Lattice distortion has been deemed as one of the most distinctive structural features of high-entropy alloys. However, despite its fundamental importance, the notion of lattice distortion and its characterization is still an issue under debate. In this work, based on the recent work on theoretical modeling, atomistic simulations and experiments, the physical origin of lattice distortion in high-entropy alloys and its resultant intrinsic residual stresses or strains are discussed.
High-entropy alloys (HEAs) or multiprincipal-element alloys have exceptional properties those may be better than the properties of conventional alloys and exhibit different deformation mechanisms. However, some issues pertaining to what may make HEAs distinct from the conventional alloys remain to be resolved. Many studies have found that heterogeneity in chemical distribution due to unique atomic features may be common in HEAs. Therefore, the relationship between the structure and properties cannot be explained completely by the traditional solid solution strengthening mechanism. In this short review, classified by their crystal structures, such as face-centered-cubic, body-centered-cubic, and dual-phase, studies on element distribution in HEAs, including concentration wave and short-range order, are summarized. The influence of heterogeneity in chemical distribution on dislocation behaviors and mechanical properties is described. Further, a brief perspective of the research directions for element distribution is proposed.
Nanocrystalline alloys (NAs) with nano-sized fine grains and high density of grain boundaries exhibit promising properties, such as high strength and hardness. However, industrial applications of NAs at high or even room temperature have been limited, owing to their thermal instability, which originates from the high proportion of grain boundaries in NAs. Recently, nanocrystalline high-entropy alloys (NC-HEAs) have emerged and have been rapidly developed, which are expected to alleviate thermal instability. In this study, design strategies for the thermal stability of NC-HEAs and related progress are investigated and summarized. In addition, the underlying mechanism for the high thermal stability of NC-HEAs is discussed by utilizing high-entropy effects, based on entropy engineering. These high-entropy design strategies may provide a new methodology for dramatically increasing the thermal stability of NAs.
High-entropy alloys are designed based on the concept of multi-principle elements and high-configuration entropy. They exhibit excellent mechanical, high-temperature, and irradiation-tolerant properties, indicating their great potential for high-performance structural materials in the recent decade. Since the discovery of high-entropy alloys, most of the related research work was based on the classical assumption of ideal solid solution. However, recent investigation have showed the local chemical order in high-entropy alloys and how such atomic-level structure tunes the deformation mechanism, which has attracted significant attention. This study reviewed the recent progress in the theoretical description and experimental characterization of the local chemical order as well as its impact on the mechanical properties of high-entropy alloys. Besides, a brief perspective on the research of understanding and optimizing high-entropy alloys from the local chemical order is proposed.
Metallic glasses (MGs) are formed by the deep undercooling of high-temperature melt up to the glass transition temperature, and this process avoids the crystallization of the melt into ordered configurations of atoms. The atomic packing of MGs lacks a long-range periodicity. MGs reside at metastable energy states far away from the equilibrium of thermodynamics, but they are jammed in dynamics. These features provide MGs with remarkable mechanical, physical, and chemical properties, such as very high strength that is close to the ideal limit. However, the plastic deformation of MGs at room temperature is easily localized to form nanoscale shear bands, thereby resulting in limited macroscopic plasticity. Moreover, physical ageing spontaneously reduces their energies toward an equilibrium state, thereby further weakening the plastic deformation ability of MGs, which is known as ageing-induced brittleness. Recent studies have shown that MGs can be rejuvenated with external energy injection into more disordered high-energy states in structure. This process, which is the inverse of physical ageing, can effectively improve the global plasticity of MGs and is expected to solve the problems of shear banding and physical ageing that restrict the applications of such materials. Therefore, the relevant aspects of the rejuvenation of MGs have attracted increasing interest. This article first introduces methods for the rejuvenation of MGs starting from the concepts of ageing and rejuvenation of glasses, and then summarizes the influencing factors of rejuvenation and the effects of rejuvenation on plasticity and other mechanical behaviors of MGs. Furthemore, the physical mechanism of rejuvenation is discussed briefly. Finally, several conclusions are drawn in this field, and some important problems that deserve further investigation are proposed.
Metallic glasses (MGs) are one of the most attractive topics in the field of condensed physics and materials science because of their unique structure and excellent properties. As a metastable material, MGs tend to present a transition toward a more stable low-energy state under applied stress or high-temperature, known as aging or structural relaxation, accompanied by a decrease in deformability at room temperature. Rejuvenation of MGs is a converse process of aging/relaxation, which transforms the materials to their previous and higher-energy states. Rejuvenation greatly expands the energy range of MGs, which not only significantly improves the deformation capability of MGs, but also provides a new opportunity to explore the atomic structure, glass transition, and deformation mechanisms of MGs. This article reviews the recent progress in the study of rejuvenation, including the methods of rejuvenation of MGs, the effect of rejuvenation behavior on microstructures, mechanical properties, and functional characteristics. Finally, a brief outlook on the study of the rejuvenation behavior of MGs is presented.
Plastic deformation of metallic glasses (MGs) at room temperature usually localizes into shear bands. The shear banding behavior dominates the deformation and fracture mechanisms of MGs and affects their mechanical properties (e.g., strength, plasticity, fracture toughness, fatigue properties, etc.). The shear banding behavior is of vital importance for understanding and improving mechanical properties of MGs; therefore, studies on shear bands have been long-standing hot topics in the MG field. In this paper, based on the previous studies of shear banding behaviors in various MGs, the mechanisms of shear band propagation, cracking, and unstable fracture under monotonic and cyclic loadings are elucidated. The experimental evidence of progressive shear band propagation under uniaxial loading is provided and the mechanisms of shear band cracking under compression are revealed. It is found that the “cold” fracture of shear band can occur when reducing sample size, adding external confinement, or decreasing the testing temperature to stabilize the shear band propagation. Under cyclic loading, the shear-band-mediated fatigue crack initiation and cracking mechanisms are confirmed. Furthermore, the fragmentation in brittle MGs under compression should be caused by split cracking from defects. The effect of sample size on competition between shearing and splitting for brittle MGs is also discussed. Finally, the significant role of external defects (e.g., notches) on shear banding behavior is described and a “defect engineering” strategy for tailoring the mechanical properties of MGs is proposed.
Metallic glass thin film (MGTF) is a new thin film material developed based on metallic glasses. It has excellent physical, chemical, and mechanical properties owing to its disordered atomic packing structure. Moreover, nanostructured MGTF can be synthesized by adjusting the process of physical vapor deposition to construct nanostructure interfaces and overcome the intrinsic brittleness of bulky metallic glasses. Recently, the rapid development of MGTF in various fields has attracted extensive attention and has become a new research hotspot. In this paper, by reviewing the influence of preparation parameters on the microstructure and properties of MGTF, and sample size effect. In this manner, research progress is analyzed and summarized, and research prospects are briefly proposed to provide reference for researchers engaged in the study of MGTF.
Metallic glasses possess densely packed and disordered atomic structures linked by non-directional metallic bonds. Within these structures, the superior properties of conventional glasses and crystalline metals can be combined with excellent physical, chemical, and mechanical properties for widespread applications. Metallic glasses also offer a unique model system for fundamental studies on amorphous materials. For these reasons, they have attracted global interest. Phase-transition studies can deepen people's understanding of the atomic structures of materials and can realize materials with tunable properties. The polyamorphic transitions in conventional amorphous materials are not expected in metallic glasses because the latter are already densely packed. However, in situ high-pressure synchrotron X-ray probing techniques have recently detected polyamorphic transitions in metallic glasses. This new phenomenon, its underlying mechanism, and the related property changes have recently sparked much excitement. This paper reviews the recent progress in polyamorphic transitions in metallic glasses and the influence of such transitions on their atomic structure and properties.
Owing to limitations in the spatial and temporal resolution of the current experimental research technologies, the heterogeneity of a disordered structure poses a great challenge to the experimental study of atomic-level behaviors of amorphous alloys. Computational simulation can be a powerful tool in the understanding of such amorphous structures and their response at the atomic level. However, owing to the limitations of multielement interactions, computational approaches, and computational capability, there is still an insurmountable gap between the model systems used in computational simulation and real amorphous alloy materials. Combining the power of the modern computing technology, software, and algorithms, the exploration and development of hihgly effective computational approaches that can be applied to the simulation of amorphous alloys is a potential way to address this long-term challenge. This article reviews recent progress in the computational study of atomic structure and structural instability in metallic glasses, the role that such computational approaches can play in the understanding and the modification of material properties, and in the optimization of material preparation. A brief perspective on the research areas of the computational simulation of metallic glasses is also proposed.
Bulk amorphous alloys possess a metastable structure, which is difficult to process and manufacture into components or parts by conventional forging or welding. Instead, components or parts from bulk amorphous alloys can be fabricated by vacuum casting with the fluidity of bulk amorphous-alloy melts. Based on the casting forming of bulk amorphous alloys, this paper briefly introduces the fluidity and filling ability of bulk amorphous-alloy melts. In addition, the technical methods and applications of vacuum die casting, vacuum suction casting, gravity casting in a water-cooled copper crucible, and phase-change refrigeration casting are also mentioned. The theoretical problems and technical bottlenecks to be resolved in the forming process of bulk amorphous alloys are then discussed. Finally, the engineering application prospects of bulk amorphous alloys are suggested.
The application of bulk metallic glasses (BMGs) as structural materials not only involves the challenge of room temperature brittleness but also bottlenecks related to formation and manufacturing. Solving these issues has become one of the research hotspots and difficulties in the material field recently. The recently developed 3D printing technology has gradually become one of the key methods to solve the existing difficulties of BMGs to realize their engineering applications. However, because BMGs have a completely different atomic structure than crystalline materials, the basic theories of material microstructure evolution, defect formation and suppression, and performance adjustment in 3D printing are completely different. The in-depth analysis of the abovementioned scientific issues is very important for the development of BMG 3D printing technology. This article is mainly focused on the research trends at home and abroad with a comprehensive analysis of the above problems and looks forward to the development trends of 3D printing technology.
The functional groups in traditional energetic materials typically contain C, N, and O. These elements are usually unstable and sensitive to external stimuli. Moreover, the chemicals used during the preparation process of traditional energetic materials are toxic and pose many safety and environmental issues. As one of the metastable materials, high-energy-state amorphous alloys are potential candidates for new energetic materials with high combustion heat, low ignition temperature, non-toxicity, and improved safety. In this work, the combustion mechanism of Fe-based amorphous ribbons with an anomalous exothermic phenomenon was systematically studied using in situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, and X-ray photoelectron spectroscopy. Experimental results show that compared to Fe-Nb-B-Y amorphous alloys with normal thermophysical and combustion behaviors, (Fe0.72B0.24Nb0.04)95.5Y4.5 amorphous ribbons with anomalous exothermic phenomena possess low ignition temperature and self-propagating combustion behavior due to the catalysis of the rapid crystallization exothermic process. The anomalous exothermic phenomenon and associated liquid-liquid phase transitions can cause rapid crystallization at elevated temperatures during heating, followed by multi-step oxidation. On the other hand, it was possible to significantly reduce the activation energy of high-temperature oxidation. The liquid-liquid phase transition can lower the energy barrier of the oxidation reaction. In this way, the oxidation reaction at high temperatures can be promoted. The results suggest that the liquid-liquid phase transition has an “induced activation” effect on the combustion of Fe-based amorphous alloys.
Nickel-phosphorus metallic glass (MG) is one of the most typical MGs, and is also one of the most popular studied MG components, especially in the process of establishing the structure model of amorphous alloys and computer simulations. Although it appears throughout the history of amorphous alloy research, its glass transition temperature and liquid properties have not yet been determined due to its low ability to form glass. In this work, we bypassed the crystallization of Ni80P20 MG during the glass transition process by an ultrafast calorimeter with heating rates up to thousands of Kelvin per second and directly detected its glass transition process. This method offers an opportunity to expose the nature of the Ni80P20 MG supercooled liquid. The liquid fragility of the metallic glass Ni80P20 was determined by the Vogel-Fulcher-Tammann equation based on the dependence of the glass transition temperature at different heating rates. The results show that the liquid fragility of Ni80P20 MG is 93 ± 4, which is obviously a very “fragile” liquid compared with other bulk MGs. It is suggested that this “fragile” liquid behavior of Ni80P20 MG may lead to its low glass-forming ability.
In mechanical systems, friction and wear lead to energy loss and machine failure. Lubricants are widely used to minimize friction and wear between moving components. Additives in lubricants significantly improve the quality of the lubricants. In recent years, nanoparticles have started to play more important roles as lubricant additives because of their ability to minimize friction and wear reduction. Despite the advantages of nanoparticles as additives, there are also some challenges to their applications. The most significant challenge is that because of the strong van der Waals force, nanoparticles aggregate in solutions. In addition, their complex process of preparation and high costs limit application in large-scale fields. Metallic glasses (MGs) with long-range disorder structure exhibit novel physical and chemical properties, e.g., high strength and hardness, high elastic limitation, high hardness/elasticity ratio, which make them potentially suitable for use as additives for lubricants. This work presents the tribological properties of friction and wear behaviors of polyalphaolefin (PAO6) oil modified with Mn55Fe25P10B7C3 MG particles at different concentrations (0~0.5%, mass fraction). Four ball tests were performed with an MMW-1A tribotester, XRD was used to examine the structure of the prepared Mn-based powders, SEM was used to observe morphologies of Mn55Fe25P10B7C3 particles and worn surfaces; OM was used to measure the wear scar diameters (WSD) and its roughness was measured with a white light interferometer (WLI). The results show a significant decrease of up to 57.1% and 15.6% for the coefficient of friction (COF) and WSD, respectively, as the addition of 0.5% MG particles in PAO6. The addition of MG particles leads to a decrease of worn surface roughness. With a high hardness/elasticity ratio and similar modulus to the friction pairs, the MG particles show a “smearing-type” wear mechanism, thus enhancing the antifriction and antiwear performance of PAO6 lubricants.
Metallic glasses (MGs) with a long-range disordered structure and without crystallographic defects have attracted great research attention. Owing to the disordered structure, MGs usually exhibit excellent physical and chemical properties, comprehensive mechanical performances, and high thermal stability. Minor doping of elements can effectively enhance the glass-forming ability of MGs and then seriously affect the yielding strength and plasticity. In this study, a family of Cu45Zr45Al10-xAgx (x = 1, 2, 3, and 5, atomic fraction, %) MG with minor Ag-addition was prepared by suction casting. A nanoindentation test was used to investigate the influence of Ag content on the nanoplastic deformation behavior of the Cu-Zr-Al-based MG. In terms of the empirical equation, the strain rate sensitivity index (m) was acquired to calculate the shear transformation zone volume during the nanoindentation creep process. Based on the Kohlrausch-Williams-Watts equation, the relaxation evolution was obtained. As Ag content approaches 5%, m attains its minimum, indicating that the creep resistance of the Cu-Zr-Al-based MG is the largest. The creep behavior of the system depends on the loading rate, i.e., the faster the loading rate, the lower the creep resistance. With an increase in the Ag content, the exponent stretch is increased, and the hardness of the Cu-Zr-Al-based MG was enhanced combined with an increased plasticity. This study presents the fundamental information of the relationship between the thermal dynamic and mechanical properties of the Cu-Zr-Al-based MG.
