China has the most abundant magnesium resources in the world. Magnesium and its alloys have the advantages of low density, high specific strength, good damping property, and exceptional electromagnetic-shielding and energy-storage characteristics. They are one of the most promising lightweight materials. The enhanced applications of magnesium alloys can save energy and reduce emissions and are significant to the new Chinese energy strategy. However, magnesium alloys have a hexagonal close-packed structure and exhibit relatively low ductility. A bottleneck in expanding the application of magnesium alloys is improving the ductility of magnesium alloys. For more than ten years, efforts have been made to improve the ductility and plastic deformation ability. Progress has been made in plastic-processing technologies of magnesium alloys. The novel alloy design theory “solid solution strengthening and ductilizing” and advanced preparation technologies such as “melt self-purification through varying temperature” have been established. Series of new magnesium alloys with good ductility and corresponding alloy grades have been developed, where the impurity content of iron can be reduced to below 10 × 10-6; the elongation was more than 60% for ultrahigh plasticity magnesium alloys and is above 10% for the ultrahigh-strength magnesium alloys (UTS > 550 MPa). New plastic-processing technologies, such as asymmetric extrusion, asymmetric rolling, asymmetric modification, cyclical multipass upsetting and squeezing, expansion control large ratio forging, and extrusion and forging composite forming, have been developed. These newly developed magnesium alloys and processing technologies weaken the basal texture in wrought magnesium alloys, improving the formability of sheets, tubes, profiles, and forgings and their product quality and reducing their product cost. These technologies have been successfully applied in the processing of magnesium sheets, pipe profiles, and forgings.

我国是世界上镁资源最为丰富的国家，镁及镁合金具有质轻、比强度高、阻尼减振、电磁屏蔽性能优良、储能特性好等优点，是最有潜力的轻量化材料之一，其推广应用对节能减排和能源转型战略具有重要意义。但镁合金具有密排六方晶体结构，塑性变形能力较差。如何改善镁合金的塑性变形能力是扩大镁合金应用的瓶颈问题之一。本文综述了10多年来，世界各国在改善镁合金塑性、提升镁合金塑性变形能力等方面所做的大量工作，及在镁合金塑性加工技术等方面取得的重要进展。发展了“固溶强化增塑”新型镁合金设计理论和“熔体变温自纯化”等关键制备技术，形成了一批高塑性变形镁合金材料和牌号，其中杂质Fe含量可降到10 × 10^-6以下，超高塑性镁合金延伸率可达到60%以上，超高强度(抗拉强度大于550 MPa)镁合金延伸率可以达到10%以上；开发了非对称挤压、非对称轧制、非对称改性、往复循环多道次镦挤开坯技术、扩收控制大比率锻造技术、挤锻复合成形技术等一批镁合金新型塑性加工技术。这些合金和技术使变形镁合金基面织构显著弱化，明显提高了变形镁合金板材、管型材和锻件的塑性成形能力和制品质量，产品成本大幅度降低，在板材、管型材和锻件制备加工中实现了成功应用。

Mg and Mg alloys, as important lightweight metal materials, have attracted great attention due to their excellent properties, such as low density, high specific strength, and good damping properties; however, their extensive applications are strictly limited by their low strength. The strength of Mg and Mg alloys can be effectively increased by introducing reinforcement phases into their matrices, i.e., via fabricating Mg-based composites. Nevertheless, the mechanical properties of Mg-based composites demonstrate a strong dependence on their microstructures. Here, new Mg-based composites reinforced by SiC whisker scaffolds with three-dimensional interpenetrating-phase architecture were fabricated through pressureless infiltration of the melt of pure Mg or AZ91D Mg alloy into the porous scaffolds of SiC whiskers. These whiskers were preferentially stacked in-plane within lamellae in the composites using gravity-assisted sedimentation and subsequent densification during the fabrication process. The microstructures and mechanical properties of the composites, particularly their fracture toughness, were characterized and analyzed. The wettability between the SiC whisker scaffolds and the melt was improved by introducing surface reactions between them which was accomplished by a pre-oxidation treatment of the scaffolds before infiltration. The pre-oxidation temperature was adjusted to ensure an adequate filling of the scaffolds without voids while avoiding the formation of excessive reaction products. The resulting composites exhibited a high flexural strength with a certain extent of fracture toughness as evidenced by stable crack propagation with rising R-curve behavior. In comparison to pure Mg, the composites infiltrated with AZ91D Mg alloy as the matrix contained a larger amount of coarsened precipitates, resulting in apparent brittleness despite increased strength.

选用SiC晶须骨架作为增强相,利用重力辅助沉降、压缩致密化、骨架烧结及无压熔渗的方法制备了具有微观三维互穿结构的SiC晶须骨架增强镁基复合材料,并对其微观组织结构与力学性能(特别是断裂韧性)进行了表征与分析。通过对SiC晶须骨架进行预氧化处理,提高了增强相与基体之间的润湿性,并通过调节预氧化温度控制析出相含量和孔洞等缺陷,优化了复合材料的弯曲强度与断裂韧性,获得了稳定的裂纹扩展行为(上升的R曲线)。相比于以纯Mg为基体的复合材料,以AZ91D镁合金为基体的复合材料中析出相含量高且发生粗化,使得材料强度提高的同时断裂韧性降低。

China has been developed into one of the most active regions in terms of both fundamental and applied research on magnesium (Mg) and its alloys in the world from a solid base laid by its prominent metallurgist and materials scientists over the past decades. Nowadays, a large number of young-generation researchers have been inspired by their predecessors and become the key participants in the fields of Mg alloys, which consequently led to the establishment of China Youth Scholar Society for Magnesium Alloys Research in 2015. Since then, the first two China Youth Scholars Symposiums on Mg Alloys Research had been held at Harbin (2015) and Chongqing (2016) China, respectively. A number of crucial research interests related to fundamental and applied Mg research were discussed at the conferences and summarized in this short perspective, aiming to boost far-reaching initiatives for development of new Mg-based materials to satisfy the requirements for a broad range of industrial employments. Herein, four main aspects are included as follows: i) Plastic deformation mechanism and strengthening strategy, ii) Design and development of new Mg-based materials, iii) Key service properties, and iv) New processing technologies.

Advances that have been made in understanding the mechanisms underlying the mechanical behavior of a number of biological materials (namely mollusk shells and sponge spicules) are discussed here. Attempts at biomimicry of the structure of a nacreous layer of a mollusk shell have shown reasonable success. However, they have revealed additional issues that must be addressed if new synthetic composite materials that are based on natural systems are to be constructed. Some of the important advantages and limitations of copying from nature are also described here.

The notion of mimicking natural structures in the synthesis of new structural materials has generated enormous interest but has yielded few practical advances. Natural composites achieve strength and toughness through complex hierarchical designs that are extremely difficult to replicate synthetically. We emulate nature's toughening mechanisms by combining two ordinary compounds, aluminum oxide and polymethyl methacrylate, into ice-templated structures whose toughness can be more than 300 times (in energy terms) that of their constituents. The final product is a bulk hybrid ceramic-based material whose high yield strength and fracture toughness [ approximately 200 megapascals (MPa) and approximately 30 MPa.m(1/2)] represent specific properties comparable to those of aluminum alloys. These model materials can be used to identify the key microstructural features that should guide the synthesis of bio-inspired ceramic-based composites with unique strength and toughness.

Fish scales from modern teleost fish are high-performance materials made of cross-plies of collagen type I fibrils reinforced with hydroxyapatite. Recent studies on this material have demonstrated the remarkable performance of this material in tension and against sharp puncture. Although it is known that teleost fish scales are extremely tough, actual measurements of fracture toughness have so far not been reported because it is simply not possible to propagate a crack in this material using standard fracture testing configurations. Here we present a new fracture test setup where the scale is clamped between two pairs of miniature steel plates. The plates transmit the load uniformly, prevent warping of the scale and ensure a controlled crack propagation. We report a toughness of 15 to 18kJm(-2) (depending on the direction of crack propagation), which confirms teleost fish scales as one of the toughest biological material known. We also tested the individual bony layers, which we found was about four times less tough than the collagen layer because of its higher mineralization. The mechanical response of the scales also depends on the cohesion between fibrils and plies. Delamination tests show that the interface between the collagen fibrils is three orders of magnitude weaker than the scale, which explains the massive delamination and defibrillation observed experimentally. Finally, simple fracture mechanics models showed that process zone toughening is the principal source of toughening for the scales, followed by bridging by delaminated fibrils. These findings can guide the design of cross-ply composites and engineering textiles for high-end applications. This study also hints on the fracture mechanics and performance of collagenous materials with similar microstructures: fish skin, lamellar bone or tendons. Copyright © 2014 Elsevier Ltd. All rights reserved.

Arapaima gigas, a fresh water fish found in the Amazon Basin, resist predation by piranhas through the strength and toughness of their scales, which act as natural dermal armour. Arapaima scales consist of a hard, mineralized outer shell surrounding a more ductile core. This core region is composed of aligned mineralized collagen fibrils arranged in distinct lamellae. Here we show how the Bouligand-type (twisted plywood) arrangement of collagen fibril lamellae has a key role in developing their unique protective properties, by using in situ synchrotron small-angle X-ray scattering during mechanical tensile tests to observe deformation mechanisms in the fibrils. Specifically, the Bouligand-type structure allows the lamellae to reorient in response to the loading environment; remarkably, most lamellae reorient towards the tensile axis and deform in tension through stretching/sliding mechanisms, whereas other lamellae sympathetically rotate away from the tensile axis and compress, thereby enhancing the scale's ductility and toughness to prevent fracture.

The Bouligand structure, which is found in many biological materials, is a hierarchical architecture that features uniaxial fiber layers assembled periodically into a helicoidal pattern. Many studies have highlighted the high damage-resistant performance of natural and biomimetic Bouligand structures. One particular species that utilizes the Bouligand structure to achieve outstanding mechanical performance is the smashing Mantis Shrimp, Odontodactylus Scyllarus (or stomatopod). The mantis shrimp generates high speed, high acceleration blows using its raptorial appendage to defeat highly armored preys. The load-bearing part of this appendage, the dactyl club, contains an interior region [16] that consists of a Bouligand structure. This region is capable of developing a significant amount of nested twisting microcracks without exhibiting catastrophic failure. The development and propagation of these microcracks are a source of energy dissipation and stress relaxation that ultimately contributes to the remarkable damage tolerance properties of the dactyl club. We develop a theoretical model to provide additional insights into the local stress intensity factors at the crack front of twisting cracks formed within the Bouligand structure. Our results reveal that changes in the local fracture mode at the crack front leads to a reduction of the local strain energy release rate, hence, increasing the necessary applied energy release rate to propagate the crack, which is quantified by the local toughening factor. Ancillary 3D simulations of the asymptotic crack front field were carried out using a J-integral to validate the theoretical values of the energy release rate and the local stress intensity factors.Copyright © 2017 Elsevier Ltd. All rights reserved.

Twisted or oscillated plywood structure can be often found in biological composites such as claws of lobsters, bone of mammals, dactyl club of mantis shrimps, and exoskeleton of beetles, which exhibits a combination of high stiffness, high fracture toughness and low density. However, there lacks a quantitative understanding of the relationship between the fracture toughness of the composite and its internal geometry. In this article, we propose that a combination of crack tilting and crack bridging determines the effective fracture toughness of the fiber-reinforced composite with the plywood structure. During the fracturing process, a crack plane initially propagates in the matrix-fiber interface following the twisted fiber alignment. Such crack tilting mechanism can significantly enlarge the area of cracking surface and thus enhance the effective fracture toughness of the composite. With the propagation of the tilted crack plane, the local energy release rate becomes too small to maintain the growth of the tilted crack plane, leading the crack to grow into the matrix, crossing the fibers. Because of the high strength of the fiber, a few fibers can maintain unbroken behind the crack tip, corresponding to crack bridging mechanism. Based on our quantitative analysis, it is found that the effective fracture toughness of the composite can be maximized for a certain pitch angle of the oscillated/twisted plywood structure, which agrees well with experiments. STATEMENTS OF SIGNIFICANCE: Fiber-reinforced composites can be widely found in nature and engineering applications. Recently, it has been discovered that many fiber-reinforced composites in biology have twisted or oscillated plywood structure with high fracture toughness, high mechanical stiffness and low density. Detailed experiments have indicated that an optimal pitch angle may exist for the plywood structure to maximize the fracture toughness of the composites. However, there lacks a quantitative model of revealing such pitch angle-dependent fracture toughness. In this work, we propose a hybrid toughening mechanism and predict the optimal pitch angle in twisted/oscillated plywood structure for maximizing the fracture toughness. Our predictions agree reasonably well with experimental results. As such, the theory may help the design of better fiber-reinforced composites.Copyright © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Through evolutionary processes, biological composites have been optimized to fulfil specific functions. This optimization is exemplified in the mineralized dactyl club of the smashing predator stomatopod (specifically, Odontodactylus scyllarus). This crustacean's club has been designed to withstand the thousands of high-velocity blows that it delivers to its prey. The endocuticle of this multiregional structure is characterized by a helicoidal arrangement of mineralized fiber layers, an architecture which results in impact resistance and energy absorbance. Here, we apply the helicoidal design strategy observed in the stomatopod club to the fabrication of high-performance carbon fiber-epoxy composites. Through experimental and computational methods, a helicoidal architecture is shown to reduce through-thickness damage propagation in a composite panel during an impact event and result in an increase in toughness. These findings have implications in the design of composite parts for aerospace, automotive and armor applications. Copyright © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The microscopic Bouligand-type architectures of fish scales demonstrate a notable efficiency in enhancing the damage tolerance of materials; nevertheless, it is challenging to reproduce in metals. Here bioinspired tungsten-copper composites with different Bouligand-type architectures mimicking fish scales were fabricated by infiltrating a copper melt into woven contextures of tungsten fibers. These composites exhibit a synergetic enhancement in both strength and ductility at room temperature along with an improved resistance to high-temperature oxidization. The strengths were interpreted by adapting the classical laminate theory to incorporate the characteristics of Bouligand-type architectures. In particular, under load the tungsten fibers can reorient adaptively within the copper matrix by their straightening, stretching, interfacial sliding with the matrix, and the cooperative kinking deformation of fiber grids, representing a successful implementation of the optimizing mechanisms of the Bouligand-type architectures to enhance strength and toughness. This study may serve to promote the development of new high-performance tungsten-copper composites for applications, e.g., as electrical contacts or heat sinks, and offer a viable approach for constructing bioinspired architectures in metallic materials.

Bioinspired architectures are effective in enhancing the mechanical properties of materials, yet are difficult to construct in metallic systems. The structure-property relationships of bioinspired metallic composites also remain unclear. Here, Mg-Ti composites were fabricated by pressureless infiltrating pure Mg melt into three-dimensional (3-D) printed Ti-6Al-4V scaffolds. The result was composite materials where the constituents are continuous, mutually interpenetrated in 3-D space and exhibit specific spatial arrangements with bioinspired brick-and-mortar, Bouligand, and crossed-lamellar architectures. These architectures promote effective stress transfer, delocalize damage and arrest cracking, thereby bestowing improved strength and ductility than composites with discrete reinforcements. Additionally, they activate a series of extrinsic toughening mechanisms, including crack deflection/twist and uncracked-ligament bridging, which enable crack-tip shielding from the applied stress and lead to "Γ"-shaped rising fracture resistance R-curves. Quantitative relationships were established for the stiffness and strengths of the composites by adapting classical laminate theory to incorporate their architectural characteristics.© 2022. This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.

The continuing development of X-ray light sources, optical devices and image analysis technologies enables us to nondestructively analyze internal structures of materials by using 3D imaging with a spatial resolution of micron, even dozens of nanometers. Innovations in materials science and technology will benefit from the new achievements of the X-ray tomography with higher resolution. This article devotes to review the origination and development of the X-ray tomography techniques for 3D imaging and introduce the principles and specifications of three imaging methods, including absorption imaging, phase contrast imaging and holographic imaging. The differences between synchrotron-based and laboratory-based X-ray tomography are also discussed. To explore new opportunities of the high resolution X-ray tomography in the development of material science and technology, particular emphasis is laid on the applications for traditional materials with a 3D view, such as the characterizations of holes, cracks, corrosion, composites, and in situ testing etc..

随着X射线光源、光学器件及图像分析技术的不断发展,微米甚至数十纳米空间分辨X射线三维数字化成像成为可能. 在此基础上,提供了高分辨无损探测材料内部结构的技术和方法,预计高分辨X射线三维成像新技术将会进一步促进材料科学技术的发展.本文将简述X射线三维成像的产生背景和发展过程,介绍吸收衬度成像、相位衬度成像和全息成像的原理与特点,着重分析高分辨透射X射线三维成像在材料孔洞、裂纹与腐蚀、复合材料以及原位测试等方向的应用及其特点,比较同步辐射与实验室X射线高分辨透射三维成像技术的不同,以探讨高分辨透射X射线三维成像在材料科学研究中进一步应用的可能性.

Seeking strategies to enhance the overall combinations of mechanical properties is of great significance for engineering materials, but still remains a key challenge because many of these properties are often mutually exclusive. Here we reveal from the perspective of materials science and mechanics that adaptive structural reorientation during deformation, which is an operating mechanism in a wide variety of composite biological materials, functions more than being a form of passive response to allow for flexibility, but offers an effective means to simultaneously enhance rigidity, robustness, mechanical stability and damage tolerance. As such, the conflicts between different mechanical properties can be "defeated" in these composites merely by adjusting their structural orientation. The constitutive relationships are established based on the theoretical analysis to clarify the effects of structural orientation and reorientation on mechanical properties, with some of the findings validated and visualized by computational simulations. Our study is intended to give insight into the ingenious designs in natural materials that underlie their exceptional mechanical efficiency, which may provide inspiration for the development of new man-made materials with enhanced mechanical performance. STATEMENT OF SIGNIFICANCE: It is challenging to attain certain combinations of mechanical properties in man-made materials because many of these properties - for example, strength with toughness and stability with flexibility - are often mutually exclusive. Here we describe an effective solution utilized by natural materials, including wood, bone, fish scales and insect cuticle, to "defeat" such conflicts and elucidate the underlying mechanisms from the perspective of materials science and mechanics. We show that, by adaptation of their structural orientation on loading, composite biological materials are capable of developing enhanced rigidity, strength, mechanical stability and damage tolerance from constrained flexibility during deformation - combinations of attributes that are generally unobtainable in man-made systems. The design principles extracted from these biological materials present an unusual yet potent new approach to guide the development of new synthetic composites with enhanced combinations of mechanical properties.Copyright © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.