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With multiple elements mixed at equal or near-equal molar ratios, the emerging, high-entropy alloys (HEAs), also named multi-principal elements alloys (MEAs), have posed tremendous challenges to materials scientists and physicists, e.g., how to predict high-entropy phase formation and design alloys. In this paper, we propose some guidelines in predicting phase formation, using thermodynamic and topological parameters of the constituent elements. This guideline together with the existing ones will pave the way toward the composition design of MEAs and HEAs, as well as property optimization based on the composition–structure–property relationship.
Quasi-static and dynamic deformation behaviors of Zr-based bulk-metallic-glass-matrix composites, fabricated by Bridgman solidification, were investigated in this study. Upon quasi-static compressive loading, the composites exhibit ultrahigh strength, accompanied by considerable plasticity. The multiplication of shear bands on the lateral surface of deformed samples, and the highly-dense liquid drops on the fracture surface, are in agreement with the improved plasticity. However, upon dynamic loading, the mechanical properties of the composites deteriorate considerably, due to insufficient time to form profuse shear bands. The strain-rate responses of the mechanical properties of the crystalline alloys and the in situ and ex situ bulk metallic glass composites are compared, and the different deformation mechanisms of the in situ composites upon quasi-static and dynamic loading are explained.
Using an infrared camera, the temperature evolution of as-cast and relaxed bulk metallic glasses during compression was measured. Substantial variations in the temperatures of both glasses during plastic deformation were observed, which are conjectured to result at least partially from shear-banding phenomena. The relaxed glass has a larger temperature rise than the as-cast glass, which can be attributed to a reduction in the free volume. The larger temperature increase in the relaxed glass may be responsible for the observed work softening. The relaxed glass also has a higher maximum temperature than the as-cast, which can be attributed to a stronger strain-rate dependence of the temperature rise rate, and a shorter dissipation time scale for the heat due to conduction. The experimental data follow the well-known model behavior, and suggest the possibility of a statistical correlation between the fluctuations of strain rates and the rates of the temperature variation.
There is often a tradeoff between strength and ductility, and the low ductility of ultrafine-grained (UFG) materials has been a major obstacle to their practical structural applications despite their high strength. In this study, we have achieved a ∼40% tensile ductility while increasing the yield strength of FeCrNiMn steel by an order of magnitude via grain refinement and deformation-induced martensitic phase transformation. The strain-rate effect on the inhomogeneous deformation behavior and phase transformation was studied in detail.
Different bulk metallic glasses (BMGs) were prepared in ductile Cu47.5Zr47.5Al5, Zr62Cu15.4Ni12.6Al10, and brittle Zr55Ni5Al10Cu30 alloys by controlling solidification conditions. The achieved microstructures were characterized by x-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and synchrotron- based high-energy x-ray diffraction. Monolithic BMGs obtained by high-temperature injection casting are brittle, while BMGs bearing some nanocrystals with the size of 3 to 7 nm and 2 to 4 nm, obtained by low-temperature injection casting and in situ suction casting, respectively, exhibit good plasticity. It indicates that the microstructures of BMGs are closely affected by the solidification conditions. Controlling the solidification conditions could improve the plasticity of BMGs.
In this study, we report the growth of metallic tungsten nanowires induced by alloy catalysts (Fe–Ni) at a temperature of 850 °C. The synthesized tungsten nanowires have bottom diameters of 100 to 400 nm and tip diameters of <80 nm, and show a well-defined single-crystalline structure. The formation of the (Fe,Ni)-catalyzed W nanowires should be controlled by the vapor–solid–solid mechanism, rather than the traditional vapor–liquid–solid mechanism, because the growth temperature is significantly below the lowest eutectic temperature (1455 °C) of the Fe–Ni–W ternary system. Our study demonstrates the feasibility of synthesizing metallic nanowires via metal-catalyzed methods, which may be extended to the synthesis of some other metallic nanowires.
We investigated the effect of strain rate on the plastic-flow stress of a Zr-based bulk-metallic glass in quasistatic compression. The results indicate that the plastic-flow stress is dependent on the strain rate: an increase in the strain rate leads to a decrease in the plastic-flow stress, and vice versa. However, simply loading, unloading, and reloading at a constant strain rate do not change the plastic-flow stress. This strain-rate dependence of the plastic-flow stress may be related to shear-banding operations.
A bulk nanocrystalline (nc) Ni–Fe alloy was subjected to tensile deformation, which leads to grain growth. The nanoindentation study indicates that the hardness, H, and Young’s modulus, E, of the nc alloy before and after tensile deformation did not show a clear indentation-rate effect. However, the tensile deformation results in a decrease in the E values of about 15%, which might be attributed to the grain rotation, leading to texture development during the stress-induced grain growth.
The stress-life fatigue behavior and fracture morphology of a (Cu60Zr30Ti10)99Sn1 bulk-metallic glass alloy was investigated under both three-point and four-point bending conditions. For all stress levels tested, the fatigue lifetimes tended to be higher for the three-point loading condition. The fatigue endurance limits (defined as 107 cycles without failure), based on the applied stress range, for three-point and four-point loading conditions were approximately 475 MPa and 350 MPa, respectively. All fracture surfaces were found to be composed of four main regions: a crack-initiation site, a stable crack-growth region, an unstable fast-fracture region, and a melting region. Finely spaced parallel marks, similar to fatigue striations found in crystalline alloys, oriented somewhat perpendicular to the direction of crack propagation were observed in the stable crack-growth region. Analyses of these marks found that their spacing increased with increasing stress-intensity-factor range. Damage was found to initiate from preexisting defects present on or near the surface.
Using an infrared camera, the plastic deformation of a relaxed Zr52.5Cu17.9Ni14.6Al10.0Ti5.0 bulk-metallic glass in a moderately high strain rate compression was observed in situ. The specimen exhibits an inhomogeneous deformation, which is manifested by serrated plastic flow, shear banding, and obvious work softening. Shear-banding operations were observed throughout the plastic deformation. Shear-banding operations started before the nominal yielding; shear bands could not block each other, but their interaction seems to accelerate the plastic deformation. A significant increase in the specimen’s temperature was observed due to shear banding.
Zr50Cu40Al10, Zr50Cu30Al10Ni10, and Zr50Cu37Al10Pd3 (in at.%) are bulk metallic glasses (BMGs) with partial crystallization that were characterized by x-ray diffraction (XRD). The study of mechanical properties was conducted in compression at room temperature. Four-point-bend fatigue experiments were performed on the zirconium (Zr)-based BMGs in air. Under compressive loading, after the elastic deformation, no obvious plasticity occurred before the final shear fracture. The compression strengths are comparable to those of fully amorphous alloys. However, the fatigue-endurance limits of these BMGs were much lower than those of fully amorphous alloys. These results suggested that the fatigue behavior of a BMG is very sensitive to the microstructure, while the compression strength is not.
The mechanical behavior of Zr-Cu-Al bulk metallic glasses (BMGs) and in situ Ta-particle-containing composites (BMGCs) was investigated at 77 K. Their strengths increased significantly whereas the plastic strains remained at comparable levels, when compared to that at 298 K. The interaction between shear bands and particles shows that shear extension in particles has limited penetration, and shear bands build up around particles. Pair distribution functions (PDFs), which carried out at cryogenic and ambient temperatures on the as-cast Zr-Cu-Al bulk metallic glasses, were studied, and simulations with reverse Monte Carlo (RMC) were performed by combining icosahedral and cubic structures as the initial structures. Based on the studies of the pair distribution functions and the Reverse Monte Carlo simulations, the concept of free volume was defined—spaces between clusters with longer bond lengths of atom pairs; the structural model of BMGs was proposed—the strongly bonded clusters correlated with each other and separated by free volume. An attempt has been made to connect the relationship between amorphous structures and their mechanical properties.
Four-point-bend fatigue experiments were conducted on the Fe48Cr15Mo14Er2C15B6 bulk metallic glass (BMG), amorphous steel, under load control, employing an electrohydraulic machine, at a frequency of 10 Hz (using a sinusoidal waveform) with an R ratio of 0.1, where R = σmin./σmax. (σmin. and σmax. are the applied minimum and maximum stresses, respectively). The test environment was laboratory air. Fe48Cr15Mo14Er2C15B6 exhibited a high fatigue-endurance limit (682 MPa), which is found to be greater than those of the Zr-based BMG, Al-alloy, and high-nitrogen steel. However, the stress versus number of fatigue cycles curve of Fe48Cr15Mo14Er2C15B6 has a significantly brittle fracture mode. Some fatigue cracks initiated from the inclusions or porosities, and the fatigue-crack propagation region was large. However, other cracks initiated from the outer tensile surface of the specimen, and the fatigue-crack propagation region was very small. The mechanisms of fatigue-crack initiation are suggested.
Using an infrared camera, we observed in situ dynamic shear-banding operations during compression of a bulk metallic glass at various strain rates. We demonstrated that the shear-banding events are highly dependent on strain rates, either intermittent at the lower strain rate or successive at the higher strain rate. Serrated plastic-flow behaviors are a result of shear-banding operations. These observations provide a new insight into inhomogeneous deformation of metallic glasses.
Using geometrically constrained specimens, the plastic flow behaviors of the as-cast and the relaxed Zr52.5Cu17.9Ni14.6Al10.0Ti5.0 bulk metallic glass in the dynamic compression were investigated. Both alloys exhibit a significant plasticity in the dynamic compression. The plastic deformation in both alloys is still inhomogeneous, which is characterized by the serrated plastic flow and the formation of shear bands. Free volumes affect the shear banding and the plastic flow. The reduced free volume results in the deviation of the shear banding direction from the maximum shear stress. The relaxed alloy exhibits the obvious stress overshoot, which is consistent with the theoretical prediction using a free volume model.
In this study, we demonstrated that the failure of bulk metallic glasses (BMGs) results from a sudden temperature rise within a shear band. Using a shear transformation zone model, we successfully calculated the temperature within a shear band and found it consistent with the observation from an in situ infrared thermographic system. The instantaneous temperature within a shear band at fracture agrees remarkably well with the glass transition temperature (Tg providing a new criterion to determine the strength of BMGs from their Tg. This agreement also discloses the fact that catastrophic failure of BMG is caused by the sudden drop in viscosity inside the shear band when the instantaneous temperature within a shear band approaches Tg.
Combinatorial experiments provide a means of generating large amounts of experimental data; however that does not necessarily lead to high throughput interpretation of that data. In this paper we provide a brief summary of how one can use informatics techniques to accelerate data interpretation from high throughput experiments. We provide examples from high throughput nanoindentation and diffraction experiments.
A two-step forced-flow, thermal-gradient, chemical vapor infiltration process (FCVI) was proposed to reduced processing time while maintaining uniformly high densities. GTCVI, a finite-volume computer code developed specifically for the FCVI process was used to model thermal gradient effects on processing time and density. An optimum thermal gradient was determined and used to process material with uniformly infiltrated bundles.