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The synthesis of a novel multicomponent perovskite was carried out by substituting 5 cations in the A-site of BaTiO3 lattice. The mechanochemical synthesis involved 5 h of ball milling followed by annealing at 800 °C for 2 h and the compound was sintered at 1200 °C for 2 h to form the cubic perovskite phase adopting the P m 3 ¯ m space group. The lattice parameter was found to be 0.391 nm. The reduction in the tetragonality and the local distortion in the compound were observed in the 2D charge distribution maps. The band gap of the compound was found to be 2.94 eV with a dielectric constant value of 270. Ferroelectric hysteresis loops were recorded and the leakage current density was found to be in the order of 10−5 A/cm2.

Sn-doped TiFe0.5Ni0.5Sb1−xSnx (x = 0, 0.05, 0.1, 0.2) were synthesized by vacuum arc melting (VAM). In addition to the half-Heusler phase, secondary phases of Fe–Sb-rich compound and Ti-rich compounds were obtained after VAM. The alloys were then subjected to ball milling for 1 h and 5 h. Ball milling for 1h led to microcrystalline grains, while that for 5 h led to nanocrystalline grains. Ball milling followed by spark plasma sintering (SPS) at 1173 K led to significant reduction in size of secondary phases in the microstructure. The undoped sample exhibited a ZT of 0.008 at 873 K for both 1h and 5h BM-SPS samples.

Rapid solidification during metal additive manufacturing (AM) leads to non-equilibrium microsegregation, which can result in the formation of detrimental phases and cracking. Most of the microsegregation models, assume a Scheil-type solidification, where the solidification interface is planar and there exists local equilibrium at the interface along with either zero or infinite solute diffusion in the respective participating phases - solid and liquid. This assumption leads to errors in prediction. One has to account for finite solute diffusion and the curvature at the dendritic tip for more accurate predictions. In this work, we compare different microsegregation models that do and do not consider finite diffusion and dendrite tip kinetics against the experiments. We also propose a method to couple dendrite tip kinetics with the diffusion module (DICTRA®) implemented in Thermo-Calc®. The models which accounted for both finite diffusion and dendrite tip kinetics matched well with the experimental data.

Kinetics of face-centered cubic (FCC) phase evolution in equiatomic AlCoCrFeNi alloy has been studied in this work, measuring the phase fraction from X-ray diffraction (XRD). As-cast alloy showed a body-centered cubic (BCC)+B2 structure. Heat treatments performed at different temperatures showed the formation up-to 30 to 35 pct FCC phase between 1073 K and 1373 K. A systematic decrease in hardness from 540 to 390 HV10 with increase in temperature suggested an increase in the FCC volume fraction. Kinetics of FCC evolution were analyzed using the Johnson–Mehl–Avrami–Kolmogorov equation and Arrhenius law to calculate the activation energy for the phase transformation. Furthermore, a time-temperature-transformation diagram was constructed from the isothermal transformation studies. Detailed microstructural investigation suggests faster kinetics of FCC phase formation near dendritic boundaries compared to interdendritic regions. The Kurdjumov–Sachs orientation relationship between FCC and BCC phases suggested a coherent interface between these phases. Results of the present study pave the way to decide on heat treatment practices in AlCoCrFeNi alloy.

In this work, crystallization kinetics and phase formation of the Hf64Cu18Ni18 glassy ribbon were studied. It was observed that the ribbon crystallizes via two exothermic peaks. Further, the activa...

A composite of two different medium entropy alloys (MEAs, i.e., CoCrFeNi and AlCoCrFe) was synthesized using ball milling and spark plasma sintering. The composite microstructure contains a homogenous distribution of fcc and bcc phases with submicron-sized grains and exhibits excellent microstructural and phase stability even after 100 h heat treatment at 800 °C. The composite provides a combination of high compressive strength, adequate plastic strain, and multiple strain-hardening stages at room temperature. This first exploratory study on a MEA composite can be used as a template to other systems and illustrates the feasibility of combining two or more MEAs.

High entropy alloys (HEAs) have brought a renewed interest to thermodynamics and kinetics. Proper understanding and treatment of relevant quantities are important for the effective theoretical design framework. Since the ideal mixing treatment is useful, its validity is evaluated through checking the Lupis and Elliot effect on extensive binary data and temperature dependency effect on limited HEA data. The physical basis of different configurational entropy contributions and mismatch entropy are elucidated. New alloy possibilities such as immiscible HEAs are revealed. Influence of elemental atmospheres and crystalline nature of matrices on diffusion in HEAs are discussed.

The thermoelectric efficiency of skutterudite materials can be improved by lowering the lattice thermal conductivity via the uniform dispersion of a nanosized second phase in the matrix of filled C...

The effect of Al addition to CoFeMnNi on the phase evolution and magnetic properties was studied for vacuum arc-melted AlxCoFeMnNi (x = 0, 0.3, 0.7, 1) alloys. These alloys were subsequently homogenized at 1050° C for 50 h and water quenched. The CoFeMnNi and Al0.3CoFeMnNi alloys showed single-phase FCC in as-cast and homogenized conditions. The Al0.7CoFeMnNi alloy showed BCC + FCC phases in as-cast condition, and the phase fraction of FCC phase increased upon homogenization. The AlCoFeMnNi alloy had B2 phase in as-cast condition and had BCC + FCC phases after homogenization. Scheil simulation was performed to predict the phase evolution during casting, and calculation of phase diagram (Calphad) approach was used to predict the phase evolution during homogenization. CoFeMnNi alloy was ferromagnetic in as-cast condition, whereas it exhibits spin or cluster glass behaviour after homogenization. The Al0.3CoFeMnNi alloy remained paramagnetic in both as-cast and homogenized condition. In Al0.7CoFeMnNi alloy, the saturation magnetization increased from 20 to 60 emu/g upon homogenization. In the as-cast alloys, AlCoFeMnNi had a maximum saturation magnetization of 126.2 emu/g, and upon homogenization, the saturation magnetization decreased to 94.3 emu/g due to the phase change associated with it. The BCC/B2 phase with equiatomic or near-equiatomic composition had a high saturation magnetization. The change in magnetic properties is correlated to the phase change associated with Al addition and homogenization.

A new set of half-Heusler high-entropy alloys Ti2NiCoSn1-xSb1+x (x = 0.5, 1), with a valence electron count higher than 18, were investigated for thermoelectric applications. Vacuum arc melting was employed for synthesis. Atom probe analysis confirmed single-phase at atomic level. The alloys were subsequently ball milled for 1 h followed by spark plasma sintering for consolidation. In 1 h BM cases, the alloy with x = 0.5 exhibited a low lattice thermal conductivity of 2.48 Wm−1K−1, and a ZT of 0.29 at 873 K.

W-Cu functionally graded composites (FGCs) up to six layers have been developed using high energy ball milling and spark plasma sintering (SPS) at a lower temperature of 900 °C. The relative density of W-Cu composites increased from 85.4% (W80Cu20 layer) to 95.7% (W20Cu80 layer) with increasing Cu content. All the W-Cu FGCs exhibited a graded structure even after SPS and showed a gradual change in hardness, Young’s modulus, and coefficient of thermal expansion (CTE). Furthermore, W-Cu composites showed a CTE and modulus between those of W and Cu and could be used as an intermediate layer between W and Cu in plasma facing components. The thermal cycle testing at 800 °C has confirmed that the W-Cu FGCs developed in this study can withstand thermal shock and showed a superior performance over directly bonded W-Cu sample. The W-Cu FGCs developed in the present study are not only suitable for plasma facing components but can also be used where the thermal stresses are introduced due to the large mismatch in CTE or elastic modulus.

The synthesis of flower-like morphology of ZnO nanorods labelled as 'ZnO flowerets' via a novel approach comprising of dealloying of mechanically alloyed powder particles of Cu-Zn of mean size 2.9 ± 0.6 μm is reported in the present work. The mechanical alloying of the powder blend was performed in a planetary ball mill. Subsequently, the dealloying was performed in a basic solution of NaOH. The optimization of type of basic solution, pH of the solution, dose of alloy powder and time of dealloying was performed to find the best possible process parameters for dealloying. The ZnO flowerets were characterized by X-ray diffraction (XRD) and field emission electron microscope (FE-SEM) attached with energy dispersive spectrophotometer (EDS). The aspect ratio of ZnO nanorods and the EDS atomic ratio of Zn/Cu in the ZnO flowerets were measured to be ∼ 11 (length = ∼520 nm and width = 48 nm) and ∼ 1.2, respectively. The results of inductively coupled plasma mass spectroscopy (ICP-MS) showed the concentration of Cu and Zn ions to be 25 and 1200 ppm, respectively, in NaOH solution after experimentation for 24 h. It also indicated the predominant dissolution of Zn relative to Cu from the alloyed powder.

The present work reports the effect of Ta on densification and microstructure evolution during non-isothermal and spark plasma sintering of nanocrystalline W. Nanocrystalline W-5wt.%Ta alloy powder was synthesized using mechanical alloying. The nanocrystalline powder was characterized thoroughly using X-ray diffraction line profile analysis. Furthermore, the shrinkage behavior of nanocrystalline powder was investigated during non-isothermal sintering using dilatometry. Subsequently, the alloy powder was consolidated using spark plasma sintering up to 1600 {\deg}C. The role of Ta on stabilizing the microstructure during spark plasma sintering of nanocrystalline W was investigated in detail using electron backscatter diffraction. The average grain size of spark plasma sintered W-5wt.%Ta alloy was observed as 1.73 micron., Comment: 14 pages, 3 figures

Two different processing routes of mechanical alloying followed by the spark plasma sintering (powder metallurgy) and vacuum arc melting (casting route) were employed to understand the role of processing routes on the phase and microstructural evolution in an equiatomic CrMoNbTiW refractory high-entropy alloy. Besides a major BCC solid solution, a small fraction of carbide, σ phase, nitride, and oxide phases were observed in the alloys prepared by the powder metallurgy route in contrast to a single-phase BCC solid solution in the casting route. The milling atmosphere (dry milling in air and Ar) has significantly influenced the phase and microstructural evolution, illustrating the substantial role of contaminants. Good thermal stability of microstructure at high homologous temperatures was shown based on the long-term heat treatment at 1300 °C for 240 h. The phase evolution predictions via Calphad studies were found to be in reasonable agreement with the experimental observations, albeit with some limitations.

High-entropy alloys (HEAs) are a new generation of materials that exhibit unique characteristics and properties, and are demonstrating potential in the form of thermal spray coatings for demanding environments. The use of HEAs as feedstock for coating processes has advanced due to reports of their exceptional properties in both bulk and coating forms. Emerging reports of thermal sprayed HEA coatings outperforming conventional materials have accelerated further exploration of this field. This early-stage review discusses the outcomes of combining thermal spray and HEAs. Various synthesis routes adopted for HEA feedstock preparation and their properties are discussed, with reference to the requirements of thermal spray processing. The HEA feedstock is then compared and correlated with coating microstructure and phase composition as a function of the thermal spray processing route. Subsequently, the mechanical behavior of thermal spray HEA coatings is summarized in terms of porosity, hardness, and tribological properties, along with their oxidation and electrochemical properties, followed by their potential applications. The thermal spray methods are contrasted against laser cladding and surface alloying techniques for synthesizing thick HEA coatings. Furthermore, HEAs that have displayed excellent properties via alternative processing routes, but have not been explored within the framework of thermal spray, are recommended.

Thermal stability of mechanically alloyed nanocrystalline AlxCoCrFeNi (x = 0, 0.3, 0.6, 1 mol) high entropy alloys (HEAs) has been investigated for the low and intermediate temperature range of 673–1073 K. Single phase FCC structure is observed in the as milled CoCrFeNi. A mixture of FCC and BCC phases is exhibited by × = 0.3, 0.6 and 1, alloys where the volume fraction of BCC increases with increasing Al content. Phase evolution in heat-treated AlxCoCrFeNi HEAs proceeds via increasing BCC fraction at 673 K, followed by subsequent reduction at elevated temperatures. For each alloy, the major phase observed in as milled condition and it is retained even after prolonged exposure at the 1073 K. Al favors the formation of the BCC phase due to its high affinity to form ordered B2 structures with constituent elements Co, Fe and Ni. Thermal exposure of AlxCoCrFeNi HEAs also leads to the formation of Cr7C3, owing to the higher negative free energy of carbide formation for Cr among other constituents. Transmission electron microscopy (TEM) investigations substantiated that nanostructure of milled powder is maintained even after the heat treatment. Grain growth factor for quinary HEAs is relatively lower than quaternary CoCrFeNi owing to their slower rates of diffusion.

Self-diffusion in ordered pseudo-binary multicomponent aluminides is investigated. Equiatomic substitution of transition metal sublattice in aluminides is performed to produce multicomponent alloys Al(CoNi), Al(FeNi), Al(CoFe), Al(CoFeNi), Al(CoFeMnNi) and Al(CoCuFeMnNi). X-ray diffraction and electron microscopy confirm the presence of single phase ordered BCC structure for all the alloys. Radioactive isotopes of 57Co, 59Fe, 54Mn, 63Ni and 65Zn are used to measure the tracer diffusion coefficients in all the alloys at 1273 K. The influence of multi-principal element alloying of the transition metal sublattice on self-diffusion is mapped to its impact on the defect structure in these ordered alloys.

Entropy stabilization was attempted through the addition of 5 different rare earth cations, 5 different transition metal cations and all these 10 cations in the form of three compounds based on NiFe2O4. Equiatomic ratios of these cations were used to maximize the entropy stabilization effect. The compounds attempted and investigated were NiFe1.9(Dy0.02Er0.02Gd0.02Ho0.02Tb0.02)O4, (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)Fe2O4 and (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)Fe1.9(Dy0.02Er0.02Gd0.02Ho0.02Tb0.02)O4. Synthesis of these compounds were carried out by sol–gel technique and all the compounds were found to crystallize in F d 3 ¯ m spinel structure indicating the entropy stabilization. An appropriate calculation of configurational entropy of mixing employing the sublattice model is demonstrated for such entropy stabilized oxides.

Filling the voids of cage forming compounds with loosely bound electropositive elements and by incorporating nano-sized secondary phases are promising approaches to enhance the thermoelectric figure of merit of these materials. Hence, in this work, by combining these two approaches-inserting In into the voids of skutterudite Co4Sb12 as well as dispersing nanoparticles (GaSb)-we have synthesized various samples via ball-milling and spark plasma sintering. InSb as the secondary phase of the matrix, mixed with GaSb, forms the solid solution Ga1-xInxSb. Nanocrystalline grains together with a few larger grains (10-30 μm) are found to be spread in In0.2Co4Sb12. The former is comprised of either InSb, GaSb or Ga1-xInxSb. Because of their identical space group and similar lattice parameters, InSb, GaSb and Ga1-xInxSb could not be detected separately in EBSD. High resolution transmission electron microscopy (HRTEM) was used to resolve different phases, which showed GaSb grains of size ∼10-30 nm and InSb grains of size ∼30-100 nm. Scattering of charge carriers at the interfaces of InSb, GaSb and Ga1-xInxSb as well as the matrix phases increased both the electrical resistivity and Seebeck coefficient. The multi-scale size distribution of grains, of both the matrix phase and the secondary phases, scattered phonons within a broad wavelength range, resulting in very low lattice thermal conductivities. As a result, an enhanced figure of merit of 1.4 was achieved for the (GaSb)0.1 + In0.2Co4Sb12 nanocomposite at 773 K.

Developing ductile refractory BCC alloys has remained a challenge. The intrinsic ductility (D) of an alloy is the ratio of surface energy ($\gamma_s$) and unstable stacking fault energy ($\gamma_{usfe}$). Lowering the valence electron concentration has been shown to improve the intrinsic ductility of refractory alloys. However, Re has been widely used to ductilize W, contrary to the low valency criteria suggested in the literature. Here we use density functional theory to calculate the enthalpy of formation, $\gamma_{usfe}$ and $\gamma_s$ of Group IV, V, VI elements and their 25 equiatomic binary alloys in BCC crystal structure. We found that positive enthalpy leads to a considerable reduction in $\gamma_{usfe}$ compared to composition averaged value, resulting in improved intrinsic ductility. Enthalpy is maximum at the equiatomic concentrations indicating the highly repulsive interaction between the alloy constituents and vicer-versa. We found that the repulsive interaction between the alloy constituents leads to a reduction in $\gamma_{usfe}$, making alloys intrinsically ductile., Comment: 19 pages, 6 figures

High entropy alloys (HEAs) are an established new class of materials exhibiting excellent mechanical and functional properties. The well-studied AlCoCrFeNi HEA has been modified to AlCoCr0.5Ni HEA in view of the latter's better oxidation resistance. The elimination of Fe and reduction in Cr content is expected to improve the HEA's corrosion and tribological performance. Thus, an AlCoCr0.5Ni HEA coating was fabricated via the atmo-spheric plasma spray (APS) process using mechanically alloyed (MA) feedstock. The microstructural characteristics of both the MA feedstock and coating have been investigated. The sliding wear behaviour of the HEA coating was evaluated at both room temperature and 500 ?C. In addition, the electrochemical compatibility of the coating was analyzed in seawater. The microstructural results revealed that the MA feedstock was composed of BCC/B2 and FCC phases, which were retained in the HEA coating along with minor oxides. The wear resis-tance of the AlCoCr0.5Ni HEA coating was superior to the AlCoCrFeNi HEA coating both at room temperature and 500 ?C. The high wear resistance of the coating was attributed to the formation of equal concentrations of the BCC/B2 and FCC phases. The coating exhibited a greater tendency of being cathodic (passive) than both an AlCoCrFeNi HEA coating and SS316L with identical polarization behaviour. Refereed/Peer-reviewed

In the present investigation, nanocrystalline W and W-xRe (x = 3, 5 wt.%) alloy powders were produced by mechanical milling/alloying using high energy ball milling. The nanocrystalline nature (∼50 nm) of these powders was validated by the Rietveld refinement of their respective X-Ray diffraction patterns. Subsequently, spark plasma sintering of the ball milled powders was carried out. It was observed that pure W was not able to densify completely (relative density of 93%) at a temperature of 1500 °C. However, the addition of 5 wt.% Re resulted in near complete densification (relative density of 97%) at the same sintering temperature. The enhanced densification of W-Re powders is mainly attributed to the ductilising effect of Re assisted by the nanocrystallinity of powders, and the application of pressure during sintering.

In the present work, the concept of mechanically activated annealing (MAA) has been applied to produce nanocrystalline Al0.3CoCrFeNi high-entropy alloys (HEAs) with reduced contamination levels. Phase evolution during conventional mechanical alloying (MA), MAA and subsequent consolidation by spark plasma sintering (SPS) have been studied in detail. Complete alloying is obtained after 15 h of MA, while milling time of 5 h and annealing at 1100 °C for 1 h have been used to achieve alloy formation during MAA. Both the MA–SPS and MAA–SPS routes have shown major FCC phase. The contamination of WC observed during MA was successfully eliminated during MAA, while the volume fraction of Cr7C3 reduced from 20% during MA–SPS to 10% after MAA–SPS. This method can serve as an effective way to produce nanostructured HEAs with minimum contamination.

This is the first report discussing the synthesis of hydrophobic alloy powders consisting of Ni and transition metal nitride (vanadium nitride (VN)) at different proportions through mechanical alloying. The milled alloy powder showed very good resistance to wetting when it was placed in a beaker containing water. The maximum contact angle of 150° with water was recorded for the alloy composition of Ni-75 (wt.%) VN when the powder was loosely sprayed on a glass slide. Few working examples also elucidated the hydrophobic nature of the as-prepared alloy powder. The optimised alloyed powder composition and its phase and morphology as well as the time of milling for maximum hydrophobicity were established with the help of X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) for phase and morphology analysis, respectively. The unique chemistry of toluene with elemental Ni and transition metal nitride VN as characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy led to the development of hydrophobicity in the ball milled powder.

Nanocrystalline CoCrFeNi and CoCrFeMnNi high entropy alloys have been processed by mechanical alloying followed by spark plasma sintering. Grain growth kinetics has been estimated for both the alloys by subjecting them to heat treatment in the temperature range 1073–1373 K. These alloys possess a thermally stable single phase FCC structure along with Cr7C3 contamination. Electron back scattered diffraction (EBSD) has been used to determine grain size of all the heat treated samples. Both CoCrFeNi and CoCrFeMnNi alloys exhibit a grain growth exponent, n = 3, suggesting long-range diffusion-controlled grain growth in these alloys. Activation energies for grain growth are 134 and 197 kJ/mol for CoCrFeNi and CoCrFeMnNi, respectively, which are significantly lower than the activation energy of lattice diffusion in these alloys. Hardness is measured for CoCrFeMnNi alloy as function of grain size and is found to follow the Hall-Petch type relation. The strength coefficient (slope of Hall-Petch relation) is calculated as 1.92 GPa, which is nearly three times that of the value reported in literature for coarse grained CoCrFeMnNi. Presence of carbides enhances the hardness of these HEAs. The maximum contribution to strengthening comes from the FCC-carbide phase boundaries.

Mechanical alloying (MA) followed by sintering has been one of the most widely adopted routes to produce nanocrystalline high-entropy alloys (HEAs). Enhanced solid solubility, room temperature processing, and homogenous alloy formation are the key benefits provided by MA. Spark plasma sintering has largely been used to obtain high-density HEA pellets from milled powders. However, there are many challenges associated with the production of HEAs using MA, which include contamination during milling and high propensity of oxidation. The present review provides a comprehensive understanding of various HEAs produced by MA so far, with the aim to bring out the governing aspects of phase evolution, thermal stability, and properties achieved. The limitations and challenges of the process are also critically assessed with a possible way forward. The paper also compares the results obtained from high-pressure torsion, another severe plastic deformation technique.

Cold-sprayed high-entropy alloy (HEA) coatings have been generated for the first time. Mechanically alloyed (MA) AlCoCrFeNi powder was chosen as feedstock, owing to the extensive literature on this alloy. Coatings were synthesized under various gas temperature and pressure conditions. Isothermal oxidation was conducted at 1100 °C for 25 h on the coating cold-sprayed at 400 °C and 10 bar on a Ni-base superalloy substrate. The as-sprayed coating retained the MA phases and formed a protective alumina layer upon oxidation. An interdiffusion zone at the interface and unanticipated Mo diffusion from the superalloy substrate into the coating were observed after oxidation. A comprehensive characterization at the coating—substrate interface suggests that diffusion in HEAs is not sluggish. The factors governing the coating’s oxidation are elucidated, and a plausible oxidation mechanism is discussed. These studies are aimed at developing oxidation-resistant HEA coatings for potential applications at high operating temperatures.

The present work focuses on the phase evolution and thermal stability of CoCrFeNi and CoCrFeMnNi equiatomic high entropy alloys in the temperature range 1073–1373 K. For comprehensive understanding, phase stability of ternary CoFeNi has also been studied in the same temperature domain. These alloys have been prepared by vacuum arc melting followed by homogenization at 1473 K for 50 h. All the three alloys show a single phase FCC structure, which is retained even after thermal exposure at 1073, 1173 and 1373 K for 96 h. Electron microscopy investigations confirm single phase structure, coarse grain size and equiatomic composition for all the as-processed and heat treated alloys. Binary Gibbs energy-composition (G-x) curves, obtained using a CALPHAD database, have been used to assess the thermal stability of CoFeNi, CoCrFeNi and CoCrFeMnNi. Driving force (D) has been calculated to measure the tendency for BCC and σ phase formation in each of the alloy. These calculations indicate that the FCC phase is the most stable in all the three alloys. Phases in heat treated CoCrFeNiV alloy reported in literature, are compared with the present alloys. Our results indicate that increase in configurational entropy need not always correlate with enhanced stabilization of solid solution phases. G-x diagrams present a powerful approach to elucidate the phase formation and stability of multicomponent alloys.

In the present study, the phase evolution and microstructure of CrMoNbTiW, a new equi-atomic refractory high-entropy alloy, are studied. The alloy was synthesized through mechanical alloying (MA) followed by consolidation using spark plasma sintering. After MA, a major BCC solid solution along with residual Cr and Nb were observed. However, secondary phases such as Laves and carbides were also observed in addition to the major BCC solid solution after sintering. Unsolicited contamination from the milling media is found to be one of the reasons for the formation of secondary phases. The high hardness of 8.9 GPa after sintering was attributed to the presence of secondary phases along with the nanocrystalline nature of the alloy. To understand the phase evolution, calculation of phase diagram was carried out using CALPHAD. Further, binary phase diagram inspection and simple empirical parameters were also used to assess their effectiveness in predicting phases.

Present work reports the synthesis of ZnO flowerets consisting of nanorod petals of ZnO having a mean aspect ratio ∼9, supported on micron sized Ni-enriched particles of size in the range of ∼1–4 μm and also free-standing pure ZnO nanorods from mechanically alloyed Ni-Zn powder particles via selective leaching in NaOH solution. Optimization of the composition of the initial alloy powder, the concentration of NaOH and time of exposure in the solution was carried out to get to the ZnO flowerets with an increment in the surface area of the order of 400%. The mechanism and reaction chemistry of ZnO nanorods formation and growth were explained based on the evidence of various characterization techniques including inductive coupled plasma mass spectroscopy (ICP-MS) and electrochemical measurements. Free-standing pure ZnO nanorods were also synthesized by ultrasonically breaking the rod petals of ZnO from the base of the flowerets. Free-standing ZnO nanorods have a mean aspect ratio of 8.5 ± 4, where the length and average diameter are ∼356 ± 64 nm and 42 ± 16 nm, respectively, and the specific surface area of 12.5 m2 g−1 with an increment of ∼650% as compared to the mechanically alloyed powder particle.

High entropy alloys (HEAs) have emerged as promising class of materials having equiatomic or near equiatomic multicomponent configurations. Nanostructured HEAs have added a new facet to the development of these alloys, showing remarkable strength and functional properties. The present work examined the phase evolution and thermal stability of nanocrystalline CoCrFeNi and CoCrFeMnNi HEAs prepared through mechanical alloying (MA) followed by spark plasma sintering (SPS). After MA, both the alloys showed single phase FCC structure with minor fractions of tungsten carbide arising due to contamination from milling media. After SPS, the major phase remained as FCC in both the alloys along with Cr7C3 evolution. Phase stability of CoCrFeNi and CoCrFeMnNi HEAs, (MA powders and SPS pellets) were investigated in the temperature range 1073–1373 K up to 96 h. Formation of Cr7C3, concomitant with Cr-depletion in FCC matrix, was observed on heat treatment of MA powders. SPS alloys retained their mixture of FCC + Cr7C3 on thermal exposure with minimal change in the fraction of carbides. The presence of single phase field in their respective phase diagrams, similar atomic sizes of constituents and non-equilibrium nature of MA combined to lend a highly stable FCC phase in the nanocrystalline HEAs studied.

The present article presents the effect of Sc addition on the secondary dendritic arm spacing, eutectic Si modification, intermetallic phase modification, aging behavior and subsequently the mechanical behavior in A356-5TiB2 in-situ composite. The A356-5TiB2 in-situ composite showed a 40% reduction in SDAS compared with A356 alloy, whereas addition of 0.4 wt% Sc and 5 wt% TiB2 in A356 alloy resulted in a 65% reduction in SDAS. The eutectic Si in A356-5TiB2 composite had a plate like morphology similar to A356 alloy but showed a reduction in length, whereas the addition of Sc resulted in a fibrous and particulate morphology of eutectic Si. The needle like β-Al5FeSi phase in A356-5TiB2 composite changed to Al5Fe(Si,Sc) phase having smaller size and irregular morphology with the addition of Sc. The intermetallic phase formation and phase modification was confirmed and characterized using X-ray diffraction, Transmission electron microscopy - Energy dispersive spectroscopy and Scanning electron microscopy. Addition of Sc to A356-5TiB2 in-situ composite resulted in improved mechanical properties. Further, T6 heat treatment revealed a significant improvement in mechanical properties for A356-5TiB2 in-situ composite with and without Sc addition.

The present work reports the role of different atomic mobility induced localized pore evolution on densification during spark plasma sintering of nanocrystalline W-Mo alloy powder. The shrinkage (or expansion) behavior of cold compacted milled powders was studied using dilatometry during non-isothermal sintering up to 1600 {\deg}C. Subsequently, the milled powders were densified to ~95% relative density using spark plasma sintering up to 1600 {\deg}C. The enhanced localized Joule heating due to dynamically evolved porous structure could be attributed for the densification during spark plasma sintering., Comment: 13 pages, 4 figures

The combination of technical advantages of high entropy alloys (HEAs) and manufacturing capabilities of thermal spray (TS) offer potential towards new protective coatings to address extreme engineering environments. In this research, equi-atomic AlCoCrFeNi HEA coatings were synthesized via atmospheric plasma spray (APS) using mechanically alloyed feedstock, and a correlation between microstructure and mechanical properties in terms of both hardness and wear were established at multiscale levels. In addition, electrochemical performance in sea water and the overall residual stress distribution in the HEA coatings were also assessed. Superimposition of scanning electron micrographs and statistically analysed heat and contour maps using nanoindentation datasets revealed deviations in localized properties within and across individual phases; which were supported by Weibull plots of individual phases. Scanning wear tests revealed superior nanowear resistance of oxide phases developed by in-flight oxidation during APS process. In comparison, the HEA phases in the coating exhibited significant localized plastic deformation. The outcome of macroscale wear testing postulated that plasma sprayed AlCoCrFeNi HEA coatings exhibited superior wear resistance at high temperature (500 °C) than at room temperature, signifying high thermal stability of the coating. Residual stress generated due to plasma spray was measured using neutron diffraction and was tensile in nature. The corrosion resistance of the coating was slightly lower than that of SS316L, however, the anodic and cathodic polarization behaviour of HEA coating were identical to that of SS316L, indicating that the AlCoCrFeNi-based HEAs have prospects as corrosion resistant materials. Refereed/Peer-reviewed

We report a systematic investigation of the microstructure and thermoelectric properties of refractory element-filled nanostructured Co4Sb12 skutterudites. The refractory tantalum (Ta) metal-filled Co4Sb12 samples (Ta x Co4Sb12 (x = 0, 0.4, 0.6, and 0.8)) are synthesized using a solid-state synthesis route. All the samples are composed of a single skutterudite phase. Meanwhile, nanometer-sized equiaxed grains are present in the Ta0.2Co4Sb12 and Ta0.4Co4Sb12 samples, and bimodal distributions of equiaxed grains and elongated grains are observed in Ta0.6Co4Sb12 and Ta0.8Co4Sb12 samples. The dominant carrier type changes from electrons (n-type) to holes (p-type) with an increase in Ta concentration in the samples. The power factor of the Ta0.6Co4Sb12 sample is increased to 2.12 mW/mK2 at 623 K due to the 10-fold reduction in electrical resistivity. The lowest lattice thermal conductivity observed for Ta0.6Co4Sb12 indicates the rattling action of Ta atoms and grain boundary scattering. Rietveld refinement of XRD data and the analysis of lattice thermal conductivity data using the Debye model confirm that Ta occupies at the voids as well as the Co site. The figure of merit (ZT) of ∼0.4 is obtained in the Ta0.6Co4Sb12 sample, which is comparable to single metal-filled p-type skutterudites reported to date. The thermoelectric properties of the refractory Ta metal-filled skutterudites might be useful to achieve both n-type and p-type thermoelectric legs using a single filler atom and could be one of replacements of the rare earth-filled skutterudites with improved thermoelectric properties.

A novel tracer-interdiffusion couple technique is utilized to measure the concentration-dependent tracer- (59Fe) diffusion coefficients in the B2-ordered Al50(Ni,Fe)50 alloys at 1373 K in a single experiment. The influence of variable molar volume on the diffusion coefficients is examined. The tracer diffusion coefficients determined under chemical gradients are shown to be in an excellent agreement with the diffusivities measured independently for homogeneous alloys. The tracer diffusion coefficients of Fe are observed to increase strongly with an increase of the Fe concentration. Strong up-hill diffusion of Al hinders a reliable determination of the interdiffusion coefficients with a single diffusion couple. The results confirm a high potential of the tracer-interdiffusion couple technique for producing highly accurate concentration-dependent mobility data in multi-component systems.

For the first time, multicomponent alloys belonging to a B2-ordered single phase were designed and fabricated by melting route. The design concept of high entropy alloys is applied to engineering the transition metal sublattice of binary B2 aluminide. The equiatomic substitution of transition metal elements in the Ni sublattice of binary AlNi followed to produce Al(CoNi), Al(FeNi), Al(CoFe), Al(CoFeNi), Al(CoFeMnNi), and Al(CoCuFeMnNi) multicomponent alloys. CALculation of PHAse Diagrams (CALPHAD) approach was used to predict the phases in these alloys. X-ray diffraction and transmission electron microscopy were used to confirm the B2 ordering in the alloys. Thermal stability of the B2 phase in these alloys was demonstrated by prolonged heat treatments at 1373 K and 1073 K up to 200 h. © 2020 by the author. Licensee MDPI, Basel, Switzerland.