Thermal stability of interfaces in Ti-6Al-4V reinforced by SiC Sigma fibres

Interfacial reactions in Ti-6Al-4V/SiC Sigma fibres (coated with carbon and TiB₂) were studied at different temperatures (600, 700 and 1000 °C). Interface microstructure was investigated by scanning electron microscopy and Auger electron spectroscopy. A simulation of the chemical phenomena occurring at the interfaces was carried out using powders of pure titanium, carbon and TiB₂; the reaction products were identified by X-ray diffraction. The double coating of Sigma fibres is effective in delaying detrimental reactions with the matrix. At the interfaces matrix/TiB₂ and TiB₂/C, the TiB and TiC_x phases form, respectively. The protective coating of fibres shows a lifetime greater than 1000 and 750 h at 600 and 700 °C, respectively.     

Effect of the filament nature on fatigue crack growth in titanium based composites reinforced by boron,B(B4C) and SiC filaments

Crack propagation testing has been applied to synthetic metal matrix composites (MMC) in order to compare failure mechanisms in Ti-6Al-4V alloy reinforced by uncoated boron, B(B₄C) and chemical vapour deposition (CVD) SiC filaments. The impeding effect of the fibres leads to low crack growth rates, compared to those reported for the unreinforced Ti-6Al-4V alloy and to higher toughness despite the presence of the reinforcing brittle phases. After long isothermal exposures at 850° C, the MMC crack growth resistance is reduced mainly due to fibre degradation, fibre-matrix debonding and an increase in matrix brittleness. However, for short-time isothermal exposures (up to about 10 h for B/Ti-6Al-4V, 30 h for B (B₄C)/Ti-6Al-4V and 60 h for SiC/Ti-6Al-4V) the crack growth resistance is significantly increased. This improvement is related to the build up of an energy-dissipating mechanism by fibre microcracking in the vicinity of the crack tip. This damaging mechanism allowing matrix plastic deformation is already effective for boron and B(B₄C) in the as-fabricated state, but occurs only after 10 h of thermal exposure at 850° C in the case of SiC/Ti-6Al-4V composites.     

The effect of the interface on the thermal conductivity of titanium-based composites

Thermal conductivity measurements have been made on composites of unalloyed Ti reinforced with short fibre SiC, particulate SiC and particulate TiB₂, and Ti-6Al-4V reinforced with long fibre SiC. Measurements were also made on the unreinforced matrices and on the reinforcements themselves. The results have been compared with predictions from the Hasselman and Johnson model for spherical and long fibre composites and from the Eshelby model for ellipsoids of any aspect ratio. It is shown that, while an enhancement of the thermal conductivity is expected to arise from the presence of both types of particulate, this effect is in fact produced only with the TiB₂ reinforcement. This is explained in terms of the large thermal resistance of the SiC/Ti interface, which is in turn related to the nature of the reaction layer formed there during processing. The transverse conductivity of the Ti-6Al-4V reinforced with long SiC fibres was also impaired by the high thermal resistance of the interface, but in this case the effect was less significant because of the larger diameter of the reinforcement, which reduces the frequency with which transport of heat across an interface must occur. Finally, similar characteristics were exhibited by the short SiC fibre/Ti composite, except that the axial conductivity was rather lower than expected. This effect is attributed to matrix porosity localized at the fibre ends.     

Interfacial reactions in a Ti-6Al-4V based composite: role of the TiB2 coating

The characterization of TiB₂/C-coated SiC fibres and their interface region in a Ti-6Al-4V based composite has been performed by using scanning electron microscopy (SEM), energy-dispersion X-rays (EDX) and Auger electron spectroscopy (AES). The features of the as-received fibre and the reactivity between fibre and matrix occurring during preparation of the composite have been studied in this paper. The interaction of the TiB₂ external coating of the fibre with both the adjacent carbon layer and the titanium-based matrix is already appreciable in the as-received composite: TiB needles grow from TiB₂ towards the matrix and a new layer containing C, Ti and B appears between TiB₂ and C. The thicknesses of the original carbon and TiB₂ fibre coatings decrease in the composite from 1000 nm to 400 and 800 nm, respectively. The TiB₂ inhibits the reaction between SiC and Ti: there is no evidence of Si _x Ti _y brittle phases.     

The CVD of Ceramic Protective Coatings on SiC Monofilaments for Use in Titanium Based Composites

TiB₂, TiC and TiN protective coatings have been deposited onto SiC monofilament fibers by the CVD technique using a cold-wall reactor at reduced pressure. The effect of deposition conditions on the morphology, microstructure, phase composition and adherence of the coatings were studied. The physical and chemical compatibility of these ceramic coatings with SiC filaments together with titanium-alloy matrices were assessed and compared. Dense and uniform TiB₂ coatings have been deposited successfully onto SiC monofilaments. The coating is stable on the SiC fiber and acts as an effective: barrier against the vigorous SiC/Ti-6Al-4V interfacial reaction. The adhesion of TiC on SiC fibers is comparatively weaker than the TiB₂ coating. There was no significant reaction found at the interface of TiC/Ti except at the interface of TiC/SiC. Examination of the TiN coatings showed severe cracking and spalling, hence TiN could not provide protection in a Ti-alloy matrix.     

Fabrication of TiC/TiB/Ti3AlC2 phases reinforced coatings on Ti-6Al-4V substrate

Intermetallic matrix composite coatings reinforced by TiC, TiB₂, and Ti₃AlC₂ were fabricated by laser cladding the mixed power Ti, Al, and B₄C on the Ti-6Al-4V alloy. X-ray diffraction, scanning electron microscope, and energy dispersive spectroscopy were chosen to investigate the structures and morphologies of the coatings. Results showed that the coatings mainly consisted of the reinforcements of TiC, TiB₂, and Ti₃AlC₂ and the matrix of Ti₃Al, TiAl, TiAl₃, and α-Ti. The hardness and wear-resisting property of the prepared specimens of Ti-45Al-10B₄C and Ti-45Al-20B₄C were studied contrastively. It was found that the coating was metallurgical bonded to the Ti-6Al-4V substrate. The micro-hardness and dry sliding wear-resisting properties of the specimen of Ti-45Al-20B₄C were enhanced further. And the micro-hardness of Ti-45Al-20B₄C was from 900 HV_0.2 to 1225 HV_0.2. The wear-resisting property of Ti-45Al-20B₄C was four times as large as that of the Ti-6Al-4V alloys.     

Interface microstructures in Ti-based composites using TiB2/C-coated and uncoated SiCf after short-term thermal exposure

The extent of interfacial reaction after short-term thermal exposure during vacuum plasma spraying (vps) and vacuum hot-pressing (vhp) of Ti-based metal-matrix composites (mmcs) using TiB₂/C-coated and uncoated SiC fibres has been investigated by a combination of scanning and transmission electron microscopies. There is no interfacial reaction after short-term thermal exposure during vps manufacture of SiC_f/Ti mmcs using either TiB₂/C-coated or uncoated SiC_f. There is only limited interfacial reaction after short-term thermal exposure during vhp manufacture of SiC_f/Ti-6Al-4V mmcs using TiB₂/C-coated SiC_f. In the initial stage of the interfacial reaction, TiB needles are formed by preferential nucleation and growth at β particles and grain boundaries in the Ti-6Al-4V matrix.     

Chemistry effects on interface microstructure and reaction in titanium-based composites

The effects of fibre and matrix chemistry on the interface microstructure and stability of Ti-based composites were investigated using Sigma SiC monofilaments as reinforcements and different titanium alloys, namely Ti-6Al-4V, Ti-1100 and Super-α₂, as matrices. Both monomatrix composites and ‘hybrid’ composites were processed by a solid-state diffusion-bonding technique, respectively incorporating either one type of matrix or different matrices into a single sample. Interface microstructures and reaction were analysed with respect to the reactivities of the interfaces and the diffusivities of the relevant elements involved. When the coated fibre was used, TiB needles were found in Ti-6Al-4V and Ti-1100 matrices but not in the Super-α₂ matrix. The reaction rate was slower in a more heavily alloyed matrix. The question of optimizing interface microstructure through matrix chemistry and hybrid composites is addressed.     

Assessment of diffusion barriers for Ti–SiC composites

Abstract

The application of chemical vapour deposition and physical vapour deposition coatings, either singly or in combination, onto SiC fibres is discussed in terms of their ability to enhance the high temperature stability of Ti–SiC composites. The thermal stability and success of potential barrier layers was assessed by studying the fibre-matrix interdiffusion and measurement of the mechanical properties of individual fibres following coating and thermal exposure. Measurements of the level of strength retention have proved to be a reliable method of assessing the effectiveness of potential diffusion barriers. Failures may result from one of three sources. For high strength fibres failures are SiC–core reaction zone initiated, for intermediate strength fibres failures are surface defect (SiC) initiated and for low strength fibres, failures are fibre–matrix reaction zone or coating initiated. To ensure high strength (i.e. core failures) it is essential that a carbon layer is retained at the SiC surface. The most successful barriers have been shown to be TiB₂ and PtAl₂ coatings preventing outward diffusion of carbon and minimising the interaction with the titanium matrix. From these results a life prediction model has been developed based on the fibre–coating interaction, which will predict fibre strength as a function of time at a given temperature.

MST/3001     

Laser-beam welding of SiC fibre-reinforced Ti-6Al-4V composite

Three- and ten-ply SiC fibre-reinforced Ti-6Al-4V composites were joined using a laser beam. With a 300 m thick Ti-6Al-4V filler metal, fully penetrated welds without apparent fibre damage, could be obtained in welding directions both parallel and transverse to the fibre direction by controlling the welding heat input. Excess heat input resulted in the decomposition of SiC and subsequent TiC formation, and also caused degradation of joint strength. The welding of the three-ply composite in which full penetration was achieved at lower laser power, exhibited higher flexibility in heat input than that of the ten-ply composite. Heat treatment at 1173 K after welding improved the joint strength because of the homogenization of the weld metal and decomposition of TiC. The strengths of the transverse weld joints after the heat treatment were approximately 650 and 550 MPa for the three- and ten-ply composites, respectively. With the welding direction parallel to the fibre direction, the strengths both parallel and transverse to the weld joint were equivalent to those of the base plate.     

Manufacturing Titanium Metal Matrix Composites by Consolidating Matrix Coated Fibres

1.IntroductionTitanium metal matrix composites(TiMMCs)rein-forced by continuous SiC fibre are being developed foraerospace applications in several countries,including theUSA,UK,France and China.TiMMCs possess excellentcombination of specific strength,stiffness,fatigue andcreep resistance at elevated temperatures which may al-low remarkable reduction in component weight[1].Own-ing to the active nature of Ti,several solid-state process-ing techniques have been developed to date for manu-f…     

Fabrication of Co-Based Coatings on Titanium Alloy by Laser Cladding with CeO2 Addition

In this study, Co-based laser cladding coatings reinforced by multiple phases were fabricated on titanium alloy. Co42 Co-based self-fluxing alloy, B₄C, and CeO₂ mixed powders were used as the precursor materials. The coatings were mainly composed of γ-Co/Ni, CoTi₂, CoTi, NiTi, TiC, Cr₇C₃, TiB₂, and TiB phases. A typical TiB₂/Cr₇C₃/TiC composite structure was chosen. It was found that CeO₂ did not influence the phase types of the coating significantly, but was effective in refining the microstructure and enhancing the microhardness and dry sliding wear resistance. Compared with the Ti-6Al-4V titanium alloy, the microhardness and wear resistance of the composite coatings were enhanced by 3.44–4.21 times and 14.26–16.87 times, respectively.     

Charpy impact behaviour of Sigma fibre reinforced titanium composites

Abstract

Charpy impact behaviour of SiC fibre reinforced titanium composites (TMCs)has been examined at a range of temperatures on specimens with fibres orientated longitudinally [0]₈ and transversely [90]₈ to the impact direction. Corresponding monolithic alloys have also been studied. Although monolithic Ti–6Al–4V has the superior ‘bend’strength at room temperature, Timetal 834 is superior at the operating temperatures of gas-turbine components. The TMCs follow the same trends as the monolithic alloys, but their impact strengths are inferior to the monolithic alloys by an order of magnitude. This is attributed to the low impact strength of the SiC fibres, which dominates in [0]₈ specimens, and the low strength of the fibre/matrix interface, which dominates in [90]₈ specimens. For [0]₈ specimens, Ti-6-4 reinforced with carbon coated SiC fibres (Sigma SM1140+) has consistently superior impact strength compared with the other three TMCs. For [90]₈ specimens, Timetal 834 reinforced with TiB_x/carbon coated SiC fibres (SM1240) exhibits the best impact strength at intermediate temperatures.     

SiC纤维增强钛基复合材料界面研究及构件研制

首先介绍了作者近年来在SiC纤维增强钛基复合材料界面反应机理、应力分布和界面调控方面的工作进展,然后介绍了作者在磁控溅射法和箔-纤维-箔法复合材料工艺及构件研制方面的研究工作。     

Fatigue behaviour of duplex metal coated SiC fibre reinforced Ti-15-3 matrix composites

Abstract

Duplex metal (Cu/Mo and Cu/W) coated SiC(SCS–6) fibre reinforced Ti-15-3 matrix composites have been prepared using a hot isostatic pressing process. The effect of the duplex metal coatings on the fatigue behaviour of unnotched SiC(SCS–6) fibre reinforced Ti-15-3 matrix composite has been studied. The fatigue resistance of this fibre reinforced composite is improved by use of the duplex metal coatings. The Cu/Mo and Cu/W duplex metal coating layers prevent debonding of the SCS coating layer from the SiC fibre surface, thus also effectively preventing a reduction in strength of the fibre. During the fatigue test, fibre bridging behind the matrix crack tip reduces the crack growth rate of the matrix; this mechanism is difficult to achieve with the pristine fibre composite. Evolution of the fatigue damage can be quantitatively evaluated by means of a fatigue damage parameter. Matrix crack propagation is the dominant factor responsible for the increase in damage parameter of the composites.     

Interface contribution to the SiC-titanium and SiC-aluminium tensile strength prediction

Fragmentation tests of single SiC filaments embedded in an aluminium (1050 and 5083 alloys) or a titanium (Ti-6Al-4V) matrix have been analysed in an effort to obtain the interface contribution in terms that could be incorporated into a tensile fracture model for unidirectional composites. Depending on the matrix, two regimes of interfacial stress transfer can be distinguished within the whole range of tested temperatures. For the SCS2/5083 system, plastic deformation of the alloy limits the stress transfer, and the interface contribution thus finds its expression in the shear stress of the matrix. for the SCS6/Ti-6Al-4V system, friction is the leading process and the interface contribution strongly depends on the stress state around the fibre. Assuming a temperature dependent compressive radial stress up to 925C, an effective transfer shear stress may be easily calculated for unidirectional SCS6/Ti-6Al-4V composites.     

TEM investigations of interfacial processes in SCS-6 SiC/TiB2/Super α2 composites

Interfacial processes in SCS-6 SiC/TiB₂/Super α₂ composites were investigated by means of analytical transmission electron microscopy (TEM). No reaction between the TiB₂ coating and the carbon coating of the fibres can be found in the as-processed specimen. At the TiB₂/matrix interface the TiB₂ transforms into both a homogeneous layer and needles of TiB which are distributed in the matrix. This process takes place according to the reaction TiB₂+Ti=2TiB. The TiB₂ coating is consumed completely in the specimen heat treated at 900°C for 64 h. At this time TiC and Ti₃AlC form at the TiB/C-coating and the TiB/matrix interface, respectively. Prolonging the holding time at 900°C, reaction layers thicken continuously following parabolic reaction kinetics. The paper additionally discusses the formation mechanisms of the reaction products.     

颗粒增强Ti基复合材料的特性

本文研究了采用粉末冶金工艺制备的碳化钛颗粒(TiCp)增强 Ti-Al-V 复合材料的显微组织、弹性模量、高温强度、蠕变行为及断裂特征。研究结果表明,选用合适的不连续增强相及 Ti合金基体可以获得增强相与基体结合良好、具有高比强、高比模、良好高温性能的不连续增强 Ti基复合材料。这些 Ti 基复合材料具有各向同性,可用于复杂应力状态,可通过热压、热挤压等一般热加工工艺加工成型。     

TiO2 coatings on silicon carbide and carbon fibre substrates by electrophoretic deposition

Electrophoretic deposition (EPD) has been used to obtain TiO₂ coatings on three dimensional (3-D) SiC fibre (Nicalon ®-type) and carbon fibre substrates. Colloidal suspensions of commercially available TiO₂ nanoparticles in acetylaceton with addition of iodine were used. The EPD parameters, i.e., deposition time and voltage, were optimised for each fibre type. Strongly adhered TiO₂ deposits with high particle packing density were obtained. Scanning electron microscopy observations revealed high penetration of the titania nanoparticles into the fibre preforms. The TiO₂ deposits were sintered at 800°C for 1 h in order to produce relatively dense and uniform TiO₂ coatings covering completely the SiC or carbon fibres. For the carbon fibre/TiO₂ system, an effort was made to produce a 3-D titania matrix composite by further infiltration of the porous fibrous preform with TiO₂ by slurry dipping and subsequent pressureless sintering. The 3-D carbon fibre reinforced TiO₂ matrix composites fabricated contained residual porosity, indicating further infiltration and densification steps are required to produce dense composites of adequate structural integrity. For SiC fibre fabrics, oxidation tests in air established the effectiveness of the TiO₂ coating as oxidation protective barrier at 1000°C. After 120 h the increase of weight due to oxidation of coated fibres was more than twice lower than that of the uncoated fibres. TiO₂ coated SiC fibre preforms are attractive materials for manufacturing hot gas filters and as reinforcing elements for ceramic matrix composites.     

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.