Hexagonal Close-packed Phase Research Articles

Irreversible phase transition of the Fe50Mn30Cr10Co10 high entropy alloy under stress

The Fe50Mn30Cr10Co10 high entropy alloy has attracted research interest in recent years due to its ability to overcome the strength-ductility trade-off. A recent study reported that a nanolaminate dual-phase microstructure, derived from the bidirectional transformation of Fe50Mn30Cr10Co10 alloy under stress, might be the main reason for its exceptional mechanical properties. Here, we report a unidirectional and irreversible phase transition from a face-centered-cubic to a hexagonal-close-packed (HCP) structure in the Fe50Mn30Cr10Co10 alloy under stress, using the in situ high-pressure x-ray diffraction method. An almost pure HCP phase is obtained at pressures exceeding 20 GPa. It remains stable in further loading and unloading processes. Transmission electron microscopy analysis indicates that dislocation motion along the {111}⟨11 2¯⟩ slip system results in the irreversible phase transition and the formation of nanolamellar microstructures in the Fe50Mn30Cr10Co10 alloy. Our study provides insights into understanding the deformation mechanism of Fe50Mn30Cr10Co10 alloy and suggests the potential to design the alloy through high-pressure manufacturing.

Compositional effect on pressure-induced polymorphism in high-entropy alloys

Recently, pressure-induced polymorphic phase transitions were recently discovered in several fragmented high-entropy alloys (HEAs), offering a valuable opportunity to deepen our understanding of these materials. However, the chemical and physical factors that govern these transitions are still unclear. Here, we combined in situ high-pressure and high-temperature synchrotron X-ray diffraction, X-ray emission spectroscopy (XES), and high-resolution transmission electron microscopy (HRTEM) to systematically study the evolution of the atomic and electronic structures in the Cantor alloy and its face-centered-cubic (fcc) subset alloys (CoCrFeMnNi, CoCrFeNi, CoCrMnNi, CoFeMnNi, CoCrNi, CoFeNi, CoMnNi, CrFeNi, and FeMnNi). Surprisingly, diverse behavior was observed among these closely related alloys during compression and decompression, which includes irreversible, reversible fcc to hexagonal close-packed (hcp) phase transitions, or even no detectable phase transitions up to ∼40 GPa. HRTEM measurements confirmed that the fcc and hcp phases abided by the classic Shoji-Nishiyama orientation relationship during the transitions. XES data indicated that high-pressure suppresses the local magnetic moments in all the studied alloys, suggesting that magnetic states do not significantly influence the polymorphic transitions. By comparing the effects of the atomic size difference, entropy, valence electron concentration, and stacking fault energy across all the compositions studied, only the stacking fault energy shows a strong correlation with the phase transitions, indicating it plays a key role in inducing polymorphism in HEAs.

Deformation mechanism of a metastable medium entropy alloy strengthened by the synergy of heterostructure design and cryo-pre-straining

Face-centered cubic (FCC) medium entropy alloys (MEAs) have received considerable attention due to their impressive mechanical properties and responses. However, their practical application is limited by their modest yield strengths. The potential enhancement of the mechanical properties of single-phase MEAs was explored in this study through a synergistic approach combining heterogeneous structure design with subsequent cryo-pre-straining. A heterogeneous lamella structure was produced in a single-phase Fe55Mn20Cr15Ni10 MEA via two-step rolling and annealing. Cryo-pre-straining at varying degrees (6, 12, 21, and 36%) introduced hexagonal close-packed (HCP) phase, high-density dislocations, twins, and stacking faults, leveraging the reduced stacking fault energy at cryogenic temperatures. This process enhanced the alloy's yield strength from 353 MPa to 1.2 GPa (compared to the baseline uniform coarse-grained structure), while maintaining an acceptable total elongation of 8.4%. The impact of cryo-pre-straining on the microstructure and mechanical properties of the MEA was assessed using in-situ synchrotron X-ray diffraction analysis. Cryo-pre-straining (36%) achieved a higher dislocation density (6.1 × 1015m−2) compared to room-temperature straining (2.5 × 1015m−2). The stress contribution from HCP-martensite and the evolution of dislocation density during loading were quantified, along with observations of negative stacking fault probability and strain-induced HCP→FCC reverse transformation in cryo-pre-strained samples under loading conditions. Furthermore, the contributions of regulated microstructures to the enhancement of yield strength were quantitatively assessed.

Enhancing the Mechanical Properties of Co-Cr Dental Alloys Fabricated by Laser Powder Bed Fusion: Evaluation of Quenching and Annealing as Heat Treatment Methods.

Residual stresses and anisotropic structures characterize laser powder bed fusion (L-PBF) products due to rapid thermal changes during fabrication, potentially leading to microcracking and lower strength. Post-heat treatments are crucial for enhancing mechanical properties. Numerous dental technology laboratories worldwide are adopting the new technologies but must invest considerable time and resources to refine them for specific requirements. Our research can assist researchers in identifying thermal processes that enhance the mechanical properties of dental Co-Cr alloys. In this study, high cooling rates (quenching) and annealing after quenching were evaluated for L-PBF Co-Cr dental alloys. Cast samples (standard manufacturing method) were tested as a second reference material. Tensile strength, Vickers hardness, microstructure characterization, and phase identification were performed. Significant differences were found among the L-PBF groups and the cast samples. The lowest tensile strength (707 MPa) and hardness (345 HV) were observed for cast Starbond COS. The highest mechanical properties (1389 MPa, 535 HV) were observed for the samples subjected to the water quenching and reheating methods. XRD analysis revealed that the face-centered cubic (FCC) and hexagonal close-packed (HCP) phases are influenced by the composition and heat treatment. Annealing after quenching improved the microstructure homogeneity and increased the HCP content. L-PBF techniques yielded superior mechanical properties compared to traditional casting methods, offering efficiency and precision. Future research should focus on fatigue properties.

Study on the Microstructure, Mechanical Properties, and Corrosion Behavior of 900 °C-Annealed CoCrFeMnNiSix (X = 0, 0.3, 0.6, 0.9) High-Entropy Alloys.

In this study, a series of CoCrFeMnNiSix (x = 0, 0.3, 0.6, 0.9) high-entropy alloys (HEAs) were prepared by suspension melting of cold crucible, annealed at 1000 °C, and then quenched at 900 °C. The changes in the microstructure of the HEAs after the addition of Si were analyzed using X-ray diffraction (XRD), metallographic microscope, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The hardness, room-temperature friction, and wear behavior, room-temperature compressive properties, and corrosion resistance of the annealed CoCrFeMnNiSix HEAs were also studied. The results show that when the Si content is 0 and 0.3, the annealed CoCrFeMnNiSix HEA exhibits a single face-centered cubic (FCC) structure. As the silicon content increases, a face-centered orthorhombic (FCO) phase appears. At a Si content of 0.9, a hexagonal close-packed (HCP) phase is observed. After heat treatment, the hardness of the CoCrFeMnNiSix HEAs increases continuously with the addition of Si. The HEA with a Si content of 0.9 achieves the highest hardness of 974.8 ± 30.2 HV. The HEA with a Si content of 0.6 reaches the highest compressive strength and yield strength, which are 1990.3 MPa and 1327.5 MPa. When the Si content is 0.9, the HEA shows the smoothest surface after wear, with the best wear resistance, achieving a value of 0.21 mm-1. In the CoCrFeMnNiSix HEAs after 900 °C heat treatment, the HEA with a Si content of 0.6 exhibits the lowest self-corrosion current density of 0.23 µA/cm2 and the highest pitting potential of 157.65 mV, indicating the best corrosion resistance.

Facile Synthesis of Rhodium-Based Nanocrystals in a Metastable Phase and Evaluation of Their Thermal and Catalytic Properties.

Controlling the polymorphism of metal nanocrystals is a promising strategy for enhancing properties and discovering new phenomena. However, previous studies on Rh nanocrystals have focused on their thermodynamically stable face-centered-cubic (fcc) phase. Herein, a facile synthesis of Rh-based nanocrystals featuring the metastable hexagonal close-packed (hcp) phase is reported by using Ru seeds in their native hcp phase to template the deposition of Rh atoms. The success of such phase-controlled synthesis relies on the templating effect promoted by the small lattice mismatch between Ru and Rh and the slow dropwise titration of the precursor at an elevated temperature, ensuring the layer-by-layer growth mode and thus the formation of a conformal hcp-Rh shell. Faster injection rate of Rh(III) precursor leads to the formation of a rough Rh shell in the conventional fcc phase due to accelerated reaction kinetics. Considering both thermodynamic and kinetic aspects of this system, the hcp-Rh phase is favored when the low surface energy from smooth overlayers balances the high bulk energy of the metastable phase, achieved through tight control of reaction rates and deposition patterns. These Ruhcp@Rhhcp core-shell nanocrystals demonstrate thermal stability up to 400°C, while exhibiting higher catalytic activity toward ethanol oxidation reaction compared to Ruhcp@Rhfcc counterparts.

Design of novel structural eutectic high entropy alloys (EHEAs) containing high-temperature resistant alloying elements

This article employs the “Simple Mixing Enthalpy Method” to add high-temperature alloying elements Mo/V and Nb to the Fe-Co-Cr-Ni-based alloy system, and utilizes a model of the relationship between elements and phase structures. Finally, EHEAs are successfully designed with the help of phase diagrams, namely FeCoNi2.0Cr1.2Mo0.2Nb0.63 (Mo0.2Nb0.63) and FeCoNi2.0Cr1.2V0.2Nb0.68 (V0.2Nb0.68). In EHEAs, a typical fully eutectic microstructure is observed, consisting of alternating FCC (face-centered cubic) and HCP (hexagonal close-packed) phases. Evaluation of mechanical performance indicators reveals that nano-hardness and elastic modulus increase from the FCC phase to the HCP phase, while the hardness and elastic modulus of the eutectic phase originate from modulation of these two phases. Meanwhile, the microhardness of both alloy systems increases linearly with increasing Nb content. In addition, EHEAs exhibit high strength and ductility but with differences attributed to the significant influence of Mo and V elements on eutectic phase spacing and phase size. Different phase interface types and strain gradients during deformation affect the mechanical properties. This study tests the high-temperature compression performance and fracture morphology of novel high-entropy alloys, paving the way for subsequent research on high-temperature performance.

Phase Engineering-Mediated D-Band Center of Ru Sites Promote the Hydrogen Evolution Reaction Under Universal pH Condition.

The rational design of pH-universal electrocatalyst with high-efficiency, low-cost and large current output suitable for industrial hydrogen evolution reaction (HER) is crucial for hydrogen production via water splitting. Herein, phase engineering of ruthenium (Ru) electrocatalyst comprised of metastable unconventional face-centered cubic (fcc) and conventional hexagonal close-packed (hcp) crystalline phase supported on nitrogen-doped carbon matrix (fcc/hcp-Ru/NC) is successfully synthesized through a facile pyrolysis approach. Fascinatingly, the fcc/hcp-Ru/NC displayed excellent electrocatalytic HER performance under a universal pH range. To deliver a current density of 10mAcm-2, the fcc/hcp-Ru/NC required overpotentials of 16.8, 23.8 and 22.3mV in 1M KOH, 0.5M H2SO4 and 1M phosphate buffered solution (PBS), respectively. Even to drive an industrial-level current density of 500 and 1000mAcm-2, the corresponding overpotentials are 189.8 and 284mV in alkaline, 202 and 287mV in acidic media, respectively. Experimental and theoretical calculation result unveiled that the charge migration from fcc-Ru to hcp-Ru induced by work function discrepancy within fcc/hcp-Ru/NC regulate the d-band center of Ru sites, which facilitated the water adsorption and dissociation, thus boosting the electrocatalytic HER performance. The present work paves the way for construction of novel and efficient electrocatalysts for energy conversion and storage.

Plastic deformations in NiCoFe medium-entropy alloy investigated using nanoindentation simulations

Medium-entropy alloys (MEA) have potential load-bearing applications as high-performance structural materials. In this work, the plastic deformations in NiCoFe MEAs were investigated using nanoindentation atomistic simulations. The nanoindentation responses of three typical orientations of NiCoFe single-crystals were investigated, i.e., [001], [011] and [111]. The results show that, during nanoindentation, Shockley partials on (111) slip planes remarkably affect dislocation-related activities including nucleation, gliding, and interactions. The form of atomic pile-ups is highly non-uniform and strongly asymmetrical due to the presence of multi-principal elements. The dislocation nucleation mechanism of the alloys during nanoindentation is proposed in detail. We find evidence that a hexagonal close-packed phase is formed from the face-centered cubic structure during nanoindentation.

The lattice friction stress driven temperature-dependent tensile deformation behaviors of CoNiCr2 eutectic medium-entropy alloy

A heterogeneous CoNiCr2 eutectic medium-entropy alloy (EMEA), comprising soft face-centered cubic (FCC) and hard body-centered cubic (BCC) lamellae, associated with minor acicular hexagonal close-packed (HCP) phase precipitated in BCC phase, was synthesized towards excellent tensile strength and ductility synergy. The tensile mechanical properties demonstrated that this alloy was temperature-dependent, i.e., when the testing temperature reduced from room temperature (RT) to liquid nitrogen temperature (LNT), the yield strength, ultimate strength, and uniform elongation were enhanced from 449 MPa, 821 MPa, and 5.0% to 702 MPa, 1174 MPa, and 8.4%, respectively. The prominent elevation of yield strength at LNT mainly resulted from the dramatically enhanced lattice friction stress (σ0) and the FCC-BCC interfacial strengthening, while the improved ductility was attributed to the superior crack-arrest capability of FCC matrix stemmed from the accumulation of stacking faults (SFs) and enhanced σ0 at LNT. Additionally, although the deformation mechanisms were dominated by planar dislocation glides and SFs at both temperatures, the initiation of premature cracks in the BCC phase due to the inferior deformation capability at LNT constrained the better strength-ductility trade-off. The cracks in the BCC phase tended to propagate along the BCC-HCP interfaces because of the strain incompatibility. Furthermore, the sub-nanoscale L12 particles in the FCC matrix could not only strengthen this alloy but also improve the stacking fault energy leading to no deformation twinning even at LNT. This work may provide a guide for the design of remarkable strength and ductility synergy EMEAs combined with outstanding castability for applications at cryogenic temperatures.

Facet-Controlled Synthesis of Unconventional-Phase Metal Alloys for Highly Efficient Hydrogen Oxidation.

Facet control and phase engineering of metal nanomaterials are both important strategies to regulate their physicochemical properties and improve their applications. However, it is still a challenge to tune the exposed facets of metal nanomaterials with unconventional crystal phases, hindering the exploration of the facet effects on their properties and functions. In this work, by using Pd nanoparticles with unconventional hexagonal close-packed (hcp, 2H type) phase, referred to as 2H-Pd, as seeds, a selective epitaxial growth method is developed to tune the predominant growth directions of secondary materials on 2H-Pd, forming Pd@NiRh nanoplates (NPLs) and nanorods (NRs) with 2H phase, referred to as 2H-Pd@2H-NiRh NPLs and NRs, respectively. The 2H-Pd@2H-NiRh NRs expose more (100)h and (101)h facets on the 2H-NiRh shells compared to the 2H-Pd@2H-NiRh NPLs. Impressively, when used as electrocatalysts toward hydrogen oxidation reaction (HOR), the 2H-Pd@2H-NiRh NRs show superior activity compared to the NiRh alloy with conventional face-centered cubic (fcc) phase (fcc-NiRh) and the 2H-Pd@2H-NiRh NPLs, revealing the crucial role of facet control in enhancing the catalytic performance of unconventional-phase metal nanomaterials. Density functional theory (DFT) calculations further unravel that the excellent HOR activity of 2H-Pd@2H-NiRh NRs can be attributed to the more exposed (100)h and (101)h facets on the 2H-NiRh shells, which possess high electron transfer efficiency, optimized H* binding energy, enhanced OH* binding energy, and a low energy barrier for the rate-determining step during the HOR process.

Chemical Imaging of Carbide Formation and Its Effect on Alcohol Selectivity in Fischer Tropsch Synthesis on Mn-Doped Co/TiO2 Pellets.

X-ray diffraction/scattering computed tomography (XRS-CT) was used to create two-dimensional images, with 20 μm resolution, of passivated Co/TiO2/Mn Fischer-Tropsch catalyst extrudates postreaction after 300 h on stream under industrially relevant conditions. This combination of scattering techniques provided insights into both the spatial variation of the different cobalt phases and the influence that increasing Mn loading has on this. It also demonstrated the presence of a wax coating throughout the extrudate and its capacity to preserve the Co/Mn species in their state in the reactor. Correlating these findings with catalytic performance highlights the crucial phases and active sites within Fischer-Tropsch catalysts required for understanding the tunability of the product distribution between saturated hydrocarbons or oxygenate and olefin products. In particular, a Mn loading of 3 wt % led to an optimum equilibrium between the amount of hexagonal close-packed Co and Co2C phases resulting in maximum oxygenate selectivity. XRS-CT revealed Co2C to be located on the extrudates' periphery, while metallic Co phases were more prevalent toward the center, possibly due to a lower [CO] ratio there. Reduction at 450 °C of a 10 wt % Mn sample resulted in MnTiO3 formation, which inhibited carbide formation and alcohol selectivity. It is suggested that small MnO particles promote Co carburization by decreasing the CO dissociation barrier, and the Co2C phase promotes CO nondissociative adsorption leading to increased oxygenate selectivity. This study highlights the influence of Mn on the catalyst structure and function and the importance of studying catalysts under industrially relevant reaction times.

Oxygen-induced decomposition of the body-centered cubic HfNbTaTiZr high-entropy alloy

The factors affecting the stability of an initially single-phase body-centered cubic (BCC) HfNbTaTiZr solid solution are investigated by heat treatments at 900 °C up to 1000 h in different atmospheres. The BCC solid solution is stable but oxygen (O) contamination originating from the aging atmosphere leads to the co-precipitation of an O-enriched hexagonal close-packed (HCP) Hf-Zr-rich phase and a second Nb-Ta-rich BCC phase. As O from the atmosphere diffuses from the surface to the core of the sample at a much faster rate along grain boundaries than through the grain interior, this results in an inhomogeneous distribution of precipitates, which formed at grain boundaries by a monotectoid reaction. In contrast, coincidence-site-lattice boundaries are not affected by this transformation owing to their lower diffusivities and lower capacity for heterogeneous nucleation. Similar phases were observed in a HfNbTaTiZr enriched with 3 at.% O, thus confirming the role of oxygen in phase stability.In both alloys, the orientation relationship (OR) between the HCP and BCC phases is predominantly of Burgers type but our analyses further revealed the presence of two minor ORs. More interestingly, both alloys exhibit rod-shaped HCP precipitates in contrast to the plate-shaped precipitates observed in conventional Ti and Zr-based alloys. This result can be rationalized by calculations of elastic strain energy density, which suggest that the rods should grow along 〈100〉BCC to minimize the elastic strain energy. Deviations of up to 20° from this theoretical direction were observed and are likely related to relaxation by misfit dislocations or by plastic deformation.

Atomic Simulation of Deformation Behavior of Polycrystalline Co30Fe16.67Ni36.67Ti16.67 High Entropy Alloy Under Uniaxial Loading

Mechanical behavior and plastic deformation mechanism of a new type of Co30Fe16.67Ni36.67Ti16.67 high entropy alloys (HEAs) have been calculated by the molecular dynamics method. The results show that the polycrystalline Co30Fe16.67Ni36.67Ti16.67 HEA has remarkable tensile plasticity and anisotropy. When the crystallographic orientation of the grain is <001>, the plastic deformation mechanism is face‐centered cubic (FCC)→body‐centered cubic (BCC) transformation and deformation twins. Grain boundary and vacancy reduce the nucleation energy of FCC→BCC phase transition, making BCC phase nucleation easy and growing in a shear manner, eventually forming deformation twins in the BCC phase. When the crystallographic orientation of grain is <110> and <111>, the plastic deformation mechanism is stacking faults, FCC→hexagonal‐close‐packed (HCP) phase transformation, and deformation twins. The motion of Shockley dislocation leads to the stacking fault, intrinsic stacking fault leads to the FCC→HCP phase transition, extrinsic stratification fault leads to the twin deformation, and the grain refining occurs during the tension process. Temperature and strain rate also have strong effects on tensile strength and elastic modulus. These results will provide a theoretical basis for the development of the HEAs and expand their application.

Interstitial engineering enabling superior mechanical properties of nitrogen-supersaturated Fe50Mn30Co10Cr10 high-entropy alloys

Interstitial atoms are key in modifying microstructures and enhancing mechanical properties of metals. Traditionally, the introduction of interstitial elements into metal matrices has been limited to low levels (< 2 at.%) to avoid the formation of brittle ceramics, constraining the exploitation of their full strengthening potential. This study introduces nitrogen-supersaturated high-entropy alloys (HEAs) with up to 28.9 at.% nitrogen, achieving substantial interstitial strengthening and phase adjustment. Remarkably, these HEAs remain solid solution phases without nitride formation, even at exceptionally high nitrogen levels. The microstructural evolution with increasing nitrogen content transitions from a single face-centred cubic (FCC) structure to a dual-phase structure of FCC and hexagonal close-packed (HCP) phases, and ultimately reverts to a predominantly FCC structure. These alloys achieve an impressive hardness of ∼ 20 GPa, comparable to ceramics, while maintaining exceptional damage-tolerance and plasticity. The outstanding mechanical properties are attributed to massive solid solution strengthening from a high nitrogen level, a hierarchical dual-phase structure, and stress-induced phase transformation from FCC to HCP. Contrary to the brittleness typical of nitrides, these nitrogen-supersaturated HEAs exhibit substantial plastic deformation akin to metallic materials, thus opening up a new pathway for enhancing the mechanical performance of advanced alloys under extreme loading conditions.

Strength Weakening and Phase Transition Mechanisms in Nanoindentation of Al/Mg-Layered Nanocomposites: A Molecular Dynamic Study

Molecular dynamics (MD) simulations were performed to investigate the nanoindentation behavior of Al/Mg-layered nanocomposites with varying layer thicknesses and Mg layer orientations in this study. The aim is to understand the weakening mechanisms at low layer thicknesses and the phase transition mechanisms associated with the dislocation slip angle in the Mg layer. Results indicate that the nanoindentation strength of nanocomposites increases with the layer thickness in the range of 1–10 nm, with the strength of 9.5 × 10−7 N at 10 nm being approximately 73% higher than that at 1 nm. This strength increase is mainly attributed to high interfacial stress, the higher percentage of amorphous atoms, weakened interatomic interactions, and the transition of adjacent interfaces to fully coherent interfaces that significantly reduce their ability to hinder dislocations at the low-layer thickness range. Additionally, in the initial deformation process, the hexagonal close-packed (HCP) phase of the Mg layer firstly transforms into the body-centered cubic (BCC) phase due to its lower energy barrier, followed by the emergence of a faced-centered cubic (FCC) phase driven by 1/3&lt;1−100&gt; dislocations. In the late stage of deformation, new dislocations are generated in the FCC phase and move along its slip planes, altering the dislocation direction. The FCC/HCP interfacial configuration also affects the HCP phase transition mechanism in the Mg layer. When the dislocation slip angle is 0°, the primary phase transition is the BCC phase, whereas a 45° slip angle results in the FCC phase. These findings will provide a guide for the preparation and manufacturing of new high-quality layered nanocomposites.

Dynamic response of equiatomic and non-equiatomic CrMnFeCoNi high-entropy alloys under plate impact

A typical Fe-rich non-equiatomic CrMnFeCoNi high-entropy alloy (HEA), Cr10Mn10Fe60Co10Ni10, and the equiatomic Cr20Mn20Fe20Co20Ni20 HEA, are investigated via plate impact experiments along with in situ free surface velocity measurements. Both the initial and postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. These two HEAs contain a single face-centered cubic phase, are texture-free, and have similar grain sizes. Dynamic yield stress and spall strength of the non-equiatomic CrMnFeCoNi are lower by ∼55 % and 15 % than those of its equiatomic counterpart, respectively. After shock compression, dislocation slips, stacking faults, Lomer-Cottrell locks and nanotwins are found in the postmortem samples of both HEAs. Due to the reduced stacking fault energy in the non-equiatomic HEA, more deformation twins and newly formed hexagonal close-packed phases are found. For spallation, intergranular voids are larger in size but smaller in quantity than intragranular voids for both HEAs. Both intragranular or intergranular voids show strong dependence on grain boundary misorientation.

Cryogenic mechanical behavior and FCC → HCP phase transformation mechanism in a Si-added CrCoNi medium-entropy alloy under quasi-static and dynamic tension

The CrCoNiSi0.3 MEA exhibits excellent cryogenic mechanical properties upon both quasi-static and dynamic tension. Under quasi-static tension at cryogenic temperature, the engineering yield and ultimate tensile strengths (UTS) reach 980 MPa and 1800 MPa, respectively, with notable ductility (62 %). The product of UTS and total elongation (TE) is 111.6 GPa %, surpassing most cryogenic high strength-ductility alloys. The significant mechanical strength enhancement is attributed to the denser deformation twins (DTs), multiple twinning, and extensive face-centered-cubic to hexagonal-close-packed (HCP) phase transitions, resulting in a high work hardening capacity. Upon dynamic tension at cryogenic temperature, the strength and strain hardening are further improved, which originates from the thickening DTs and HCP sequence and localized plastic deformation. The effects of temperature and strain rate on phase transition are studied. It is proposed that there is a competing relationship between high strain rate and increased stacking fault energy (SFE) due to temperature rise. The coupling effect of cryogenic temperature and high strain rate inhibits phase transition due to the deformation inhomogeneity in CrCoNiSi0.3 MEA. The findings make a valuable contribution to understand the influence of temperature and strain rate on the mechanism of FCC-to-HCP phase transition.

Implications of FCC and HCP cobalt phases on wear performance of weld deposited cobalt-based coating

SS316L possesses comparatively low wear and abrasion resistance properties and thus needs proper surface modification to sustain under severe industrial applications. Cobalt-based Stellite 6, a suitable coating for such applications, contains 4–5 % of tungsten that provides solid solution strengthening to the cobalt-based matrix, due to which it shows good wear resistance in abnormal working conditions. Stellite 6 is dominated by face-centered cubic (FCC) and hexagonal close packed (HCP) cobalt along with different carbide phases such as M7C3, M23C6, MC and M6C where M is Cr, W, Mo, Co and Fe. Weld deposited cobalt-based Stellite 6 coating on SS316L substrate, contains FCC structure as primary phases. Pure cobalt contains a stable HCP phase at temperature below 417 °C and FCC phase at and above 417 °C up to melting point of pure cobalt i.e., 1495 °C. Under the frictional stresses caused during wear test, impact more on FCC cobalt than HCP cobalt due to the high slip system of FCC. Because of this, FCC cobalt underwent through high wear losses compared to HCP cobalt in cobalt-based coating. Present study revealed that, high wear losses are seen in samples where FCC cobalt phases are dominant. The observed mechanism by SEM shows abrasive wear in which material removal takes place by ploughing, micro cutting and indentation. The targeted industrial applications where such wear losses are supposed to happen includes slurry transportation applications such as pipes, sea engineering components, submersible pumps, control valves.

Metal Doped Unconventional Phase IrNi Nanobranches: Tunable Electrochemical Nitrate Reduction Performance and Pollutants Upcycling.

Electrochemical nitrate reduction (NO3RR) provides a new option to abate nitrate contamination with a low carbon footprint. Restricted by competitive hydrogen evolution, achieving satisfied nitrate reduction performance in neutral media is still a challenge, especially for the regulation of this multielectron multiproton reaction. Herein, facile element doping is adopted to tune the catalytic behavior of IrNi alloy nanobranches with an unconventional hexagonal close-packed (hcp) phase toward NO3RR. In particular, the obtained hcp IrNiCu nanobranches favor the ammonia production and suppress byproduct formation in a neutral electrolyte indicated by in situ differential electrochemical mass spectrometry, with a high Faradaic efficiency (FE) of 85.6% and a large yield rate of 1253 μg cm-2 h-1 at -0.4 and -0.6 V (vs reversible hydrogen electrode (RHE)), respectively. In contrast, the resultant hcp IrNiCo nanobranches promote the formation of nitrite, with a peak FE of 33.1% at -0.1 V (vs RHE). Furthermore, a hybrid electrolysis cell consisting of NO3RR and formaldehyde oxidation is constructed, which are both catalyzed by hcp IrNiCu nanobranches. This electrolyzer exhibits lower overpotential and holds the potential to treat polluted air and wastewater simultaneously, shedding light on green chemical production based on contaminate degradation.