High temperature creep of silicon carbide particulate reinforced aluminum

The effect of stress on the creep properties of 30 vol.% silicon carbide particulate reinforced 6061 aluminum (SiC_p-6061 Al), produced by powder metallurgy, has been studied in the temperature range of 618–678 K. The experimental data, which extend over seven orders of magnitude of strain rate, show that the creep curve exhibits a very short steady-state stage; that the stress exponent, n, is high (n > 7.4) and increases with decreasing the applied stress; and that the apparent activation energy for creep, Q_a, is much higher than the activation energy for self-diffusion in aluminum. The above creep characteristics of SiC_p-6061 Al are similar to those reported for dispersion strengthened (DS) alloys, where the high stress exponent for creep and its variation with stress are explained in terms of a threshold stress for creep that is introduced by the dispersoid particles. Analysis of the creep data of SiC_p-6061 Al using the various threshold stress models proposed for DS alloys indicates that the threshold stresses introduced by the SiC particulates are too small to account for the observed creep behavior of the composite. By considering an alternate approach for the source of the threshold stress in SiC_p-6061 Al, an explanation for the asymptotic behavior of the creep data of the composite is offered. The approach is based on the idea that the oxide particles present in the Al matrix, as a result of manufacturing the composite by powder metallurgy, serve as effective barriers to dislocation motion and give rise to the existence of a threshold stress for creep.     

Residual stresses in silicon carbide particulate reinforced aluminum composites

In metal matrix composites (MMCs) residual stresses are unavoidable during cooling from high temperature in fabrication or heat treatment because of the difference in the thermal expansion coefficients between the matrix and the reinforcement. In particle reinforced MMC the residual stresses have been proved to be hydrostatic in this study by both experiments and mathematical analysis. A very slight surface effect on the measured stresses was predicted in the case Cu Kα radiation was used. The residual stresses were determined to be tensile in the Al matrix and compressive in the reinforcement. A reduction in residual stress magnitudes of both the matrix and reinforcement was observed after the sample was cooled into liquid nitrogen and heated back to room temperature, which is believed to be caused by plastic deformation of the matrix in low temperature treatment.     

Mechanical properties of a metallic composite material based on an aluminum alloy reinforced by dispersed silicon carbide particles

The mechanical properties of a composite material with a matrix of aluminum alloy D16 reinforced with dispersed silicon carbide particles have been studied. The physicomechanical properties (density, elastic modulus, ultimate tensile strength, and limiting strains) of the composite material with various filler contents are determined experimentally. The experimental results are compared to the results of a theoretical simulation obtained using elastic and elastoplastic models of the composite material. The experimental and the calculated mechanical properties of the composite material with the volume content of the filler up to 30% agree well with each other.     

Recrystallization microstructure in cold-rolled aluminum composites reinforced by silicon carbide whiskers

Recrystallization behavior has been studied in 50 and 90 pct cold-rolled, silicon carbide whisker-reinforced aluminum composites containing fine aluminum-oxide particles. The micro-structure in the cold-worked state, in the early stage of recrystallization, and after the completion of recrystallization was examined by transmission and scanning electron microscopy and light microscopy. Recrystallization kinetics were studied by microhardness measurement. In the cold-worked state, it was found that the presence of SiC whiskers reduced the cell size, increased the hardness, and altered the distribution of dislocations. During recrystallization, the SiC whis-kers, often present in groups, showed a strong tendency to stimulate nucleation, increasing the number of nuclei and lowering the recrystallization temperature. The recrystallization kinetics in the composite were accelerated; however, the grain refinement effect of SiC was limited, apparently due to the presence of the fine aluminum-oxide particles. The structural observation and kinetics have been discussed and related to results from previous studies of dispersion-strengthened aluminum/aluminum-oxide materials and aluminum of commercial purity.     

High temperature deformation of an alumina composite reinforced with silicon carbide whiskers

Four-point bending creep tests were carried out in air on an alumina matrix composite reinforced with 9.3 vol.% of silicon carbide whiskers. Typical three-stage creep was observed. In the temperature range of 1673–1823 K, the composite exhibited an average stress exponent of 3.8. The activation energy for creep was estimated as ∼820–830 kJ mol⁻¹. Microstructure of the composite was characterized before and after deformation. Dislocation networks and other configurations were observed in samples deformed to large strains. It is concluded that the deformation mechanism consists of intragranular dislocation movement controlled by the lattice diffusion of oxygen ions.     

SiC颗粒增强铝基复合材料的制备工艺和性能研究

采用粉末冶金法制备SiCp/6061Al复合材料,研究热压温度、球磨工艺参数和SiC颗粒(SiCp)体积分数对SiC颗粒增强铝基复合材料性能的影响,测试其力学性能及物理性能,用扫描电镜对材料的微观组织和断口进行观察。结果表明:540℃是较适合的热压温度;随着SiCp含量的增加,复合材料的致密度、热膨胀系数下降,抗拉强度先提高后迅速降低。     

The infiltration of aluminum into silicon carbide compacts

Although liquid-metal processing of metal matrix composites offers economic advantages, problems related to the nonwetting nature of the ceramic discontinuous reinforcement create obstacles to its ready implementation. Infiltration can occur only if a threshold pressure is applied to overcome the unfavorable interfacial forces in the system. The research reported in this paper has been devoted primarily to experiments on infiltrating silicon carbide compacts with pure aluminum, aluminum-1 wt pet magnesium, and aluminum-1 wt pet silicon. The major finding has been that an incubation time is necessary before infiltration can proceed, even though the threshold pressure is exceeded. Thus, while the model equations available for predicting the infiltration rate of compacts appear to be adequate, the incubation time can represent the rate-determining step in the process. It is suggested that the mechanism responsible for the incubation phenomenon may be related to a surface modification produced by either reaction of liquid aluminum with an oxide film on the surface of the particles or coverage of the surface by a capillarity-induced aluminum condensate.     

Optimum parameters for wetting silicon carbide by aluminum alloys

The effect of magnesium and silicon additions to aluminum, free silicon on the SiC substrate, nitrogen gas in the atmosphere, and process temperature on the wetting characteristics of SiC by aluminum alloys are investigated using the sessile drop technique. The contribution of each of these parameters and their interactions to the contact angle, surface tension, and driving force for wetting are determined. In addition, an optimized process for enhanced wetting is suggested and validated. Results show that the presence of free silicon on the surface of SiC significantly reduces the contact angle between the molten alloy and the substrate. The positive effect of silicon on the contact angle is attributed to a chemical reaction in which both SiC and aluminum are active participants. The results also indicate that nitrogen gas in the atmosphere positively influences the liquid/vapor surface tension, and the presence of magnesium in the aluminum alloy favorably affects the overall driving force for wetting. A mechanism is proposed to explain the beneficial role that the interaction of nitrogen with magnesium plays in enhancing wetting. Magnesium significantly reduces the surface tension of aluminum melts but has a low vapor pressure. Consequently, it readily volatilizes during holding at the processing temperature and is lost from the alloy. It is proposed that a series of chemical reactions in the system Al-Mg-N are responsible for reintroducing magnesium into the melt, thus, maintaining a low melt surface tension. Interactions between the aluminum alloy and the silicon carbide substrate that may lead to the dissolution of the substrate and the formation of undesirable reaction products, particularly Al₄C₃, are examined, and means for mitigating their formation are outlined.     

鱼雷罐用铝碳化硅碳砖中碳化硅的氟硅酸钾滴定法研究

探讨了测定鱼雷罐用铝碳化硅碳砖中碳化硅的氟硅酸钾滴定方法。用KOH和KNO₃混合熔剂在镍坩埚中熔融鱼雷罐用铝碳化硅碳砖,选取熔融温度为700 ℃,熔融时间为15 min的熔融条件,在此条件下进行浸取易于操作,减少了对镍坩埚的损害。采用自制的抽滤装置,即将普通抽滤瓶进行改装,将玻璃砂芯漏斗部分用塑料代替,只需用直径约为40 mm、相当于四分之一张普通滤纸的滤纸进行过滤,过滤时抽滤速度快,滤纸不会穿漏,且吸附的残余酸量很少,易于洗涤和中和。方法用于自制标准样品的测定,回收率在98%～101%之间,相对标准偏差小于5%,能够满足日常分析检测需要。     

Steady state creep behaviour of silicon carbide particulate reinforced aluminium composites

The roles of volume fraction and size of reinfrecement on the steady state creep behaviour of pure aluminium matrix-silicon carbide particulate composites have been studied in the temperature range 623–723 K. The observed apparent stress exponents are higher than 15 and apparent activation energy is 249 kJ mol⁻¹. By considering the existence of a threshold stress, the data for 1.7 μm particulate reinforced composites with different volume fraction can be rationalized according to the substructure invariant model. The effective stress-strain rate behaviour of composites with 10 vol.% of coarser particulates (14.5 and 45.9 μm), however, agree with the stress dependent substructure model. The present analysis is validated by constructing a new type of “dislocation creep mechanism map”. The observed threshold stress varies with the volume fraction of reinforcement and is independent of particulate sizes and test temperatures. It is suggested that a model based on applied stress independent load transfer is required to explain the origin of such a threshold stress.     

Creep and rupture of an advanced fiber strengthened eutectic composite superalloy

Creep and rupture tests have been conducted on NiTaC-13, an advanced TaC fiber strengthened composite. A simple equation is developed to describe the creep behavior in argon for strains up to about 1 pct at temperatures between 871 and 1093°C. This equation may readily be incorporated in a nonlinear analysis of the deformation of a body subjected to nonsteady and nonuniform stresses and temperatures. The creep rates in air show a progressive increase relative to those in argon due to a loss in cross-section resulting from oxidation. The Larson-Miller parameter is shown to be unreliable for either correlation or extrapolation of rupture data. This is especially true for air tests. A modified parameter is, however, shown to give a good correlation with all the data. Although metallurgical instabilities are present, they have no clear effect on rupture strength and are not uniquely linked with the parametric representation. There is a systematic increase in ductility with increase in temperature and the generally high level of ductility is reflected in pronounced notch strengthening. Some load relaxation tests indicate that fiber failure occurs in excess of 1 pct composite strain. It is suggested, therefore, that 1 pct could be an appropriate design limiting strain for this class of material.     

Composites with zirconium matrix reinforced with boron and silicon carbide fibers

Structural features of a composite material (CM) consisting of a plastic zirconium foil-like matrix reinforced with continuous high-strength fibers of boron or silicon carbide with a diameter of 100 μm (25 to 30 vol.%) are examined at testing temperatures up to 950 °C. Model specimens are compacted by diffusion welding in vacuum at 1100 °C. Structural studies reveal a diffusion interaction area at the fiber-matrix interface. Cracks and pores appear in the area at 1100 °C. The effect of the diffusion area thickness on CM mechanical properties is discussed. It is established that reinforcement of zirconium with B or SiC fibers provides 7 and 5 times higher strength at 950 °C, respectively. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 48–53, 2007.     

Creep rupture in a nickel-based superalloy

Smooth bar creep rupture tests were conducted under constant loads at 1033 and 1144 K using RENé 80. Creep deformation occurred dominantly by the dislocation creep mechanism, but creep rupture proceeded by the continual nucleation and growth of cavities. Rupture times were well predicted by the constrained diffusive growth model of Riedel, particularly at low applied stresses, but their temperature dependence and the Monkman-Grant product were better related to the unconstrained diffusive growth models. Interruption tests showed development of micro- cracks in the tertiary stage of creep and sometimes revealed edge cracks, the growth of which provided the creep crack growth rate.