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High-entropy alloys (HEAs) with multiple principal elements represent a paradigm shift in structural alloy design and show excellent surface degradation resistance in corrosive environment. Here, the tribo-corrosion response of laser-engineered net-shaped CoCrFeMnNi HEA was evaluated in 3.5 wt% NaCl solution at room temperature. The additively manufactured (AM-ed) CoCrFeMnNi showed five times lower wear rate, regenerative passivation, and nobler corrosion potential during tribo-corrosion test compared to its arc-melted counterpart. A significant anisotropy was seen in the tribo-corrosion response with 45° to the build direction showing better performance compared to tests along the build direction and perpendicular to it. The open circuit potential curves were characterized by a sharp drop to more negative values as wear began, followed by continuous change for the active tribo-corrosion duration and finally a jump to nobler value at the end of the test indicating excellent surface re-passivation for the AM-ed alloy. The superior tribo-corrosion resistance of AM-ed CoCrFeMnNi was attributed to the refined microstructure and highly protective surface passivation layer promoted by the sub-grain cellular structure formed during additive manufacturing. These results highlight the potential of utilizing additive manufacturing of HEAs for use in extreme environments that require a combination of tribo-corrosion resistance, mechanical durability, extended service life, and net shaping with low dimensional tolerance.

Lightweight energy-saving magnesium (Mg) nanocomposites could be titled materials of future. Remarkable specific strength along with acceptable ductility is two key features of these materials which have made them a unique class of emerging materials. However, magnesium nanocomposites are at the initial stages of development and therefore systematic research is required to establish microstructure/property relationships at different potential conditions (i.e. temperatures and strain rates). In this paper, a recently introduced magnesium-samarium oxide (Mg–Sm2O3) nanocomposite, with different volume fractions of nanosize Sm2O3 particles as reinforcement, is employed to study ambient temperature rate-dependent plastic response of the material. To study the rate-dependent response of the materials, a depth-sensing nanoindentation measurement technique was used which is considered a non-destructive, robust, and convenient method to assess the controlling mechanisms of creep phenomenon in small scales. Tests were performed on both nanocomposites and pure Mg for the purpose of comparison. Microstructural observations and nanoindentation data indicate that the creep resistance of the materials is directly associated with the volume fraction of the nanoparticles. The results of this study would provide the required reference lines for the forthcoming elevated temperature creep assessment of the materials.

Multi-principal element alloys represent a new paradigm in structural alloy design with superior mechanical properties and promising ballistic performance. Here, the mechanical response of Al0.3CoCrFeNi alloy, with unique bimodal microstructure, was evaluated at quasistatic, dynamic, and ballistic strain rates. The microstructure after quasistatic deformation was dominated by highly deformed grains. High density of deformation bands was observed at dynamic strain rates but there was no indication of adiabatic shear bands, cracks, or twinning. The ballistic response was evaluated by impacting a 12 mm thick plate with 6.35 mm WC projectiles at velocities ranging from 1066 to 1465 m/s. The deformed microstructure after ballistic impact was dominated by adiabatic shear bands, shear band induced cracks, microbands, and dynamic recrystallization. The superior ballistic response of this alloy compared with similar AlxCoCrFeNi alloys was attributed to its bimodal microstructure, nano-scale L12 precipitation, and grain boundary B2 precipitates. Deformation mechanisms at quasistatic and dynamic strain rates were primarily characterized by extensive dislocation slip and low density of stacking faults. Deformation mechanisms at ballistic strain rates were characterized by grain rotation, disordering of the L12 phase, and high density of stacking faults.

The effect of ion irradiation on the hardness and yield strength of Zr57Nb5Cu15.4Ni12.6Al10 bulk metallic glass has been studied using nanoindentation and micropillar compression tests. The results for the amorphous alloy were compared with those for 304 stainless steel. After Ni2+ ion irradiation, the metallic glass was found on an average to have 16% lower hardness and 13% lower yield strength. In contrast, 304 stainless steel showed significant hardening (~ 50% hardness and strength increase) after irradiation under identical conditions. The irradiation-induced hardening of the steel was attributed to hindrance of dislocation movement from the defects generated, while the softening behavior of the metallic glass was attributed to structural change in the irradiated region while retaining a fully amorphous state. The present work paves the way for better understanding of the irradiation response of metallic glasses to develop degradation-resistant alloys for next-generation nuclear applications.

An AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA), comprising of FCC phase and a high volume fraction of plate and irregularly shaped BCC-ordered intermetallic B2 domains, was subjected to normal impact by spherical Tungsten–Carbide projectiles. Depending on projectile velocity, the impacted AlCoCrFeNi2.1 plates were partially penetrated (at the lowest velocity of 803 m/s), plugged (intermediate velocity of 1159 m/s), and fully penetrated (highest velocity of 1388 m/s). Electron microscopy was utilized to characterize the residual damage or deformation features in the recovered specimens. Failure in the partially penetrated conditions was dominated by interfacial decohesion at the plate-like B2 and FCC interfaces. At higher velocities where plugging occurred, failure was dominated by crack formation in regions containing adiabatic shear band. These results indicated a transition in failure modes as a function of projectile velocities, where the FCC-B2 microstructure dominate failure at lower velocities, while such microstructural features do not influence dynamic failure at higher velocities.

The strain gradient plasticity of multiprincipal element alloys, CoCrNi and CoCrFeMnNi, was investigated in work-hardened and annealed conditions using nano-indentation. The hardness of the alloys demonstrated significant dependence on indentation depth, which was explained by geometrically necessary dislocations resulting from the high strain gradients due to the shape of the indenter. The characteristic length-scale of the indentation size effect for the multiprincipal element alloys was found to be larger than pure nickel due to higher friction stress, which constrains the storage volume of geometrically necessary dislocations.

High entropy alloys represent a new paradigm of structural alloy design consisting of (near) equal proportions of constituent elements resulting in a number of attractive properties. In particular, eutectic high entropy alloys offer a remarkable combination of high strength and good ductility from the synergistic contribution of each phase in the eutectic, thereby circumventing the strength-ductility trade-off in conventional structural materials. In the present study, wear and corrosion behavior were evaluated for the AlCoCrFeNi2.1 eutectic high entropy alloy consisting of BCC (B2), and FCC (L12) lamellae. A transition from adhesive to oxidative wear was observed in reciprocating wear analysis. The L12 phase with lower hardness preferentially deformed during the wear test. The ratio of hardness to modulus was almost two times higher for the B2 phase as compared to L12. The overall corrosion resistance of the eutectic high entropy alloy was comparable to 304 stainless steel in 3.5 wt% NaCl solution. However, detailed microscopy revealed preferential dissolution of the B2 phase. Phase-specific scanning kelvin probe analysis showed relatively higher electropositivity for the B2 phase as compared with L12, supporting the selective corrosion and higher coefficient of friction of B2.

Hypereutectoid SAE 52100 steel is extensively used in bearing applications. Microstructure modification in terms of dislocation martensite and carbide refinement was achieved for SAE 52100 steel through a simple duplex heat treatment. Refinement of prior austenite grains to less than 5 μm resulted in the conversion of conventional high-carbon twinned martensite to dislocation martensite. The concurrent refinement of austenite grains and carbide precipitates was accomplished by high-temperature austenitization followed by low-temperature tempering. This resulted in nanoscale nonstoichiometric e-carbides within a heavily twinned martensitic structure. These nanoscale carbides acted as grain boundary pinning agents after their transformation to θ-carbides during the final austenitization process. The resulting microstructure was characterized by a fine dispersion of θ-carbides within dislocation martensite and showed roughly 24% increase in tensile strength and 30% better wear resistance compared to conventional structure.

Lamellar eutectic structure of Al0.7CoCrFeNi high-entropy alloy (HEA) is emerging as a promising candidate for structural applications because of its high strength-ductility combination. The alloy consists of a fine-scale lamellar fcc+B2 microstructure with high flow stresses >1500 MPa under quasi-static conditions. The response to shear loading was not investigated so far. This is the first report on the shear deformation of an eutectic structured HEA and effect of precipitation on shear deformation. The dynamic shear response (DSR) of the eutectic HEA was examined in two microstructural conditions, with and without the presence of L12 precipitates. A split-Hopkinson pressure bar (SHPB) was used to compress the hat-shaped specimens to study the local DSR of the alloy. The adiabatic shear bands (ASBs) in two different microstructural conditions were characterized after deformation at dynamic strain rates. The adiabatic shear localization occurs at low strains for the high strength material, and the eutectic microstructure does not delay cracking. The width of ASBs and the extent of plastic deformation around them has been correlated with the rate of straining. Dynamic recrystallization within ASBs and profuse twinning around it was observed. Local mechanical response of individual lamellae before and after shear deformation was examined using nano-indentation.

Lamellar eutectic structure in Al0.7CoCrFeNi high-entropy alloy (HEA) is emerging as apromising candidate for structural applications because of its high strength-ductility combination.The alloy consists of a fine-scale lamellar fcc + B2 microstructure with high flow stresses &gt, 1300 MPaunder quasi-static tensile deformation and &gt, 10% ductility. The response to shear loading was notinvestigated so far. This is the first report on the shear deformation of a eutectic structured HEA andeffect of precipitation on shear deformation. A split-Hopkinson pressure bar (SHPB) was used tocompress the hat-shaped specimens to study the local dynamic shear response of the alloy. Thechange in the width of shear bands with respect to precipitation and deformation rates was studied.The precipitation of L12 phase did not delay the formation of adiabatic shear bands (ASB) or affectthe ASB width significantly, however, the deformed region around ASB, consisting of high densityof twins in fcc phase, was reduced from 80 &mu, m to 20 &mu, m in the stronger precipitation strengthenedcondition. We observe dynamic recrystallization of grains within ASBs and local mechanicalresponse of individual eutectic lamellae before and after shear deformation and within the shearbands was examined using nano-indentation.

Eutectic high entropy alloys, with lamellar arrangement of solid solution phases, represent a new paradigm for simultaneously achieving high strength and ductility, thereby circumventing this well-known trade-off in conventional alloys. However, dynamic strengthening mechanisms and phase-boundary interactions during external loading remain unclear for these eutectic systems. In this study, small-scale mechanical behavior was evaluated for AlCoCrFeNi2.1 eutectic high entropy alloy, consisting of a lamellar arrangement of L12 and B2 solid-solution phases. The ultimate tensile strength was 1165 MPa with ductility of ~18% and ultimate compressive strength was 1863 MPa with a total compressive fracture strain of ~34%. Dual mode fracture was observed with ductile failure for L12 phase and brittle mode for B2 phase. Phase-specific mechanical tests using nano-indentation and micro-pillar compression showed higher hardness and strength and larger strain rate sensitivity for B2 compared with L12. Micro-pillars on B2 phase deformed by plastic barreling while L12 micro-pillars showed high density of slip steps due to activation of more slip systems and homogenous plastic flow. Mixed micro-pillars containing both the phases exhibited dual yielding behavior while the interface between L12 and B2 was well preserved without any sign of separation or cracking. Phase-specific friction analysis revealed higher coefficient of friction for B2 compared to L12. These results will pave the way for fundamental understanding of phase-specific contribution to bulk mechanical response of concentrated alloys and help in designing structural materials with high fracture toughness.

The performance of complex concentrated alloys (or high entropy alloys) with widely varying microstructure is evaluated by ballistically impacting targets with spheres fired at normal incidence. By changing alloy composition in the Al-Co-Cr-Fe-Ni multi-principal system, the variation in microstructure included single-phase equiaxed grains, single-phase with bimodal grain-size distribution, and eutectic two-phase lamellar microstructure. Rigorous characterization in the form of bulk mechanical testing, scanning electron microscopy, and spatially resolved nano-indentation on initial and ballistically impacted plates was used to connect microstructural details to aspects of ballistic behavior governing performance. Based on the results, it was shown that although the addition of a harder secondary phase improves strength, cracks that initiate and propagate within the harder phase and ultimately across the target plate drastically reduce the ballistic performance of the two-phase material. The single-phase alloy with bimodal grain-size distribution exhibited superior ballistic performance compared to the other high entropy alloys, although none of these materials exceeded the performance of conventional rolled homogeneous armor steel. These results pave the way for development of high-performance concentrated alloys for ballistic applications by appropriate microstructural design.

Design of high-entropy alloys with complex microstructures presents a unique opportunity to combine the best of solid solution strengthening and precipitation hardening. These alloys are potentially disruptive for several high-performance applications including aerospace, oil and gas, nuclear industry, and next-generation tribology. This study presents the thermo-mechanical treatment, microstructural evolution, and surface degradation resistance in precipitation hardenable Al0.3Cu0.3Ti0.2CoCrFeNi high-entropy alloy. The heat-treated alloy consisted of fine precipitates of L12 and L21, strengthening the face-centered cubic solid solution matrix. Potentiodynamic polarization showed nano-galvanic coupling between the intermetallic particles and the matrix. Phase-specific nano-indentation showed that hard intermetallic particles caused three-body wear and higher friction values during sliding wear. The microstructure was correlated with processing conditions and resulting surface degradation resistance using transmission electron microscopy imaging and analysis. Hardening high-entropy alloys by introducing precipitates negatively effects corrosion resistance. A team led by Rajarshi Banerjee at the University of North Texas designed a new high entropy alloy, Al0.3Cu0.3Ti0.2CoCrFeNi, whose composition promoted precipitation strengthening of nanometer-scale L12 intermetallics as well as copper clusters. By using cold rolling and annealing at different times and temperatures, the alloys ranged from a solid solution to an alloy with two different types of intermetallics. This caused two different types of corrosion patterns: the more traditional large pits, and basket-weave elongated pits resulting from the galvanic coupling between the two intermetallic phases. These hard intermetallic phases were also dislodged during sliding, contributing to increased wear. Better understanding the corrosion and wear of high-entropy alloys may hasten their industrial implementation.

Amorphous metallic alloys are widely used in bulk form and as coatings for their desirable corrosion and wear behavior. Nevertheless, the effects of heat treatment and thermal cycling on these surface properties are not well understood. In this study, the corrosion and wear behavior of two Zr-based bulk metallic glasses were evaluated in as-cast and thermally relaxed states. Significant improvement in wear rate, friction coefficient, and corrosion penetration rate was seen for both alloys after thermal relaxation. A fully amorphous structure was retained with thermal relaxation below the glass transition. There was an increase in surface hardness and elastic modulus for both alloys after relaxation. The improvement in surface properties was explained based on annihilation of free volume.

Metallic glass nanocomposite thin films were synthesized for an immiscible Ag-Cu alloy system by magnetron sputtering. The structure of the films was unique, consisting of homogeneously dispersed nanocrystallites in an amorphous matrix. The size and volume fraction of the nanocrystallites increased with increasing film thickness resulting in increased elastic modulus and hardness. The high electrical conductivity of the nanocomposite films was examined by a valence-band study, which showed that exchange interaction between Ag and Cu in the nanocomposite structure resulted in enhanced charge carrier concentration. The inverse correlation between electrical conductivity and film thickness was explained by surface and interface scattering of electrons with increasing volume fraction of nanocrystallites. The small temperature dependence of conductivity was attributed to the distorted Fermi surface of the nanocomposite films resulting in a greater contribution from structure scattering, which is temperature-independent.

CoCrFeMnNi high entropy alloys (HEAs) were additively manufactured (AM) by laser powder bed fusion and their corrosion resistance in 3.5 wt% NaCl solution was studied by potentiodynamic polarization and electrochemical impedance spectroscopy tests. A systematic study of AM CoCrFeMnNi HEAs&rsquo, porosity under a wide range of laser processing parameters was conducted and a processing map was constructed to identify the optimal laser processing window for CoCrFeMnNi HEAs. The near fully dense AM CoCrFeMnNi HEAs exhibit a unique non-equilibrium microstructure consisting of tortuous grain boundaries, sub-grain cellular structures, columnar dendrites, associated with some processing defects such as micro-pores. Compared with conventional as-cast counterpart, the AM CoCrFeMnNi HEAs showed higher pitting resistance (&Delta, E) and greater polarization resistance (Rp). The superior corrosion resistance of AM CoCrFeMnNi HEAs may be attributed to the homogeneous elemental distribution and lower density of micro-pores. Our study widens the toolbox to manufacture HEAs with exceptional corrosion resistance by additive manufacturing.

Electro-catalyst design with superior performance and reduced precious metal content (compared to state-of-the-art Pt/C) has been a challenge in proton exchange membrane fuel cells, preventing their widespread adoption. Metallic glasses have recently shown promising performance and large electrochemical surface area in catalytic reactions. The electro-catalytic behavior of recently developed Pt-, Pd-, and Pt/Pd-based metallic glasses was evaluated in this study using scanning electrochemical microscopy. The influence of chemistry and electronic structure on catalytic behavior was studied using scanning kelvin probe technique. The work function for the metallic glasses was lower by 75 mV to 175 mV compared to pure Pt. This resulted in higher catalytic activity for the amorphous alloys, which was attributed to the ease of charge transfer on the surface. The binding energy for the metallic glasses, measured using X-ray photoelectron spectroscopy, was higher by 0.2 eV to 0.4 eV. This explained easier removal of adsorbed species from the surface of amorphous alloys. The synergistic effect of Pt and Pd in alloys containing both the noble metals was demonstrated towards hydrogen oxidation reaction.

The combination of high strength and good ductility are very desirable for advanced structural and functional applications. However, measures to enhance strength typically lead to ductility reduction due to their inverse correlation, nano-grained structures for an instance. Bi-modal grain structure is promising in this regard, but its realization is limited by multiple complex processing steps. Here, we demonstrate a facile single-step processing route for the development of bimodal grain structure in austenitic stainless steel, SS316L. The bimodal structure comprised of fine martensite grains (

The mechanical behavior of nano-scale metallic glasses was investigated by in situ compression tests in a scanning electron microscope. Platinum-based metallic glass nano-pillars were fabricated by thermoplastic forming. The nano-pillars and corresponding bulk substrate were tested in compression over the range of room temperature to glass transition. Stress–strain curves of the nano-pillars were obtained along with in situ observation of their deformation behavior. The bulk substrate as well as nano-pillars showed an increase in elastic modulus with temperature which is explained by diffusive rearrangement of atomic-scale viscoelastic units.

Noble-metal based metallic glasses have recently shown excellent performance towards a range of catalytic reactions, which is attributed to their disordered atomic structure and high density of active sites on the surface. An in-depth understanding of the mechanisms that promote the catalytic performance of amorphous alloys would facilitate the design and development of highly efficient catalysts with significantly lower precious metal loading. In the present study, the electro-catalytic behavior of amorphous Pt- and Pd-based metallic glasses was studied towards hydrogen oxidation reaction by cyclic voltammetry and scanning electrochemical microscopy. The electrochemically active surface area for the metallic glasses was found to be several folds higher compared to pure Pt and Pd. Density functional theory calculations showed that hydrogen oxidation on the surface of the amorphous alloys preferentially followed the reverse Heyrovsky-Volmer pathway. The minimum over-potential for the amorphous alloys was calculated to be significantly lower compared to pure Pt and Pd, consistent with the experimental trends. The enhanced catalytic activity for the amorphous alloys was attributed to the reduced chemisorption of hydrogen on the metallic glass surface, particularly for the alloys containing both Pt and Pd.

High entropy alloys (HEAs) have attracted widespread interest due to their unique properties at many different length-scales. Here, we report the fabrication of nanocrystalline (NC) Al0.1CoCrFeNi high entropy alloy and subsequent small-scale plastic deformation behavior via nano-pillar compression tests. Exceptional strength was realized for the NC HEA compared to pure Ni of similar grain sizes. Grain boundary mediated deformation mechanisms led to high strain rate sensitivity of flow stress in the nanocrystalline HEA.

Metallic glass composites represent a unique alloy design strategy comprising of in situ crystalline dendrites in an amorphous matrix to achieve damage tolerance unseen in conventional structural materials. They are promising for a range of advanced applications including spacecraft gears, high-performance sporting goods and bio-implants, all of which demand high surface degradation resistance. Here, we evaluated the phase-specific electrochemical and friction characteristics of a Zr-based metallic glass composite, Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5, which comprised roughly of 40% by volume crystalline dendrites in an amorphous matrix. The amorphous matrix showed higher hardness and friction coefficient compared to the crystalline dendrites. But sliding reciprocating tests for the composite revealed inter-phase delamination rather than preferred wearing of one phase. Pitting during potentiodynamic polarization in NaCl solution was prevalent at the inter-phase boundary, confirming that galvanic coupling was the predominant corrosion mechanism. Scanning vibration electrode technique demonstrated that the amorphous matrix corroded much faster than the crystalline dendrites due to its unfavorable chemistry. Relative work function values measured using scanning kelvin probe showed the amorphous matrix to be more electropositive, which explain its preferred corrosion over the crystalline dendrites as well as its characteristic friction behavior. This study paves the way for careful partitioning of elements between the two phases in a metallic glass composite to tune its surface degradation behavior for a range of advanced applications.

Friction stir processing of a ternary immiscible alloy was investigated as a pathway to thermally stable nanostructured alloys. The processed alloy was characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), nano-indentation, and thermal stability analysis. Both XRD and high-magnification SEM confirmed the formation of solid solution at the shear edge of the friction stir layer. Microstructural inhomogeneity was observed in the processed region that resulted in hardness variation. Upon annealing treatment, a significant improvement in hardness was observed at 200°C that was due to the formation of nano-precipitates. Remarkably, the Cu-Ag-Nb immiscible system displayed exceptional thermal stability up to 500°C.

Temperature-induced variation in magnetic properties for cobalt-based metallic glass was investigated. The formation of metastable nanocrystalline phases prior to complete devitrification and their effect on magnetic properties for Co72B19.2Si4.8Cr4 metallic glass was studied. The nature, shape, and distribution of the intermediate nanocrystalline phases were characterized using transmission electron microscopy and x-ray diffraction. A drastic change in magnetic properties was found in going from a fully amorphous state to different stages of nanocrystallization. The coercivity changes from amorphous soft magnetic state (H c ~ 0.12 Oe) to a nanocrystalline-dispersed hard magnetic state (H c ~ 187 Oe), with no significant change in saturation magnetization. This suggests potential use in futuristic magnetic switches, fluxgate sensors, and electromagnetic shielding devices.

The nanomechanical behavior of the Co20Cr20Fe20Mn20Ni20 high-entropy alloy was investigated in as-cast, rolled, annealed, and thin-film forms. Dislocation nucleation was studied by repeated indents at a low load for each of the different processing conditions. Distinct displacement bursts (pop in) were observed in the loading curve marked by incipient plasticity for all the samples. The as-cast and annealed samples showed pop ins for 100% of the indents, whereas the rolled and thin-film samples showed a much lower fraction of displacement bursts. This was explained by the high density of dislocations for the cold-worked and thin-film conditions. The strong depth dependence of hardness was explained by geometrically necessary dislocations. The nanomechanical behavior and twinned microstructure indicate low stacking-fault energy for this high-entropy alloy.

Cavitation erosion and corrosion of structural materials are serious concerns for marine and offshore industries. Durability and performance of marine components are severely impaired due to degradation from erosion and corrosion. Utilization of advanced structural materials can play a vital role in limiting such degradation. High entropy alloys (HEAs) are a relatively new class of advanced structural materials with exceptional properties. In the present work, we report on the cavitation erosion behavior of Al 0.1 CoCrFeNi HEA in two different media: distilled water with and without 3.5 wt% NaCl. For comparison, conventionally used stainless steel SS316L was also evaluated in identical test conditions. Despite lower hardness and yield strength, the HEA showed significantly longer incubation period and lower erosion-corrosion rate (nearly 1/4th) compared to SS316L steel. Enhanced erosion resistance of HEA was attributed to its high work-hardening behavior and stable passivation film on the surface. The Al 0.1 CoCrFeNi HEA showed lower corrosion current density, high pitting resistance and protection potential compared to SS316L steel. Further, HEA showed no evidence of intergranular corrosion likely due to the absence of secondary precipitates. Although, the degradation mechanisms (formation of pits and fatigue cracks) were similar for both the materials, the damage severity was found to be much higher for SS316L steel compared to HEA.

Slurry erosion is a major problem in hydroturbines and other fluid machineries wherein erodent particles entrained in carrier fluid impinge upon the target surface to cause its degradation. Several possible alternative solutions are available to control slurry erosion; however, surface coatings developed using thermal spraying are mostly used owing to their versatile nature. In this work, Ni based thermal spray coatings were deposited on a commonly used hydroturbine steel (CA6NM), with Al2O3 mixed in different proportions. Coatings were developed using high velocity flame spray process (HVFS). Slurry erosion performance of prepared coated and bare steel was investigated using a specially designed jet type test rig with sand as erodent particles. To investigate the effect of concentration on erosion behavior, sand was mixed in water at two different levels, 0.1 wt % and 0.5 wt. %. It was observed that all the coatings helped in improving the erosion resistance of the steel with one containing 40 wt % Al 2 O 3 showing the maximum improvement. This was related to the microstructure, which controlled the micro hardness and fracture toughness of the coatings. Shielding effect of the rebounding particles was found to have profound effect as erosion rates did not show similar augmentation with concentration. Fracturing of alumina phase along with removal of coating in form of splats was observed to be prominent erosion mechanism.

There has been tremendous interest in recent years in a new class of multi-component metallic alloys that are referred to as high entropy alloys, or more generally, as complex concentrated alloys. These multi-principal element alloys represent a new paradigm in structural material design, where numerous desirable attributes are achieved simultaneously from multiple elements in equimolar (or near equimolar) proportions. While there are several review articles on alloy development, microstructure, mechanical behavior, and other bulk properties of these alloys, then there is a pressing need for an overview that is focused on their surface properties and surface degradation mechanisms. In this paper, we present a comprehensive view on corrosion, erosion and wear behavior of complex concentrated alloys. The effect of alloying elements, microstructure, and processing methods on the surface degradation behavior are analyzed and discussed in detail. We identify critical knowledge gaps in individual reports and highlight the underlying mechanisms and synergy between the different degradation routes.

The atomic distribution in catalytically active metallic glass alloys, Pd43Cu27Ni10P20and Pt57.5Cu14.7Ni5.3P22.5, was investigated using three-dimensional atom probe microscopy. Atom probe analysis showed uniform distribution of constituent elements for both the starting amorphous alloys, with no phase separation. Both the crystallized alloys showed eutectic microstructure with a very sharp interface (~0.5 nm as determined from atom probe). The atomic distribution in the devitrified state is explained based on the “fragile liquid” behavior for these noble-metal glassy alloys.

The trends in glass formation among bulk metallic glass-forming alloys are investigated within the framework of viscosity and specific volume measurements. This investigation was carried out using four alloys (Zr41.2Ti13.8Cu12.5Ni10Be22.5, Zr57Cu15.4Ni12.6Al10Nb5, Zr52.5Cu17.9Ni14.6Al10Ti5 and Ni59.5Nb40.5) that have widely different glass forming abilities (GFAs). This study shows that the viscosity at the melting temperature is correlated with volume change upon crystallization in accordance with Cohen–Grest free-volume theory. The viscosity of the best glass former (Zr41.2Ti13.8Cu12.5Ni10Be22.5) is found to be an order of magnitude larger compared to the worst glass former (Ni59.5Nb40.5) at their respective liquidus temperatures. The other two alloys have intermediate values. The specific volume results also support the trend in GFA with the best glass former showing the smallest volume change upon crystallization and worst glass former showing the largest change. This study suggests that high viscosity and correspondingly small free volume of the liquid at the melting temperature contribute significantly to improve the GFA.

We demonstrate the refinement and uniform distribution of the crystalline dendritic phase by friction stir processing (FSP) of titanium based in situ ductile-phase reinforced metallic glass composite. The average size of the dendrites was reduced by almost a factor of five (from 24 μm to 5 μm) for the highest tool rotational speed of 900 rpm. The large inter-connected dendrites become more fragmented with increased circularity after processing. The changes in thermal characteristics were measured by differential scanning calorimetry. The reduction in crystallization enthalpy after processing suggests partial devitrification due to the high strain plastic deformation. FSP resulted in increased hardness and modulus for both the amorphous matrix and the crystalline phase. This is explained by interaction of shear bands in amorphous matrix with the strain-hardened dendritic phase. Our approach offers a new strategy for microstructural design in metallic glass composites.