Ti6Al4V Matrix Research Articles

Structure evolutions in a Ti–6Al–4V matrix composite reinforced with TiB, characterised using high energy X-ray diffraction

A titanium matrix composite reinforced with TiB was produced using powder metallurgy. A Ti–6Al–4V alloy was chosen to be the matrix, and 12wt.% of TiB2 was used as the boron source for the solid state formation of TiB. The TiB2 to TiB conversion reaction was studied using an in situ high energy X-ray diffraction technique while heat treating the composite. The TiB2 (space group: P6/mmm) converts into TiB-B27 (Pnma), via TiB-Bf (Cmcm). The metastable character of Bf is confirmed here; it is the first phase formed during the conversion and it progressively converts into B27 during elevated temperature heat treatment. A modification of the phase transformation kinetics in the matrix and of the composite β transus temperature (Tβ=1275°C) was also observed, mainly due to gas contamination and intensive work hardening as a result of the mechanical alloying process used to manufacture the material and to a modification of the matrix equilibria.

In Situ Reinforcement of Ti6Al4V Matrix Composites by Polymer‐Derived‐Ceramics Phases

Ti6Al4V‐based metal matrix composites (MMCs) are successfully synthesized via a novel process route. Ti6Al4V powder is premixed with a solid preceramic polymer used as precursor for the in situ formation of a ceramic phase dispersed at the micrometer range. Ball‐milling is used to intimately mix the powders. After the pyrolysis and sintering step, a Ti5Si3 and TiC dispersion within the Ti6Al4V equiaxed grains matrix is formed. The effect of the volume fraction of the polymer‐derived ceramic phase in the Ti6Al4V matrix is investigated in terms of Vickers hardness and wear resistance. The introduction of 10 vol% of ceramic reinforcement produces a homogeneous structure and results in a compact material with improved integrity and enhanced mechanical properties compared to standard cast alloy or Ti6Al4V‐based composites reported in the literature.

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiCf/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10Hz under a maximum applied stress ranging from 600 to 1200MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiCf/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: Smax/μ=1.381−0.152×lgNf. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.

Rapid preparation of TiC reinforced Ti6Al4V based composites by carburizing method through spark plasma sintering technique

A rapid method, using woven carbon fiber cloth as carbon supplier, combining with spark plasma sintering (SPS) technique, was proposed to fabricate TiC reinforced Ti6Ai4V based composites. By this way, carburized layer with the thickness of 4mm can be obtained within 20min. Microstructure analysis reveals that the composites exhibit advantage of good interfaces between the uniformly distributed TiC precipitates and matrix. Investigations on mechanical properties show that compared with Ti6Al4V matrix alloy, the TiC reinforced Ti6Al4V based composites exhibits higher yield strength, higher ultimate compressive strength, and similar ductility under both quasi-static compression and dynamic compression. The failure mechanisms of the composites and its influence on mechanical properties were discussed.

Microstructure and Properties of 3.5 vol.% TiBw/Ti6Al4V Composite Tubes Fabricated by Hot-hydrostatic Extrusion

Tubes of 3.5 vol.% TiB whiskers reinforced Ti6Al4V matrix composites (TiBw/Ti6Al4V) were successfully fabricated by a two-step hot-hydrostatic extrusion process: β extrusion at 1100 °C and subsequent near-β extrusion at 950 °C. The dimensions of tubes were about 7 mm in diameter and 2 mm in thickness. A refined basket-weave structure in Ti6Al4V matrix was achieved at ambient temperature after the extrusion process. Besides, the original network structure formed by TiB whiskers synthesized was broken, while the TiB whiskers were preferentially aligned in the extruding direction. Meanwhile, a fibrous texture was evolved finally, resulting from partial dynamic recrystallization during the β extrusion and the involvement of α phase during the near-β extrusion. The tensile and compressive tests results showed that both the strength and ductility of the tubes were significantly improved. In particular, the tubes exhibited good mechanical properties at elevated temperatures.

Synthesis of a TiBw/Ti6Al4V composite by powder compact extrusion using a blended powder mixture

A Ti–6wt%Al–4wt%V alloy (Ti6Al4V) matrix composite, reinforced by in situ synthesized TiB whiskers (TiBw) has been successfully fabricated by powder compact extrusion using a blended powder mixture. The microstructural characterization of the various extruded samples showed that the different starting powders, pre-alloyed powder plus boron powder or titanium plus Al–40V master alloy powder plus boron powder, had a significant effect on the morphology of the in situ synthesized TiB whiskers. It is also evident that the TiB whiskers affect the microstructural evolution of the Ti6Al4V matrix. The tensile test results indicated that the composite with a dispersion of fine TiB whiskers with high aspect ratios exhibited a high ultimate tensile stress (UTS) and yield stress (YS) of 1436MPa and 1361MPa, respectively, a reasonably good tensile ductility reflected by an elongation to fracture of 5.6% was also achieved. This is a significant improvement compared with as-extruded monolithic Ti6Al4V alloy produced in this study.

On the exploitation of thermoelectric coupling for characterization of elliptical inclusions in metals

A comparison between reported analytical results with experimental data of the magnetic flux density on cylindrical tin inclusions of elliptical cross-section embedded in a copper matrix under external thermal excitation is presented. By changing the aspect ratios b/a designated by e of the elliptical inclusions, a wide range of real situation such as slender inclusions can be simulated. The aspect ratio of the elliptical cylindrical inclusions varied from 0.50 to 3.250. A fairly modest 2.3°C/cm temperature gradient in the specimen produced magnetic flux densities ranging from 2 to 100μT at 2mm lift-off distance between the tip of the magnetometer probe and the specimen. The experimental magnetic field distribution illustrated the potential for the non contacting thermoelectric technique to detect and characterize metallic inclusions of different geometries based on their magnetic signature. Preliminary results on a cylindrical hard alpha (TiN) inclusion embedded in Ti–6Al–4V matrix is also presented to demonstrate that the proposed method might be applicable to a wide range of alloys including high-strength, high-temperature engine materials.

SEM in situ study on the mechanical behaviour of SiC f/Ti composite subjected to axial tensile load

Ti–6Al–4V matrix composites reinforced by SiC fibers with and without carbon coating (SiC f/Ti–6Al–4V) were prepared by Foil–Fiber–Foil (FFF) method and subjected to heat treatment at 900 °C for 50 h. Subsequently in situ Scanning Electron Microscopy (SEM) technique was used to study fracture mechanism of the composites under axial tensile loads. The influence of fiber coating, interfacial reaction zone and titanium alloy microstructure on initiation and propagation of cracks was investigated in detail. The results show that upon loading, the non coated fibers crack and fracture into smaller fragments. However, the carbon coated fiber does not develop any fracture during the entire experiment and a part of the fiber is pulled out during the final fracture.

Texture induced anisotropy in extruded Ti–6Al–4V–xB alloys

The tensile properties of extruded Ti–6Al–4V–xB alloys (wt.%) were evaluated in an orientation perpendicular to the extrusion direction at room-temperature and 455°C. The extrusion process preferentially oriented the basal plane of α-Ti perpendicular to the extrusion axis. This strong α-phase texture resulted in tensile anisotropy. The tensile strength in the transverse orientation was lower than that in the longitudinal orientation, but it remained greater than that for the as-cast Ti–6Al–4V. The TiB phase was aligned in the extrusion direction and increased B content was found to weaken the α-phase texture, causing a weakening of tensile anisotropy. Debonding was not observed during the tensile tests in the transverse orientation, indicating a strong interface bond exists between the TiB phase and the two-phase (α+β) Ti–6Al–4V matrix.

Room temperature tensile fracture characteristics of in situ TiBw/Ti6Al4V composites with a quasi-continuous network architecture

In situ TiB-whisker-reinforced Ti6Al4V matrix composites with a quasi-continuous network reinforcement distribution exhibited an improved combination of room temperature tensile properties compared to the composites with a uniform reinforcement distribution. The unique microstructure consisting of a whisker-rich boundary region and a whisker-lean matrix region leads to unique fracture characteristics . Under tensile stress, the main crack follows the TiBw-rich region, which imparts the superior strengthening effect of TiBw. The relatively large TiBw-lean region contributes positively to the elongation to failure of the composites.

Laser Cladding of TiC for Better Titanium Components

Pure commercial titanium is widely used because of its high corrosion resistance and lower cost compared with other titanium alloys, in particular when there is no high wear requirements. Nevertheless, the wear resistance is poor and surface damage usually occurs in areas under contact loadings. Laser cladding is a suitable technique for manufacturing precise and defect free coatings of a dissimilar material with higher wear and corrosion resistance. In this work a good understanding of laser metal deposition mechanisms allowed to obtain defect free coatings of Ti6Al4V and TiC metal matrix composite (MMC) using a flash lamp pumped Nd:YAG laser of 1kW. A complete investigation of the process parameters is discussed and resultant wear properties are shown. The results show the feasibility to apply the process for manufacturing, improving or repairing high added value components for a wide range of industrial sectors.

Structure and mechanical properties of Ti–6Al–4V with a replicated network of elongated pores

Ti–6Al–4V, with a network of elongated, open pores aligned along two perpendicular directions, is produced by a two-step replication process: (i) Ti–6Al–4V powder or foil preforms containing low-carbon steel wire meshes are densified by hot pressing under transformation superplasticity conditions; (ii) porosity is created by electrochemical dissolution of the low-carbon steel wires and the adjacent Fe-containing Ti–6Al–4V matrix. If high-carbon steel wires are used, Fe diffusion into Ti–6Al–4V is inhibited by a carbide layer forming at the wire/matrix interface, and pores exactly replicate the shape of the wires. Ti–6Al–4V with ∼19% and 34% porosity, without and with Fe–Ti interdiffusion respectively, shows low oxygen contamination and good compressive ductility. Strength and stiffness, as measured by compression testing and ultrasonic measurements, are compared with simple analytical models and numerical finite-element models.

Interfacial shear strength behaviour of Ti/SiC metal matrix composites at room and elevated temperature

Synchrotron X-ray strain measurements have been used to follow the distribution of elastic strain, and hence interfacial shear stress, along single SCS-6 and Sigma SM2156 Ti–6Al–4V matrix coated SiC monofilaments sandwiched between Ti–6Al–4V foils during single-fibre-fragmentation testing. The interfacial shear strength behaviours were characteristically different. For the SCS-6 system, the interfacial response was dominated by classical frictional sliding, initiating near the ends and progressing along most of the length of the fragments, in common with previous observations. By contrast, for the SM2156 fibre system, a significant threshold stress was evident which must be exceeded before sliding could occur. As a result, sliding was only found near the ends of the fibre fragments. Upon unloading, reverse frictional sliding was found to take place, initiating from the fibre ends at a shear stress somewhat lower than that for forward sliding. Finite element modelling suggests that this is due to a reduction in the radial fibre clamping stress upon unloading rather than a change in the friction coefficient. For both the systems, the frictional sliding strength fell approximately linearly with increasing temperature towards zero at ∼600–700 °C, consistent with a Coulomb friction coefficient of ∼0.4 and thermal clamping residual stresses that approach zero at these temperatures. By contrast, the threshold stress required to initiate sliding for SM2156 falls at a slower rate with increasing temperature, such that it would still be significant at these temperatures. Some evidence was found for a decrease in interfacial shear stress with decreasing fragment length.

In situ (TiBw + TiCp)/Ti6Al4V composites with a network reinforcement distribution

Hybrid TiB whiskers (TiBw) and TiC particles (TiCp) reinforced Ti6Al4V matrix composites (TiBw + TiCp)/Ti64 (TiB:TiC = 1:1) with a novel network reinforcement distribution were successfully fabricated by in situ reaction hot pressing (RHP). TiBw and TiCp are in situ synthesized around the as-received Ti64 particles, and subsequently formed into a TiBw and TiCp hybrid network structure. In this reinforcement network structure, TiB whiskers act like dowel connectors that join the neighbouring Ti64 matrix particles. The existence of TiC particles increases the local volume fraction and continuity of reinforcement in the boundary region which contributes positively to the strength and elastic modulus of the composites. The in situ synthesized (TiBw + TiCp)/Ti64 composites exhibit superior mechanical properties without any subsequent processing such as extrusion, which indicates that the strengthening effect of reinforcements and the toughness of the Ti64 matrix can be better exploited through this novel network structure.

Anelastic Phenomena at the Fibre-Matrix Interface of the Ti6Al4V-SiC<sub>f</sub> Composite

The composite, consisting of Ti6Al4V matrix reinforced by unidirectional SiC fibres (SCS-6), has been investigated by mechanical spectroscopy at temperatures up to 1,173 K. For comparison, the same experiments have been performed on the corresponding monolithic alloy. The internal friction (IF) spectrum of the composite exhibits a new relaxation peak superimposed to an exponentially increasing background. This peak, which is not present in the monolithic alloy, has an activation energy H = 186 kJ mol-1 and a relaxation time 0 = 2.3 x 10-15 s. The phenomenon has been attributed to a reorientation of interstitial-substitutional pairs in the  phase of Ti6Al4V matrix around the fibres. This explanation is supported by the results of micro-chemical characterization carried out by X-ray photoelectron spectroscopy (XPS) combined with Ar ion sputtering.

Hardness and wear resistance improvement of surface composite layer on Ti–6Al–4V substrate fabricated by powder sintering

A wear resistant surface composite layer on Ti–6Al–4V substrate was fabricated using powder sintering method. The surface composite layer consisted of Ti–6Al–4V matrix and different fractions of TiN particles as reinforcement phase. The surface layer and the substrate were directly bonded together while the powders were cold formed and then sintered at an elevated temperature. The two layers showed good metallurgical bond. In this study, 5%, 10% and 15% TiN weight fractions were adopted to fabricate the surface composite layer. Effects of TiN addition on the microstructure, hardness and wear resistance were investigated. It was found that the wear resistance of the surface composite layer was improved due to the addition of TiN compared to that of pure Ti–6Al–4V.

Ti–6Al–4V with micro- and macropores produced by powder sintering and electrochemical dissolution of steel wires

Ti–6Al–4V powder preforms containing parallel layers of steel wire meshes are sintered into composites. Elongated macropores are created by electrochemical dissolution of the steel wires, and equiaxed micropores by partial powder densification. The macropore diameter and fraction is tailored by the creation of a Fe-containing diffusion zone in the Ti–6Al–4V matrix, which is removed electrochemically together with the wires. Ti–6Al–4V with 21–41% porosity shows compressive stiffness and strength attractive for biomedical implants.

Effect of hydrogen on superplastic deformation of (TiB+TiC)/Ti–6Al–4V composite

Abstract Ti–6Al–4V matrix composite reinforced with TiB plus TiC was prepared and hydrogenated. The phases were identified by X-ray diffraction (XRD). Microstructures were examined by optical microscopy (OM). Dependence of transus temperatures of the composite on hydrogen concentration was determined. The result shows hydrogen decreases transus temperatures of the composite significantly and increases the amount of β phase. Superplastic deformation of the hydrogenated composites was performed. Hydrogen decreases the optimum superplastic temperatures and increases the optimum superplastic strain rate. The flow stresses of hydrogenated composites decrease greatly compared to unhydrogenated composite. The strain rate sensitivity index m of the composite increases with increasing hydrogen concentration and reaches maximum 0.327 at 0.85 wt.% H. Meanwhile, the activation energy Q decreases with increasing hydrogen concentration and ranges in 488–382 kJ/mol. Hydrogen promotes recrystallization of the composite and refines the microstructure during superplastic deformation.

High temperature damping behaviour of Ti6Al4V–SiC f composite

The Ti6Al4V–SiC f composite, reinforced by unidirectional SiC fibres (SCS-6), has been investigated by internal friction (IF) and dynamic modulus measurements in the temperature range 300–1173 K. Experiments have been carried out by using a vibrating reed technique with electrostatic excitation and frequency modulation detection of flexural vibrations in the frequency range from 600 to 1800 Hz. For comparison the monolithic Ti6Al4V alloy has been submitted to the same experiments. The IF spectrum of the composite exhibits a relaxation peak at about 870 K superimposed to an exponentially increasing background. The peak, not present in the spectrum of matrix alloy, has activation energy H = 186 kJ mol −1 and relaxation time τ 0 = 2.3 × 10 −15 s. The peak has been explained by an interstitial-substitutional pair reorientation mechanism in the α phase of Ti6Al4V matrix around the fibres.

Composite of Ti6Al4V and SiC fibres: evolution of fibre–matrix interface during heat treatments

AbstractThe changes in fibre–matrix interface during the heat treatment of Ti6Al4V matrix reinforced with carbon‐coated SiC fibres, have been studied using XPS, AES and SEM/EDS (energy dispersion spectroscopy) techniques. The samples were characterized before and after heating at different temperatures up to 600 °C with long exposure times up to 1000 h. Particular attention was paid to the C/matrix interface, where high temperature treatment could lead to the formation of titanium carbide. Due to the experimental difficulties to separate the contributions of TiC and SiC, the interaction between titanium and carbon was simulated by heating Ti6Al4V and pure Ti foils covered with graphite. Obtained interfaces have been analysed by XPS depth profiling. Experimental results were supported by theoretical calculations of carbon diffusion. Copyright © 2008 John Wiley & Sons, Ltd.