蜂窝结构力学超材料弹性及抗冲击性能的研究进展

具有手性蜂窝结构的力学超材料是近年来发展起来的高性能工程材料,它具有轻质、高比刚度、负泊松比、结构参数可调以及力学性能稳定等优点。其不仅可以实现面内变形,面外承载的双重力学作用,还具有出色的隔振、吸声降噪以及控制弹性波的传播等工程应用潜质,在智能结构、车辆船舶、航空航天等领域具有巨大的发展潜力。本文从其弹性和抗冲击两个力学性能方面进行综述。首先介绍并评述了近年来蜂窝结构力学超材料的面内杨氏模量、负泊松比特性以及面外剪切模量等弹性性能的理论分析研究进展。在抗冲击性能方面,从力学模型建立和有限元分析的角度出发,对手性蜂窝结构力学超材料在冲击载荷作用下的整体变形及其抗冲击性能的研究现状分别进行了评述。最后指出针对蜂窝结构力学超材料弹性及冲击性能的研究,可进一步建立内部韧带变形及力的传递力学模型以及深入探索冲击过程吸能机理等,以期为该类力学超材料内部韧带和节点环结构的优化设计提供参考。     

Crystal–Glass High-Entropy Nanocomposites with Near Theoretical Compressive Strength and Large Deformability

High-entropy alloys (HEAs) and metallic glasses (MGs) are two material classes based on the massive mixing of multiple-principal elements. HEAs are single or multiphase crystalline solid solutions with high ductility. MGs with amorphous structure have superior strength but usually poor ductility. Here, the stacking fault energy in the high-entropy nanotwinned crystalline phase and the glass-forming-ability in the MG phase of the same material are controlled, realizing a novel nanocomposite with near theoretical yield strength (G/24, where G is the shear modulus of a material) and homogeneous plastic strain above 45% in compression. The mutually compatible flow behavior of the MG phase and the dislocation flux in the crystals enable homogeneous plastic co-deformation of the two regions. This crystal–glass high-entropy nanocomposite design concept provides a new approach to developing advanced materials with an outstanding combination of strength and ductility.     

Composite bending-dominated hollow nanolattices: A stiff,cyclable mechanical metamaterial

Manufacturing ultralight and mechanical reliable materials has been a long-time challenge. Ceramic-based mechanical metamaterials provide significant opportunities to reverse their brittle nature and unstable mechanical properties and have great potential as strong, ultralight, and ultrastiff materials. However, the failure of ceramics nanolattice and degradation of strength/modulus with decreasing density are caused by buckling of the struts and failure of the nodes within the nanolattices, especially during cyclic loading. Here, we explore a new class of 3D ceramic-based metamaterials with a high strength–density ratio, stiffness, recoverability, cyclability, and optimal scaling factor. Deformation mode of the fabricated nanolattices has been engineered through the unique material design and architecture tailoring. Bending-dominated hollow nanolattice (B-H-Lattice) structure is employed to take advantages of its flexibility, while a few nanometers of carbonized mussel-inspired bio-polymer (C-PDA) is coherently deposited on ceramics’ nanolayer to enable non-buckling struts and bendable nodes during deformation, resulting in reliable mechanical properties and outperforming the current bending-dominated lattices (B-Lattices) and carbon-based cellulose materials. Meanwhile, the structure has comparable stiffness to stretching-dominated lattices (S-Lattices) while with better cyclability and reliability. The B-H-Lattices exhibit high specific stiffness (>10⁶?Pa·kg^?1·m^?3), low-density (～30?kg/m³), buckling-free recovery at 55% strain, and stable cyclic loading behavior under up to 15% strain. As one of the B-Lattices, the modulus scaling factor reaches 1.27, which is lowest among current B-Lattices. This study suggests that non-buckling behavior and reliable nodes are the key factors that contribute to the outstanding mechanical performance of nanolattice materials. A new concept of engineering the internal deformation behavior of mechanical metamaterial is provided to optimize their mechanical properties in real service conditions.     

Current Progress of Mechanical Properties of Metals with Nano-scale Twins

Focus on face-centered cubic （fcc） metals with nano-scale twins lamellar structure, this paper presents a brief overview of the recent progress made in improving mechanical properties, including strength, ductility, work hardening, strain rate sensitivities, and in mechanistically understanding the underling deformation mechanisms. Significant developments have been achieved in nano-twinned fcc metals with a combination of high strength and considerable ductility at the same time, enhanced work hardening ability and enhanced rate sensitivity. The findings elucidate the role of interactions between dislocations and twin boundaries （TBs） and their contribution to the origin of outstanding properties. The computer simulation analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating TBs as internal interfaces and allowing special slip geometry arrangements that involve soft and hard modes of deformation. Parallel to the novel mechanical behaviors of the nano-twinned materials, the investigation and developments of nanocrystalline materials are also discussed in this overview for comparing the contribution of grain boundaries/TBs and grain size/twin lamellar spacing to the properties. The recent advances in the experimental and computational studies of plastic deformation of the fcc metals with nano-scale twin lamellar structures provide insights into the possible means of optimizing comprehensive mechanical properties through interfacial engineering.     

Nanoscale deformation of multiaxially forged ultrafine-grained Mg-2Zn-2Gd alloy with high strength-high ductility combination and comparison with the coarse-grained counterpart

Cold processing of magnesium(Mg) alloys is a challenge because Mg has a hexagonal close-packed(HCP)lattice with limited slip systems, which makes it difficult to plastically deform at low temperature. To address this challenge, a combination of annealing of as-cast alloy and multi-axial forging was adopted to obtain isotropic ultrafine-grained(UFG) structure in a lean Mg-2Zn-2Gd alloy with high strength(yield strength: ~227 MPa)-high ductility(% elongation: ~30%) combination. This combination of strength and ductility is excellent for the lean alloy, enabling an understanding of deformation processes in a formable high strength Mg-rare earth alloy. The nanoscale deformation behavior was studied via nanoindentation and electron microscopy, and the behavior was compared with its low strength(yield strength: ~46 MPa)-low ductility(% elongation: ~7%) coarse-grained(CG) counterpart. In the UFG alloy, extensive dislocation slip was an active deformation mechanism, while in the CG alloy, mechanical twinning occurred.The differences in the deformation mechanisms of UFG and CG alloys were reflected in the discrete burst in the load-displacement plots. The deformation of Mg-2Zn-2Gd alloys was significantly influenced by the grain structure, such that there was change in the deformation mechanism from dislocation slip(non-basal slip) to nanoscale twins in the CG structure. The high plasticity of UFG Mg alloy involved high dislocation activity and change in activation volume.     

夹杂及弹性耦联对手性蜂窝复合材料吸振带隙的影响

为了研究手性蜂窝复合材料的振动特性与其振动传播带隙之间的关系,首先,建立了该种材料离散多自由度的夹杂-韧带振动力学模型,该模型考虑了其内嵌夹杂的局部振动与由微结构韧带连接的节点环之间的弹性耦联及诱发共振模态。然后,重点研究了微结构元件之间的耦联程度和微结构元件的尺寸参数对材料吸振带隙低频段和高频段的影响,并结合有限元方法对模型进行了验证分析。结果表明:除柔性包覆的夹杂以外,耦联诱发振动、节点环和韧带的材料及尺寸参数都对手性蜂窝复合材料的固有振动频率产生显著影响,从而控制带隙的位置和带宽。随着节点环内、外弹性耦联程度的减小,夹杂的模态频率将控制带隙的低频段,且随着夹杂质量的增大,低频段的频率降低;高频段由韧带振动表征;当节点环内、外弹性耦联程度增大时,带隙的低频段对韧带和框架的模态更加敏感,从而出现比夹杂模态频率更低的带隙。无论弹性耦合程度强弱,当韧带和节点环的厚度减小时,材料第三阶较高的包覆层变形频率将被相对更低的韧带振动频率取代。所得结论可为小尺寸、低频宽带隙手性蜂窝型隔振材料的设计研究提供理论指导。     

低屈服点钢材与Q345B和Q460D钢材本构关系对比研究

为进一步探讨材料本构行为对构件及结构受力性能的影响,首先,进行了LYP100低屈服点钢材的本构关系试验研究,分析此材料的单调性能、滞回性能、耗能能力及循环本构模型等。在此基础上,全面对比LYP100和LYP160低屈服点钢材、普通钢材（Q345B）及高强度钢材（Q460D）的本构关系。最后,通过对比不同钢材的循环本构模型以及理想弹塑性模型对结构构件滞回行为的预测结果,深入研究材料本构关系对构件及结构的重要影响。结果表明:低屈服点钢材单调以及循环强屈比均在2.0~3.0以上,是普通钢材以及高强度钢材的2.0倍~3.0倍。同时,低屈服点钢材具有更好的延性和耗能能力。由于低屈服点钢材具有显著的各向同性强化行为,其采用循环本构模型和理想弹塑性模型的计算结果差异更大。因此,在结构计算分析中,需要根据所采用的钢材选取适当的本构关系模型。     

Nanostructured Composite Materials with Improved Deformation Behavior

The recent progress in the development of nanostructured composites is described for Zr‐base multicomponent alloys as a typical example for such materials. These advanced composite materials are attractive candidates for structural as well as functional applications. The combination of high strength with high elastic strain of fully nanocrystalline and glassy alloys renders them quite unique in comparison to conventional (micro‐)crystalline materials. However, one major drawback for their use in engineering applications is the often limited macroscopic plastic deformability, despite the fact that some of these alloys show perfectly elastic‐plastic deformation behavior. To improve the room temperature ductility of either fully nanocrystalline or amorphous alloys, the concept of developing a heterogeneous microstructure combining a glassy or nanostructured matrix with second‐phase particles with a different length‐scale, has recently been employed. This review describes the composition dependent metastable phase formation in the Zr‐(Ti/Nb)‐Cu‐Ni‐Al alloy system, which in turn alters the mechanical properties of the alloys. We emphasize the possibilities to manipulate such composite microstructures in favor of either strength or ductility, or a combination of both, and also discuss the acquired ability to synthesize such in‐situ high‐strength composite microstructures in bulk form through inexpensive processing routes.     

Design,Fabrication, and Mechanics of 3D Micro‐/Nanolattices

Over the past several decades, lattice materials have been developed and used as engineering materials for lightweight and stiff industrial structures. Recent advances in additive manufacturing techniques have prompted the emergence of architected materials with minimum characteristic sizes ranging from several micrometers to hundreds of nanometers. Taking advantage of the topological design, structural optimization, and size effects of nanomaterials, various 3D micro‐/nanolattice materials composed of different materials exhibit combinations of superior mechanical properties, such as low density, high strength (even approaching the theoretical limits), large deformability, good recoverability, and flaw tolerance. As a result, some micro‐/nanolattices occupy an unprecedented area in Ashby charts with a combination of different material properties. Here, recent advances in the fabrication and mechanics of micro‐/nanolattices are described. First, various design principles and advanced techniques used for the fabrication of micro‐/nanolattices are summarized. Then, the mechanical behaviors and properties of micro‐/nanolattices are further described, including the compressive Young's modulus, strength, energy absorption, recoverability, and tensile behavior, with an emphasis on mechanistic insights and origins. Finally, the main challenges in the fabrication and mechanics of micro‐/nanolattices are addressed and an outlook for further investigations and potential applications of micro‐/nanolattices in the future is provided.     

Designing metallic glasses with optimal combinations of glass-forming ability and mechanical properties

It is a long-standing challenge to search for metallic glasses(MGs)with optimal combinations of glassforming ability(GFA),strength and toughness in the vast compositional space.By taking into account both recently developed ellipse criterion and temperature-based GFA criterion,here we established quantitative correlations among compositions,elastic constants,GFA and mechanical properties of MGs,which enable to predict the GFA,fracture strength and fracture surface simultaneously in advance once the compositions of MGs are determined.Experimental data confirm the validity of this approach in prediction.Finally,a strategy for designing MGs with optimal combinations of strength,toughness and GFA is proposed,which allows for high-throughput discovering glass formers with excellent mechanical properties.     

The metastable constituent effects on size-dependent deformation behavior of nanolaminated micropillars: Cu/FeCoCrNi vs Cu/CuZr

Metastable high entropy alloys(HEAs) and amorphous metallic glasses(MGs), with the chemical disordered character, are intensively studied due to their excellent performance. Here, we introduce Cu to separately constrain these two metastable materials and comparatively investigate their deformation behaviors and mechanical properties of Cu/HEA Fe Co Cr Ni and Cu/MG Cu Zr nanolaminated micropillars in terms of intrinsic layer thickness h and extrinsic pillar diameter D. The metastable HEA layers, as the hard phase in Cu/HEA micropillars, are stable and dominate the deformation, while transformation(crystallization) occurs in MG which plays a minor role in deformation of Cu/MG micropillars. The h-controlled deformation mode transits from the D-independent homogenous-like deformation at large h to the Ddependent shear banding at small h in both Cu/HEA and Cu/MG micropillars. Although both Cu/HEA and Cu/MG micropillars exhibit a maximum strain hardening capability controlled by h, the former manifests much lower hardening capability compared with the latter. The intrinsic size h and extrinsic size D have a strong coupling effect on the strength of Cu/HEA and Cu/MG micropillars. The strength of strength of Cu/HEA micropillars exhibits the D-dependent transition from "smaller is stronger" to "smaller is weaker"with increasing h. By contrast, the strength of Cu/MG micropillars exhibits the transition from bulk-like D-independent behavior at large h to small volume D-dependent behavior(smaller is stronger) at small h.     

The influence of the nitrogen/nickel‐ratio on the cyclic behavior of austenitic high strength steels with twinning‐induced plasticity and transformation‐induced plasticity effects

Austenitic high nitrogen (AHNS) and austenitic high interstitial steels (AHIS) are of interest for mechanical engineering applications because of their unique combination of mechanical (strength, ductility), chemical (corrosion resistance) and physical (non‐ferromagnetic) properties. But despite their high strength values e. g. after cold deformation up to 2 GPa in combination with an elongation to fracture of 30 %, which is based on twinning‐induced plasticity (TWIP) mechanisms and transformation‐induced plasticity (TRIP) mechanisms, the fatigue limit remains relatively small. While for chromium‐nickel steels the fatigue limit rises with about 0.5‐times the elastic limit it does not at all for austenitic high‐nitrogen steels or only to a much smaller extent for nickel‐free austenitic high‐interstitial steels. The reasons are still not fully understood but this behavior can roughly be related to the tendency for planar or wavy slip. Now the latter is hindered by nitrogen and promoted by nickel. This contribution shows the fatigue behavior of chromium‐manganese‐carbon‐nitrogen (CrMnCn) steels with carbon+nitrogen‐contents up to 1.07 wt.%. Beside the governing influence of these interstitials on fatigue this study displays, how the nitrogen/nickel‐ratio might be another important parameter for the fatigue behavior of such steels.     

Mechanical behaviour and texture of annealed AZ31 Mg alloy deformed by ECAP

Abstract

AZ31 Mg alloy samples were processed by equal channel angular pressing (ECAP) at 220°C for four passes. An average grain size of ～1·9 μm with reasonable homogeneity was obtained. The ECAP process imparted large plastic shear strains and strong deformation textures to the material. Subsequent annealing of the equal channel angular pressed samples produced interesting mechanical behaviours. While yield strength increased and ductility decreased immediately after undergoing ECAP, annealing at temperatures <250°C restored ductility significantly at a small decrease in of yield strength. Annealing at temperatures >250°C reduced yield strength without additional improvement in ductility. It is believed that the combination of stress relief via dislocation elimination, refined microstructure and the retention of a strong ECAP texture at low annealing temperatures enhance ductility. High temperature annealing breaks down the ECAP texture resulting in no further improvement in ductility. The results show that the mechanical properties of the alloy can be positively influenced by annealing after ECAP to achieve a combination of strength and ductility.     

The improvement of strength and ductility in ultra-fine grained 5052 Al alloy by cryogenic- and warm-rolling

The effects of deformation temperatures and post-deformation annealing on mechanical properties, in conjunction with microstructural evolution in the 5052 Al alloy, were investigated. The combination of cryogenic-rolling with warm-rolling effectively increased tensile strength and yield strength without the decrease of ductility through the formation of ultra-fine grains with dynamic recovery in the 5052 Al alloy. And static annealing, as a post-heat treatment, enhanced the ductility. Therefore, ultra-fine grained 5052 Al alloy with high strength and a moderate level of ductility could be made by the combination of cryogenic-rolling with warm-rolling and the additional static annealing process.     

GH625合金的冷变形及其对力学性能的影响

研究了冷加工变形量对GH625合金板材力学性能的影响。研究表明,随冷加工变形量的增加,GH625合金的拉伸强度增加,但塑性降低。冷加工变形量对待持久寿命和冷热疲劳性能影响显著,20％左右的冷变形量可使合金具有最挂的持久寿命和疲劳性能及良好的综合力学性能。合金冷作硬化效果与合金的回复和再结晶程度及固溶化处理温度有关。     

裂纹尖端形变功与韧性启裂

研究了高强度结构钢典型强化组织板条马氏体、粒状贝氏体和板条马氏体加粒状贝氏体混合组织的韧性启裂.结果表明;高强度结构钢的韧性启裂依赖于强度、塑性和形变硬化能力,表现为裂纹尖端应力应变场中消耗的弹性应变能和塑性形变功不同.其相对量可用无量纲参量H表示,并与材料的流变应力有较好的对应关系.     

THE INFLUENCE OF AGEING AT INTERMEDIATE TEMPERATURES ON THE MONOTONIC STRESS-STRAIN BEHAVIOUR AND FRACTURE TOUGHNESS OF A DUPLEX STAINLESS STEEL

Abstract— The mechanical behaviour of the duplex stainless steel AISI 329 has been investigated for ageing times up to 15,000 h at 475, 425, 375, 325 and 275°C. The study has concentrated on changes in the monotonic stress-strain behaviour and fracture toughness as a function of ageing temperature and time. It is shown that the tensile behaviour of the steel changes strongly due to ageing. A large increase in yield strength and reductions in ductility and fracture toughness are observed. The deformation hardening behaviour of the aged steel is explained by using a model based on a modified rule of mixtures. Finally it is shown that the higher toughness of aged duplex stainless steels, in comparison with ferritic stainless steels aged under the same ageing conditions, may be associated with the increase in crack growth resistance induced by ductile ligaments of austenite which bridge the crack faces.     

High strength-ductility nano-structured high manganese steel produced by cryogenic asymmetry-rolling

A bulk nanostructured twinning-induced plasticity (TWIP) steel with high ductility and high strength was fabricated by cryogenic asymmetry-rolling (cryo-ASR) and subsequent recovery treatment. It was found that the cryo-ASRed TWIP steels exhibit simultaneous improvements in the ductility, strength and work hardening. Typical microstructures of the cryo-ASR TWIP steel were characterized by shear bands and intensive mechanical nano-sized twins induced by cryogenic deformation. These mechanical nano-scale twins remain thermally stable during the subsequent recovery treatment. It is believed that the ductility enhancement and high work-hardening ability for the cryo-ASR TWIP steels should be mainly attributed to the high-density pre-existing nano-scale twins.     

Influences of residual stress and micro-deformation on microstructures and mechanical properties for Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy produced by laser powder bed fusion

Laser powder bed fusion(LPBF)yields unique advantages during the fabrication of titanium alloys.In the present work,Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy specimens with excellent mechanical performances were fabricated by LPBF.The as-built specimens displayed relatively high strength and ductility under modest volume energy densities(VEDs),whereas they manifested high strength with low ductility under high VEDs.To investigate the key reason of this phenomenon,the specimens were designed with two VEDs ranges of 60 J/mm3 and 85J/mm3.Special attention was paid to the influences of residual stress and micro-deformation on microstructures and mechanical properties for the first time.The results indicated that the residual stresses and relative density of the 60 J/mm3 range specimens were higher than that of the 85 J/mm3 range specimens.Dislocation multiplication and dislocation movement promoted by the residual stress were hindered by the initial α'phase grain boundary(prior-α'GB),leading to the formation of α'metastable structures.The mean tensile strength and elongation of the 60 J/mm3 range specimens were 1248.1 MPa and 12.3％,respectively,whereas the corresponding values for the 85 J/mm3 range specimens were 1405.3 MPa,5.0％,respectively.During deformation,the strength and ductility of the specimens were first improved by lamellar structures generated from prior-α'phases,and then effectively enhanced by the interaction between the{10-12}twins and dislocations.However,pores significantly reduced the ductility;hence,high VED specimens with large twins and numerous large pores increased the strength and reduce the ductility.