Multiple strengthening mechanisms in nanoparticle-reinforced copper matrix composites

The multiple hardening mechanisms of a copper matrix have been presented and discussed. The pre-alloyed ball milled Cu–3 wt.%Al and the atomized Cu–0·6 wt.%Ti–2·5 wt.%TiB ₂ powders have been used as starting materials. Dispersoid particles Al ₂ O ₃ and TiB _2\thinspace _{{\bf 2}\thinspace }were formed in situ. The powders have been hot consolidated. Optical microscopy, SEM, TEM, and X-ray diffraction analysis were performed for microstructural characterization. Increase in microhardness of Cu–3 wt.%Al compacts is a consequence of the crystallite size refinement and the presence of Al ₂ O ₃ particles. High hardening of Cu–0·6 wt.%Ti–2·5 wt.%TiB ₂ is a consequence of the presence of modular structure, Cu ₄ Ti_(m), and TiB ₂ particles.     

Size dependent strengthening mechanisms in carbon nanotube reinforced metal matrix composites

The matrix grain size plays a dual role in metal matrix composites (MMCs). Contrary to enhance the strength of matrix, grain refinement can weaken the thermal expansion mismatch strengthening induced by the reinforcement. In this article, a dislocation density based model is developed to describe the factors affecting the strengthening mechanisms in Carbon nanotube (CNT)-reinforced MMCs with different matrix grain sizes. Two kinds of thermal expansion mismatch strengthening mechanisms are considered, i.e., geometrically necessary dislocations (GNDs) are distributed in entire matrix and GNDs are limited in dislocation punched zones (DPZs). In addition, comparisons between the predictions and some available experimental results are also performed.     

Comparison of strengthening in wire-drawn or rolled Cu-20% Nb with a dislocation accumulation model

Strengthening after large deformations by wire-drawing or rolling of Cu, Nb and Cu-20% Nb was compared with the predictions of a proposed modified substructural strengthening model for ductile two-phase alloys. The comparisons indicate that the more extensive and refined model of Funkenbusch and Courtney offers no improvement over the original model of Ashby in predicting the strengthening with increased deformation processing or the dislocation densities necessary to produce the observed strengthening in Cu-20% Nb. Both models can predict the strengthening behaviour of Cu-20% Nb. However, neither model is in accord with the observations that the dislocation density in the Cu matrix is essentially independent of the degree of deformation processing, and that the magnitudes of the dislocation density are much the same in the Cu in Cu-20% Nb and pure Cu identically deformation-processed. In addition, there is no experimental support for the Funkenbusch and Courtney model prediction of an order of magnitude greater dislocation density in the Nb filaments than in the Cu matrix in Cu-20% Nb. It appears that a mechanism that does not require an accumulation of dislocations for strengthening, such as the difficulty in propagating dislocations between closely spaced barriers, is more likely to be responsible for strengthening in Cu-Nb-type deformation-processed composites.     

Dual strengthening mechanisms induced by carbon nanotubes in roll bonded aluminum composites

The dual role of carbon nanotubes (CNTs) in strengthening roll bonded aluminum composites has been elucidated in this study. An increase in the elastic modulus by 59% has been observed at 2 vol.% CNT addition in aluminum, whereas tensile strength increases by 250% with 9.5 vol.% CNT addition. CNTs play a dual role in the strengthening mechanism in Al–CNT composite foil, which can be correlated to the degree of dispersion of CNTs in the matrix. Better CNT dispersion leads to improvement of elastic properties. In contrast, CNT clusters in the aluminum matrix impede dislocation motion, causing strain hardening and thus improvement in the tensile strength. Dislocation density of the composites has been computed as a function of CNT content to show the effect on strain hardening of the metal matrix–CNT composite.     

纳米颗粒分布对镁基复合材料强化机制的影响

Orowan强化、 热错配强化和Hall-Petch强化是纳米颗粒增强镁基复合材料的主要强化机制, 纳米颗粒在基体中的分布状态对起主导作用的强化机制具有重要影响。本文中对现有强化机制模型进行了适当修正, 以纳米SiC颗粒增强AZ91D复合材料为例, 通过理论计算分析了纳米颗粒完全分布于晶内、 完全分布于晶界、 在晶内晶界上均有分布的三种状态对镁基复合材料屈服强度的影响, 并与实验结果进行对比。结果表明: 颗粒完全分布于晶内时, 增强效果最好, 主要增强机制为Orowan强化; 颗粒完全分布于晶界上时, 增强效果最差, 主要增强机制为Hall-Petch强化。颗粒在晶内晶界上均有分布时, 多种强化机制共同发挥作用, 增强效果随着晶内与晶界上颗粒比例的减小而逐渐减弱。     

纳米颗粒分布对镁基复合材料强化机制的影响

Orowan强化、热错配强化和Hall-Petch强化是纳米颗粒增强镁基复合材料的主要强化机制,纳米颗粒在基体中的分布状态对起主导作用的强化机制具有重要影响.本文中对现有强化机制模型进行了适当修正,以纳米SiC颗粒增强AZ91D复合材料为例,通过理论计算分析了纳米颗粒完全分布于晶内、完全分布于晶界、在晶内晶界上均有分布的三种状态对镁基复合材料屈服强度的影响,并与实验结果进行对比.结果表明:颗粒完全分布于晶内时,增强效果最好,主要增强机制为Orowan强化;颗粒完全分布于晶界上时,增强效果最差,主要增强机制为Hall-Petch强化.颗粒在晶内晶界上均有分布时,多种强化机制共同发挥作用,增强效果随着晶内与晶界上颗粒比例的减小而逐渐减弱.     

Hot-working and strengthening in metal carbide-graphite composites

A study of the hot-pressing of graphite-metal powder mixes up to 2700° C has been effected, concentrating on metals such as titanium, vanadium, niobium, tantalum and zirconium which form stable refractory carbides. In particular, it is shown that titanium/vanadium and graphite/electrographite powder compacts can be deformed plastically and even die-moulded rapidly above 2000° C in a one-stage process to form strong, shock-resistant composite artefacts consisting of a graphite matrix hardened by finely divided metal carbide. The compressive strength is increased by a factor of 10 over a typical electrographite. Densification and strengthening are induced at much lower temperatures than those required for pure carbons and graphite.     

Physical and mechanical behavior of hot rolled HDPE/HA composites

In the present study hydroxyapatite (HA) reinforced high-density polyethylene (HDPE) composites were synthesized with possible application as orthopedic implants. HDPE was reinforced with HA particles using a novel hot rolling technique that facilitated uniform dispersion and blending of the reinforcements in the matrix. Composites were processed with up to 50 wt.% HA particles. Scanning Electron Microscopy studies confirmed uniform particle distribution of the reinforcement. Mechanical properties of the composites were examined by tensile tests. Increasing volume fraction of reinforcement from 10–50 wt.% resulted in a 150% increase in elastic modulus and 20% increase in tensile strength. Differential Scanning Calorimetry, Fourier Transformed Infrared spectroscopy and X-ray Diffraction studies indicate that the new blending process can be used to synthesize a crystalline, uniformly reinforced composite having chemical affinity between the matrix and reinforcement.     

A combined microstructure strengthening analysis of SiCp/Al metal matrix composites

A combined microstructure strengthening analysis was made to predict the yield strength of SiC_p/Al metal matrix composites. The modified shear lag theory can be used to predict the yield strength of particlereinforced high strength aluminium composites, but the yield strength is underestimated for composites prepared from pure and cast aluminium matrices because the change of matrix microstructure after particle incorporation is not considered. The increased yield strength due to the change of matrix microstructure represents a large increment of the total increased yield strength of SiC particle-reinforced low strength aluminium composites. Several microstructure strengthening mechanisms are examined to evaluate their effects on the matrix yield strength. The result of the combined microstructure strengthening analysis is consistent with experimental results.     

Mechanical properties of rolled A356 based composites reinforced by Cu-coated bimodal ceramic particles

Three kinds of A356 based composites reinforced with 3 wt.% Al₂O₃ (average particle size: 170 μm), 3 wt.% SiC (average particle size: 15 μm), and 3 wt.% of mixed Al₂O₃–SiC powders (a novel composite with equal weights of reinforcement) were fabricated in this study via a two-step approach. This first process step was semi-solid stir casting, which was followed by rolling as the second process step. Electroless deposition of a copper coating onto the reinforcement was used to improve the wettability of the ceramic particles by the molten A356 alloy. From microstructural characterization, it was found that coarse alumina particles were most effective as obstacles for grain growth during solidification. The rolling process broke the otherwise present fine silicon platelets, which were mostly present around the Al₂O₃ particles. The rolling process was also found to cause fracture of silicon particles, improve the distribution of fine SiC particles, and eliminate porosity remaining after the first casting process step. Examination of the mechanical properties of the obtained composites revealed that samples which contained a bimodal ceramic reinforecment of fine SiC and coarse Al₂O₃ particles had the highest strength and hardness.     

Hazard mitigation and strengthening of unreinforced masonry walls using composites

An experimental investigation was conducted to study the behavior of unreinforced masonry (URM) walls retrofitted with composite laminates. The first testing phase included testing 24 URM assemblages under different stress conditions present in masonry walls. Tests included prisms loaded in compression normal and parallel to bed joints, diagonal tension specimens, and specimens loaded under joint shear. In the second testing phase, five masonry-infilled steel frames were tested with and without retrofit. The composite laminates increased the stiffness and strength and enhanced the post-peak behavior by stabilizing the masonry walls and preventing their out-of-plane spalling. Tests reported in this paper demonstrate the efficiency of composite laminates in improving the deformation capacity of URM, containing the hazardous URM damage, preventing catastrophic failure and maintaining the wall integrity even after significant structural damage.     

Current practice and open issues in strengthening historical buildings with composites

Modern techniques and innovative materials are often quite rapidly proposed and allowed in current practice, even for restoration of historical constructions, in which essential preservation criteria must be taken into account. The considerable variability and complexity of masonry structures and types means that choosing the most appropriate structural models and interventions is particularly difficult, since they must be based on suitable knowledge of both existing and new materials, and on their interactions in environmental and loading conditions. This paper discusses the potentials and limitations of externally bonded composite materials in masonry structures and components, in the light of knowledge acquired from research in the field, together with the requirements and recommendations of codes and restoration documents. The analysis of some case studies is presented, to highlight the advantages and constraints in the use of composites for strengthening historical buildings.     

微纳米SiO2/PP复合材料增强增韧的实验研究

为了研究无机刚性颗粒对通用塑料聚丙烯(PP)的力学性能的影响,采用熔融共混方法制备了经硅烷偶联剂A-151处理的SiO2/PP复合材料,并通过其缺口冲击、拉伸、弯曲试验和冲击断面的形貌观察,分析研究了微纳米SiO2颗粒大小、填充量、表面改性以及不同颗粒大小SiO2混合物对PP复合材料增韧、增强效果的影响.实验结果表明:纳米SiO2的加入可以同时改善其韧性、刚性和强度;填充量相同,颗粒越细,SiO2/PP复合材料的力学性能越好.SiO2经改性后填充到PP基体中,明显改善了颗粒在基体中的分散性及基体与颗粒之间界面结合性能,使复合材料的综合力学性能得到提高.不同颗粒大小的SiO2混合后填充到PP基体中,混合SiO2的协同效应使复合材料拉伸、弯曲性能进一步提高,对PP基体具有更好的增强效果,但其冲击性能下降.     

A research on the creep properties of titanium matrix composites rolled with different deformation degrees

(TiB + La₂O₃)/Ti composites were in situ synthesized and deformed with different deformation degrees. The influence of TiB whisker orientation and grain refinement on the creep properties of titanium matrix composites (TMCs) are discussed. The creep test reveals that the steady state creep rate of TMCs first decreases and then increases with the increase of deformation degree, which can be attributed to competing effects: TiB whisker rotating to the rolling direction, α plate grain boundary hindering and pinning dislocations can all decrease the creep rate, however, dislocation movement on the α plate grain boundary and dislocation emitting from the α plate grain boundary can both increase the creep rate.     

Precipitation of MC phase and precipitation strengthening in hot rolled Nb–Mo and Nb–Ti steels

Hot rolled Nb–Mo steel of yield strength 600 MPa and Nb–Ti steel of yield strength 525 MPa with polygonal and acicular ferrite microstructure have been developed. Using physicochemical phase analysis, XRD, TEM and EDS, the distribution, morphology, composition, crystal structure and particle size of precipitates were observed and identified in these steels. The results revealed that the steels containing both Nb and Mo exhibited fine and uniformly distributed MC-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb and Ti. The physicochemical phase analysis showed MC-type carbides contain both Nb and Mo, and the ratio of Mo/Nb was 0.41. Meanwhile, the mass% of the fine particles (<10 nm in size) of Nb–Mo steel was 58.4%, and higher than that of Nb–Ti steel with 30.0%. Therefore, the results of strengthening mechanisms analysis showed the higher strength of Nb–Mo steel than that of Nb–Ti steel is attributed to its relatively more prominent precipitation strengthening effect. The yield strength increments from precipitation hardening of Nb–Mo steel attained 182.7 MPa and higher than that of Nb–Ti steel.     

Role of residual stress field interaction in strengthening of particulate-reinforced composites

A quantitative analysis was conducted on the effect of residual thermoelastic stress concentrations on the strength of particle-reinforced brittle matrix systems. The analysis is derived from the stress intensity factor for a periodic array of coplanar cracks emanating from the matrix-particle interface. It is shown that the major drop in strength occurs at smaller volume fractions of second phase where the residual stress field interaction effects are minimal. The effect of volume fraction on strength becomes important at larger volume fractions (normally above 10–15%). The theory is compared with experimental measurements of strength for glass and alumina matrix composites as a function of the particle volume fraction, its size, and thermal mismatch .     

Surface modification and adhesion mechanisms in woodfiber-polypropylene composites

The interfacial adhesion between wood fiber and thermoplastic matrix polymer plays an important role in determining the performance of wood-polymer composites. The objectives of this research were to elucidate the interaction between the anhydride groups of maleated polypropylene (MAPP) and hydroxyl groups of wood fiber, and to clarify the mechanisms responsible for the interfacial adhesion between wood fiber and polypropylene matrix. The modification techniques used were bulk treatment in a thermokinetic reactive processor and solution coating in xylene. FT-IR was used to identify the nature of bonds between wood fiber and MAPP. IGC and wood veneer pull-out test was used to estimate the interfacial adhesion. Mechanical properties of injection molded woodfiber-polypropylene composites were also determined and compared with the results of esterification reaction and interfacial adhesion tests. Confocal Microscopy was employed to observe the morphology at the wood fiber-polypropylene interface, and the dispersion and orientation of wood fiber in the polypropylene matrix, respectively. The effectiveness of MAPP to improve the mechanical properties (particularly the tensile strength) of the composites was attributed to the compatibilization effect which is accomplished by reducing the total wood fiber surface free energy, improving the polymer matrix impregnation, improving fiber dispersion, improving fiber orientation, and enhancing the interfacial adhesion through mechanical interlocking. There was no conclusive evidence of the effects of ester links on the mechanical properties of the composites.     

Transformation structures and strengthening mechanisms of laser processed Fe-Cr alloys

The microstructures and microhardness of laser processed Fe-Cr surface alloys were investigated as functions of composition (5 to 50 wt % Cr) and melt penetration depth (100 to 1500 m). The transformation structures were characterized by optical metallography and thin foil transmission electron microscopy. The microstructures were ferritic irrespective of compositions and melt depths. The alloys containing chromium up to 12 % (within the -phase field) exhibited a massive ferritic morphology while the alloys containing chromium more than 12% (beyond the -phase field) showed an equiaxed ferritic morphology. Transmission electron microscopy studies revealed that the substructure of ferrite consisted of dislocations, the dislocation density increased with increased chromium content. Melt depth, unlike composition, did not have a significant effect on the morphology and substructure of ferrite grains. Small amounts of -carbide and M₃C carbide phases were observed in these alloys. Both the carbides were found to decrease with an increase in the chromium of the fusion zone. Microhardness measurements indicated that there was an increase in hardness with an increase in the chromium content of the alloy.