Advanced Alloys Research Articles

Tailoring the chemical environments and magnetic moments of the interstitials of high-entropy alloys

The interstitial defects play a crucial role in controlling the irradiation performance of high-entropy alloys (HEAs) particularly of CrMnFeCoNi. However, the site-to-site chemical and magnetic disorder of HEAs prohibits understanding the behavior of interstitials in HEAs. Herein we demonstrate that the chemical bonding dominates the formation of dumbbells at the weak magnetic effect, whereas the strong magnetic effect can perform over the chemical bonding in determining the formation of dumbbells. Accordingly, Ni-containing dumbbells are particularly unstable due to its weak chemical bonding and magnetic effect, while the spin-down Cr and the magnetic adaptive Mn are essential for stabilizing CrMnFeCoNi. Note that the chemical bonding and magnetic effects can be quantitatively characterized by the cohesive and band-filling descriptors. Our study elucidates a physical understanding of the dumbbell interstitials of HEAs and provides a rational guidance for designing advanced structural alloys with optimal irradiation resistance.

In-situ investigation of tensile anisotropy mechanism in an advanced Ti2AlNb-based alloy associated with CRSS ratio and damage model

Combined with in-situ tensile test and electron backscatter diffraction technology, the critical resolved shear stress (CRSS) ratio of O phase and the damage model of grain boundaries were proposed to reveal the tensile anisotropy mechanism of Ti–22Al–25Nb alloy isothermally forged in B2 phase region. In-situ observation suggests that the single slip mode of lamellar O phase is the major deformation behavior, and its preferential slip dominates the yield of the material. Based on the types, number, Schmid factor (SF) and orientation of slip activation, the CRSS ratio of basal <a>, prism <a>, first-order and second-order pyramidal <c+a> slips for O phase is estimated to be 1.19: 1: 1.37: 1.39. Meanwhile, basal and prism <a> slips mainly appear the radial and axial directions (RD and AD) samples with [012] and [100] texture, while the [001], [010] and [212] texture of OD (45° to RD) sample shows an increase in pyramidal <c+a> slips. Thus, the slip activation of O phase depends on the grain orientation and CRSS, resulting in significant strength anisotropy. Furthermore, the microcracks and secondary cracks at grain boundaries are significantly affected by the slip accumulation of lamellar O phase at the O/O and O/B2 interfaces. Considering the stress state of the grain boundary, a damage model about crack nucleation and propagation is proposed to explain the ductility anisotropy. From this, the RD sample has a higher crack nucleation and propagation resistance than the AD and OD samples due to the flattened morphology of B2 grains.

Excellent dynamic properties and corresponding deformation mechanisms in a microband-induced plasticity steel with dual-heterogeneous structure

A dual-heterogeneous structure with dual phases and heterogeneous grains was designed in a high Mn microband-induced-plasticity (MBIP) steel. The heterogeneous structured sample exhibits similar dynamic shear yield strength, while exhibiting enhanced dynamic uniform shear strain and dynamic shear toughness by 36 % and 47 %, respectively, as compared to the homogeneous structured sample. The strain hardening mechanisms contributing to the excellent dynamic shear properties in the heterogeneous structured sample have been revealed. Firstly, the superior hetero-deformation-induced hardening capability of the heterogeneous structured sample arises from a higher density of geometrically necessary dislocations induced in each phase. Secondly, a pronounced strain rate effect is observed in body-centered cubic (BCC) grains and face-centered cubic (FCC) ultra-fine grains, leading to an elevated level of strain rate sensitivity. Finally, the MBIP effect is also observed in FCC coarse grains under dynamic loading. Moreover, the MBIP effect in FCC ultra-fine grains is more likely to occur at high strain rates due to the emergence of single-wall domain boundaries, which is not observed during quasi-static tensile testing. The present results provide new routes for designing impact-tolerant structures in advanced metals and alloys.

Minor additions of Sn suppress the omega phase formation in beta titanium alloys

A critical characteristic of β-Ti alloys is the inevitable formation of ω-precipitates during certain heat treatments which leads to embrittlement, or even to a complete loss of ductility. Therefore, alloy design with the goal to inhibit the elementary ω-formation process is of utmost importance. Here, we propose a design strategy for prototypical β-type Ti–Cr–(Mo) alloys to alleviate this problem using only minor additions of Sn. Upon addition of Sn, we observed an extensive deceleration or even suppression of the ω-formation kinetics during isothermal ageing. Furthermore, the internal friction response of the elementary formation process indicated a decisive reduction of potential ω-nucleation sites, while the activation energy of the process remained almost unchanged. The results show, that the addition of Sn can significantly increase the width of the time–temperature process window and the long-time ageing resistance of β-Ti alloys, opening up huge opportunities for advanced alloy design and manufacturing routes.

Cooperative regulation of mechanical properties and magnetoresistance effect in high-entropy alloys by spinodal decomposition

Adjusting the phase composition of high-entropy alloys is an effective approach to change its mechanical and magnetic properties. In this study, FeCoNiCr high-entropy alloys with Al addition (AlxFeCoNiCr; x = 0.0, 0.25, 0.5, 0.75, 1.0, 1.25, 1.75) were prepared by argon arc melting to obtain a special spinodal decomposition structure. There precipitated body-centered cubic (BCC) phase in the face-centered cubic (FCC) phase matrix for the samples with Al (x = 0.25, 0.5, 0.75) addition. Furthermore, the increase in the Al content (x = 1.0, 1.25, 1.75) transformed the alloy into a BCC phase, and resulted in a spinodal decomposition in the alloy. This spinodal decomposition created a low-misfit coherent nanostructure combining the enriching AlNiCo ordered B2 matrix with enriching Fe–Cr BCC disordered nanoprecipitates. The difference in the distribution of chemical elements of the two phases leads to different magnetic properties and induces the magnetoresistance (MR) effect. This provides a pathway for the design of advanced functional high-entropy alloys.

Powdered Hierarchically Porous Silica Monoliths for the Selective Extraction of Scandium.

Scandium (Sc) is a high value Critical Material that is most commonly used in advanced alloys. Due to current and potential supply limitations, there has been an international effort to find new and improved ways to extract Sc from existing and novel resources. Solid-phase extraction (SPE) is one promising approach for Sc recovery, particularly for use with low-grade feedstocks. Here, unfunctionalized, powdered hierarchically porous silica monoliths from DPS Inc. (DPS) are used for Sc extraction in batch and semicontinuous flow systems at model conditions. The sorbent exhibits excellent mass transfer properties, much like the whole monoliths, which should permit Sc to be rapidly recovered from large volumes of feedstock. The Sc adsorption capacity of the material is ∼142.7 mg/g at pH 6, dropping to ∼12.0 mg/g at pH 3, and adsorption is furthermore highly selective for Sc compared with the other rare earth elements (REEs). Under semicontinuous flow conditions, recovery efficiency is limited by a kinetic process. The primary mechanism responsible for the system's slow approach to equilibrium is the Sc adsorption reaction kinetics rather than inter- or intraparticle diffusion. Overall, this unmodified hierarchically porous silica powder from DPS shows great promise for the selective extraction of Sc from various feedstocks.

Preliminary Characterization of Alumina-Forming Austenitic–Type Advanced Alloys as Structural Materials for LFRs

The lead-cooled fast reactor (LFR) is one of the most promising Generation-IV nuclear designs currently under development in Europe, China, and the United States. LFRs can ensure enhanced performance and minimal waste production thanks to a closed fuel cycle, but they also have some issues that need to be addressed. One of the most critical is the long-term degradation process initiated in structural materials exposed to liquid Pb. The present state of the art has shown that commercial austenitic steels, such as American Iron and Steel Institute 316L and 15-15Ti can be adopted as structural materials in Pb environments up to 480°C, beyond which they start to experience the dissolution of constituting alloying elements (Ni, Cr, and Fe) if not protected by a coating or by surface modification. In more recent years, a lot of research effort has been done in order to develop new coating technologies and new base materials for operation with liquid Pb at higher temperatures. Among the newest alloys, alumina-forming austenitic (AFA) steels have gained interest in the research community because of their promising corrosion resistance results even at temperatures of 600°C. In this framework, an experimental campaign has been run at the Research Center ENEA of Brasimone that aims to characterize the behavior of two different AFA steels (with low and high Ni content in their composition) in static Pb at 650°C and 750°C with a moderate low oxygen concentration (10−6 wt %). After exposure, the AFA steels were characterized from the point of view of the morphology and composition, and the results are presented and discussed here.

Accelerating formation of α-Al2O3 scale on a nanograined Ni-Al alloy by self-generated NiO top layer

Alfa (α)-Al2O3 is widely used to resist oxidation at high temperature. However, presence of some transient phases can reduce the oxidation resistance. Several strategies have been proposed to solve this issue. Here, we found that nucleation of α-Al2O3 can be accelerated by a NiO layer on the surface of nanostructured Ni-7 wt%Al alloy. The oxidation rate constant is an order of magnitude lower than that of β-NiAl after 1000 °C oxidation. The enhanced growth compressive stress in scale is supposed to facilitate the α nucleation. This provides a novel strategy to develop advanced engineering alloys with high temperature applications.

D022 precipitates strengthened W-Ta-Fe-Ni refractory high-entropy alloy

Refractory high-entropy alloys have recently emerged as promising candidates for high-temperature structural applications. However, their performance is compromised by the trade-off required between strength and ductility. Here, a novel W30Ta5(FeNi)65 refractory high-entropy alloy with an outstanding combination of strength and plasticity at both room and elevated temperatures is designed, based on the multi-phase transitions design strategy. The alloy comprises a body-centered cubic dendrite phase, a topologically close-packed μ rhombohedral phase, and a high-density coherent nano-precipitate γ″ phase with the D022structure (Ni3Ta type) embedded in a continuous face-centered cubic matrix. Owing to precipitation strengthening of D022, the yield stress of the alloy is determined as high as 1450 MPa, which is a significant improvement (∼100%) in comparison with the D022-free alloy, without a loss of ductility. This alloy exhibits an excellent high-temperature strength, with the yield strengths of 1300 MPa at 600 °C and 320 MPa at 1000 °C. Detailed microstructural characterization using transmission electron microscopy, high-angle annular dark-field imaging, and three-dimensional atom probe tomography analyses indicated that this superior strength–plasticity combination stems from the synergy of a multiple-phase structure. These results provide a new insight into the design of RHEAs and other advanced alloys.

Designing Nuclear Fuels with a Multi-Principal Element Alloying Approach

Previous research has shown that multi-principal element alloys (MPEAs) using chromium, molybdenum, niobium, tantalum, titanium, vanadium, and zirconium can form stable body-centered-cubic (BCC) structures across a large temperature region (25°C to 1000°C). This is the same crystal structure as γ-uranium (U), which has shown desirable thermal and irradiation behavior in previous alloy fuel research. It is hypothesized then that the MPEA alloying approach can be used to produce a stable BCC uranium-bearing alloy and to retain its stability throughout anticipated operating regimes of power-producing reactors. Candidate elements were assessed using Monte Carlo N-Particle (MCNP) analysis to determine uranium densities necessary to make the alloy an economically viable fuel compared to conventional fuel forms. Following neutronic considerations, materials property databases and empirical predictors were used to determine the compositions with a high potential to form a BCC solid solution alloy. The final four alloys were MoNbTaU2, MoNbTiU2, NbTaTiU2, and NbTaVU2, which were cast using arc melting of raw elemental foils and chunks. Characterization of the fabricated alloys included scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, and transmission electron microscopy. The results showed a two-phase system with a U-rich matrix phase surrounding the refractory precipitates. The U phase was found to contain varying concentrations of the alloying elements and was a BCC γ-U phase. These results warrant further research to identify ideal compositions for use as an advanced alloy fuel.

Effect of WC Particle Size on Microstructure and Mechanical Properties of WC‐10CoCrFeNiAl Composites

The present study focuses on the synthesis of three distinct WC‐HEA hard alloys using the SPS method, using three different grades of tungsten carbide with varying particle sizes. Subsequently, an investigation is conducted to analyze the influence of WC particle size on the microstructure and properties of WC‐10CoCrFeNiAl hard alloys. Additionally, an investigation was conducted to assess the influence of small particle size WC addition content on both the microstructure and properties of this advanced alloy. The results indicate that a reduction in the particle size of WC leads to an increase in hardness, while exhibiting a nonmonotonic change in fracture toughness of WC‐10CoCrFeNiAl hard alloy. The presence of smaller WC particles promotes the formation of N7C3 and η phase (M3W9C4), which compromises its toughness while simultaneously enhancing its hardness. The mechanical properties are optimized when the particle size of WC is ≈1 μm. The incorporation of fine WC particles significantly enhances hardness while compromising fracture toughness. When the content of small particle WC reaches 25 wt%, the mechanical properties of the dual‐scale hard alloy are optimized, the hardness and fracture toughness are 18.8 GPa and 11.1 MPa.m1/2, respectively.

Miniaturised experimental simulation of open-die forging

This study presents a novel experimental set-up for laboratory-scale simulation of cogging and open-die forging processes during ingot-to-billet conversion of advanced engineering alloys. The experimental set-up is designed to be cost-effective, employing a remotely operated manipulator assembly constructed from readily available “off-the-shelf” components used in conjunction with a conventional uni-axial load-frame - equipment that is available in most materials testing laboratories. Small test-bars of C101 copper alloy were subjected to multi-stroke cogging operations with intermittent rotation at ambient and elevated temperatures (20–600 °C). Prior to forging, the as-received material underwent heat treatments to coarsen the starting grain structure, and to help demonstrate the capability of the apparatus to achieve grain refinement via recrystallisation (dynamic and static) and recovery processes within the deformed material. The resulting microstructural evolution and mechanical property changes of the forged material have been investigated using light microscopy (LM), Vickers hardness (HV) testing, and electron backscatter diffraction (EBSD). The deformed C101 alloy exhibited measurable grain refinement after forging at elevated temperatures, thus demonstrating the effectiveness of the designed miniaturised open-die forging set-up.

Composition design of high-performance copper alloy by coupling artificial neural network and genetic algorithm

To investigate the complex nonlinear relationship between the composition and properties of high-performance copper alloys, a double-layer artificial neural network model based on a genetic algorithm (ANN-GA) was developed using MATLAB tools. The dataset showed that the correlation coefficient of the model was greater than 95%. Quantitative calculation formulas for the alloy composition, ultimate tensile strength (UTS), and conductivity were established using the transfer function, weights, and thresholds of the ANN-GA model. The effects of the main elements on the mechanical and electrical properties of the alloys were analysed. The simulation results were similar to the experimental results obtained by the ANN-GA, confirming the excellent prediction ability of the model. Four advanced copper alloys with high strength and conductivity were successfully simulated to meet the targets of UTS (≥540 MPa) and conductivity (≥80% IACS). Two alloys were selected for melting, heat treatment, and performance testing. The error between the experimental results and predicted results was within 5%, demonstrating the applicability of the model. The proposed framework is useful for designing high-performance Cu alloys.

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.

A L12 precipitation strengthened Co-free medium-entropy alloy with superior high-temperature performance

In present work, the superior high-temperature properties in a novel (Ni, Cr, Fe)3(Al, Ti, Cr)-γʹ (L12) type precipitation-strengthened Co-free medium-entropy alloy (MEA) was systematically evaluated. Experimental results indicate that this γʹ-strengthened alloy demonstrates excellent structure thermal stability and no other brittle intermetallic phases formed at grain boundaries or grain interior during long-term thermal exposures. The γʹ phase in this Co-free MEA exhibits an extraordinary sluggish coarsening rate, which is much lower than most conventional superalloys and recently developing γʹ-strengthened high/medium-entropy alloys (HEAs/MEAs). This γʹ-strengthened alloy also exhibits a good combination of high γʹ-solvus temperature (1068 °C) and low mass density (7.79 g/cm3). More significantly, this γʹ-strengthened MEA shows excellent high-temperature tensile strength, especially outstanding specific strength, which is superior to most Ni/Co-based superalloys. Our present work provides a useful illustration for designing advanced high-temperature alloys based on γʹ(L12)-strengthened HEAs/MEAs, rather than Ni-based/Co-based superalloys.

Flash electropolishing of BCC Fe and Fe-based alloys

The preparation of transmission electron microscopy (TEM) samples is a critical step in the characterization of materials, and the focused ion beam (FIB) technique is a commonly used method. However, a significant limitation of this technique is the FIB-induced damages on the foil surfaces, which can obscure the real features of interest, particularly in radiation effects studies. To overcome this limitation, this study presents a detailed description of the flash electropolishing technique, which can be used to remove the FIB damage from samples. The flash electropolishing technique has been successfully applied to a range of materials, including Fe, Fe-based model alloys, commercial Fe-Cr alloys, and advanced Fe-Cr alloys, both in their as-received and ion-irradiated states. Furthermore, the parameters used for Fe-Cr model alloys can be adjusted for commercial and advanced alloys with minimal modifications. The study also examined the effects of electropolishing variables, such as perchloric acid concentration, electropolishing temperature, Cr concentration, and voltage. Qualitatively, a general trend and scoping test strategy is explored in our experiments. Overall, the flash electropolishing technique offers a promising solution to the challenges posed by FIB-induced damages in the preparation of TEM samples.

Probing passivity of corroding metals using scanning electrochemical probe microscopy

AbstractPassive films are essential for the longevity of metals and alloys in corrosive environments. A great deal of research has been devoted to understanding and characterizing passive films, including their chemical composition, uniformity, thickness, porosity, and conductivity. Many characterization techniques are conducted under vacuum, which do not portray the true in‐service environments passive films will endure. Scanning electrochemical probe microscopy (SEPM) techniques have emerged as necessary tools to complement research on characterizing passive films to enable the in situ extraction of passive film parameters and monitoring of local breakdown events of compromised films. Herein, we review the current research efforts using scanning electrochemical microscopy, scanning electrochemical cell microscopy (or droplet cell measurements), and local electrochemical impedance spectroscopy techniques to advance the knowledge of local properties of passivated metals. The future use of SEPM for quantitative extraction of local film characteristics within in‐service environments (i.e., with varying pH, solution composition, and applied potential) is promising, which can be correlated to nanostructural and microstructural features of the passive film and underlying metal using complementary microscopy and spectroscopy methods. The outlook on this topic is highlighted, including exciting avenues and challenges of these methods in characterizing advanced alloy systems and protective surface films.

Microstructure and fracture analysis on MIG welding of Nimonic alloys

Advanced alloy has an essential role in welding process to enhance the performance characteristics. Nimonic alloy has outstanding welding characteristics. The purpose of this work is to analyze the microstructure and fracture analysis of the welded Nimonic alloy joints by employing metal inert gas welding (MIG) welding. In this work, Taguchi analyze was effectively utilized to anticipate the ideal parameters of the input variables such as voltage, welding speed, welding current towards the maximized optimal value of the welded specimen's ultimate tensile strength. In this study, a L9 orthogonal array was employed, and the ultimate tensile strength for the specimen were estimated. The findings reveal that at maximum welding speed and voltage, the specimen has a satisfactory strength bonded joint. Microstructure and fracture analysis were also discussed. The objective of the work is to enhance the ultimate tensile strength of the nimonic welded joints. The optimal welding strength was attained at welding current of 110A, speed of 20 m/s and voltage of 155 V.

Weight-averaged solute segregations at grain boundaries in advanced ferritic alloys

Solute segregation can have profound effects on the atomic structures and properties of grain boundaries (GBs), and hence the overall mechanical properties of polycrystalline alloys. In this work, a two-step site- and GB-weighted average method was proposed to predict the temperature-dependent segregation behaviors and effects from first-principles. Validated by the experimental S enrichments at GBs, this method was extended to a series of alloying elements in ferrite alloys. The calculation results suggested that the equilibrium weight-averaged GB concentrations follow the general ordering of Y>La>Zr>Hf>Al>Si>Cr>Ti>W, and decrease slowly with temperature. The segregation of Y, La, Ce, Zr, Hf, Al and Si, evidently reduces the GB adhesion strength with La being the most detrimental, while Ti, Cr and W have only minimal impacts on GB adhesion. These segregation effects do not change much with temperature. The new method is not limited to ferritic alloys but can be extended to any other alloy systems.

Investigation of the Micromechanical Behavior of a Ti68Nb7Ta3Zr4Mo18 (at.%) High-Entropy Alloy.

Intense research efforts are focused on the development of advanced high-entropy alloys intended for premium aerospace components and other applications, where high strength and good formability are crucial. The mechanical properties of these alloys are closely related to the phase transformation, dislocation evolution, and grain size, and these factors are affected by the deformation temperature. The response of the retained austenite to strain-induced martensitic transformation at various temperatures was studied in an advanced Ti68Nb7Ta3Zr4Mo18 (at.%) high-entropy alloy via molecular dynamics simulation. It was found that the Ti68Nb7Ta3Zr4Mo18 alloy changes from a single crystal to a polycrystal during the tensile process, and the transition of the Ti68Nb7Ta3Zr4Mo18 (at.%) high-entropy alloy from the BCC phase to the FCC phase occurs. At high temperatures and low strain rates, grain boundary slip is the main deformation mechanism, and at low temperatures and high strain rates, dislocation slip replaces grain boundary slip as the dominant deformation mechanism, which improves the strength of the alloy. Moreover, when the grain size is too small, the strength of the alloy decreases, which does not satisfy the fine grain strengthening theory and shows an inverse Hall-Petch relationship. This study offers a new compositional window for the additive manufactured lightweight high-strength material categories for various applications including the aerospace industry.