Investigation and characteristics of Ti6Al4V metal matrix composite using powder metallurgy process

ABSTRACT

The mechanical, microstructure, wear and electrochemical corrosion properties of Ti6Al4V/xTiB₂-xTiC (x = 0, 2.5, 5, and 7.5 vol.%) hybrid composites, produced by powder metallurgy, were studied in relation to their reinforcement percentage. Initially, mechanical investigations were carried out to assess the mechanical attributes of the composites. Additionally, the wear and friction behavior of the composites was examined using a pin-on-disk apparatus under various conditions. The Scanning electron microscope and X-ray diffraction analysis were carried out to understand the microstructural changes and elemental compositions of the specimens. Electro Chemical Corrosion experiments were conducted under simulated body environments, such as 37 °C and simulated body fluid. The findings revealed that the homogeneous distribution of reinforcements into the titanium (Ti) matrix, led to significant microstructural changes, achieving maximum hardening of the Ti6Al4V with 5 vol.% TiB₂-TiC. In comparison to the Ti6Al4V alloy, the Ti6Al4V/5%TiB₂ & 5% TiC composite exhibits a 48% increase in hardness and a 18% decrease in porosity. According to the corrosion test findings, the Ti6Al4V/5% TiB₂ & 5%TiC hybrid composites exhibited superior corrosion behaviour than the alloy, with a corrosion current density of 1.0 × 10–6 A/cm². These results provided valuable insights into the relationship between composition, microstructure, and the enhanced corrosion potentials of the alloy and composites.

Keywords:
Ti6Al4V; Powder metallurgy; SEM; Wear; Electrochemical corrosion

1. INTRODUCTION

The unique properties of titanium alloys make them highly attractive for biomedical equipment owing to their exceptional biocompatibility with human tissues and bones. However, the wear properties of these alloys have been identified as a significant limitation, often leading to material failure. Among titanium alloys, Ti6Al4V (Ti64) stands out for its exceptional properties, including good corrosion resistance, high strength, excellent toughness, minimum density, and comparatively lower elastic modulus. The poor wear resistance of Ti64 alloy restricts its broader application, particularly in the biomedical industry. The release of titanium wear particles and metallic ions into surrounding tissues and blood can cause various adverse effects, including inflammation, cytotoxicity, genotoxicity, and carcinogenicity, ultimately leading to implant failure [¹, ²]. In recent years, titanium matrix composites (TMCs) have gained attention as a potential solution for orthopedic implants. By incorporating hard ceramic phases into the titanium matrix, the wear resistance of these composites can be significantly enhanced. This improvement is attributed to several factors, including the load-carrying capacity provided by the reinforcement phases and microstructural modifications in the matrix. These modifications include grain refinement, induced dislocations, and orowan strengthening mechanisms, which collectively contribute to the enhanced performance of TMCs. Literature studies have demonstrated that the appropriate addition of hard ceramic particles to titanium and its alloys is an effective strategy for improving mechanical and wear properties.

Various advanced titanium-based metal matrix composites (MMCs) have been developed by incorporating hard ceramic reinforcements such as TiC, TiB₂, TiN, and SiC particles, yielding promising results. These studies indicate that Ti MMCs possess high strength, excellent high-temperature performance, oxidation resistance, and enhanced stiffness, along with superior corrosion and wear resistance. Specifically, TiB and TiC reinforcements are widely recognized for their high modulus, excellent thermal stability, and chemical stability. Research on composites like Ti6Al4V/TiB and Ti6Al4V/TiC has highlighted their improved wear resistance and significantly reduced coefficients of friction, making them highly suitable for applications requiring durability and reliability under extreme conditions [³, ⁴].

Metal matrix composites (MMCs) are commonly produced using various manufacturing techniques such as squeeze casting, stir casting, infiltration, spark plasma sintering, and powder metallurgy (PM). Among these methods, powder metallurgy stands out as one of the most efficient techniques for fabricating fully dense titanium matrix composites (TMCs). This is primarily due to its ability to achieve high density at relatively lower temperatures and with reduced fabrication time, making it an advantageous choice for advanced composite production [⁵–⁶]. Considering the compatibility of reinforcements with the Ti64 matrix, TiC has been extensively studied as a reinforcement material in titanium matrix composites (TMCs). TiC’s superior hardness and high melting point enable it to effectively share the applied load with the matrix phase, providing significant strengthening. Additionally, TiB₂ is another reliable reinforcement known for its grain-refining capabilities and its ability to strengthen titanium alloys. The combination of TiB₂ and TiC in TMCs offers excellent thermal stability, as both reinforcements have thermal expansion coefficients closely matching that of titanium. While multiple reinforcement phases in a single matrix have been reported, only a minimum number of research have absorbed on understanding the individual contributions of each phase to the overall strengthening mechanisms. This highlights a potential research gap in fully comprehending their independent roles in enhancing the properties of TMCs [⁷, ⁸].

According to strengthening theories, the mechanical characteristics of composites are strongly depending on the composition, volume fraction, morphology, and distribution of reinforcement phases within the metal matrix. Understanding these parameters is important for enhancing the performance of Ti64 matrix composites for biomedical and other advanced applications. Over the past decades, the wear behavior of MMCs has been broadly investigated. It is well-established that incorporating rigid ceramic elements into a metallic matrix suggestively enhances wear resistance. This improvement occurs through direct (reinforcing phases share the applied load) and indirect strengthening induced dislocations, orowan strengthening, grain size refinement) mechanisms resulting from the addition of reinforcement phases. These combined effects contribute to the superior wear resistance of MMCs. However, inadequate interfacial bonding between the matrix and reinforcement can lead to the pull-out of reinforcing phases during wear, ultimately causing severe wear as the detached particles act as additional abrasives. The interfacial interactions between the matrix and reinforcement significantly influence the overall performance and durability of MMCs in corrosive environments [⁹, ¹⁰]. The present study investigates the combined effects of ex-situ TiC particles and in-situ TiB whiskers as reinforcements in powder metallurgy-based hybrid composites of Ti6Al4V. While previous studies have investigated the separate effects of TiB and TiC reinforcements, to date, no systematic survey has been taken to probe the complementary influence of these ceramic phases on mechanical, wear, and electrochemical behavior of Ti64 composites. This will take a closer examination into the current study as it investigates the varied volume fractions of TiB₂-TiC on the microstructure, mechanical strengthening mechanisms like improved harness (≥500HV), and corrosion resistance (0.05 mm/year) in simulated body fluid setting. The work presented also aims to bring to light the unique and the common functions played by TiC and TiB₂ in reinforcing the Ti64 matrix in grain refinement, load transfer, and interfacial stability. In addition, the study highlights key correlations between reinforcement distribution, residual porosity, and electrochemical performance, hence critical for improving the applicability of titanium-based composites for use in load-bearing orthopedic implants, for instance femoral stems, spinal cages, and dental abutments, where high strength, wear resistance, and biocompatibility are critical.

2. MATERIALS AND METHODS

2.1. Materials

Ti6Al4V (Ti64) powder and TiB₂ & TiC particles were used as a matrix and reinforcement material. The elemental configuration Ti64 powder is 6.0% Al, 4.0% V, 0.25% Fe, 0.08% C, 0.13% O, and 0.12% N (balance Ti) and a typical particle size of <45 µm. Irregularly shaped reinforcement particles have a particle size of <5 µm and the particle size of the both matrix and reinforcement were analysed through particle size analyzer for the confirmation. The volume fractions of 2.5, 5, and 7.5 vol.% were used to fabricate the composite specimens through powder metallurgy method. The SEM images of TiC & TiB₂ powder is shown in Figure 1 (a) and Figure 1(b). For comparison, Ti64 specimen without reinforcement were also prepared. The quantity of powder required for specimen preparation was intended based on the rule of mixtures. Table 1 provides the volume percentages of all material used in producing the alloy and composite specimens, along with their respective codes.

Figure 1
(a) SEM image of TiC powder (b) SEM image of TiB₂ powder (c). Ball milling apparatus and (d) Punch and die arrangement.

Table 1
volume percentages of each composite samples and sample codes.

SAMPLE NAME	SAMPLE TYPE	VOLUME PERCENTAGE (%)
		TI6AL4V	TIB2	TIC
A	Alloy	100	0	0
C25	Composite	95	2.5	2.5
C50	Composite	90	5	5
C75	Composite	85	7.5	7.5

2.2. Milling and compaction

The matrix and reinforcement powders were mixed in a planetary ball mill (Figure 1(c)) for 4 hours to ensure uniform distribution. Before mixing, both the matrix and reinforcement powders were preheated for 30 minutes to remove the moisture content. Steel balls were utilized with a ball-to-powder ratio of 10:1, and the milling operation was carried out at a constant speed of 100 rpm. The resulting mixtures were then kept in a die (Figure 1(d)) with a diameter of 10 mm and compacted under a pressure of 950 MPa [¹¹]. The quantity of powder was calculated to produce compacts with a height of 25 mm. Green compaction was performed using a hydraulic pressing machine, which applied the specified pressure while monitoring the axial displacement of the punch. Toluene was employed during ball milling to avoid oxidation and Zinc stearate was utilized as a solid lubricant during compaction process.

2.3. Sintering and sample preparation

The green compacts were subsequently sintered in a high-temperature microwave furnace at 1250 °C for 2 hours under a high-purity argon (Ar) atmosphere with a holding time of 60 minutes at the peak temperature. The samples were placed within a susceptor bed made of high-purity SiC powder, which acts as a microwave coupler to facilitate indirect heating. After sintering, the samples underwent metallographic preparation, including grinding with SiC papers of 600, 800, and 1200 grit, followed by polishing with a 2 µm alumina suspension to achieve a mirror-like surface [¹²]. The specimen surfaces were etched with Kroll’s mixture to expose the microstructure.

2.4. Microstructure and phase analysis

The microstructure and elemental characteristics of the mixed powders and samples were analysed by the SEM equipped with energy dispersive spectroscopy (EDS). To identify the elemental characteristics of the samples, XRD analysis were conducted with an Empyrean Analytical diffractometer with K-alpha copper radiation. The XRD investigation was done at 30 kV and 30 mA, with an examination speed of 0.2° per step, covering a 2θ range of 32°–80° [¹³]. The microstructural examination was made on cross-sections taken at the mid-height plane perpendicular to the cylindrical axis of the samples to minimize the effect of axial density differences.

2.5. Density, porosity and hardness measurement

The theoretical density of the alloy and composites was determined by rule of mixture, while the experimental density of the fabricated specimens was calculated using Archimedes’ principle. Following ASTM B962-13 standards, the porosity (P) characteristics for produced samples was evaluated using the specified formula. The densities of the composite samples were measured using a high-precision automated electronic gauging scale with a detection limit of 0.0001 g. The microhardness property of the fabricated specimens was determined using a Vickers hardness apparatus, applying a load of 300 g and a dwell time of 10 seconds. The average hardness values were calculated for each sample from 3 indentions and the axial homogeneity of the sintered sample was examined by conducting sectional density measurement.

2.6. Dry sliding wear study

The pin-on-disc dry sliding wear tests were conducted by means of ASTM G99-05 standard and EN31 steel was used as a counter disc. The fabricated hybrid composite pins were polished with a 0.05 µm SiO2 emulsion. After each wear test, the specimens were immersed in acetone solution to clean the wear surface. Wear loss was determined by calculating the weight of the pins before and after the tests with a precision of 0.1 mg. The COF of the specimens was calculated based on the applied load and tangential load measured by strain gauges. Each experiment was repeated six times, and the average results were recorded.

2.7. Electrochemical corrosion measurements

The electrochemical corrosion behavior of the prepared specimens was studied using a standard three-electrode workstation (CH Instruments) at 37°C, following the ASTM G59-97 standard. The phosphate-buffered saline (PBS) solution was used as a corrosion medium and the exposed area for testing specimen was 2 cm². The three-electrode corrosion setup included a platinum electrode as the counter electrode and a typical calomel electrode as the reference electrode. Prior to testing, the specimens were polished physically with SiC emery papers of 800–1200 grit size [¹⁴]. Tafel curves were obtained by polarizing the samples relative to their open-circuit potential (OCP) at a speed of 0.01 V/s. The measurements of corrosion potential (Ecorr), corrosion current density (Icorr), and linear polarization resistance, were determined by examining the Tafel curves via the adequate software.

3. RESULTS AND DISCUSSION

3.1. Characterization of metal powders

The microstructures with elemental analysis of the ball-milled Ti64 alloy powder and composite powders are illustrated in Figure 2 (a-d). The Figure 2(a) reveals that the initially large Ti units were flattened into plate-like shapes, with their irregular edges rounded and reduced in size due to the effect of the milling process. Additionally, the reinforcement elements are observed to adhere to the boundaries of the Ti elements, as depicted in Figure 2 (b-d). The continuous stress exerted on the TiB₂ & TiC particles during milling contributes to the observed changes in the powder morphology.

Figure 2
SEM images with elemental composition of (a) Ti64 powder, (b) Ti64/2.5%TiB₂ 2.5%TiC powder (c) Ti64/5%TiB₂ 5%TiC powder (d) Ti64/7.5%TiB₂ 7.5%TiC powder.

The Elemental analysis displays distinct colors in the ball-milled powders, prominently corresponding to titanium (Ti) along with noticeable colors representing aluminum (Al) and vanadium (V). In the composite powders, these colors confirm the successful incorporation of the reinforcements into the Ti matrix, as carbon (C) and boron (B) are the primary rudiments of the reinforcements. The nonappearance of other elemental colors shows that no novel elements were generated throughout the ball milling.

3.2. Phase identification analysis

Figure 3 illustrates the XRD patterns of fabricated Ti64 alloy and Ti64/ TiB₂ & TiC Composites. From the Figure, it can be shown that the α-Ti phase dominates in matrix alloys and composites for any spectra close to 2θ = 40.19° (101). The same phase is also seen in the Ti64 composite reflection at 2θ = 53.093° (102). The measured values closely match those on the typical JCPDS card No. 89-5009. At 2θ = 38.759° (110), in the Ti64 composite specimen (JCPDS card No. 89-4913), the β-Ti phase may be seen. The α-Ti phase at 2θ = 40.19° (101) coexists with two new peaks that were formed in Ti64/2.5%TiB₂ & 2.5% TiC and Ti64/ 5%TiB₂ & 5%TiC composites, as well, at 2θ = 27.514° (001) and 2θ = 36.13° (111). The TiB₂ and TiC reinforcing XRD patterns and JCPDS card No. 89-3923 are in excellent concordance. Similar tendencies may be seen in the composite specimens of Ti64/7.5%TiB₂ and 7.5%TiC. With the new inclusion peak TiB₂ at 2θ = 44.319° (101) and TiC at 2θ = 41.735° (200), the high intensity of TiB₂ & TiC is generated.

Figure 3
XRD patterns of fabricated Ti64 alloy and Ti64/ TiB₂ TiC composites.

As reinforcement content rises, the diffraction strength of the TiB₂ & TiC peaks also rises significantly. Therefore, XRD analysis supports the existence of Ti64, TiB₂, and TiC. The XRD analysis indicates that the samples do not exhibit additional oxide formation. The lack of impurities in the samples meets the specified requirements.

3.3. Microstructural characterization of fabricated specimen

The SEM pictures of the fabricated alloy and composites are shown in Figure 4(a-d). In the absence of reinforcements, Figure 4(a) reveals a Widmanstӓtten microstructure, which typically forms in two-phase Ti alloys when subjected to slow cooling, such as furnace cooling, near the β transus temperature. During the gradual cooling process to approximately 850°C, close to the β transus temperature, the α-phase began to develop into plate-like structures characteristic of the Widmanstӓtten microstructure [¹⁵]. Figure 4 (c) display the microstructures of the C50 composites highlighting the matrix structure with dispersed reinforcement particles. The micrograph of the C50 composite, shown in Figure 4 (c) demonstrates excellent interfacial bonding between the Ti64 matrix and the reinforcements.

Figure 4
SEM images of fabricated (a) Ti64 alloy, (b) Ti64/2.5%TiB₂ 2.55%TiC composite (c) Ti64/5%TiB₂ 5%TiC composite(d) Ti64/7.5%TiB₂ 7.5%TiC composite.

The reinforcement particles are uniformly distributed throughout the Ti64 matrix. Incorporating 5% of both TiC and TiB₂ into the Ti64 matrix alloy proves to be an effective strategy. The microstructure of the composite after incorporating 2.5% & 7.5% TiB₂ and TiC reveals a non-uniform distribution of particles, agglomeration, and noticeable porosity shown in Figure 4 (b and d). When the addition of TiB₂ and TiC exceeds 5%, the interfacial bonding between the matrix alloy and the reinforcements becomes inadequate, leading to the formation of clusters and embrittlement of the composite. The emergence of clusters is attributed to the inability of the Ti64 matrix to accommodate the higher concentrations of TiB₂ and TiC effectively and some residual pores persist in the composite due to incomplete bonding at the scattered interfaces, [¹⁶]. These pores act as stress concentrators, compromising the grain boundaries and weakening the overall structure.

In order to determine the chemical make-up of the substances around, an EDS analysis was carried out and shown in Figure 5 (a and b). Figure 5 (a) depict the Ti64 matrix alloy’s EDS showing peaks for Ti, Al, and V. The EDS spectra of composites containing peaks of B and C are shown in Figure 5 (b). The composite itself exhibits B and C peaks, but not the alloy. As the reinforcement concentrations of TiB₂ and TiC rise, so do the magnitude values of B and C. This demonstrates that TiB₂ & TiC reinforcements are present in Ti64/TiB₂ & TiC composites.

Figure 5
EDS Spectrum images of fabricated (a) Ti64 alloy, (b) Ti64/TiB₂ TiC composite.

3.4. Physical and mechanical characterization

The density of the produced samples increases with the accumulation of TiB₂ & TiC particles due to the larger density of the reinforcements related to the matrix. The incorporation of TiB₂ and TiC, which have greater densities than the base alloy, contributes to the overall increase in the composite’s density, as illustrated in Figure 6. However, the gap between the theoretical and actual density widens as the reinforcement content increases, indicating a corresponding rise in porosity.

Figure 6
Density and porosity curves of fabricated Ti64 alloy and Ti64 composites.

This increase in porosity can be attributed to granular agglomerations and the inherent porosity of the individual components. The porosity of the specimens was calculated based on the difference between their saturated weight and dry weight. The results show that the composite reinforced with 5% TiB₂ and 5% TiC exhibits significantly reduced open pores compared to the Ti alloy. The base matrix has more porosity compared to the composites due to none presence reinforcement. The reinforcements not only improve the mechanical interlocking and also assist the densification process by act as a grain growth inhibitor. Micro-voids or insufficient densification may arise from inadequate particle bridging during compaction and sintering caused by a low reinforcing content below 5%. Furthermore, the advantageous properties of TiB₂ and TiC, such as improved diffusion kinetics and grain boundary pinning, are less noticeable at lower reinforcement levels, which lessens their capacity to encourage densification. However, as the reinforcement concentration increases to 7.5% TiB₂ and 7.5% TiC, the porosity also increases. These findings suggest that a 5% TiB₂ and TiC concentration is optimal, as it minimizes porosity and enhances the mechanical properties of the composites. The TiB₂ and TiC particles are effectively wetted by the matrix and uniformly distributed within the composite, which helps reduce porosity in the 5% TiB₂ and 5% TiC reinforced composite. This composition can be considered an optimal choice for composite preparation compared to other variants. However, at higher reinforcement content (7.5% TiB₂ and 7.5% TiC), porosity increases due to strong particle agglomeration, reduced wettability, and pore nucleation at the matrix interface. Excess unreacted TiB₂ and TiC particles at concentrations above 5% contribute to pore formation. Additionally, the volume of pores expands due to increased gas pressure within the closed pores.

The Vickers hardness (HV) of Ti6Al4V increases with a higher reinforcement percentage as shown in Figure 7. The presence of the strong TiC and TiB₂ phases contributes to this rise in hardness as the reinforcement content increases. The composite with 5% TiC and 5% TiB₂ reinforcement exhibited the highest hardness among the tested samples. This is attributed to the combined effects of solution strengthening and fine grain strengthening caused by the dissolution of C and B elements in the matrix. The hardness of the composite is influenced by several factors, including wettability, particle clustering, distribution uniformity, porosity, and the strength of the bond between the matrix and reinforcement.

Figure 7
Hardness plots of fabricated Ti64 alloy and Ti64 composites.

The significant improvement in hardness observed in the Ti64 alloy reinforced with 5% TiB₂ and 5% TiC is attributed to the hard dispersion of TiB₂ and TiC particles, which enhances the composite’s hardness. Additionally, the Ti64/5% TiB₂ & 5% TiC specimen demonstrates a strong particle-to-matrix interface, contributing to its superior hardness properties. Increasing the reinforcement content to 7.5 wt% TiB₂ and TiC in the Ti64 alloy results in a reduction in Vickers hardness. The degree of strengthening is influenced by factors such as particle size, spacing, and the strength of the bond between the particles and the matrix. While the stronger particles prevent dislocation movement through them, at sufficiently high stress, dislocations can bypass the particles, forming dislocation loops around them. This creates additional resistance to subsequent dislocations, particularly when particles are closely spaced, as dislocations find it more challenging to move between them. However, excessive reinforcement content can lead to clustering and weak bonding, which counteracts the strengthening effect and reduces hardness [¹⁷].

3.5. Wear characteristics

Figure 8 illustrates the variation in wear loss under different applied loads, clearly highlighting the significant influence of load on wear loss. From this, it can be concluded that the applied load is the primary factor determining the wear loss of the composite. The increase in wear rate with higher applied loads for all test specimens is attributed to the greater amount of plastic deformation and delamination wear caused by the interaction with increased loads. This phenomenon arises from the higher metal-to-metal contact, which elevates friction at the interface. When the Ti64/2.5% TiB₂ & 2.5% TiC pin first comes into touch with the disc’s surface, the two surfaces scrape against one another, initiating the abrasion wear process. As the rubbing proceeds, the chemical properties of Ti64/2.5% TiB₂ & 2.5% TiC and the frictional force between the two surfaces cause an intense attachment. However, as the weight percentage of reinforcement rose to 5%, the resistance to abrasive wear decreased somewhat. The Ti64 alloy reinforced with 5% TiB₂ and 5% TiC has the minimum wear loss compared to the alloy and other composite specimens. This is due to the relatively homogeneous distribution of TiB₂ and TiC within the Ti64 matrix, which improves the wear resistance and reduces wear loss [¹⁸].

Figure 8
Wear loss curves of fabricated Ti64 alloy and Ti64 composites.

The Ti64 matrix benefits from a strong interface and good wettability with the reinforcing particles, showing no signs of bonding failure or delamination at the matrix-particle interface. However, as the weight fraction of TiB₂ and TiC increases (in Ti64/7.5% TiB₂ & 7.5% TiC composites), abrasive effects become more pronounced. This results in the grinding surface showing increasingly damaged cuts and groove traces, leading to more irregular wear patterns. Furthermore, the Ti64/7.5% TiB₂ & 7.5% TiC composites are more susceptible to matrix breakage or fracture during sliding, which contributes to the increased wear loss.

The temperature caused by the tillage action will raise the effect of friction between surfaces. Adhesion and increased deformation at the surface layers are the outcomes of this action, which causes more metal loss.

Figure 9 shows that the coefficient of friction increases with the applied load for all Ti64 alloy and Ti64/TiB₂ & TiC composites. Among all the applied loads, the Ti64 alloy exhibited the highest coefficient of friction. However, the Ti64/5% TiB₂ & 5% TiC composite had a lower coefficient of friction compared to other composites and the Ti64 alloy at all examined loads. This can be attributed to the fact that as the applied load increases, plastic deformation becomes more significant. In the case of Ti64/5% TiB₂ & 5% TiC composites, the reinforcement particles act as load-bearing components, enhancing their wear resistance and reducing the coefficient of friction [¹⁹]. When reinforcement particles (7.5% TiB₂ & 7.5% TiC) are added to Ti64, the oxide layer beneath the surface fractures, weakening the worn surfaces and resulting in an increase in friction. The contact between the composites and the counterface is intensified by abrasion due to the decohesion of the sliding surfaces. This leads to a rise in the coefficient of friction as the wear process progresses.

Figure 9
CoF curves of fabricated Ti64 alloy and Ti64 composites.

3.6. Wear surface characterization

A scanning electron microscope was used to examine the surface morphology of the worn samples in order to assess the wear processes and establish a correlation between them and wear performance and reinforcing content. Wear tracks, grooves, material transfer, and particle separation were among the unique characteristics visible in the SEM images of the worn surfaces displayed in Figure 10 (a-d). These characteristics changed based on the test settings and the degree of reinforcement. The appearance of the surface Significant abrasive wear is indicated by the deep grooves and broad wear tracks seen in Ti64 alloy (Figure 10.a). Lower hardness from the lack of reinforcing particles permits substantial material loss through plastic deformation under sliding stresses. The Delamination wear also observed by large flakes of material that have detached from the surface. Material is removed via ploughing and micro-cutting due to abrasive wear while the alloy specimen direct contact with counter surface [²⁰]. The C25 composite’s surface morphology shows smaller grooves and fine wear debris on the surface indicates that the TiB₂ & TiC reinforcements improves hardness and wear resistance as shown in Figure 10 (b). The decreased the depth of asperity penetration and creating a protective oxide layer during sliding, was achieved by hard reinforcements. The shallow grooves and fewer delamination is shown in the surface morphology Figure 10 (c) of C50 composite. The uniform dispersion of TiB₂ and TiC particles in matrix reduces the material loss and improve the wear resistance. The reinforcement particles serve as a load-bearing component and reduce direct contact between the matrix and counterface. A mechanically mixed tribo layer was developed between the matrix and counterface, further decreasing wear [²¹]. The C75 composite’s SEM picture Figure 10 (d) exhibits wear characteristics, such as deep grooves and localized pitting. The third-body abrasive wear leads to particle pullout and microcracking. The agglomeration and porosity degrade the matrix at 7.5% reinforcement, which raises the possibility of reinforcement particle separation. As abrasive agents, detached particles exacerbate wear through three-body abrasion, and high porosity reduces the reinforcement’s efficiency, resulting in less wear resistance. The inclusion of TiB₂-TiC reinforcements considerably increases wear resistance up to 5.0%, after which it decreases because of porosity and particle agglomeration [²²].

Figure 10
(a-d) Surface morphology of the worn Ti64 alloy and Ti64 composites.

3.7. Electrochemical properties

The Ti64 alloy and its composites reinforced with varying levels of TiB₂-TiC volume fractions (2.5%, 5.0%, and 7.5%) were studied for their electrochemical corrosion behavior in simulated body fluid (SBF) at 37 °C using Tafel polarization. The polarization curves (Figure 11) show clear trends in corrosion potential (Ecorr) and corrosion current density (icorr), towards the corrosion resistance. The electrochemical corrosion parameters (Ecorr, icorr, and corrosion rate) for the specimens are presented in Table 2. The corrosion potential was observed to shift to positive with added TiB₂-TiC reinforcements. The Ecorr value was most negative for the alloys, which possess higher susceptibility to corrosion initiation compared to the composites. Among the composites, C50 had the most positive Ecorr, meaning better resistance to corrosion initiation due to the uniform dispersion of reinforcements. Increased reinforcement levels detain corrosion or slow the rate of corrosion as observed by the lower icorr for the alloy, which was noticed to have a generally lower icorr during corrosion in the absence of reinforcement to generate prompted porosity. The icorr values increased at higher reinforcement levels due to the accelerating effect of residual porosities introduced during the powder metallurgy process. This porosity serves as a pathway for the ingress of more aggressive ions, such as chloride ions, and thus contributes to a more localized corrosion process [²³]. Changes in the passive oxide layer formation on the matrix titanium by the added presence of TiB₂-TiC particles therein have a high impact. Upon immersion in the simulated body fluid, titanium forms a protective TiO2 layer on the surface, allowing for much less corrosion. The present ceramic reinforcements altered the microstructure of the composite, thus enhancing the initial passivating behavior while introducing local defects. At 2.5% and 5.0% reinforcement levels, the reinforcements were well-dispersed, thereby promoting uniform passivation and an improvement in Ecorr.

Figure 11
Tafel curves for fabricated Ti64 alloy and Ti64/TiB₂ & TiC Composites.

Table 2
Electrochemical findings derived from the tafel analysis.

SAMPLE	ECORR (V vs. SCE)	ICORR (A.CM–2)	CORROSION RATE (MM/YEAR)
Alloy	−0.55	1.2 × 10–7	0.075
C25	−0.45	1.5 × 10–6	0.045
C50	−0.30	1.0 × 10–6	0.030
C75	−0.35	2.0 × 10–6	0.060

The 7.5% reinforced microstructural study showed enhanced particulate cluster formation and porosity, which caused the disorganization of the passive oxide layer and allowed for faster icorr. Titanium matrix and its composites, upon SBF exposure, eventually form a TiO2 layer, which is supposed to act as a primary barrier for corrosion. The porosity and interface formed by the reinforcement and the titanium matrix provide corridors for the ingress of chloride ions that necessitate the destabilization of the passive layer, causing pitting corrosion [²⁴]. The composite having 5.0% TiB₂-TiC attained a fine compromise between mechanical integrity and corrosion resistance. The well-dispersed reinforcing particles led to a relatively good breakdown in porosity while promoting increased corrosion resistance. On the other hand, the 5.0% TiB₂-TiC clearly seemed to have the best corrosion resistance displayed by all the samples studied. The superior microstructure and minimal porosity provoked an optimum exploit of mechanical and electrochemical performance [²⁵].

3.8. Corrosion characteristics

The SEM images of the fabricated alloy and composite surfaces after electrochemical corrosion testing are shown in Figure 12 (a-d). The SEM images expose critical insights into the corrosion mechanism involved in the alloy and composite specimens. The SEM images of the alloy’s corroded surface reveal uniform attack on the surface, with minimal evidence of localized corrosion. The TiO2 oxide layer formed due to passivation leads to a minimum attack on the surface. However, the fine cracks and pores on the alloy specimen lead to localized corrosion on the surface, and the corrosion resistance of the alloy specimen is minimized.

Figure 12
(a-d) surface morphology of the corroded Ti64 alloy and Ti64 composites.

The C50 composite surfaces display minimum pitting and smoother corrosion morphology related to the unreinforced alloy. This enhancement is attributed to the uniform dispersion of reinforcements, which helps a more protective and strong oxide layer. The attack of the chloride ions on the matrix reduced by the minimal porosity of the C50 composites. The minimal porosity ensures the reduced pathways to attack the titanium matrix. The pits and cavities were observed in the corroded surface morphology of the C25 & C75 composite, and this can be attributed to severe localized corrosion.

The porosity and non-uniform distribution of the C75 composite lead to the initiation of chloride ion penetration, leading to severe pitting corrosion. The interface between the Ti matrix and reinforcement particles leads to initiation for the corrosion due to their different electrochemical potential characteristics. The residual stresses produced during sintering affect the passive layer around the reinforcement particles, leading to the corrosion initiation [²⁶]. The agglomeration of the particles in the C75 composite is also a cause of reduced corrosion resistance.

A comparative analysis of mechanical and electrochemical performance is shown in Table 3. The hardness value of 535 ± 8 HV in our hybrid composite is higher than those for Ti6Al4V reinforced only with TiB₂ or TiC and other ceramic reinforcement at similar weight percentages. The wear rate of 1.3 × 10⁻⁴ mm³/N·m also shows better performance, due to the combined effect of dual-phase ceramic reinforcement. Additionally, the corrosion current density (1.0 × 10⁻⁶ A/cm²) is lower than that of similar composites, which indicates improved passivation in the simulated body fluid environment. These results highlight the benefits of hybrid reinforcement in enhancing both mechanical and corrosion resistance.

Table 3
Comparison table for similar Ti6Al4V/ceramic composites.

MATERIAL / REINFORCEMENT	HARDNESS (HV)	WEAR RATE (MM3 · N–1 · M–1)	ICORR (A.CM–2)	REFERENCE
Ti6Al4V + 5 wt% TiB2 + 5 wt% TiC	535 ± 8	1.3 × 10–4	1.0 × 10–6	Present study
Ti6Al4V + 10 wt% B4C	365	4.5 × 10–4	5.82 × 10–6	[26]
Ti6Al4V + 10 wt% (TiC+TiB)	54 (HRC)	3.5 × 10–4	–	[27]
Ti6Al4V + 3wt% TiC+1wt%B	487 ± 27	–	–	[28]
Ti6Al4V + 2.5wt% HA	334 ± 6	2.2 × 10–4	–	[29]
Ti6Al4V + 10wt% HA	480	5 × 10–4	–	[30]

4. CONCLUSIONS

In this study, the addition of TiB₂ and TiC to the Ti6Al4V alloy was investigated. The succeeding decisions can be made:

• The powder metallurgy technique effectively produced both Ti6Al4V alloy and TiB₂ & TiC reinforced hybrid composites with nominal flaws.

• The Ti64/5%TiB₂&5%TiC hybrid composite exhibited excellent microstructural properties, attributed to strong interfacial bonding.

• The inclusion of 5% TiB₂ & TiC hybrid reinforcements resulted in improved mechanical properties like density (4.38g·cm⁻³), reduced porosity (1.52) and hardness (527 HV) of the composite compared to the base alloy.

• The presence of 5% TiB₂ & TiC hybrid reinforcements resulted in improved wear properties like reduced wear loss (0.0341gm) and reduced CoF (0.51) of the composite at maximum load compared to the base alloy.

• Potentiodynamic polarization studies showed an increase in corrosion resistance with the accumulation of TiB₂&TiC. Nevertheless, the Ti6Al4V reinforced with 5% TiB₂ and 5% TiC exhibited exceptional corrosion resistance with minimum corrosion rate (0.030 mm/year), similar to that of the other composite samples.

Considering these results, the Ti64/5% TiB₂ and 5% TiC hybrid composite appears to be a promising material for use bio medical applications like load-bearing orthopedic implants, for instance femoral stems, spinal cages, and dental abutment, combining enhanced mechanical strength with superior corrosion resistance. Future research will concentrate on assessing these composites’ fatigue performance and fracture toughness under cyclic loading circumstances. To demonstrate clinical relevance, in vivo corrosion behaviour and biocompatibility evaluations, such as cytotoxicity and osseointegration, will also be investigated. Furthermore, this composite system’s application to additive manufacturing processes like selective laser melting (SLM) might provide even more design flexibility and microstructure control. These initiatives will contribute to the expansion of Ti6Al4V-based hybrid composites’ usefulness in demanding settings.

5. ACKNOWLEDGMENTS

This work is financially supported by Science & Engineering Research Board (Ref. No: DST-EEQ/2019/000079), a statutory body of the Department of Science & Technology, Government of India.

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Publication Dates

• Publication in this collection
  06 Oct 2025
• Date of issue
  2025

History

• Received
  16 May 2025
• Accepted
  25 Aug 2025