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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.

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.

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

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.

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.

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

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.

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.

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.

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.

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 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

Low-temperature synthesis of aluminium boride (AlB12) powders by mechanochemical synthesis of Al and B powder blend has been carried out. The influence of milling time on the phase formation temperature and annealing time of AlB12 was investigated. X-ray diffraction of the milled and annealed powder samples revealed the formation of AlB2 without any oxide phases. AlB12 powder was synthesized at a significantly low temperature of 1030 K. The processed AlB12 powder was nanocrystalline in nature confirmed by transmission electron microscopy investigations. It has been shown that mechanochemical synthesis could be an attractive route for the synthesis of borides at low temperatures.

AlSb is a low-cost, high-band-gap semiconducting material, yet largely limited in its potential due to difficulty in the synthesis of single-phase AlSb. The present work compares different processing routes in their potency to produce bulk and nanocrystalline AlSb. Vacuum arc melting has been successfully employed, for the first time, to synthesize single-phase bulk AlSb. Owing to high vapor pressure of Sb, an optimum amount of excess Sb (3%) needs to be added to achieve single-phase line compound AlSb. Two methods, viz. mechanical alloying of elemental powders and mechanical milling of cast alloy, have been used to synthesize nanocrystalline AlSb. Nanocrystalline AlSb could be produced after ball milling the cast alloy; however, prolonged milling results in the appearance of Sb along with a non-stoichiometric Al-Sb-O oxide. Mechanical alloying of Al and Sb does not lead to appearance of single-phase AlSb, and the mixture largely consists of starting material and amorphous products. Therefore, the effectiveness of a synthesis route in producing single-phase AlSb is governed by the tendency of oxygen pickup during the processing.

A simple and completely predictive model has been developed to predict whether a multicomponent equiatomic alloy will form a single phase BCC, FCC, HCP or a combination of two or more solid solution phases or intermetallic compounds (IM) or an amorphous phase. This approach is based on the viscosity of alloys as a function of temperature, utilising the viscosities of its constituting elements, and suitably incorporating the crystal structure information. Some other parameters affecting viscosity of an alloy like atomic size of constituting elements, packing density of the unit cell, etc., are suitably incorporated into the model. The temperature-time-transformation (TTT) diagrams were generated with the help of the viscosity data of five widely experimentally examined alloys, CoCrCuFeNi, CoCrFeMnNi, AlCoCrFeNi, AlCuMgMnZn and ZrTiCuNiBe. The chance of formation of preferable lower order alloys has also been considered. In this regard, all the possible binary to quinary alloys that can form from the constituting elements have been studied. The formation of the single phase BCC, FCC, or formation of multi phases, IMs or an amorphous phase in these alloys has been excellently predicted by the model. It has also been revealed that AlCuMgMnZn alloy prefers to form a number of IMs with a rare HCP phase, which matches excellently with the experimental evidence. The most important part of the present work is that it acts as an efficient guide about the processing route that should be used to form an intended phase in a particular alloy via the critical cooling rate Rc obtained through the predicted TTT diagrams. Further, two alloys (AlCoCrFeNi and CoCrFeMnNi) could not be vitrified even via melt spinning route as predicted by the model. Almost all the equiatomic alloys found so far ranging from ternary to octanary according to literature have been studied by the present model. The phase formation in most of the alloys has been predicted correctly by the model.

The mechanism and kinetics of dealloying of mechanically alloyed Ag25Zn75 powder particles was investigated at four different temperatures under free corrosion conditions in freely aerated 2 N HCl. The activation energies of 30.65 ± 2.08 kJ/mole and 41.51 ± 3.30 kJ/mole, corresponding to the dissolution of Zn and pore coarsening, respectively, were obtained. The pore formation is controlled by Zn dissolution from the alloy and coarsening is controlled by diffusion of Ag atoms on the alloy/electrolyte interface. Ag rearrangement was found to be the rate controlling phenomenon for the overall dealloying.

Microstructural and magnetic properties of AlNiCo based high entropy alloys have been investigated. Equiatomic AlNiCo alloy exhibits superparamagnetic like behavior but addition of elements like Cu and Fe significantly alter the microstructure and magnetic properties. From the present study, it is clear that the addition of Cu leads to phase separation and significant increase in coercivity. On the other hand, addition of Fe leads to enhancement in ferromagnetic exchange interaction that in turn results in the development of soft magnetic behavior in AlNiCoCuFe alloy.

High entropy alloys (HEAs) have emerged as a promising class of equiatomic or near equiatomic multicomponent alloys, which garner fundamental curiosities and interest in high temperature applications. Understanding diffusion kinetics of HEAs is critical to assess their phase stability and deformation behaviour, particularly at elevated temperatures. For the first time, bulk tracer diffusion coefficients of Co, Cr, Fe and Mn are determined in polycrystalline CoCrFeNi and CoCrFeMnNi HEAs using the radiotracer method in the temperature interval of 1073–1373 K. Material homogeneity and the absence of any phase decomposition in CoCrFeNi and CoCrFeMnNi HEAs were established by electron microscopy and atom probe tomography investigations. Both bulk and grain boundary diffusion contributions to penetration profiles are observed for diffusion of Co, Cr, Fe and Mn tracers in both HEAs. The temperature dependencies of bulk diffusion for all tracers show Arrhenius behaviour. The corresponding activation energies (Q) and the logarithm of pre-exponential factors (D0) show a linear relationship, thus following the “compensation rule”. An increase of the configurational entropy leads to reduced diffusion rates only when a homologous temperature scale is used for comparison. The increase of activation energy barrier and lower frequency factors both contribute to the decreased diffusion rates. A cross-over temperature (Tc = 1020 K) is observed for Co diffusion (on slight extrapolation of Arrhenius plot) in CoCrFeNi and CoCrFeMnNi HEAs, while Cr and Fe exhibit almost parallel Arrhenius lines. Above Tc, the Co diffusivity is higher in CoCrFeMnNi than in CoCrFeNi, which suggests that diffusion in HEAs need not be assumed to retard with an increasing number of elements. The existence of a cross-over temperature correlates with the change in binding energy (or enthalpy) of the constituents from CoCrFeNi to CoCrFeMnNi.

Out of several methods, one of the most explored strategies to decrease the lattice thermal conductivity of Co4Sb12-based materials are either filling suitable electropositive elements into the voids or the formation of nanocomposites. These two approaches were combined in this work by filling Yb into the void of Co4Sb12 and preparing nanocomposites of Yb0.2Co4Sb12 and InSb according to the formula (InSb)x + Yb0.2Co4Sb12 (where x = 0.1, 0.2, 0.3, 0.4), via ball-milling and spark plasma sintering. Yb2O3 and CoSb2 as impurity phases were found at the grain boundaries. EBSD and TEM micrographs showed nanocrystalline InSb phase (20–200 nm) dispersed in the matrix grains. The charge transfer from Yb filler with an oxidation state of +3 to Co4Sb12 yielded a low electrical resistivity (ρ) of the matrix. An increase in ρ and Seebeck coefficient (S) in the composites with x = 0.1 and 0.3 occurred due to the higher amount of oxide impurities in these two samples and the scattering of charge carriers at the interfaces induced by the secondary phases. The other two composites with x = 0.2 and 0.4 exhibited ρ(T) and S(T) similar to the Yb0.2Co4Sb12 matrix. The dispersion of the InSb and Yb2O3 phases at the grain boundaries combined with the anharmonicity introduced by the fillers (Yb) in the voids enhanced the scattering of phonons within a broad wavelength range and reduced the lattice thermal conductivity significantly. Hence, a highest zT of ~1.2 at 773 K with a thermoelectric efficiency of 8.89% and 8.28% (423–773 K) were obtained for (InSb)0.1 + Yb0.2Co4Sb12 and (InSb)0.2 + Yb0.2Co4Sb12 nanocomposites, respectively.

The conventional way to form a multicomponent alloy requires mixing individual elements in a single step. Phase formation, therefore, is governed by inherent thermodynamic and kinetic factors of the system. We propose here a new approach for multicomponent alloy synthesis, which involves step by step addition of constituent elements. In the present work, this is illustrated through the formation of nanocrystalline equiatomic AlCoCrFeNi by mechanical alloying. For example, first, binary CoNi is formed by milling elemental Co and Ni powders. In the subsequent steps, Fe, Cr and Al are added to form ternary (CoNiFe), quaternary (CoNiFeCr) and quinary (CoNiFeCrAl) alloy, respectively.Three different classes of binaries have been selected as initial phases, namely, B2 (AlNi, AlCo and AlFe), BCC (FeCr) and FCC (CoNi and FeNi). Remaining constituent elements are added step-wise in varying sequences and equiatomic AlCoCrFeNi is obtained in the concluding step. The final AlCoCrFeNi alloy at the end of each sequence has varied fractions of BCC and FCC phases. For instance, AlNi+Co+Fe+Cr sequence is single phase BCC, whereas the sequence FeNi+Co+Cr+Al results in a mixture of BCC (75%) and FCC (25%) phases. The extent to which a particular element stabilizes a structure (BCC/B2/FCC) has also been elucidated. Keywords: High entropy alloys, Mechanical alloying, X-ray diffraction, Nanocrystalline, Sequential alloying

In this study, dense fine-grained ZrB2 and TiB2 were fabricated using reactive spark plasma sintering (RSPS) of ball-milled Zr/B and Ti/B mixtures. Systematic investigations were carried out to understand the mechanisms of reactive sintering. Two densification mechanisms were found to be operating during RSPS. The first stage of densification was due to self-propagating high temperature synthesis reaction leading to formation of ZrB2 and TiB2 compacts having relative density of similar to 48 and similar to 65%, respectively. The second stage of densification occurred at temperatures more than 1100 degrees C and resulted in final relative density of more than 98%. Electron backscatter diffraction and electron microscopy studies on interrupted RSPS samples as well as dense samples showed deformed grains and presence of slip steps while grain orientation spread map and pole figure analysis confirmed plastic flow. Plastic flow-aided pore closure is shown as major mechanism during reactive sintering.

The present study reports the effect of Sn substitution on the structural and thermoelectric properties of synthetic tetrahedrite (Cu12Sb4S13) system. The samples were prepared with the intended compositions of Cu12Sb4-xSnxS13 (x = 0.25, 0.35, 0.5, 1) and sintered using spark plasma sintering. A detailed structural characterization of the samples revealed tetrahedrite phase as the main phase with Sn substituting at both Cu and Sb sites instead of only Sb site. The theoretical calculations using density functional theory revealed that Sn at Cu(1) 12d or Cu(2) 12e site moves the Fermi level (E-F) toward the band gap, whereas Sn at Sb 8c site introduces hybridized hole states near E-F. Consequently, a relatively high optimum power factor of 1.3 mW/mK(2) was achieved by the x = 0.35 sample. The Sn-substituted samples exhibited a significant decrease in the total thermal conductivity (kappa(T)) compared to the pristine composition (Cu12Sb4S13), primarily because of reduced electronic thermal conductivity. Due to an optimum power factor (1.3 mW/mK(2)) and reduced thermal conductivity (0.9 W/mK), a maximum zT of 0.96 at 673 K was achieved for x = 0.35 sample, which is nearly 40% increment compared to that of the pristine (Cu12Sb4S13) sample.

Oxidation behaviour of five high entropy alloys (HEAs): AlCoCrFeNi, Al2CoCrFeNi, Al2CoCr1.5FeNi, AlCoCrNi and AlCoCr0.5Ni, has been studied at 1100 °C in the cast-annealed condition. Al2CoCrFeNi and AlCoCr0.5Ni HEAs exhibit the lowest oxidation rates of the order of 10−12 g2 cm−4-s−1. Al-rich nanoscale precipitates are observed to aid protective oxide growth. The Al-lean phase fraction and its Al content also have a major effect on the alloy’s oxidation behaviour. A parameter ‘X’ taking the ratio of these two has been introduced, which coupled with CALPHAD predictions can be a potential tool to shortlist HEAs for oxidation resistance applications.

High entropy alloys (HEAs) have drawn considerable attention owing to their unique properties such as high fracture toughness, good strength–ductility combination and enhanced corrosion resistance. In this study, the corrosion resistance dependence on the crystal structure and grain size of AlCoCrFeNi HEA is investigated. AlCoCrFeNi HEA with different mixture of body centered cubic (BCC) and face centered cubic (FCC) phases is produced using sequential alloying and tested for corrosion resistance in 3.5 wt% NaCl solution. CoNi+Fe+Cr+Al (84% BCC), FeCr+Ni+Al+Co (62% BCC) and AlNi+Co+Cr+Fe (38% BCC) alloy sequences have corrosion potential of −454, −299 and −524 mV (vs. SCE), and corrosion current of 14, 0.4 and 12 µA, respectively. FCC is a tight binding lattice with higher packing fraction than BCC which made it better for corrosion resistance, but FCC is rich in elements like Co, Ni, and Fe which are easily corroded. These two competing effects lead to a nearly optimum corrosion resistance for FeCr+Ni+Al+Co alloy sequence with 62% BCC and 32% FCC. It is also observed that an increase in grain size improves corrosion resistance. The influence of different chemical elements, crystal structure and microstructure (coarse vs. nanocrystalline) on corrosion resistance is discussed.

High entropy alloys (HEAs) are multicomponent alloys in equiatomic or nearly equiatomic composition. Anticipated sluggish atomic diffusion is reported to be one of the core effects in HEAs which is presumably responsible for their many unique properties. For the first time, in the present study, tracer (Ni) diffusion in CoCrFeNi and CoCrFeMnNi alloys is measured by the radiotracer technique in the temperature range of 1073–1373 K using the 63 Ni isotope. Chemically homogeneous alloys of equiatomic composition were prepared by a vacuum arc melting route. The microstructure and phase stability of the alloys in the given temperature range is confirmed by differential thermal analysis and X-ray diffraction. Ni diffusion in both CoCrFeNi and CoCrFeMnNi alloys is found to follow Arrhenius behavior. When plotted against the homologous temperature, a tendency to a successive slow down of the tracer diffusion rate with an increased number of components in equiatomic alloys is unambiguously established. Both the entropy term as well as the energy barriers is revealed to contribute to this trend. The current results indicate that diffusion in HEAs cannot a priori be considered as sluggish.

A357 (Al–7Si–0.6Mg)-xTiB 2 (x = 5, 10 wt.%) in-situ composites processed by the salt-melt reaction method were successfully foamed by melt processing with the addition of heat treated TiH 2 as blowing agent. Holding time was varied in order to identify critical holding time beyond which cell wall collapse and drainage dominate. X-ray tomography was employed to characterize the geometric structure of cells. Cell structure with high degree of uniformity evolved in 5TiB 2 composite foams compared with 10TiB 2 composite foams for a given holding time. 5TiB 2 composite foams showed better expansion compared to 10TiB 2 composite foams. However, the number of cell wall remnants (defects) was more for highly expanded 5TiB 2 composite foams. TiB 2 particles present in the foam promoted foam stability.

Present work studies 126 quaternary and 126 quinary equiatomic refractory high entropy alloys (RHEAs), made from Group IV (Ti, Zr, Hf), Group V (V, Nb, Ta) and Group VI (Cr, Mo, W) elements. Rule-of-mixtures (ROM) technique is used to calculate liquidus temperature, density (ρ), Young's modulus (E), % atomic size difference (δ), valence electron concentration (VEC) and specific heat at constant pressure and at 1273 K (Cp). CALPHAD technique is used to predict the number of phases formed at 298 K, ρ and liquidus temperature. ROM calculated densities are matching perfectly with CALPHAD values. Densities and E are directly proportional to the VEC and liquidus temperature of the alloys. Ti, Zr, Hf are ductilizing the alloys and making them light; whereas Cr, Mo and W, are reducing the alloys' ductility and making them heavy. For quinary RHEAs, Cp shows six distinct groups with δ , but a similar trend is not observed in quaternary RHEAs. A methodology is developed to screen a large number of alloys based on various properties. Correlations between those properties are also studied.

Y2Ti2O7, Y2Zr2O7, Y2Hf2O7 have pyrochlore structure and are found to be promising dispersoids for steel-based oxide dispersed strengthened (ODS) alloys, which are used in nuclear reactor and supercritical boilers applications. They should be nanocrystalline and possess high strength for better performance of ODS alloys. In the present study, oxides are successfully synthesized through Mechanically Activated Annealing (MAA) and Reverse Co-precipitation (RCP) techniques. The RCP technique could yield pyrochlore phase at lower temperatures with finer crystalline size. The structural analysis was done through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectra to confirm the presence of single phase and nanocrystallinity of oxides. Raman spectra probes into local disorderness in the structure and bond length. Y2Hf2O7 synthesized through MAA has the highest hardness (20 GPA) among all the oxides synthesized in the present study and those reported so far. Further, Y2Ti2O7 has higher Young’s modulus than the Y2Zr2O7, Y2Hf2O7 due to the orderness in the structure.

In the present work, a novel Al–Ti–Zr light-weight alloy was designed using CALPHAD (CALculation of PHAse Diagram) method and validated with experiments. Based on the melting points obtained from differential scanning calorimetry and CALPHAD, the as-cast alloy was homogenised at 475 °C for 24 h. For the chosen nominal composition of Al87.5Ti6.25Zr6.25 (at.%), a microstructure with FCC and Al3(Ti, Zr) as the major phases was observed experimentally in the homogenised (equilibrated) microstructure in contrast to the three phases (FCC, Al3Ti(h) and Al3Zr) predicted by the thermodynamic calculations. The homogenised alloy showed a significant hardness improvement when compared to the as-cast alloy. The hardness of the alloy (∼2200 MPa) is ∼30% superior to that of 7075 Al alloy and better than other commercial Al alloys.

Eutectic high entropy alloys synthesized via conventional casting have been intensely studied recently owing to their superior mechanical properties. In the current work, AlCoCrFeNi2.1 eutectic high entropy alloy has been synthesized for the first time via a combination of mechanical alloying and spark plasma sintering (MA-SPS) route. The alloy primarily showed FCC and B2 phases in addition to minor secondary phases. As opposed to previous literature on MA-SPS, WC contamination from milling media was eliminated by adopting a shorter milling time. Apart from the elimination of WC, the adopted strategy is an effective method to obtain a heterogeneous microstructure, which has been demonstrated to enhance both strength and compressive strain to failure. Heat treatment of the alloys for a short duration of 10 min at 1200 °C was found to improve both strength and compressive plasticity. This was attributed to the reprecipitation of secondary phases and the presence of heterogeneous microstructure.

In the present study, the evolution of microstructure, texture and high-temperature mechanical behavior of the additively manufactured (AM) eutectic high entropy alloys (EHEA) AlCoCrFeNi2.1 have been investigated. The material was manufactured through laser engineered net shaping (LENS) process. The microstructural investigation revealed constituent phases consisting of dendritic and eutectic features, with the phases having ordered FCC (L12) and BCC crystal structures. The phase fraction of L12 was more across the build (X) face and that of the BCC was more along build plane (Z face). In both the phases, nickel constituted the base element with L12 phase being deficient in Al and the BCC deficient in Cr and rich in Al. The stability of L12 phase was attributed to Co, Cr, and Fe with near equiatomic distribution. Kurdjumov-Sachs (KS)orientation relationship was followed between the ordered L12 and BCC phases along and across the build-up direction. Strain partitioning was observed more in the BCC phase than in the L12 phase during uniaxial compression at all temperatures. Difference in hardness was observed along Z and X directions which further resulted in yield anisotropy during compression test at room temperature. High-temperature compression tests at temperatures 400 °C, 600 °C, 700 °C, and 800 °C revealed that the yield strength increased from room temperature till 400 °C and then started decreasing till 800 °C.

The extent of oxidation and mechanism involving glassy and nanocrystalline phases are found to be different than that in case of normal crystalline metals and alloys. A short review on oxidation behavior of metallic glass and nanocrystalline alloys is presented in the light of work by the present authors and other researchers. The main factors, which are important in controlling the oxidation of metallic glass, are: (i) structure of glass and its devitrified states, (ii) composition of the alloy, and (iii) structure of the oxide layer. Hence, these factors are discussed with the help of isothermal and non-isothermal oxidation both in air and in the presence of oxygen of a number of Zr-based glassy alloys. The oxidation resistance of the glassy alloys decreases in the following order: microcrystalline alloy�>�fully nanocrystalline alloy�>�partially nanocrystalline alloy�>�relaxed amorphous alloy�>�quenched amorphous Zr-based alloy. � 2015, The Indian Institute of Metals - IIM.

The energetics behind transformation of liquid structure into subsequent intermediate phases during solidification is expected to play a decisive role in glass/crystal formation. A great deal of experimental and simulation work on supercooled liquids has indicated that, there exists a close link between the liquid structure and icosahedral clusters, especially for bulk metallic glass forming liquids. Pertinently, icosahedral clusters are also found to be energetically favorable to form upon devitrification of Zr60Cu10Al15Ni15 glassy alloy. Such evolution of icosahedral clusters upon devitrification in this alloy invariably proves their manifestation at the intermediate stage during transition of supercooled liquid into glass. Hence understanding the energetics behind restructuring of these clusters into glass or crystal during solidification, aids in microstructure optimization of glass/crystal composites for structural and functional applications. In this paper, it has been attempted to investigate the energetics behind the evolution of Zr�Ni and Zr�Al binary intermetallic phases during crystallization of Zr60Cu10Al15Ni15 glassy alloy. Ascalaph Designer Molecular Modeling Suite is used to generate different models of clusters to understand the formation of Zr�Ni and Zr�Al phases. We propose solidification mechanism in this alloy via two steps, namely, formation of intermediate Zr�Cu icosahedral clusters which is structurally restricted process and precipitation of crystalline phases as thermodynamically favorable process. � 2015, The Indian Institute of Metals - IIM.

Lead-free Na0.5Bi0.5TiO3 (NBT) fine-grained ceramics were prepared by spark plasma sintering method. Single phase ceramics with an ABO3 perovskite structure were confirmed by X-ray diffraction method. The grain morphology was analyzed by using a scanning electron micrograph. The crystal particles arranged compactly and the typical grain size was about ≈200 nm. The depolarization temperature T d, the rhombohedral–tetragonal phase transition temperature T R−T and the temperature T m of the maximum dielectric constant corresponding to tetragonal–cubic phase transition were determined to be 453 K, approximately 623 and 793 K, respectively, from the temperature dependence of dielectric properties. The P-E hysteresis loop at different temperatures indicated the existence of antiferroelectric state at the depolarization temperature T d, which explained the antiferroelectric phase above T d. The values of P s, P r and E c reduced slightly with increasing temperature in the range 303–453 K.

Multiferroic composites of x Ni 0.5 Zn 0.5 Fe 1.9 O 4− δ −(1− x ) Na 0.5 Bi 0.5 TiO 3 ( x NZF−(1− x ) NBT, where x =0.05, 0.15 and 0.25 mol fraction) were synthesized by spark plasma sintering (SPS) in conjunction with high-energy ball milling. The presence of NZF and NBT phases in the composites was confirmed by X-ray diffraction. The dielectric constant was studied as a function of frequency (0.5 kHz to 1 MHz) and temperature (30–500 °C). It was found that the 0.25 NZF–0.75 NBT composite possessed the most promising dielectric properties with its dielectric constant being 5–10 times higher than those for the other two composites in the full range of frequency. The magnetic and ferroelectric properties were examined at room temperature and all composite samples exhibited both pronounced ferromagnetic and ferroelectric characteristics. A maximum ME voltage coefficient of ~870 μV/cm Oe was obtained at the magnetic field of ~1.25 kOe for the 0.25 NZF–0.75 NBT composite, which was well comparable with the ME output for the lead-containing NZF–PZT composites.

The grain size dependency of lattice parameter during high-energy ball milling of W has been investigated. The lattice parameter varies non-monotonically with grain size during milling with a lattice contraction initially followed by an expansion. The lattice parameters were calculated in view of the non-equilibrium grain boundary structure that evolved during milling using excess free volume and the interfacial stresses at the grain boundaries. The calculated lattice parameters closely match the experimentally observed values. � 2014 Acta Materialia Inc.

In situ composites are a class of composite materials in which the reinforcement is formed within the matrix by reaction during the processing. In situ method of composite synthesis has been widely followed by researchers because of several advantages over conventional stir casting such as fine particle size, clean interface, and good wettability of the reinforcement with the matrix and homogeneous distribution of the reinforcement compared to other processes. Besides this, in situ processing of composites by casting route is also economical and amenable for large scale production as compared to other methods such as powder metallurgy and spray forming. Commonly used reinforcements for Al and its alloys which can be produced in situ are Al2O3, AlN, TiB2, TiC, ZrB2, and Mg2Si. The aim of this paper is to review the current research and development in aluminum-based in situ composites by casting route.