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.     

The effect of SiC particle reinforcement on the creep behavior of 2080 aluminum

The influence of SiC particle reinforcement on the creep behavior of 2080 aluminum is investigated between 150 °C and 350 °C. The effect of particle size (F-280, F-600, and F-1000), volume fraction (10, 20, and 30 vol pct), and heat treatment (T6 and T8) on creep behavior is studied. In both the T6 and T8 conditions all composites are less creep resistant than similarly heat-treated monolithic materials when crept at 150 °C. These results contradict continuum mechanics predictions for steady-state creep rate, which predict composite strengthening. A high dislocation density is observed near SiC particles. It is proposed that strain localization near the reinforcements leads to microstructural breakdown and the subsequent reduction in creep resistance. When both materials are severely overaged or when they are tested at very high temperatures (350 °C), composite materials exhibit improved creep resistance relative to monolithic material. In these cases, the strengthening is consistent with continuum predictions for direct composite strengthening.     

Effect of particle size and volume fraction on hot extrusion reaction synthesis of SiC particle reinforced NiAl

Hot extrusion reaction synthesis (HERS) was used to fabricate nickel aluminide/SiC _p composites from elemental powder mixtures of nickel, aluminum, and silicon carbide. The effect of extrusion temperature, silicon carbide particle size, and volume content on the developed microstructures and on the peak extrusion pressure was investigated using a miniature extrusion rig. Matrix microcracking and loss of aluminum were observed in the final microstructures. The large surface area to volume ratio of the miniature extrudate “wires” in conjunction with a shorter reaction time at temperature reduced the reaction between the matrix and the SiC reinforcements. Although extrusion should have eliminated reaction synthesis related porosity, considerable levels of porosity were still generally present in the final extrudates, because all the elemental powder extrusions reacted after emerging from the die exit instead of before, thus by-passing the consolidation stage of extrusion. A novel two-stage extrusion method has been identified to overcome this problem.     

Effect of aluminum particle size on the volume changes experienced by compacts from a mixture of aluminum and copper powders during liquid-phase sintering

Conclusions The process of liquid-phase sintering of aluminum-copper powder compacts comprises two main stages — growth and shrinkage. Growth is due to diffusion of copper atoms from the liquid phase into the solid, with the formation of liquid interlayers at the grain and subgrain boundaries. Compacts from powder mixtures containing large aluminum particles exhibit increased growth in the first stage of sintering, which may be due to nonuniform reaction of the liquid phase on the peripheries of the particles, resulting in the appearance of local strains and further separation of the solid-phase particles.Translated from Poroshkovaya Metallurgiya, No. 9(285), pp. 23–27, Septmeber, 1986.     

SiC粒度及粉末装载量对SiCp/Al复合材料热循环行为及力学性能的影响

选用4种不同粒度的SiC分别与平均粒度为14 μm的SiC粉末混合,混炼后得到4种不同粉末装载量(体积分数)的喂料,采用粉末注射成形方法制备成SiC坯体,再无压熔渗Al～12%Si～8%Mg合金,获得高体积分数SiCp/Al复合材料,研究SiC粉末粒度及SiCp/Al复合材料中SiC的体积分数(即注射成形喂料中SiC粉...     

Thermal shock behavior of a three-dimensional SiC/SiC composite

Thermal shock behavior of a three-dimensional (3-D) SiC/SiC composite was studied using the water-quenched method. Thermal shock damage of the composite was assessed by scanning electron microscopy characterization and residual three-point-bending strength. In the thermal shock process, the composite displayed the same bending mechanical behaviors as those of the original composite and retained 80 pct of the original strength in the longitudinal direction after being quenched from 1200°C to 25°C in water for 100 cycles. However, the composite displayed anisotropy in resistance to thermal shock damage. The observed microdamage processes were as follows: (1) formation of micropores and long crack, (2) transfer and growth of pores, (3) saturation of the dimension and the density of pores, and (4) accelerated growth of the long crack along the longitudinal direction. The critical thermal shock number for the composite was about 50. When thermal shock was less than 50 cycles, the residual flexural strength of the composite decreased with thermal shock cycles increasing. When the number was greater than 50, the strength of the composite did not decrease further.     

The influence of reinforcement particle size distribution on the mechanical behavior of a stainless steel/tin composite

A stainless steel-based composite reinforced with TiN particles has been developed for potential applications in industry. The results show that the compatibility between the matrix and reinforcement is good and the bonding between the two phases is strong and of the diffusion type. The ductility and toughness of the composites are lower than those of the matrix. The strengthening behavior of the composites, however, is not straightforward. It is also found that reinforcements that differ only in particle-size distribution can bring about a large change in the mechanical properties of the composites. It is believed that the microstructural change of the matrix after incorporating the reinforcement, together with the load-bearing effect of the reinforcement, gives rise to the results.     

Effect of powder particle size on the growth of titanium compacts during liquid-phase sintering with aluminum

Conclusions In the liquid-phase sintering of a Ti+30 at. % Al powder mixture compact growth increases with increasing mean particle size. The volume of a compact from such a mixture exhibits an anomalous increase (surpassing its growth due to Kirkendall flow during unipolar diffusion of aluminum atoms from the molten phase to the titanium). The increase is attributable to growth of layers of an intermetallic compound, which sets up a stress in the particles disturbing the continuity of their material. A fall in the density of particles linked with the disturbance of the continuity of their material is characteristic of particles exceeding in size a certain critical value — in the case under consideration, about 45m. Smaller particles grow only as a result of Kirkendall flow, whose extent is determined by the concentration of aluminum in the mixture. The reason why no disturbance in the continuity of material occurs in fine particles is apparently that the intermetallic compound layers forming on them are very thin, so that the stress set up in the particles does not reach a level sufficient for the initiation and growth of discontinuities.Translated from Poroshkovaya Metallurgiya, No. 9 (225), pp. 33–37, September, 1981.     

The effect of particle reinforcement on the creep behavior of single-phase aluminum

The effect of TiC particle reinforcement on the creep behavior of Al (99.8) and Al-1.5Mg is investigated in the temperature range of 150 °C to 250 °C. The dislocation structure developed during creep is characterized in these materials. The addition of TiC increases creep resistance in both alloys. In pure aluminum, the presence of 15 vol pct TiC leads to a factor of 400 to 40,000 increase in creep resistance. The creep strengthening observed in Al/TiC/15_p is substantially greater than the direct strengthening predicted by continuum models. Traditional methods for explaining creep strengthening in particle-reinforced materials(e.g., threshold stress, constant structure, and dislocation density) are unable to account for the increase in creep resistance. The creep hardening rate(h) is found to be 100 times higher in Al/TiC/15_p, than in unreinforced Al. When incorporated into a recovery creep model, this increase inh can explain the reduction in creep rate in Al/TiC/15_p. Particle reinforcement affects creep hardening, and thus creep rate, by altering the equilibrium dislocation substructure that forms during steady-state creep. The nonequilibrium structure generates internal stresses which lower the rate of dislocation glide. The strengthening observed by adding TiC to Al-1.5Mg is much smaller than that found in the pure aluminum materials and is consistent with the amount of strengthening predicted by continuum models. These results show that while both direct (continuum) and indirect strengthening occur in particle-reinforced aluminum alloys, the ratio of indirect to direct strengthening is strongly influenced by the operative matrix strengthening mechanisms. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

The bauschinger effect in a SiC/Al composite

An interesting mechanical property of SiC/Al composites is that the tensile yield stress is less than the compressive yield stress, even though the apparent modulus in tension is greater than that in compression. An investigation was undertaken to determine if the Bauschinger effect (BE) in a SiC/Al composite is asymmetrical. It was found that the BE is indeed asymmetrical in the case of the composite and the magnitude of the BE increases with total forward strain. These results can be explained in terms of the changes in “back stress” caused by the changes in the residual stress and the work hardening during forward strain.     

碳化硅体积分数对SiCp/Al-30Si复合材料组织与性能的影响

采用真空热压烧结法制备SiC颗粒体积分数分别为20%、25%和30%的SiCp/Al-30Si复合材料。利用扫描电镜对复合材料的微观组织进行表征,并检测其力学性能及物理性能,运用Turner、Kerner理论模型对材料的热膨胀系数进行计算,分析碳化硅体积分数对SiCp/Al-30Si复合材料组织及性能的影响。研究结果表明:随SiC含量的增加,复合材料的组织中会出现SiC颗粒的团聚,使材料的致密度及抗拉强度下降,在50～100℃之间的热膨胀系数降低,其平均值与Kerner模型计算值很接近。     

The effect of volume percent and morphology of phases on the damping behavior of epoxy/aluminum composites

In this investigation, the finite element method (FEM) has been employed to predict the effects of volume percent and morphology, including size, shape, and continuity of phases, on damping behavior of epoxy/Al composites. It is shown that for a given volume percent of phases, the loss factor of the composite increases with an increase in particle size. The effect of matricity was obtained by selecting a composite with 50 vol pct of each phase and arranging, in one case, aluminum as the particle phase and, in the other case, aluminum as the matrix phase. The loss factor obtained for the former was found to be much higher. This was attributed to the ability of the epoxy phase when it is in the form of matrix to damp/deform relatively freely. The normal stress distributions and two-dimensional (2-D) hydrostatic stress distributions were also predicted. In general, the stresses were found to be higher in the stiffer aluminum phase and the stress gradients were found to increase with an increase in particle size for a given volume percent of phases. The 2-D hydrostatic stresses were also found to be higher in the stiffer aluminum phase and the stress gradients were found to increase with an increase in particle size as well. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee.     

Mechanical behavior and microstructure characterization of sinter-forged SiC particle reinforced aluminum matrix composites

A novel, low-cost sinter-forging approach to processing particle reinforced metal matrix composites for high-performance applications was examined. The microstructure of the sinter-forged composites exhibited relatively uniform distribution of SiC particles, which appeared to be somewhat aligned perpendicular to the forging direction. The degree of alignment and interparticle bond strength was not as high as that observed for the extruded composite. The sinter-forged composite exhibited higher Young’s modulus and ultimate tensile strength than the extruded material, but lower strain-to-failure. The higher modulus and strength were attributed to the absence of any significant processing-induced particle fracture, while the lower strain-to-failure was caused by poorer matrix interparticle bonding compared to the extruded material. Fatigue behavior of sinter-forged composites was similar to that of the extruded material. Fe-rich inclusions were extremely detrimental to fatigue life. Cleaner processing, which eliminated the inclusions, enhanced the fatigue life of the sinter-forged composites to levels similar to that of the extruded material.     

Effect of yttrium oxide volume fraction and particle size on elevated temperature strength of a dispersic strengthened superalloy

The effect of yttrium oxide dispersoid volume fraction and particle size on the 1400†F (1033 K) and 1900†F (1311 K) rupture strength of a dispersion strengthened nickel-base superalloy, made by mechanical alloying, was investigated. Yttrium oxide Contents ranged from 0 pct to 4.5 pct by volume, and average oxide particle sizes varied from about 150Å (15 nm) to 580Å (15 nm) to 580Å (58 nm). High volume fractions and small particle sizes gave low grain aspect ratios and poor 1900†F (1311 K) and 1400†F (1033 K) stress rupture properties following heat treatment. Rupture strengths at 1400†F (1033 K) were otherwise relatively unaffected by dispersoid parameters. At grain aspect ratios less than 6.0, 1900†F (1311 K) rupture strength was controlled by grain geometry while at higher values rupture strength was directly influenced by dispersoid parameters.     

The influence of buoyant forces and volume fraction of particles on the particle pushing/entrapment transition during directional solidification of Al/SiC and Al/graphite composites

Directional solidification experiments in a Bridgman-type furnace were used to study particle behavior at the liquid/solid interface in aluminum metal matrix composites. Graphite or siliconcarbide particles were first dispersed in aluminum-base alloysvia a mechanically stirred vortex. Then, 100-mm-diameter and 120-mm-long samples were cast in steel dies and used for directional solidification. The processing variables controlled were the direction and velocity of solidification and the temperature gradient at the interface. The material variables monitored were the interface energy, the liquid/particle density difference, the particle/liquid thermal conductivity ratio, and the volume fraction of particles. These properties were changed by selecting combinations of particles (graphite or silicon carbide) and alloys (Al-Cu, Al-Mg, Al-Ni). A model which considers process thermodynamics, process kinetics (including the role of buoyant forces), and thermophysical properties was developed. Based on solidification direction and velocity, and on materials properties, four types of behavior were predicted. Sessile drop experiments were also used to determine some of the interface energies required in calculation with the proposed model. Experimental results compared favorably with model predictions. BRU K. DHINDAW Visiting Scholar with the Solidification Laboratory at the time this work was performed.     

颗粒尺寸及分布均匀性对SiC/Al-30Si复合材料组织性能的影响

采用真空热压法制备SiCp/Al-30Si复合材料.利用扫描电镜对材料的微观组织进行表征,检测力学性能.结果表明:随着SiC颗粒平均粒径的增大,材料的组织中SiC颗粒的团聚现象逐渐消失,其在基体中的分布更加均匀.抗拉强度与增强体颗粒尺寸有关,SiC颗粒平均粒径为13 μm时,材料的抗拉强度最大.材料的断裂方式为脆性断裂,SiC颗粒粒径为4μm时,断口表面有团聚、裸露的SiC颗粒;SiC颗粒粒径为13μm时,断口SiC颗粒表面包覆着一层铝硅合金;SiC颗粒粒径为30μm时,断口处有断裂的SiC颗粒,部分SiC颗粒从基体中被拔出.