SiC matrix composites reinforced with internally synthesized TiB2

A new process of preparing particulate-reinforced ceramic composites by internal synthesis has been developed. SiC powder mixed with TiN and amorphous boron was hot-pressed above 2000° C in an argon atmosphere. The boron molar content in the mixture was designed to be more than twice that of TiN. In the process of hot-pressing, the following reaction took place between 1100 and 1700° C TiN+2B TiB₂+1/2N₂ The synthesis of TiB₂ was followed by the densification of SiC matrix with the aid of the excess boron. The new process provides SiC matrix composites in which fine TiB₂ particulates are dispersed. Compared with hot-pressed monolithic SiC, the composite containing 20 vol % TiB₂ exhibits a 80% increase in fracture toughness and about the same flexural strength of 490 MPa at 20° C in air and 750 MPa at 1400° C in a vacuum.     

Thermal treatment effects in SiC/Al metal matrix composites

A short-fibre-reinforced SiC/Al-7% Si-0.6% Mg composite has been subjected to thermal cycling and elevated temperature isothermal exposure treatments. The microstructure and residual mechanical properties have been determined as a function of these treatments. It was found that isothermal or cycling treatment at 350 °C caused severe room-temperature strength degradation while treatment at 525 °C caused little change. Strength changes are attributed principally to precipitation and dissolution effects and to transfer of magnesium from the matrix into the fibre/matrix interfacial reaction layer. There was no evidence of mechanical damage resulting from the cycling treatments.     

The production of Al-Mg alloy/SiC metal matrix composites by pressureless infiltration

Metal matrix composites have been produced by pressureless infiltration of Al-Mg alloys into SiC preforms at 900°C under N₂ for different infiltration times. The wettability of the ceramic reinforcement by the Al-Mg alloy is crucial in determining whether an MMC can be produced by pressureless infiltration. Sessile drop results show that Al alloys with Mg contents greater than 8 wt% had a contact angle lower than 90°C after 5 minutes contact time. This was in agreement with the pressureless infiltration results as MMCs have been produced after 30 minutes with these alloys. Sessile drop experiments also show that SiC is similarly wetted by Al-Mg alloys under both N₂ and Ar. It is concluded that the infiltration process does not involve the intermediate nitride phase suggested by other authors.     

三维网络SiC/Cu金属基复合材料的凝固显微组织

用挤压铸造法制备了三维网络SiC/Cu金属基复合材料,研究了铸造压力、网络SiC骨架预热温度、浇注温度等工艺条件对复合材料凝固显微组织的影响.结果表明,三维网络SiC陶瓷骨架在晶体生长和结晶过程中有重要作用,在一定条件下在网孔内可形成垂直于骨架表面的枝晶网络,或形成粒度细小且分布均匀的等轴晶组织;骨架的孔径对显微组织的影响也很大,细小的孔径有利于晶粒细化和组织均匀化,粗大的孔径助长宏观偏析和铅的偏聚.骨架减轻了复合材料中锡的反常偏析,使锡的偏析主要发生在骨架表面附近的微小区域,从而避免了在铸件表层的集中偏析.     

Simple model for consolidation of metal matrix coated SiC fibre composites

Abstract

A simple approach to modelling the consolidation of matrix coated fibre composites is presented. It employs an existing porous material constitutive model for monolithic materials. It is argued that in the consolidation of metal coated SiC fibres, the deformation primarily occurs in an outer layer of the fibre coating, and the internal core remains undeformed, largely because of the generally hydrostatic compressive loading, and because of the incompressible nature of the material in creep. The consolidation process is therefore not vastly different to that occurs for monolithic metal fibres, and similar equations can therefore be used for the composite consolidation. The constitutive equations have been implemented into general purpose non-linear finite element software within a large deformation formulation by means of two different user subroutines, one providing a general implementation, and the other a cpu time efficient approach. The manufacture and testing of SiC continuous fibre, Ti-6Al-4V metal matrix composite specimens is described and the results of the tests compared with the model calculations, showing that good agreement can be achieved with a simple model. The dependence of volume fraction of fibres and temperature can be introduced empirically through the specification of just two material constants. The model is therefore useful in the development of consolidation processes.     

Microstructural analysis of isothermally exposed Ti/SiC metal matrix composites

Specimens of diffusion-bonded titanium metal matrix composites have been subjected to thermal exposure treatments and examined principally by transmission electron microscopy. The fibres investigated were SCS-6 and Sigma. The fibre/matrix reaction layers have been shown to consist of titanium carbide and two titanium silicides. The reaction proceeds by the initial formation of a layer of TiC followed by a layer of mixed silicides, Ti₅Si₄ and Ti₅Si₃. Extensive porosity is generated during the reaction and this prevents the formation of a completely protective interfacial layer.     

Interfacial reaction kinetics of SiC fiber-reinforced Ti3Al matrix composites

• 1.(1) SiC fibers reacted with Ti₃Al + Nb to form multilayer reaction products during consolidation and extended isothermal exposure. Complex carbides and silicides genertated from the reactions between Ti, Nb, Al and SiC appears to be the major components in the reaction zone. The C-rich layer on the surface of a SiC fiber affects the development of the reaction zone and the distribution of the reaction products.
• 2.(2) The fiber/matrix interfacial reaction is diffusion-controlled with an activation energy of 271.49 KJ/mole and 218.11 KJ/mole for the SCS-6/Ti-24A1-11Nb and Sigma/Ti-24Al-11Nb composites, respectively, at the early stage of reaction.
• 3.(3) The results of this study indicate that in the SCS-6/Ti-24Al-11Nb composite, the activation energy is higher, the growth rate of reaction zone is slower, and the consumption of C-rich layer is much slower than that in the SCS-6/Ti-6Al-4V composite.

     

Interactions between tungsten carbide (WC) particulates and metal matrix in WC-reinforced composites

A variety of experimental techniques have been used to investigate the interactions between tungsten carbide (WC–Co 88/12) particulates and the matrix in some new wear resistant cobalt-based superalloy and steel matrix composites produced by hot isostatic pressing. The results show that the chemical composition of the matrix has a strong influence on the interface reaction between WC and matrix and the structural stability of the WC particulates in the composite. Some characteristics of the interaction between matrix and reinforcement are explained by the calculation of diffusion kinetics. The three-body abrasion wear resistance of the composites has been examined based on the ASTM G65-91 standard procedure. The wear behavior of the best composites of this study shows great potential for wear protection applications.     

Precursor routes to Group 4 metal borides, and metal boride/carbide and metal boride/nitride composites

Precursor routes to Group 4 metal borides, as well as metal-boride/carbide and metal-boride/nitride composites, that employ metal-oxide/polymer and metal/polymer dispersions are described. The metal boride precursors were initially obtained by dispersing metal oxides in a decaborane-dicyanopentane polymer, but better results have now been achieved using a newly developed polyhexenyldecaborane polymer. Subsequent pyrolyses of the metal-oxide/polymer dispersions afford metal borides, including TiB₂, ZrB₂, and HfB₂ in high chemical and ceramic yields. On the other hand, pyrolyses of hafnium/polyhexenyldecaborane dispersions provide an efficient route to hafnium-boride/carbide composites. Metal-boride/nitride composites, including TiB₂/TiN and HfB₂/HfN materials, are also readily obtained by pyrolyses of metal dispersions in the boron-nitrogen polymer polyborazylene, (B₃N₃H₄) _X . The high ceramic yields of the metal-boride/nitride systems (98%), enable their use for the controlled formation of shaped monolithic TiB₂/TiN or HfB₂/HfN materials by pyrolysis of pressed precursor green bodies.     

Synthesis of mixed disilicides/SiC composites by displacement reaction between metal carbides and silicon

Displacement reactions between metallic carbides (of Mo, W, Ta and Nb) and Si have been used for the preparation of mixed disilicide/SiC composite materials (Mo–W)Si₂/SiC and (Nb–Ta)Si₂/SiC. Pieces were consolidated by hot pressing and subsequently analysed by X-ray diffraction, scanning electron microscopy and microhardness testing. Mixed-disilicide/SiC composites were easy to obtain, pieces exhibiting a very homogeneous microstructure and strength and toughness.     

Wettability of transition metal boride eutectic alloys to graphite

The wetting of nickel, platinum and palladium boride eutectics (Ni₅₅B₄₅, Ni₆₂B₃₈, Pd₇₂B₂₈, and Pt₇₂B₂₈) to polycrystalline graphite has been studied by sessile drop and surface analytical techniques. The wettability is critically influenced by surface segregation of low-level bulk impurities in the alloys. At the melting temperature, the impurities segregate to the surface of the liquid alloy and inhibit wetting. A poorly-wetted droplet with a large contact angle results. Good wetting results by introducing materials into the contact system which possess high chemical reactivity to the major segregant species, either as a surface coating or by incorporation into the alloy during manufacture. The materials act to suppress segregation by tying up the alloy impurities through compound formation. The results indicate that the use of surface sensitive techniques in the study of wetting is essential.     

Joining of SiC fibre reinforced borosilicate glass matrix composites to molybdenum by metal and silicate brazing

The brazing of SiC fibre reinforced borosilicate glass matrix composites with Mo plates has been investigated. Molybdenum was chosen as the metallic partner under consideration of system requirements, e.g. thermomechanical stability at temperatures of interest (500–750^∘C), and physical properties, e.g. coefficient of thermal expansion close to that of the glass matrix composite. Two brazing filler materials were investigated: a glass braze (Schott G018-174) and an active filler metal (Incusil ABA, brazing temperature = 740^∘C). When using the glass braze the surface of the metal had to be roughened to ensure a bond of significant strength. Vacuum brazing with the active filler metal resulted in joints with high strength, which allows to fully utilise the mechanical competence of the glass matrix composite when the joint configuration is adapted to the relevant loading conditions. A novel design of a tool for hot glassware handling, made of glass matrix composite/Mo joints, is presented.     

Cast metal matrix composites

Abstract

Metal matrix composites have been available in certain forms for at least two decades, e.g. boron fibre reinforced aluminium and various dispersed phase alloys and cermets. Recently, a range of alumina and silicon carbide fibres, whiskers, and particles with diameters <20 μm have become available. The possibilities of incorporating these materials into metals to improve stiffness, wear resistance, and elevated temperature strength without incurring weight penalties have attracted the attention of design engineers in the aerospace and automobile industries. The aim of the present paper is to outline the manufacturing processes for such composites, in particular those based upon liquid metal technology, e.g. squeeze casting and spray forming. Some of the mechanical and physical properties which have been determined for these materials are described. An analysis of how matrix alloy selection may influence tensile and fracture behaviour of short fibre and particle reinforced composites is attempted.

MST/770     

Interface microstructure and reaction in Gr/Al metal matrix composites

The interface structure in Gr/Al composites fabricated with liquid metal infiltration has been studied using transmission electron microscopy (TEM). Morphologies of interfacial reaction product, aluminium carbide Al₄C₃, formed at different manufacturing parameters were compared and, the growth mechanism of the carbide was studied by means of high resolution electron microscopy (HREM). It has been shown that the morphology of the carbide is intimately related to the processing parameters with which the composites were produced. There are two kinds of interfaces between the carbide and the aluminium matrix. They have different growth mechanisms and relative growth rates under different growth driving forces. Several crystal orientation relationships between the carbide and the aluminium matrix have been observed.     

SiC泡沫／Al双连续相复合材料连续性的研究

运用挤压铸造法制备了SiC泡沫／Al双连续性复合材料，研究了SiC泡沫、复合压力和合金成分对复合材料连续性的影响。结果表明，SiC泡沫陶瓷的加入阻碍了基体合金流动，降低了复合材料的连续性。随着复合压力的增加，复合材料的连续性逐渐增强，当压力为150MPa时，复合材料的连续性最好。随着含硅量的增加，基体合金的热膨胀系数逐渐降低，基体和增强体之间的热膨胀匹配增强，复合材料中残余应力降低，复合材料的连续性增强。