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Effects of Constituent Elements and Fabrication Methods on Mechanical Behavior of High-Entropy Alloys: A Review
High-entropy alloys (HEAs) have become a research hotspot in recent years. The nature of the multi-principal elements, high mixing entropy, and mutual interactions between elements render this novel material outstanding mechanical and functional properties, in which most research efforts are focused on mechanical properties. There are many aspects that can influence the mechanical behavior, such as constituent elements and fabrication methods. This paper will mainly summarize and discuss the effects of constituent elements and fabrication techniques on the mechanical properties of HEAs, by reviewing relevant papers, to have a better understanding of the variation ranges resulting from the above two factors and the reasons for the properties changes. Future directions are provided at the end of this article.
Journal Article
Bulk Nanostructured Materials
This paper will address three topics of importance to bulk nanostructured materials. Bulk nanostructured materials are defined as bulk solids with nanoscale or partly nanoscale microstructures. This category of nanostructured materials has historical roots going back many decades but has relatively recent focus due to new discoveries of unique properties of some nanoscale materials. Bulk nanostructured materials are prepared by a variety of severe plastic deformation methods, and these will be reviewed. Powder processing to prepare bulk nanostructured materials requires that the powders be consolidated by typical combinations of pressure and temperature, the latter leading to coarsening of the microstructure. The thermal stability of nanostructured materials will also be discussed. An example of bringing nanostructured materials to applications as structural materials will be described in terms of the cryomilling of powders and their consolidation.
Journal Article
Bauschinger Effect and Back Stress in Gradient Cu-Ge Alloy
Using surface mechanical attrition treatment (SMAT), a gradient structure composed of two gradient structure (GS) layers and a coarse grain (CG) layer was generated from a Cu-5.7 wt pct Ge alloy, significantly improving the yield strength of the sample. Unloading–reloading tests showed an unusual Bauschinger effect in these GS samples. The back stresses caused by the accumulated geometrically necessary dislocations (GNDs) on the GS/CG border increased with increasing strain. As found by electron backscatter diffraction (EBSD), the GNDs are mainly distributed in the gradient structured layer, and the density of the GNDs increase with increasing SMAT time. The effect of the back stress increased with increasing SMAT processing time due to the increase in the strain gradient. The pronounced Bauschinger effect in a GS sample can improve the resistance to forward plastic flow and finally contributes to the high strength of GS samples.
Journal Article
The Evolution of Strain Gradient and Anisotropy in Gradient-Structured Metal
Gradient-structured metals have been reported to possess superior mechanical properties, which were attributed to their mechanical heterogeneity. Here we report
in-situ
observation of the evolution of strain gradient and anisotropy during tensile testing of a gradient-structured metal. Strain gradients and anisotropy in the lateral directions were observed to increase with increasing applied tensile strain. In addition, the equivalent Poisson’s ratio showed gradient, which evolved with applied strain. The gradient structure produced higher deformation anisotropy than coarse-grained homogeneous structure, and the anisotropy increased with increasing tensile strain. The strain gradient and anisotropy resulted in strong back-stress hardening, large strain gradients, and a high density of geometrically necessary dislocations, which helped with increasing the ductility.
Journal Article
In Situ Studies on the Irradiation-Induced Twin Boundary-Defect Interactions in Cu
Polycrystalline Cu films with nanoscale annealing twins are subjected to
in situ
Kr
++
ion irradiation at room temperature inside a transmission electron microscope up to a dose of 1 displacement-per-atom. Radiation induces prominent migration of incoherent twin boundaries. Depending on twin thickness, three types of twin boundary evolutions are observed, including rapid detwinning, gradual detwinning, and self-healing. The mechanism of twin thickness-dependent evolution of microstructures is discussed. This study provides further evidence on twin boundary-defect interactions and may assist the design of radiation-tolerant twinned metallic materials.
Journal Article
Mechanical Properties of Gradient Structure Mg Alloy
In this work, a surface mechanical attrition treatment (SMAT) process was applied to AZ31B magnesium alloy at room temperature. This method produced a gradient structure on the treated AZ31B, in which the grains of the topmost layer are refined to nanoscale sizes. A combination of nanocrystallites at the surface and coarse-grains in the center are the main features of this structure. This structure results in an excellent combination of both strength and ductility. The highest yield strength for the 30 minutes SMAT AZ31B samples increased to 249 ± 5 MPa and the uniform elongation decreased to 9.3 ± 0.8 pct, whereas the original yield strength was only 147 ± 4 MPa and the uniform elongation was 15.4 ± 1.1 pct. Microstructural observations, stress relaxation tests, and hardness tests were used to verify the results. Additionally, there is a specific volume fraction of gradient structure to achieve the best mechanical performance, which is shown to be in the range of 9.3 to 14 pct for the AZ31B alloy.
Journal Article
Discrete Element Simulation of the Macro-Meso Mechanical Behaviors of Gas-Hydrate-Bearing Sediments under Dynamic Loading
Under the action of dynamic loadings such as earthquakes and volcanic activities, the mechanical properties of gas-hydrate-bearing sediments will deteriorate, leading to a decrease in the stability of hydrate reservoirs and even inducing geological disasters such as submarine landslides. In order to study the effect of dynamic loading on the mechanical properties of hydrate sediments, triaxial compression tests of numerical specimens were carried out by using particle flow code (PFC2D), and the macro-meso mechanical behaviors of specimens were investigated. The results show that the loading frequency has a small effect on the stiffness of the hydrate sediment, while it has a large effect on the peak strength. The peak strength increases and then decreases with the increase in loading frequency. Under the same loading frequency, the peak strength of the hydrate sediment increases with the increase in loading amplitude, and the stiffness of the specimen decreases with the increase in loading amplitude. The maximum shear expansion of the specimen changes with the movement of the phase change point and the rearrangement of the particles. The maximum shear expansion of the specimen changes with the movement of the phase change point and the change of the bearing capacity of the particles after the rearrangement, and the more forward the phase change point is, the stronger the bearing capacity of the specimen in the plastic stage. The shear dilatancy angle and the shear dilatancy amount both increase linearly with the increase in loading amplitude. The influence of loading frequency and amplitude on the contact force chain, displacement, crack expansion, and the number of cementation damage inside the sediment is mainly related to the average axial stress to which the specimen is subjected, and the number of cracks and cementation damage of the sediment specimen increases with the increase in the average axial stress to which the sediment specimen is subjected. As the rate of cementation damage increases, the distribution of shear zones becomes more obvious.
Journal Article
New Type of Composite Girder Design and Research on Key Mechanical Behavior of JiaoJiang Second Bridge
JiaoJiang second bridge is a 5-span continuous cable-stayed bridge with double pylons and double cable planes whose main span is 480m. The main girder’s cross section has used a new type of composite beams --Semi-enclosed steel box composite girder, this article describes the design concept ofto this new type of composite girder, researched and analyzed the overall mechanical behavior of the composite girder, the spatial bearing behavior of the composite girder’s connecting piece, the segmental assembling technology of the composite girder and the mechanical behavior of its connecting piece by means of experiment and finite element calculation, then looks forward to the popularization and application prospect of this new type of composite girder.
Journal Article
The Effect of Joint Dip Angle on the Mechanical Behavior of Infilled Jointed Rock Masses under Uniaxial and Biaxial Compressions
Due to the complex formation process of a rock mass, a large number of fissures, joints, faults, other defects exist and the defects commonly contain infilled materials. The jointed rock masses are in a complex geological environment, in which the geometric distribution and the boundary condition can greatly affect the mechanical behavior of the infilled jointed rock mass. In this study, the infilled jointed rock mass specimens with different dip angles are prepared using similar materials, and the uniaxial and biaxial compression tests on the specimens are conducted. The effect of the joint dip angle on the mechanical behavior of the infilled jointed rock mass under uniaxial and biaxial compressions is investigated. The results show that the uniaxial compressive strength shows a W-shaped variation, and the biaxial compressive strength shows a V-shaped variation with an increase in the dip angle. Most of the cracks appear in pairs around the joint and occur symmetrically in a bilateral distribution, and the existence of the infilled joints induces a nonlinear mechanical behavior in the specimen. In addition, the specimens exhibit three failure modes under uniaxial compression: splitting failure, step-path failure and planar failure. The specimens present two failure modes under biaxial compression: splitting failure and planar failure.
Journal Article