Single-phase Alloys Research Articles

Electroplating and Characterization of Invar Fe–Ni Alloy Films from Plating Baths Containing Organic Acids

Practical plating baths and plating conditions for preparing Fe–Ni alloy electroplated films with compositions in the Ni content range 36–100 mass% have been proposed. The addition of saccharin in sulfate-chloride bath is indispensable for preparation of Fe-30–45 mass% Ni alloy electroplated films, i.e., those in invar composition region. The addition of tartaric acid or malonic acid to Fe–Ni alloy plating baths (containing saccharin) prevents a reduction in the toughness of the plated film by masking Fe3+ generated by oxidation. It was possible to suppress the internal stress in invar Fe–Ni alloy electroplated films to low levels. Resultantly, the invar electroplated films obtained from these optimized plating baths show promise for electroforming. On the contrary, the addition of citric acid, which forms a more stable complex with Fe3+, causes very large internal stress in the resultant invar Fe–Ni alloy electroplated films, making them unsuitable as MEMS materials. It was confirmed that Fe–Ni alloy electroplated films with Ni contents of 45–100 mass% comprise a single-phase alloy having an fcc structure but the invar Fe-36–42 mass% Ni alloy electroplated film is a metastable bcc phase or a mixture of bcc and fcc phases. Therefore, to impart low thermal expansion characteristics to as-plated invar alloy films, it is necessary to induce phase transformation to an fcc phase, which is the equilibrium phase at high temperature, by the means of heat treatment.

Thermal Stability of Ni3Al-Based Composite with Honeycomb Structure

By powder metallurgy, materials in the form of a single-phase alloy based on Ni3Al and the corresponding composite Ni3Al + W with honeycomb structure are produced. The structural unit of the composite is a round granule (mean size 25 μm) of nickel alloy surrounded by a continuous tungsten coating (thickness ~0.4 μm) applied by chemical gas-phase deposition. Compressive tests at room temperature show that the yield point of the composite Ni3Al + W at 20–1000°C exceeds that of the single-phase Ni3Al-based alloy (by a factor of as much as 1.7). However, at higher temperatures, the yield points of the alloy and composite are comparable. The unit yield point (standardized at a density of 7.8 g/cm3 for the alloy and 9.5 g/cm3 for the composite) behaves analogously. At 1300°C, the single-phase Ni3Al-based alloy exhibits solid–liquid behavior in compression. Creep tests with compression in vacuum at 1000–1200°C are conducted. By pair and parametric analysis of the creep processes according to Hollomon, regression equations for the creep rate, stress, and test temperature are obtained. The creep limit is calculated from the tolerances on the steady creep rate and its inverse. At all the test temperatures, the composite is characterized by lower creep rate (by a factor of seven) and higher creep limit (by a factor of 2.5) than for the nickel alloy on which it is based. The activation energy of creep is determined for the alloy and composite on the basis of an exponential relationship between the experimental quantities. The activation energy of creep for the nickel alloy is close to the activation energy of nickel self-diffusion in Ni3Al and materials based on it (230–310 kJ/mol). For the composite, it is close to the activation energy of tungsten self-diffusion (503 kJ/mol).

Development of advanced materials via entropy engineering

The recent and growing interests in the metals community regarding the concept of entropy stabilization of single-phase alloys has resulted in a surge of development of other high-entropy materials via entropy engineering. This paper first briefly introduces the basic concept of entropy engineering and then summarizes its application to materials design. Moreover, perspectives regarding the entropy engineering and possible research directions are also surmised.

Dependence of pop-in behavior of a high-entropy alloy FeCoCrMnNi on tip radius

Our previous work has established that the dislocation nucleation during the onset of plasticity, the so-called pop-in, in face-centered cubic single-phased high-entropy alloy FeCoCrMnNi is controlled by vacancy defects. This implies that the tip radius would affect the pop in behavior as the number of vacancy, the available sites for dislocation nucleation, within the stressed volume is proportional to radius. To verify this in current work, a wide range of nanoindenter tips with radius across from 200 to 2013 nm were used. It was found that when tip radius is smaller than 638 nm, the pop-in or displacement burst size increases linearly with it, and that when tip radius is larger than 638 nm, the pop—in size became essentially constant. These experimental findings confirm the effect of tip radius on pop-in behavior. A theoretical model based on image force has been developed to rationalize the above observations.

Influence of chemical composition of wear-resistant hard-facing materials on the tendency to formation of hot cracks during weld facing

Abstract At present, extreme cases of intercrystalline rupture at high temperatures in non-equilibrium crystallizing alloys (built-up layers) are clearly distinguished. Solidifying cracks are formed in enriched inter-axial and inter-dendritic areas at the moment when the latter are in a liquid state. This type of destruction is associated with crystallization of the last portions of the liquid phase enriched with low-melting alloying elements and impurities and is observed in alloys, where the second phase in the form of eutectics and chemical compounds is formed directly during the crystallization process. Subsolidus cracks, which are most often formed in single-phase alloys with a narrow temperature interval of crystallization, not in liquid intercrystalline interlayers, but after solidification of the last portions of the liquid phase, i.e. below the temperature of the real solidus.

Жаропрочные сплавы на основе RuAl. I. Литейные сплавы

Refractory ruthenium monoaluminide RuAl, a heat-resistant material with the melting point tmp = 2100°C, lighter than nickel superalloys—the density ρ = 7.97 g/cm3—is considered to be a promising candidate for high-temperature applications under relatively low loads in high-speed oxidizing gas flows at temperatures higher than not only the operating (top) but also the melting (tmp) temperatures of both nickel superalloys and nickel and titanium aluminides. RuAl is also an ideal candidate for protective coatings of critical components of the hot gas path of gas turbine and rocket engines. In the first part of the article, the authors consider the principles of selecting the alloying systems and the methods for production of single-phase or heterophase RuAl cast alloys and changes in their structural and phase states under cold and hot deformation and provide data on the mechanical properties within a wide range of temperatures, the oxidation resistance and the resistance to some corrosive media, and the plasma erosion resistance.

Design of single-phase high-entropy alloys composed of low thermal neutron absorption cross-section elements for nuclear power plant application

Two quinary high-entropy alloys MoNbCrVTi and MoNbCrZrTi were designed and prepared for the two following goals. The first goal is to search for single-phase high-entropy alloys composed of low thermal neutron absorption cross-section elements, and the second goal is to verify the validity of the empirical parameters calculation and CALPHAD (acronym of CALculation of PHAse Diagrams) calculation in the initiatory selection process of desired alloys from numerous candidates. The density, hardness, microstructure, and compressive mechanical properties of the two alloys were preliminarily investigated, and the phase formation of these two alloys was also discussed. The MoNbCrVTi alloy consists of a single BCC phase with typical dendritic microstructure, while the MoNbCrZrTi alloy mainly consists of a BCC phase plus a C15 Laves phase. The densities of the as-cast MoNbCrVTi and MoNbCrZrTi alloys were determined to be 7.30 ± 0.01 and 7.33 ± 0.01 g/cm3, respectively. The MoNbCrVTi alloy exhibits a hardness of 494.4 ± 7.7 Hv, a high yield strength of 1281 MPa and a fracture strain of 9.4% at room temperature. By comparison, the MoNbCrZrTi alloy exhibits a hardness of 552.9 ± 8.6 Hv, a higher yield strength of 1454 MPa but a lower fracture strain of 2.7%. The proposed two goals are basically achieved based on the preliminary results.

Modelling of Creep in Alloys Strengthened by Rod-Shaped Particles: Al-Cu-Mg Age-Hardenable Alloys

In recent years, a creep model that does not involve adjustable parameters has been successfully applied to coarse-grained aluminum. The main feature of this model is that it is fully predictable. On the other hand, in the case of age-hardenable alloys, any physically-based creep model should take into account the changes in the volume fraction, size and distribution of strengthening precipitates, and the effect of grain size. With this aim in view, in this paper, the original model previously applied to single phase-alloys has been modified to describe the effects of the grain size and of the consequences of the high-temperature exposure on the strengthening role of precipitates. To this end, phenomenological equations describing the coarsening phenomena and their dependence on the applied stress have been introduced. The modified model has given an excellent description of the experimental behavior of an AA2024-T3 alloy tested at 250 and 315 °C and has provided a sound explanation of the difference observed when comparing the minimum creep rate obtained using two different testing techniques.

Composition-Controllable AuPt Alloy Catalysts for Electrooxidation of Formic Acid

AuPt alloy catalysts with various compositions have been successfully prepared simply by one-step co-reduction of Au and Pt precursors involving sodium citrate as stabilizer and reductant. XRD, TEM and EDX element mapping analysis confirmed that the resulting AuPt nanoparticles are single-phase alloys rather than random mixtures of tiny Au and Pt particles. Compared with Pt/C, alloying Au with Pt can effectively alter the kinetic process of formic acid oxidation, reducing the generation of CO-like intermediates. Au81Pt19 displays superior electrocatalytic activity and durability, ~11 times in the mass activity better than commercial Pt/C and may be of practical significance for the commercialization of direct formic acid fuel cell.

Entropy Determination of Single-Phase High Entropy Alloys with Different Crystal Structures over a Wide Temperature Range.

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, contributes less on the absolute scale. Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order. Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy.

Entropy contributions to phase stability in binary random solid solutions

High entropy alloys contain multiple elements in large proportions that make them prone to phase separation. These alloys generally have shallow enthalpy of mixing which makes the entropy contributions of similar magnitude. As a result, the phase stability of these alloys is equally dependent on enthalpy and entropy of mixing and understanding the individual contribution of thermodynamic properties is critical. In the overall vision of designing high entropy alloys, in this work, using density functional theory calculations, we elucidate the contributions of various entropies, i.e., vibrational, electronic and configurational towards the phase stability of binary alloys. We show that the contribution of electronic entropy is very small compared to the vibrational and configurational entropies, and does not play a significant role in the phase stability of alloys. The configurational and vibrational entropies can either destabilize or can collectively contribute to stabilize the solid solutions. As a result, even those systems that have negative mixing enthalpy can show phase instability, revealed as a miscibility gap; conversely, systems with positive mixing enthalpy can be phase stable due to entropic contributions. We suggest that including entropic contributions are critical in the development of theoretical framework for the computational prediction of stable, single-phase high entropy alloys that have shallow mixing enthalpies, unlike ordered intermetallics.

Comparative study of the surface oxide films on lean duplex and corresponding single phase stainless steels by XPS and ToF-SIMS

The surface oxide films formed after mechanical polishing on duplex stainless steel and corresponding single phase alloys were characterized by XPS and ToF-SIMS. The native oxide films are mixtures of oxides and hydroxides composed of Fe(III), Mo(IV) and Mo(VI) in the outer region and Cr(III) in the inner region. Nickel, molybdenum and nitrogen are enriched in the underlying metal while chromium and molybdenum are enriched in the oxide layer. The contents of alloying elements in the oxide layer and/or in the underlying metal are found to be directly related to their content in the bulk of the alloys.

Thermal spin fluctuations in CoCrFeMnNi high entropy alloy

High entropy alloys based on 3d transition metals display rich and promising magnetic characteristics for various high-technology applications. Understanding their behavior at finite temperature is, however, limited by the incomplete experimental data for single-phase alloys. Here we use first-principles alloy theory to investigate the magnetic structure of polymorphic CoCrFeMnNi in the paramagnetic state by accounting for the longitudinal spin fluctuations (LSFs) as a function of temperature. In both face-centered cubic (fcc) and hexagonal close-packed (hcp) structures, the LSFs induce sizable magnetic moments for Co, Cr and Ni. The impact of LSFs is demonstrated on the phase stability, stacking fault energy and the fcc-hcp interfacial energy. The hcp phase is energetically preferable to the fcc one at cryogenic temperatures, which results in negative stacking fault energy at these conditions. With increasing temperature, the stacking fault energy increases, suppressing the formation of stacking faults and nano-twins. Our predictions are consistent with recent experimental findings.

Directionally-solidified dendrite morphology with eight secondary arms in an FCC ordered phase alloy

We found a new dendritic solidification morphology with secondary arms growing in eight directions rather than the 〈1 0 0〉 directions expected for a face-centered cubic (FCC) alloy. A bulk single crystal of a Ni-base γ′ single-phase alloy was cast by withdrawing method in a directional solidification furnace. The solidification structure was observed using scanning electron microscopy and analyzed using electron back-scatter diffraction. These results demonstrated that the growth direction of the primary dendrites was [0 0 1], which is known to be the preferred growth direction for FCC metals and alloys. However, the primary dendrite had eight secondary arms growing in the high-Miller-index directions between 〈1 0 0〉 and 〈1 1 0〉. The main factor behind this new dendrite morphology is likely to be the anisotropic atomic stacking of L12 ordered phase, which is the primary phase of the investigated alloy.

Magnetic properties and exchange interaction of rapidly quenched La or Ce substituted nanocrystalline NdFeB alloys with various compositions

The effects of La and Ce substitutions for Nd on the magnetic properties and intergranular exchange coupling of nanocrystalline (Nd1-xMx)yFe94-yB6, (M = La and Ce; y = 10 and 12; x = 0–0.4) alloys have been investigated and presented in order to improve room temperature magnetic properties and reduce the rare earth content for NdFeB based alloys. Optimum magnetic properties such as remanent magnetization Mr, maximum energy product (BH)max and coercivity Hc up to 104 emu/g, 151 kJ/m3 and 9.0 kOe have been obtained for as-quenched single phase (Nd0.95La0.05)12Fe82B alloy. The decrease in Mr for La/Ce substituted alloys is attributed to saturation magnetization reduction. An unusual increase in Hc for 5% La substituted single phase alloy is speculated to the change in microstructure and magnetic phase separation. The analysis of remanence ratio, susceptibility curve, recoil loops and Henkel plots reveal that La substituted alloys exhibit strong ferromagnetic exchange coupling between hard magnetic phases or hard and soft phases than Ce substituted alloys due to the increase of exchange length and refine microstructure by La substitution. The present results indicate that partial substitution of Nd by La or Ce leads to various change behaviors for magnetic properties and inter-grain exchange coupling.

Synthesis of CuInS2 Compound Solar Cell Nanoparticles in an Aqueous Solution By Controlling the Concentration of Metal Complexes

CuInS2(CIS) is one of the most effective photovoltaic devices for developing high stability and low cost solar cell. Bandgap of CIS is 1.5eV while ideal band gap for solar cell is 1.4eV, so CIS is suitable as solar cell materials. Moreover, unlike Cu(In,Ga)Se2(CIGS), it does not contain toxic materials, such as Se [1]. Until now, CIS solar cells have been synthesized by utilizing gas phase method. However, the gas phase evaporation process needs high energy consumption, high initial cost, and large amount loss of resources. For spreading the solar cells greatly, it is required to develop a method which can resolve the above problems, so it is necessary to synthesize CIS solar cells in liquid method. Here, we have already developed a synthesizing method of single phase and homogeneous Copper-Indium (CI) alloy nanoparticles in an aqueous solution. In this method, the concentration ratio of the metal complexes in the solution are calculated by using the critical stability constants, and the reduction reaction rate is also controlled. Synthesized CI nanoparticles were spin coated onto substrate and selenized, consequently it shows the photo voltaic effect. Thus, printable solar cell can be formed by using this method. However, CIS2 nanoparticles were not synthesized until now, because of ligand exchange reaction between the complexing agents and Cu and In. Therefore, in this study, to prevent these undesirable reaction, we tried to control the condition of Cu-In-complexing reagent(s) system precisely by using the calculation. By using these calculation results, CIS2 particles were tried to synthesized in an aqueous solution Results of calculation indicated that iminodiacetic acid (IDA) was appropriate complexing reagent. In the ternary system of Cu-In-IDA, it was calculated that the Copper comlexes and Indium complexes can be restricted to homogenized species, [(Cu2+)(IDA)] (pH3) and [(In3+)(IDA)2] (pH3) under the condition that the total molar concentration of Copper and Indium ions were 5 mM, respectively. Based on this calculation, a solution under the above condition was prepared, and both metal complexes were tried to co-sulfurize. As a results, particles with amorphous like morphology were obtained. Therefore, in order to improve the crystallinity, the particles were heated for 30 min at 500 °C under N2 atmosphere. XRD results of heat treated samples indicated that several peaks with a dominant CIS2 peak were observed. Thus, it can be said that CIS2 nanoparticles were successfully synthesized in an aqueous solution, without undesirable ligand exchange reaction during reaction. These results indicated that homogenize metal complexes system of Cu-In-IDA are essential for the synthesis of CIS2 particles. In our presentation, detailed and another results will be introduced. [1]Recent Development of Thin Film Compound Semiconductor Photovoltaic Cells, Wada, Publication of CMC, 2007

Deformation-induced changes in single-phase Al-0.1Mg alloy

ABSTRACTAl-0.1wt.%Mg has been chosen to explore the effects of deformation on the microstructure of nominally single-phase materials. For these materials, the rate of static grain growth is much higher compared to that of Zener pinned systems and dual-phase alloys, where growth is hindered due to the pinning force exerted by second-phase particles on grain boundaries. Therefore, deformation-induced microstructural changes in single-phase alloys occur without any restricting pinning pressure. This paper illustrates the complex effect of deformation on the microstructural changes mainly associated with dynamic recovery. The process affects the initial microstructure due to an intensive substructure development. Dynamic recrystallisation, associated with the formation of new grains, is considered to be a transient phenomenon that quantitatively influences the mean grain size formed during straining. This work also aims to estimate the stored energy of the deformed regions and texture components to explore their contribution to the migration of high-angle grain boundaries.

Effect of Immersion Time on Corrosion Behavior of Single-Phase Alloy and Nanocomposite Bismuth Telluride-Based Thermoelectrics in NaCl Solution

The corrosiveness of bismuth telluride-based thermoelectric materials (n-type single-phase alloy and a nanocomposite with MoS2 nanoinclusions), in 0.1 molar solution of sodium chloride (NaCl), was investigated. The electrochemical impedance spectroscopy curves obtained after 1, 24, 48 and 72 h immersion time revealed the enhancement of the corrosion resistance of the nanocomposite specimen in a 0.1 molar NaCl solution in comparison with the single-phase bismuth telluride-based alloys, and the passivity increased by immersion time up to 72 h. The nanocomposite sample with submicron grains provided suitable nucleation sites for passive film nucleation that led to higher protective behavior.

Abnormal band bowing effects in phase instability crossover region of GaSe1-xTex nanomaterials

Akin to the enormous number of discoveries made through traditional semiconductor alloys, alloying selected 2D semiconductors enables engineering of their electronic structure for a wide range of new applications. 2D alloys have been demonstrated when two components crystallized in the same phase, and their bandgaps displayed predictable monotonic variation. By stabilizing previously unobserved compositions and phases of GaSe1−xTex at nanoscales on GaAs(111), we demonstrate abnormal band bowing effects and phase instability region when components crystallize in different phases. Advanced microscopy and spectroscopy measurements show as tellurium is alloyed into GaSe, nanostructures undergo hexagonal to monoclinic and isotropic to anisotropic transition. There exists an instability region (0.56 < x < 0.67) where both phases compete and coexist, and two different bandgap values can be found at the same composition leading to anomalous band bowing effects. Results highlight unique alloying effects, not existing in single-phase alloys, and phase engineering routes for potential applications in photonic and electronics.

Effect of processing temperature on the microstructural characteristics of Cu-Ag nanocomposites: From supersaturation to complete phase decomposition

Initially micrometer-sized powder blends of different compositions (Cu-6at%Ag, Cu-37 at%Ag and Cu-84 at%Ag corresponding to Cu-9vol%Ag, Cu-46 vol%Ag and Cu-89 vol%Ag) were deformed by high-pressure torsion for varying applied strain and temperature in order to study their effect on the degree of supersaturation and the prevailing deformation mechanisms. The resulting microstructures were comprehensively characterized with synchrotron X-ray diffraction and transmission electron microscopy. A gradual transition from complete supersaturation to complete phase separation was observed with increasing processing temperature. The critical temperature for full supersaturation strongly depended on the composition. In composites with low Ag and Cu contents single-phase alloys could be obtained up to processing temperatures of 100 °C. In the medium composition range strain localization in shear bands prevented full supersaturation for room temperature deformation. Only in regions deformed by shear bands a complete single-phase supersaturated solid solution was obtained, while the lamellar matrix retained a dual-phase structure. By lowering the processing temperature, using liquid nitrogen as coolant, a homogenous single-phase alloy could be also attained at medium compositions. The present results unravel a clear correlation between dominating deformation mechanisms and the degree of deformation-induced supersaturation.