Chapter 5 CVD-Produced Boron Filaments F. E. Wawner, F. E. Wawner Department of Materials Science University of Virginia, Charlottesville, Va. 22903Search for more papers by this authorH. E. DeBolt, H. E. DeBolt AVCO Corp., Specialty Materials Div. 2-T Industrial Ave., Lowell, Mass. 01851Search for more papers by this authorR. D. Suplinskas, R. D. Suplinskas AVCO Corp., Specialty Materials Div. 2-T Industrial Ave., Lowell, Mass. 01851Search for more papers by this author F. E. Wawner, F. E. Wawner Department of Materials Science University of Virginia, Charlottesville, Va. 22903Search for more papers by this authorH. E. DeBolt, H. E. DeBolt AVCO Corp., Specialty Materials Div. 2-T Industrial Ave., Lowell, Mass. 01851Search for more papers by this authorR. D. Suplinskas, R. D. Suplinskas AVCO Corp., Specialty Materials Div. 2-T Industrial Ave., Lowell, Mass. 01851Search for more papers by this author Book Editor(s):William J. Smothers, William J. SmothersSearch for more papers by this author First published: 01 January 1980 https://doi.org/10.1002/9780470291030.ch5Citations: 1Book Series:Ceramic Engineering and Science Proceedings AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Proceedings of the 2nd and 3rd Annual Conference on Composites and Advanced Materials: Ceramic Engineering and Science Proceedings, Volume 1 RelatedInformation

Chapter 4 Some Aspects of Boron Filament Elongation F. E. Wawner, Department of Materials Science University of Virginia, Charlottesville, Va. 22903Search for more papers by this authorJ. W. Eason, Department of Materials Science University of Virginia, Charlottesville, Va. 22903Search for more papers by this authorH. E. DeBolt, AVCO Specialty Materials Div. 2-T Industrial Ave., Lowell, Mass. 01851Search for more papers by this authorR. D. Suplinskas, AVCO Specialty Materials Div. 2-T Industrial Ave., Lowell, Mass. 01851Search for more papers by this author F. E. Wawner, Department of Materials Science University of Virginia, Charlottesville, Va. 22903Search for more papers by this authorJ. W. Eason, Department of Materials Science University of Virginia, Charlottesville, Va. 22903Search for more papers by this authorH. E. DeBolt, AVCO Specialty Materials Div. 2-T Industrial Ave., Lowell, Mass. 01851Search for more papers by this authorR. D. Suplinskas, AVCO Specialty Materials Div. 2-T Industrial Ave., Lowell, Mass. 01851Search for more papers by this author Book Editor(s):William J. Smothers, Search for more papers by this author First published: 01 January 1980 https://doi.org/10.1002/9780470291030.ch4Citations: 2Book Series:Ceramic Engineering and Science Proceedings AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Citing Literature Proceedings of the 2nd and 3rd Annual Conference on Composites and Advanced Materials: Ceramic Engineering and Science Proceedings, Volume 1 RelatedInformation

The interaction between dislocations and semicoherent precipitates (the Omega phase) is discussed in this study. The investigation shows that the semicoherent precipitate is cut by dislocations during deformation. Conventional TEM and HRTEM observations demonstrate that shearing of precipitates by dislocations has occurred. The multiple cutting of the precipitate was observed. A cutting model was proposed to interpret the multiple cutting phenomena. The general theory is not only applicable to the Omega precipitate in Al-Cu-Mg-Ag alloys, but also to semicoherent precipitates in other alloys.

The current investigation involves fabrication and characterization of squeeze-cast aluminum metal matrix composites reinforced with sintered metal compacts. Four types of metallic powder compacts were used: plain carbon steel, 409 stainless steel, A6 tool steel, and a wear-resistant stainless steel. All the composites were cast under the same nominal conditions, but only the plain carbon steel-reinforced composites contained a significant amount of reaction phase. When tested in tension, the composites exhibited very limited ductility, with the exception of the 409 stainless steel-reinforced composites. The higher ductility demonstrated by these composites is attributed to the high relative density of the reinforcing compact and the low amount of reaction phase formed. When tested in compression, the 0.2% offset yield strength of the composites were observed to correlate directly with the microhardness measured on the reinforcement particles within the composite.

The interaction between dislocations and semicoherent precipitates (Ω phase) is discussed. The investigation shows that the semicoherent precipitate is cut by dislocations during deformation. Conventional TEM and HRTEM observations demonstrate that shearing of precipitates by dislocations occurs by multiple cutting of the precipitate. A strengthening mechanism is proposed based on this observation. The general theory is not only applicable to the Ω precipitate in Al–Cu–Mg–Ag alloys, but also to semicoherent precipitates in other alloys.

The current investigation involves the fabrication and characterization of an aluminium metal matrix composite reinforced with sintered metal preforms. Two types of metallic preforms were used (steel and stainless steel) and a variety of squeeze casting conditions were investigated using systematic design-of-experiments techniques to determine the effect of casting conditions on the composite microstructure and mechanical properties. It was observed that a detrimental reaction phase containing iron, aluminium and silicon formed around the metallic preform particles, with a lower volume fraction of reaction phase forming at the lower melt casting temperature. This reaction phase appears to promote premature fracture by facilitating crack initiation and propagation. The stainless steel-reinforced composites had a smaller volume fraction of reaction phase and exhibited superior properties compared to the steel-reinforced composites.

It is known that Al2O3, Al2O3+TiO2, Cr2C3+NiCr, NiAl, NiCrAl, ZrO2 and MgZrO3 coatings are resistant to specific corrosion media which are widely employed in industry. In the material environment configuration, the component surface is a vital parameter in determining its optimum performance. For this purpose, the corrosion behaviour of ceramics, cermet and metallic coatings on AISI 304L stainless steel substrates were investigated in 1 N H2SO4 solutions. A plasma spray process was employed on a substrate with deposition of ceramic, cermet, and metallic powders such as Al2O3, Al2O3+TiO2, Cr2C3+NiCr, NiCrAl, ZrO2+NiAl and MgZrO3+NiAl. The porosity of coatings was measured by water impregnation, electrochemical measurements and optical methods. Potentiodynamic polarization measurements were also carried out to determine the corrosion behaviour of the plasma sprayed coatings. The microstructure of coatings and corroded coatings were investigated during the corrosion test by optical microscope, SEM and X-ray diffraction (XRD). It is concluded that the corrosion resistance depends strongly on the porosity and coating type. Besides, it was found that the corrosion resistances of Al2O3, Al2O3+TiO2, ZrO2+NiAl and MgZrO3+NiAl coatings are higher than those of Cr2C3+NiCr, NiAl and NiCrAl coatings.

The current investigation involves fabrication and characterization of squeeze-cast aluminum metal matrix composites reinforced with sintered metal compacts. Four types of metallic powder compacts were used: plain carbon steel, 409 stainless steel, A6 tool steel, and a wear-resistant stainless steel. All the composites were cast under the same nominal conditions, but only the plain carbon steel-reinforced composites contained a significant amount of reaction phase. When tested in tension, the composites exhibited very limited ductility, with the exception of the 409 stainless steel-reinforced composites. The higher ductility demonstrated by these composites is attributed to the high relative density of the reinforcing compact and the low amount of reaction phase formed. When tested in compression, the 0.2% offset yield strength of the composites were observed to correlate directly with the microhardness measured on the reinforcement particles within the composite.

The overall objective of this study is to investigate the microstructural stability and mechanical properties of Al-Cu-Mg-Ag alloys subjected to elevated temperatures. The addition of Ag to Al-Cu-Mg alloys with correct Cu-to-Mg ratio has been shown to generate a precipitate phase, designated Ω, which displays superior thermal stability compared with the normally occurring S′ and θ′. Samples produced for this study contained the expected Ω, θ′ and S′. In addition a cubic phase, previously designated σ(Al5Cu6Mg2), was obtained. The σ phase was seen to be a semicoherent and coplanar phase with the Al matrix, i.e., {1 0 0}σ//{1 0 0}Al and 〈0 1 0〉σ//〈0 1 0〉Al. The coarsening rate of the σ phase was found to be much lower than the θ′ phase at 200°C. An ingot was produced of the σ phase, which was verified by X-ray diffraction. The ultrasonic technique was used to determine Young's modulus and the shear modulus. Estimates for the structural interfacial energy were determined. A hot-stage Vickers hardness measurement on the equilibrium σ phase indicates a high yield strength up to 350°C. The data from the present study indicate that an Al alloy with the σ phase may exhibit superior elevated-temperature stability.

The tensile strength of unidirectional SiC-fiber/titanium composites (SCS-6/Ti-1100) of varying fiber volume fraction (0.15–0.35) was measured in the as-produced condition. Additionally, fibers were etched from the panels and tensile tested to determine their strength distribution, interface properties were measured with fiber push-out tests, and the tensile properties of the matrix (without fiber) were also determined. From the measured constituent properties, the composite strength was predicted by using both global and local load-sharing (GLS and LLS) models, and these predictions were compared with the measured values. The composite strength was lower than predicted by Curtin's GLS model, but greater than predicted if it is assumed that the first fiber failure initiates composite failure (i.e. severe LLS). Metallographic examination of the fractured specimens revealed that the failure was consistent with a non-cumulative (localized damage) failure mode, which, as suggested by previous theoretical and experimental studies, is promoted by matrix plasticity.

The strength distribution of SiC fibers has been measured after chemical extraction from a titanium-alloy matrix composite both in the as-consolidated state and after heat-treatment at 950°C for 430 hours. The strength and fracture mode of fibers extracted from the as-consolidated composite was comparable to that of virgin fiber. However, fibers from the heat-treated composite demonstrated lower tensile strength and fractured from surface flaws created by chemical reaction between the fiber and matrix. A fracture mechanics analysis of the relationship between the size of these surface flaws and fiber tensile strength enabled the calculation of the fiber fracture toughness (2.2 MN V4). The validity of this toughness value for internally-initiated tensile failures was verified by comparison of observed and predicted flaw sizes.

The ring groove areas of squeeze-cast Al-12% Si alloy pistons can be selectively reinforced with Saffil (Al2O3) fibres or SiC whiskers to provide local high temperature strength and wear resistance. Since the reinforced region and the unreinforced alloy typically have different coefficients of thermal expansion, cyclic residual stress may occur at the macro-interface between them when it experiences thermal cycling. This could conceivably result in fatigue induced damage at the macro-interface, making it susceptible to failure. To investigate this, the strength of the macro-interface has been measured before and after thermal cycling using bimaterial tensile samples. Prior to thermal exposure, samples typically failed at the macro-interface with an average strength less than that of the unreinforced alloy alone. The low initial strength has been attributed to several factors, including poor alloy-reinforcement bonding and an accumulation of brittle particles or other material at the macro-interface. After being thermally cycled 1000 times between 50 °C and 275 °C or given an equivalent isothermal exposure, samples typically failed in the unreinforced alloy or at the macro-interface with average strengths less than those measured prior to thermal exposure. However, there was no clear evidence that fatigue induced damage had occurred as a result of thermal cycling and the strength drop associated with thermal exposure has been attributed to alloy overageing.

An interesting cubic-shaped precipitate was observed in the matrix of a squeeze-cast Al-4.3 wt % Cu-2.0 wt % Mg/SiC composite which was heat-treated to a T7 condition. Although this phase had been observed by a few investigators in the past, it had never been examined in detail until now. This cubic phase generally had an edge length ranging from 30 to 50 nm and existed in volume fractions as high as 3.8%. Theoretical strengthening models predicted this phase to have good potential for precipitate strengthening. In addition, the cubic phase exhibited a low rate of coarsening at temperatures as high as 250 °C; apparently due to its low interfacial energy. Consequently, this cubic precipitate shows potential for increasing the useful temperature range of aluminium alloys and composites and could be of great importance to the aerospace and automotive industries.

Unidirectionally-reinforced Timetal® 21S composite specimens were subjected to elevated temperature heat treatments. The SiC fibers were then chemically extracted from the matrix, and their tensile strengths were measured at room temperature. A Weibull statistical analysis of fiber strength distribution was performed to compare the Weibull parameters of fibers from the as-consolidated and heat-treated composites. Fractographic analysis of the tested fibers was used to identify the flaws which caused failure in each condition. Surface flaws were found to initiate low strength failures in all conditions, and the number of surface initiated failures increased with an increase in severity of heat-treatment. A relationship between the fiber/matrix chemical reaction and surface flaw development is demonstrated. A fracture mechanics analysis that explains the relationship between surface flaw size, fiber fracture toughness, and the measured tensile strengths is suggested.

Various metal oxides are potential reaction barriers in titanium/silicon carbide composites. A sol-gel process utilizing metal alkoxides was developed to coat silicon carbide fibres with relatively thick, crack-free, adherent layers of yttrium and calcium oxide. A multiple-dip technique was employed in order to avoid cracking due to large residual thermal and drying stresses produced in the film. The influence of several processing parameters and the geometry of the substrate on the coating thickness is discussed.

The effect of thermal exposure on the mechanical properties of Timetal 1100/SCS-6 (SiC) fiber composites has been evaluated. Unidirectional composites were exposed and tensile tested in the transverse (90°) and longitudinal (0°) orientation. Two panels were sectioned into samples, thermally exposed, and tested in the transverse orientation; one contained well-spaced fibers, while the other had a high degree of fiber-fiber contact. Likewise, two groups were tested in the longitudinal orientation. These panels all had good fiber spacing, but some contained large regions of poor matrix-ply bonding, while others were predominantly well-consolidated. The poor spacing in the transverse samples and the poor matrix-ply bonding in the longitudinal samples strongly affected the residual properties of the composite following exposure. In effect, these defects increased the volume of the matrix that was embrittled by the environment during exposure in air, thereby resulting in degradation of the matrix and the fiber-matrix interface. Well-consolidated composites with good fiber spacing retained a large percentage of their as-fabricated properties following exposure.

An Al-4.3 wt% Cu-2.0 wt% Mg alloy reinforced with 20 vol% reinforcing fibres was examined after a T7 heat treatment. The expected precipitate phase was equilibrium S′ (Al2CuMg), which was confirmed to form in the monolithic alloy. However when this Al-Cu-Mg alloy was squeeze-cast into a fibre preform and given an identical T7 heat treatment a number of other phases also nucleated; these included θ′ (Al2Cu), β′ (Mg2Si) and the cubic σ phase (Al5Cu6Mg2). These additional phases were determined to nucleate and grow rapidly during the water-quench following solution treatment. The existence of excess Si (approximately 0.5 wt%) in the matrix was determined to be responsible for nucleation of these additional phases. This extra Si entered the composite matrix during squeeze-casting through breakdown of an SiO2 layer which existed at the fibre interfaces. During quenching Si clusters rapidly form and provide nucleation sites for the σ and θ′ phases. The Si clusters apparently created a compressive strain in the matrix which attracted a high concentration of small Cu atoms to their interface. The σ phase nucleated in this high-Cu region since, on a localized scale, σ became the equilibrium phase. This type of nucleation process may also explain the enhanced precipitate nucleation which occasionally takes place in other alloy systems when trace amounts of certain elements are added.