Functional gradients and heterogeneities in biological materials: Design principles,functions, and bioinspired applications

Living organisms have ingeniously evolved functional gradients and heterogeneities to create high-performance biological materials from a fairly limited choice of elements and compounds during long-term evolution and selection. The translation of such design motifs into synthetic materials offers a spectrum of feasible pathways towards unprecedented properties and functionalities that are favorable for practical uses in a variety of engineering and medical fields. Here, we review the basic design forms and principles of naturally-occurring gradients in biological materials and discuss the functions and benefits that they confer to organisms. These gradients are fundamentally associated with the variations in local chemical compositions/constituents and structural characteristics involved in the arrangement, distribution, dimensions and orientations of the building units. The associated interfaces in biological materials invariably demonstrate localized gradients and a variety of gradients are generally integrated over multiple length-scales within the same material. The bioinspired design and applications of synthetic functionally graded materials that mimic their natural paradigms are revisited and the emerging processing techniques needed to replicate the biological gradients are described. It is expected that in the future bioinspired gradients and heterogeneities will play an increasingly important role in the development of high-performance materials for more challenging applications.     

Coatings on Mg alloys and their mechanical properties: A review

Poor corrosion resistance is a serious drawback of Mg alloys, restricting their practical applications. Coating is one of the effective techniques for improvement in the poor corrosion resistance. In this paper, the coating processes for Mg alloys so far developed are reviewed. Among several processes, the coating processes based on mechanical energy, including metal forming, are attractive because the corrosion resistance and formability of Mg alloys are simultaneously improved.     

Performance of ground clay brick in ASR-affected concrete: Effects on expansion, mechanical properties and ASR gel chemistry

The effects of ground clay brick (GCB) on alkali-silica reaction (ASR) expansion as well as on mechanical properties of ASR-affected concrete are investigated. Crushed red clay brick originated from demolished masonry was ground in a laboratory ball mill and replaced for portland cement at levels of 15% and 25% by weight in concrete mixes produced with alkali reactive sand. ASR expansion, compressive strength, flexural strength, and modulus of elasticity of the concrete mixes were evaluated. Effect of GCB on ASR gel chemistry was also studied on Pyrex glass-paste specimens using SEM/EDS (scanning electron microscope equipped with energy dispersive X-ray spectroscopy). The results indicate that GCB effectively reduces ASR expansion in concrete: associated cracking and loss on mechanical properties are also significantly reduced. SEM study suggests that GCB alters alkali silica gel chemistry thus resulting in a less expansive product.     

Structure, thermal properties and mechanical properties of sPS-based nanocomposites with and without styrenic elastomer inclusions

Syndiotactic polystyrene (sPS)-based nanocomposites with and without toughener inclusions were successfully prepared. One organo-montmorillonite (20A) and two styrenic elastomers (SBS and SEBS) served as the reinforcing filler and as tougheners, respectively. XRD and TEM results confirmed the achievement of intercalated and partially exfoliated sPS/20A nanocomposites. The presence of SBS or SEBS slightly depressed the dispersibility of 20A. DSC results indicated that 20A inhibited the crystallization of sPS. The presence of SBS or SEBS further retarded the crystallization of sPS; this effect was more apparent with SEBS. The presences of 20A and SBS/SEBS facilitated the formation of α-form sPS crystals. The thermal stability enhancement of sPS/20A nanocomposites was confirmed, and was further improved with the inclusion of SBS or SEBS. The stiffness of sPS increased with the sole addition of 20A. The addition of SBS or SEBS greatly increased the impact strength of the composites, especially with the addition of SEBS. The achievement of toughened sPS-based nanocomposites was confirmed.     

Influence of Ti content on microstructure,mechanical properties and castability of directionally solidified superalloy DZ125L

Directionally solidified DZ125L alloys of various Ti content were developed by low segregation technology. The microstructure, mechanical properties and castability of directionally solidified superalloy DZ125L were investigated. With decreasing Ti content in DZ125L alloy, the size, volume fraction and separation temperature of γ′ decreased. While the tensile strength and rupture life of directional solidified cylindrical samples is also obviously decreased. Lowering Ti content in alloy resulted in free hot cracking casting blades. Compared with IN792 alloy, DZ125L alloy needs much lower Ti/Ta ratio to achieve good castability.     

C/C复合材料飞机刹车盘的结构与性能

采用企业、行业及国家相关标准的试验方法，对超码复合材料公司，英国Dunlop公司，法国CarbonIn-dusty公司，美国B．F．Goodrich、ALS公司等生产的9种C／C复合材料飞机刹车盘的物理、力学、热学、摩擦磨损的性能特征，以及中南大学生产的C／C复合材料刹车盘的有关性能，进行了对比分析。结果表明，选择适宜的炭纤维预制体结构，控制热解炭基体微观结构为光学粗糙层结构，合理的热处理温度是获得高性能炭刹车盘材料的关键。我国拥有自主知识产权研发的大型民机炭刹车盘在高摩擦特性方面获得了重大突破，已用于波音757—200型飞机，实现了国内C／C复合材料具有里程碑意义的第四个重大突破。     

Structure and mechanical properties of the annealed TZAV-30 alloy

The Ti–30Zr–5Al–3V (wt.%, TZAV-30) alloy having good mechanical properties is a potential structural material to apply in the aerospace industry. The microstructure and mechanical properties of ZTAV-30 alloy underwent various annealing heat treatments were investigated. The specimens annealed from 500 to 800 °C are composed of α and β two phases. No compound is detected in specimens annealed in that temperature range. The microstructure of annealed specimens is characterized as a typical basketweave microstructure. Three microstructural parameters, thickness of plate α phase, relative fraction of β phase and aspect ratio of α grains, were measured in those annealed specimens. As the alloy annealed in the range from 500 to 800 °C, the average thickness of plate α grains increases with the increasing annealing temperature from 500 to 700 °C but decreases while annealed at 800 °C. The fraction of retained β phase increases with annealing temperature. And the aspect ratio of plate α grains decreases firstly but increases while the annealing temperature is higher than 700 °C. As the variation of those three microstructural parameters, the strength of examined alloy varies from 1269 to 1355 MPa for tensile strength and from 1101 to 1190 MPa for yield strength, inversely, the elongation changes in the range from 12.7% to 8.4%. The strengthening and toughening mechanism of the TZAV-30 alloy with basketweave microstructure is also discussed in this paper.     

Deformation behavior and mechanical properties of braided rectangular pipes

The purpose of this study is to clarify the deformation behavior and the relationship between the braiding angle and mechanical properties for the braided rectangular pipe. Three-point bending test was performed for each specimen. Furthermore, the effect of the solid-core bar on the deformation state of the rectangular pipe was studied by finite element method (FEM). Moreover, the effect of the braiding angle on the deformation behavior in detail was investigated. The bending modulus and the bending strength for each specimen were decreased with an increase in the braiding angle. The local deformations occurred at the compression side under loading nose and decreased with an increase in stiffness properties in circumferential direction. From FEM analysis, using the aluminum solid-core bar was effective to restrain the local deformation. The difference between the bending moduli of experiments and FEM was decreased with an increase in the modulus in transverse direction.     

A first-principles investigation on mechanical and metallic properties of titanium carbides under pressure

The titanium carbides are potential candidates to achieve both high hardness and refractory property. We carried out a structural search for titanium carbides at three pressures of 0 GPa, 30 GPa and 50 GPa. A phase diagram of the Ti-C system at 0 K was obtained by elucidating formation enthalpies as a function of compositions, and their mechanical and metallic properties of titanium carbides were investigated systematically. We also discussed the relation of titanium concentration to the both mechanical and metallic properties of titanium carbides. It has been found that the average valence electron density and tractility improved at higher concentrations of titanium, while the degree of covalent bonding directionality decreased. To this effect, the hardness of titanium carbide decreases as the content of titanium increases. Our results indicated that the titanium content significantly affected the metallic properties of the Ti-C system.     

Structure and mechanical properties of reaction-sintered ceramic composite materials based on titanium and hafnium diborides

The structure and mechanical characteristics of ceramic composite materials based on titanium and hafnium diborides produced by reaction sintering in the course of hot pressing have been studied. It has been shown that ultradispersed structure is responsible for high mechanical characteristics of the synthesized composites.     

Effect of residual stresses on mechanical properties and interface adhesion strength of SiN thin films

Residual stresses play a significant role in the mechanical reliability of thin films. Thus in this study, the mechanical properties and interface adhesion strengths of SiN thin films containing different residual stresses have been investigated by using nanoindentation and nanoscratch tests. With varied residual stresses from compressive to tensile, the penetration depth of nanoindentation tests shifted to a higher value. The hardness and elastic modulus decreased from 11.0 and 95 GPa, respectively, for the film containing a compressive stress of 235 MPa to 9.6 and 84 GPa for the film with a tensile stress of 86 MPa. With decreasing compressive stress and increasing tensile stress, the interface adhesion energy decreased from 1.8 to 1.5 J/m². Compressive stresses were expected to blunt crack tips and inhibit crack propagation, while tensile stresses enlarged crack opening and facilitated crack propagation, thus changing the mechanical properties of the SiN thin films.     

Structural,thermal, mechanical and dynamic mechanical properties of cenosphere filled polypropylene composites

Polypropylene (PP)/cenosphere based composites were fabricated and characterized for their structural/morphological and mechanical properties such as tensile, flexural, impact and dynamic mechanical properties such as storage and loss moduli as a function of temperature. The morphological attributes were characterized by scanning electron microscopy (SEM) and wide-angle X-ray diffraction (WAXD) while the thermal characterizations were done by conducting differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA). The morphological investigations have revealed a uniformly distributed/dispersed state of the cenosphere in the bulk PP matrix of the composites. The WAXD/DSC studies have revealed a decrease in crystallinity of the composites with increase in cenosphere content. Dynamic mechanical analysis (DMA) revealed an enhancement in the energy dissipation ability of the composite with 10 wt.% of cenosphere and an increase in the storage modulus up to ∼30% in the composites relative to the soft PP-phase. The tensile modulus increased up to ∼43% accompanied by a nominal decrease in tensile strength while the strain at break remained largely unaffected. The impact strength of the composites marginally reduced compared to PP indicating a low-cost material-concept with maximized stiffness–toughness combination. The theoretical modeling of the tensile data revealed appreciable extent of phase-adhesion despite the cenospheres lack any surface modification indicating better extent of mechanical interlocking and surface-compatibility between polymer and filler. Fractured surface morphology indicated that the failure mode of the composites undergoes a switch-over from matrix-controlled shear deformation to filler-controlled quasi-brittle modes above a cenosphere loading of 10 wt.% in the composites.     

新型稀土镁基贮氢电极合金的结构与性能

系统研究了La0．7Mg0．3(Ni0．85Co0.15)x(x=2．5，3．0，3．5，4．0，4．5，5．0)贮氢电极合金的结构和储氢性能．该类型合金由(La，Mg)Ni3相(PuNi3型结构)和LaNi5相(CaCu5型结构)组成，两相的a轴参数和晶胞体积都随着x值的增大而减小．(La，Mg)Ni3相的丰度先从x=2．5时的48．4％增加到x=3．5时的78．2％，然后又减小到x=5．0时的12．2％，而LaNi5相的丰度在x=2．5-3．5时基本保持不变(-20％)，在x=4．0时突然增加到71.9％．合金的吸氢量从x=2．5时的0．86％(质量分数)增加到x=3．5时的1．50％然后又减小到x=5．0时的1．19％．合金的放氢平台压力在x=2．5-3．5时保持基本不变(-65．9kPa)，然后逐渐增加到x=5．0时的0．30MPa．随着x的增加，吸放氢过程的滞后效应先增大后减小，而合金的放氢平台变得更加平坦．     

Rheology,fiber distribution and mechanical properties of calcium carbonate (CaCO3) whisker reinforced cement mortar

Calcium carbonate (CaCO₃) whiskers are a new kind of microfiber used in cementitious composites and have proved to provide excellent effect on strengthening and toughening. In order to further improve the mechanical properties of CaCO₃ whisker-reinforced cementitious composites, rheological properties of fresh mixtures and the CaCO₃ whisker distribution in the hardened matrix were investigated. The yield stress and plastic viscosity increased with an increasing content of CaCO₃ whisker and a decreasing water-cement ratio. Also, the rheological properties were affected by the distribution of CaCO₃ whisker in the matrix. The largest increments in flexural and compressive strength were 27.59% and 12.60% for the mortars with CaCO₃ whisker contents of 2.0% and 1.5%, respectively. The properties responsible for the mechanical response were explained in terms of the effects of CaCO₃ whisker reinforcement, the distribution of CaCO₃ whiskers, and the porosity as well as pore size distribution.     

Densification behavior,mechanical properties and thermal shock resistance of tungsten alloys fabricated at low temperature

The low-temperature shrinkage of tungsten was greatly accelerated by the addition of trace Nb and Ni, and the addition of trace Nb and Ni also significantly promoted the final sintering density. The 99.1% of theory density for W–0.1 wt.%Nb–0.1 wt.%Ni material sintered at 1600 °C was obviously greater than 93.7% of theory density for W material sintered at 2000 °C. Ball milling treatment played an important role in promoting the sintering densification of W–0.1 wt.%Nb–0.1 wt.%Ni powder, and the powder milled for 10 h (W10) could be sintered to near full density (99.4% of theory density) at 1600 °C. The ball milling for 15 h has no effect in improving the sintering density, but it induced rapid growth of tungsten grains. The microhardness and tensile strength of the sintered tungsten alloys were highly dependent on its sintering density and grain size. Improving the sintering density while controlling the grain growth could effectively promote the microhardness and tensile strength. Furthermore, the improvement of thermal shock resistance of the W10 alloy was due to good microstructure and the increase in the tensile strength.     

Molecular dynamics study of binding energies, mechanical properties, and detonation performances of bicyclo-HMX-based PBXs

To investigate the effect of polymer binders on the monoexplosive, molecular dynamics simulations were performed to study the binding energies, mechanical properties, and detonation performances of the bicyclo-HMX-based polymer-bonded explosives (PBXs). The results show that the binding energies on different crystalline surfaces of bicyclo-HMX decrease in the order of (010)>(100)>(001). On each crystalline surface, binding properties of different polymers with the same chain segment are different from each other, while those of the polymers in the same content decrease in the sequence of PVDF>F(2311)>F(2314) approximately PCTFE. The mechanical properties of a dozen of model systems (elastic coefficients, various moduli, Cauchy pressure, and Poisson's ratio) have been obtained. It is found that mechanical properties are effectively improved by adding small amounts of fluorine polymers, and the overall effect of fluorine polymers on three crystalline surfaces of bicyclo-HMX changes in the order of (010)>(001) approximately (100). In comparison with the base explosive, detonation performances of the PBXs decrease slightly, but they are still superior to TNT. These suggestions may be useful for the formulation design of bicyclo-HMX-based PBXs.     

Microstructure,mechanical and degradation properties of equal channel angular pressed pure magnesium for biomedical application

Abstract

Magnesium is a biocompatible and biodegradable metal, which has attracted much interest in biomedical engineering. Pure magnesium shows the low strength and plasticity at ambient temperature. Microstructure, mechanical properties and degradation properties of the equal channel angular pressed pure magnesium have been investigated for biomedical application in detail by optical microscopes, mechanical properties testing and corrosion testing. The results have revealed that the processing temperature and routes are important factors that affect the properties of pure Mg by equal channel angular pressing. The two-step equal channel angular pressing processing (one pass at 360°C and three passes at 200°C) has been successfully applied to control the microstructure, mechanical and degradation properties of the pure Mg. Optical microscopy observation has indicated that the grain size of the as cast pure magnesium has been significantly decreased after equal channel angular extrusion, which has mainly contributed to the high tensile strength and good elongation. Equal channel angular pressed pure magnesium has provided moderate corrosion resistance, which has opened a new window for materials design, especially for biomedical.     

氟化类金刚石膜结构、性能及应用

氟化类金刚石膜是基于传统类金刚石膜的基础上发展起来的一种改性材料,该膜具有许多良好的性能,包括疏水性能、电学性能、光学性能、机械性能、热稳定性以及生物相容性,因此引起了人们的极大关注.本文介绍了氟化类金刚石膜的结构、性能、应用领域和发展趋势.     

Effect of cooling rate on microstructure,microsegregation and mechanical properties of cast Ni-based superalloy K417G

The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the temperature range of solidification.The casting mould with different casting runners was designed to obtain three different cooling rates.The microstructures were observed and the microsegregation was investigated.Also,high temperature tensile test was performed at 900?C and stress rupture test was performed at 950?C with the stress of 235 MPa.The results showed that the secondary dendrite arm spacing,microsegregation,the size and volume fraction of γ'phase and the size of γ/γ'eutectic increased with decreasing cooling rate,but the volume fraction of γ/γ' eutectic decreased.In the cooling rate range of 1.42?C s~(-1)–0.84?C s~(-1),the cast micro-porosities and carbides varied little,while the volume fraction and size of phase and γ/γ' eutectic played a decisive role on mechanical properties.The specimen with the slowest cooling rate of 0.84?C s~(-1) showed the best comprehensive mechanical properties.