Whisker Concentration Research Articles

Plastic Deformation Mechanisms in SiC‐Whisker‐Reinforced Alumina

SiC‐whisker‐reinforced Al2O3 samples (SiCw/Al2O3), obtained from three different manufacturers, containing 0–30 vol% SiC have been crept under compression at 1400°C in flowing argon. Compressive creep tests and microstructural observations have been used to characterize the plastic deformation mechanisms. The presence of whiskers decreased the creep rate by reducing grain‐boundary sliding. Damage formation was increased, however, because the whiskers acted as stress concentration sites. For specimens with whisker loadings greater than 15%, the absolute creep rate was not strongly dependent on whisker concentration, and the formation of cavitation damage was negligible below a critical stress that depended on the fabrication procedure of the specimen. This creep regime was characterized by a stress exponent of approximately 1, in which deformation occurred primarily by diffusional flow. For the materials with less SiC, the deformation occurred primarily by grain‐boundary sliding.

High temperature mechanical properties of SIC whisker reinforced Al<SUB align=right>2O<SUB align=right>3 composites

High–temperature constant load and constant strain rate compressive tests have been performed on Al2O3 reinforced with different concentrations of SIC whiskers. The samples were obtained from three sources, but contained whiskers from the same source. The temperature range was from 1300 to 1500°C and experiments were conducted in air and argon. Strain rates varied from 3x10−8 to 5x10−5 s−1 with stresses between 3 and 500 MPa. Stress exponents were found to vary between 1 and 5 and depended on microstructure, whisker concentration and stress, indicating a change in the mechanism controlling plastic deformation. Change from diffusional creep to a damage accumulation mechanism was supported by TEM observations on samples as–received and deformed both in the n=l regime and in the n=5 regime. Degradation of Sic whiskers occurred when testing in air.

Preparation of Alumina Hydrate-Coated SiC Whisker

Alumina hydrate-coated SiC whisker, a starting powder for fabrication of SiC whisker reinforced alumina ceramics, was prepared from SiC whisker and aluminum sulfate by homogeneous precipitation method using urea. Effects of reaction conditions on the coalescence of coated bodies and formation of independent alumina hydrate particles were studied. Observation of coated bodies during reaction showed that coalescence of coated bodies occurs by collision and agglomeration followed by deposition of alumina hydrate on the collided specimens. Coalescence of coated bodies was suppressed by intensive stirring, use of a flow reactor, high [SiC]/[Al2(SO4)3] ratio, low hydrolysis rate of urea, and high urea concentration. Generation of independent alumina hydrate particles was suppressed at a high SiC whisker concentration, a low aluminum sulfate concentration, and a low hydrolysis rate of urea. When the deposition rate of alumina hydrate was very low, coating was not uniform.

Effect of dispersion parameters and cold deformation on recrystallisation of Al–SiC composites

The effect of SiC whisker concentration (1–4 vol.-%), Al2O3 fine particle size and concentration (f/r=0·25–0·5 μm−1f volume concentrationr radius of particles), and degree of deformation by cold rolling (50–90%) on the recrystallisation behaviour and grain size in Al–SiC composites was studied. The result is of general interest as the Al–Al2O3–SiC composite is an alloy system with stable coarse and fine particles, the parameters of which can be varied independently over a wide range. The microstructure in the cold rolled state and initial stages of recrystallisation was examined using transmission electron microscopy. The recrystallisation temperature in different composites was determined by microhardness measurement after an isochronal heat treatment. The average grain size and grain size distribution in fully recrystallised composites were measured. It was found that the SiC whiskers act as strong nucleation sites and also enhance dynamic recovery, whereas the Al2O3 fine particles have the opposite effect. The number density of nuclei and the recrystallisation temperature were greatly affected by Al2O3 concentration and degree of cold deformation. The average grain size and the width of the grain size distribution decreased with increasing SiC content and decreasing f/r ratio of Al2O3 particles. These results were discussed and related to the effect of the parameters studied on critical nuclei size, nucleation rate, nuclei-whisker interaction, and grain growth.MST/1270

Effect of dispersion parameters and cold deformation on recrystallisation of Al–SiC composites

AbstractThe effect of SiC whisker concentration (1–4 vol.-%), Al2O3 fine particle size and concentration (f/r=0·25–0·5 μm−1f volume concentrationr radius of particles), and degree of deformation by cold rolling (50–90%) on the recrystallisation behaviour and grain size in Al–SiC composites was studied. The result is of general interest as the Al–Al2O3–SiC composite is an alloy system with stable coarse and fine particles, the parameters of which can be varied independently over a wide range. The microstructure in the cold rolled state and initial stages of recrystallisation was examined using transmission electron microscopy. The recrystallisation temperature in different composites was determined by microhardness measurement after an isochronal heat treatment. The average grain size and grain size distribution in fully recrystallised composites were measured. It was found that the SiC whiskers act as strong nucleation sites and also enhance dynamic recovery, whereas the Al2O3 fine particles have the oppo...

Pressureless Sintering of SiC‐Whisker‐Reinforced Al2O3 Composites: I, Effect of Matrix Powder Surface Area

Pressureless sintering of SiC‐whisker‐reinforced Al2O3 composites was investigated. In Part I of the study, the effect of the matrix (Al2O3) powder surface area on densification behavior and microstructure development is reported. Compacts prepared with higher surface area Al2O3 powder showed enhanced densification at lower whisker concentrations (5 and 15 vol%). Samples with 15 vol% whiskers could be pressureless sintered to ∼97% relative density with zero open porosity and ∼1.6‐μm matrix average grain intercept size.

Pressureless Sintering of SiC‐Whisker‐Reinforced Al2O3 Composites: II, Effects of Sintering Additives and Green Body Infiltration

Pressureless sintering of SiC‐whisker‐reinforced Al2O3 composites was investigated. In Part II of the study, the effects of Y2O3/MgO sintering additives and green body infiltration on densification behavior and microstructure development are reported. Both sintering additives and green body infiltration resulted in enhanced densification. However, the infiltration approach was more effective for samples with high SiC whisker concentrations. Samples with 27 vol% whiskers could be pressureless sintered to ∼93% relative density and ∼3% open porosity. Fracture toughness values and microstructural features (e.g., grain size) for the infiltrated samples remained approximately the same as observed in the uninfiltrated samples.

Compressive Creep of SiC‐Whisker‐Reinforced Al2O3

Compressive creep of SiC‐whisker‐reinforced Al2O3 composites (0, 5, 15, and 25 wt% SiC) was measured in the temperature range of 1300° to 1500°C in air and argon. The creep resistance increased with increasing whisker concentration. The results indicated that the whiskers degraded in air, increasing strain rates compared to those in argon. Stress exponents between 1.0 and 2.0 and an activation energy of 620 ± 100 kJ/mol were measured. Transmission electron microscopy observations indicated that cavitation was minimal and that the deformed composites had the same dislocation structure as did the as‐received samples.

A Continuous Process for the Production of Whisker Reinforced Composites

Various techniques have been proposed and developed for aligning the whisker reinforcing phase of metal matrix composites. Most of these techniques involve multi-step alignment processes, and are not suitable for economical, continuous production of metal-whisker com posites. A laboratory scale, prototype system for the continuous dielectro phoretic deposition and alignment of micron sized, ultra high strength whiskers has been designed, developed and tested. This process consists of the deposition of whiskers onto one of two electrodes placed in a suspension of the whiskers, when a voltage is applied between the electrodes. By employing a metal matrix foil as the deposition elec trode, this technique has demonstrated that the continuous production of metal-whisker combinations suitable for fabrication into advanced whisker reinforced composites is feasible. The volumetric loading of the reinforcing whiskers on the metal foil can be controlled by varying such process parameters as voltage, foil speed, electrode separation and geometry, and the concentration of whiskers in the supporting media. A discussion of these alignment and deposition process variables is presented. The metal foils containing the deposited, aligned whiskers are then sandwiched into a preform structure consisting of successive layers of metal foil and fibers. Consolidation of the stacked layers into a com posite is accomplished by hot pressing and diffusion bonding. Photo micrographs of the consolidated composite, showing alignment of the whiskers and the microscopic composite structure are shown and discussed for a range of whisker compositions. Preliminary mechanical properties data for selected composite compositions are presented and discussed.