Characteristics of AA6061/BN composite fabricated by pressureless infiltration technique

Spontaneous infiltration and strengthening behaviors were analyzed in terms of microstructures and tensile properties of AA6061/BN composite fabricated by pressureless infiltration technique in the presence of both Mg and nitrogen. The microstructure and properties were compared with control AA6061 without BN fabricated by the same method. The Mg₃N₂ formed by the reaction of Mg vapor and nitrogen gas, which coated the particles in the powder bed, is believed to induce spontaneous infiltration through greatly enhancing wetting by means of the reaction Mg₃N₂ + 2Al → 2AlN + 3Mg. This was identified by the finding of AlN particle layers on the surfaces of prior Al particles in the powder bed, which made contact with the infiltrating melt. In addition, unreacted Mg₃N₂ was observed outside the composite, where the Al melt did not come into direct contact. Fine AlN particles formed in situ resulted in significant strengthening, even in the control alloys with no addition of BN. In the composite reinforced with BN, additional AlN was formed by the interfacial reaction of the BN and Al melt as well as by the in situ reaction. Consequently, both the BN particles and the additional AlN particles formed by the interfacial reaction led to a further strengthening in the composite, as compared to the control alloy, which was strengthened only by the AlN particles formed in situ. In addition, the flake shape of BN may have lent considerable strength, due to the high aspect ratio it demonstrates, as compared with that of a spherically shaped particle.     

Fabrication of SiC-Particles-Shielded Al Spheres upon Recycling Al/SiC Composites

Wettability of liquid A359 alloy on SiC particles under molten salt NaCl-KCl-NaF is found at 180 deg, meaning that SiC particles prefer the molten salt phase against the Al phase or the Al/molten salt interface. Thus, this molten salt can be used for recycling, i.e., to separate the phases in the SiC reinforced Al matrix composites. If the separation process is interrupted, Al droplets (submillimeter solidified powder) can be produced, stabilized/surrounded by a monolayer of shielding SiC particles.     

Analysis of interfacial shear strength of SiC fiber reinforced 7075 aluminum composite by pushout microindentation

The interfacial shear strength of continuous silicon carbide fiber reinforced 7075 aluminum matrix composite (SiC_f/7075Al) has been investigated in this research by pushout microindentation. The SiC_f/7075Al composite specimens were processed by diffusion bonding alternate layers of SiC fibers and 7075Al alloy plates. From the measured stress-displacement curves of indentation tests, the interfacial shear strengths of the composite specimens were obtained, and the stress-displacement curves were basically divided into two regions: (1) elastic deformation and (2) interface decohesion and fiber sliding. With increasing aging time, the interfacial shear strength of the composite increased to 167 MPa for T6-treated specimens, and the variation of the interfacial shear strength well followed that of the ultimate tensile strength of 7075Al matrix alloy. With decreasing specimen thickness, the interfacial shear strength of the composite and the amplitude of stress fluctuation slightly decreased because of the stress relaxation effect near specimen surfaces. Under higher indentation velocities, both the interfacial shear strength and the amplitude of stress fluctuation became smaller.     

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.     

On the infiltration behavior of Al, Al-Li, and Mg meltas through SiC p bed

Aluminum, Al-Li(8090), and Mg matrix composites with uniform distributions of SiC _p reinforcement have been prepared by the vacuum infiltration technique. The infiltration kinetics have been found to increase in the order of Al, Al-Li, and Mg. The Al-Li alloy and Mg as matrix materials have shown improved wettability with SiC _p in comparison to Al, leading to enhanced infiltration kinetics and reduced reinforcement degradation in the former cases. The infiltration kinetics are insensitive to preheat temperature beyond a critical temperature, which is close to the melting point of the matrix. A marginal improvement in infiltration kinetics could be obtained with Cu and Ni coating on SiC and on its dynamic oxidation. The improvement is significant at a higher preheat temperature of SiC. The Vickers hardness, measured on the SiC particles, has been shown to be an index of the strength of the interface between the matrix and reinforcement in the composite.     

Microstructure and mechanical properties of SiC particles reinforced Mg–8Al–1Sn magnesium matrix composites fabricated by powder metallurgy

The new type of Mg–8Al–1Sn (AT81) magnesium matrix composites reinforced with different volume fractions (5, 10, 15, 20, 25 and 30 vol.-%) of SiC particles (average size of 10 μm) was fabricated by powder metallurgy. With the increasing volume fraction of SiC particles (SiC_p), the particles gradually show more homogeneous distribution. Compared with the AT81 alloy, the yield strength (YS) and ultimate compressive strength of the SiC_p/AT81 composites are improved simultaneously. With the increasing SiC_p from 0 to 30 vol.-%, the YS and ultimate compressive strength increase from 69 to 239 MPa and 286 to 385 MPa respectively, while the corresponding fracture strain (ε) decreases from 19·3 to 4·8%. The improvement of the YS and ultimate compressive strength of the SiC_p/AT81 composites benefits from the more homogeneous microstructure due to the increase in the SiC particles.     

Superplasticity in a high strength powder aluminum

The superplastic properties of a rapidly solidified, high strength P/M Al alloy and the same alloy reinforced with SiC particulates (SiC _p ) have been studied. To prepare superplastic test materials, a matrix alloy powder of composition 7.2Zn-2.4Mg-2Cu-0.2Zr-0.12Cr-0.2Co (Kaiser PM-64) and the powder mixed with 10 to 20 vol pct SiC _p (~5 μm diameter) were thermomechanically processed to very fine equiaxed grain structures of ~6 μm and ~8 μm, respectively. Superplasticity in these materials was evaluated by characterizing (1) high temperature stability, (2) dynamic grain growth, (3) strain rate sensitivity, (4) flow stress behavior, (5) cavitation and cavitation control, and (6) total superplastic strain. It was observed that the PM-64 alloy could achieve a total elongation of over 800 pct, while the SiC_p reinforced alloy could attain an elongation greater than 500 pct before failure. Also, it was shown that with the use of hydrostatic pressure during superplastic flow, cavitation could be controlled. Observations were made of the effect SiC _p reinforcement particles had on the superplastic flow stress behavior. Interpretations are proposed to explain the role of particulates during superplastic straining.     

纳米SiC增强纯Al基复合材料的微观组织和力学性能

与采用微米尺度SiC颗粒为增强相制备的Al基复合材料相比,以纳米SiC颗粒为增强相制备的Al基复合材料具有更加优异的力学性能,可极大提高SiC增强Al基复合材料的服役可靠性及应用范围。采用传统粉末冶金方法制备纳米SiC颗粒增强纯Al基复合材料,研究烧结温度和增强相体积分数对复合材料微观结构和力学性能的影响。研究表明,烧结温度和增强相体积分数均对复合材料的微观结构和力学性能有重要影响。随烧结温度升高,复合材料中的残留微孔减少,密度和强度均得到显著提高。含体积分数为3%纳米SiC颗粒的复合材料在610℃具有最高的强度,进一步提高纳米SiC颗粒的含量并不能提高材料的力学性能,这主要是由于当纳米SiC颗粒的体积分数超过3%时将出现明显的团聚,从而降低强化效应。     

Fabrication and characteristics of AA6061/Si3N4p composite by the pressureless infiltration technique

The tensile properties and microstructures of AA6061/Si₃N₄ particle composites fabricated by pressureless infiltration under a nitrogen atmosphere were analyzed. In addition, the control AA6061 without Si₃N₄ particles fabricated by the same method was investigated to separate the effect of Si₃N₄ particle addition. It was found that AlN particle layers formed on the surface of Al particles in the powder bed, which replaced the Mg₃N₂ coated layers through the following reaction: Mg₃N₂ + 2Al → 2AlN + 3Mg. Thus, the spontaneous infiltration results from a great enhancement of wetting via the formation of Mg₃N₂ by the reaction of Mg vapor and nitrogen gas. The increased tensile strength and 0.2 pct offset yield strength in the control AA6061 were largely due to fine AlN particles formed by the aforementioned in situ reactions, as compared to commercial AA6061. In the composite reinforced with Si₃N₄ particles, of course, the AlN was also formed through the following additional reaction at the Si₃N₄ particle/Al melt interfaces: Si₃N₄ + 4Al → 4AlN + 3Si. However, this AlN may not contribute to the increase in strength because its formation is compensated by the consumption of Si₃N₄ particles. Consequently, the strength increase of the composite fabricated by the present method is attributed to the fine AlN particles formed in situ, as well as the fine reinforcing Si₃N₄ particles, as compared to commercial AA6061.     

碳含量对690合金组织和力学性能的影响及强化机制

 研究了不同碳含量对脱敏态690合金显微组织和力学性能的影响,并对合金的强化机制进行了分析。研究结果表明：碳质量分数在0.011%～0.033%的范围内时,随着其含量的增加,脱敏态690合金的晶界析出物形貌由细小半连续的颗粒逐渐转变为粗大分散的颗粒;主要析出物为M23C6富铬碳化物,M23C6析出物的质量分数从0.181%增加到0.541%。同时,随碳含量的增加,脱敏态690合金的平均晶粒尺寸从23μm减少到10μm;室温抗拉强度从702.5MPa增加到810MPa;屈服强度从300MPa增加到402.5MPa。细晶强化是690合金的主要强化方式。     

Processing,microstructure, and mechanical properties of cast <Emphasis Type="Italic">in-Situ</Emphasis> Al(Mg,Mn)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>(MnO<Subscript>2</Subscript>) composite

Cast particulate composites, containing in-situ generated reinforcing particles of alumina, have been developed by solidification of slurry obtained by dispersion of externally added manganese dioxide particles (MnO₂) in molten aluminum, and alumina is formed by reaction of manganese dioxide with molten aluminum. The chemical reaction also releases manganese into molten aluminum. Magnesium is added to the melt in order to help wetting of alumina particles by molten aluminum and to retain the particles inside the melt. The present work aims to understand the influence of key parameters such as processing temperature, time, and the amount of MnO₂ particles added on the microstructure and mechanical properties of the resulting cast in-situ composites. The sequence of addition of MnO₂ particles and magnesium has significant influence on the microstructure and mechanical properties. Increasing processing temperature and time increases the extent of reduction of MnO₂ particles, generating more alumina particles as well as releasing more manganese to the matrix alloy. Alumina helps to nucleate finer and sometimes blocky MnAl₆ in the matrix of the composite and thereby results in relatively higher ductility and increased strength in the composite as compared to the base alloy of similar composition. Even in the presence of relatively higher porosity of 8 to 9 vol pct, one observes a percent elongation not below 7 to 8 pct, which is considerably higher than those observed in cast Al(Mg)-Al₂O₃ composite synthesized by externally added alumina particles.     

Effect of Annealing on Thermal Stability, Precipitate Evolution, and Mechanical Properties of Cryorolled Al 7075 Alloy

The effect of annealing on microstructural stability, precipitate evolution, and mechanical properties of cryorolled (CR) Al 7075 alloy was investigated in the present work employing hardness measurements, tensile test, X-ray diffraction (XRD), differential scanning calorimetry (DSC), electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). The solution-treated bulk Al 7075 alloy was subjected to cryorolling to produce fine grain structures and, subsequently, annealing treatment to investigate its thermal stability. The recrystallization of CR Al 7075 alloys started at an annealing temperature of 423 K (150 °C) and completed at an annealing temperature of 523 K (250 °C). The CR Al 7075 alloys with ultrafine-grained microstructure are thermally stable up to 623 K (350 °C). Within the range of 523 K to 623 K (250 °C to 350 °C), the size of small η phase particles and AlZr₃ dispersoids lies within 300 nm. These small precipitate particles pin the grain boundaries due to the Zener pinning effect, which suppresses grain growth. The hardness and tensile strength of the CR Al 7075 alloys was reduced during the annealing treatment from 423 K to 523 K (150 °C to 250 °C) and subsequently it remains constant.     

Tribological Behavior of SiC Particulate Reinforced AA5754 Matrix Composite Under Dry and Lubricated Conditions

The present work dealt with an investigation on tribological behavior of AA5754 matrix reinforced with 10 wt% SiC particles composite by using a pin on disc machine. Sliding tests were conducted under dry and oil lubricated conditions against hardened DIN 100Cr6/EN31 steel counter surface. Wear rates of the matrix alloy and the metal matrix composite (MMC) were measured over load of 100 N under a speed of 4.71 m/s. Detailed scanning electron, optical microscopy analyses were undertaken to clarify the effect of SiC particles on tribological behavior. SiC reinforced AA5754 alloy exhibited lower wear rate than unreinforced alloy.     

Powder metallurgy approach for control of microstructure and properties in high strength aluminum alloys

High-strength products made from atomized Al-Zn-Mg-Cu-Co alloy powders have good combinations of strength, ductility, resistance to stress-corrosion cracking and fracture toughness. Powder Metallurgy (PJM) methods produce fine metallurgical structures and compositions which cannot be produced by Ingot Metallurgy (IJM) methods. Fine structures result from very rapid solidification and from the effect of fine dispersoids in restricting grain growth. Stress-corrosion cracking (SCC) performance is favored by grain morphology of PJM products. Co₂Al₉ particles in PJM products are 0.02 to 2.0 μm spheroids occurring frequently on grain boundaries where they may serve several functions in slowing SCC attack. Oxide particles are irregular shapes, 0.01 to 0.04 μm in size, occurring in clusters at grain boundaries and in grain bodies. Some of the oxide particles are magnesium oxide and alter the environment in a SCC crack to arrest attack. Porosity is not a significant factor in the structure of PJM products made by a vacuum compacting process. P/M wrought products have superior combinations of high strength and stress-corrosion cracking resistance compared to IJM 7075 and 7050 alloys. While equaling the fracture toughness of 7075 alloy, the PJM products at present have somewhat lower fracture toughness than 7050 alloy, due in part to a larger amount of second-phase particles in the form of Co₂Al₉ and oxide.     

MgO/HA复合材料的强韧化与冷处理

以多种不同粒径的MgO颗粒为第二相,以HA为基体,采用无压烧结法制备MgO/HA复合材料;研究MgO粒径与MgO/HA复合材料的抗弯强度和断裂韧性之间的关系,探讨冷处理对复合材料性能的影响。结果表明:添加适宜粒径的MgO颗粒能够提高HA复合材料的抗弯强度和断裂韧性,其断裂韧性可达基体断裂韧性的1.5倍,抗弯强度可达基体抗弯强度的1.29倍,MgO颗粒增韧的粒径范围为15～35μm,增强的粒径范围为<25μm。冷处理可以进一步提高复合材料的强度和韧性,而且可以改变增韧和增强的MgO粒径范围,使增强与增韧粒径的重叠范围变宽。     

颗粒尺寸对真空浸渗SiC/Al复合材料性能的影响

采用无压浸渗法制备了SiC/Al复合材料,考察了复合材料中SiC颗粒尺寸对复合材料的组织结构、抗弯强度、摩擦磨损性能的影响.结果表明:随着SiC颗粒尺寸的减小,SiC/Al复合材料的残余气孔率逐渐减小,密度和抗弯强度逐渐增加;粒度配比有利于提高复合材料的抗弯强度.与灰铸铁配副时,材料的摩擦系数与磨损率明显依赖于碳化硅颗粒尺寸,二者均随颗粒尺寸的增大而先降低后增大.粒度配比能明显改善复合材料的干摩擦磨损性能.粗细颗粒的粒度配比具有相互强化的作用,有利于降低摩擦系数和磨损率,并使其趋于稳定.     

Growth behavior of microstructurally short cracks in the 6061 aluminum alloy with and without 22 Vol Pct SiC whiskers

Short crack growth behavior of the 6061 Al alloy with and without SiC whiskers was investigated. Fluctuations in the growth rate of short cracks converge with growth of the cracks and become substantially constant between 25 and 40 μm in the metal matrix composite (MMC) and 110 and 183 μm in the unreinforced alloy. This is attributed to the release of the short cracks from the microstructural effects,i.e., the interaction with reinforcement structure in the MMC and grain boundaries in the unreinforced alloy. Furthermore, there exists slowing down of short crack advance in the MMC, and this was explained from rapid development of crack closure obtained in this study.     

混料时间和挤压对SiC增强纯Al基复合材料显微组织和力学性能的影响

采用粉末冶金法制备SiC颗粒增强工业纯Al基复合材料,研究混料时间和挤压对复合材料显微组织和力学性能的影响。研究表明:机械混粉过程存在最佳的混料时间,混料时间为16 h时SiC颗粒分布均匀,复合材料的密度高、力学性能好。挤压可以改善复合材料的界面结合强度、减少孔洞的数量,从而提高材料的致密度和力学性能。烧结态复合材料的断裂机制以基体的脆性断裂以及增强相与基体的界面脱粘为主。挤压态复合材料的断裂以基体的韧性断裂以及SiC颗粒的脆性断裂为主,伴随着少量的基体与SiC颗粒的界面脱粘。     

Al-5 Ti-1 B细化剂对7075铝合金组织与性能的影响

研究了Al-5 Ti-1 B细化剂对7075铝合金组织与性能的影响。结果表明：添加微量的Al-5 Ti-1 B细化剂可使7075铝合金的铸态组织从粗大的枝晶细化为细小均匀的等轴晶,随着Al-5 Ti-1 B细化剂添加量的逐渐增加,7075铝合金的晶粒平均直径逐渐减小,抗拉强度和伸长率逐渐提高。当添加0．3％的Al-5Ti-1B细化剂时,7075铝合金被细化为平均直径44．7μm的等轴晶,抗拉强度和伸长率分别提高至311 MPa和4．1％,与未添加Al-5 Ti-1 B细化剂相比,7075铝合金的晶粒平均直径下降了64．3％,抗拉强度和伸长率分别提高了13．1％和45．4％。     

液态反应合成Mg-Li-MgO/Mg_2Si复合材料的组织与性能

用ＤＴＡ对ＳｉＯ２ 与ＭｇＬｉ 合金反应合成复合材料的热力学进行了研究, 证明反应能够进行。检测结果表明反应生成的粒子尺寸细小且分布均匀。复合材料的强度、硬度、弹性模量明显提高; 该复合材料的延伸率低于基体合金, 但仍可达到较高水平（ ＞ ４％） , 高于Ａｌ２Ｏ３ 及ＳｉＣ纤维增强复合材料。