Effects of Tungsten Addition on the Microstructure and Mechanical Properties of Near-Eutectic AlCoCrFeNi<Subscript>2</Subscript> High-Entropy Alloy

The effects of tungsten addition on the microstructure and mechanical properties of near-eutectic AlCoCrFeNi₂ high-entropy alloy were investigated in this paper. The AlCoCrFeNi₂W _x alloys comprised the primary BCC phase plus eutectic FCC/BCC phases. It was found that W element can both promote the formation of the primary BCC phase and act as a solid solution strengthening element. The hardness of the AlCoCrFeNi₂W _x alloys increased from HV 293 to HV 356.2 with the increase in W content. The addition of W element improved the strength of alloys but reduced ductility. Thereinto, the AlCoCrFeNi₂W_0.2 alloy showed the most excellent compressive properties with the fracture strength of 2785.9 MPa and the plastic strain of 0.42, respectively, which implied the potential industrial application values.     

Properties of the Ti<Subscript>40</Subscript>Zr<Subscript>10</Subscript>Cu<Subscript>36</Subscript>Pd<Subscript>14</Subscript> BMG Modified by Sn and Nb Additions

The results of investigation of the influence of additions of 2 and 3 at.% of Sn and simultaneously of Sn and 3 at.% Nb on microstructure and properties of the bulk metallic glasses of composition (Ti₄₀Cu_36?x Zr₁₀Pd₁₄Sn _x )_100?y Nb _y are reported. It was found that the additions of Sn increased the temperatures of glass transition (T _g), primary crystallization (T _x ), melting, and liquidus as well as supercooled liquid range (ΔT) and glass forming ability (GFA). The nanohardness and elastic modulus decreased in alloys with 2 and 3 at.% Sn additions, revealing similar values. The 3 at.% Nb addition to the Sn-containing amorphous phase decreased as well all the T _g, T _x , T _L, and T _m temperatures as ΔT and GFA; however, relatively larger values of this parameters in alloys containing larger Sn content were preserved. In difference to the previously published results, in the case of the amorphous alloys containing small Nb and Sn additions, a noticeable amount of the quenched-in crystalline phases was not confirmed, at least of the micrometric sizes. In the case of the alloys containing Sn or both Sn and Nb, two slightly different amorphous phase compositions were detected, suggesting separation in the liquid phase. Phase composition of the alloys determined after amorphous phase crystallization was similar for all compositions. The phases Cu₈Zr_3, CuTiZr, and Pd₃Zr were mainly identified in the proportions dependent on the alloy compositions.     

Microstructure and Mechanical Properties of a Refractory CoCrMoNbTi High-Entropy Alloy

In this work, a new refractory high-entropy alloy, the Co-Cr-Mo-Nb-Ti system, was proposed as a family of candidate materials for high-temperature structural applications. CoCrMoNbTi _x (x values in terms of molar ratios, x = 0, 0.2, 0.4, 0.5 and 1.0) alloys were prepared by vacuum arc melting. The effects of variations in the Ti content on the phase constituents, microstructure and mechanical properties of the alloys were investigated using x-ray diffractometry, scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy and compressive testing. The results showed that the CoCrMoNbTi_0.4 alloy possessed a typical cast dendritic microstructure consisting of a single body-centered cubic (BCC) solid solution. Laves phases (Cr₂Nb and Co₂Ti) were formed in other alloys with different Ti contents. The results were discussed in terms of the mixing enthalpy, atomic size difference, electronegativity difference and valance electron concentrations among the elements within alloys. The alloy hardness exhibited a slightly decreasing trend as the Ti content increased, resulting from the coarser microstructure and reduced amount of Laves phases. Augmented Ti content increased the compressive strength, but decreased the ductility. Particularly, for the CoCrMoNbTi_0.2 alloy, the hardness, compressive strength and fracture strain were as high as 916.46 HV_0.5, 1906 MPa and 5.07%, respectively. The solid solution strengthening of the BCC matrix and the formation of hard Laves phases were two main factors contributing to alloy strengthening.     

Superior Mechanical Properties of AlCoCrFeNiTi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High-Entropy Alloys upon Dynamic Loading

High-entropy alloys with composition of AlCoCrFeNiTi _x (x: molar ratio; x = 0, 0.2, 0.4) under quasi-static and dynamic compression exhibit excellent mechanical properties. A positive strain-rate sensitivity of yield strength and the strong work-hardening behavior during plastic flows dominate upon dynamic loading in the present alloy system. The constitutive relationships are extracted to model flow behaviors by employing the Johnson-Cook constitutive model. Upon dynamic loading, the ultimate strength and fracture strain of AlCoCrFeNiTi _x alloys are superior to most of bulk metallic glasses and in situ metallic glass matrix composites.     

Microstructure and mechanical properties of multi-principal component AlCoCrFeNiCu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloy

By introducing Cu, AlCoCrFeNiCu _x (x values in molar ratio, x = 0, 0.1, 0.5, 1.0, 1.5, 2.0, and 2.5) alloys were designed and prepared. The effects of Cu on microstructure and properties of AlCoCrFeNi alloy were investigated. The introduction of Cu results in the formation of Cu-rich FCC solid solution phase when Cu content is low. There are two FCC solid solution phases, i.e., Cu-rich FCC solid solution phase and phase transformation-induced FCC solid solution phase, when the Cu content is more than 1.0. Both the yield stress and plastic strain of alloy show a turning point when the Cu content is 0.5. Among the seven alloys, Cu_0.5 alloy exhibits the largest yield stress of 1187 MPa and the lowest plastic strain of 16.01 %.     

Magnetoresistive properties of Ni-doped La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganites

La_0.7Sr_0.3Mn_1?x Ni _x O₃ (x = 0, 0.025, 0.050 and 0.075) ceramics were prepared by the conventional solid-state reaction method. The partial substitution of Mn by Ni²⁺ leads to a decrease in cell volume as well as a structural transition from the rhombohedral to the orthorhombic structure. Ni²⁺ doping increases the electrical resistivity, decreases the semiconductor–metal transition temperature (T _ms) and relatively enhances the room temperature magnetoresistance (MR), especially in x = 0.025 and around T _ms. With respect to conduction mechanism, the small polaron hopping (SPH) and the variable range hopping (VRH) models were used to examine conduction in the semiconducting region.     

Microstructure and Properties of FeAlCrNiMo<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High-Entropy Alloys

FeAlCrNiMo _x high-entropy alloys were prepared. The effect of Mo content on the microstructure and the properties of the alloys were investigated. When the Mo content was 0.1, the alloys were composed of single BCC solid solution; when Mo content reaches 0.25, the alloys were composed of BCC solid solution and ordered B2 solid solution. When Mo content is more than 0.75, some σ phases emerged. The volume fraction of the second phase increases with the increasing Mo content, and the crystal grains became coarsening. The yield strength, fracture strength, and hardness increase with the increasing Mo content and reach 2252, 2612 MPa, and 1006 Hv, respectively. The magnetic transformation undergoes from the ferromagnetism to paramagnetism with the increasing Mo content. The saturation intensity and remnant magnetism are decreased with the increasing Mo content.     

Effect of Titanium Additions on the Thermophysical Properties of Glass-Forming Cu<Subscript>50</Subscript>Zr<Subscript>50</Subscript> Alloy

Differential scanning calorimetry, laser flash method, and dilatometry were used to study the thermophysical properties of quenched Cu₅₀Zr_50–xTi_x (x = 0, 2, 4, 6, 8) alloys in the temperature range from room temperature to 1100 K. Data obtained on the heat capacity, thermal diffusivity, and density have been used to calculate the coefficient of thermal conductivity. Temperatures corresponding to the stability of martensite CuZr phase, its eutectoid decomposition, and formation in Cu₅₀Zr_50–xTi_x alloys with different Ti contents upon heating have been determined. It has been found that the thermal diffusivity and thermal conductivity of the studied alloys are low and a typical of metallic systems. As the titanium content increases, the coefficients of thermal conductivity and thermal diffusivity vary slightly. It has been shown that the low values of thermophysical characteristics correspond to the better capability of amorphization and can be a criterion for the glass-forming ability of Cu–Zr-based alloys.     

Structure,magnetic and magnetocaloric properties of nonstoichiometric TbCo<Subscript>2</Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> compounds

The structure and magnetic and magnetocaloric properties of new nonstoichiometric TbCo₂Ni _x compounds (0 ≤ x ≤ 0.4) have been studied. The alloys with х ≤ 0.1 have been shown to be single-phase with the MgCu₂-type structure; in alloys with х > 0.1, an additional phase with a PuNi₃-type structure has been formed. It has been found that the concentration dependences of the Curie temperature and magnetic moment of the 3d-metal sublattice have a maximum at x = 0.025. The magnetocaloric effect magnitude for the TbCo2Nix compounds has been estimated using the results of magnetic and heat-capacity measurements.     

Effect of Cu and Mo Additions on the Crystallization Behavior and Magnetic Properties of Fe<Subscript>80</Subscript>Zr<Subscript>10</Subscript>B<Subscript>10</Subscript> Alloy

Fe₈₀Zr₁₀B₁₀, Fe₈₀Zr₁₀B₉Cu₁, and Fe₈₀Zr₈Mo₂B₁₀ amorphous alloys were prepared by melt-spinning and annealed at various temperatures. The effect of Cu and Mo additions on the thermal property, microstructure and magnetic properties of Fe₈₀Zr₁₀B₁₀ alloy is studied. Both Cu and Mo additions decrease the crystallization activation energy. The crystallization process of Fe₈₀Zr₁₀B₁₀ alloy is very complex. Both Cu and Mo additions simplify the crystallization process. But a few α-Mn-type phase is still observed in the initial crystallization stage of Mo-containing alloy. Both Cu and Mo additions increase saturation magnetization (M _s) and decrease coercivity (H _c) of alloys. The addition of Cu is beneficial to decrease H _c in the initial crystallization stage, and the addition of Mo is beneficial to decrease H _c at high temperatures.     

Structure,Magnetic and Magnetocaloric Properties of Nonstoichiometric TbCo<Subscript>2</Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Compounds

TbCo₂Mn_x (x ≤ 1) alloys were synthesized and their crystal structure, heat capacity, magnetic and magnetocaloric properties were studied. Single-phase compounds with the MgCu₂-type structure were formed at х < 0.4. In alloys with х > 0.4, additional phases with the PuNi₃- and Th₆Mn₂₃-type structures form. It was shown that there is a substantial increase in the Curie temperature and magnetic moment of 3d?metal sublattice of the nonstoichiometric compounds when compared to those of the TbCo₂ binary compound. The magnetocaloric effect of single- and multiphase alloys were estimated based on magnetic and heat capacity measurements.     

Effect of silicon on the phase formation in mechanically activated systems based on Fe<Subscript>75</Subscript>C<Subscript>25</Subscript>: Mechanosynthesis of composite states

The methods of X-ray diffraction analysis, Mögsbauer spectroscopy, and measurement of dynamic magnetic susceptibility have been used to study stationary phase states that develop at the later stages of mechanical alloying in a planetary ball mill. In the Fe(100 ? x)C(x), Fe(75)C(25 ? x)Si(x), and Fe(75 ? x)C(25)Si(x) (x ≤ 25) systems, the processes of phase formation are determined by the dynamic equilibrium between the crystalline and amorphous phases. Depending on the composition of the alloys, the conditions of this equilibrium are changed, which is reflected in the sets of the crystalline phases that are formed.     

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.     

Phase Evolution and Mechanical Properties of AlCoCrFeNiSi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High-Entropy Alloys Synthesized by Mechanical Alloying and Spark Plasma Sintering

In the current investigation, AlCoCrFeNiSi_x (x?=?0, 0.3, 0.6 and 0.9 in atomic ratio) high-entropy alloy systems are prepared by mechanical alloying and subsequently consolidated by spark plasma sintering. The microstructural and mechanical properties were analyzed to understand the effect of Si addition in AlCoCrFeNi alloy. The x-ray diffraction analysis reveals the supersaturated solid solution of the body-centered cubic structure after 20 h of ball milling. However, the consolidation promotes the transformation of body-centered phases partially into the face-centered cubic structure and sigma phases. A recently proposed geometric model based on the atomic stress theory has been extended for the first time to classify single phase and multi-phases on the high-entropy alloys prepared by mechanical alloying and spark plasma sintering process. Improved microhardness and better wear resistance were achieved as the Si content increased from 0 to 0.9 in the present high-entropy alloy.     

Influence of Feedstock Powder Modification by Heat Treatments on the Properties of APS-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-40% TiO<Subscript>2</Subscript> Coatings

The formation and decomposition of aluminum titanate (Al₂TiO₅, tialite) in feedstock powders and coatings of the binary Al₂O₃-TiO₂ system are so far poorly understood. A commercial fused and crushed Al₂O₃-40%TiO₂ powder was selected as the feedstock for the experimental series presented in this paper, as the composition is close to that of Al₂TiO₅. Part of that powder was heat-treated in air at 1150 and 1500 °C in order to modify the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including EDS of metallographically prepared cross sections. Only a maximum content of about 45 wt.% Al₂TiO₅ was possible to obtain with the heat treatment at 1500 °C due to inhomogeneous distribution of Al and Ti in the original powder. Coatings were prepared by plasma spraying using a TriplexPro-210 (Oerlikon Metco) with Ar-H₂ and Ar-He plasma gas mixtures at plasma power levels of 41 and 48 kW. Coatings were studied by XRD, SEM including EDS linescans of metallographically prepared cross sections, and microhardness HV1. With the exception of the powder heat-treated at 1500 °C an Al₂TiO₅-Ti₃O₅ (tialite–anosovite) solid solution Al_2?xTi_1+xO₅ instead of Al₂TiO₅ existed in the initial powder and the coatings.     

Structural and phase transformations,mechanical properties,and shape-memory effects in quasibinary Ni<Subscript>50</Subscript>Ti<Subscript>38</Subscript>Hf<Subscript>12</Subscript> alloy obtained by quenching from the melt

Methods of transmission and scanning electron microscopy and chemical microanalysis, electron diffraction, and X-ray diffraction were used to systematically study the structure and the chemical and phase composition of the Ni₅₀Ti₃₈Hf₁₂ alloy synthesized by rapid quenching from the melt and subjected to various heat treatments. The critical temperatures of the devitrification of the initially amorphous rapidly quenched alloy and the B2 ? B19′ thermoelastic martensitic transformations have been determined. The lattice parameters of the B2 austenite and thermoelastic B19′ martensite have been measured. The main features of the formation of an ultrafine-grained structure in the alloy and the subsequent phase transformations (martensitic transformation and the decomposition with the formation of an intermetallic phase of the (Ti,Hf)₂Ni type) have been studied depending on the regimes of heat treatment. Based on the results of measurements of mechanical properties upon tension (σ_M, σ_u, and δ) and the shape-memory effects (degree of shape recovery depending on the deformation by bending; and magnitude of the reversible strain ε_rev), regimes for obtaining high-strength and plastic states of the alloy with a shape-memory effect have been established.     

Simulation of hydrogen diffusion in TiH<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> structures

Quantum-mechanical calculations of the energies of the formation of Frenkel pairs and barriers for hydrogen migration via different mechanisms in the titanium hydride δ-TiH _x and in the α phase of titanium have been carried out. Using the potential of interaction developed for the molecular-dynamic simulation, diffusion coefficients of hydrogen in fcc and hcp lattices of TiH _x were calculated depending on the temperature. The opportunity to approximate the coefficients of hydrogen self-diffusion has been analyzed in terms of the model of non-interacting point defects. For δ-TiH _x , the range of concentrations and temperatures was separated where the self-diffusion of hydrogen becomes liquid-like (ceases be dependent on the hydrogen concentration), upon the transition into which there takes place a sharp increase in the isochoric heat capacity. The effect of defects in the Ti sublattice on the coefficient of self-diffusion of H has been investigated.     

Effect of Hydrogen Intercalation on the Structure of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> with a Low Oxygen Content

We used Raman spectroscopy to study the interaction of YBa₂Cu₃O_y (123) (y ≤ 6.5) with hydrogen at temperatures of 150–200°С. In contrast to the YBa₂Cu₃O_6.96 compound, compositions with low oxygen contents show a lower tendency to form stacking faults. We found that the hydrogenation of these compositions does not lead to the transformation of the 123 phase to the pseudo-124 phase. Absorbed hydrogen does not react with oxygen in 123 and does not form hydroxyl groups. The H_xYBa₂Cu₃O_y oxyhydride is the hydration-reaction product.     

Oxidation Kinetics of UZr<Subscript>2.3</Subscript> and U<Subscript>2</Subscript>Ti Alloys in Dry Air

The isothermal oxidation behavior of UZr_2.3 and U₂Ti alloys in dry air was studied by thermogravimetric technique in the temperature ranges of 773–848 and 548–623 K, respectively. The oxidation products were found to be U₃O₈ and ZrO₂ for UZr_2.3 and U₃O₈ and TiO₂ for U₂Ti by X-ray diffraction analysis. The oxidation kinetics were found to best fit into linear rate law for both the alloys in the temperature range of investigation and the lowest value of α (0.11 for UZr_2.3 and 0.12 for U₂Ti), which corresponds to the fraction of completed reaction. The linear rate constants (k) were evaluated from the oxidation data. The activation energies of oxidation reactions were calculated using the Arrhenius equation and found to be 161 kJ mol^?1 for UZr_2.3 and 88 kJ mol^?1 for U₂Ti.     

Synthesis and Structural Characterization of Sb-Doped TiFe<Subscript>2</Subscript>Sn Heusler Compounds

Ternary Heusler compounds form a numerous class of intermetallics, which include two families with general compositions ABC and AB₂C, usually referred to as half- and full-Heusler compounds, respectively. Given their tunable electronic properties, made possible by adjusting the chemical composition, these materials are currently considered for the possible use in sustainable technologies such as solar energy and thermoelectric conversion. According to theoretical predictions, Sb substitution in the TiFe₂Sn full-Heusler compound is thought to yield band structure modifications that should enhance the thermoelectric power factor. In this work, we tested the phase stability and the structural and microstructural properties of such heavily doped compounds. We synthesized polycrystalline TiFe₂Sn_1?xSb_x samples, with x?=?0, 0.1, 0.2 and 1.0 by arc melting, followed by an annealing treatment. The structural characterization, performed by x-ray powder diffraction and microscopy analyses, confirmed the formation of the pseudo-ternary Heusler structure (cF16, Fm-3m, prototype: MnCu₂Al) in all samples, with only few percent amounts of secondary phases and only slight deviations from nominal stoichiometry. With increasing Sb substitution, we found a steady decrease in the lattice parameter, confirming that the replacement takes place at the Sn site. Quite unusually, the as-cast samples exhibited a higher lattice contraction than the annealed ones. The fully substituted x?=?1.0 compound, again adopting the MnCu₂Al structure, does not form as stoichiometric phase and turned out to be strongly Fe deficient. The physical behavior at room temperature indicated that annealing with increasing temperature is beneficial for electrical and thermoelectrical transport. Moreover, we measured a slight improvement in electrical and thermoelectrical properties in the x?=?0.1 sample and a suppression in the x?=?0.2 sample, as compared to the undoped x?=?0 sample.