Layered Ti-Al metal composite (LMC) was designed and fabricated by hot-rolling and annealing of pure Ti and Al sheets. The as-prepared composite exhibits high tensile ductility, being superior to any ...individual Ti or Al sheets. The stress/strain evolution and fracture behavior of the LMC were analyzed by in-situ observations during the tensile deformation. Three deformation stages of LMC were clearly observed by neutron diffraction: elastic stage, elastic-plastic stage and plastic stage. It is found that stress partitioning at the elastic-plastic deformation stage improves the strain balance of LMC, but leads to an internal stress accumulated at the interface. Additionally, a strain-transfer from Ti to adjacent Al layers relieves the strain localization of Ti layers in LMC, which improves the ductility of Ti. Both stress partitioning and strain localization of Ti layers facilitate the nucleation of cracks at a low macro strain. However, the crack propagation is constrained by layered structure. In terms of the Al layers, the constrained micro-cracks relieve the stress concentration in Al layer and improve the ductility of Al layers, so that cracking indirectly affects the plastic deformation behavior of LMC, then improving its entire ductility. This work provides a new structural strategy towards simultaneously improving strength and ductility to develop high performance LMC by structural design.
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Bulk nanostructured (ns)/ultrafine-grained (UFG) metallic materials possess very high strength, making them attractive for high strength, lightweight and energy efficient applications. The most ...effective approach to produce bulk ns/UFG metallic materials is severe plastic deformation (SPD). In the last 30 years, significant research efforts have been made to explore SPD processing of materials, SPD-induced microstructural evolutions, and the resulting mechanical properties. There have been a few comprehensive reviews focusing mainly on SPD processing and the mechanical properties of the resulting materials. Yet no such a review on SPD-induced microstructural evolutions is available. This paper aims to provide a comprehensive review on important microstructural evolutions and major microstructural features induced by SPD processing in single-phase metallic materials with face-centered cubic structures, body-centered cubic structures, and hexagonal close-packed structures, as well as in multi-phase alloys. The corresponding deformation mechanisms and structural evolutions during SPD processing are discussed, including dislocation slip, deformation twinning, phase transformation, grain refinement, grain growth, and the evolution of dislocation density. A brief review on the mechanical properties of SPD-processed materials is also provided to correlate the structure with mechanical properties of SPD-processed materials, which is important for guiding structural design for optimum mechanical properties of materials.
Despite being the most promising third-generation semiconductor materials, the deformation and removal mechanisms of gallium nitride (GaN) single crystals involved in the ultra-precision machining ...process are not well revealed and few investigations on the grinding of GaN crystals have been reported, which hinders the development of high-efficiency and ultra-precision manufacturing of GaN components. Self-rotating grinding tests of GaN crystals were performed, and the results indicated that abrasive size had a significant influence on the surface morphology and roughness, in comparison with wheel rotational speed and feed speed. As the abrasive size decreased from 18 μm to 1.6 μm, the brittle fracture-dominated surface gradually changed to a full-plastic surface without brittle fractures and cracks. An ultra-smooth surface with a roughness of 1 nm in Sa was acquired using #8000 grinding wheels and a spark-out time of 10 min, which indicated that the machining technology of “grinding instead of polishing” of GaN crystals was achieved in this work. The plastic deformation mechanism of GaN crystals induced by ultra-precision machining was investigated using a cross-sectional TEM method and MD simulation, and both experimental and simulation results indicated that the plastic deformation involved in the scratching process was caused by the formation of polycrystalline nanocrystals, high-angle lattice misorientations, and close-to-atomic-scale defects, including stacking faults, dislocations and serious lattice distortions, along with a small amount of amorphous and phase transitions. There was an obvious delamination phenomenon in the plastic deformation zone. This research enhances the understanding of the deformation and damage mechanisms of GaN crystals involved in the ultra-precision machining process and is of significance for achieving the high-efficiency and high-accuracy manufacturing of GaN components.
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•Full plasticity deformation of GaN is obtained during grinding and scratching.•Ultra-smooth surface with a roughness of 1 nm in Sa is acquired.•Plastic damage mechanism induced by ultra-precision machining is revealed at close-to-atomic scale.•There is obvious delamination at plastic damage zone of scratched subsurface.•High-pressure phase transition from hexagonal to cubic is induced in grinding and scratching.
Precipitate evolution in an AA5024 Al-based alloy containing Sc and Zr is investigated in different states: as-cast, extruded and severely deformed. An overview of the microstructure is given and the ...evolution of particle size and morphology as a function of mechano-thermal processing is investigated, with an emphasis on the Al3(Sc,Zr)-based precipitates. The precipitates are found to maintain their coherence with the matrix after the interaction with dislocations and low-angle grain boundaries as it is revealed by atomistic simulations and by the so-called Geometric Phase Analysis of the local strain fields. It is established that the precipitates can loose their coherency with the matrix during the severe plastic deformation, that decreases generally grain boundary pinning effect and promotes recrystallization.
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Bulk nanostructured (NS) materials processed by severe plastic deformation (SPD) have received considerable attention for several decades. The physical origin of this processing philosophy is to ...enable substantial grain refinement from a micrometer to a nanoscale level mainly through the activation of fundamental deformation mechanisms: dislocation glide, deformation twinning, and their sophisticated interactions. The formation of nanostructures in NS metallic materials is significantly governed by the quintessential dominance of these two plasticity carriers during SPD, and their mechanical properties are thereby correspondingly affected. According to conventional crystal plasticity, the stacking fault energy (SFE) of materials is one of the most crucial factors primarily controlling which deformation mechanism plays an overwhelming role in accommodating the plasticity. Therefore, a profound understanding of the vital significance of SFE in NS materials can extend and enrich our comprehension of their structure-property relationship, lead to the design of NS metallic materials with superior properties, and pave the path for their perspective applications. Choosing Cu and its binary alloys as model systems, this review extensively surveys the principal influences of SFE on the preferred choice of deformation mechanisms during SPD, microstructural evolution, grain refinement, deformation behavior, and mechanical properties of NS material including tensile properties and cyclic deformation responses.
Twenty-five years ago, in 1988, there appeared a classic description of the application of severe plastic deformation (SPD) to bulk solids in order to achieve exceptional grain refinement to the ...submicrometer level. This report and later publications initiated considerable interest in materials science laboratories around the world and many experiments were subsequently performed to evaluate the principles and practice of SPD processing. The present report provides an overview of the more recent developments in this field, with special emphasis on the opportunities for achieving homogeneity in the as-processed materials and on the general characteristics of the mechanical properties achieved after SPD processing. For simplicity, special emphasis is placed on the two techniques of equal-channel angular pressing and high-pressure torsion as these are currently the most popular procedures for applying SPD processing.
High strain rate superplasticity by the friction stir processing (FSP), an adaptation of the friction stir welding (FSW), is summarized in this overview article. As a common severe plastic ...deformation (SPD) processing technique, the microstructures prepared by FSP are characterized by fine grain sizes, being homogeneous with fragmented and dispersed particles, and having a high proportion of high-angle grain boundaries. These attributes are beneficial to the superplastic forming operations at high strain rates and low temperatures. In this monograph, the principles of superplasticity are reviewed, where the importance of grain boundary sliding (GBS), strain rate sensitivity index, grain refinement, and deformation temperature on the remarkable enhancement of ductility is emphasized. Afterwards, FSP is introduced and the effects of the main processing parameters on the heat input and grain size are critically discussed. Finally, the recent progress in the application of FSP for processing of superplastic materials is thoroughly overviewed and the influence of thermal stability against grain growth, addition of alloying elements to form pinning particles, external cooling for obtaining ultrafine grained (UFG) microstructure, FSP process variables such as tool rotation rate and traverse speed, and multi-pass FSP is summarized. Accordingly, this overview presents the opportunities that FSP can offer for controlling the superplastic behavior of materials.
Abstract
The article briefly reviews analytical works on methods for calculating the parameters of contact interaction of rough surfaces. The influence of information technology development on ...scientific research and engineering problem solving is described. The contact of simulated surfaces is described. A number of assumptions are accepted and the principle of distribution of the protrusions of the rough surface is established. Further, the processes occurring during the contact interaction of rough surfaces in friction are described. Calculation relations are derived to determine the number of contact interactions between the protrusions of the surfaces under consideration. The volume of deformed material in the case of perfectly plastic deformation of its surface by absolutely rigid irregularities of the second surface is determined. The results of the work can be used for building calculation models of tribocouple wear and for the development of computer programs for modeling the processes of contact interaction of rough surfaces in friction.
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