Multi-principal Element Research Articles

A new synergy to overcome the strength-ductility dilemma in Al-Si-Cu alloy by adding AlZrNiTi master alloy

With the increase in lightweight components, Al-Si-Cu alloys are extensively applied in the automotive field. In this study, a multi-principal element alloy (MPEA) AlZrNiTi was designed to modify the Al-9Si-3Cu alloy, and the modification effects of individual additions of Zr, Ni, and Ti elements to the alloy were assessed. Moreover, the microstructure evolution, fracture behavior, and the mechanisms of strengthening and toughening during the modification process were investigated. Experimental results showed that when the addition amount of AlZrNiTi alloy was 0.8 wt%, significant refinements were made to the α-Al, eutectic Si, and Al2Cu in the Al-9Si-3Cu alloy, and the mechanical properties were significantly improved. In addition, the AlSiZr phase served as the heterogeneous nucleation core of α-Al was discovered. Furthermore, the AlZiTi phase was enriched at the front edge of the solid-liquid interface in solidification, hindering the growth of α-Al grains. It was also found that Al(Ni, Cu) particles with a size of 336 nm promoted the formation of high-density twins in Si crystal through the impurity-induced twin (IIT) mechanism, thereby modifying the Si phase. Comparative experiments reveal that the modification effect and mechanical properties of AlZrNiTi master adding to Al-9Si-3Cu alloy was obviously better than that of individual adding Zr, Ni, and Ti elements.

Experimental investigation of comprehensive performance by tailoring the proportion of late transition metals in quinary Fe-Co-Ni-Si-B amorphous alloys

We fabricated the Fe-Co-Ni-Si-B multi-principal element alloys by vacuum melt-spinning method. The effects of late transition metals proportion in quinary alloy ribbons on the formation, phase structure, thermal stability, microhardness, corrosion resistance and soft magnetic properties were investigated. The melt-spun structure consists of an amorphous single phase for all the specimens. The surfaces of glassy ribbons are rather flat and flawless and the average surface roughness is 0.351 nm or below. The initial crystallization temperature (Tx) and supercooled liquid region (ΔTx) of alloy system decrease by degrees with increasing Co and Ni content, showing the maximum values of 753.0 K and 60.0 K for Fe60Co10Ni10Si6B14 alloy. But the glass transition temperature (Tg) seems to barely change and maintain approximately 692.0 K. All the as-received alloy ribbons exhibit good bending ductility and acceptable Vickers microhardness ranging from 671 to 785 HV0.5. The amorphous alloy ribbons present a gradually wide passive potential range (ΔE) with the decrease of Fe content, but the corrosion current density (Icorr) becomes larger. The smallest Icorr value is 1.785 μA·cm-2 for Fe60Co10Ni10Si6B14 alloy and the widest ΔE value is about 0.272 V for Fe26Co27Ni27Si6B14 alloy. The saturation magnetization of these glassy alloys reduces from 165.0 to 112.0 emu/g as the Fe content decreases. Meanwhile, the coercivity also changes from 14.25 to 6.70 A/m. This paper offers a brand-new perspective to understand the role of late transition metals on the various physicochemical properties of quinary Fe-Co-Ni-Si-B amorphous alloys.

On novel Ni-rich phase formed during aging of an additively manufactured ultra-high strength CoFeNi alloy

The precipitation mechanism of strengthening phase of a body-centered-cubic (BCC) structured CoFeNi multi-principal element alloy produced by additive manufacturing was studied by transmission electron microscopy (TEM) and atom probe tomography (APT). The hardness of CoFeNi alloy achieved the peak value of 554.79 ± 9.86 HV after aging for 160 h at 400 °C. The CoFeNi alloy exhibited an ultrahigh compressive yield strength and ultimate compressive strength up to 1.8 GPa and 2.3 GPa respectively, whilst maintains a strain of 18.9 %. The microstructure of the alloy exhibited nanoscale lamellar Ni-rich phase with high stability homogeneously distributed in BCC matrix after aging at 400 °C. The novel Ni-rich phase containing about 50 at. % Ni with a hexagonal structure is the mainly precipitation strengthening phase, and has coherent orientation relationship with the BCC matrix in the<101>bcc∥<112¯0>Niand 111bcc∥{0001}Ni. Based on the experimental results, the precipitation mechanisms of the Ni-rich phase were discussed in detail. The current work provides a possible new direction to design high-strength alloy by introducing the novel Ni-rich phase.

On the microstructure, corrosion behavior and surface films of the multi-principal element alloy CrNiTiV

A multi-principal element alloy consisting of equi-atomic concentrations of Cr, Ni, Ti, and V, was produced by arc melting. The CrNiTiV alloy was characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and scanning transmission electron spectroscopy (STEM), revealing a complex microstructure containing five phases. The corrosion performance of the alloy was evaluated with cyclic potentiodynamic polarization tests, indicating a low corrosion current density, a high breakdown potential and high repassivation potential. X-ray photoelectron spectroscopy and STEM were carried out after constant immersion in 0.6 M NaCl solution, revealing the passive (surface) film for each of the five phases. The Cr and V dominant phase consisted of an oxidized Cr/V-rich surface film, while the remaining four Ti-containing phases were predominantly TiO2-rich surface films. Ni enrichment and Ti deficiency were observed at the metal/oxide interface in certain Ti-containing phases.

Multiple potential phase-separation paths in multi-principal element alloys

It is now well established that multi-principal element alloys (MPEAs) offer ample opportunities for exploring new compositions beyond those accessed previously by conventional alloys. However, there is one more realm of possibility presented by MPEAs that has not been touch upon thus far. Here we show that, different from conventional alloys based on a single host element, a given starting MPEA solid solution on its way towards equilibrium can take a rich variety of potential decomposition pathways via multi-stage phase separation, offering a wide range of composition destinations. If/when some of them are reached, assuming kinetically allowed, the multiple phase separation reactions one after another would lead to domains that are compositionally complex and spatially localized. This hypothetical scenario is demonstrated in this paper using a model that mimics Cr-Co-Ni MPEA, showing a preponderance of multiplicity even when assuming only fcc-based phases can form. The complex chemical heterogeneities created as such are expected to be an additional knob to turn for tuning spatially variable composition and chemical order and therefore mechanical properties. Our results thus advocate multiple phase separation possibilities with many potential paths and terminal chemical heterogeneities as yet another important characteristic that distinguishes MPEAs from conventional alloys.

Corrosion resistance and passive film stability of laser direct energy deposited TiVZrNbAl lightweight multi-principal element alloy

This work reports the corrosion behaviour of a novel laser direct energy deposited Ti45V30Zr12Nb12Al1 lightweight multi-principal element alloy (MPEA) in a 0.6 M NaCl solution. The MPEA exhibits superior corrosion resistance to typical titanium alloys, including Ti80. Its excellent corrosion resistance is attributed to a passive film with bipolar semiconducting behaviour and an amorphous structure. This unique feature inhibits anion permeation from the electrolyte and cation dissolution from the passive film. Further analyses revealed that Nb alloying effectively reduces the point defect density in the passive film, enhancing its ability to resist attacks by chloride ions.

Unveiling the precipitation behavior and mechanical properties in G-phase strengthened CrFe2Ni2Ti0.2Six multi-principal element alloys

In order to develop Co-free CrFeNi based multi-principal element alloys (MPEAs) with good synergies between strength and plasticity, the effects of Si content on the microstructures and mechanical properties of CrFe2Ni2Ti0.2Six (x = 0, 0.125, 0.25, 0.375, 0.5 and 0.625) MPEAs are studied in this work. The results show that the addition of Si induces a phase transformation from a single-phase face-centered cubic (FCC) structure to a dual-phase structure, comprising an FCC solid solution phase and G-phase. The outcomes from phase formation theory and phase diagram calculations (CALPHAD) also confirm that CrFe2Ni2Ti0.2Six MPEAs are prone to form dual-phase structures. With increasing Si content, the yield strength and hardness of the alloys increase by 47.2 % and 57.3 %, respectively, while still maintaining a strain of 34.2 %, which is indicative of commendable overall mechanical properties. This is ascribed to the coherence between FCC matrix and G-phase, enabling an enhancement in strength concurrently with the retention of considerable plasticity. Moreover, grain boundary strengthening and precipitation strengthening play a dominant role in the strengthening of CrFe2Ni2Ti0.2Six MPEAs. The present work offers new insights for the further development of high-performance non-metallic MPEAs, so as to achieve a combination of targeted microstructures and excellent properties.

Ultra-high hard and fracture-resistant multi-phase nanocrystalline AlCrFeMoNbNi multi-principal element alloy

Multi-phase nanocrystalline AlCrFeMoNbNi multi-principal element alloy was developed using mechanical alloying followed by spark plasma sintering. Mechanical alloying resulted in a two-phase microstructure (BCC and FCC), whereas subsequent sintering resulted in a multi-phase microstructure consisting of FCC, Rhombohedral and B2 phases. Extremely high hardness values in the range of 35.79–16.05 GPa were measured as a function of indentation load between 25 and 5000 g. A highly pronounced indentation size effect was observed. From classic Nix-Gao analysis, a load independent hardness of 13.82 GPa was estimated. The high hardness arises from a combination of finer grain size and solid solution strengthening (FCC phase) and multi-phase structure consisting of B2 and Rhombohedral phases. The major phase being FCC (83%) offers nearly 85% of the total flow stress realized for this MPEA and the remaining 15% of the flow stress must be arising from both the B2 and Rhombohedral phases. It is to be noted that the phase fractions of B2 and Rhombohedral phases are 10% and 7% respectively. Indentation fracture toughness, evaluated using the cracks developed during Vickers indentation, yielded encouraging numbers in the range 12.28–13.85 MPam. The excellent combination of ultra-high hardness and indentation fracture toughness of this AlCrFeMoNbNi MPEA surpasses the reported data on several related harder materials, including silicides, nitrides, carbides, etc.

Orientation-related and temperature-dependent continuous grain boundary migration in multi-principal element alloys

Grain boundaries (GBs) significantly affect the mechanical properties of metals and alloys. In this study, we investigated using molecular dynamic simulations the migration behavior of Σ25 (710), Σ5 (310), and Σ37 (750) [001] symmetric tilt GBs in CoCrCuFeNi multi-principal element alloy (MPEA) and Cu samples subjected to shear deformation. In Cu, the migration of the GBs exhibits a coupled migration pattern, consistent with the Cahn model; while in MPEA, the migration pattern varies with GB angle and temperature. Both Σ25 (710) and Σ37 (750) GBs, along with higher temperatures, induce GB roughening and continuous migration in the MPEA samples. Further investigation to the effects of GB angle and temperature was conducted through microstructure evolution tracing and quantitative analysis. A model was developed to describe the temperature-dependent continuous GB migration and average flow stress in MPEA samples with Σ25 (710) or Σ37 (750) GBs. This work can help understand the mechanical behavior of GB in MPEA and provide valuable insights for the development of high-performance materials.

Bidirectional Phase Transformations in Multi-Principal Element Alloys: Mechanisms, Physics, and Mechanical Property Implications.

The emergence of multi-principal element alloys (MPEAs) heralds a transformative shift in the design of high-performance alloys. Their ingrained chemical complexities endow them with exceptional mechanical and functional properties, along with unparalleled microscopic plastic mechanisms, sparking widespread research interest within and beyond the metallurgy community. In this overview, a unique yet prevalent mechanistic process in the renowned FeMnCoCrNi-based MPEAs is focused on: the dynamic bidirectional phase transformation involving the forward transformation from a face-centered-cubic (FCC) matrix into a hexagonal-close-packed (HCP) phase and the reverse HCP-to-FCC transformation. The light is shed on the fundamental physical mechanisms and atomistic pathways of this intriguing dual-phase transformation. The paramount material parameter of intrinsic negative stacking fault energy in MPEAs and the crucial external factors c, furnishing thermodynamic, and kinetic impetus to trigger bidirectional transformation-induced plasticity (B-TRIP) mechanisms， are thorougly devled into. Furthermore, the profound significance of the distinct B-TRIP behavior in shaping mechanical properties and creating specialized microstructures c to harness superior material characteristics is underscored. Additionally, critical insights are offered into key challenges and future striving directions for comprehensively advancing the B-TRIP mechanism and the mechanistic design of next-generation high-performing MPEAs.

Wear-resistant CrCoNi nanocrystalline film via friction-driven surface segregation

Revealing the frictional behavior through the lens of structural and chemical evolution is crucial for comprehending the exceptional wear-resistance of alloys with complex composition. Here, we propose that superior wear resistance can be achieved via dynamic surface segregation during sliding at room temperature. This strategy was demonstrated in CrCoNi multi-principal element alloy (MPEA) films with nano-grain structure, which exhibit a remarkably low wear rate that is <50 % of that for their VCoNi counterpart. Such distinct wear behavior is attributed to the specific friction-driven Ni segregation on the CrCoNi surface, which facilitates the preferential oxidation and formation of a nanocomposite protective layer with equiaxed nanograins uniformly embedded in an amorphous matrix. This wear-induced unique microstructure accommodates sliding-induced plastic deformation against damage and is responsible for the superior wear-resistance. Having revealed these fundamental mechanisms by experiment and simulation, this study provides a brand-new perception for designing self-adaptive MPEA surfaces. This involves adjusting the evolution of deformation layers with specific structure and chemistry, precisely engineered for tribological applications.

Irradiation response of microstructure and mechanical properties of NbMoVCr multi-principal element alloy coatings

In this study, NbMoVCr multi-principal element alloy (MPEA) coatings with near-equal atomic ratios were deposited onto the ferritic/martensitic (F/M) steel substrate by magnetron sputtering and then irradiated using 6 MeV Au2+ at room temperature (RT) and 550 °C, respectively. The results demonstrate that the NbMoVCr coatings have excellent irradiation resistance in terms of phase stability. RT-irradiation introduces both interstitial- and vacancy-type dislocation loops, and the dislocation loop density increases with the increase of irradiation damage dose. Meanwhile, RT-irradiation could also induce grain growth of the coating. However, under high-temperature irradiation, more recombination and annihilation of irradiation-induced defects, and recrystallization occur in the coating. In addition, irradiation also promotes the desegregation of V and Cr elements at the grain boundaries and enhances the uniformity of the distribution of coating elements. Irradiation softening and hardening in NbMoVCr coatings are also found to depend strongly on both irradiation damage dose and irradiation temperature. The creep deformation mechanism of the coating is dominated by diffusion; thus, the irradiation-enhanced diffusion reduces the creep resistance of the coating.

Non-equilibrium solidification behavior and mechanical properties of undercooled Fe7(CoNi)77B16 muti-principal element alloy

Multi-principal element alloys (MPEAs) have attracted growing attention owing to the vast compositional field and the comprehensive properties. In this work, Fe7(CoNi)77B16 MPEA was undercooled by vacuum melting furnace with low (61 K, 95 K), medium (115 K, 148 K), and high undercoolings (200 K). The maximum growth velocity of the alloy was 0.21 m/s due to the sluggish kinetics effect. The regular eutectic transformed to anomalous eutectic and the primary dendrite refined from 9.1 μm to 3.5 μm with the increase of undercooling. Meanwhile, the phase selection behavior at high undercooling was observed between the stable (Fe, Co, Ni) phase and the metastable (Fe, Co, Ni)23B6 phase. The hardness of primary phase and compressive strength of the undercooled MPEAs were increased to 952.19 HV and 2061.72 MPa, which is much higher than the as-cast alloy. The enhancement of the mechanical properties was attributed to microstructure evolution. The decrease in plasticity resulted from the increasing volume fraction of brittle intermetallic phases. This study helps to theoretically understand the solidification mechanism of the Fe-Co-Ni-B MPEAs and provides a promising way to enhance the mechanical properties of the novel MPEAs.

Ubiquitous short-range order in multi-principal element alloys

Recent research in multi-principal element alloys (MPEAs) has increasingly focused on the role of short-range order (SRO) on material performance. However, the mechanisms of SRO formation and its precise control remain elusive, limiting the progress of SRO engineering. Here, leveraging advanced additive manufacturing techniques that produce samples with a wide range of cooling rates (up to 107 K s−1) and an enhanced semi-quantitative electron microscopy method, we characterize SRO in three CoCrNi-based face-centered-cubic (FCC) MPEAs. Surprisingly, irrespective of the processing and thermal treatment history, all samples exhibit similar levels of SRO. Atomistic simulations reveal that during solidification, prevalent local chemical order arises in the liquid-solid interface (solidification front) even under the extreme cooling rate of 1011 K s−1. This phenomenon stems from the swift atomic diffusion in the supercooled liquid, which matches or even surpasses the rate of solidification. Therefore, SRO is an inherent characteristic of most FCC MPEAs, insensitive to variations in cooling rates and even annealing treatments typically available in experiments.

Corrosion of irradiated NbMoVCr coatings in lead-bismuth eutectic

NbMoVCr multi-principal element alloy (MPEA) coatings were irradiated by 6 MeV Au-ions at room-temperature and 550 ℃ to the peak irradiation doses of 20 dpa and 80 dpa. Then the effect of irradiation on corrosion of coatings in liquid lead-bismuth eutectic (LBE) was investigated. By comparing the microstructure of the surface oxide film and grain boundary corrosion region in the irradiated and unirradiated coatings, it is revealed that Au-ions irradiation promotes both the overall corrosion and intergranular corrosion of NbMoVCr coatings, which is attributed to the irradiation-induced defects as well as the desegregation of V elements at grain boundaries.

Understanding stacking fault energy of NbMoTaW multi-principal element alloys by interpretable machine learning

Stacking fault energy (SFE) is a crucial property influencing the deformation mechanisms of multi-principal element alloys (MPEAs). However, experimentally measuring SFE and exploring composition space of MPEAs is challenging. This study explores the SFE in NbMoTaW by integrating machine learning force fields (ML-FF), molecular dynamics simulations (MD), neural networks, and symbolic regression (SR) methods. A SFE dataset containing 2000 different NbMoTaW compositions were generated by combining ML-FF and MD. Then the neural-network model was developed for SFE prediction with an accuracy of 0.981 and a mean absolute error of 0.020. Using SR, the valence electron concentration (VEC), average shear modulus (G), radii gamma (γ) were identified as the key descriptors to determine SFE with the relationship of SFE=0.307·VEC+0.352·(G+γ). This work demonstrated a significant impact of chemical composition on SFE and established an accurate mathematical expression for SFE prediction, enhancing the understanding and alloy design of MPEAs.

Achieving excellent mechanical and robust lubrication behavior in the CoCrNi medium-entropy alloy via in-situ graphite

The unsatisfactory wear resistance and lubrication performance of CoCrNi multi-principal elements alloys (MPEAs) have emerged as critical challenges, impeding their extensive utilization as advanced engineering materials, despite their desirable mechanical and physical properties. Incorporating graphite to obtain MPEAs-based self-lubricating composites is believed to improve the wear resistance. However, it is challenging to prepare composites with high strength, large plasticity, and excellent anti-friction performance simultaneously. The present study reports the fabrication of novel CoCrNi MPEA-based self-lubricating composites, consisting of in-situ graphite and high-hardness silicides/carbides as reinforcement, prepared by powder metallurgy and reactive sintering. The 20.6 vol% Gr@CoCrNi–SiC composite exhibits an optimal trade-off between strength and ductility, along with anti-friction and wear resistance which is superior to the reported composites with different solid lubricants. The wear mechanism is attributed to the coordinated deformation mechanism between the CoCrNi FCC matrix and silicides/carbides, and the stabile lubrication effect supported by the structural transformation from the original polycrystalline lattice structure to core-shell nanocomposite structure (similar to diamond-like carbon) of the in-situ graphite. This simple, economical and practical strategy contributes to the development of MPEA self-lubricating composites with excellent comprehensive properties.

High strength-ductility synergy in refractory multi-principal element alloys via special deformation mechanisms and dislocation behaviors

High strength-ductility synergy in refractory multi-principal element alloys via special deformation mechanisms and dislocation behaviors

L-PBF processing and characterization of a Ti35Nb30Zr29Mo3Ta3 multiprincipal element alloy for medical implants

This Letter reports on the processing, mechanical properties, electrochemical performance, and toxicity behavior of a multiprincipal element Ti35Nb30Zr29Mo3Ta3 alloy. The recovered centimeter-sized cubes or parallelepipeds were polished before analysis. It was found that the homogeneity and yield stress of the alloys get increased when higher volumetric energy densities are applied during processing. The synthesized alloys also exhibited a Young modulus closer to those of bones than those usually reported on the conventional orthopedic metals (60–80 GPa for this alloy against 110 and 4–30 GPa for the Ti-6Al-4V and bone, respectively). The electrochemical corrosion assays performed at 25 and 37 °C, in sodium chloride (NaCl, 0.1 M), deoxygenated high glucose Dulbecco's Modified Eagle Medium (DMEM), and deoxygenated DMEM implemented with fetal bovine serum showed corrosion potentials (Ecorr) ranged in the order NaCl(25 °C) ≈ DMEM + PBS(37 °C) &amp;gt; DMEM(37 °C) and impedances much larger in the DMEM and DMEM + PBS media than in NaCl. Finally, pre-osteoblasts cells cultured in the medium conditioned by the alloy did not evidence cytotoxicity effect, because no effects on the cell proliferation and morphology were observed. These data suggest the biocompatibility of this alloy, indicating no acute toxicity, which is a prerequisite for the integration of an alloy as a bioimplant.

Tuning chemical short-range order for stainless behavior at reduced chromium concentrations in multi-principal element alloys

Single-phase multi-principal element alloys hold promise for improved mechanical properties as a result of multiple operative deformation modes. However, the use of many of these alloys in structural applications is limited as a consequence of their poor aqueous corrosion resistance. Here we introduce a new approach for significantly improving the passivation behavior of alloys by tuning the chemical short-range order parameter. We show that the addition of only 0.03 to 0.06 mole fraction of Al to a (FeCoNi)0.9Cr0.1 alloy changed both the magnitude and sign of the Cr-Cr order parameter resulting in passivation behavior similar to 304L stainless steel containing twice the amount of Cr. Our analysis is based on comparing electrochemical measures of the kinetics of passive film formation with chemical short-range order characterizations using time-of-flight neutron scattering, cluster expansion methods, density functional theory and Monte Carlo techniques. Our findings are interpreted within the framework of a recently proposed percolation theory of passivation that examines how selective dissolution of the non-passivating alloy components and short-range order results in excellent passive films at reduced levels of the passivating component.