversible temperature-induced martensitic and inter-martensitic phase transformations with thermal hysteresis of about 25 K. The conclusions from the detailed study of the phase transformations lead to new possibilities to enhance the magnetocaloric effect by utilizing the entropy associated with multi-structural transformations.

The Heusler alloys demonstrate magnetically induced strain and magnetocaloric properties, but the mechanical properties are poor. Therefore, in this work, the influence of thermomechanical treatment on the properties of Ni-Mn-Ga-Si Heusler alloys is considered. The effect of multi-axial isothermal forging on functional properties of the Ni2.30Mn0.73Ga0.90Si0.07 alloy at 973 K and true strain (e) of 3.9 is presented. It is shown that a unique two-phase microstructure is formed as a result of forging. The large grains in the size range of 100–200 µm are surrounded by a fine-grained structure. The study of the thermomechanical properties by the three-point bend test has shown that the alloy demonstrates a single-stage reversible deformation of 3.1% at a constant stress of 860 MPa as compared to the same alloy in the as-cast condition which shows 2% at 380 MPa. The specimen demonstrates a reversible deformation of 5% without any degradation during thermal cycling (with a base of up to 4000 thermal cycles) under a stress of 550 MPa and up to 5% with degradation occurring at 700 thermal cycles under a stress of 650 MPa. Thus, forging makes it possible to obtain a material with higher operational properties and greater resistance to fracture during multiple cycles of martensitic transformation. In this case, it is possible to obtain anisotropy of properties equal or close to that of the specimen in the as-cast state.

The results of investigation of functional characteristics of the Mn2NiGa Heusler alloy prepared by argon-arc melting are presented. Its electric resistance, magnetization, magnetocaloric and shape memory effects were studied using direct experimental techniques in a wide temperature range 100–400 K. The inverse magnetocaloric effect, which does not reach saturation in pulsed magnetic fields up to 50 T, was observed in both martensite and austenite phases. The value of reversible deformation in the alloy was as high as 0.35% under bending mechanical loads up to 247 MPa.

We have performed comparative analysis of functional properties and functional fatigue of alloys of the Ni–Mn–Ga system in the initial molded state and the Ni–Ti system in the initial hot-forged state without thermal treatment. It is shown that the Ni2MnGa alloy in the molded state is preferable for low-loaded actuating elements (up to 100 MPa) and low recoverable strains (up to 1.5%) in the temperature range from –85 to –75°C; the Ni–Ti system alloy in the hot-forged state should be used for high-loaded actuating elements (up to 500 MPa) and high recoverable strains (up to 8.5%) in the temperature range from –40 to 40°C.

Tiny chromian spinel precipitations associated with plastic deformed enstatite crystals from spinel peridotite were studied. It is defined that there are several localizations of the precipitations: 1) in recrystallization zones where they coexist with olivine and enstatite neoblasts, 2) in the volume of polygonized primary enstatite crystals as lamellae and holly leaf grains, 3) in lamellae and neoblasts of diopside and pargasite which were formed during a plastic deformation of primary orthopyroxene. In the first two cases, we suggest the solid-state mechanism of their formation. First, it is a result of an impurities segregation on the structure defects (dislocation → subgrain boundary → grain boundary). Second, it is a result of the nucleation and growth of the new-formed mineral phases (spinel, forsterite, enstatite II) in most distorted places of porphyroclasts. In the third case, it is possible that chromian spinel were appears either as a result of the solid-state crystallization or as a cumulate of «diopside» or «pargasite» melts formed by frictional melting of orthopyroxene porphyroclasts in micro-chambers.

The formation of a sharp crystallographic texture in a Ni–Mn–Ga Heusler alloy by the multiple isothermal forging has been studied. An analysis of the thermal expansion near the martensitic transformation temperatures in the as-cast and forged states of the alloy shows that the thermomechanical treatment leads to an increase in the anisotropy of the sample geometric size changing during the phase transformation, which favors an increase in the functional characteristics of the alloy. The structural studies show that the alloy structure after multiple isothermal forging has the bimodal distribution of grain sizes. The formation of the bimodal structure by forging is assumed to make it possible to enhance the stability of the functional properties of the alloy during repeated cycles of the phase transformation.

The plastic behavior during deformation by upsetting and its effect on the microstructure in the polycrystalline Ni2.19Fe0.04Mn0.77Ga alloy are studied. The temperatures of martensitic and magnetic phase transformations were determined by the method for analyzing the temperature dependence of the specific magnetization as M F = 320 K, A S = 360 K, and T C = 380 K. Using differential scanning calorimetry, it is shown that the phase transition from the ordered phase L21 to the disordered phase B2 is observed in the alloy during sample heating in the temperature range of 930–1070 K. The melting temperature is 1426 K. An analysis of the load curves constructed for sample deposition at temperatures of 773, 873, and 973 K shows that the behavior of the stress–strain curve at a temperature of 773 K is inherent to cold deformation. The behavior of the dependences for 873 and 973 K is typical of hot deformation. After deforming the alloy, its microstructure is studied using backscattered scanning electron microscopy. Plastic deformation of the alloy at study temperatures results in grain structure fragmentation in the localized deformation region. At all temperatures, a recrystallized grain structure is observed. It is found that the structure is heterogeneously recrystallized after upsetting at 973 K due to the process intensity at such a high temperature. The alloy microstructure after plastic deformation at a temperature of 873 K is most homogeneous in terms of the average grain size.

The temperatures ductility of the polycrystalline Ni 2.19 Fe 0.04 Mn 0.77 Ga alloy was investigated. The analysis of the temperature dependence of the specific magnetization shows that temperatures of the martensitic and magnetic phase transformations have the following values: M F =325 K, A S =343 K, T C =365 K. The differential scanning calorimetry shows that in the temperature range of 930 to 1070 K in the alloy the phase transition of the ordered L2 1 phase to disordered B2 phase is observed. The stress-strain curves of cylindrical samples of alloy at temperatures of 773 K, 873 K and 973 K were constructed.

The structure and properties of a polycrystalline Ni–Mn–In Heusler alloy have been studied after a plastic deformation by upsetting. An analysis of points of a martensitic and magnetic phase transformations shows that the martensite transformation takes place at temperatures lower than the Curie point. At high temperatures in the range 930–1110 K the alloy undergoes the phase transformation of ordered phase L21 to disordered phase B2, and the melting temperature of the alloy is 1245 K. The flow curves of the alloy cylindrical samples at temperatures 773, 873, and 973 K have been built. An analysis of the alloy microstructure after upsetting at a temperature of 773 K leads to the conclusion that many macrocracks are initiated in the sample. The treatment at 873 and 973 K causes a fragmentation of the grains with grain sizes from several to several dozen micrometers. However, the upsetting at 873 K leads to insignificant scatter in the grain sizes, and the microstructure is more homogeneous and worked out.

Heusler alloys demonstrate various effects, such as the ferromagnetic shape memory effect, the magnetocaloric effect, etc. But these alloys are also liable to destruction during multiple cycles of martensitic transformation. In order to increase stability of functional properties of the alloys, thermo-mechanical treatment by various methods is carried out. In this work, a Heusler Ni2.26Mn0.80Ga0.89Si0.05 alloy was subjected to combined deformation processing: multiple isothermal forging and subsequent direct extrusion. The multiple isothermal forging was carried out at 700 °C in 5 upset transitions and a total true degree of deformation of e ≈ 2.42. The extrusion was also carried out at 700 °C with an extrusion ratio of 2:1 (e = 1.39). Analysis of the microstructure shows that, resulting from combined treatment, metallographic and crystallographic textures are formed. The metallographic texture is characterized by coarse grains of approximately 1 mm in length and hundreds of microns in diameter. The coarse grains are in turn surrounded by a layer of fine-grained structure. The crystallographic texture is axial of the 〈1 1 0〉 type. This kind of structure enhances mechanical properties of the alloy. According to the study of thermal expansion anisotropy in the region of martensitic transformation, contribution of the martensitic transformation to the relative change in the length of the extruded sample is 0.13% in the direction, normal to the extrusion axis, and 0.09% - along it. These values are comparable to those observed on Heusler alloys in the as-cast state. Thus, it is found that thermo-mechanical treatment by the combined method allows obtaining a significant amount of thermal expansion anisotropy while remarkably increasing mechanical properties of the material.

are presented of a study of the temperature dependence of the magnetization σ(Т) of the polycrystalline Ni52Mn24Ga24 alloy in various structural states: in the initial coarse-grained state, after severe plastic deformation by high pressure torsion, and after stepped annealing of the deformed specimen at temperatures from 200 to 700°С for 30 min. As a study of the σ(Т) curve shows, in an alloy possessing a coarse-grained initial structure, a martensitic phase transition and a magnetic phase transition are observed in the room temperature interval. The martensitic transformation takes place in the ferromagnetic state of the alloy. This transformation is accompanied by an abrupt lowering of the magnetization of the material, associated with a lowering of the symmetry of the crystalline lattice and a high value of the magnetocrystalline anisotropy constant of the alloy in the martensitic phase. It is shown that as a result of plastic deformation there takes place a destruction of ferromagnetic order and a suppression of the martensitic transformation. Consecutive annealing after deformation leads to a gradual recovery of ferromagnetic order and growth of the magnetization of the material. Recovery of the martensitic transformation begins to be manifested only after annealing of the alloy at a temperature of 500°С, when the mean grain size in the recrystallized structure reaches a value around 1 μm.

Composites with a copper matrix, reinforced by carbon nanotubes, have been produced by the multistage method including functionalization of carbon nanotubes, their dissolution, mixing with a copper powder, preliminary warm isostatic compaction, and severe plastic deformation by torsion. The microstructure and microhardness of composites containing to 4 wt % of carbon nanotubes have been studied. It has been shown that the matrix grain size after severe plastic deformation significantly decreases, while the internal stress level, dislocation density, and composite microhardness increase with increasing concentration of nanotubes.

Hybrid-silica materials containing octyl, phenyl and urea-propyl functional groups were reported. These functionalized silicas were prepared by a via one-pot synthesis method through co-condensation of tetraethyl ortosilicate with organosilanes co-precursors. The obtained materials were analyzed by a number of techniques including scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction pattern and nitrogen sorption measurement. The main elements: morphology and surface properties of the final products that are basically dependent on the type and the quantity of functional-containing groups were fully discussed. The adsorption properties of the modified silicas were investigated against bilirubin in aqueous solution at pH 7.4, the effect of time was studied as well. The modified silicas showed a greatly increasing of adsorption capacity for bilirubin compared to pure amorphous silica. The results indicate that the type of functional-containing groups and surface properties affect the adsorption behavior. These materials show a potential application as effective adsorbents for bilirubin removal.

Polyvinylpyrrolidone (PVP) modified silica coated with albumin as host matrices for bilirubin removal were synthesized through sol–gel method. Such materials were characterized by scanning electron microscopy, nitrogen adsorption, Fourier transform spectroscopy and elemental analysis. It was shown that PVP grafted onto silica provides strong and high immobilization of BSA in comparison with pure silica. These modified silicas with different content of BSA were used to study bilirubin adsorption from the bilirubin–phosphate solutions (pH 7.4). The effect of time, ionic strength and concentration on the bilirubin adsorption were investigated as well. The results demonstrate that the adsorption capacity of tested samples increases after surface modification. The maximum adsorption capacity for bilirubin was 2.82 mg/g.

Grafting with poly(N-vinyl-2-pyrrolidone) (PVP) was used to functionalize silica. The functionalized silicas were characterized by scanning electron microscopy, nitrogen adsorption, energy-dispersive X-ray analysis, Fourier transform spectroscopy, and thermogravimetric analysis. The Barrett–Joyner–Halenda analysis confirmed that PVP-modified silica samples have a mesoporous structure. Bovine serum albumin (BSA) was immobilized onto the surface of PVP-modified silica systems through physical adsorption. Also it was shown that the adsorption capacity of the tested samples depends on the quantity of PVP inside the silica matrix, which confirmed the formation of a specific complex between BSA and PVP. In addition, the effects of pH, time, and concentration on BSA adsorption were investigated. The maximum BSA adsorption capacity was 71.54 ± 2.0 mg/g. The desorption and reusability of all prepared samples were studied as well. PVP-modified silicas are capable of holding more BSA compared to nonmodified silica. ...

The results of a study of the martensitic transformation in the polycrystalline Ni 52 Mn 24 Ga 24 alloy in different structural states are presented in the article. In the initial state in the alloy magnetic and martensitic phase transformation is observed with the following characteristic points: M S =25°C; M F =15°C; A S =35°C; A F =45°C; T C =127°C. The alloy in the initial state, after intensive plastic deformation by high pressure torsion and after the stepped annealing of the deformed material at temperatures of 400°C, 500°C and 600°C was investigated. The microstructure of the alloy was investigated by means of scanning electron microscope equipped by the detector sensitive to orientation contrast of material. Analysis of the microstructure of the alloy in the initial state shows that the average grain size is 270 µm. In the alloy after plastic deformation and subsequent annealing at 400°C, this value is 180 nm. Annealing at 500°C and 600°C leads to an increase in the average grain size up to 1.08 µm and 2.33 µm, respectively. The results of the study of the microhardness of the alloy in different structural states are presented. As a result of plastic deformation of the alloy the microhardness increases from 2.1 GPa in the initial state to 5.1 GPa in the deformed state. After annealing, a gradual decreasing of the microhardness down to 2.8 GPa is found. The temperature dependence of magnetization of the alloy in different structural states shows that as a result of severe plastic deformation the ferromagnetic order is destroyed and the martensitic transformation is suppressed. After stepped annealing at 400°C, 500°C and 600°C, there is a gradual recovery of ferromagnetic order. The martensitic transformation is observed in the alloy only after annealing at 500°C. In this case the average grain size is increased to 1 µm.

The morphology and fine structure of high-temperature austenitic and low-temperature martensitic phases in a ferromagnetic Ni2.08Mn0.96Ga0.96 alloy and the effect of magnetic field on the peculiarities of the martensite formation have been studied. The Ni2.08Mn0.96Ga0.96 alloy in the initial cast and annealed states was found to undergo martensitic and magnetic phase transformations at temperatures M s = 280 K, M f = 265 K, A s = 280 K, A f = 295 K, and T C = 375 K, respectively. Above M s, the alloy is in a metastable premartensitic state, which leads to a characteristic diffuse scattering and tweed contrast when studying by electron microscopy. An applied magnetic field of up to 600 kA/m was found to affect the martensitic transformation in the alloy. The field application leads to changes in the morphology and fine structure of martensite due to orientation along the field direction of the magnetizations of the energetically advantageous (in terms of the directions of the magnetization vectors) c domains of the existing orientation variants of the martensite crystals having a packet pairwise-twinned morphology. The martensitic structure of the Ni2.08Mn0.96Ga0.96 alloy already formed previously during cooling is not affected by an external magnetic field with a strength of up to 600 kA/m.

The temperature dependence of the thermal expansion of polycrystalline Ni2.08Mn0.96Ga0.96 alloy samples is studied in the as-cast state and after homogenizing annealing. A sample in which twins are mainly oriented normal to the sample length shortens jumpwise during the martensitic transformation. A sample in which twins are parallel to its length elongates jumpwise. After annealing, both samples do not exhibit a jumplike change in length during the phase transformation. When an annealed sample is cooled in a magnetic field, its length again changes jumpwise in the phase transformation range.

Results of investigation of the temperature dependence of magnetization of Ni2.08Mn0.96Ga0.96 alloy in the magnetic fields of various intensities are reported. An abrupt change in magnetization at transformation of low temperature phase to the high temperature one is observed. Magnetization increases during the phase transition in the magnetic field having intensity below 500 kA/m and decreases at higher intensities. The explanation is based on zigzag configuration of domains in twinned structure. In the Curie temperature region the ferromagnetic ? paramagnetic phase transition occurs sharply at low field strength, while at higher field strength the transition is smooth. It is concluded that the increase in flatness of the curve σ = f (T) and the increase of ferromagnetic state destruction temperature with increase of the intensity of the magnetic field is indicative of the main role of Mn in magnetization of the alloy.

A study of the relationship of the crystallographic texture, the preferential orientation of martensite twins and the geometry changes during the martensite transformation of polycrystalline NiMnGa alloy is shown. Two states of alloy are investigated: as-cast state and the state after annealing at 923 K for 5 hours. It is shown that in the initial state the austenite phase has crystallographic axial texture , while martensite has the two-component texture of the and . The preferential orientation of the martensitic twins in the martensitic structure is found. The crystallographic texture of the alloy after annealing is not changed, however, no preferential orientation of the martensitic twins is observed. In the martensitic phase structure martensitic twins are oriented randomly. Anisotropy of the thermal expansion during the martensitic transformation is shown, while in the annealed state such effect is not found.

The effect of a magnetic field on the temperature dependence of thermal expansion of Ni2.08Mn0.96Ga0.96 alloy in the range of structural-phase transformation is investigated. It is found that, when the sample is cooled in a magnetic field with an intensity of 110 kA/m, it shortens by 0.21% more strongly than upon cooling in the absence of the magnetic field. The additional shortening of the sample under the action of the magnetic field depends on the intensity of the magnetic field applied to the sample.