L10 Phase Research Articles

Reliable identification of the complex or superlattice nature of intrinsic and extrinsic stacking faults in the L12 phase by high-resolution imaging

Planar defect-based deformation mechanisms are becoming increasingly important for understanding and tuning the mechanical properties of superalloys. Typically, to elucidate such deformation mechanisms, the type of planar defect is determined by conventional transmission electron microscopy or by high-resolution imaging along 〈110〉 crystallographic directions. However, an unambiguous identification of the complex or non-complex nature of intrinsic and extrinsic stacking faults in the L12 phase may not be possible by analyzing only one projection. To overcome this limitation, we present a straightforward approach to identify the superlattice or complex character of intrinsic and extrinsic faults in the L12 structure in high-resolution scanning transmission electron microscopy by tilting to a different zone axis and examining the shift of superlattice planes. Using this approach, two key aspects of the Kolbe mechanism for the formation of superlattice extrinsic stacking faults are verified for the first time: the presence of a complex intrinsic fault segment at the leading end and the occurrence of re-ordering.

Thermal Stability and Hot Corrosion Performance of the AlCoCrFeNi2.1 High-Entropy Alloy Coating by Laser Cladding.

Both crack-free AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) and Y and Hf co-doping AlCoCrFeNi2.1 EHEA (YHf-EHEA) coatings were prepared by laser cladding. The solidification microstructure, thermal stability, and hot corrosion performance of the coatings at 900 °C under 75% Na2SO4 + 25% NaCl molten salts were investigated. The experimental results showed that the structure of the as-deposited coatings consisted of FCC and BCC/B2 phases. After heat treatment, an Al-rich L12 phase was precipitated in the FCC phase of all coatings. The grain sizes of the EHEA and YHf-EHEA coatings after heat treatment at 900 °C for 10 h increased by 27.5% and 15.7%, respectively, compared to the as-deposited coatings. Meanwhile, after hot corrosion, the spallation areas of the YHf-EHEA and EHEA coatings accounted for 14.98% and 5.67% of the total surface area, respectively. In this study, the Y and Hf co-doping did not change the microstructure morphology and phase structure of the coatings but did improve the thermal stability and resistance of the hot corrosion oxide scale spallation, providing a certain amount of data and theoretical support for the application of EHEA coatings as high-temperature protective coatings.

3D-printed strong and ductile high-entropy alloys with orientation arranged nanostructure complex

The rapid development of 3D printing technologies demands new materials with novel properties, as many high-strength metallic materials (up to over 1000 MPa) suffer from a significant weakness in ductility despite their enhanced mechanical properties, particularly in tension strength. To address this issue, we fabricated precipitation-hardened high-entropy alloy (Fe28Co29.5Ni27.5Al8.5Ti6.5) and employed aging post-treatment to obtain a duplex multicomponent intermetallic nanoprecipitate arrangement while retaining the in-situ dislocation network features. This approach resulted in the formation of a novel orientation arranged nanostructure complex (NSC) in the alloy. The NSC structure is composed of duplex multicomponent intermetallic nanoprecipitates (L12 and L21 phases), disordered multicomponent FCC matrix, and in-situ dislocation network. This resulted in ultra-high tensile strengths of 1430 MPa and an excellent tensile elongation close to 16%, surpassing the performance of other advanced commercial steels and alloys. These findings offer an effective approach to facilitate the design of high-strength and high-ductility structural materials for industrial applications. This is achieved by tailoring the microstructure through a combination of in-situ generation of high-density dislocations structure and multicomponent intermetallic nanoprecipitates in additive manufacturing. As such, this study represents an important step forward in the development of advanced materials for 3D printing technologies.

Ab initio investigation on the ordering mechanism of L12 phase in Al–Zr–Y(Sc) alloy

The L12-Al3Y(Sc) phase promotes the precipitation kinetics of Al3Zr. However, although Al3Y and Al3Sc are both with L12 structures, the mechanisms are not entirely the same due to different structures of the compound composites: Al3(Zr, Sc) has a core-shell structure, while Al3(Zr, Y) does not. Formation mechanisms of core-shell and non-core-shell structures were revealed in this work through analyzing surface energy, interface binding energy, and interface energy. The results showed that the interface energy and the lattice mismatch of Al/Al3Sc and Al/Al3Y are 0.011 J/m2,1.9% and 0.512 J/m2 and 5% respectively. During the growth of Al3(Zr, Sc) with Al3Sc as the heterogeneous core, the interface energy decreased linearly with increasing Zr contents at the interface and eventually a Zr rich shell formed. However, during the formation of Al3(Zr, Y), the interface energy fluctuated with changes in the composition and Zr(Y) content at the interface, eventually forming a composite phase shell of Al3Y(Zr). The diffusion migration energy for Zr atoms in pure Al was 1.102 eV, and it increased by at least 2% near the Al/Al3Sc interface, while it decreased by up to 46.3% near the Al/Al3Y interface. The impact of the L12-Al3Y(Sc) heterogeneous core on the diffusion ability of Zr atoms is crucial for its influence on the kinetics of Al3Zr precipitation.

Phase-Field Study of Exchange Coupling in Co-Pt Nonstandard Nanochessboards.

The Co-Pt binary system can form a two-phase nanochessboard structure comprising regularly aligned nanorods of magnetically hard tetragonal L10 phase and magnetically soft cubic L12 phase. This Co-Pt nanochessboard, being an exchange-coupled magnetic nanocomposite, exhibits a strong effect on magnetic domains and coercivity. While the ideal nanochessboard structure has tiles with equal edge lengths (a = b), the non-ideal or nonstandard nanochessboard structure has tiles with unequal edge lengths (a ≠ b). In this study, we employed phase-field modeling and computer simulation to systematically investigate the exchange coupling effect on magnetic properties in nonstandard nanochessboards. The simulations reveal that coercivity is dependent on the length scale, with magnetic hardening occurring below the critical exchange length, followed by magnetic softening above the critical exchange length, similar to the standard nanochessboards. Moreover, the presence of unequal edge lengths induces an anisotropic exchange coupling and shifts the coercivity peak with the length scale.

Influence of chemical composition on coarsening kinetics of coherent L12 precipitates in FCC complex concentrated alloys

In this study, we report the experimental coarsening kinetics at 850, 900 and 950 °C of four complex concentrated alloys in the Al–Ti–Cr–Fe–Co–Ni senary system with different chemical compositions but a similar γ’ (L12) volume fraction (∼35 % at 950 °C) in a face-centered cubic (γ, FCC) matrix. The selected alloys were specifically designed to investigate the influence of Fe additions and Ni–Co substitutions on Ostwald ripening kinetics. Atom Probe tomography (APT) was used to determine the compositions of the FCC and L12 phases, which agree very well with Calphad calculations at thermodynamic equilibrium. Thermo-kinetic modeling of L12 precipitation was carried out using the Prisma module developed by Thermo-Calc and compared with experimental results. Apparent activation energies were determined and discussed in light of diffusion-controlled coarsening models to identify the key parameters affecting Ostwald ripening. We suggest that the abnormally high apparent activation energies results from composition-dependent parameters. When the latter are accounted for, the corrected activation energies for coarsening are in better agreement with available diffusion data.

Ultrastrong High‐Ductility Ni35Co35Fe10Al10Ti8B2 High‐Entropy Alloy Strengthened with Super‐High Concentration L12 Precipitates

L12 phase hardening alloys with excellent mechanical properties are of great significance for structural applications. However, low volume fractions of L12 precipitates in conventional alloys (nearly lower than 60%) tend to limit their practical usage, while the strengths of the alloys generally increase with L12 precipitation contents. Herein, a novel high‐entropy alloy (HEA) Ni35Co35Fe10Al8Ti10B2 with ultrahigh concentration L12 precipitates is successfully designed aided by the calculation of phase diagrams (CALPHAD). The volume fraction of L12 precipitates in this HEA is up to 75% and outperforms that of most of traditional superalloys. The novel L12‐strengthened Ni35Co35Fe10Al8Ti10B2 has an ultrahigh tensile yield strength of ≈1.45 GPa, ultimate tensile strength of ≈1.9 GPa, and great ductility of ≈23% at room temperature. The desirable strength–ductility combination is superior to most of conventional superalloys and reported HEAs, mainly due to the presence of ultrahigh concentration L12 precipitates that act as dislocation obstacles and the formation of numerous stacking faults and deformation twining. This work is expected to provide guidance for developing new high‐performance HEAs with an excellent combination of strength and ductility.

Achieving excellent strength-ductility synergy via high density dislocation and nano-twinning in cryo-rolled FeCoCrNiMo0.2 high-entropy alloy

High-density dislocations and nano-twinning are known to significantly influence the work hardening ability of materials, presenting a potent approach for enhancing their overall strength. Here, a superior tensile strength-ductility synergy in a FeCoCrNiMo0.2 high entropy alloy (HEA) with high density dislocation and nano-twinning was achieved by employing cryo-rolling followed by subsequent annealing. Through electron microscope observation, the effects of geometrically necessary dislocation density and nano-twinning on tensile strength and plasticity are elucidated. The microstructure analysis reveals a substantial presence of dislocations and nano-twinning within the deformed grains following short-time annealing at 600 °C and 800 °C. The strategic presence of nano-twins serves as effective impediments to dislocation slip, resulting in the storage of dislocations within the grains, which contributes to the alloy's superior strength-ductility synergy and remarkable work hardening ability. Impressively, the obtained tensile strength-ductility matching exhibits yield strength and ductility values of 992.1 MPa/0.19 and 719 MPa/0.39, respectively. Upon increasing the annealing temperature to 1000 °C, high-density annealing twins emerge in the recrystallized grains, accompanied by a concomitant reduction in the geometrically necessary dislocation density due to dislocation rearrangement and annihilation. Furthermore, a small amount of topological close-packed (TCP) phase and L12 phase precipitation during annealing are observed, contributing to increased work hardening ability of the alloy. The findings underscore the importance of microstructural engineering as a valuable strategy for optimizing the mechanical properties of HEAs with a face-centered cubic structure.

Twin boundary defects-assisted dual-nanoprecipitation in a selective-laser-melted Al alloy

A novel phenomenon of twin boundary-assisted dual-nanoprecipitation in a selective-laser-melted (SLM) Al alloy is systematically investigated. The dual-nanoprecipitation of Mg-Zn-rich η’ and Al-Sc-Zr-rich L12 phases is observed at a twin boundary. This twin boundary can be decomposed into incoherent twin boundaries with 9R phases, whereas provide nucleation sites for the dual-nanoprecipitation during in-situ heating at 120 °C. Such dual-nanoprecipitation is primarily driven by the Gibbs-free energy difference between the incoherent twin boundaries with 9R phases and the adjacent matrix. The results offer primary guidance to design a novel SLM Al alloy with dense twins and precipitations.

Enriching Pt3 ensemble with isolated 3-fold hollow site by crystal-phase engineering of Pt3Fe single-nanoparticle for acetylene hydrogenation

Tuning the crystal-phase of bimetallic particles enables a precise control of catalytically active sites but is experimentally challenged by the averaging ensemble effects that are caused by the particle communications occurred during the crystal-phase transformation. Here, H2-treatment at 773–973 K of Pt3Fe single-nanoparticle, confined by a permeable silica shell, achieved crystal-phase transformation from the disordered A1 phase to the chemically ordered L12 phase. Detailed microscopic and spectroscopic characterizations have identified that the disordered A1 particle possessed contiguously connected large Pt domains while the ordered L12 particle afforded regularly populated Pt3 ensembles surrounded by Fe single-atoms. The isolated 3-fold hollow Pt site in the three-atom ensemble efficiently boosted the adsorption of acetylene, yielding a much higher hydrogenation activity. The reaction rate of the L12 particle was twice of that for the A1 particle. Moreover, the charge transfer from the neighboring Fe single-atoms to the Pt atoms on the L12 particle expedited the desorption of ethylene, promoting the selectivity as well. Overall, both geometric and electronic interactions in the ordered crystal-phase synergistically facilitated the selective hydrogenation of acetylene to ethylene.

Hot deformation characteristics and microstructure evolution of Al20Co36Cr4Fe4Ni36 eutectic high entropy alloy

Eutectic high entropy alloys (EHEAs) have garnered much research attention due to their excellent mechanical properties, superior castability, good oxidation resistance, and stable microstructure in a wide range of temperatures. Inspires from mature Ni and Co-based superalloys, a novel Al20Co36Cr4Fe4Ni36 EHEA was designed and prepared to obtain good mechanical properties at both room temperature and high temperatures in this study. The novel EHEA consists of a L12 phase and a B2 phase with a superior ultra-tensile strength of 1005 ± 40 MPa and a ductility of 8.8 ± 0.4% at room temperature. The engineering compressive stress–strain curves at 1073 K indicated that the current EHEA performs a high strength of 321 ± 9 MPa without fracture even at a high temperature of ~0.6 TE, which is even superior to some refractory high entropy alloys (RHEAs). The hot deformation behavior and microstructure evolution of the new EHEA was investigated at the temperature range of 1073 K to 1273 K and various strain rates from 10−3 s−1 to 10−1 s−1. The constitutive equation that describes the correlations between the flow stress, strain rate, and temperatures was constructed. Continuous dynamic recrystallization (CDRX) was the primary dynamic recrystallization (DRX) mechanism in both the L12 and B2 phases during hot deformation. However, the proportion of dynamic recrystallization grains in the L12 phase is higher than that in the B2 phase according to the high angle grain boundaries (HAGBs) under the same deformation condition, suggesting that the CDRX kinetics was faster and stronger in the L12 phase. The present study provided a new thought for designing EHEAs and an insight into the hot deformation behavior of EHEAs with lamellar morphologies.

High-strength medium-entropy alloy designed by precipitation-strengthening mechanism via machine learning

Precipitation-hardened high/medium entropy alloys (PH-HEAs/MEAs) have attracted extensive interest due to their excellent mechanical properties at both room and high temperatures. Strengthening mechanism models can be used to guide the discovery and design of new alloys, however, the parameters of the models are often lacking, limiting their use for compositional screening from a large space. We proposed a strategy combining machine learning with precipitation strengthening mechanism for high-performance PH-HEAs/MEAs. Using this strategy, we successfully synthesized an alloy Al5Ti8Co40Fe7Ni40 with a yield strength approaching 1.15 GPa, 10% higher than the best value of the existing AlTiCoCrFeNi HEAs/MEAs strengthened by L12-type nanoprecipitates. The contribution of precipitation strengthening from the L12 phase is approximately 717 MPa. Our strategy that unites the strengthening mechanism and machine learning may inspire the rapid design and discovery of precipitation-hardened alloys.

Non-classical nucleation of ordered L12 precipitates in the FCC based Al0.25CoFeNi high entropy alloy

The complex interplay between competing phase stabilities of FCC, L12, BCC, and B2 phases in the Al0.25CoFeNi (7Al-31Co-31Fe-31Ni in at. %) high entropy alloy (HEA) leads to non-classical phase transformation pathways and resultant novel microstructures. Specifically, the competition between the homogenous precipitation of L12 and heterogenous precipitation of BCC/B2 can be studied at a temperature of 500 °C in the Al0.25CoFeNi alloy. Upon isothermally annealing the single FCC phase microstructure of this HEA at 500 °C up to 50 h, the transformation initiates with the formation of a transient ordered L12 phase with minor Ni–Al enrichment, which is far-from equilibrium, as revealed by atom probe tomography, and can be considered non-classical nucleation. The near equilibrium L12 phase eventually replaces the transient L12 during continued annealing at the same temperature. However, the resultant FCC + L12 microstructure is metastable because the true equilibrium for the Al0.25CoFeNi alloy at 500 °C is a mixture of L12 + B2 phases, as revealed by solution thermodynamics modeling. The higher nucleation barrier for the BCC-based ordered B2 phase coupled with the slower kinetics at 500 °C leads to the homogeneous precipitation of L12, while the B2 phase appears to sluggishly grow from grain boundaries acting as heterogeneous nucleation sites.

Photoinduced Nonthermal Reduction of the Coercive Field in FePt and FePt0.84Rh0.16 Epitaxial Thin Films in the L10 Phase

The time-resolved magneto-optical Kerr effect in epitaxial thin films of the FePt compound and the FePt0.84Rh0.16 solid solution with the perpendicular magnetic anisotropy on MgO (001) substrates has been studied. The evolution of hysteresis loops at short (100 fs–1 ns) and long (1–20 ms) time scales after the excitation by a femtosecond light pulse has been studied. Long-lived nonthermal reduction of the coercive field has been detected. The coercive field is recovered in several milliseconds. It has been proposed to explain the observed phenomenon by the excitation of high-Q-factor acoustic resonances in the substrate/film system and to the strong magnetoelastic interaction in FePt and FePt0.84Rh0.16 films.

Enhanced strength in Co-free Ni47.5-xFe25Cr25AlxTi2.5 high entropy alloys via introducing dual precipitates

Synergistic strengthening mechanisms (e.g. solid solution strengthening, grain/twin boundary strengthening, dislocation strengthening, and precipitation strengthening) have been shown to effectively achieve a balance between strength and ductility of mechanical properties of metal materials. In this study, Co-free AlxNi47.5-xFe25Cr25Ti2.5 high entropy alloys (HEAs) mainly strengthened by grain boundary/twin-precipitation were successfully prepared by combining rolling and annealing. The phase-field method was used to demonstrate that twins enhance the Hall-Petch effect by introducing additional twin boundaries. Only a small amount of lamellar L12 phase is precipitated in the aged Al2.5 HEA. The tensile strength is 1086 MPa and the elongation is 30.8%. With the increase of Al content, the volume fraction of L12 phase in the alloy increases, and the bulk B2 phase is precipitated. When the Al content increases to 7.5 at. %, the strength of the alloy increases to 1757 MPa, but the plasticity is catastrophically reduced to only 3.4%. The excellent strength of aged Al7.5 HEA is mainly due to the good combination of grain/twin boundary strengthening and precipitation strengthening.

The Structure and Mechanical Properties of Rolled Sheets of the Multicomponent Al–2.5Ca–2.5Mg Alloy Doped with Scandium and Zirconium

Using the Al–2.5% Mg–1% Mn–0.4% Fe base alloy (wt %) as an example, the effects of calcium, scandium, and zirconium additives (at about 2.5, 0.1, and 0.2 wt %, respectively) on the strength of rolled sheets has been studied. Ingots of the alloy of the chosen composition and the hot and cold rolled sheets obtained from them were used as objects for the experimental study. Based on microstructural studies, it isshown that calcium binds iron into compact inclusions (presumably, into the Al10CaFe2 phases), which has a positive effect on the mechanical properties of the alloy, as well as on the technological plasticity during therolling operation. Zirconium and scandium additives make it possible to form thermally stable nanoparticles of the Al3(Zr, Sc)–L12 phase, which is favorable for maintaining a partially unrecrystallized structure in coldrolled sheets during annealing at least up to 400°C. Using the alloy of the selected composition as an example, the fundamental possibility for producing sheet products from cast (unhomogenized) ingots with the properties of thermally hardenable alloys of the AlSiMgMn and AlZn4.5Mg1.5Mn types without using quenching has been demonstrated.

Microstructural evolution and mechanical properties of (FeCoNi)85·84Al7·07Ti7.09 high-entropy alloy by selective laser melting with aging treatment

Faced-centred cubic nanoparticles-strengthened FeCoNiAlTi system high-entropy alloy (HEA) has demonstrated unique mechanical performance advantages with recently developed HEA alloy compositions. One of the key factors determining the performance of an alloy is the efficient manipulation of precipitation. However, the traditional aging method used to achieve this can be problematic. It is time-consuming and difficult to accurately control the distribution of the precipitated phase, which restricts the development of new, high-performance structural materials. Consequently, researchers have been exploring alternative methods to more precisely and effectively manipulate precipitation in alloys. Here, we have utilized both high-efficiency additive manufacturing processes and a short-term aging process to produce (FeCoNi)85·84Al7·07Ti7.09 high-entropy alloys that exhibit outstanding mechanical properties with an ultimate tensile strength of 1625 MPa, yield strength of 1214 MPa and an elongation to failure of 11.6%. Our findings reveal the formation of L12 and L21 nanoparticles phases after aging treatment, and their contents gradually decrease with increasing aging treatment temperature. And, we introduce two strengthening mechanisms of dislocation strengthening and precipitation strengthening into the alloys, and realized the coexistence of high-density dislocations and precipitated phases, thereby exhibiting excellent comprehensive mechanical properties. This approach represents a promising new direction in the development of high-performance alloys, offering a more efficient and effective means of producing materials with improved properties.

Thermal stability and deformation mechanisms in Ni-Co-Fe-Cr-Al-Ti-Nb-type nanoparticle-strengthened high-entropy alloys

The precipitate morphologies, coarsening kinetics, elemental partitioning behaviors, grain structures, and tensile properties were explored in detail for L12-strengthened Ni39.9Co20Fe15Cr15Al6Ti4–xNbxB0.1 (x = 0 at.%, 2 at.%, and 4 at.%) high-entropy alloys (HEAs). By substituting Ti with Nb, the spheroidal-to-cuboidal precipitate morphological transition, increase in the coarsening kinetics, and phase decomposition upon aging at 800 °C occurred. The excessive addition of Nb brings about the grain boundary precipitation of an Nb-rich phase along with the phase decomposition from the L12 to lamellar-structured D019 phase upon the long-term aging duration. By partially substituting Ti with Nb, the chemically complex and thermally stable L12 phase with a composition of (Ni58.8Co9.8Fe2.7)(Al12.7Ti5.8Nb7.5Cr2.3) ensures the stable phase structure and clean grain boundaries, which guarantees the superb high-temperature mechanical properties (791 ± 7 MPa for yielding and 1013 ± 11 MPa for failure) at 700 °C. Stacking faults (SFs) were observed to prevail during the plastic deformation, offering a high work-hardening capability at 700 °C. An anomalous rise in the yield strength at 800 °C was found, which could be ascribed to the multi-layered super-partial dislocations with a cross-slip configuration within the L12 particles.

Correlation between the nanomechanical characteristic and the phase transformation of BCC-based high entropy alloys produced via powder metallurgy

ABSTRACT In this study, the effects of Cu addition on AlFeMnTiSi0.75Cu x (x = 0, 0.25, 0.5, 0.75, 1.00; in molar ratios) high entropy alloys (HEAs) prepared via mechanical alloying and spark plasma sintering were investigated. The structure, phase, morphology and composition of HEA powders were analysed and the results revealed that the AlFeMnTiSi0.75Cu x HEAs exhibited a multiphase structure. Additionally, after sintering at 900 °C, the formation of BCC, µ and L21 phases in the densified HEAs was enhanced. The investigation of the hardness, nanoindentation and compressive properties revealed that the microstructural and mechanical properties of AlFeMnTiSi0.75Cu x HEAs were improved at the optimal Cu fraction (0.25 molar ratio). The nanoindentation results revealed that the AlFeMnTiSi0.75Cu x HEAs exhibited the highest hardness and elastic modulus (HIT = 19.2 GPa, EIT = 336 GPa). These results improve the current understanding of multiphase HEAs and may pave way for the development of advanced HEAs with superior mechanical properties.

Ductility deterioration induced by L21 phase in ferritic alloy through Ti addition

Ductility deterioration induced by L21-Ni2AlTi precipitates in the aged ferritic alloys was examined systematically by using a combination of scanning transmission electron microscope (STEM), mechanical tests and first-principles thermodynamic calculations. The experimental studies revealed that the strength and hardness of the aged Fe–10Cr–5Ni–1Al–1Ti ferritic alloy containing B2–NiAl and L21-Ni2AlTi precipitates were higher than that of the aged Fe–10Cr–5Ni–1Al ferritic alloy containing NiAl precipitates, whereas the elongation-to-failure decreased dramatically from 9.3% to 0.3% indicating an obvious ductility deterioration due to the formation of L21-Ni2AlTi precipitates. This was also confirmed by the observation of fracture transition mode from dimpled failure to cleavage failure. The first-principles calculations, concerning the precipitate/matrix interface, were carried out to provide a theoretical analysis for the ductile–brittle transition by means of empirical ductility criteria ratios G/B and (C12–C44)/B as well as cleavage energy. The cleavage energy results indicated an intrinsic brittleness of the L21-Ni2AlTi phase and the L21-Ni2AlTi/BCC-Fe interface. Our analysis revealed that the intrinsic brittleness of L21-Ni2AlTi phase and L21-Ni2AlTi/BCC-Fe interface plays a vital role in determining the deformation behavior of the aged Fe–10Cr–5Ni–1Al–1Ti alloy.