Papers by Keyword: Compressive Deformation

Abstract: The Harmonic Structure (HS) is a recently introduced concept that paves the way for engineering metallic materials to achieve superior mechanical performance. They consist of soft, coarse-grained regions surrounded in three dimensions by an interconnected network of hard, ultra-fine grained regions. In addition, from a structural materials point of view, high entropy alloys have attracted attention due to their unique mechanical properties. In the present study, the HS design was applied to a high entropy CrMnFeCoNi alloy (also called "Cantor alloy"). The HS-designed Cantor alloy was successfully fabricated by mechanical milling, which is one of the surface severe plastic deformation processes, and the subsequent sintering process. The mechanical properties of these HS and homogeneous (Homo) Cantor alloy compacts were investigated by high-temperature compression tests in the temperature range of room temperature (RT) and 1173K, under initial strain rates of 0.01 s^-1, 0.001 s^-1, and 0.0001 s^-1. The stress-strain curves of the HS compacts showed a large initial increase in stress and then a rapid decrease with strain, while that of the Homo compact showed a gentle increase and a gradual decrease. EBSD observation of the deformed compacts revealed that the HS compacts were probably deformed not only by dynamic recrystallization, but also by grain boundary sliding during deformation. The strain rate sensitivity value m of the HS compacts was 0.541 (true strain: 0.2) at 1173 K. In other words, the HS compacts exhibited pseudo-superplastic deformation at these temperatures.

Abstract: Harmonic Structure (HS) materials, a class of heterogeneously structured materials, are known to exhibit unique and superior mechanical properties. The HS consists of soft, coarse-grained regions (Core) that are three-dimensionally surrounded by an interconnected network of hard, ultrafine grained (UFG) regions (Shell). The unique UFG network structure of the Harmonic Structure increases the dislocation density of the core regions in contact with the Shell, resulting in increased strength and work hardening rate in the Core regions. These contribute to the high strength of the HS materials and suppress the plastic instability of the Shell regions, resulting in higher ductility of the HS materials. In the present research, the HS design is applied to a high-entropy CrMnFeCoNi alloy, also known as the Cantor alloy, to study the microstructure change during high temperature deformation at 1073 K and 1173 K. Although the alloy exhibits high strength and high ductility at cryogenic temperature due to the twinning deformation, the high temperature properties are not clear, especially in the case of the HS design. As a result, the alloy with or without HS design did not show twinning deformation at these temperatures, and it is noteworthy that the alloy with HS showed preferential recrystallization in the UFG network region, and thus the recrystallized UFGs played an important role in grain boundary sliding to demonstrate the pseudo-superplastic deformation behavior.

Abstract: Functionally graded structures from natural rubber foams were constructed and evaluated in this study. The structures were assembled based on density gradient achieved by stacking layers of natural rubber foams with different densities. Both physical and computational structures were constructed and assembled through experimental work and computer-aided design (CAD) modelling, respectively. Finite Element Analysis (FEA) simulation were conducted on two different assembled structures under compressive deformation mode using Ogden foam hyper-elastic model available in the simulation software package. Experimental result found that the middle section foam with 4/16/4 stacking sequence was deformed significantly compared to that of foam with 16/4/16 stacking sequence. The FEA simulation results indicated that Ogden foam hyper-elastic model is useful in representing deformation at low strain region as the results were in good agreement with those of obtained from the experimental foam compression tests especially for foam with 16/4/16 stacking. The study was able to show that FEA analysis is a good approach to supplement limitation that are encountered by experimental approach in understanding deformation behaviour of functionally graded natural rubber foams.

Abstract: The effect of layer thickness on hardness and buckling behavior was investigated on Ni-Co-Cu/Cu multilayered films. The Ni-Co-Cu/Cu multilayered films were grown on annealed copper substrates by electrodeposition. We fabricated the multilayered films with various layer thicknesses ranging from 10 nm to 1000 nm. First, dependence of Vickers hardness on the Cu layer thickness was investigated. When the Ni-Co-Cu layer had the constant thickness of 75 nm and the Cu layer thickness was smaller than 75 nm, the hardness increased rapidly with decreasing Cu layer thickness. Subsequently, compressive tests were conducted on the multilayered films having the component layers ranging from100 nm to 1000 nm, where the hardness values did not change rapidly with layer thickness. The copper substrates coated with the multilayered films were compressed until 20% strain. From SEM surface observations after the compressive tests, formations of band-like structures having a certain thickness were recognized. Cross-sectional observation revealed that some band-like structures were formed as a result of local buckling of the multilayered film. The vertical thickness of the bank-like structures increased linearly with increasing component layer thickness.

Abstract: Discrete Element Method (DEM) computer simulation is used to examine the influence of contact force between two-dimensional aggregates of polystyrene microsphere formed on the air-liquid interface. Colloidal aggregates have been treated as the granular material or discontinuum materials. The interaction force models are related to experiment which had done by digital video microscopy. The interaction mechanisms of the contact forces between particles in the colloidal system can be considered as a combination of spring and dashpot force and van der Waals force. According to the DEM, the interaction forces are evaluated to introduce relations between particles and the result comparison between the computer simulation and the experimental work. This study indicates that the behavior of the colloidal aggregates depends on the long-ranged (spring and dashpot) and the short-ranged interaction force (van der Waals). Besides, the behaviors shown in both computer simulation and the experiment are in good agreement. Thus, this computer simulation method can mimic the behavior of colloidal aggregates forming as a monolayer at the air-liquid interface.

Abstract: Coarse-grained soil is widely used in railway construction, and it is of great significance to take research on how compression deformation characteristics affect deformation of coarse-grained soil in high filled subgrade. To analyze compression deformation characteristics of coarse-grained soil under different moisture content and different grain compositions conditions, influence pattern of moisture content and grain compositions was researched through uniaxial compression test. The result indicates that compressive deformation is in logarithm relation with time; moisture content and grain compositions are important factors that affect the characteristics of compressive deformation of sandy slate coarse-grained soil and it is better to control subgrade settlement with dry or saturated coarse-grained soil which contains 70% coarse particles.

Abstract: The deformation micromechanism of Zr50Cu50 bulk metallic glasses under compress loading is studied by means of molecular dynamics. The modified self part of the Van Hove function is used to evaluate the transport properties of clusters in deformation behaviors. It is found that the instable clusters are restricted by the ‘backbone’, a kind of three dimensional network structure formed by close-pack clusters. Local shear transformation zone (STZ) is initially formed in these instable clusters, merge with each other and generate eventually a whole STZ, which penetrates the glasses matrix when the backbone is destroyed with increasing loading.

Abstract: The temperature-dependent deformation and damage behaviors of ultrafine-grained (UFG) Cu and Ti produced by equal channel angular pressing (ECAP) were investigated and compared. It was found that ECAPed materials with different crystalline structures, e.g. the present fcc Cu and hcp Ti, exhibited significantly distinctive high-temperature deformation and damage characteristics. As the testing temperature is below recrystallization, small- and large-scale cracks or voids formed along the shear bands (SBs) on the surface of UFG Cu, whereas only a few fine shear lines and some non-propagation voids appeared on the surface of UFG Ti. As the temperature is above recrystallization, some small cracks (or voids) formed along grain boundaries and slip deformation took place in many coarsened grains, while only extrusions and intrusions instead of obvious cracks or voids are observable for UFG Ti. The corresponding microstructual changes after compressive deformation, e.g. grain coarsening, were also examined and confirmed by TEM observations.

Abstract: The individual or joint effects of annealing and equal channel angular pressing (ECAP) treatments on the high-temperature compressive deformation and damage behavior of cast LY12 Al alloys were studied. The compressive deformation behavior and surface deformation and damage characteristics of differently treated LY12 Al alloy samples were examined at temperatures ranging from 25°C to 400°C. It is found that the LY12 Al alloy exhibits different compressive mechanical behavior depending on different treatments, and the compressive deformation and damage characteristics of LY12 Al alloys with different microstructural states are closely related with the testing temperature. As compared to other samples (e.g. as-cast, annealed, as-cast+2ECAPed), the sample annealed at 450°C for 3h followed by ECAPed for 2 passages (i.e. annealed+2ECAPed) shows a better high-temperature deformation stability.

Abstract: Fracture behavior and the microstructure of Ti3AlC2 ceramics prepared by SHS/PHIP method were studied. Stress-strain curves at different temperature with a strain rate of 1×10-3 s-1 were obtained. Fracture toughness, flexural strength, crack propagation behavior and compressive deformation of specimens were investigated. Results show that the microstructure of the large size Ti3AlC2 ceramics prepared by SHS/PHIP method has typical layered feature of ternary carbide compound. The bridge-link phenomena induced by the flaky grains occurred in three-point bending test. It restrained the crack propagation and improved the fracture toughness of the material. Cylindrical specimens under axial compression usually smashed into chips for most of the ceramics materials, however, for the Ti3AlC2 ceramics prepared by SHS/PHIP, shear fracture along 45º incline of the specimen occurred at room temperature, and bulging deformation without any crack exhibited when temperature was high. It is concluded that the Ti3AlC2 ceramics prepared by SHS/PHIP has better fracture-resistance properties.