Multi-principal Element Research Articles

A review on high-throughput development of high-entropy alloys by combinatorial methods

High-entropy alloys (HEAs) are an emerging class of alloys with multi-principal elements that greatly expands the compositional space for advanced alloy design. Besides chemistry, processing history can also affect the phase and microstructure formation in HEAs. The number of possible alloy compositions and processing paths gives rise to enormous material design space, which makes it challenging to explore by traditional trial-and-error approaches. This review highlights the progress in combinatorial high-throughput studies towards rapid prediction, manufacturing, and characterization of promising HEA compositions. This review begins with an introduction to HEAs and their unique properties. Then, this review describes high-throughput computational methods such as machine learning that can predict desired alloy compositions from hundreds or even thousands of candidates. The next section of this review presents advances in combinatorial synthesis of material libraries by additive manufacturing for efficient development of high-performance HEAs at bulk scale. The final section of this review discusses the high-throughput characterization techniques that are used to accelerate the material property measurements for systematic understanding of the composition-processing-structure-property relationships in combinatorial HEA libraries.

Excellent strength-ductility combination in Co36Cr15Fe18Ni18Al8Ti4Mo1 multi-principal element alloys by dual-morphology B2 precipitates strengthening

Precipitation strengthening provides one of the most widely-used mechanisms for strengthening multi-principal-element alloys (MPEAs). Here, we report dual-morphology B2 precipitates in Co36Cr15Fe18Ni18Al8Ti4Mo1 MPEA obtained by thermo-mechanical processing. Electron microscopy characterization reveals that the dual-morphology B2 precipitates are either recrystallized B2 particles formed at the grain boundaries or triple junctions with recrystallization process, or rod-like within the non-recrystallized FCC matrix. The dual-morphology B2 precipitates enhance the yield strength and ultimate tensile strength up to 1120 MPa and 1480 MPa, respectively. This work suggests the mechanical properties of the alloy can be optimized by B2 precipitation strengthening to meet the needs of engineering applications.

Microstructure characterisation of multi-principal element alloys welds produced by electron beam welding

This work explores the feasibility of producing bead-on-plate welds of a CrCoNi medium entropy alloy and a CrMnFeCoNi high entropy alloy using electron beam welding. The alloys were welded in two states: one in an as-cold-rolled condition and the other in an annealed condition. In addition, the materials are welded with two different parameters. The FCC microstructure of the welds is investigated using scanning electron microscopy assisted by energy-dispersive X-ray spectroscopy and electron-backscattered diffraction. The impact of the weld on the microstructure is discussed. The heat-affected zone is negligible for the annealed condition of both medium and high entropy alloys since there is no driving force for recrystallisation and the exposure time to high temperature is insufficient for grain coarsening. The texture formed in the fusion zone is also discussed and compared to the texture in the base metal and the heat-affected zone. Although the grain growth along the 〈100〉 crystallographic direction is preferential in all cases, the crystallographic texture type differs from each weld. Higher hardness values are measured in the medium entropy alloy's base metal and fusion zone than in the high entropy alloy.

Quantifying chemical fluctuations around medium-range orders and its impact on dislocation interactions in equiatomic CrCoNi medium entropy alloy

Equiatomic multi-principal element alloys manifest unique mechanical properties derived from the single-phase solid solutions with compositional disorders. Recently discovered medium-range orders (MROs) with a size of 1–5 nm in CrCoNi medium entropy alloy acclaimed measurable impact on mechanical properties, which, however has been questioned by density functional theory calculations. Here we report that the formation of MROs is accompanied by the redistribution of constituent elements, and the subtle chemical composition fluctuations could by directly probed and quantified by using a state-of-the-art aberration-corrected transmission electron microscope. The presence of such MROs contributes to an appreciable increase in yield strength (∼40 MPa) with a higher work hardening rate, originating from the strong dislocation interactions with MROs at the incipient plastic deformation. These findings demonstrate that MROs have a significant impact on reducing the mean free path of full/partial dislocations along a specific slipping plane, offering a new avenue for strengthening of equiatomic element alloys by tuning local composition and atomic configurations.

Helium irradiation-induced swelling resistance of heterogeneous nanoparticles in an iron-based multi-principal element alloy

In this work, we studied the resistance to helium-irradiation-induced swelling of the Fe6Cr1.2Mn0.8Cu1.5Mo0.3V0.2 alloy containing highly thermally stable Cu nanoprecipitates for nuclear applications using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The results show that the Cu nanoprecipitate/matrix interface and the nanoprecipitate itself can trap helium bubbles, creating open volume sinks of helium bubbles; large helium bubbles tend to be dispersed at the interface, while small helium bubbles are distributed in the matrix. The SAXS results indicate that the average size of helium bubbles is 1.3–1.7 nm, and the distribution of helium bubbles in the two samples varies due to the difference in Cu nanoprecipitation/matrix interface density, which was also verified via TEM. In addition, a low swelling ratio (0.05%/dpa) of the alloy was calculated, further confirming the feasibility of heterogeneous nanoprecipitation management of helium damage in irradiation tolerant materials. These nanostructured materials have the potential to reduce the helium effect by providing more nucleation sites for helium bubbles.

Exceptional Soft Magnetic Properties of an Ordered Multi-principal Element Alloy with Disordered Nanoprecipitates

In this work, a new non-equiatomic multi-principal element alloy (MPEA) Fe40Co30Mn15Al15 with excellent soft magnetic performance was fabricated. The microstructure and magnetic behavior of the as-cast alloy were investigated experimentally and theoretically using synchrotron X-ray diffraction, transmission electron microscopy, atom probe tomography, vibrating sample magnetometry, and density functional theory calculations. The alloy consists of an ordered B2-phase matrix enriched with Co/Al and uniformly distributed BCC nanoprecipitates enriched with Fe/Mn. The new alloy exhibits a high saturation magnetization of ~ 158 Am2/kg, a low coercivity of ~ 108 A/m, a high electrical resistivity of ~ 238 µΩ cm, and a high Curie temperature of ~ 1020 K, which shows an excellent combination of soft magnetic properties compared to previously reported MPEAs and conventional FeCo alloys. The findings thus provide a new perspective for designing high-performance materials for soft magnetic applications.

Effects of lattice distortion and chemical short-range order on creep behavior of medium-entropy alloy CoCrNi

Creep behavior of multi-principal element alloys (MPEAs) is an intriguing topic to explore for their potential high-temperature applications. The challenge on this topic is to elucidate what effect the local chemical fluctuation has on the creep behavior of MPEAs. By using large-scale molecular dynamics (MD) simulations, we investigate the creep performances of CoCrNi medium-entropy alloys (MEAs) with and without chemical short-range order (CSRO) as well as the Average-atom (A-atom) counterpart (no lattice distortion (LD)) under different uniaxial tensile stresses at various elevated temperatures. A power-law model is adopted to analyze the implicit stress exponent and activation energy, which are associated with the creep mechanism and creep resistance, respectively. The results reveal CSRO rather than LD plays an important role in creep performance. Specifically, with the introduction of CSRO, the activation energy for creep in CoCrNi MEA is significantly increased and finally close to the activation energy for the diffusion of the Cr element that is the highest among the three elements (Co, Cr, and Ni). The CoCr clusters in the MEA with CSRO make it difficult to increase the shear strain in the creep process, resulting in a much lower creep rate than that of the CoCrNi MEA without CSRO. For the CoCrNi MEA without CSRO, grain boundary diffusion is the main creep mechanism. For the MEAs with CSRO, the predominant creep mechanism depends on the applied stress. At low stress, grain boundary diffusion still prevails and there is no obvious dislocation glide or grain boundary sliding. When the applied stress is high enough, grain boundary diffusion and dislocation slip dominate the creep behavior, accompanied by the occurrence of grain boundary sliding.

Thermally sprayed multi-principal element Cr40Co40Ni20 coatings – Oxidation upon coatings' build-up and electrochemical corrosion

Thermally sprayed Cr40Co40Ni20 coatings were produced by high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF). The phases formed upon coatings' build-up were sensitive to oxidation at high temperatures, leading to the formation of various oxygen-rich compounds such as chromia and spinel. Chromia tended to be formed at the inner layer, in contact with the metal, and the spinel forming the outer layer. HVAF coatings suffered much less oxidation compared to the HVOF coatings. The pronounced Cr-depletion at the metallic and oxygen compounds caused by formation of spinel and oxide upon deposition of coatings created regions susceptible to readily corrode in chloride-rich electrolyte.

Dual enhancement in strength and ductility of Ti-V-Zr medium entropy alloy by fracture mode transformation via a heterogeneous structure

The formation of a homogeneous body center cubic (BCC) solid solution and short-range clustering (SRC) allow refractory multi-principal elements alloys to reach incredibly high yield strengths, but their uses are severely constrained by their inherent brittleness. In this study, Ti-V-Zr medium entropy alloys with SRC are designed and prepared, and the heterogeneous grain structures (HGS) with multiphase are formed. By tunning microstructure, movable dislocations provide deformation ability, since shear beads formation is depressed. The strength and ductility of Ti40Zr60-xVx alloys are enhanced by the increasing proportion of V. Ti40Zr15V45 alloy achieves a high yield tension strength of 1067 MPa and ductility of >7 % at room temperature. With a 1793 MPa compressive strength, it outperforms other BCC alloys with a homogeneous structure. Thermodynamic stability of phases indexed by parameters tends to form heterogeneous microstructure in grain scale, which realizes dual enhancement of strength and ductility by altering the ratio of phases. Dislocation reactions, which are also visible in SRC, result in shear band fractures with dimples on the cleavage plane surfaces. The <001> immovable dislocation is formed in the SRC structure and provides extra strengthening. The formation process model has been developed and validated. The model for the formation process has been built and proven.

Influence of chemistry and structure on interfacial segregation in NbMoTaW with high-throughput atomistic simulations

Refractory multi-principal element alloys exhibiting promising mechanical properties such as excellent strength retention at elevated temperatures have been attracting increasing attention. Although their inherent chemical complexity is considered a defining feature, a challenge arises in predicting local chemical ordering, particularly in grain boundary regions with an enhanced structural disorder. In this study, we use atomistic simulations of a large group of bicrystal models to sample a wide variety of interfacial sites (grain boundary) in NbMoTaW and explore emergent trends in interfacial segregation and the underlying structural and chemical driving factors. Sampling hundreds of bicrystals along the [001] symmetric tilt axis and analyzing more than one hundred and thirty thousand grain boundary sites with a variety of local atomic environments, we uncover segregation trends in NbMoTaW. While Nb is the dominant segregant, more notable are the segregation patterns that deviate from expected behavior and mark situations where local structural and chemical driving forces lead to interesting segregation events. For example, incomplete depletion of Ta in low-angle boundaries results from chemical pinning due to favorable local compositional environments associated with chemical short-range ordering. Finally, machine learning models capturing and comparing the structural and chemical features of interfacial sites are developed to weigh their relative importance and contributions to segregation tendency, revealing a significant increase in predictive capability when including local chemical information. Overall, this work, highlighting the complex interplay between the local grain boundary structure and chemical short-range ordering, suggests tunable segregation and chemical ordering by tailoring grain boundary structure in multi-principal element alloys.

The role of chromium content in aqueous passivation of a non-equiatomic Ni38Fe20CrxMn21-0.5xCo21-0.5x multi-principal element alloy (x = 22, 14, 10, 6 at%) in acidic chloride solution

The effect of the Cr content on the corrosion behavior in a series of single-phase non-equiatomic Ni38Fe20CrxMn21-0.5xCo21-0.5x (6 < x < 22 at%) multi-principal element alloys (MPEAs) was investigated in acidified NaCl solutions. Comparisons were made with binary solid solution Co-Cr, Ni-Cr, and Fe-Cr alloys over a similar range of Cr contents. The corrosion behavior was evaluated using in-situ AC and DC electrochemical methods and ex-situ surface sensitive characterization techniques. Passivity and various levels of local corrosion resistance were obtained in the MPEA with 10 at% Cr or above. The binary Ni-Cr alloys with 12–30 at% Cr behaved similarly. The MPEA with 6 at% Cr and binary alloys with 5, 6, or 10 at% Cr were marginally passive or active and underwent localized corrosion during both linear sweep voltammetry and potentiostatic hold experiments. Passive films formed during potentiostatic hold experiments were characterized with X-ray photoelectron spectroscopy and atom probe tomography. Cr cation enrichment in the passive films was observed for all alloys and similar enrichment factors were obtained as a function of Cr content regardless of whether MPEA or binary alloy except for the Fe-Cr alloy. Moreover, passive current density was correlated with the Cr cation fraction in the passive film. The degree of Cr enrichment was attributed to a combination of thermodynamic and kinetic factors, such as selective chemical dissolution of alloying elements as well as limitation due to solute depletion at the metal oxide interface in alloys with a bulk Cr content ≤ 10 at%.

Charge-density based evaluation and prediction of stacking fault energies in Ni alloys from DFT and machine learning

A combination of high strength and high ductility has been observed in multi-principal element alloys due to twin formation attributed to low stacking fault energy (SFE). In the pursuit of low SFE alloys, a key bottleneck is the lack of understanding of the composition–SFE correlations that would guide tailoring SFE via alloy composition. Using density functional theory (DFT), we show that dopant radius, which have been postulated as a key descriptor for SFE in dilute alloys, does not fully explain SFE trends across different host metals. Instead, charge density is a much more central descriptor. It allows us to (1) explain contrasting SFE trends in Ni and Cu host metals due to various dopants in dilute concentrations, (2) explain the large SFE variations observed in the literature even within a given alloy composition due to the nearest neighbor environments in “model” concentrated alloys, and (3) develop a machine learning model that can be used to predict SFEs in multi-elemental alloys. This model opens a possibility to use charge density as a descriptor for predicting SFE in alloys.

Temperature-dependent tensile behavior of the HfNbTaTiZr multi-principal element alloy

HfNbTaTiZr is a refractory multi-principal element alloy (RMPEA) that exhibits high strength combined with good ductility at room temperature. An insufficient understanding of RMPEA tensile behavior, however, limits their ability to be thermo-mechanically processed and optimized for engineering applications. In this investigation, uniaxial tensile tests were conducted at room temperature, 800 °C, and 1200 °C under vacuum on sheet materials fabricated by hot and cold rolling combined with annealing treatments. At testing temperatures of 25 °C and 1200 °C, the HfNbTaTiZr alloy exhibited high tensile ductility (16.0% and 121.0%, respectively). At 800 °C, the HfNbTaTiZr alloy exhibited significantly decreased ductility (2.0%) compared to room temperature, and coincided with a transition from ductile to intergranular fracture. SEM and TEM studies revealed precipitation reactions at grain boundaries following thermal exposure and during tensile testing at 800 °C, resulting in tantalum and niobium-rich BCC precipitates, and finer-scale hafnium and zirconium-rich precipitates. After annealing at 800 °C for 100 h, nanoindentation revealed soft and stiff tantalum and niobium-rich BCC precipitates located at grain boundaries. Thermodynamic calculations corroborated the existence of two BCC phases and predicted an HCP phase to be stable between 703–804 °C, which was consistent with TEM observations. At 1200 °C, high tensile ductility resulted from the combined effects of dynamic recovery and partial dynamic recrystallization. These experimental and computational results underscore a need to understand the high temperature phase equilibria and complex deformation behaviors of refractory MPEs so that their microstructure and mechanical performance can be better controlled across the elevated temperature ranges of interest.

γ′′ Phase transformation, precipitation hardening, hetero-deformation induced hardening and deformation mechanisms in a Nb-alloyed medium-entropy alloy

In present work, a novel γ′′-strengthened Nb-alloyed medium-entropy alloy (MEA) was successfully fabricated and systematically investigated. Our results evidence that the γ′′ phase brings out significant precipitation hardening effect (∼10.2 %volume fraction contributes ∼ 685 MPa strength enhancement). The metastable γ′′ phase could abnormally transform to ε phase, and the morphology and distribution of ε phase shows great impacts on mechanical properties. A hetero-lamellae grain coupling with dual-precipitation (HLG + dual-P) structure was successfully constructed in this Nb-alloyed MEA, which could effectively combine precipitation hardening and hetero-deformation induced (HDI) hardening effects. Deformation characters investigations reveals that the absence of mechanical twinning in γ′′-strengthened MEA originates from the improved critical twinning stress related to the fine dislocation source size and the difficulty of high-energy complex SFs reordering. The combination of massive stacking-faults, Lomer-Cottrell locks and dynamically reinforced HDI hardening contribute to the excellent ductility and work-hardening capacity at high-level strength in the HLG + dual-P structural alloy. Our work provides a useful insight and helpful guidance for developing high-performance precipitation-hardened multi-principal element alloys.

Improved strength and ductility of Ti2(CrFeNi)98 multi-principal element alloy at cryogenic temperature with deformation twins induced by BCC precipitates

The mechanical properties of a Ti2(CrFeNi)98 multi-principal element alloy (MPEA) with heterogeneous microstructures were systematically investigated at room (293 K) and cryogenic (77 K) temperatures, together with the corresponding microstructure evolution. As the temperature decreased from 293 K to 77 K, the yield strength, ultimate tensile strength and ductility increased from 800 MPa to 980 MPa, 1.1 GPa–1.5 GPa and 20%–25%, respectively. Microstructure characterization indicates that the increase of ductility at 77 K is owing to the arising of deformation twins (DTs) in the face-centered cubic matrix adjacent to body-centered cubic precipitates. Finite element analysis confirm that the DTs is induced by locally concentrated stress which exceeds the critical twinning stress, owing to the low stacking fault energy at cryogenic temperature. This work provides a designing strategy for promising Co-free MPEAs with both good strength and ductility at extreme low temperature conditions.

Study on the microstructural evolution in NbMoTaW series refractory multi-principal element alloy films by low-energy and high-flux He ions irradiation

In this study, the nanocrystalline NbMoTaW and NbMoTaWV refractory multi-principal element alloy (RMPEA) films were selected for the research of low-energy (50 eV), high-flux (~1021 ions m−2 s−1) helium ions irradiation response and thermal conductivity at 1050 K. The NbMoTaW and NbMoTaWV RMPEA films maintained the single-phase body-centered cubic (BCC) structures after irradiation. The electrical and thermal conductivities did not decrease, and the phenomena like “fuzz” structures of W films or “fuzz-like” structures of NbMoTaW RMPEA bulks were not found, indicating that the nanocrystalline RMPEA films have inhibition effects on the growth of “fuzz” structures. The inhibition of “fuzz” structures in the RMPEA films could be attributed to: (i) The high-density columnar crystal grain boundaries could accommodate more helium and provide helium release channels. (ii) The sluggish diffusion kinetic effects of RMPEA films suppressed the helium diffusion and maintained the helium release channels at high temperatures. Moreover, the NbMoTaWV RMPEA films were discovered to produce smaller helium bubbles and less pronounced microcracks compared to NbMoTaW RMPEA films. It was due to the solid solution incorporation of V which further enhanced the lattice distortion and toughness as well as the profound suppression of helium diffusion.

Flexible stretchable low-energy X-ray (30–80 keV) radiation shielding material: Low-melting-point Ga1In1Sn7Bi1 alloy/thermoplastic polyurethane composite

A highly flexible stretchable thermoplastic polyurethane (TPU) composite loaded with a low-melting-point Ga1In1Sn7Bi1 multiprincipal element alloy (LMPEA) was prepared, and its radiation shielding performance was evaluated. The fluid characteristic of LMPEA and the flexibility of TPU enable good interface compatibility. Ga1In1Sn7Bi1 LMPEA consists of two eutectic structures, and the liquid gallium-rich phases are distributed at the boundary of the InBi intermetallic compound and Sn solid solution. In the low-photon energy range of 30–80 keV, LMPEA has a theoretical specific lead equivalent of 0.803 mmPb/mm and a theoretical weight reduction of 17.27% compared with lead. To evaluate the photon attenuation capability for the LMPEA/TPU composites, the Phy-X procedure and Monte Carlo simulations were used to determine the shielding parameters, such as the mass attenuation coefficient, linear attenuation coefficient, half-value layer, tenth-value layer, mean free path, effective atomic number, and fast neutron removal cross section. The attenuation performance test of X-ray protective materials measured the actual lead equivalent. At the same thickness, the LMPEA/TPU composite (66.667, 50.000 wt% LMPEA loading) has a higher measured lead equivalent than the in-service medical shielding materials, which meets the lead equivalent requirements of X-ray protective clothing. LMPEA/TPU composites are nontoxic, lightweight, and have excellent low-energy X-ray shielding ability, offering great potential for application in medical wearable materials.

Lightweight Multiprincipal Element Alloys with Excellent Mechanical Properties at Room and Cryogenic Temperatures.

Lightweight multiprincipal element alloys (MPEAs) are promising candidates for potential application as engineering materials due to their high strength and low density. In this work, lightweight Ti70Al15V15 and Ti80Al10V10 MPEAs were fabricated via vacuum arc melting. The phases of the Ti70Al15V15 alloys consisted of a BCC phase and a small amount of B2 phase while the Ti80Al10V10 alloys displayed a dual-phase structure with BCC and HCP phases. The different phase compositions led to differences in their mechanical properties. When the temperature changed from 298 K to 77 K, the strength of the alloys further increased and maintained a certain plasticity. This is attributed to the increasing lattice friction stress at cryogenic temperature. TEM observation demonstrated that dislocation played a crucial role in plastic deformation for both the Ti70Al15V15 and Ti80Al10V10 alloys. In addition, Ti80Al10V10 exhibited significant work-hardening capabilities. By analyzing the strengthening mechanism of the alloys, the theoretical yield strength was calculated, and the results agreed with the experimental values. The present results provide new insight into developing lightweight MPEAs containing Ti and Al.

Spark Plasma Sintering (SPS) of Multi-Principal Element Alloys of Copper-Niobium-Titanium-Di-Boride-Graphite, Investigation of Microstructures, and Properties

A near-equiatomic multi-principal element alloy of Cu40Nb30(TiB2)20C10 with both nano-particle size (14 nm) and micron-particle sizes (−44 µm) of Nb was designed and made via the spark plasma sintering technique at two different sintered temperatures of 650 °C and 700 °C with other SPS parameters being constant. The sintering mode, microstructures, microhardness, density, relative density, wear behavior, and corrosion properties of the alloys were investigated and compared to ascertain the best for aerospace applications. The SPS technique was applied to produce the tested samples in this study. The results showed that the alloys with nano-particles of Nb sintered faster, with the lowest wear rate, and their microstructure shows a dendritic configuration with the existence of graphite-rich and niobium-rich nano-segregations in the inter-dendritic areas with the lowest coefficient of friction, Cu-NbTiB2C with nano-particles of Nb sintered at 650 °C recorded the highest microhardness value (786.03 HV0.2), and CuNbTiB2C with micro-particles of Nb sintered at 700 °C exhibited the best anti-corrosion characteristics in a sulphuric acid environment. The results obtained in this study correspond to the requirements for high-performance engineering materials, which will make the novel materials relevant in the aerospace industry.

A Process Parameter Predictive Framework for Laser Cladding of Multi-principal Element Alloys

• A framework is constructed to predict the processing parameters for laser additive manufacturing of MPEAs • A set of dimensionless objective functions correlated elemental diffusion and atomic radii to the macroscopic process parameters. • A multi-objective cuckoo search optimization enables a Pareto front to identify optimal processing parameters. Surface engineering by additive manufacturing of multi-principal element alloys (MPEAs) has generated significant attention recently for the range of remarkable material properties that can be achieved. A challenge exists in determining the optimum processing parameters for fabricating alloys of various compositions, as they govern the quality of the deposited material. Nevertheless, only limited models are available to predict the initial parameter window for the processing parameters. Using AlCoCrFeNi MPEA as a testbed for laser metal deposition, we present a framework correlating material properties to processing variables coupling predictions from fundamental molecular simulations and meta-heuristic optimization approaches. A set of dimensionless objective functions are constructed to connect elemental diffusion and atomic radii to the macroscopic process parameters, viz., cooling rate, energy density and powder deposition density. Our results suggest that the diffusion coefficient varies exponentially with cooling rate, when the MPEA assumes a crystalline phase upon solidification due to the formation of crystal point defects and a high activation energy rate required for diffusion during rapid cooling. However, the absence of these defects in an amorphous phase of the alloy renders no definitive correlation for the elemental diffusion coefficients with varying cooling rates. Through a multi-objective cuckoo search optimization, we construct a Pareto front to identify optimal values for processing variables, which concur with the parameters adopted in the literature for laser cladding of complex alloys.