Characterization of the martensitic transformation in Ni50−xTi50Cux alloys through pure thermal measurements

The addition of a third element to the Ni-Ti system often changes the product and the path of the martensitic transformation of the alloy, which is a direct B2-B19′ transformation for the NiTi alloy in the fully annealed state. In this study we investigate the martensitic transformation of fully annealed Ni_50−xTi₅₀Cu_x (x = 3-10 at%) shape memory alloy (SMA) samples using differential scanning calorimetry (DSC) and the four-probe electrical resistance (ER) measurements under stress-free conditions. DSC and ER data show that the ternary alloy goes through a direct B2-B19′ transformation for Cu content between 3 and 7 at% and through the two-stage B2-B19-B19′ transformation for Cu content between 8 and 10 at%. We find good agreement between the two techniques as regards the detection of the phase transformation temperatures. B19′ starting and finishing temperatures decreases with the increases of Cu content and show a significant reduction starting from 7 at%; the range of temperatures in which B19 is stable increases with increasing Cu content.     

Embedding of Superelastic SMA Wires into Composite Structures: Evaluation of Impact Properties

Shape memory alloy (SMA) represents the most versatile way to realize smart materials with sensing, controlling, and actuating functions. Due to their unique mechanical and thermodynamic properties and to the possibility to obtain SMA wires with very small diameters, they are used as smart components embedded into the conventional resins or composites, obtaining active abilities, tunable properties, self-healing properties, and damping capacity. Moreover, superelastic SMAs are used to increase the impact resistance properties of composite materials. In this study, the influence of the integration of thin superelastic wires to suppress propagating damage of composite structures has been investigated. Superelastic SMAs have very high strain to failure and recoverable elastic strain, due to a stress-induced martensitic phase transition creating a plateau region in the stress-strain curve. NiTi superelastic wires (A _f = ?15 °C fully annealed) of 0.10 mm in diameter have been produced and characterized by SAES Getters. The straight annealed wire shows the typical flag stress-strain behavior. The measured loading plateau is about 450 MPa at ambient temperature with a recoverable elastic strain of more than 6%. For these reasons superelastic SMA fibers can absorb much more strain energy than other fibers before their failure, partly with a constant stress level. In this paper, the improvement of composite laminates impact properties by embedding SMA wires is evaluated and indications for design and manufacturing of SMA composites with high-impact properties are also given.     

A Study of the Origin of Weak Ferromagnetism in Zn1−x Co x O

To study the origin of ferromagnetism in Zn_1?x Co _x O thin films, its thermal diffusivity, in addition to its magnetization measurements, were analyzed. Thin films of Zn_1?x Co _x O (x = 0.03) were deposited on Si (100) substrates through ultrasonic spray pyrolysis. Magnetization M(B) measurements at low temperature showed a hysteresis loop that indicated the existence of ferromagnetic ordering in Zn_0.97Co_0.03O. However, the magnetic moment per Co ion was much lower than expected. A comparison of M(T) measured at zero-field-cooled and field-cooled conditions showed a superparamagnetic-like behavior, with a blocking temperature of about 130 K. Temperature dependence on the thermal diffusivity of Zn_0.97Co_0.03O showed a pronounced lambda-shaped minimum at 130 K, which indicated the existence of a second-order phase transition at this temperature. The weak ferromagnetism in the Zn_0.97Co_0.03O with the Curie temperature of 130 K was ascribed to the uncompensated magnetic moment at the surface of CoO nanoclusters.     

The 400 °C Isothermal Section of the La-Co-Mg Ternary System

The isothermal section of the La-Co-Mg system at 400 °C was determined by characterization of about thirty ternary alloys synthesised by induction melting in sealed Ta crucibles and then annealed. Scanning electron microscopy (SEM) coupled with energy dispersive x-ray spectroscopy (EDXS) and x-ray powder diffraction (XRPD) were used to analyze microstructures, identify phases, measure their compositions and determine their crystal structures. Phase equilibria are characterized by the absence of ternary solid solutions and by the presence of three ternary phases. The existence and the crystal structure of the La_4?x CoMg_1 + x (τ₁, 0 ≤ x ≤0.15, cF96-Gd₄RhIn) were confirmed and its homogeneity region determined; the new phases La_23?x Co₇Mg_4 + x (τ₂, ?0.50≤ x ≤0.60, hP68-Pr₂₃Ir₇Mg₄) and ~La₃₈Co₅₅Mg₇ (τ₃, unknown crystal structure) were detected.     

Microstructures,Martensitic Transformation,and Mechanical Behavior of Rapidly Solidified Ti-Ni-Hf and Ti-Ni-Si Shape Memory Alloys

In this work, the microstructure and mechanical properties of rapidly solidified Ti_50?x/2Ni_50?x/2Hf _x (x = 0, 2, 4, 6, 8, 10, and 12 at.%) and Ti_50?y/2Ni_50?y/2Si _y (y = 1, 2, 3, 5, 7, and 10 at.%) shape memory alloys (SMAs) were investigated. The sequence of the phase formation and transformations in dependence on the chemical composition is established. Rapidly solidified Ti-Ni-Hf or Ti-Ni-Si SMAs are found to show relatively high yield strength and large ductility for specific Hf or Si concentrations, which is due to the gradual disappearance of the phase transformation from austenite to twinned martensite and the predominance of the phase transformation from twinned martensite to detwinned martensite during deformation as well as to the refinement of dendrites and the precipitation of brittle intermetallic compounds.     

Shape memory behavior of Ti–20Zr–10Nb–5Al alloy subjected to annealing treatment

The effects of annealing temperature on microstructures, phase transformation, mechanical properties, and shape memory effect of Ti–20Zr–10Nb–5Al alloy were investigated. X-ray diffraction(XRD) patterns show that the alloy is composed of single hexagonal ɑ'-martensite phase for both as-rolled sample and sample annealed at773 K for 30 min, while single orthorhombic ɑ' phase exists in the samples annealed at 873 and 973 K for30 min. The optical observations indicate that the alloy is recrystallized when annealed at 873 K, and the grain size of the sample annealed at 973 K is about five times larger than that annealed at 873 K. Both of the samples annealed at 873 and 973 K show almost the same reverse martensite transformation start temperature of 483 K as demonstrated by thermal dilatation tests. The critical stress values for martensite reorientation(σ_M) are 392 and 438 MPa for the alloys annealed at 873 and 973 K, respectively. The maximum shape memory strain is 2.8 %, which is obtained in the alloy annealed at 873 K due to the lower σ_M. Moreover,the sample annealed at 873 K exhibits larger tensile stress and tensile strain due to the smaller grain size.     

Effects of solidification rate and excessive Fe on phase formation and magnetoclaoric properties of LaFe11.6xSi1.4

The effects of solidification rate and excessive Fe on phase formation and magnetocaloric properties of LaFe_11.6xSi_1.4 (x=1.1, 1.2) were investigated by XRD, SEM and VSM measurements. The XRD results show that the amount of LaFeSi phase in the as-cast melt-spun ribbons prepared by a copper wheel at a speed of 10 m/s is less than that in the as-cast arc melting buttons with the same x values. The annealed melt-spun ribbons contain smaller amount of La(Fe, Si)₁₃ (1:13) phase than the corresponding annealed arc melting buttons. Although the melt-spun sample has finer crystalline grains of α-Fe, as indicated by SEM analysis, its crystalline size has not reached nano-scale. Therefore, the magnetic exchange-coupling between 1:13 phase and α-Fe phase has not been observed in melt-spun ribbons. Further, the maximum negative magnetic entropy change (-S_Max) and relative cooling power (RCP) of annealed melt-spun ribbons under a field change of 0-2 T are weaker than those of the corresponding annealed arc melting buttons.     

1:13 phase formation mechanism and first-order magnetic transition strengthening characteristics in (La,Ce)Fe<Subscript>13–<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys

The effects of the introduction of Ce to La_1?x Ce _x Fe_11.5Si_1.5 alloys on 1:13 phase formation mechanism, the first-order magnetic phase transition strengthening characteristics, and magnetocaloric property were studied, respectively. The results show that the formation mechanisms of 1:13 and LaFeSi phases in La_1?x Ce _x Fe_11.5Si_1.5 alloys are the same as those of Ce₂Fe₁₇ and CeFe₂ phases in Ce–Fe binary system, respectively. The substitution of Ce in 1:13 phase which is limited can make the first-order magnetic phase transition characteristics strengthen, which can make thermal and magnetic hysteresis increase, the temperature interval of temperature-induced phase transition decrease, and the critical magnetic field of field-induced magnetic phase transition (H _C) increase, respectively. Owing to the lattice shrink of 1:13 phase with the increase in Ce content, the Curie temperatures (T _C) show a linear decrease. The maximum change in magnetic entropy gradually increases due to the decrease in temperature interval of temperature-induced phase transition, but the relative cooling capacities are all about 80 J·kg^?1 at magnetic field of 2 T.     

Microstructure and martensitic transformation behavior of Ni56-xMn25FexGa19 shape memory alloys简

Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni56-x Mn25 Fex Ga19(x = 0, 2, 4, 6, 8) shape memory alloys were investigated using optical microscopy(OM), X-ray diffraction analysis(XRD), differential scanning calorimeter(DSC), and compressive test. It is found that these alloys are composed of single non-modulated martensite phase with tetragonal structure at room temperature, which means substituting Fe for Ni in Ni56 Mn25 Ga19 alloy has no effect on phase structure. These alloys all exhibit a thermoelastic martensitic transformation between the cubic parent phase and the tetragonal martensite phase. With the increase of Fe content, the martensitic transformation peak temperature(Mp) decreases from 356 °C for x = 0 to 20 °C for x = 8, which is contributed to the depressed electron concentration and tetragonality of martensite. Fe addition remarkably reduces the transformation hysteresis of Ni–Mn–Ga alloys. Substituting Fe for Ni in Ni56 Mn25 Ga19 alloy can decrease the strength of the alloys and almost has no influence on the ductility and shape memory property.     

Experimental Investigation of the Isothermal Section at 800 °C of the Nd-Fe-Co Ternary System

The phase equilibrium of the ternary Nd-Fe-Co system at 800 °C was investigated by means of powder x-ray diffraction and scanning electron microscopy–energy dispersive x-ray spectroscopy. Seven binary compounds, i.e., Nd₂Co₁₇, NdCo₅, Nd₅Co₁₉, Nd₂Co₇, NdCo₃, NdCo₂, Nd₂Fe₁₇ were identified to exist at this isothermal section. This isothermal section consists of ten single-phase, ten two-phase and six three-phase regions. All measured compositions and unit-cell refinements were performed at room temperature from quenched samples annealed at 800 °C for one week. The maximum solubility at 800 °C of Fe in NdCo_2?x Fe _x (MgCu₂-type structure, Fd-3 m), NdCo_3?x Fe _x (PuNi₃-type structure, R-3 m space group), Nd₂Co_7?x Fe _x (Ce₂Ni₇-type structure, R-3 m), Nd₅Co_19?x Fe _x (CeCo₁₉-type structure, R-3 m space group), NdCo_5?x Fe _x (CaCu₅-type structure, P6/mmm), Nd₂Co_17?x Fe _x (Th₂Zn₁₇ type structure, R-3 m) and Nd₂Fe_17?x Co _x (Th₂Zn₁₇ type structure, R-3 m) are about 31.6 at.% Fe, 47.9 at.% Fe, 13.3 at.% Fe, 8.6 at.% Fe, 10.37 at.%, 36.35 at.% Fe, and 58.23 at.% Fe respectively. The solid solubility range of Co in Nd₂Fe₁₇ form discontinuous series of 2 ranges is about 0-30.14 at.% Co, and 51.9-100 at.% Co and the solid solubility range of Fe in Nd₂Co₁₇ is about 0-48.1 at.% Fe, and 69.86-100 at.% Fe.     

Experimental Investigation on Phase Equilibria of Al-Fe-Y System at 773 K

The phase diagram determination of Al-Fe-Y is helpful for development of magnetic and amorphous materials. The entire isothermal section of Al-Fe-Y at 773 K has been determined by means of x-ray powder diffraction, scanning electron microscopy with energy dispersive analysis and optical microscopy. The existences of 14 binary compounds in the Al-Fe, Al-Y and Fe-Y binary systems were confirmed. The binary phases Al₂Y, Fe₂Y, Al₂Y₃, Fe₂₃Y₆ and Fe₁₇Y₂ in this study present appreciable ternary solubility: Fe_17?x Al _x Y₂ (0 ≤ x ≤ 8.5), Y(Fe_1?x Al _x )₂ (0 ≤ x ≤ 1), Al₂(Fe _x Y_1?x )₃ (0 ≤ x ≤ 0.3), and (Fe,Al)₂₃Y₆ with Al up to 22 at.%. The ternary compounds: τ₁-Al₁₀Fe₂Y, τ₂-Al_8?x Fe_4+x Y (0 ≤ x ≤ 2) were confirmed. The isothermal section consists of 19 single-phase regions, 33 two-phase regions and 17 three-phase regions.     

Experimental Investigation of the Al-Fe-Gd System at 773 K

The phase equilibrium of the Al-Fe-Gd plays an important role of development of bulk amorphous alloys and magnetocaloric materials. The entire isothermal section of the phase diagram for Al-Fe-Gd ternary system at 773 K has been investigated by means of x-ray powder diffraction, scanning electron microscopy with energy dispersive analysis, and optical microscopy. The existences of 13 binary compounds for the Al-Fe, Al-Gd, and Fe-Gd binary systems were confirmed. The binary phases, i.e., Al₂Gd, Fe₂Gd, and α-Fe₁₇Gd₂ present appreciable ternary solubility. Three ternary compounds: τ₁-Al₁₀Fe₂Gd, τ₂-Al_8+x Fe_4-x Gd (0 ≤ x ≤ 2), and λ-Al_2?x Fe _x Gd (0.951 ≤ x ≤ 1.245) were determined. The isothermal section consists of 19 single-phase regions, 40 binary-phase regions, and 20 ternary-phase regions.     

Influence of Nd-Co Substitution on Structural,Electrical, and Dielectric Properties of X-Type Hexagonal Nanoferrites

A series of single phase X-type hexagonal ferrites with concentration Sr_2?x Nd _x Ni₂Fe_28?y Co _y O₄₆ (x = 0.02, 0.04, 0.06, 0.08, 0.10 and y = 0.1, 0.2, 0.3, 0.4, 0.5) has been prepared by sol-gel method sintered at 1250 °C for 6 h. The x-ray diffraction analysis reveals the single phase of X-type hexagonal ferrites. The particle size was calculated by using SEM and TEM. The ferrite substituted with Nd³⁺ and Co²⁺ has average particle size in the range of 40-50 nm. The room temperature electrical resistivity experiences the significant enhancement from a value of 1.1 × 10⁷ to 2.03 × 10⁸ Ωcm with the increase in Nd³⁺ and Co²⁺ concentration. The dielectric constant exhibits high value at low frequencies and decreases with the increase of frequency. The tangent dielectric loss shows the abnormal behavior which can be explained on the basis of hopping between the Fe²⁺ and Fe³⁺ ions on octahedral sites. The maximum value of tangent loss at low frequencies reflects the application of these materials in medium frequency devices (MF).     

Effect of Single and Double Sintering on Microstructure and Magnetic Properties of Gd-Substituted Ni-Cd Ferrites

Two different groups, I and II, which are fired one and two times, respectively, from Ni-Cd ferrites of chemical formula Ni_0.7Cd_0.3Gd _x Fe_2?x O₄ (x = 0 to x = 0.1 with step = 0.025) have been prepared by conventional ceramic method. The effect of Gd-substitution on the microstructure and magnetic properties of the two groups has been studied. X-ray patterns indicated the presence of a minor secondary phase with the spinel phase at Gd-concentrations with x = 0.075 and 0.1. SEM and VSM are used to investigate the microstructure and to measure the magnetization of the samples at room temperature, respectively. The initial permeability is measured as a function of temperature and Curie temperature is determined. It was found that there are considerable differences in the microstructure and the magnetic properties of the two groups. Although the Gd-substitution decreased the values of both saturation magnetization M _s and initial permeability μ_i, the value of Curie temperature T _C has been improved. Moreover, the double sintering process improved the magnetization and densification of samples.     

LixZrS2 intercalation compounds

The disulfide ZrS₂ has been intercalated with lithium by means of the butyllithium method. Two phases have been characterized. The first (0? x < 0.20), of the NiAs type, presents no parameter variation. The second (0.30 < x ? 1) is rhombohedral at room temperature but undergoes a phase transition to a spinel structure in the 0.30 < x < 0.50 range at 250°C. Electrical and magnetic measurements have shown that the first phase is semiconducting, the second being of a metallic type. Comparisons are made with the Li_xZrSe₂ system.     

Isothermal Sections of Pd-Cu-Sn System at 500 and 800 °C

The isothermal sections of the Pd-Cu-Sn system at 500 and 800 °C up to 50% Sn were constructed. Two ternary phases τ and (Pd,Cu)₃Sn of tetragonal and orthorhombic syngony respectively were discovered in the palladium-rich region. At both temperatures, solubility of tin in the FCC solid solution phase (α) has a pronounced minimum around 75-85 at.% Cu. γ-Pd_2?x Sn and η-Cu₆Sn₅ phases of Ni₂In type form a continuous (Pd,Cu)_2?x Sn phase.     

Microstructural studies and electrical properties of Mg-doped SrTiO3 thin films

The structure–property relations of Mg-doped SrTiO₃ (ST) sol–gel thin films deposited on Pt/TiO₂/SiO₂/Si substrates have been investigated in order to determine the effect that Mg dopants have on the dielectric properties of SrTiO₃. It has been predicted that Mg-doped SrTiO₃ should exhibit a dielectric anomaly similar to that observed recently in Bi doped SrTiO₃ but, to date, no polar state has been reported. It has been suggested that this may relate to the low solubility of Mg on the A-site in bulk ceramics (<0.05 at.%). However, for Sr_1−xMg_xTiO₃ (SMT) (x ⩽ 0.30) films annealed at 750 °C, all Mg ions were accommodated in the perovskite lattice and for SMT films annealed at 900 °C, the solubility limit of Mg was x = 0.10, above which a Mg-rich ilmenite second phase was observed. Irrespective of the higher solid solubility limit of Mg in the ST lattice for sol–gel ST films compared to equivalent ceramics, no ferroelectric or relaxor phase transition was observed, refuting previous predictions for this dopant.     

Phase Equilibria in the Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>–InSb System

The phase diagram of the (Sb₂Te₃)_100?x –InSb _x system was determined based on x-ray diffraction (XRD) analysis, differential thermal analysis (DTA), and microhardness and density measurements. An intermediate compound with composition Sb₂Te₃·2InSb was formed as a result of syntectic reaction, melting incongruently at 553 °C. This compound has tetragonal lattice with unit cell parameters of a = 4.3937 Å, b = 4.2035 Å, c = 3.5433 Å, α = 93.354°, and β = γ = 90°. Sb₂Te₃·(2 + δ)InSb (?1 ≤ δ ≤ +1) and (Sb₂Te₃)_100?x (InSb) _x (90 ≤ x ≤ 100) solid solutions exist in the investigated system, based on the intermediate compound Sb₂Te₃·2InSb and on InSb, respectively. Also, two invariant equilibria exist in the system, with eutectic point coordinates at compositions of x = 60 and x ≈ 85 mol% InSb and eutectic temperatures of T _E = 541 and T _E ≈ 501 °C, respectively.     

Thermal and transport properties of as-grown Ni-rich TiNi shape memory alloys

Electrical resistivity, Seebeck coefficient, specific heat and thermal conductivity measurements on the Ti_50−xNi_50+x (x = 0.0–1.6 at.%) shape memory alloys are performed to investigate their thermal and transport properties. In this study, anomalous features are observed in both cooling and heating cycles in all measured physical properties of the slightly Ni-rich TiNi alloys (x ≤ 1.0), corresponds to the transformation between the B19′ martensite and B2 austenite phases. Besides, the transition temperature is found to decrease gradually with increasing Ni content, and the driving force for the transition is also found to diminish slowly with the addition of excess Ni, as revealed by specific heat measurements. While the signature of martensitic transformation vanishes for the Ni-rich TiNi alloys with x ≥ 1.3, the characteristics of strain glass transition start to appear. The Seebeck coefficients of these TiNi alloys were found to be positive, suggesting the hole-type carriers dominate the thermoelectric transport. From the high-temperature Seebeck coefficients, the estimated value of Fermi energy ranges from ∼1.5 eV (Ti_48.4Ni_51.6) to ∼2.1 eV (Ti₅₀Ni₅₀), indicating the metallic nature of these alloys. In addition, the thermal conductivity of the slightly Ni-rich TiNi alloys with x ≤ 1.0 shows a distinct anomalous feature at the B19′ → B2 transition, likely due to the variation in lattice thermal conductivity.     

Alloying Behavior and Properties of FeSiBAlNiCo<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High Entropy Alloys Fabricated by Mechanical Alloying and Spark Plasma Sintering

In this paper, FeSiBAlNiCo_x (x = 0.2, 0.8) high-entropy alloy (HEA) powders were fabricated by mechanical alloying process, and the powders milled for 140 h were sintered by spark plasma sintering (SPS) technique. The microstructures and properties of as-milled powders and as-sintered samples were investigated. The results reveal that the final milling products (140 h) of both sample powders present the fully amorphous structure. The increased Co contents obviously enhance the glass forming ability and thermal stability of amorphous HEA powders, which are reflected by the shorter formation time of fully amorphous phase and the higher onset crystallization temperature, respectively. According to coercivity, the as-milled FeSiBAlNiCo_x (x = 0.2, 0.8) powders (140 h) are the semi-hard magnetic materials. FeSiBAlNiCo_0.8 HEA powders possess the highest saturation magnetization and largest remanence ratio. The SPS-ed products of both bulk HEAs are composed of body-centered cubic solid solution, and FeSi and FeB intermetallic phases. They possess the high relative density above 97% and excellent microhardness exceeding 1150 HV. The as-sintered bulks undergo the remarkable increase in saturation magnetization compared with the as-milled state. The SPS-ed FeSiBAlNiCo_0.8 HEA exhibits the soft magnetic properties. The electrochemical corrosion test is carried out in 3.5% NaCl solution. The SPS-ed FeSiBAlNiCo_0.2 HEA reveals the better passivity with low passive current density, and the higher pitting resistance with wide passive region.