New strengthening technology for comprehensive str

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Traditional materials comprehensive strength and toughness new strengthening technology

Science published a specially invited review paper, which elaborated on the research results of using nano scale coherent interface strengthening materials, and how to improve the strength of materials without losing their plasticity, which is a major challenge faced by many material scientists. Science published a specially invited review paper, detailing the research results of using nano scale coherent interface strengthening materials, and how to improve the strength of materials without losing their plasticity, which is a major challenge faced by many materials scientists. In order to obtain good comprehensive strength and toughness properties after strengthening, the strengthened interface should have three key structural characteristics: (1) there is a crystallographic coherent relationship between the interface and the matrix; (2) The interface has good thermal stability and mechanical stability; (3) The characteristic size of the interface is in the order of nanometer (100nm). Furthermore, luco and other researchers proposed a new material strengthening principle and approach, using nano scale coherent interface to strengthen materials

improving the strength of materials has been the core issue of material research for centuries. So far, the ways of strengthening materials can be divided into four categories: solid solution strengthening, second phase dispersion strengthening, processing (or strain) strengthening, and crystal controller and software particle refinement strengthening. The essence of these strengthening techniques is to improve the strength of materials by introducing various defects (point defects, line, plane and volume defects, etc.) to hinder the dislocation movement, making it difficult for materials to produce plastic deformation. However, the strengthening of materials is often accompanied by a sharp decline in plasticity or toughness, resulting in the lack of plasticity and toughness of high-strength materials, while the strength of high-plastic ductile materials is often very low. For a long time, the inverted relationship between strength and toughness of this material has become a major scientific problem in the field of materials and an important bottleneck restricting the development of materials

traditional material strengthening techniques use ordinary non coherent grain boundaries or phase boundaries to block dislocation movement to improve strength. When a large number of incoherent grain boundaries are introduced into the material, the strength is significantly improved (for example, the strength of nanocrystalline materials is one order of magnitude higher than that of coarse crystalline materials). However, with the increasing number of obstacles to dislocation movement (i.e. incoherent grain boundaries), the lattice dislocation movement is seriously hindered or even completely suppressed, and the plastic deformation cannot be coordinated, so the material becomes brittle

coherent grain boundary or phase boundary is a special and common low-energy interface. The structural feature is that the atoms on the interface are located at the nodes of the lattices on both sides of the interface at the same time, that is, the lattice lattice lattice on both sides of the interface are connected with each other, and the atoms on the interface are shared by both. Some coherent grain boundaries (such as small angle tilted grain boundaries) have weak resistance to dislocation movement, so they can not effectively strengthen the material; While other coherent or semi coherent grain boundaries can effectively hinder dislocation movement and have strengthening effect, such as precipitation strengthening, GP zone in Al Cu alloy is equal to or precipitate in Ni based alloy. However, the stability of the coherent interface in these precipitates is low, and the coherent relationship disappears when the precipitates grow up; Twin boundary is a special coherent grain boundary, and the lattice on both sides of it is mirror symmetric. Although the research shows that the blocking effect of a single twin boundary on dislocation in some annealed alloys is equivalent to that of ordinary grain boundaries, due to the small number of twin boundaries, the overall strengthening effect is much weaker than other strengthening mechanisms (such as solution strengthening and fine grain strengthening). Therefore, for a long time, the coherent interface has not been used as an interface that can effectively strengthen materials

however, the unique structure of the coherent interface makes it have some special mechanical behaviors. Some coherent interfaces (such as twin grain boundaries) can not only hinder the movement of dislocations, but also act as the slip surface of dislocations to absorb and store dislocations in the process of deformation, thus contributing to improving the toughness and plasticity of materials. If we can effectively improve the stability of the coherent interface and increase the density of the coherent interface, we can use the coherent interface to improve the strength of the material and improve its toughness and plasticity at the same time

luco et al. Found that the nano scale twin interface has the three basic structural characteristics of the above strengthened interface. Using pulse electrodeposition technology, they successfully prepared twin structures with high-density nano scale (twin lamella thickness of 100nm) in pure copper samples. It is found that the strength and tensile plasticity of the samples increase synchronously with the decrease of twin lamella thickness. When the thickness of the laminate is 15nm, the tensile yield strength is close to 1.0gp. In the formula: S1 - scanning time (s or min) a (more than ten times that of ordinary coarse-grained Cu), the tensile uniform elongation can reach 13%. Obviously, this kind of nano twin strengthening, which improves the strength and plasticity synchronously, is quite different from other traditional strengthening technologies. Theoretical analysis and molecular dynamics simulation show that the ultra-high strength and high plasticity of high-density twin materials are due to the unique interaction between nano scale twin boundaries and dislocations. For example, when an edge dislocation meets a twin grain boundary, the reaction between the dislocation and the twin boundary can generate a new edge dislocation to slip in the twin lamella. At the same time, the electronic tensile testing machine at the twin boundary is the main equipment for material detection now. Because it has the advantages of easy operation and high precision, it generates a new incomplete dislocation, which can slip on the twin boundary. When the twin lamellae are in the nano scale, dislocation interacts with a large number of twins to continuously improve the strength. At the same time, a large number of movable incomplete dislocations are produced at the twin boundary, and their sliding and storage bring high plasticity and high processing strengthening to the samples. It can be seen that the meter scale twins with small strain deformation can strengthen metal materials and improve toughness and plasticity at the same time

nanoscale twin boundaries in materials can be obtained through a variety of preparation technologies, such as electrolytic deposition, magnetron sputtering deposition, plastic deformation or annealing recrystallization, which can produce nanoscale twins in metals. The results show that the faster the deposition rate is, the thinner the twin lamella is formed. For example, in pulse electrodeposition, when the deposition rate exceeds 4nm/s, the average twin lamella in Cu samples is less than 20nm. Twinning induced by plastic deformation is very common in medium and low layer fault energy materials (such as Cu, Cu alloy and stainless steel). Increasing the strain rate or reducing the deformation temperature are conducive to the formation of twinning. The dynamic plastic deformation (DPD) technology recently developed by luco and others can form a large number of nano scale twin boundaries in materials, which has become an effective way to prepare massive nano twin structures. Using nano scale coherent grain boundary strengthening materials can also bring excellent electrical properties. The research shows that the ultra-high strength nano twin Cu sample has the same high conductivity as oxygen free high-purity copper, and can achieve high strength and high conductivity at the same time. The nano twin structure can effectively reduce the diffusion mobility of the electrically induced atoms in Cu, thus greatly reducing the electromigration effect, which finds a new solution to reduce the electromigration damage of copper wires in microelectronic devices. Some scholars also found that the nano twin structure can effectively improve the damping performance of materials, opening up a new way for the development of high-performance damping materials

using nano scale coherent interface strengthening materials has become a new way to improve the comprehensive properties of materials. Although there are still many challenges in the preparation technology of nano scale coherent interface, growth control, and the exploration of various physical and chemical properties, mechanical properties and causative behavior, this new strengthening approach shows great development potential and broad application prospects in improving the comprehensive properties of engineering materials

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