The effect of microstructure and composition on the properties of vapour quenched AlCr alloys—II. Tensile properties

The effect of chromium and iron additions and of annealing and working on the microstructure and tensile properties of vapour quenched AlCr and AlCrFe alloys has been determined. Tensile strengths of the worked AlCrFe alloys were in the range 568–831 MPa. Chromium in solid solution or iron present as iron-rich precipitates increased the yield stress by 44.7 MPa/at.%Cr and 333 MPa/at.%Fe respectively. The contributions to the yield strength of AlCr alloys were solid solution 40% and dislocation density/cell size 60% and to the yield strength of AlCrFe alloys were solid solution 25%, iron-rich precipitates 42% and dislocation density/cell size 33%. Vapour quenching may allow the more efficient use of alloying elements in the strengthening of Al-alloys and greater flexibility in obtaining the desired combination of solute concentration, particle volume fraction and particle size.     

伪半固态触变成形制备SiCp/Al电子封装材料的组织与性能

采用伪半固态触变成形工艺制备了40%、56%和63%三种不同SiC体积分数颗粒增强Al基电子封装材料,并借助光学显微镜和扫描电镜分析了材料中Al和SiC的形态分布及其断口形貌,测定了材料的密度、致密度、热导率、热膨胀系数、抗压强度和抗弯强度.结果表明,通过伪半固态触变成形工艺可制备出的不同SiC体积分数Al基电子封装材料,其致密度高,热膨胀系数可控,材料中Al基体相互连接构成网状,SiC颗粒均匀镶嵌分布于Al基体中.随着SiC颗粒体积分数的增加,电子封装材料密度和室温下的热导率稍有增加,热膨胀系数逐渐减小,室温下的抗压强度和抗弯强度逐渐增加.SiC/Al电子封装材料的断裂方式为SiC的脆性断裂,同时伴随着Al基体的韧性断裂.      

Plastic relaxation of thermoelastic stress in aluminum/ceramic composites

The dislocation generation due to a thermoelastic stress in 2024 Al/ceramic (SiC or TiC) composites was studied using transmission electron microscopy Composites containing different ceramic particulates, ceramic volume fraction, and particle size were investigated. Dislocation density profiles were measured as a function of the distance from an Al/ceramic interface and compared with those calculated from an elastoplasticity model which accounts for the volume fraction of the ceramic particles. The intensity of dislocation generation showed a strong particle size dependence: as the ceramic particle size became of the order of a micron, the intensity of dislocation generation increased significantly. With an increase in the volume fraction of the ceramic particles, the dislocation density also increased, and the dislocation structure became a more tangled arrangement. If heat dissipation was taken into account as part of the plastic work, the predicted dislocation densities of the elastoplasticity model were found to be in reasonable agreement with the measured dislocation densities of 10⁹ to 10¹⁰ cm⁻².     

Effect of SiC volume fraction and particle size on the fatigue resistance of a 2080 Al/SiC p composite

The effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated. Matrix microstructure in the composite and the unreinforced alloy was held relatively constant by the introduction of a deformation stage prior to aging. It was found that increasing volume fraction and decreasing particle size resulted in an increase in fatigue resistance. Mechanisms responsible for this behavior are described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion in the form of S’ precipitates, and the decrease in strain localization with decreasing reinforcement interparticle spacing as a result of reduced particle size. Microplasticity was also observed in the composite, in the form of stress-strain hysteresis loops, and is related to stress concentrations at the poles of the reinforcement. Finally, intermetallic inclusions in the matrix acted as fatigue crack initiation sites. The effect of inclusion size and location on fatigue life of the composites is discussed.     

陶瓷颗粒增强扩散合金化钢复合材料的微观结构和力学性能

采用传统粉末冶金工艺制备了陶瓷颗粒增强Fe?0.5Mo?1.75Ni?1.5Cu?0.7C扩散合金化钢复合材料,选用的陶瓷颗粒为SiC、TiC和TiB₂。采用光学显微镜和扫描电子显微镜观察了烧结材料微观结构,并对烧结材料的硬度、强度和摩擦磨损性能进行了测试。结果表明,由于SiC和TiB₂与基体的化学相容性好,陶瓷颗粒与基体界面结合良好;由于TiC颗粒具有极高的化学稳定性,TiC颗粒与基体界面结合情况不理想。随着陶瓷相含量（质量分数）的增加,添加SiC和TiC的烧结试样相对密度降低;添加TiB₂的烧结试样相对密度先增加后降低,当添加TiB₂质量分数为0.9%时达到最大值。随着陶瓷含量增加,添加SiC和TiB₂烧结试样的硬度增大,当陶瓷相质量分数超过1.2%时,硬度增加缓慢;添加TiC烧结试样的硬度先增加后降低,当添加TiC质量分数为0.9%时达到最大值。随着陶瓷相含量增加,添加SiC和TiC烧结试样的强度降低,少量添加SiC对强度没有明显损害;添加TiB₂烧结试样的强度先增加后降低,当添加TiB₂质量分数为0.6%时达到最大值（971.7MPa）,比基体提高了14.1%以上。添加陶瓷相对烧结钢性能的积极影响依次是TiB₂、SiC和TiC。     

A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—II. Haz microstructure and strength evolution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part II the heat and material flow models presented in the first paper (Part I) are utilized for prediction of the HAZ subgrain structure and strength evolution following welding and subsequent natural ageing. The modelling is done on the basis of well established principles from thermodynamics, kinetic theory and simple dislocation mechanics. The models are validated by comparison with experimental data, and are illustrated by means of novel mechanism maps. These show the competition between the different process variables that contribute to microstructural changes and strength losses during friction welding of AlMgSi alloys and AlSiC metal matrix composites.     

Effect of reinforcement volume fraction on mechanical alloying of Al–SiC nanocomposite powders

Abstract

Mixtures of aluminium powder and nanoscaled SiC particles (n-SiC) at various volume fractions of 0, 1, 3, 5, 7 and 10 are comilled in a high energy planetary ball mill under an argon atmosphere to produce nanocrystalline Al–SiC nanocomposites. High resolution scanning electron microscopy (HRSEM), X-ray diffraction (XRD) method, laser particle size analysis and powder density measurement were used to study the morphological changes and microstructural evolution occurred during mechanical alloying. Al–SiC composite powder with microscaled SiC particles (1 m m) was also synthesised and characterised to examine the influence of reinforcement particle size on the milling process. It was found that with increasing volume fraction of n-SiC, a finer composite powder with more uniform particle size distribution is obtained. The morphology of the particles also became more equiaxed at shorter milling times. Furthermore, the analysis of XRD patterns by Williamson–Hall method indicated that the crystallite size of the aluminium matrix decreases with increasing reinforcement volume content while the lattice strain changes marginally. As compared with microscaled SiC particles, it appeared that the effect of n-SiC on the milling stages is more pronounced. The results clearly show that the reinforcement particles influence the work hardening and fracture of the metal matrix upon milling, affecting the structural evolution. With decreasing size of the ceramic particles to nanoscale, this influence becomes more pronounced as the surface to volume fraction increases.     

Study of Tribological and Mechanical Properties of A356–Nano SiC Composites

In the present investigation, the microstructural, wear, tensile and compressive properties of Al?C7Si alloy matrix nano composites have been discussed. It is noted that the composites contain higher porosity level in comparison to the matrix and increasing amount of porosity is observed with the increasing volume fraction of the reinforcement phase in the matrix. The wear sliding test disclosed that the wear resistance of the nano SiC reinforced composites is higher than that of the unreinforced alloy. It is believed that the presence of SiC particles could shield the matrix and silicon phase from directly experiencing the applied load from the counterface. It was revealed that the presence of nano-SiC reinforcement also enhanced the hardness, tensile and compressive yield strength of Al?C7Si alloy which can be attributed to small particle size and good distribution of the SiC particles and grain refinement of the matrix. The highest yield strength and UTS was obtained by the composite with 3.5?vol% SiC nano-particles. The results show that the addition of nano-particles reduces the elongation of A356 alloy.     

不同粒径的SiC体积配比对SiC_p/Al基复合材料显微组织和拉伸性能的影响

为了研究不同粒径的Si C体积配比对SiC_p/Al基复合材料显微组织及拉伸性能的影响,采用高压扭转法(High-pressure torsion,HPT)将3.5μm(小)、7.0μm(大)SiC颗粒体积比分别为4∶1、1∶1、1∶4的SiC颗粒和纯Al粉末混合物制备成10%SiC_p/Al基复合材料(体积分数)。用金相显微镜、万能试验机、扫描电镜等分析2种粒径的Si C体积比对SiC_p/Al基复合材料显微组织和拉伸性能的影响。结果表明,随扭转半径增大,各试样的SiC颗粒分布更加均匀,颗粒团聚、偏聚现象减少,其中小、大SiC颗粒体积比为1∶1的试样性能最优,伸长率、相对密度最高,分别达到14.3%和99.1%,拉伸断裂形式为塑性断裂。     

SiC含量对SiCp/Al复合材料摩擦性能的影响

采用粉末冶金技术制备了SiC_p/Al复合材料,探讨了SiC颗粒质量分数对SiC_p/Al复合材料密度、布氏硬度、微观形貌以及摩擦磨损性能的影响。结果表明,SiC颗粒表面形成了少量可提高界面结合性的Al₄C₃化合物。随着SiC质量分数增加,SiC_p/Al复合材料的密度没有明显的变化,当SiC质量分数增加至25%时,密度明显下降。SiC_p/Al复合材料的布氏硬度随着SiC质量分数的增加呈先增长后减小的变化趋势。当SiC质量分数为20%时,材料的硬度最优（HBW 114）,平均摩擦系数达到最大值（0.3425）,摩擦后试样表面形貌平整且犁沟较浅,SiC颗粒未出现明显剥落。     

增强颗粒对颗粒增强铝基复合材料强度的影响

通过ASHALBY等效夹杂理论分析复合材料受载时作用在增强体上的应力,并假设增强体的断裂符合WEIBULL分布,在综合考虑复合材料各种强化机制的基础上引入增强体断裂对材料屈服强度的影响,建立了一个复合材料的屈服强度模型,将其应用于SiC颗粒增强AJ基复合材料,发现在屈服状态下复合材料的颗粒断裂分数随着增强体的体积含量和粒度的增加而增加,但增强体粒度变化对颗粒断裂影响更大。同时发现WEIBULL常数m取3时模型预测强度值与实测强度吻合得很好。     

Anin situ HVEM study of dislocation generation at Al/SiC interfaces in metal matrix composites

ANNEALED aluminum/silicon carbide (Al/SiC) composites exhibit a relatively high density of dislocations, which are frequently decorated with fine precipitates, in the Al matrix. This high dislocation density is the major reason for the unexpected strength of these composite materials. The large difference (10:1) between the coefficients of thermal expansion (CTE) of Al and SiC results in sufficient stress to generate dislocations at the Al/SiC interface during cooling. In thisin situ investigation, we observed this dislocation generation process during cooling from annealing temperatures using a High Voltage Electron Microscope (HVEM) equipped with a double tilt heating stage. Two types of bulk annealed composites were examined: one with SiC of discontinuous whisker morphology and one of platelet morphology. In addition, control samples with zero volume percent were examined. Both types of composites showed the generation of dislocations at the Al/SiC interface resulting in densities of at least 10¹³ m^-2. One sample viewed end-on to the whiskers showed only a rearrangement of dislocations, whereas, the same material when sectioned so that the lengths of whiskers were in the plane of the foil, showed the generation of dislocations at the ends of the whiskers on cooling. The control samples did not show the generation of dislocations on cooling except at a few large precipitate particles. The results support the hypothesis that the high dislocation density observed in annealed composite materials is a result of differential thermal contraction of Al and SiC. The SiC particles act as dislocation sources during cooling from annealing temperatures resulting in high dislocation densities which strengthen the material.     

细晶强化和位错强化对中锰马氏体钢的强化作用

 研究了碳和锰含量对淬火中锰马氏体钢的位错密度、残余奥氏体含量、晶粒尺寸等组织结构以及室温力学性能的影响。借助于SEM、EBSD、TEM和XRD表征了材料的微观组织,探讨了马氏体钢的强化机制。结果表明:随着碳含量增加,淬火中锰钢的位错密度和残余奥氏体体积分数逐渐增加,板条束和板条块尺寸逐渐细化,大角晶界百分数逐渐增加,强度逐渐升高;增加锰含量能够提高马氏体钢的位错密度和抗拉强度。分析认为,位错强化和细晶强化是淬火中锰马氏体钢的主要强化机制。马氏体板条尺寸是马氏体抗拉强度的结构控制单元,而原奥氏体晶粒尺寸则是马氏体屈服强度的结构控制单元。     

Mechanical Behavior of Al-SiC Nanocomposites Produced by Ball Milling and Spark Plasma Sintering

Al-SiC nanocomposites were prepared by high energy ball milling of mixtures of pure Al and 50-nm-diameter SiC nanoparticles, followed by spark plasma sintering. The final composites had grains of approximately 100 nm dimensions, with SiC particles located mostly at grain boundaries. The samples were tested in uniaxial compression by nano- and microindentation in order to establish the effect of the SiC volume fraction, stearic acid addition to the powder, and the milling time on the mechanical properties. The results are compared with those obtained for pure Al processed under similar conditions and for AA1050 aluminum. The yield stress of the nanocomposite with 1 vol pct SiC is more than ten times larger than that of AA1050. The largest increase is due to grain size reduction; nanocrystalline Al without SiC and processed by the same method has a yield stress seven times larger than AA1050. Adding 0.5 vol pct SiC increases the yield stress by an additional 47 pct, while the addition of 1 vol pct SiC leads to 50 pct increase relative to the nanocrystalline Al without SiC. Increasing the milling time and adding stearic acid to the powder during milling lead to relatively small increases of the flow stress. The hardness measured in nano- and microindentation experiments confirms these trends, although the numerical values of the gains are different. The stability of the microstructure was tested by annealing samples to 423 K and 523 K (150 °C and 250 °C) for 2 hours, in separate experiments. The heat treatment had no effect on the mechanical properties, except when treating the material with 1 vol pct SiC at 523 K (250 °C), which led to a reduction of the yield stress by 13 pct. The data suggest that the main strengthening mechanism is associated with grain size reduction, while the role of the SiC particles is mostly that of stabilizing the nanograins.     

Investigation on compressibility of Al–SiC composite powders

Abstract

The consolidation behaviour of particulate reinforced metal matrix composite powders during cold uniaxial compaction in a rigid die was studied. Al–SiC powder mixtures with varying SiC particle size, ranging from nanoscale (50 nm) to microscale (40 µm), at different volume fractions up to 30% were used. Based on the experimental results, the effect of the reinforcement particles on the densification mechanisms, i.e. particle rearrangement and plastic deformation, was studied using modified Cooper–Eaton equation. It was found that by increasing the reinforcement volume fraction or decreasing its size, the contribution of particle rearrangement on the densification increases while the plastic deformation becomes restricted. In fact, when percolation network of the ultrafine reinforcement particles is formed, the rearrangement could be the dominant mechanism of consolidation. It was also shown that at tap condition and at the early stage of compaction where the particle rearrangement is dominant, the highest density is achieved when the reinforcement particle size is properly lower than the matrix (0˙3<the size ratio<0˙5) and the fraction of hard particles is relatively low (<10%). At high compaction pressures, the reinforcement particles significantly influence the yield pressure of composite powders, thereby retarding the densification.     

Fracture at elevated temperatures in a particle reinforced composite

The tensile fracture behavior of a cast and extruded 2014 aluminum alloy metal matrix composite (MMC) reinforced with 10, 15, and 20 vol.% aluminum oxide particles was investigated as a function of temperature between 100 and 300°C and hold time, and compared with the unreinforced alloy. In addition, the effect of aging condition was investigated in a 15 vol.% composite tested at 200°C. At lower temperature the composites have higher yield strength and UTS than the unreinforced material, and both decrease with increasing temperature. At higher temperatures all the materials have similar strength levels. The elongation is lower in the composites, decreasing with increasing level of reinforcement and increasing with increasing temperature, except at the highest temperature where all the composites are about the same. The microstructural damage in the composites also varies with temperature: particle fracture dominates at lower temperatures and interparticle voiding is the main damage feature at elevated temperatures. The time at temperature, and hence the degree of overaging, has little effect on the observed trends in the composite, in contrast with the unreinforced material where the density of voids decreases with increasing hold times. The transition temperature where the major damage changes from particle cracking to interparticle voiding increases with volume fraction and particle size, and decreases with overaging. The cracked particle density and void density both increase with strain, and the highest rate of increase occurs in the overaged material. In general, the tendency for particle cracking is reduced and for interparticle voiding is increased by any factor which permits accomodation of strain by the matrix, such as lower volume fraction of particles, small particle size, nonclustered particle distribution, and matrix softening from underaging or overaging.     

Residual stresses in alumina-SiC nanocomposites

Residual stresses in an alumina-SiC particulate composite were studied as a function of SiC content by X-ray diffraction. The average microstresses in each phase and the stress fluctuations in the matrix were evaluated from a combination of X-ray reflection shift and line broadening analysis. The measured average microstresses show good agreement with those calculated theoretically from a simple model. The average microstresses in the matrix increases with the SiC content while the fluctuations of the stress field decrease. In effect, the mean dislocation density in the alumina matrix increases with the SiC volume fraction with more dislocations forming networks and subboundaries, as was confirmed by transmission electron microscopy.     

Enhancement of strength and superplasticity in a 6061 Al alloy processed by equal-channel-angular-pressing

Pre-equal-channel-angular-pressing (ECAP) solution treatment combined with post-ECAP aging treatment has been found to be effective in enhancing the room-temperature strength of 6061 aluminum alloy. The largest increase in ultimate tensile strength (UTS) (=460 MPa) and yield stress (YS) (=425 MPa) is obtained in post-ECAP aged 6061 Al with six pressings. The strength increases by a factor of 1.4 when compared to T6 treated commercial 6061 Al. The strength of 6061 Al obtained in the present research is higher than that of ECA-pressed 6061 Al with pre-ECAP peak-aging treatment studied by other investigators. The more effective strengthening of post-ECAP low-temperature aging may be linked with the higher dislocation accumulation rate in the solutionized matrix and the presence of higher density particles in the aged matrix. Modest low-temperature (523 K) and high-temperature (813 K) superplasticity is observed in the ECAP 6061 Al, which may be a result of increased grain bundary area from grain refinement.