Al_2O_3/Cu复合材料的高温变形行为

采用Gleeble-1500热模拟试验机和透射电子显微镜研究了变形温度为300~900℃,应变速率为0.01~10s-1条件下Al_2O_3/Cu复合材料的高温流变行为和组织演变规律,并利用Arrhenius关系和Zener-Hollomn参数构建了合金的峰值屈服应力、变形温度和应变速率三者之间的本构方程。结果表明:Al_2O_3/Cu复合材料的流变应力-应变曲线为典型的动态再结晶类型,其曲线由加工硬化、动态软化和稳定流变3个阶段组成,当变形温度一定时,流变应力随应变速率的增大而增大,而当应变速率固定时,流变应力随变形温度的升高而减小;求解得到复合材料的结构因子lnA为15.2391,应力水平参数a为0.020788mm~2/N,应力指数n为5.933035,变形激活能Q为2.1697×10~5kJ/mol;随着变形温度的升高,基体内位错密度逐渐下降,并呈现出明显的再结晶特征,而当固定变形温度时,随着应变速率的增大,基体内位错密度呈先增大后下降趋势。基于微观组织演变和热加工图,Al_2O_3/Cu复合材料的最佳热加工参数范围为热加工温度500~850℃、应变速率低于0.1s-1。     

Atomistic simulation and modeling of localized shear deformation in metallic glasses

Since the end of 1980s, bulk metallic glasses became available for various multi-component alloys. Because bulk metallic glasses are applicable to structural materials, their mechanical properties have become a matter of great interest in these decades. A characteristic feature of plastic deformation of metallic glasses at the ambient temperature is the localized shear deformation. Since we have no appropriate experimental technique, unlike crystalline matter, to approach microscopic deformation process in amorphous materials, we have to rely on computer simulation studies by use of atomistic models to reveal the microscopic deformation processes. In this article, we review atomistic simulation studies of deformation processes in metallic glasses, i.e., local shear transformation (LST), structural characterization of the local shear transformation zones (STZs), deformation-induced softening, shear band formation and its development, by use of elemental and metal-metal alloy models. We also review representative microscopic models so far proposed for the deformation mechanism: early dislocation model, Spaepen’s free-volume model, Argons’s STZ model and recent two-state STZ models by Langer et al.     

颗粒形状对复合材料单轴棘轮行为及其细观塑性变形特征的影响

在细观有限元模型基础上 , 利用 ABAQUS有限元程序对具有不同颗粒形状(球形、 立方体、 短棱柱和短圆柱)的 SiC P/ 6061Al 合金复合材料的单调拉伸行为和单轴棘轮行为进行数值模拟 , 讨论颗粒形状对复合材料棘轮行为的影响。 结果表明: 颗粒形状对复合材料的弹性模量、 单拉行为和单轴棘轮行为均有较大影响。 在所讨论的几种颗粒形状中 , 球形颗粒的增强效果最弱 , 抵抗棘轮变形的能力最差 ; 不同短棱柱颗粒的增强效果与其拥有的棱边数有关 , 即五棱柱颗粒的增强效果最好 , 然后随棱边数的增加逐渐下降 , 最后接近于短圆柱形颗粒。通过有限元分析结果讨论了不同颗粒形状下基体的细观塑性变形特征及其演化规律 , 这些结果有助于分析该类复合材料损伤和失效机制。      

Fabrication of Al/Al2O3 composites and FGM

Al/Al2O3 composites of different ratios were hot-press sintered at 575 similar to 640℃ under a pressure of 30 MPa for 2 h in a vacuum furnace. It was found that the relative density of the Al/Al2O3 composites could be increased evidently with the rise of sinter temperature. No reaction occurred between Al and Al2O3 at the sinter temperatures. Under 640℃-30 MPa-2 h experimental condition, Al/Al2O3 system FGM was successfully fabricated, and its density range changed quasi-continuously from 2.887x10(3) kg/m(3) to 3.1909x10(3) kg/m3 within the middle 1.0 mm thickness range.     

Simulations of deformation and damage processes of SiCp/Al composites during tension

The deformation, damage and failure behaviors of 17 vol.% SiCp/2009Al composite were studied by microscopic finite element (FE) models based on a representative volume element (RVE) and a unit cell. The RVE having a 3D realistic microstructure was constructed via computational modeling technique, in which an interface phase with an average thickness of 50 nm was generated for assessing the effects of interfacial properties. Modeling results showed that the RVE based FE model was more accurate than the unit cell based one. Based on the RVE, the predicted stress-strain curve and the fracture morphology agreed well with the experimental results. Furthermore, lower interface strength resulted in lower flow stress and ductile damage of interface phase, thereby leading to decreased elongation. It was revealed that the stress concentration factor of SiC was ～2.0: the average stress in SiC particles reached ～1200 MPa, while that of the composite reached ～600 MPa.     

Multiscale constitutive modeling for plastic deformation of nanocrystalline materials

Macroscopic and microscopic constitutive modeling that can display large plastic deformation with shear band were presented for nanocrystalline materials subjected to uniaxial load over a wide strain rate range. The macroscopic model implemented with parameters microscopic meaning was established based on the theory of plastic dissipation energy. The microscopic model based on deformation mechanisms was composed of two parts: hardening and softening stages. In the hardening stage, the phase mixture model was used and a shear band deformation mechanism was proposed in the softening stage. Numerical simulations shown that the predications were in good agreement with experimental data. Finally, a parameter of normalized softening rate was proposed and its characteristics were evaluated quantitatively. It can be concluded that the failure strain could be prolonged when the normalized softening rate decrease through changing the softening path.     

Evolution of internal strain during plastic deformation in magnesium matrix composites

Synchrotron radiation diffraction during in situ tensile tests has been used to evaluate the internal elastic strains within the grains of magnesium alloy, AZ31, unreinforced and reinforced with 5 and 10% volume of SiC particles. Composites present initial thermal residual stresses, which are positive (tensile) in the matrix and negative (compressive) in the reinforcing particles. Internal elastic strains evolve in a similar behaviour in the unreinforced AZ31 and in both composites. However, the accumulated elastic strains are reduced in the case of the composite because a part of the applied load is borne by the ceramic particles.     

Multilayer composites in Al2O3/MoSi2 system

A new multilayer composite with a super-plastic layer, a hard layer and a weak interface was proposed. The hard layer can provide the strength of the multilayer composites at high temperature, the plastic layer can deform plastically at high temperature and disperse the applied stress and stop the advance of the crack, and the weak interface can deflect the propagating crack at room temperature. Such multilayer composites were prepared by tape casting in the Al₂O₃/TiC/MoSi₂–Mo₂B₅ system. It was found that such design is effective on the increase of fracture energy both at room temperature and at high temperature, and the strength at high temperature could be remained in a relatively high revel.     

Flow rule for the plastic deformation of particiulate metal-matrix composites

Finite element calculations are performed on models of particulate metal-matrix composites to study the applicability of a quadratic yield function and an associated flow rule. The matrix, here taken to be Al, is described by a J₂ flow rule for an isotropic material and the reinforcing particles, either SiC or TiC, are taken to be elastic. Two different types of three-dimensional models of the composite are considered: (1) a simple cubic lattice of spherical particles and (2) random digital models that are approximately isotropic. Only in the second type of model can the existence of a flow rule be established. The validity of a flow rule in random models is associated with the absence of shear planes that extend throughout the solid without intercepting any particles. Such planes can be drawn in the simple cubic lattice for some shear deformations and particle volume fractions. Localized shear bands occurring on these planes results in a shear response essentially identical to that of the unreinforced matrix material, which precludes the determination of the shear response from the uniaxial deformation of the composite.     

Elevated temperature slow plastic deformation of NiAl-TiB2 particulate composites at 1200 and 1300K

Elevated temperature compression testing has been conducted in air at 1200 and 1300K with strain rates varying from 10^–4 to 10^–7sec^–1 on NiAl-TiB₂ particulate composites. These materials, which consisted of a B2 crystal structure intermetallic Ni-50at% Al matrix and from 0 to 30 vol % of approximately 1 m diameter TiB₂ particles, were fabricated by XD synthesis and hot pressed to full density. Flow strength of the composites increased with volume fraction of the strengthening phase with NiAl-30TiB₂ being approximately three times stronger than NiAl. Comparison of the light optical and transmission electron microstructures of asreceived and tested samples revealed that reactions did not occur between the two phases, and NiAl-TiB₂ interfaces were not cracked during deformation. Additional transmission electron microscopy indicated that the particles stabilize a vastly different microstructure in the NiAl matrix of the composites than that formed in unreinforced NiAl.     

Slip casting of Al2O3 and Al2O3/ZrO2 composites

Al₂O₃ and Al₂O₃/ZrO₂ composites have been fabricated by slip casting from aqueous suspensions. The physical and structural characteristics of the starting powders, composition of the suspensions, casting behaviour, microstructure of the green and fired bodies and the mechanical properties of the products were investigated. The addition of ZrO₂ to Al₂O₃ leads to a significant increase in fracture toughness when ZrO₂ particles are retained in the tetragonal form (transformation-toughening mechanism) but when microcracking (due to the spontaneous transformation of ZrO₂ from the tetragonal phase to the monoclinic one) is dominant, an excellent toughness value is accompanied by a drastic drop in strength and hardness.     

Cyclic oxidation of FeCrAlY/Al2O3 composites

Abstract

Three-ply composites consisting of a FeCrAlY matrix and continuous single crystal Al₂O₃ (sapphire) fibers were cyclically oxidized at 1,000° and 1,100°C for up to 1,000 1-h cycles. FeCrAlY matrix only samples were also fabricated and tested for comparison. Fiber ends were exposed at the ends of the composite samples. Following cyclic oxidation, cracks running parallel to and perpendicular to the fibers were observed on the large surface of the composite. In addition, there was evidence of increased scale damage and spallation around the exposed fiber ends, particularly around the middle ply fibers. This damage was more pronounced at the higher temperature. The exposed fiber ends showed cracking between fibers in the outer plies, occasionally with Fe and Cr-rich oxides growing out of the cracks. Large gaps developed at the fiber–matrix interface around many of the fibers, especially those in the outer plies. Oxygen penetrated many of these gaps resulting in significant oxide formation at the fiber–matrix interface far within the composite sample. Around several fibers, the matrix was also internally oxidized showing Al₂O₃ precipitates in a radial band around the fibers. The results show that these composites have poor cyclic oxidation resistance due to the CTE mismatch and inadequate fiber–matrix bond strength at temperatures of 1,000°C and above.     

Impact of graphite particle on milling of Al/15Al2O3 and Al/15Al2O3/5Gr composites

ABSTRACT

Hybrid Metal Matrix Composites (MMCs) are a new class of composites, formed by a combination of the metal matrix and more than one type of reinforcement having different properties. Machining of MMCs is a difficult task because of its heterogeneity and abrasive nature of reinforcement, which results in excessive tool wear and inferior surface finish. This paper investigates experimentally the addition of graphite (Gr) on cutting force, surface roughness and tool wear while milling Al/15Al₂O₃ and Al/15Al₂O₃/5Gr composites at different cutting conditions using tungsten carbide (WC) and polycrystalline diamond (PCD) insert. The result reveals that feed has a major contribution on cutting force and tool wear, whereas the machined surface roughness was found to be more sensitive to speed for both composite materials. The incorporation of graphite reduces the coefficient of friction between the tool–workpiece interfaces, thereby reducing the cutting force and tool wear for hybrid composites. The surface morphology and worn tool are analyzed using scanning electron microscope (SEM). The surface damage due to machining extends up to 200 µm for Al/15Al₂O₃/5Gr composites, which is beyond 250 µm for Al/15Al₂O₃ composites.     

基于同步辐射X射线的SiC颗粒/Al复合材料变形损伤

通过原位X射线成像系统研究了两种SiC粒径配比（45 μm和（45+100）μm）对70vol% SiC颗粒（SiC_P）/Al复合材料变形损伤行为的影响。在准静态压缩加载下,利用X射线数字图像相关方法（XDIC）计算了SiC_p/Al复合材料在不同变形阶段的应变场分布。宏观应力-应变曲线表明,因颗粒尺寸引起的SiC_p/Al复合材料的强度差异较小,但粒径配比为45 μm的SiC_P/Al的延展性明显优于（100+45）μm的SiC_P/Al。细观应变场分析表明,粒径配比为（100+45）μm的SiC_P/Al比45 μm的SiC_P/Al更早出现变形损伤带,且前者在变形后期其应变场不均匀性更高。这是由于（100+45）μm SiC_P/Al中更早在大颗粒附近出现应变集中点,而且这些集中点会迅速长大和汇聚进而形成宏观裂纹,导致材料更早失效和破坏。因此,减小颗粒尺寸和促进颗粒均匀分布有利于提高颗粒增强金属基复合材料的延展性。断口回收分析表明:两种颗粒尺寸的SiC_P/Al复合材料的断裂模式都属于脆性断裂,且断口中都发现有颗粒破坏和界面脱粘现象存在。      

The mechanical properties of laminated microscale composites of Al/Al2O3

The tensile properties, at both room and elevated temperatures, of laminated thin films containing alternate layers of aluminium and aluminium oxide were investigated. At room temperature the strength of the films followed a Hall-Petch type relationship dependent on the interlamellar spacing, and the strength could be extrapolated from data for conventional grain size aluminium. At the finest interlayer spacing of 50 nm, the strength was equivalent to/70, where is the shear strength of aluminium and the samples exhibited very extensive ductility. At elevated temperatures, cavitation became an important deformation mechanism but it occurred preferentially at Al/Al rather than Al/Al₂O₃ boundaries. The microstructure of the films was probed using transmission electron microscopy and fractography was used to investigate deformation and fracture mechanisms.     

The fibre distribution of Al2O3/Al–Cu alloy composites

Two methods which rely on direct microstructural measurements to assess the fibre distribution in alumina continuous fibre-reinforced Al–Cu alloy composites produced via an infiltration process, are outlined. The first is based on distance analysis, i.e. the distance distribution of nearest neighbours; and the second is based on fibre–cell structures. Specimens with two fibre volume fractions, 0.39 and 0.50, were employed in this study. It was found that the fibres in both kinds of specimen appear to have a rough thread-like distributions, and the local volume fraction of the fibres varies over a larger range in the specimen with lower fibre volume fraction than does that in the specimen with the larger one. Quantitative relationships between fibre distribution and the composite defects are deduced. Some data on the microsegregation of copper and the macrosegregation of eutectic phase are given in relation to the fibre distributions. The reasons for the uneven fibre distributions are also discussed.     

A study of the failure mechanisms in Al/Al2O3 and Al/SiC composites through quantitative microscopy

The failure mechanisms in tension and fatigue of three Al alloys reinforced with ceramic particulates were studied by means of optical and scanning electron microscopy. Damage was concentrated in the reinforcements, which failed in a brittle fashion during deformation, leading to the specimen fracture when a critical fraction of broken particulates was reached in a given section of the specimen. This critical fraction was measured on polished longitudinal sections of broken specimens for each composite, temper, and loading condition, and was mainly dependent on the matrix alloy. It was also found that the reinforcement fracture probability was controlled by the particulate size and aspect ratio: large and elongated particulates oriented in the loading direction were more prone to fail than small, equiaxed ones. Finally, a significant fraction of the reinforcements in the cast materials was broken prior to testing. They were shattered during extrusion rather than fractured, and associated with clusters of particulates formed during solidification.