Phase transformation and microstructure and mechanical properties of as cast NiTiRe shape memory alloys

Abstract

The microstructure, martensitic transformation and mechanical properties of as cast Ni₅₂Ti_48?xRe_x shape memory alloys (SMAs) were investigated. The microstructure of these alloys consists of B19′ martensite phase as a matrix and B2 austenite in small percentages in addition to some precipitations of NiTi intermetallic compounds. There are two types of NiTi precipitates: the first one is Ti₂Ni, which can be seen in the all microstructures of the three alloys, and the other is Ni₂Ti, which is found only in the microstructure of Ni₅₂Ti_47·7Re_0·3 alloy. According to differential scanning calorimetry measurements, one stage of transformation reaction B2 to B19′ accompanied with forward and backward martensitic transformation was observed. The martensitic peak and the austenitic peak were increased with the addition of rhenium. Both are increased as the number of valence electron per atom increase and the valence electron concentration decrease. Hardness measurements of Ni₅₂Ti_48?xRe_x SMAs are improved by the Re additions.     

The evolution of structure and properties in GdMn(1−x)TixO3 ceramics

In this work, the effects of Ti doping on the microstructure, dielectric, and magnetic properties of GdMn_(1?x)Ti_xO₃ (x?=?0.00–0.15) ceramic samples synthesized using a solid-state reaction were investigated. All the experimental samples formed a single-phase structure, and no structural transformation occurred within the experimental doping range; however, Ti doping caused lattice shrinkage. Ti doping reduced the grain size, and the microstructure of the synthesized samples appeared more compact in scanning electron microscopy images. The lattice distortion of GdMn_(1?x)Ti_xO₃ caused by Ti substitution at the Mn sites resulted in changes in the Raman vibration modes. X-ray photoelectron spectroscopy results showed that the valence state transition of the Ti and Mn ions occurred and the concentration of Ti⁴⁺, Mn³⁺ ions and oxygen vacancies changed due to the charge compensation induced by Ti doping. Ti doping had a significant influence on the size and concentration of cation vacancies in the GdMn_(1?x)Ti_xO₃ samples. Appropriate Ti doping was shown to reduce the dielectric loss, improve the frequency stability of the dielectric constant, and significantly affect the long-range ordering of Gd³⁺ magnetic moments and clearly reduce magnetization.

     

Electron Concentration Dependence of Phase Transition and Magnetic Properties in NiMnGa Alloys

The phase transition, Curie temperature and magnetic properties of NiMnGa alloys represented by S1, S2, S3, and S4 in different atomic percentage were investigated from TG/DTA and PPMS measurements. It was found that the electron concentration (e/a) ratio of S1 and S4 as well as S2 and S3 are equal to each other. The transformation temperatures and magnetization values are similar for these alloys having the same e/a. Results show that room temperature magnetization depends on valence electron concentration of alloys and saturation of alloys increases with the decrease of e/a ratio. The measured coercivity values for all alloys were above 125 Oe and they had a long saturation field, i.e., above 10000 Oe, by exhibiting, the characteristics of hard magnetic materials.     

Synthesis,structure, and room-temperature ferromagnetism of Ni-doped ZnO nanoparticles

In this paper, Ni-doped ZnO (Zn_1−x Ni _x O, in which 0 ≤ x ≤ 0.05) diluted magnetic semiconductors nanoparticles are prepared by an ultrasonic assisted sol–gel process. Transmission electron microscopy shows sphere-like nanoparticles with an average size of about 25 nm. From the analysis of X-ray diffraction, the Ni-doped ZnO nanoparticles are identified to be a wurtzite structure, but impurity phases are observed when the Ni content x reaches 0.05. Sample structures are further studied by Raman spectra, from which a broad and strong Raman band in the range of 500–600 cm⁻¹ is observed in Zn_1−x Ni _x O. With the increment of x, wurtzite structures degrade gradually. The magnetic properties are measured using superconducting quantum interference device at room temperature; the Zn_1−x Ni _x O (x ≤ 0.02) nanoparticles show ferromagnetism. However, for the sample of Zn_0.95Ni_0.05O, paramagnetism is observed, which may be ascribed to ferromagnetic–antiferromagnetic competition.     

Transformation of precipitated phases in Zr50·5Cu34·5−xNi4Al11Agx alloy master ingots with adding Ag

Abstract

The transformation of precipitated phases of Zr_50·5Cu_34·5?xNi₄Al₁₁Ag_x alloy master ingots with Ag substitution of Cu was studied in detail by phase analysis. The precipitated (Zr–Cu) rich phases deteriorate the glass forming ability (GFA) of Zr_50·5Cu_34·5Ni₄Al₁₁. Two new (Zr–Cu) rich phases, A1 with bcc structure and a?=?0·339 nm and A2 with fcc superlattice structure and a?=?1·21 nm, were identified by a transmission electron microscope. When x?=?2, A1 phase disappears, and A2 phase remains and is suppressed gradually with further Ag addition. When x?=?13, one precipitated phase with Ag more than 13 at-% begins to deteriorate GFA. In the view of chemistry, the precipitation of (Zr–Cu) rich phases means that the interaction between Cu and Zr atoms is rather drastic. The addition of Ag weakens the interaction between Cu and Zr. The similar competition mechanism proposed by the authors plays an important role in suppressing precipitated phases and improving GFAs.     

Correlation between microstructures and mechanical properties in Ni-rich Ni–Mn–Ga high-temperature shape-memory alloys

Microstructures and mechanical properties of polycrystalline and single-crystalline Ni_55+xMn₂₅Ga_20?x (x?=?1, 2, 3) alloys are investigated. The alloys exhibit a single martensite phase for x?=?1, and two phases containing tetragonal martensite phase and face-centered γ phase for x?=?2 and 3. The appearance of γ phase can apparently raise the strength and plasticity of the alloys. Moreover, single crystals show better mechanical properties compared with the polycrystals with the same composition. Furthermore, a kind of net-like γ phase is observed in directionally solidified Ni₅₈Mn₂₅Ga₁₇ single crystal. This alloy exhibits excellent plasticity due to that the crack propagation is seriously hindered by the plastic net-like γ phase.     

Electronic structure and possible martensitic transformation in Ni2FeIn alloy

The electronic structure and magnetic properties of Heusler alloys (Ni₂FeIn) have been studied by first principle calculations. The possible tetragonal martensitic transformation has been predicted and the structure optimization was made on cubic austenitic Ni₂FeIn in Cu₂MnAl type. The equilibrium lattice constant of austenitic Ni₂FeIn is 6.03 Å. In tetragonal phase, the global energy minimum occurs at c/a = 1.29. The corresponding equilibrium lattice constants for martensite Ni₂FeIn are a = b = 5.5393 Å and c = 7.1457 Å, respectively. In the austenitic phase, E _F is located at the peak in the minority DOS for c/a = 0.96 to 1.20, but in the martensitic phase, E _F moves to the bottom of the valley in the minority DOS, reducing the value of N(E _F ) effectively. Both austenitic and martensitic phases are ferromagnetic and the Ni and Fe partial moments contribute mainly to the total moments. Therefore, the martensitic transformation behavior in Ni₂FeIn is predicted.     

High Pressure Synthesis of (La1−<Emphasis Type="Italic">x</Emphasis>Sr<Emphasis Type="Italic">x</Emphasis>)8O20, (0.25 ≤ x ≤ 0.65) with a Wide Solubility Range and High Cu-Valence

With an attempt to increase the Cu valence, we synthesized the (La_1?xSr _x )₈Cu₈O_20?δ (8-8-20 phase) compounds with large x using an O₂-HIP apparatus. By heat treatment in a high-pressure of oxygen gas (40 MPa), the 8-8-20 compounds were stabilized in the range of 0.25 ≤ x ≤ 0.65. Their crystal structures, characterized by x-ray Rietveld refinement and electron diffraction, are tetragonal, space group P4/mbm. The formal copper valence was found to increase with Sr concentration from 2.24 at x = 0.25 to 2.63 at x = 0.65, keeping the oxygen content constant around 20.0. Electrical resistivity and magnetic susceptibility showed only metallic behavior from room temperature down to 5 K with no sign of superconductivity.     

Microstructure and martensitic transformation of Ti49Ni51 − xHfx high temperature shape memory alloys

In present work, the microstructure and martensitic transformation of Ti₄₉Ni_51 − xHf_x (x = 3-15) alloys were studied. The microstructure of Ti₄₉Ni_51 − xHf_x alloys consists of B19′ martensite and (Ti,Hf)₂Ni phase at room temperature. The martensitic transformation behavior is characterized by a single-stage transformation. With increasing Hf content, the transformation temperature increases from 75 to 279 °C resulting from the reduced valence electron concentration, indicating that the replacement of Hf for Ni is effective in increasing the transformation temperatures. The results suggest that the Ti₄₉Ni_51 − xHf_x shape memory alloy is one of potential candidates for high temperature applications.     

Local strains,calorimetry, and magnetoresistance in adaptive martensite transition in multiple nanostrips of Ni39+xMn50Sn11−x(x ≤ 2) alloys

Abstract

Ni_39+xMn₅₀Sn_11?x (x = 0.5, 1.0, 1.5 and 2) alloys comprise multiple martensite nanostrips of nanocrystallites when cast in small discs, for example, ～15 mm diameter and 8 mm width. A single martensite phase with a L1₀ tetragonal crystal structure at room temperature can be formed at a critical Sn content of 9.0 at.% (x = 2), whereas an austenite cubic L2₁ phase turns up at smaller x ≤ 1.5. The decrease in the Sn content from x = 2 to 0.5 also results in a gradual increase in the crystallite size from 11 to 17 nm. Scanning electron microscopy images reveal arrays of regularly displaced multiple martensite strips (x ≤ 1.5) with an average thickness of 20 nm. As forced oscillators, these strips carry over the local strains, magnetic dipoles, and thermions simultaneously in a martensite–austenite (or reverse) phase transition. A net residual enthalpy change ΔH_M?A = ?0.12 J g^?1 arises in the process that lacks reversibility between the cooling and heating cycles. A large magnetoresistance of (–)26% at 10 T is observed together with a large entropy change of 11.8 mJ g^?1 K^?1, nearly twice the value ever reported in such alloys, in the isothermal magnetization at 311 K. The ΔH_M?A irreversibility accounts for a thermal hysteresis in the electrical resistivity. Strain induced in the martensite strips leads them to have a higher electrical resistivity than that of the higher-temperature austenite phase. A model considering time-dependent enthalpy relaxation explains the irreversibility features.     

Effect of Fe Substitution on Ni2MnGe Heusler Alloys: A First Principles Study

The effect of Fe substitution on electronic structure and magnetic properties of Ni₂MnGe alloy has been investigated by using full-potential local-orbital minimum-basis method, which is used in the VCA form. The theoretical lattice parameters are obtained from the dependence of the total energy on the lattice parameter. A decreasing trend of total magnetic moment with Fe substitution is found for Ni₂Mn_1?x Fe _x Ge for studied concentrations x. The pressure–magnetization behavior is obtained for Ni₂Mn_1?x Fe _x Ge for applied pressure from 0 to 2000 GPa, and the changes of total magnetic moment gradually approach linear with the increase of x concentration. The value of equilibrium bulk modulus of Ni₂Mn_1?x Fe _x Ge (for x=0.0–1.0) obtained from Murnaghan equation of state (EOS) shows that the studied compounds are less compressible than Ni₂MnGe.     

Preparation of NixTiy compound via Ni plating followed by thermal treatment of Ti powder

In this research, Ni_xTi_y compound was prepared by thermal treatment of Ni-plated Ti powder. For this purpose, Ti powder was plated in an electroless Ni bath for various times (120, 225, 300, and 720?min). Hydrazine hydrate was used as a reductant for the deposition of pure Ni on the Ti particles. The plated powder (225?min) was heat treated under argon atmosphere to achieve Ni_xTi_y powder. Finally, the heated/plated powder was pressed by CIP followed by sintering at 980°C for prepare the Ni_xTi_y bulk sample. The plated powders as well as sintered one were characterized using scanning electron microscopy, energy dispersive spectrometer, X-ray fluorescence, X-ray diffraction and differential scanning calorimetric. The NiTi₂, NiTi, and Ni₃Ti phases were detected in the XRD patterns of heated/plated Ti powder. According to DSC data, the heated/plated Ti powder showed reversible martensitic transformation at temperature range of ?38.0°C to +38.1°C, while sintered/heated/plated Ti powder displayed reversible transformation at temperature range of 16.0°C–15.4°C.     

Transport,magnetic and spectroscopic studies of nickel-gallium ferrites

X-ray, electrical conductivity, magnetic hysteresis and infrared spectroscopic studies for the system NiGa_2−2xFe_2xO₄ were carried out. All the compounds, 0 x⩽1 showed cubic symmetry. The activation energy, thermoelectric coefficient values decreased with increasing Fe³⁺ concentrations but with increasing number of Fe³⁺ ions, the values of lattice constant and magnetic moment increased. Compounds with x⩽0.5 are p-type while those with x⩾0.75 are n-type semiconductors. Magnetic hysteresis loops indicated that the compounds with x⩾0.25 are ferrimagnetic while the compound NiGa₂O₄ (x=0.0) is anti-ferromagnetic. Constant and low values of coercive force indicated that the compounds with x⩾0.50 exhibit multidomain behaviour. X-ray intensity calculations, electrical conductivity, magnetic hysteresis and infrared studies indicate the presence of Ga³⁺ ions at the tetrahedral site, Ni²⁺ ions at the octahedral site and Fe³⁺ ions are present in both tetrahedral and octahedral sites. The probable ionic configuration for the system NiGa_2−2xFe_2x O₄ is suggested to be Ga _1−x ³⁺ Fe _x ³⁺ [Ni²⁺ Fe³⁺ O ₄ ²⁻ .     

Preparation and Magnetic Properties of?(La1?xSr1+x)(Mn0.5Co0.5)O4

A series of single phase (La_1−x Sr_1+x ) (Mn_0.5Co_0.5)O₄ (0≤x≤0.40) materials with a tetragonal K₂NiF₄ structure was prepared by a solid state reaction method at 1400 or 1450 °C with a short period of heating time. Powder X-ray diffraction (XRD) and the Rietveld refinement method were employed for the structural analysis. Unit cell a- and c-axes decrease with increasing amount of Sr substitution. A discrepancy between the zero-field-cooled and the field-cooled magnetization is observed in all samples investigated below a characteristic temperature, T ^*, which likely arises from the canted nature of spins or the random freezing of spins. It appears that T ^* decreases with increasing amount of Sr substitution, which is possibly due to the enhancement of chemical pressure induced by Sr and a corresponding increases of the valence of Mn and/or Co.     

Effect of Ni-substitution on Ni2MnGe Heusler alloys: Ab initio study

The effect of Ni-substitution on magnetic properties of Ni₂MnGe alloy has been investigated by first principles method FPLO, which is used in the CPA form. The theoretical lattice parameters obtained from the dependence of the total energy on the lattice parameter are comparable with experimental ones. A decreasing trend of total magnetic moment with Ni-substitution is found for Ni_2+xMn_1−xGe for studied concentrations of x. The linear pressure-magnetization behaviour is obtained for Ni_2+xMn_1−xGe for applied pressure from 0 to 9 GPa, and the pressure derivative of the total magnetic moment is calculated. The value of equilibrium bulk modulus of Ni_2+xMn_1−xGe (for x = 0.1 – 0.9) obtained from Murnaghan equation of state (EOS) shows that the studied compounds are less compressible than Ni₂MnGe.     

Magnetic and electrical properties of hot-pressed Ni-Zn-Li ferrites

The magnetic properties of Ni-Zn ferrites have been upgraded by preparing hot-pressed Ni_0.4Zn_0.6–2xLi_xFe_2+xO₄ ferrites wherein 2xZn²⁺ ions have been substituted byxLi¹⁺ andxFe³⁺ ions. This results in an increase of saturation magnetization, Curie temperature and dielectric constant, whereas resistivity and initial permeability are reduced. The values of saturation magnetization, Curie temperature and dielectric constant are improved due to hot pressing in which grain growth and densification are controlled simultaneously. The variations of saturation magnetization and Curie temperature can be explained by the preferential site occupancy of Li¹⁺ and Ni²⁺ ions at the tetrahedral site, sublattice magnetization with canted spin structure, and magnetic exchange interactions. The inferences drawn from the bulk magnetic properties of these ferrites conform with the conclusions drawn from variations of internal magnetic field, obtained from Mössbauer studies of these samples. The decrease in d.c. resistivity due to substitution of Li¹⁺ ions is attributed to increased hopping due to formation of Fe²⁺ and Ni³⁺ ions.     

Effect of Ni Doping on Structural,Morphological, Optical and Magnetic Properties of Zn1−x Ni x O Dilute Magnetic Semiconductors

Ni²⁺-doped ZnO diluted magnetic semiconducting materials (Zn_1?x Ni _x O with x=0.01,0.02,0.03,0.04,0.05) were synthesised by the co-precipitation method. All synthesised samples were sintered at 600 °C for 6 hours. The effects of Ni²⁺ ion-doping on the structural, morphological, optical and magnetic properties of ZnO were investigated using powder X-ray diffraction, field emission SEM, UV–DRS spectroscopy, photoluminescence and vibrating sample magnetometry. The XRD patterns of pure and Ni-doped ZnO samples revealed single phase hexagonal wurtzite structure. The SEM analysis revealed the morphology of prepared samples, and the chemical compositions of all samples were analysed using exhibit energy density X-ray analysis (EDAX) characterisation. The absorption and emission properties revealed the effect of Ni²⁺ doping in ZnO samples. All Ni²⁺ ion-doped samples showed ferromagnetism at room temperature. The observed results are here analysed and reported.     

Thermal and structural study of nanocrystalline Fe(Co)NiZrB alloys prepared by mechanical alloying

Three nanocrystalline alloys, Fe_75−x Co _x (Ni₇₀Zr₃₀)₁₅B₁₀ (x = 0, 10, and 20), were synthesized from elemental powders in a planetary high-energy ball mill. Their microstructure, magnetic properties, and thermal stability were characterized by X-ray diffraction, transmission M?ssbauer spectroscopy, transmission electron microscopy, scanning electron microscopy, induction coupled plasma, vibrating sample magnetometry, and differential scanning calorimetry. After 80 h of milling, the nanocrystallites size of alloys is in the range 6–10 ± 1 nm. The lattice parameter decreases when increasing (decreasing) milling time (Fe content). Furthermore, the thermal stability of the nanocrystalline phase increases when increasing Co concentration. The activation energy of the main crystallization process, between 275 ± 8 and 311 ± 10 kJ mol⁻¹, is associated with grain growth. Slight contamination from milling tools and milling atmosphere was detected. Minor differences were detected after M?ssbauer analysis.     

Room Temperature Ferromagnetism in Ni Doped ZnO Powders

Zn_1−x Ni _x O (x=0.1%, 0.4%, 0.7%, 1.0%) powders were prepared using the sol-gel technique, and the structural, optical, and magnetic properties of the samples were investigated. X-ray diffraction measurements show that all samples have a wurtzite structure and that the c-axis lattice constant decreases as the Ni content increases. X-ray photoelectron spectroscopy studies reveal that the doped Ni ions are in divalent states in all samples. Optical absorption spectra show that the band energy of Zn_1−x Ni _x O powders decreases with increasing Ni concentration. Photoluminescence measurements of Zn_1−x Ni _x O show a broad peak at a wavelength centered at about 467 nm which indicates the presence of a fraction of defects in ZnO. The room temperature ferromagnetism observed in all samples is intrinsic in nature.     

Theoretical study of Ni–Al nanoalloy clusters using particle swarm optimisation algorithm

Abstract

The global structural optimisations for Ni–Al nanoalloy clusters at different compositions have been investigated using particle swarm optimisation combined with simulated annealing method. The second moment approximation of the tight binding potential has been used in describing the interatomic interactions. Some stable structures were obtained for Ni_xAl_x(x=1–8), Ni_3xAl_x(x=1–4) and Ni_xAl_3x(x=1–4) nanoalloy clusters. The simulation results show that the lowest energy isomers of nanoalloy clusters with the approximate composition 'NiAl, Ni₃Al and NiAl₃' generally have structures based on icosahedral packing. It is confirmed that segregation is favoured for Ni–Al nanoalloy clusters, with the surface becoming richer in Al and the core becoming richer in Ni.