Severe Lattice Distortion Research Articles

Enhancing the high-temperature oxidation performance of AlCrTiSiN coating by in-situ generation of multi-layer antioxidant composite structures

A novel AlCrTiSiN high-entropy coating with superior thermal stability and high-temperature oxidation resistance, has been produced. The coating is characterized by unique fcc single-phase solid solution structure with uniform composition, severe lattice distortion and low fcc-AlN solubility, which results in the superior thermal stability. The excellent high-temperature oxidation resistance of coating is attributed to the multi-layer composite structures generated by in-situ reaction of element selective diffusion during the oxidation process. The inner dense SiO2-typed amorphous sub-layer and the intermediate (Al,Cr)2O3-typed sub-layer effectively block the O inward diffusion, and thus enhance the high-temperature oxidation resistance of the coating.

Fabrication and improved properties of ZrC-SiC-ZrB2 ceramics by ultra-high pressure sintering

ZrC-SiC-ZrB2 ceramics were prepared via ultra-high pressure sintering under 15 GPa at 1200 °C. The densification mechanism, microstructures, and properties of the prepared ceramics were investigated. The sintered ceramics with ZrB2 content of 15 vol% showed the improved comprehensive mechanical properties, including Vickers hardness of 26.6 GPa, Young's modulus of 469.1 GPa, and fracture toughness of 5.0 MPa·m1/2, which was 86.7% higher than that of pure ZrC ceramics. In the sintered sample, the ultra-high pressure induced new microstructure, such as L-C lock in ZrC phase, stacking faults and nanotwins in SiC phase, and severe lattice distortion in ZrB2 phase, jointly contributed to the improvement in Vickers hardness. Deflection and bridging during crack propagation were ascribed to the uniform combination of various phases.

Constructing Bipolar Dual-Active Sites through High-Entropy-Induced Electric Dipole Transition for Decoupling Oxygen Redox.

It remains a significant challenge to construct active sites to break the trade-off between oxidation and reduction processes occurring in battery cathodes with conversion mechanism, especially for the oxygen reduction and evolution reactions (ORR/OER) involved in the zinc-air batteries (ZABs). Here, using a high-entropy-driven electric dipole transition strategy to activate and stabilize the tetrahedral sites is proposed, while enhancing the activity of octahedral sites through orbital hybridization in a FeCoNiMnCrO spinel oxide, thus constructing bipolar dual-active sites with high-low valence states, which can effectively decouple ORR/OER. The FeCoNiMnCrO high-entropy spinel oxide with severe lattice distortion, exhibits a strong 1s→4s electric dipole transition and intense t2g(Co)/eg(Ni)-2p(OL) orbital hybridization that regulates the electronic descriptors, eg and t2g, which leads to the formation of low-valence Co tetrahedral sites (Coth) and high-valence Ni octahedral sites (Nioh), resulting in a higher half-wave potential of 0.87 V on Coth sites and a lower overpotential of 0.26 V at 10 mA cm-2 on Nioh sites as well as a superior performance of ZABs compared to low/mild entropy spinel oxides. Therefore, entropy engineering presents a distinctive approach for designing catalytic sites by inducing novel electromagnetic properties in materials across various electrocatalytic reactions, particularly for decoupling systems.

High friction stress and weak grain-refinement strengthening effect induced by severe lattice distortion in ZrTiNb medium entropy alloys

In this work, we have studied the grain-refinement strengthening effect of body-centered cubic Zr50Ti35Nb15 medium entropy alloy. The results suggest that the yield strength increases as grain size decreases from 530 μm to 0.51 μm, reaching the maximum value of 651 MPa, and still having decent ductility. Moreover, it is found that the alloys exhibit a high Hall–Petch intercept of 564 MPa and a low Hall–Petch slope, which means strong lattice friction stress and weak grain-refinement strengthening. The high lattice friction stress originates from the obstruction of dislocation motion caused by atomic size and modulus misfit. Dislocation interaction and multiplication in grain interior is difficult due to the high dislocation slip resistance, leading to a weak dislocation pile-up capability at grain boundary. Instead, dislocations nucleation and emission become the dominant deformation process since low shear modulus promotes dislocation source activation.

Thermal stability optimization of single-leg skutterudite-based thermoelectric devices based on lattice distortion effects

Skutterudite-based (SKD-based) thermoelectric materials are well-known for their high figure-of-merit (zT value) in the intermediate temperature region. Based on the urgent need for long-term high-temperature service, the poor interfacial thermal stability when connected with the Cu electrodes has greatly limited its industrial application. In this work, we propose a novel alloying route for the barrier layers for p-type SKDs. A Fe80Cr17.5Mo2.50/p-SKD junction with matched coefficients of thermal expansion (CTE), high mechanical reliability, and low contact resistivity is obtained. The addition of large-scale Mo causes severe lattice distortion in the barrier alloy, which contributes to the sluggish elemental diffusion. Thus, after aging at 823 K for 600 h, the Fe80Cr17.5Mo2.5 junction has a thinner reaction layer (∼25 μm), lower contact resistivity (∼3.8 μΩ·cm2), and higher shear strength (∼14 MPa), compared with the Mo-free (Fe80Cr20) barrier junction. Our finding opens a new insight for fabricating long-term high thermally stable SKD-based thermoelectric devices with desirable mechanical stability.

Medium- and High-Entropy Rare Earth Hexaborides with Enhanced Solar Energy Absorption and Infrared Emissivity.

The development of a new generation of solid particle solar receivers (SPSRs) with high solar absorptivity (0.28-2.5 μm) and high infrared emissivity (1-22 μm) is crucial and has attracted much attention for the attainment of the goals of "peak carbon" and "carbon neutrality". To achieve the modulation of infrared emission and solar absorptivity, two types of medium- and high-entropy rare-earth hexaboride (ME/HEREB6) ceramics, (La0.25Sm0.25Ce0.25Eu0.25)B6 (MEREB6) and (La0.2Sm0.2Ce0.2Eu0.2Ba0.2)B6 (HEREB6), with severe lattice distortions were synthesized using a high-temperature solid-phase method. Compared to single-phase lanthanum hexaboride (LaB6), HEREB6 ceramics show an increase in solar absorptivity from 54.06% to 87.75% in the range of 0.28-2.5 μm and an increase in infrared emissivity from 76.19% to 89.96% in the 1-22 μm wavelength range. On the one hand, decreasing the free electron concentration and the plasma frequency reduces the reflection and ultimately increases the solar absorptivity. On the other hand, the lattice distortion induces changes in the B-B bond length, leading to significant changes in the Raman scattering spectrum, which affects the damping constant and ultimately increases the infrared emissivity. In conclusion, the multicomponent design can effectively improve the solar energy absorption and heat transfer capacity of ME/HEREB6, thus providing a new avenue for the development of solid particles.

Radiation-enhanced precipitation and the impact on He bubble formation in V-Ti-based refractory alloys containing interstitial impurities

V-Ti-based refractory high entropy alloys (RHEAs), e.g., VTiTa and VTiTaNb, have been found with outstanding mechanical properties and irradiation resistance, granting them application potential in nuclear engineering. Nonetheless, similar to conventional V-Ti-based alloys such as V-4Ti-4Cr, these alloys are sensitive to C/N/O interstitial impurities. Under irradiation, such impurities may promote the formation of precipitates which may affect the defect evolution, especially at the appearance of helium. Previous studies have observed different helium behavior in the above alloys, including the heterogeneous distribution of helium bubbles in VTiTa, but the origin was not clear. In this work, detailed microstructural characterization is performed to analyze the irradiation induced precipitates and their impact on helium bubble formation in these alloys. Under helium ion irradiation at 700°C, the TiN precipitates with the NaCl-type structure are formed in all the three alloys. Nonetheless, the interfaces between precipitates and matrix in VTiTa and VTiTaNb are semi-coherent while those in V-4Ti-4Cr are coherent, due to the various lattice parameters of the alloys. Moreover, severe lattice distortion is also found inside the precipitates in the two RHEAs. Consequently, significant heterogeneous nucleation of He bubbles is observed in these two RHEAs but not in V-4Ti-4Cr. Among the three alloys, the growth of precipitates is the fastest in VTiTa, causing the formation of bubble clusters. Furthermore, the growth direction as well as the shape of precipitates affects the shape of bubbles, leading to the {001} cuboids shape in V-4Ti-4Cr and the truncated {110} dodecahedra shape in V-Ti-Ta and V-Ti-Ta-Nb.

Design and characterization of (TiZr)95-xHfxMo5 multi-principal element alloys with excellent properties for biomedical applications

Developing new bio-metallic materials with improved performance has attracted intensive research focus over the last decades. In the present work, a new series of (TiZr)95-xHfxMo5 (x = 0, 10, 15, 20, 25, 31.67) multi-principal elements alloys (MPEAs) have been designed for potential implant materials. The mechanical properties, deformation characteristics, tribological properties, corrosion resistance, and in vitro biocompatibility of the MPEAs were characterized. All the TiZrHfMo alloys possess a high yield strength (913–––1083 MPa) and tensile strength (931–––1118 MPa) and a large microhardness (296–––333 Hv). The addition of Hf into the alloys induced severe lattice distortion, which increased the solid-solution strengthening effect. It also resulted in a finer grain size and led to excellent wear resistance. Meanwhile, the results showed that the dislocation glide mode transformed from planar slip to cross-slip and the transformation became apparent with the decrease of Hf. Also, the MPEAs showed a good corrosion resistance compared to CP-Ti, which was credited to the superior passivation films. The in vitro biocompatibility experiments displayed that the TiZrHfMo alloys exhibited high cell viability ratios, which were close to that of CP-Ti. The present work demonstrates the huge potential of TiZrHfMo MPEAs as implant material for biomedical applications.

Recent Progress in High‐Entropy Alloy Electrocatalysts for Hydrogen Evolution Reaction

AbstractHigh‐entropy alloys (HEAs) materials, as promising nanomaterials, have garnered significant attention from researchers due to their excellent performance in the field of hydrogen evolution reaction (HER). The four core effects of HEAs, including the high‐entropy effect, severe lattice distortion effect, sluggish diffusion effect, and cocktail effect, are pivotal in underpinning their remarkable mechanical and thermodynamic properties. Nevertheless, the intricate geometric and electronic structures of HEAs make their catalytic mechanisms exceptionally complex and challenging to decipher. In particular, a thorough analysis of the underlying factors responsible for the outstanding catalytic activity, selectivity, and the ability to maintain stable hydrogen production, even at high current densities, in HEAs is lacking. To provide a systematic exploration of the design and application of HEAs in HER systems, this review commences with an examination of the physicochemical properties of HEAs. It covers a wide range of topics, including the synthesis methods of HEAs, and the major reaction mechanisms of HERs, and presents innovative methods and approaches for designing HEAs specifically in the context of HERs.

Preparation of AlCoCrFeNi/W-TiC HEA composite coating by laser cladding

In this paper, AlCoCrFeNiW2-x(TiC)x high-entropy alloy coatings (HEA) (x: mass fraction, x = 0, 0.5, 1.0, 1.5, and 2) were prepared on 316 stainless steel (316 L SS) substrate by laser cladding technique. The microstructure, phase, microhardness, wear resistance and corrosion resistance of the HEA composite coatings were discussed by adding different mass fractions of W and TiC particles. The results show that the coatings are metallurgically bonded under the experimental parameters, and the grain size of the coating decreases from the interface to the surface, which is in accordance with the constitutional undercooling criterion. Meanwhile, the contents of W and TiC can adjust the mechanical properties of the coatings. The microhardness of the coatings increases and then decreases with the variation of x value, which can be explained by the large number of TiC particles leading to severe lattice distortions and high internal stresses, which results in the fluctuation of microhardness in the cross-section. The coating exhibits the best microhardness, with an average microhardness of 452 HV when x = 0.5. The wear resistance of the coating is better with an average coefficient of friction of 0.4474 when x = 0.5. However, due to the presence of phase-boundary defects in the composite coatings, which contain cationic and anionic vacancies, When the passivation film is damaged by Cl-, the electrolyte will penetrate along the phase boundary defects and generate corrosive microcells.

Unveiling deformation behavior and damage mechanism of irradiated high entropy alloys

The oxide dispersion strengthening (ODS) high entropy alloy (HEA) exhibits the high elevated temperature performance and radiation resistance due to severe atomic lattice distortion and oxide particles dispersed in matrix, which is expected to become the most promising structural material in the next generation of nuclear energy systems. However, microstructure and damage evolution of irradiated ODS HEA under loading remain elusive at submicron scale using the existing simulations owing to a lack of atomic-lattice-distortion information from a micromechanics description. Here, the random field theory informed discrete dislocation dynamics simulations based on the results of high-resolution transmission electron microscopy are developed to study the dislocation behavior and damage evolution in ODS HEA considering the influence of severe lattice distortion and nanoscale oxide particle. Noteworthy, the damage behavior shows an unusual trend of the decreasing-to-increasing transition with the continuous loading process. There are two main types of damage micromechanics generated in irradiated ODS HEA: the dislocation loop damage in which the damage is controlled by irradiation-induced dislocation loops and their evolution, the strain localization damage in which the damage comes from the dislocation multiplication in the local plastic region. The oxide particle hinders the dislocation movement in the main slip plane, and the lattice distortion induces the dislocation sliding to the secondary slip plane, which promotes the dislocation cross-slip and dislocation loop annihilation, and thus reduces the material damage in the elastic damage stage. These findings can deeply understand atomic-scale damage mechanism and guide the design of ODS HEA with high radiation resistance.

Microstructural analysis of the cast and melt-spun high entropy noble alloy Ag20Pd20Pt20Cu20Ni20

Abstract High entropy alloys (HEA) represent a relatively new class of materials with promising properties for various applications. In recent years, these alloys have received considerable attention as potential heterogeneous catalysts in chemical and electrochemical reactions. Their enhanced catalytic activity is controlled by chemical composition, surface atomic coordination, electronic configuration and degree of microstructural metastability. Single-phase HEAs are of particular importance, because they possess a uniform microstructure that is useful for designing and prediction of mechanical and potential functional properties. The cooling rate has a significant impact on the formation of the microstructure, affecting the size of grains, as well as distribution and composition of precipitates and phases that are being formed during solidification. The influence of different cooling rates on the microstructure of the alloy Ag20Pd20Pt20Cu20Ni20 was studied in our research work. The microstructure and phase constituents were characterised by Scanning Electron Microscopy and X-ray Diffraction. Liquid phase separation with a consequential monotectic reaction resulted in an anomalous multiphase cast microstructure. With an increase in the cooling rate using the melt spinning technique, the number of formed phases and the size of the grains decreased and the high entropy supersaturated solid solution was attained, as the atoms in the alloy did not have enough time to diffuse and to rearrange themselves into a stable, ordered structure. It was also confirmed that higher cooling rates cause severe lattice distortion and create coordinatively unsaturated sites at the surface which are essential for the bonding and activation of the reactants and therefore improve the potential catalytic properties of the Ag20Pd20Pt20Cu20Ni20 alloy significantly.

Synergistic Effects of PtRhNiFeCu High Entropy Alloy Nanocatalyst for Hydrogen Evolution and Oxygen Reduction Reactions.

The unique properties of high entropy alloy (HEA) catalysts, particularly their severe lattice distortion and the synergistic effect of multiple components, endow them with exceptional multifunctional catalytic performance. Herein, it is revealed for the first time, that the ultrasmall PtRhNiFeCu HEA nanoparticles catalyst shows outstanding catalytic activity for both hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The catalyst exhibits an impressively low overpotential of 13mV at 10mAcm-2, a Tafel slope of 29.6mVdec-1, and high mass activity of 7.6AmgPt -1 at -50mV in alkaline media, and long-term stability of at least 20h. Moreover, the catalyst also demonstrates effective catalytic activity for acidic ORR with a commendable performance of 1.23AmgPt -1, much exceeding the commercial Pt/C catalyst. Density functional theory (DFT) calculations unveil that the efficient electrocatalytic performance for HER and ORR can be primarily attributed to the synergistic effect between components tailors and optimizes the electronic structure of PtRhNiFeCu/C HEA, which not only enhances the HER activity through increasing water capture capability, decreasing energetic barrier for water dissociation, and optimizing hydrogen absorption but also initiates non-platinum active sites with high ORR activity, achieving the improved ORR performance.

Superior strength-ductility combination in TiC/TC4 composites via In situ construction Ti2Cu nanoparticles

Synergistic improvement of strength and ductility is challenging in the engineering applications of titanium matrix composites (TMCs). Herein, we fabricated the TC4 composites reinforced with grain boundary micron-TiC and intragranular nano-Ti2Cu using spark plasma sintering (SPS) and hot rolling (HR) to achieve a good balance between strength and ductility. An interfacial reaction between Ti and C forms the grain boundary TiC, and the vast majority of intragranular nano-Ti2Cu precipitates to accommodate the severe lattice distortion caused by the solid solution of Cu in β-Ti phase. Benefiting from Ti2Cu nanoprecipitates and their ability to effectively suppress dislocation motion, precipitation strengthening, and dislocation strengthening are the main factors, in addition to grain refinement strengthening due to deformation processing, that contribute to the realization of the superior yield strength (YS) of 1360 MPa and ultimate tensile strength (UTS) of 1504 MPa in composites than TC4 matrix (YS = 986 MPa, UTS = 1160 MPa). Meanwhile, the interfacial strain-delocalization effect of TiC/Ti2Cu with Ti matrix, the shear-lag effect of TiC, and the sustained strain-hardening effect provided by TiC/Ti2Cu empower the composites with an outstanding ductility (fracture elongation of 8.4 %). This study highlights the untapped potential for improving the mechanical properties of Ti matrix composites.

Ultrastrong and High Thermal Insulating Porous High-Entropy Ceramics up to 2000°C.

High mechanical load-carrying capability and thermal insulating performance are crucial to thermal-insulation materials under extreme conditions. However, these features are often difficult to achieve simultaneously in conventional porous ceramics. Here, for the first time, it is reported a multiscale structure design and fast fabrication of 9-cation porous high-entropy diboride ceramics via an ultrafast high-temperature synthesis technique that can lead to exceptional mechanical load-bearing capability and high thermal insulation performance. With the construction of multiscale structures involving ultrafine pores at the microscale, high-quality interfaces between building blocks at the nanoscale, and severe lattice distortion at the atomic scale, the materials with an ≈50% porosity exhibit an ultrahigh compressive strength of up to ≈337MPa at room temperature and a thermal conductivity as low as ≈0.76W m-1 K-1. More importantly, they demonstrate exceptional thermal stability, with merely ≈2.4% volume shrinkage after 2000°C annealing. They also show an ultrahigh compressive strength of ≈690MPa up to 2000°C, displaying a ductile compressive behavior. The excellent mechanical and thermal insulating properties offer an attractive material for reliable thermal insulation under extreme conditions.

Elinvar-Like Effect Induced by High Lattice Distortion in Zr 6 Ta 2 O 17 Ceramics

Super strength and toughness, excellent deformation resistance, and high-temperature service performance are the key factors to determine the practical application of new thermal barrier coatings (TBCs). The limited mobility of dislocations and the internal inherent defects in ceramics will inevitably lead to the decline of strength–plasticity and the reduction of service performance. Introducing preexisting twin boundaries and stacking faults (SFs) or preparing ceramic materials with high configuration entropy has demonstrated to be an effective strategy for enhancing the mechanical properties of ceramics. However, due to the positive thermal expansion coefficient of most ceramics and the remarkable increase of structural disorder at elevated temperature, the problem of elastic softening has become a bottleneck restricting the high-temperature service life of new TBCs. In this paper, the deformation behavior of high configuration entropy Zr 6 Ta 2 O 17 ceramics at 25 to 1,200 °C was in situ monitored via digital image correlation technique and three-point bending test platform in high-temperature environment. A remarkable Elinvar-like effect appears in the Zr 6 Ta 2 O 17 ceramic. More interestingly, mechanical deformation dominates the severe lattice distortion (deformation twins, SFs) and the disorder–order transition of chemical order at the atomic scale, while temperature can further enhance the degree of lattice distortion and ordering of Zr 6 Ta 2 O 17 ceramics. Furthermore, the atomic fluctuations at high temperature promotes the comprehensive improvement of mechanical properties in the Zr 6 Ta 2 O 17 ceramics.

High Entropy Alloy Deposition from an Aqueous Bath

A new alloy design approach based on the concept of high entropy alloy (HEA) has attracted attention during the last few years. HEA is defined as an alloy composed of five or more principal elements with a concentration between 5 and 35 at% of each element. Depending on the combinations of the elements, HEA could show high resistance to corrosion, fatigue and fracture toughness besides unique electrical and magnetic responses. The outstanding characteristic of HEA is attributed to the high mixing entropies and the severe lattice distortion related to the addition of multiple components. Due to this unique combination of properties and the potential for designing a wide variety of new materials, interests have been directed toward high entropy alloy films and coatings for applications where corrosion and oxidation protection are strongly required. This study attempted to deposit five elements (Ni-Co–Cu–Mo-W) from an aqueous bath on a low-carbon steel substrate by the direct current. The goal was to combine the properties of Ni-Co-Cu with Ni-W and Ni-Mo coatings. In this case, a HEA coating can be produced, which might maintain corrosion and wear resistance even at high temperatures and be suitable for application in aggressive environments. To understand better the role of entropy on the coatings properties, three electroplating systems were designed, including electroplating of five elements (Ni-Co-Cu-Mo-W), which was compared with electroplating of four elements (Ni-Co-Cu-W) and (Ni-Co-Mo-W). In this case, systems with different configuration entropy can be produced, and by characterizing them, the relationship between the entropy and the properties of the coating might be cleared.Electrodeposition was carried out in Hull-cell to find the alloy deposition's proper working window. The role of different parameters such as pH, current density, and complex agent on the coating's composition was studied. The morphology and composition of the Hull-cell coatings were checked by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy along with X-ray fluorescence spectroscopy. Finally, the hardness and corrosion resistance of the coatings was studied by the nanoindentation technique and polarization in NaCl. This study developed a single aqueous bath for the electrodeposition of Ni-Co-Cu-Mo-W, Ni-Co-Cu-W and No-Co-Mo-W, and suitable deposition conditions were determined. It was shown that besides the metal concentration ratio, pH was the most influential parameter in the codeposition of elements with different deposition mechanisms. Ni and Co were the main inducing elements in the deposition of Mo and W than Cu. In addition, Ni, Co, Mo and W deposited uniformly, while Cu showed a different behavior. A competition between Mo and W deposition was seen, and Mo deposited more preferentially than W. The coatings showed weak passivity behavior and comparison between coating’s corrosion resistance was a challenge since their corrosion behavior does not only depend on coating composition but also on coatings compactness and integrity. By comparing the nano-hardness of the coatings, it was seen that the Ni-Co-Cu-W system with even the lowest configurational entropy has the highest hardness value. Keywords: Electrodeposition; High entropy alloy; Alloy deposition; hardness; Corrosion resistance

Nickel content-dependent microstructure and mechanical properties of TiZrNbHfNi high entropy alloy thin films

Alloying is known as an effective strategy to improve the mechanical properties of metallic materials. Here we show that the microstructure and mechanical behavior of a TiZrNbHf1.6 high entropy alloy film can be highly tuned by Ni addition. The newly designed TiZrNbHf1.6Nix (x = 0, 0.4, 1.2, 1.8, 2.5, 3.0, 4.6) alloy films are synthesized by co-sputtering Ti, Zr, Nb, Hf and Ni elemental targets. The microstructure and mechanical behavior of the alloy films are investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and nano/micro-indentations. The microstructural analysis indicates that the TiZrNbHf1.6 film has a single body-centered cubic with columnar-nanograined structure. As Ni content increases to 0.4 or above, the films show a complete amorphous structure. Our theoretical analysis shows that the transition from the crystalline to the amorphous structure can be attributed to the large atomic radius difference and the change of the thermodynamic parameters (e.g., mixing entropy and mixing enthalpy) due to the Ni addition. Compared to the TiZrNbHf1.6 film with a hardness of 4.8 GPa, a hardness increase of 8–35% is achieved in the Ni-doped alloy films (5.2–6.5 GPa) as measured by nanoindentation. The elevated hardness may be attributed to the severe lattice distortion and the amorphous structure induced by the Ni addition. Furthermore, microindentation reveals a diversity of deformation behavior in those films with different Ni content. A homogeneous deformation (slight plastic pileup) occurred in the films with Ni content of 1.2 and 4.6, which is close to the deformation behavior of the crystal TiZrNbHf1.6 film. While the film with a Ni content of 1.8 exhibits a catastrophic cracking deformation. Finally, multiple shear banding dominates the deformation of these films with Ni content of 0.4, 2.5 and 3.0. Our findings demonstrate that ultrastrong and highly deformable amorphous high entropy alloy thin films can be developed by appropriate element addition, making them promising materials for corrosion and wear-resistant coatings.

In-situ alloying of NiTiNb ternary shape memory alloys via laser powder bed fusion using pre-alloyed NiTi and elemental Nb powders: Microstructure, phase transformation behavior and functional properties

Laser powder bed fusion (L-PBF) provides not only the advantage to make geometrically complex structures, but also the potential for in-situ alloying. In this work, (NiTi)100–xNbx (x = 1, 3, 5, 9 at%) ternary shape memory alloys were in-situ synthesized through L-PBF using a mixture of pre-alloyed near-equiatomic NiTi and elemental Nb powders. Low-Nb (1, 3 at%) alloys exhibit coarse columnar grains and high cracking tendency, while high-Nb (5, 9 at%) alloys show fine microstructure and low cracking susceptibility. Increasing Nb content leads to a transition from planar to cellular/dendritic growth, as a result of the increased constitutional supercooling (ΔTCS) and thermal undercooling (ΔTt) ahead of the solidification front. The L-PBF fabricated (NiTi)91Nb9 alloy shows comparable transformation characteristics to the conventional Ni47Ti44Nb9 alloy, with martensite start temperature (Ms) of 190 K and thermal hysteresis up to ca. 170 K after 20% pre-deformation. Achieving low Ms temperature with near-equiatomic Ni/Ti composition (i.e., Ni/Ti=1.024) is attributed to severe lattice distortion caused by Nb solution in the supersaturated matrix and the depletion of Ti to form the Ti-rich β-Nb phase, which resulted from the intrinsic rapid solidification associated with L-PBF. The in-situ alloyed (NiTi)100–xNbx alloys exhibit good mechanical and functional properties (total fracture strain >7%, and shape recovery rates >85%). This work may provide insights into in-situ alloying of NiTi-based ternary SMAs through L-PBF.

Effect of dendritic structure and secondary phases on the fatigue behavior of ERNiCrMo-3 weld metal

The effect of dendritic structure and secondary phases on the fatigue crack propagation mechanism of ERNiCrMo-3 weld metal was investigated. Element segregation in the weld metal induced severe lattice distortion in the interdendritic region, creating substantial variations in mechanical properties between the dendrite core and interdendritic region. This disparity resulted in an unstable crack propagation rate. The limited plastic deformation capacity of the interdendritic region caused a contraction in the local plastic deformation zone at the crack tip, increasing stress concentration and accelerating crack propagation. The fatigue crack in the interdendritic regions exhibited a rate of approximately 2.6 μm/cycle, surpassing the 1.5 μm/cycle observed in the dendrite cores. Meanwhile, element segregation promoted the precipitation of numerous Laves phases in the interdendritic regions. With increasing stress, micro-voids formed by broken Laves phases served as rapid channels for crack propagation. Post-weld heat treatment weakened the non-uniformity of the microstructure. The fatigue crack propagated at a rate of about 1.2 μm/cycle. Furthermore, the dissolution of Laves phases and the precipitation of γ″ enhanced the fatigue performance of the weld metal. Under equivalent stress levels, the fatigue life of post-weld heat-treated weld metal increased by approximately 1.2 times compared to that of as-welded metal.