Multi‐principal elemental alloys, commonly referred to as high‐entropy alloys (HEAs), are a new class of emerging advanced materials with novel alloy design concept. Unlike the design of conventional ...alloys, which is based on one or at most two principal elements, the design of HEA is based on multi‐principal elements in equal or near‐equal atomic ratio. The advent of HEA has revived the alloy design perception and paved the way to produce an ample number of compositions with different combinations of promising properties for a variety of structural applications. Among the properties possessed by HEAs, sluggish diffusion and strength retention at elevated temperature have caught wide attention. The need to develop new materials for high‐temperature applications with superior high‐temperature properties over superalloys has been one of the prime concerns of the high‐temperature materials research community. The current article shows that HEAs have the potential to replace Ni‐base superalloys as the next generation high‐temperature materials. This review focuses on the phase stability, microstructural stability, and high‐temperature mechanical properties of HEAs. This article will be highly beneficial for materials engineering and science community whose interest is in the development and understanding of HEAs for high‐temperature applications.
In recent years, high entropy alloys (HEAs) receive wide attention due to its unique alloy design concept and outstanding properties. This review presents a general overview of HEAs as a potential candidate for high‐temperature applications. The need for the profound research on the high‐temperature properties of HEAs is highlighted.
Exfoliation of graphite is a promising approach for large-scale production of graphene. Oxidation of graphite effectively facilitates the exfoliation process, yet necessitates several lengthy washing ...and reduction processes to convert the exfoliated graphite oxide (graphene oxide, GO) to reduced graphene oxide (RGO). Although filtration, centrifugation and dialysis have been frequently used in the washing stage, none of them is favorable for large-scale production. Here, we report the synthesis of RGO by sonication-assisted oxidation of graphite in a solution of potassium permanganate and concentrated sulfuric acid followed by reduction with ascorbic acid prior to any washing processes. GO loses its hydrophilicity during the reduction stage which facilitates the washing step and reduces the time required for production of RGO. Furthermore, simultaneous oxidation and exfoliation significantly enhance the yield of few-layer GO. We hope this one-pot and fully-scalable protocol paves the road toward out of lab applications of graphene.
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•A review is presented on recent research on precipitation-hardened high-entropy alloys that can be produced by additive manufacturing.•The different strengthening mechanisms of AMed ...HEAs were reviewed with emphasis on precipitation strengthening.•The advantages of using applying post-printing heat-treatment for enhancing the strength-ductility relationship of HEAs are discussed.•Dynamic responses of precipitation during deformation are explained through the interaction between dislocations and precipitates.•The future prospects of precipitation-hardened high-entropy alloys fabricated by additive manufacturing are presented.
The growing demand for advanced metallic materials with optimum mechanical properties has led to the creation of next-generation materials based on the alloying of multiple-principal elements in high concentrations. High-entropy alloys (HEAs) have a high potential for industrial applications due to their extraordinary properties under elevated, ambient, and cryogenic conditions. Due to several limitations of conventional manufacturing methods, to develop HEAs of the maximum capability, a novel metal additive manufacturing (MAM) technique has been developed to produce defect-free HEA components with the desirable performance. The unique microstructures of MAMed HEAs provide an optimum strength-ductility relationship, even in extreme environments, by the simultaneous activation of several strengthening mechanisms. In particular, applying post-printing heat-treatment can significantly enhance the strength-ductility relationship of HEAs. Herein, a comprehensive review based on the process-microstructure-properties relationship in precipitation-hardenable HEAs fabricated by 3D printing is provided. Different kinds of precipitates formed in the microstructures of MAM-processed HEAs after applying a proper post-MAM heat treatment are presented. Moreover, the corresponding mechanical properties of these components are discussed in detail. Also, the improvement in the mechanical properties of precipitation-hardened MAM-processed HEAs due to the interaction between dislocations and precipitates is introduced, resulting in precipitate shearing and creation of Orowan/Hirsch loops.
In this study, the TiC-reinforced CoCrFeMnNi high-entropy alloy (HEA) composite was fabricated using water atomization (WA), mechanical milling (MM), and spark plasma sintering (SPS). The ...microstructural evolution and mechanical properties of TiC-reinforced HEA composite are investigated using electron backscatter diffraction, transmission electron microscopy, and room temperature compression tests. The addition of 5 wt% of TiC nano-particles to CoCrFeMnNi HEA resulted in fine grain size, high yield strength, and high strain hardening. The average grain size achieved for alloys with and without TiC after sintering is 5.1 μm and 10.6 μm, respectively. The addition of TiC increases the compressive yield strength from ∼507 MPa to ∼698 MPa and compressive fracture strength from ∼1527 MPa to ∼2216 MPa, without sacrificing the ductility. The strengthening behavior of TiC-reinforced CoCrFeMnNi HEA composite is quantitatively discussed based on grain boundary strengthening, dislocation strengthening, and dispersion strengthening. The role of TiC nano-particles in the strain hardening improvement is investigated with respect to the dislocation-particle interaction and consequently increased dislocation density.
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•Fabrication of TiC reinforced CoCrMnFeNi high entropy alloy composite by powder metallurgy route.•TiC particles prevents grain boundary movement and results in fine grain size of FCC matrix after sintering.•CoCrMnFeNi-TiC composite shows enhanced mechanical properties as compared to CoCrMnFeNi alloy.
Tensile properties of an as-cast CoCrFeMnNi high-entropy alloy were investigated at various temperatures ranging from −160 to 1000°C. The tensile strength and ductility did not vary significantly ...with loading direction, despite the alloy’s strongly preferred crystallographic orientation. The impact toughness values of the as-cast high-entropy alloy were much higher than those of many traditional alloys, particularly at low temperatures. The mechanical properties of the as-cast high-entropy alloy were compared with those of wrought high-entropy alloy and noticeable differences between the two alloys were found. The maximum tensile ductility and three different strain hardening stages were observed at 500°C in the as-cast structure. Transmission electron microscopy observations demonstrated that the initiation of deformation twinning was very active even at 500°C. A simple calculation suggests that very large grains of the as-cast structure induced a reduction in twinning stress, retarding the onset of strain localization.
Metal additive manufacturing (MAM) is an emerging technology to produce complex end-use metallic parts. To adopt MAM for manufacturing numerous engineering parts used in critical applications, a ...thorough understanding of the relationship between the complex thermal cycles in MAM and the unique heterogeneous microstructures of MAM parts need to be established. This review article provides a comprehensive overview of the evolution of heterogeneous microstructures in MAM parts, including melt pool boundaries, heterogeneous grain structure, sub-grain cellular structure, matrix supersaturation, segregation, phase transformation, oxides formation, and texture. The evolution of residual stresses and the anisotropy in MAM parts and the post-MAM heat treatment effects on the microstructural evolution are also discussed. This review covers the microstructural aspects of most engineering materials in particular steels, high entropy alloys, aluminum alloys, titanium alloys, nickel-base superalloys, and copper alloys, with a primary focus on the parts manufactured using selective laser melting, direct energy deposition, and electron beam melting processes.
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Excellent ductility is crucial not only for shaping but also for strengthening metals and alloys. The ever most widely used eutectic alloys are suffering from the limited ductility and ...losing competitiveness among advanced structural materials. Here we report a distinctive concept of phase-selective recrystallization to overcome this challenge for eutectic alloys by triggering the strain hardening capacity of the duplex phases completely. We manipulate the strain partitioning behavior of the two phases in a eutectic high-entropy alloy (EHEA) to obtain the phase-selectively recrystallized microstructure with a fully recrystallized soft phase embedded in the skeleton of a hard phase. The resulting microstructure fully releases the strain hardening capacity in EHEA by eliminating the weak boundaries. Our phase-selectively recrystallized EHEA achieves a high ductility of ∼35% uniform elongation with true stress of ∼2 GPa. This concept is universal for various duplex alloys with soft and hard phases and opens new frontiers for traditional eutectic alloys as high-strength metallic materials.
The strengthening mechanism of the metallic material is related to the hindrance of the dislocation motion, and it is possible to achieve superior strength by maximizing these obstacles. In this ...study, the multiple strengthening mechanism-based nanostructured steel with high density of defects was fabricated using high-pressure torsion at room and elevated temperatures. By combining multiple strengthening mechanisms, we enhanced the strength of Fe-15 Mn-0.6C-1.5 Al steel to 2.6 GPa. We have found that solute segregation at grain boundaries achieves nanograined and nanotwinned structures with higher strength than the segregation-free counterparts. The importance of the use of multiple deformation mechanism suggests the development of a wide range of strong nanotwinned and nanostructured materials via severe plastic deformation process.
Identifying phase information of high-entropy alloys (HEAs) can be helpful as it provides useful information such as anticipated mechanical properties. Recently, machine learning methods are ...attracting interest to predict phases of HEAs, which could reduce the effort for designing new HEAs. As research direction is in its infancy, there is still plenty of room to develop machine learning models to improve the prediction accuracy and further guide the design of HEAs. In this work, we employ deep learning-based methods regarding optimization, generation, and explanation, for enhancing the performance and identifying key design parameters for phase prediction of HEAs. We first establish regularized deep neural networks for predicting HEA phases and optimize hyper-parameters concerning model architecture, training, and regularization. To overcome data shortage of HEAs, we then focus on developing conditional generative adversarial network for generating additional HEA samples. We observe the augmentation from our generative model significantly improves model performance, achieving prediction accuracy of 93.17%. Lastly, we concentrate on understanding contributions of design parameters to identifying solid solution (SS) phase as an example. Our work delivers guidance not only for developing a reliable deep learning-based phase prediction model, but for explaining significant design parameters to assist design of novel HEAs.
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•We propose deep learning-based methods for reliable phase prediction of high-entropy alloys with three aspects.•Promising set of hyper-parameters for the regularized deep neural network are searched via Bayesian optimization process.•Generative design of neural network is established to produce realistic data from pre-existing high-entropy alloy samples.•Additional samples by generative learning framework can improve phase prediction performance of existing high-entropy alloys.•Significant design parameters for phase prediction of HEAs are revealed based on the interpretation of our proposed model.
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•Laser welding between a CoCrFeMnNi high entropy alloy and 316 stainless steel was performed.•Joint ductility increased from 5 to 10 % by changing the CoCrFeMnNi high entropy alloy ...from rolled to annealed condition.•The joints strength is preserved at ≈ 450 MPa regardless of the CoCrFeMnNi high entropy alloy condition.•The CoCrFeMnNi / 316 stainless steel joints can be considered for structural applications.
Dissimilar joining involving high entropy alloys is currently being explored to evaluate the suitability of these novel advanced engineering materials in structural applications. Recently, joining of a CoCrFeMnNi high entropy alloy to 316 stainless steel was successfully attempted. However, the joint ductility was limited by the lack of deformation experienced by the cold-rolled CoCrFeMnNi base material during tensile loading. In this work, it is shown that by simply changing the base material condition, from cold-rolled to annealed, it is possible to significantly improve the joint fracture strain from ≈ 5 to ≈ 10 %, while preserving the strength at ≈ 450 MPa. Using electron microscopy, high energy synchrotron X-ray diffraction and mechanical testing aided by digital image correlation, the microstructure evolution across the welded joint is assessed and correlated to its mechanical performance. Moreover, thermodynamic calculations considering the compositional changes across the fusion zone were used to predict the microstructure evolution of this region.
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