High-performance High-entropy Alloys Research Articles

Controlling of cellular substructure and its effect on mechanical properties of FeCoCrNiMo0.2 high entropy alloy fabricated by selective laser melting

Fine cellular substructures are typical microstructure feature of high entropy alloys (HEAs) produced via selective laser melting (SLM), playing a pivotal role in improving the mechanical properties. Nonetheless, the controlling of cellular substructure and its impact on the mechanical properties remains ambiguous. This study investigates the effect of energy densities on the cellular substructure evolution and mechanical properties of the FeCoCrNiMo0.2 HEA. It is found the increase in energy density causes a decrease in temperature gradient (G) and solidification rate (R) in molten pool, consequently leading to the increase of cellular substructure size and the intensification of Mo segregation at cellular substructure boundaries. The cellular substructure size (d) can be described by formula d=80(GR)−1/3, in which G and R can be obtained from discrete element method - computational fluid dynamics (DEM-CFD) simulation. The strength of the FeCoCrNiMo0.2 HEA is significantly affected by dislocation strengthening (σd) and segregation strengthening (σs), which are further determined by the cellular substructure size and the Mo segregation, respectively. The increase of cellular substructure size leads to the decrease of σd, while the intensification of Mo segregation results in the increase of σs. The competition between these two strengthening effects leads to an optimized and excellent tensile property of 707 MPa in yield strength, 947 MPa in ultimate tensile strength and 34 % in fracture elongation at a moderate energy density of 47 J/mm3. The findings provide guidance towards the advancement of high-performance high entropy alloys fabricated by SLM in terms of cellular substructure controlling.

Microstructure and mechanical property of Al,Ti co-adding L21-strengthened NiCrFe-based HEAs

In this study, leveraging on the machine learning, a novel high-strength Ni100-x-yCr25Fe25AlxTiy (x = 5, y = 5; x = 8, y = 7; x = 10, y = 10) high-entropy alloy (HEA) system was rationally designed, whose phase transformed from FCC + BCC to single BCC. The structural features, mechanical properties, and strengthening mechanism were systematically investigated. It has been discovered that the addition of Al and Ti can accelerate the formation of L21 phase. The Ni35Cr25Fe25Al8Ti7 HEA shows outstanding comprehensive mechanical behavior, whose yield strength, compressive strength, fracture strain, and nano-hardness is 1913.69 MPa, 3896.62 MPa, 39.94%, and 11.07 GPa, respectively. These findings will give new insights into the controllable design of novel high-performance HEAs, enabling the achievement of target microstructures and superior combinations of properties.

Laser additively manufactured crack-free aluminum-bearing high entropy alloys: alloy design, synthesis, cracking inhibition and microstructure evolution effects on their tensile properties

ABSTRACT Developing high-performance high-entropy alloys (HEAs) fabricated by laser additive manufacturing (LAM) is the pursuit of the metallic community. In the present work, we designed a series of [(Al6-xNbx)-(FeCoNi)12]Cr3 HEA compositions using a high-entropy alloying strategy based on a cluster-plus-glue-atom model. And their thin-wall-sharped bulks were fabricated by LAM and post-aging treatment. The effects of cracking inhibition and microstructure evolution on the tensile properties were researched in detail. The results show that as the Nb substitutes for Al atoms, the cracking behaviour is ameliorated, ascribed to the tiny Laves phase refined the dendrite spacings and back-filled in the inter-dendritic liquid film. Also, introducing Nb atoms improves the strength but deteriorates the ductility. Significantly, the Nb4 HEA possesses the best tensile-property combination (i.e. σs ∼ 419.2 MPa, σb ∼ 787.4 MPa, and δ ∼ 15.5%) with a strain mechanism of dislocation slip mode. After post-aging for 72 h, the microstructure comprises fully recrystallized equiaxed FCC grains and many tiny needle-like D019 precipitates, leading to high strength and sufficient ductility (i.e. σ0.2 ∼ 535.9 MPa, σb ∼820 MPa and δ value of 8.9%). These findings provide a new paradigm for the LAM of crack-free HEAs with excellent mechanical properties.

An ultra-strong and ductile crystalline-amorphous nanostructured surface layer on TiZrHfTaNb0.2 high-entropy alloy by laser surface processing

Heterogeneous crystalline-amorphous nanostructures have been documented to show superior strength-ductility synergy via the co-deformation cooperative effects of nanograins and amorphous grain boundaries. In this work, a facile laser surface remelting technique with rapid cooling rate was successfully developed to fabricate a ∼ 100 μm-thick gradient nanostructured layer accompanied by phase decomposition on a TiZrHfTaNb0.2 high-entropy alloy, where a ∼ 5 μm-thick crystalline-amorphous nanostructured top surface layer with an average grain size of ∼ 7 nm was obtained. Such crystalline-amorphous nanostructured layer shows an ultrahigh yield strength of ∼ 6.0 GPa and a compression strain of ∼ 25 % during the localized micro-pillar compression tests. The atomic observations reveal that co-deformation cooperative mechanisms include the well-retained dislocation activities in nanograins but crystallization in amorphous grain boundaries, which subsequently lead to the grain coarsening via grain boundary-mediated plasticity. This study sheds light on the development of high-performance high-entropy alloys with novel crystalline-amorphous nanostructures and provides significant insight into their plastic deformation mechanisms.

Corrosion-erosion behavior and mechanism of Cu[sbnd]Mo co-doped CoCrFeNi high-entropy alloy coating prepared by directed energy deposition

Developing cost-effective and high-performance high-entropy alloy (HEA) coatings on demand is imperative. This study designed a class of HEAs on-demand by synergistic doping of Cu and Mo elements based on the first-principles and multi-principal alloy design concept. A CoCrFeNiCu0.25Mo0.75 HEA coating with outstanding corrosion and slurry erosion wear performances was fabricated on the duplex stainless steel using the directed energy deposition technique. The corrosion mechanism of the HEA coating in 3.5 % NaCl and 0.5 M H2SO4 solutions and the slurry erosion wear mechanism under a slurry concentration of 5 wt% were analyzed in combination with the density functional theory. The results indicate that the SQS crystal model is a better predictor of phases and properties of HEA than the VCA model. The HEA coating is a cellular structure that consists mainly of a face-centered cubic phase structure and contains a small amount of homogeneous sigma nanocrystalline secondary phase precipitation. Meanwhile, the passive film of the HEA coating is mainly composed of metal oxides and hydroxides. Cu is not oxidized during the corrosion process and always exists in the metallic form, which serves as a barrier layer to prevent the transfer of point defects. Mo with a larger atomic size can induce lattice distortion, enhance the coating strength and hardness, and reduce the density of point defects. Further, the slurry erosion wear rate of the HEA coating is lower at a 30° impingement angle than 90° for brittle erosion mode. At the 30° impingement angle, micro-cutting and ploughing are the main wear mechanisms of the HEA coating, while at the 90° impingement angle, micro-indents and craters are the main wear mechanisms. In addition, the corrosion and slurry erosion wear performances of CoCrFeNiCu0.25Mo0.75 HEA coating are better than the most reported once-formed HEAs.

Effects of phase transition on tribological properties of amorphous VAlTiCrSi high-entropy alloy film by magnetron sputtering

High-entropy alloy films have shown great potential in high-temperature antifriction and anti-wear applications . Magnetron sputtering high-entropy alloy films with uniform composition will have inevitable structural changes at high temperatures, which have an impact on their tribological behavior. In this work, amorphous VAlTiCrSi high-entropy alloy films prepared by magnetron sputtering show good thermal stability and high-temperature tribological properties. Especially at 700 °C, the structural change from an amorphous phase to a mixture consisting of body-centered cubic, Al 8 Cr 5 , (V, Ti, Cr) 5 Si 3 , and amorphous phases are responsible for the improved tribological properties. The hard body-centered cubic phase with solid-solution strengthening effect and intermetallic compounds significantly enhance the mechanical properties of VAlTiCrSi film, so as to improve the wear resistance performance of the films. The insight into the relationship between structural changes and friction properties will contribute to the design of novel high-performance high-entropy alloy films in high-temperature environments. • Amorphous VAlTiCrSi high-entropy alloy film prepared by magnetron sputtering. • Structure changes from amorphous to body-centered cubic, Al 8 Cr 5 , and (V, Ti, Cr) 5 Si 3 . • The hard phases at 700 °C improve the mechanical and tribological properties.

Effects of Al and Ti co-doping on the strength-ductility- corrosion resistance of CoCrFeNi-AlTi high-entropy alloys

In order to design and develop new marine structural materials with a balance of strength-toughness-corrosion resistance, in this work, the synergistic effects of Al and Ti on the microstructure, mechanical and corrosion resistance properties of CoCrFeNi(Al0.3Ti0.2)x high-entropy alloys were systematically investigated. With the addition of Al and Ti, the L12-γ' phase formed in the alloys significantly improves the mechanical properties, but excessive addition will lead to the formation of Al-Ti-rich B2 phase structure, which reduces the ductility of the alloy. For the corrosion resistance of the alloys in 3.5 wt. % NaCl solution, the increase of Al and Ti contents improves the stability of the passive film, while effectively reduces the corrosion current density and passivation current density of the alloys. However, the precipitation with a large number of strengthening phases aggravates the segregation of elements, resulting in a decrease in the pitting corrosion resistance and the resistance to the pit propagation of the alloy. In addition, compared with conventional structural alloys, the L12-strengthened dual-phase CoCrFeNiAl0.15Ti0.1 high-entropy alloy possesses an excellent balance of strength-toughness-corrosion resistance. This work also provides guidance for the future development of high-performance high-entropy alloy systems for marine engineering.

Effect of Twin Boundary Density on Mechanical Behavior of Al0.1CoCrFeNi High-Entropy Alloy by Molecular Dynamics Simulation

Molecular dynamics simulation method was used to study the influence of co-lattice twin boundary (TB) spacing on the deformation behavior of Al0.1CoCrFeNi high-entropy alloy single crystal under uniaxial tension. Studies have shown that there is a “critical distance” between the twin boundaries, and the sensitivity of the two sides to the change of the twin spacing is different, and the influence of the evolution of the deformation mechanism of the different twin spacing is analyzed from the number of defects and the evolution of the dislocation density. The results show that as the distance between twins decreases, the deformation mechanism gradually transforms from dislocation slip accompanied by defects, such as stacking faults and secondary twins, to a deformation mode of amorphous phase transition. The research aims to provide guidance and reference for the design of high-performance high-entropy alloys.

Laser powder bed fusion of high-entropy alloy particle-reinforced stainless steel with enhanced strength, ductility, and corrosion resistance

In additive manufacturing of metal matrix composites, it is difficult to ensure high strength as well as high ductility simultaneously. In this study, a FeCoNiCr high-entropy alloy was used as the strengthening phase to improve the performance of austenitic stainless steel processed via laser powder bed fusion. The microstructure of the as-printed parts was composed of high-entropy alloy nanoparticles, a solid solution matrix, and high-density dislocations. The strengthening effect caused by the microstructures resulted in a tensile strength of 627 MPa with a maximum elongation of 50%. The semi-obstructive effect of the interface on the dislocations improved the ductility, as proved through molecular dynamics analysis. The Cr, Ni, and Co elements in the high-entropy alloy melted into the stainless-steel matrix, facilitating the formation of a passive film; this improved the corrosion resistance of the composite. Thus, simultaneous improvements in strength, ductility, and corrosion resistance of the stainless-steel composites were realized. The composites inherited the performance characteristics of high-entropy alloys. This research also provides a new strategy to select the strengthening phase for composite materials. High-performance high-entropy alloy particles with lattice structures and element types similar to the stainless steel matrix have substantial potential to be used as strengthening phases.

Effect of twin boundary on mechanical behavior of Cr<sub>26</sub>Mn<sub>20</sub>Fe<sub>20</sub>Co<sub>20</sub>Ni<sub>14</sub> high-entropy alloy by molecular dynamics simulation

The high-entropy alloys break through the traditional alloy structure and present unique and superior mechanical properties. However, the potential deformation mechanism of high-entropy alloy, which is regarded as a new member of alloy families in recent years, needs to be further investigated. In this paper, the mechanical properties of the nano-twin Cr<sub>26</sub>Mn<sub>20</sub>Fe<sub>20</sub>Co<sub>20</sub>Ni<sub>14</sub> high-entropy alloy under tensile loading are studied by molecular dynamics simulation, and the effect of twin boundary on the deformation behavior of nano-twin Cr<sub>26</sub>Mn<sub>20</sub>Fe<sub>20</sub>Co<sub>20</sub>Ni<sub>14</sub> high-entropy alloy is studied on an atomic level. The results show that the yield strength of the nano-twin Cr<sub>26</sub>Mn<sub>20</sub>Fe<sub>20</sub>Co<sub>20</sub>Ni<sub>14</sub> high-entropy alloy increases with twin boundary spacing decreasing, presenting a Hell-Petch relationship. However, there is a critical value of the twin boundary spacing, which makes the sensitivity of the yield strength of the high-entropy alloy to the twin boundary spacing change significantly before and after this value. The results also indicate that the deformation mechanism of nano-twin Cr<sub>26</sub>Mn<sub>20</sub>Fe<sub>20</sub>Co<sub>20</sub>Ni<sub>14</sub> high-entropy alloy changes from dislocation slip to amorphous phase transition with the decrease of twin boundary spacing. The research results of this paper have a certain reference value and guidance significance for designing and preparing high-performance high-entropy alloys.

Microstructural evolution and mechanical properties of AlCoCrFeNi high-entropy alloy joints brazed using a novel Ni-based filler

This paper reported a route to fabricate a high-performance high-entropy alloy (HEA) brazed joint with the fusion zone similar to the base metal, in which the brazing of a Ni-containing AlCoCrFeNi HEA using a novel Ni-based filler was investigated at 1150–1350 °C for 0–45 min. A microstructure similar to the base metal was received in the fusion zone through proper interface control, thus the joint shear strength obtained was approximately three times (687.2 ± 23.2 MPa) in contrast to that of the joint with the heterogenous interface (199.4 ± 2.8 MPa). The strengthening mechanism was attributed to the high-entropy solid solution, precipitation strengthening and grain refinement occurring in the joint. The highest joint strength of 687.2 ± 23.2 MPa was received at 1300 °C for 15 min with the fracture mainly happening in the base metal. The brazed joint exhibited desirable high-temperature stability with minor degradation of joint strength (∼3%) after annealing of 800 °C for 240 h. The excellent joint performances are potential for high temperature applications.

A new method of preparing high-performance high-entropy alloys through high-gravity combustion synthesis

A new method of high-gravity combustion synthesis (HGCS) followed by post-treatment (PT) is reported for preparing high-performance high-entropy alloys (HEAs), Cr0.9FeNi2.5V0.2Al0.5 alloy, whereby cheap thermite powder is used as the raw material. In this process, the HEA melt and the ceramic melt are rapidly formed by a strong exothermic combustion synthesis reaction and completely separated under a high-gravity field. Then, the master alloy is obtained after cooling. Subsequently, the master alloy is sequentially subjected to conventional vacuum arc melting (VAM), homogenization treatment, cold rolling, and annealing treatment to realize a tensile strength, yield strength, and elongation of 1250 MPa, 1075 MPa, and 2.9%, respectively. The present method is increasingly attractive due to its low cost of raw materials and the intermediate product obtained without high-temperature heating. Based on the calculation of phase separation kinetics in the high-temperature melt, it is expected that the final alloys with high performance can be prepared directly across master alloys with higher high-gravity coefficients.

Mechanistic origin of high strength in refractory BCC high entropy alloys up to 1900K

The body centered cubic (BCC) high entropy alloys (HEAs) MoNbTaW and MoNbTaVW show exceptional strength retention up to 1900K. The mechanistic origin of the retained strength is unknown yet is crucial for finding the best alloys across the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a theory of strengthening of edge dislocations in BCC alloys. The strength of BCC HEAs can be controlled by edge dislocations, especially at high temperatures, because the random field of solutes in these high-concentration alloys creates large energy barriers for thermally-activated edge glide. A parameter-free theory for this mechanism of edge motion in BCC alloys qualitatively and quantitatively captures the strength versus temperature for both MoNbTaW and MoNbTaVW alloys and other BCC HEAs. A reduced analytic version of the theory, embodying the edge-strengthening mechanism, then enables screening over > 600,000 compositions in the Mo-Nb-Ta-V-W family to identify promising new compositions with high retained strength and/or reduced mass density. Overall, the theory reveals an unexpected mechanism - edge dislocation motion in the random alloy - as responsible for high temperature strength in BCC alloys and paves the way for theory-guided design of new high-performance HEAs.

Design of D022 superlattice with superior strengthening effect in high entropy alloys

Precipitation strengthening is one of the most promising mechanisms to develop high-performance high entropy alloys (HEAs). However, the design of a reinforcing phase with an excellent strengthening effect is still one of the most pivotal challenges. In the present study, a design strategy based on overall valence electron concentration (OVEC) is developed, and a coherent D022 superlattice (noted as γ″ phase) with superior strengthening effect is designed. The newly developed γ″ phase is systematically characterized using transmission electron microscope and atom probe tomography. Differentiating from the traditional Ni3Nb γ″ phase, the present high-entropy γ″ phase contains ∼7.7% Co and follows the (Ni,Co,Cr,Fe)3(Nb,Fe) stoichiometry. Three γ″ phase variants are observed with crystallographic orientation relationships of [001]γ″//<001>γ and (001)γ″//{100}γ. The lenticular γ″ particles with small volume fraction (7%) causes a significant yield strength increase (670 MPa) and ductility retention (40%), resulting in excellent yield strength-ductility combination. The excellent strengthening effect of the γ″ phase is attributed to both ordering strengthening and coherency strengthening. The present study proposes a new design strategy of precipitates and develops a superior reinforcing phase for HEAs. These findings will not only promote the development of precipitation-hardened HEAs but deepen the fundamentals of precipitates design for other complex concentrated alloys as well.

Manufacture of WNbMoTa High Performance High-entropy Alloy by Laser Additive Manufacturing

Manufacture of WNbMoTa High Performance High-entropy Alloy by Laser Additive Manufacturing

Design of D0 &lt;sub&gt;22&lt;/sub&gt; Superlattice with Superior Strengthening Effect in High Entropy Alloys

Precipitation strengthening is one of the most promising mechanisms to develop high-performance high entropy alloys (HEAs). However, the design of a reinforcing phase with an excellent strengthening effect is still one of the most pivotal challenges. In the present study, a design strategy based on overall valence electron concentration (OVEC) is developed, and a coherent γ″ phase with superior strengthening effect is designed. The newly developed γ″ phase is systematically characterized using transmission electron microscope and atom probe tomography. Differentiating from the traditional Ni3Nb γ″ phase, the present high-entropy γ″ phase contains ~7.7% Co and follows the (Ni,Co,Cr,Fe)3(Nb,Fe) stoichiometry. Three γ″ phase variants are observed with crystallographic orientation relationships of [001]γ″// γ and (001)γ″//{100}γ. The lenticular γ″ particles with small volume fraction (7%) causes a significant yield strength increase (670 MPa) and ductility retention (40%), resulting in excellent yield strength-ductility combination. The excellent strengthening effect of the γ″ phase is attributed to both ordering strengthening and coherency strengthening. The present study proposes a new design strategy of precipitates and develops a superior reinforcing phase for HEAs. These findings not only promote the development of precipitation-strengthened HEAs but deepen the understanding of precipitates design in HEAs as well.

The microstructure and phase equilibrium of new high performance high-entropy alloys

In recent years, series of high-entropy alloy have been well developed with high hardness and high temperature stability. These properties could apply in hard surface welding technology. Several AlCrFeMnNi high-entropy alloys have been developed and made as welding rods to apply in the hardface welding on carbon steel using nickel-based alloy as bond coating layer. The hardness of hardface can reach Hv 900 after aging at 700 °C for 4 h. One of alloys performed resistant with the thermal effect during hardfacing operation and maintain better hardness in the multiple overlays. An electron probe microanalyzer (EPMA) was used to verify the phase compositions of hardfacing microstructure. There is nanoprecipitate phase formed within the matrix grain and might contribute the high hardness. A primitive phase diagram calculation was done by using the Calphad method. The calculated results show that the hardening mechanism is due to bcc order/disorder coherent strengthening within the matrix grain. Two high-entropy alloys were equilibrated at 700 °C for 10 days. The equilibrium phases are consistent with the calculated isopleth. The new phase may relate with FCC phase structure and still under investigation.