TiB Needles Research Articles

Investigation of the Young's modulus of TiB needles in situ produced in titanium matrix composite

In situ Ti/TiB composites with different volume fractions of discontinuous TiB reinforcements were produced by powder metallurgy. After compacting Ti+TiB 2 powders by hot unidirectional pressure, heat treatments led to the in situ formation of distinctive needles of TiB, randomly distributed in the titanium matrix. The Young's modulus of TiB was evaluated using the ASW computation method and experimental Vickers micro-indentation. Three point bend tests were performed on Ti/TiB composites as a function of the TiB volume fraction in order to extract the Young's modulus of TiB from the elastic properties of the composite. The different values obtained according to these three methods were discussed and compared with the literature.

Stabilisation of TiB x-Coated SiC Fibres by Nitridation

Titanium diboride (TiB2) is a ceramic of semi-metallic nature which has many attractive properties for high temperature structural use. Its room temperature elastic modulus is very high (540 GPa . 440 GPa for SiC or TiC). It has good wear resistance and is resistant to chemical attack. Because of these properties it has been used as a reinforcement in composite systems of metal and ceramic matrices, in the form of particles and fibres [1, 2], and as a protective coating for SiC fibres [3]. The latter is of particular interest in the development of Ti-based fibre composites in recent years, due to a persistent drive from the aero-engine industry for improved high-temperature materials, excellent specific strength offered by Ti alloys, and commercial scale production of high strength/stiffness SiC monofilaments, notably, the SCS series from Textron and the Sigma variations from DRA Sigma [3]. Unfortunately, the potential applications of these composites are shadowed by their unsatisfactory interface stability. C-coated SiC fibres tend to react with the matrices at elevated temperatures forming brittle carbides [4, 5]. The innovation of a TiB2/C-double coating on SiC, aiming to improve the stability, was marred by the formation of TiB needles found at the matrix/TiB2-coating interface for Ti-based alloys [6, 7]. Such reactions are also seen in TiB2 particle-reinforced Ti alloys [1]. The needles may enhance stress concentration and cracking at the interface and are highly undesirable. Improving the interface chemical stability is thus of high priority in order to ensure the successful use of TiB2 in composite materials. Previous analysis of the reaction mechanisms have confirmed that rate controlling step is the relatively fast boron diffusion across the reaction layer towards the Ti matrix [8, 9], particularly along rapid diffusion paths [7]. Therefore, in order to stabilise the interface, it is essential to inhibit boron diffusion. Here a method of stabilising the surface of TiBx coating and hence the interfaces it forms with other materials, particularly, Ti matrices, is reported.

In situ preparation of titanium base composites reinforced by TiB single crystals using a powder metallurgy technique

The feasibility of Ti/TiB composite by in situ precipitation of the reinforcement have been investigated. The titanium monoboride can be obtained by the chemical reaction between TiB 2 and Ti powders. The fabrication method requires two stages: the first stage corresponds to the compacting of the pre-blended powders and the second one to the nucleation and growth of the TiB needles. By controlling the second stage, it is possible to monitor the density and the aspect ratio of the reinforcement. Bend tests have been performed on these composites reinforced with 20% in volume of discontinuous TiB needles and it has been demonstrated that the load transfer is efficient to get a 50% improvement of the Young's modulus of the titanium. The simplicity of this technique makes this material very attractive for numerous applications.

The kinetics and mechanism of interfacial reaction in sigma fibre-reinforced Ti MMCs

Abstract Interfacial reaction between titanium matrix and reinforcement plays a crucial role in determining the mechanical properties of titanium metal matrix composite materials. In order to improve the mechanical properties of composite materials, it is essential to understand the thermodynamics and kinetics of such interfacial reactions. Ti-6Al-4V foils and C/TiBx-coated SiC fibres were used to fabricate composite materials by diffusion bonding. The interface formed after annealing at different temperatures has been characterized mainly by scanning and transmission electron microscopies to establish the reaction kinetics between the TiBx coating and Ti matrix. It is found that the major reaction product is TiB needles, although a TiB2 layer is also present as a transition phase during the initial stage of the reaction. Experimental results indicate that, at a temperature between 870 and 970°C, the growth rate of TiB needles along the needle direction is more than six times of that of the TiB2 layer. After a detailed analysis of the crystal structures and the growth morphologies of both TiB and TiB2, the diffusion mechanisms for B atoms in TiB and TiB2 have been identified as vacancy diffusion. However, the low activation energy path for B diffusion in TiB is in the [0 1 0]TiB direction, effectively one-dimensional, while that in TiB2 is along 〈 1 1 ¯ 00 Ȳ T i B 2 directions, which form a two-dimensional network. In addition, it is found that the estimated diffusion coefficient for B in TiB along the needle direction is about 45 times larger than that in TiB2, although the activation energies for B diffusion in both TiB and TiB2 are effectively the same, being 187–190 kJ mol−1.

Transmission electron microscopy study of Ti/SiCfcomposites fabricated by vacuum plasma spraying and vacuum hot-pressing

Abstract The microstructures of Ti/SiCf composites fabricated by vacuum plasma spraying (VPS) of Ti onto SiC fibres with a duplex TiB2/C protective coating followed by subsequent vacuum hot-pressing (VHP) have been investigated by scanning and transmission electron microscopy. After VPS, the TiB2 coating consists of three different sub-layers: (1) an inner ∼ 0·3 μm thick sub-layer of a few large acicular and round TiB2 crystals embedded in a nanocrystalline matrix TiB2; (2) a middle ∼ 0·2 μm thick sub-layer of amorphous B; and (3) an outer 0·3 μm thick sub-layer of amorphous TiB2. After VPS and VHP, microcracks are present at the Ti/SiCf interface. No interfacial reaction products are found at the interface between the outer-TiB2 coating and the sprayed Ti droplets after VPS. After VHP, TiB needles are formed at the interface between the outer-TiB2 coating and the Ti matrix. A high density of planar faults is also found in the TiB needles. The orientation relationship between the TiB and the α-Ti is: [010]γiBTiB∥[1120]αi (100)TiB∥(0001)α. After VHP, large dislocated TiC particles are formed at the TiB/C interface, and the interfacial reactions are uneven so that in some regions the C coating is consumed, and Ti reacts directly with the SiC fibres to form a fine scale mixture of Ti5Si3 and TiC particles.

Interface microstructures in Ti-based composites using TiB 2/C-coated and uncoated SiC f 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 TiB 2/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 TiB 2/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 TiB 2/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.

The effect of temporary hydrogenation on the processing and interface of titanium composites

Hydrogen as a temporary alloying element in titanium increases its workability at elevated temperatures. It can also be easily extracted from titanium under vacuum conditions. Therefore solid-state consolidation of Ti-based composites may make use of this reversible hydrogenation behaviour of titanium to either reduce the consolidation temperature or shorten the consolidation time at a given temperature, so as to avoid harmful interface reactions. This article reports findings in processing Ti-6Al-4V and Ti-1100 based composites using hydrogen-charged foils and by in situ hydrogenation. Hydrogen-aided processing was found to substantially increase the rate of consolidation at a given temperature, particularly when using a Ti-6Al-4V matrix. This enabled full consolidation at a relatively low temperature without any reactions at the interfaces. In particular, the long TiB needles that have decorated the interfaces in conventionally processed samples were not formed in the hydrogen-treated samples. Both reduced thermal activation at the low temperature and solute hydrogen in the matrix are believed to inhibit boron diffusion through the matrix and so the formation of the borides. Clean and intimate fibre/matrix interface bonding was observed in the hydrogen-treated samples, while conventionally processed samples showed less well bonded interfaces. Owing to the limited period of dehydrogenation after consolidation, hydrogen residuals were not thoroughly removed. Hydrides were found in a boundary β-phase. Further evacuation is required to drive off the excess hydrogen in the processed samples.

As received Ti–6AI–4V/σ-SiC fibre composite

A Ti–6Al–4V/σ (SM 1240) composite prepared by diffusion bonding has been studied in the as received condition, using Auger electron spectroscopy, transmission electron energy loss spectroscopy, and scanning electron microscopy. The SiC based σ fibre has a tungsten core, and a duplex coating of carbon (adjacent to the SiC deposit) and TiBx. It is shown that boron from the TiBx layer diffused into the matrix and formed TiB needles. Carbon was detected in the TiBx layer and was present in elemental free form. A continuous SiO2+ carbon layer was detected at the SiC/carbon layer interface. Analysis of in situ fracture composite surfaces in an Auger spectrometer has shown that the tensile failure was initiated within the carbon layer or at the TiBx/matrix interface. An oxide layer detected at the TiBx/matrix interface influenced the fracture behaviour of the composite.MST/2027

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- α 2, 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- α 2 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.

Interface studies in a Ti-6Al-4V/sigma fiber composite

The paper reports results from an on going study of the interfaces in diffusion bonded Ti-6Al-4V/Sigma composites. This paper concentrates on the analysis of a composite in the as-received condition using Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). The SiC based Sigma fiber has a tungsten core and a duplex coating of carbon (adjacent to the SiC deposit) and TiB x . It is shown that boron from the TiB x layer diffused into the matrix and formed TiB needles. Carbon was detected in the TiB x layer. Analysis of in-situ fracture surfaces has shown that the failure was initiated either within the carbon layers or at the TiB x -matrix interface. The oxygen detected at the TiB x -matrix interface influenced the fracture behavior of the composite

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 SixTiy brittle phases.

Reactions between Ti 3AlNb and mixed second phases

In engineered multiphase materials, the interfacial characteristics that develop between individual phases during high-temperature exposure can play a significant role in determining mechanical behavior. In titanium alloys containing silicon carbide and boron fibers, rule-of-mixtures strength values are not realized, primarily because of the chemical reactions that occur between the matrix alloys and these phases. Previous investigations of the reaction between titanium alloys and boron have shown that a uniform reaction layer of the phase TiB/sub 2/ is formed, followed by growth of TiB needles. The primary mechanism of this reaction was observed to be the one-way diffusion of boron into titanium. The use of a B/sub 4/C coating on the boron (B/sub 4/C/B) as a diffusion barrier was found to significantly slow the reaction between the titanium and boron. Certain alloy additions to titanium, such as aluminum, vanadium, and molybdenum, can increase the chemical compatibility of these two phases. For the case of SiC, the reaction zone is formed by the interdiffusion of silicon and carbon atoms with titanium. X-ray diffraction studies have shown the reaction components to include TiC, Ti/sub 5/Si/sub 3/, and TiSi/sub 2/. In the present study, the reactions between a Ti/sub 3/Al alloy and both B/submore » 4/C/B and SiC have been examined. Reaction models are proposed to describe the development of the chemical interaction zone formed between these phases.« less