Manufacturing and friction welding properties of particulate reinforced 7005 Al

This study has successfully incorporated Al₂O₃, SiC particulates into the 7005Al alloy matrix by using a drag-push method. The reinforced particulates are uniformly distributed in the matrix. This study also discusses the influence of aging treatment on the friction welding properties of 7005Al/10 wt%, 15 m and 6 m SiC_(p) composites and 7005Al/10 wt%, 15 m Al₂O_3(p) composites joint system. Experimental results show that after peak aging treatment was performed, if SiC particulates were used in the strengthening phase, the heat-affected zone (HAZ) had higher density of strengthening particulate, this resulted an increase in the hardness and stress concentration at the fully plasticized zone (Zpl) of the HAZ region, but a decrease in the width of the Zpl zone and the welding strength. And the welded fracture surface morphology had a low-ductile fracture.     

Fine silicon carbide fibers synthesized from polycarbosilane-polyvinylsilane polymer blend using electron beam curing

A fine SiC fiber is synthesized from a polymer blend of polycarbosilane (PCS) and polyvinylsilane (PVS) with electron beam curing under vacuum. The obtained SiC fiber from the PCS-PVS blend polymer has smaller average diameter of 8.5 m than that of 11.8 m from PCS, and shows higher average tensile strength of 3.2 GPa than that of 2.8 GPa from PCS after heat treatment at 1673 K in Ar gas atmosphere. However, the SiC fiber from the polymer blend decreases in tensile strength after heat treatment above 1773 K due to -SiC crystal growth near the fiber surface, because of a small amount of oxygen incorporated in the fiber.     

Pulse chemical vapour infiltration of SiC in porous carbon or SiC participate preform using an r.f. heating system

SiC was infiltrated into a porous carbon or an SiC particulate preform from a gaseous system of 6% CH₃SiCl₃-H₂ using a pulse chemical vapour infiltration apparatus and r.f. heating at 1273 to 1423 K. At 1273 K, the SiC matrix infiltrated the porous carbon initially to half the thickness of the substrate and finally over the full thickness. After 10000 pulses, three-point flexural strength saturated at about 120MPa. SiC particulate preform made from an average particle size of 4m was infiltrated by SiC. After 30000 pulses at 1273 K, the flexural strength of the composite increased to 200 to 220 M Pa.     

In-Situ Characterization of SiC–AlN multiphase ceramics

AlN and SiC can react and form a solid solution at temperatures above 1800 °C, a result that may be beneficial for sintering silicon carbide ceramics. The pressureless sintered AlN–SiC multiphase ceramics have reached high density at a temperature of 2100 °C for 1 hr in Ar. Analytical scanning transmission electron microscopy was then used to determine the grain boundary, fracture surface, and the local compositions. Because AlN has a higher solid vaporization pressure than SiC, the vaporization rate of the AlN solid would far exceed that of SiC at a sintering temperature. The vaporizing AlN was deposited on the surface of SiC powder; SiC grains then elongated in a random arrangement. The form of elongated rod crystals of 4H SiC is 5 to 8 m in length and 1 m in width. It resulted in the sample fracture section producing pulling-out and a strong tearing-open effect. The bending strength and the fracture toughness of the material obtained are 420 MPa and 4.40 MPa × m^1/2, respectively.     

Spray forming of ultra-fine SiC particle reinforced 5182 Al-Mg

This paper describes the spray forming of SiC particle reinforced Al metal matrix composites (MMCs) with particular emphasis on microstructure characterization of SiC particle distribution. A 5182 Al-Mg alloy was used as matrix material, and SiC particles with a mean diameter of 1.2 m and 2.0 m as reinforcement. The reinforcing particle distribution and microstructural characteristics of MMCs were analyzed in the current study using TEM, SEM and optical microscopy. The distribution of SiC particles in the as-spray deposited and hot-extruded conditions was characterized. SEM results indicate that the SiC particles are homogeneously distributed although some clustering was evident in the matrix. TEM and OM examinations show that most of SiC particles are present intergranularly in the Al matrix. EDS analysis indicated that Mg tends to segregate and form oxide phases in the vicinity of SiC particles and that there is no compositional variation of Mg across grain boundaries in the Al matrix.     

A note on the μ-zeros of combinations of cross-product bessel functions

Summary In the problem of diffraction of elastic waves in annular circular, or cylindrical (doubly connected) domains, the zeros of complicated combinations of cross-products of cylinder functions as analytic functions of the index =n+i (in general complex) are required. In this note, the imaginary -zeros of two such combinations of cross-products of two identical doubly connected domains, however with two different boundary conditions on one boundary, are asymptotically computed and graphically displayed. It is shown that even for such a comparison of different boundary conditions, one can obtain with a minimum effort in mathematics asymptotic distributions and the relative dispositions of the respective -zeros. It is then rather simple to determine with a computer the exact positions of the -zeros.

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Zusammenfassung Bei der Behandlung des Beugungsproblems elastischer Wellen in kreisringförmigen oder zylindrischen (zweifach zusammenhängenden) Bereichen werden die Nullstellen komplizierter Produktkombinationen von Zylinderfunktionen benötigt, und zwar als analytische Funktionen ihrer im allgemeinen komplexen Ordnung =n+i . In dieser Arbeit werden die imaginären -Nullstellen zweier solcher Produktkombinationen in zwei identischen, zweifach zusammenhängenden Bereichen, jedoch mit an einem Rand verschiedenen Randbedingungen, asymptotisch berechnet und graphisch dargestellt. Es wird gezeigt, daß man selbst bei einem solchen Vergleich verschiedener Randbedingungen mit einem Minimum an mathematischem Aufwand asymptotische Verteilungen und die relativen Lagen der entsprechenden -Nullstellen erhalten kann. Es ist dann ziemlich einfach, mit Hilfe eines Rechners die exakten Lagen der -Nullstellen zu bestimmen.

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With 1 Figure     

Observations of slip patterns on the surface of fatigued gold using the scanning tunnelling microscope

Observations of fatigue-induced slip patterns on the surface of polycrystalline gold (+99.9%) using the scanning tunnelling microscope (STM) are presented. The samples were cycled in four-point bending between zero and 0.0025 strain for 250 and 2500 cycles. STM observations of the sample fatigued for 250 cycles revealed broad slip bands 0.38–0.69 m wide containing narrow slip bands 0.015–0.123 m wide and fine slip lines 0.006 m wide. The depth of these features are 20–57 nm for broad slip bands, 3–7 nm for narrow slip bands, and approximately 2–4 nm for fine slip lines. The sample fatigued for 2500 cycles showed similar values for the width of the slip bands and slip lines except that their depth was increased by a factor of between 2 and 4 times for the narrow slip bands and the fine slip lines. Apparent persistent slip bands (PSBs) spaced 2.4 m, 300–500 nm deep were also observed. These results demonstrate that STM is a significant new tool for observing and distinguishing various types of fatigue-induced surface slip patterns on suitably prepared samples. The vertical resolution obtained with STM is vastly superior to current SEM and TEM methods.     

Rheology of stimulated whole saliva in a typical pre-orthodontic sample population

The dynamic viscosity () of stimulated whole saliva in a typical pre-orthodontic sample population was characterized as a function of temperature (T). Samples were collected from 30 adolescents or young adults, after screening for factors that are known to have an effect on salivary viscosity. Using a cone and plate viscometer, 1.5 ml of stimulated whole saliva was evaluated at a constant shear rate of 450 s^-1 from T=20°C to T=40°C. Data from the -T plots showed a negative dependence of the form, =a–bT, over a range of from 1.08 to 2.45 centipoise (cps) at 34°C. Most of the samples fell into a narrow envelope, where the mean of the saliva samples ranged from 2.42±0.61 cps at 20°C to 1.57±0.32 cps at 37°C. With regard to sample stability, viscosity-time plots indicated that a small but predictable decrease in occurred during the 3 h period. The -T plots generated from fresh and frozen saliva samples demonstrated an appreciable change in as a result of refrigeration. With regard to sample reproducibility, viscometric data obtained from a typical pre-orthodontic patient over a 1-week period fluctuated within a fairly broad envelope of values.Presented, in part, at the 21st Annual Meeting and Exhibition of the A.A.D.R., Boston, Massachusetts, March, 1992.     

Combustion synthesis and mechanical properties of molybdenum disilicide composites reinforced with SiC particulate

Intermetallic composites of molybdenum disilicide reinforced with silicon carbide were produced by combustion synthesis of the elemental powders. The combustion reaction was initiated near 700°C and completed within a few seconds. The end product was a porous composite which was subsequently hot pressed to >97% theoretical density. The grains of the matrix were 8–14 m in size surrounded by SiC particulate reinforcement of 1–5 m. The mechanical properties of the composites improved with increasing SiC reinforcement. The hardness of the materials increased from 10.1 GPa to 12.7 GPa with the addition of 20 vol% SiC reinforcement, while the strength increased from 195 MPa to 299 MPa. The fracture toughness also increased from 2.79 MPa m1/2 to 4.08 MPa m1/2 with 20 vol% SiC. © 1998 Chapman & Hall     

Preparation of gas-permeable SiC shape by pressure-pulsed chemical vapour infiltration into carbonized cotton-cloth preforms

Carbonized cotton-fibre preforms were partially infiltrated with SiC by pressure-pulsed chemical vapour infiltration (CVI) from SiCl4–CH4–H2. Fibrous carbon/SiC of 10 mm dia. and 20 mm long having the porosity of 83% was obtained after 15 000 pulses at 1150°C. Pores of the sample after 10 000 pulses distributed below 100 m (average pore size, 30 m). Pressure drop of the sample after 15 000 pulses under the axial air-flow was 11 kPa at a face velocity of 1.1 m s-1. On the samples after 15 000 pulses, 15% of preform-carbon were lost within 5 h by air oxidation at 1000°C, and tensile strength along the axis reached 10 MPa, which was close to that before oxidation. In the case of the sample after 5000 pulses, tensile strength lowered significantly after oxidation, however, the strength was recovered by application of 4000 pulses of second CVI. © 1998 Chapman & Hall     

Effect of microstructural features on the ageing behaviour of Al–Cu/SiC metal matrix composites processed using casting and rheocasting routes

In the present study, microstructural variation in Al–2 wt% Cu/SiC composites was accomplished by synthesizing them using conventional casting and partial liquid phase casting (rheocasting) routes. Microstructural characterization studies conducted on the rheocast composite samples revealed a finer grain size, minimal porosity, uniform distribution of SiC particulates, and a superior matrix – particulate interfacial integrity when compared to the conventionally cast composite samples. Furthermore, the results of interfacial characterization studies revealed that the presence of porosity associated with either individual SiC particulate or SiC clusters significantly influence the constitutional characteristics of the interfacial region. Results of ageing studies revealed an accelerated ageing kinetics in case of rheocast samples when compared to the conventionally cast composite samples. The results of ageing studies were finally rationalized in terms of the difference in microstructural characteristics of the rheocast and conventionally cast composite samples. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of Matrix Constitution on Microstructure and Mechanical Properties of Rheocast Metal Matrix Composites

In this study, aluminum-based metallic matrices with varying amounts of copper (I wt.% Cu, 3 wt.% Cu and 4.5 wt.% Cu) were reinforced with SiC particulates using a partial liquid phase casting technique. Microstructural characterization studies conducted on the composite samples revealed an increase in uniformity of distribution of SiC particulates and SiC/Al interfacial integrity and a decrease: in porosity in the metallic matrix with decreasing weight percent of copper. The results of the ageing studies revealed an accelerated ageing kinetics for the Al-1% Cu/SiC composite when compared to Al-3% Cu/SiC and Al-4.5% Cu/SiC samples. Results of ambient temperature mechanical tests demonstrate an increase in 0.2% yield strength and ultimate tensile strength of the composites and a decrease in ductility and strain hardening rate with an increasing weight percent of copper in the metallic matrix. Fracture studies revealed the presence of interfacial debonding, particulate breakage and cracks in the matrix of tensile specimens. The results of microstructural characterization, mechanical testing and fractography were finally rationalized in terms of the effect of variation in weight percent of copper in the metallic matrix.     

Particulate-fibre-reinforced glass matrix hybrid composites

This study extends the concept of hybridization of glass matrix composites to fibre and particulate reinforcements. Hybridization of Nicalon^®-glass matrix composites with silicon carbide particulates of mean diameters of 0.77, 1.5 and 6 m was studied. The microcrack stress, transverse strength and interlaminar strengths improved significantly on hybridization. At the optimum loading of 7.5 wt % for 6 m particle size the microcrack stress increased by 62%, transverse strength by 650% and interlaminar shear strength by 200%. The ultimate strength declined for this composite by 6%. The decline in ultimate properties was attributed to the damaged graphitic skin at the particle-fibre contact points.     

Processing, microstructure and mechanical properties of hot-pressed SiC continuous fibre/SiC composites

SiC (SCS-6TM) continuous fibre/SiC composites were fabricated by hot-pressing at 1700°C in vacuum using an Al sintering additive. Analytical transmission electron microscopy was used to investigate the microstructure of the composites. The room-temperature mechanical and high-temperature creep properties of the composites were investigated by four-point bending. The SiC powders used were sintered at a relatively low sintering temperature to high density (97% of theoretical density) with the addition of the Al sintering additive. It is believed that the Al additive is very efficient for the densification of SiC. The SiC fibres maintained their original form and microstructure during fabrication. The SiC matrix reacted with the outermost carbon sublayer in the fibre, forming a thin (1.8–4.8m) interfacial layer, which was composed of Al4C3, Si–Al–C, and Si–Al–O phases. The incorporation of SiC fibre into a dense SiC matrix significantly increased the room-temperature failure strain and improved the high-temperature creep properties. In addition, the incorporation of SiC fibre into a porous SiC matrix increased the room-temperature failure strain, but did not contribute to the high-temperature creep properties.     

High-temperature strengths of aluminium composite reinforced with continuous SiC fibre

Aluminium alloys were reinforced unidirectionally with 30, 35, 40 and 50 vol %SiC fibres by a liquid-pressing method. The SiC fibres for reinforcement were produced from a polycarbosilane and were yarns consisting of 500 continuous filaments of length 300 m and diameter 13m, having a tensile strength of 2000 MPa. High-temperature tensile and bending strengths of the composites were examined in air in the temperature range from room temperature to 500° C. The strengths were not influenced by temperature up to 400° C, but were decreased at 500° C. The decrease is considered to be caused by the reduction in transfer efficiency of the stress accompanying the decrease in adhesion between fibres and matrix.     

Microstructure and mechanical properties of RSP/M Al-Fe-V-Si and Al-Fe-Ce alloys

Two aluminium alloys with nominal compositions of Al-8Fe-4Ce and Al-8Fe-1V-2Si (all compositions in wt%) were rapidly solidified by ultrasonic gas atomization. The atomized powders with an average particle size (d₅₀) of 30 m were vacuum hot pressed and subsequently hot extruded. The P/M extrusion exhibited similar microstructure and elevated temperature tensile properties. The tensile and stress rupture samples of both the alloys exhibited ductile dimple failure. However, the Al-Fe-V-Si extrusion samples exhibited significantly better creep and stress rupture properties. The Al-Fe-Ce alloy was found to be more susceptible to cavitation at elevated temperatures which resulted in poor stress rupture properties.     

Tensile testing of ceramic fibres by video extensometry

A computer-assisted video extensometer was used to measure the Young's modulus and tensile strength of commercially available alumina fibres (11.5 m in diameter). The results showed excellent agreement with manufacturers reported values (384 ± 12 GPa and 3132 ± 296 MPa for Young's modulus and tensile strength, respectively). The machine was initially tested with 100 m diameter SiC monofilaments to identify optimum experimental conditions. The mechanical properties of these fibres were independent of the crosshead speed and fibre length. This non-contacting extension measuring system allowed testing of fragile materials and a submicron resolution could be achieved with a high-precision CCD camera. The results in term of precision and resolution therefore meet the requirements for strain measurements in mechanical ceramic materials testing.     

Pressureless sintering of β-SiC with Al2O3 additions

-SiC was pressureless sintered to 98% theoretical density using Al₂O₃ as a liquid-phase forming additive. The reaction between SiC and Al₂O₃ which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated -SiC grains (maximum length 10 m and width 2–3 m) in a matrix of fine equi-axed grains (2–3 m) and plate-like grains (2–5m). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the -phase was transformed to the -phase.     

Statistical analysis for strength and spatial distribution of reinforcement in SiC particulate reinforced aluminum alloy composites fabricated by die-casting

Statistical analysis for strength and spatial distribution of reinforcement in die-cast SiC_p/Al alloy composites was performed in order to predict the reliability of composites. Microstructural analysis was also done to determine the critical features of the composites. Die-casting was carried out using the preheated die at the casting temperature range of 620–750°C. It was found that the SiC pacticulates were homogeneously dispersed in die-cast Al matrix alloy, resulting from the refinement of dendritic cell size due to rapid cooling rate. The tensile strength of die-cast SiC_p/Al alloy composites was higher than that of die-cast Al matrix alloy. Also, the tensile strength was slightly increased with increasing SiC particulate volume fraction at the casting temperature range of 650–700°C. It was concluded that the die-cast temperatures of 750 and 700°C are optimum condition for the distribution of SiC particulates in consequence of good fluidity of melt for 10 and 20 vol.% SiC_p/Al alloy composites, respectively. However, the strength scattering of composites was increased with increasing SiC particulate volume fraction. For the statistical evaluation of strength, the maximum Weibull modulus of die-cast SiC_p/Al alloy composites, which was obtained at the cast temperature of 700°C, was 29.6 in Al matrix alloy, 22.2 in 10 vol.% SiC_p and 14.2 in 20 vol.% SiC_p, respectively.     

Composites of aluminium alloys: fabrication and wear behaviour

In this paper processes for fabrication of aluminium-alloy composites containing paniculate non-metals, the net shape forming of these composites, their microstructures, their friction and wear behaviours and their mechanical properties are described. Composites of two wrought (2014 and 2024) and one cast (201) aluminium alloys containing 2 to 30 wt% of Al₂O₃ and SiC particles in the size range of 1 to 142m were prepared. The non-metallic particles were added to a partially-solid vigorously-agitated matrix alloy. The particles were then retained in the matrix until interface interaction, for example, the formation of MgAl₂O₄ spinel in the case of Al₂O₃ particles, were faciliated. These composites were solidified and subsequently reheated to above their liquidus temperature and formed under high pressure in a closed-die forging type of apparatus. Composites with particulate additions of size larger than 5m possessed homogeneous structures; particles of size 1m, however, tended to cluster. The wear behaviour of the composites was studied using a pin-on-disc type machine. It was shown that composites containing large amounts of non-metals, 20 wt%, exhibit excellent wear resistance whilst those with small to moderate amounts of non-metals possess tensile properties comparable to the matrix alloy. Increasing the amount of particulate additions results in reduced ductility. Finally, a method was investigated of producing components with high weight-fractions of non-metals near their surface.