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.     

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.     

Creep rupture of a silicon carbide reinforced aluminum composite

The microstructure, texture, and whisker orientations in 6061 Al-20 wt pct SiC whisker composites have been examined using transmission electron microscopy and X-ray diffraction. Tension creep tests of the composite material have also been conducted in the temperature range 505 to 644 K (450 to 700 F). The steady state creep rate of the composite depends strongly on the temperature and applied stress. The stress exponent for the steady state creep rate of the composite is approximately 20.5 and remains essentially constant within the range of test temperatures. The activation energy is calculated to be 390 kJ/mol, nearly three times as high as the activation energy for self-diffusion of aluminum. No threshold stress was observed. Fracture surface examination using scanning electron microscopy shows that the composite fails by coalescence of voids in the aluminum matrix which originate at the aluminum-SiC interface. It is demonstrated that SiC paniculate composites are less creep resistant than SiC whisker composites.     

耐烧蚀硅基纤维布增强酚醛树脂复合材料的高温氧化性能与力学性能

针对目前广泛应用的耐烧蚀硅基纤维布增强的复合材料存在的问题,比较了三种不同的硅基纤维布增强酚醛树脂复合材料的高温氧化后的失重率、力学性能和宏观/微观形貌的变化.发现高温氧化后,混编复合材料失重率的变化明显大于单编复合材料失重率的变化;高温氧化后材料的弹性模量随温度升高而减小.在三种硅基纤维布增强的复合耐蚀材料中,单编硅基纤维布增强的复合材料具有更好的抗高温氧化性能.     

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.     

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.     

High cycle fatigue of tantalum carbide reinforced nickel base eutectics at room temperature

The high cycle fatigue response of two advanced tantalum carbide strengthened eutectic superalloys has been determined at room temperature. Since these alloys will be coated in service, the effects of variables associated with coating processes were given special attention. Both alloys showed a well defined fatigue limit. It was concluded that the maximum stress obtained in the cycle at the fatigue limit coincided with the tensile stress at which the matrix yielded. Above this stress level cyclic deformation of the matrix resulted in fiber failure, a necessary precursor of specimen failure. Detailed observation of the sequence of events leading to fiber failure and subsequent early crack growth in the matrix over a broad range of alturnating stresses confirmed that fiber fracture was the critical crack nucleation step in high cycle fatigue failure of the two alloys. It was shown that several changes in the surface condition did not affect the fatigue life of the alloys. However, when the samples were prestrained to crack the carbide fibers, they failed when cycled, at alternating stresses below the fatigue limit for virgin material. A similar loss of fatigue life was observed after heat treatment of one of the alloys. Fractographic examination indicated that this degradation resulted from enhanced crack nucleation at sigma platelets which were present as a result of the heat treatment. D. E. GRAHAM, formerly with Corporate Research and Development     

硬质合金钻头钻削碳纤维增强复合材料工艺

针对双向编织碳纤维增强复合材料钻孔过程中存在的生产效率低,易出现毛刺、撕裂、脱层缺陷等工艺难题,通过设计正交实验和单因素实验进行硬质合金钻头钻削双向编织碳纤维增强复合材料的研究,获得钻削力F随主轴转速n和进给量fr的变化规律。结果表明,轴向力随着进给量fr增加而增大,且进给量fr对轴向力的影响大于主轴转速n的影响,并进一步建立钻削力与转速以及进给量的之间关系式Fz=2 298.472n-0.290fr0.115;通过n和fr即可大致预测钻孔出口侧的撕裂长度:l=-28.5n-0.290fr0.115+3.485;孔壁表面粗糙度随轴向力增加呈小幅下降趋势。这些研究结果可对双向编制碳纤维增强复合材料钻削力和加工质量进行预估,为钻削加工参数的初选提供依据。     

碳化硅晶须对C/C复合材料表面SiC涂层的影响

以三氯甲基硅烷(CH3SiCl3,MTS)为先驱体原料,采用化学气相沉积法(CVD)在C/C复合材料表面原位生长碳化硅晶须(SiCw)及制备SiC涂层,研究SiCw对SiC涂层微观形貌,织构及力学性能的影响。结果表明:SiCw不仅可促成SiC等轴颗粒的细化、生长完善,裂纹宽度减小、偏转明显,而且可使涂层的织构发生改变;同时,大量的空洞在SiCw处形成,使得内层SiC涂层硬度低于外层,从而导致整个SiC涂层的硬度和弹性模量降低。     

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.     

添加氧化铝烧结助剂的碳化硼陶瓷无压烧结工艺研究

采用无压烧结工艺,通过添加质量分数为5%的氧化铝烧结助剂,制备得到了碳化硼陶瓷,其中烧结温度从2000℃到2250℃,保温时间为1、2和3 h。对烧结试样进行了体积密度、显气孔率、维氏硬度、显微形貌和晶体结构测试,并与2250℃下烧结得到的不添加烧结助剂碳化硼试样进行了比较。实验结果表明:由于烧结助剂与碳化硼在扩散运动中的相互作用,导致添加氧化铝助剂无压烧结碳化硼晶粒的形态变化具有温度选择性;氧化铝助剂所体现的液相扩散作用和钉扎作用,既可阻碍碳化硼晶粒长大,又可大幅度降低碳化硼的气孔率;通过烧结工艺控制氧化铝助剂成分在晶粒烧结体中的比例,可以将氧化铝成分完全包裹在碳化硼晶粒内部,有利于碳化硼烧结中的晶粒控制和空隙调整,从而避免助剂成分对烧结碳化硼可能造成的不利影响。     

High temperature hardness of tungsten carbide

The effect of crystallographic orientation and test temperature on hardness of WC single crystals was investigated along with the hot hardness of poly crystalline tungsten carbide. Also investigated was the effect of carbide grain size and the amount of binder phase on the hot hardness of some cemented tungsten carbides. The hot hardness of single crystal WC on all major crystallographic orientations evaluated decreases very rapidly for increasing temperature, and the single crystal hardness on its hardest orientation is only about half of the polycrystalline material depending on the test temperature. Because of its polycrystalline character, some cobalt bonded cemented tungsten carbides can be harder than single crystal WC over some intermediate temperature range.     

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.