High temperature creep behavior of metal matrix AluminumSiC composites (original) (raw)
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Metallurgical and Materials Transactions A, 2009
The tensile creep behavior of powder metallurgy (P/M)-processed and hot-rolled commercially pure Al and Al-5 or Al-10 vol pct SiC particulate composites has been evaluated after subjecting to 0, 2, and 8 thermal cycles between 500°C and 0°C with rapid quenching. The images of microstructures obtained using scanning and transmission electron microscopy as well as changes in the electrical resistivity, Young's modulus, and microhardness have been examined in the samples subjected to thermal cycling, in order to compare the effects of structural damage and strengthening by dislocation generation. The damage is caused by voids formed by vacancy coalescence, and is more severe in pure Al than in Al-SiC p composites, because the particlematrix interfaces in the composites act as effective sinks for vacancies. Creep tests have shown that the application of 2 thermal cycles lowers the creep strain rates in both pure Al and Al-SiC p composites. However, the creep resistance of pure Al gets significantly deteriorated, unlike the mild deterioration in the Al-5 SiC p composite, while the time to rupture for the Al-10 SiC p composite is increased. The dislocation structure and subgrain sizes in the Al and in the matrices of the Al-SiC p composites in the as-rolled condition, after thermal cycling, and after creep tests, have been compared and related to the creep behavior. The dimple sizes of the crept fracture surfaces appear to be dependent on the void density, tertiary component of strain, and time to rupture.
Creep deformation of alumina-SiC composites
Materials Science and Engineering: A, 1990
Composites of alumina reinforced with SiC whiskers have been creep tested in bending and in compression at 1200-1400 °C in an air ambient. The flexural creep data follow a power law constitutive relation with two distinct stress exponents that depend on the level of applied stress. Crept specimens were examined by transmission electron microscopy to determine the mechanisms of creep deformation and microstructural damage. The primary mechanism of creep deformation under these conditions is grain boundary and interface sliding resulting from diffusion. At high stress levels, the sliding is often unaccommodated, resulting in cavitation at grain boundary-interface junctions. Cavitation is associated with an increase in the stress exponent for flexural creep.
Effect of a solid solution on the steady-state creep behavior of an aluminum matrix composite
Metallurgical and Materials Transactions A, 1996
The effect of an alloying element, 4 wt pct Mg, on the steady-state creep behavior of an AI-10 vol pct SiCp composite has been studied. The A1-4 wt pct Mg-10 vol pct SiC, composite has been tested under compression creep in the temperature range 573 to 673 K. The steady-state creep data of the composite show a transition in the creep behavior (regions I and II) depending on the applied stress at 623 and 673 K. The low stress range data (region I) exhibit a stress exponent of about 7 and an activation energy of 76.5 kJ mol-L These values conform to the dislocation-climb-controlled creep model with pipe diffusion as a rate-controlling mechanism. The intermediate stress range data (region II) exhibit high and variable apparent stress exponents, 18 to 48, and activation energy, 266 kJ mol-~, at a constant stress, cr = 50 MPa, for creep of this composite. This behavior can be rationalized using a substructure-invariant model with a stress exponent of 8 and an activation energy close to the lattice self-diffusion of aluminum together with a threshold stress. The creep data of the AI-Mg-A1203r composite reported by Dragone and Nix also conform to the substructure-invariant model. The threshold stress and the creep strength of the A1-Mg-SiC e composite are compared with those of the A1-Mg-AIzO3/and 6061 AI-SiCe.w composites and discussed in terms of the load-transfer mechanism. Magnesium has been found to be very effective in improving the creep resistance of the AI-SiC, composite.
High temperature creep of silicon carbide particulate reinforced aluminum
Acta Metallurgica et Materialia, 1990
The effect of stress on the creep properties of 30 vol.% silicon carbide particulate reinforced 6061 aluminum (SiCp-6061 AI), 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, Qa, is much higher than the activation energy for self-diffusion in aluminum. The above creep characteristics of SiCp-6061 A1 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 SiCp-6061 AI 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 SiCp-6061 A1, 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 AI 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.
Tensile creep behavior of Al-5SiC p composite, powder metallurgy processed and hot-rolled at either 400 1C (400-HRC) or 600 1C (600-HRC), has been examined. Creep-tests have been carried out at temperature and stress ranges of 325-400 1C and 9-21 MPa, respectively. The creep behavior has been examined by analyzing steady-state creep rate, time to rupture, Larson-Miller parameter (LMP), strain fractions in primary, secondary and tertiary stages, with assessment of damage and dimple size. The creep resistance of 400-HRC with higher dislocation density is found as greater than that of 600-HRC, whereas apparent stress exponents and activation energies exceed that for dislocation climb. The results suggest possibility of life prediction using Monkman-Grant relationship and LMP variation with stress. Thermal cycling between 500 1C and 0 1C is more effective in improving creep resistance at lower temperatures, and is more beneficial for 400-HRC than for 600-HRC, confirming positive role of strainhardening.
AN INFLUENCE OF SiC CONTENT AT Al (AlMg) MATRIX COMPOSITES ON CREEP CHARACTERISTICS
Creep tests under a range of step-increasing tensile stresses were carried out on the AlMg/SiC composites produced by means of the KOBO method. The content of reinforced phase was equal to 2.5, 5, 7.5 and 10%. The AlMg/SiC creep characteristics were compared to those of the Al/SiC composites. Structural assessments of materials in the as-received state and after creep were carried out.
Compression creep of PM aluminum matrix composites reinforced with SiC short fibres
Materials Science and Engineering: A, 2006
The compression creep behaviour of Al-SiC fiber metal matrix composites (MMC), made by hot-pressing (HP), was evaluated at various temperatures and over several orders of magnitude of strain rates. The interpretation of metal flow-patterns during the whole deformation cycle was complex owing to the fact that the short-fibre distribution in the composites was roughly planar. However, every specimen showed a well-defined flow stress or plateau (σ p true ) up to the end of the tests that were associated with nearly 50% linear compression strains. Such stresses clearly increased with the volume fraction (f) of fibres and strain rates, and decreased with increasing temperatures. Cross-examination of the creep curves [log strain rate (γ) versus log shear stress (τ)] for both the HP Al matrix and composites show an apparent stress exponent n ap = [δ(lnγ)/δ(ln τ)] clearly increasing while decreasing τ. This anomalous behaviour can be attributed to the existence of a finite threshold stress (τ 0 ) for every composition. This threshold stress appears to be related to the oxide contamination (judged from TEM observations) of the matrix, as a result of the use of powder metallurgy (PM) synthesis method. Following certain approximations during deformation behaviour of PM specimens reinforced with ceramic particles, the present data, for short-fibre reinforced MMC, seems to be consistent with the mechanism of dislocation climb that is characterized by an stress exponent of around five, and an activation energy close to that for self-diffusion in pure aluminum (143.2 kJ mol −1 ). (C.J.R.G. Oliver). features: the apparent n ap exponents [slope of the log (strain rate)-log (stress) plots] and apparent activation energies Q a for creep were higher [1-12] than those expected for the unreinforced matrix metal. By incorporating a threshold stress (σ 0 ) into the normal power law creep (PLC) expression, the apparent n values became lower, and more reasonable values for the activation energy were obtained. It is therefore relevant to consider [20] the fine oxide film from powder manufacture to be dispersed as fine particles during the PM process, playing an important role as a barrier to matrix deformation at high temperatures, also probably controlling the above threshold stresses.
Analysis of creep behavior of SiC/Al metal matrix composites based on a generalized shear-lag model
Journal of Materials Research, 2004
The creep behaviors of 20 vol% SiCw/2124Al, extruded with different ratios, and SiCp/2124Al, reinforced with 10-30 vol% SiC particles, were investigated to clarify the effects of aspect ratio, alignment, and volume fraction of reinforcement on creep deformation. The effective stresses on the matrix of SiC/Al composites are calculated based on the generalized shear-lag model. The minimum creep rates of SiCw/2124Al extruded with different ratios and SiCp/2124Al reinforced with different volume fractions of SiC particles are found to be similar under a same effective stress on matrix, which is calculated by the generalized shear-lag model. The subgrain sizes in matrices of crept SiC/Al composites are dependent on the effective stress on matrix but not on the applied stress on the composite. It is suggested that the role of SiC reinforcements is to increase the creep resistance of SiC/Al composite by reducing the effective stress on matrix.
Room Temperature Creep of Sic\Sic Composites
Ceramic Engineering and Science Proceedings
During a recent experimental study, time dependent deformation was observed for a damaged Hi-Nicalon TM reinforced, BN interphase, chemically vapor infiltrated SiC matrix composites subjected to static loading at room temperature. The static load curves resembled primary creep curves. In addition, acoustic emission was monitored during the test and significant AE activity was recorded while maintaining a constant load, which suggested matrix cracking or interfacial sliding. For similar composites with carbon interphases, little or no time del_ndent deformation was observed. Evidently, exposure of the BN interphase to the ambient environment resulted in a reduction in the inteffacial mechanical properties, i.e. interfacial shear strength and/or debond energy. These results were in qualitative agreement with observations made by Eldridge of a reduction in interracial shear stress with time at room temperature as measured by fiber push-in experiments.
Journal of the American Ceramic Society, 2006
Silicon carbide fiber (Hi-Nicalon Type S, Nippon Carbon) reinforced silicon carbide matrix composites containing melt-infiltrated silicon were subjected to creep at 13151C at three different stress conditions. For the specimens that did not rupture after 100 h of tensile creep, fast-fracture experiments were performed immediately following the creep test at the creep temperature (13151C) or after cooling to room temperature. All specimens demonstrated excellent creep resistance and compared well to the creep behavior published in the literature on similar composite systems. Tensile results on the after-creep specimens showed that the matrix cracking stress actually increased, which is attributed to stress redistribution between composite constituents during tensile creep.