Characterization of pulse electric current sintered Ti-6Al-4V ternary composites : Role of YSZ-Si3N4 ceramics addition on structural modification and hydrogen desorption

In this study, we fabricated Ti-6Al-4V composites using pulse electric current (spark plasma) sintering technique and examined the influence of yttria-stabilized zirconia (YSZ) and silicon nitride (Si 3 N 4 ) particles on micro-structural, mechanical properties. Moreover, we investigated the effects of YSZ and Si 3 N 4 on hydrogen uptake rate of the fabricated composites. The formation of new phases in addition to the parent α and β phases corroborates the increased hardness property exhibited by the Ti-6Al-4V composites. Further, the improvement in the hardness property was ascribed to Orowan strengthening effect due to resistance offered by cross dislocation pinning effect of closely packed particles. From the nanomechanical test, the penetration depth of the unrein-forced Ti-6Al-4V alloy was maximum at a value of 272.57 nm, while all the reinforced alloys exhibited reduced penetration depth, thereby increasing the stiffness and strength of the Ti-6Al-4V composites. Other nano-mechanical analyses such as nanohardness, elastic modulus, and creep were also improved in the reinforced composites in comparison with the Ti-6Al-4V alloy. The amount of hydrogen absorbed by the specimens was measured, and the Ti-6Al-4V composite with the highest proportion of Si 3 N 4 reinforcement exhibited the highest hydrogen concentration.


Introduction
Engineers and researchers have adopted new fabricating techniques for overcoming several challenges associated with manufacturing and designing advanced materials for different applications.These techniques are often utilized by professionals who employ advancements in the fabrication of functional materials to solve challenges that are faced in manufacturing and other engineering-related industries.These fields have recorded various breakthroughs, improving the relationship between material processing and microstructural properties.
Ti-6Al-4V, a titanium alloy, has easy weldability, high corrosion resistance, and enhanced mechanical properties at room temperature.Therefore, it has been widely used in the production of structural components used in the aerospace, automobile, submarine, and other engineering industries.Its ability to promote adhesion between human tissue and the implants when used as a biomaterial has also extended its usage to the biomedical industry [1,2].Titanium-based alloy has been reported to deteriorate at high temperatures, limiting their applications in fields where enhanced mechanical properties at elevated temperatures are crucial [3].The need to improve the overall properties of Ti-6Al-4V alloy through chemical and physical modification has continued to attract significant attention from researchers worldwide.The choice of fabricating technique is also essential in determining the properties of the composites.
Conventional casting and other sintering consolidation techniques, including hot pressing and high-temperature extrusion, have been used to fabricate biomedical titanium components [1,[4][5][6].However, Sung et al. [7] reported on casting titanium alloy to promote the formation of an alpha case mechanism in as-cast titanium alloy.The driving force for this mechanism described in this study was attributed to the presence of oxygen in the interstitial atoms and the dissolved substitutional metal atoms present in the moulds [8,9].Moreover, the process of consolidation, through hot and high-temperature extrusion, can lead to accelerated grain growth, which has the potential to adversely impact the mechanical properties of the end product [10].Although the consolidation of metallic powders using spark plasma sintering gives a sintered compact that requires machining into the desired shape, it is considered more efficient [11].Sintering parameters such as holding time, cooling time, sintering temperature, heating rate and sintering pressure can be controlled to achieve proper densification of the sintered compacts, producing components with reduced porosity and excellent mechanical properties [12,13].This can be achieved through the incorporation of reinforcement particles with improved engineering properties.In this study, we reinforced the Ti-6Al-4 V alloy with particles of YSZ and Si 3 N 4 .
YSZ is an inert and biocompatible ceramic material with high toughness and strength.These properties are attributed to the tetragonal-monoclinic transformation, which causes a reinforcement mechanism known as phase transformation toughening [14].This transformation is accompanied by an increase in volume, resulting in compressive stress, which in turn promotes the crack resistance of the ceramic material [15,16].Studies have shown that passive layers deposited on substrates by bulk zirconia are suitable as bioactive films [17], corrosion inhibitors [18] and heat-resistant turbine blades [19].On the other hand, Si 3 N 4 , a high-performance industrial ceramic, has reportedly been used in automotive engines, gas turbines, and heat exchanger applications owing to its improved temperature resistance, toughness, and abrasion resistance [20].Despite the wide application of Si 3 N 4 , its osteoconductivity and biocompatibility extended its usage to the biomedical industry [21].Research on the possible application of Si 3 N 4 coatings and monolithic Si 3 N 4 for hip replacement in orthopedic reconstruction is ongoing [22,23].Notably, the chemical interaction between the matrix and reinforcement particles is essential in determining the microstructural and mechanical properties of the resulting composites.This interaction is based on the following factors: (i) internal stability between the impurities induced during processing and inherent stability of the constituent elements, (ii) chemical compatibility between the constituent elements of the matrix and reinforcement particles, and (iii) environmental stability, which is dependent on the interaction between the processing environment and the constituent elements [24].Researchers have extensively studied the types and properties of materials suitable for biomedical applications.
Bolzoni et al. [25] investigated the feasibility of using sintered Ti-6Al-4 V alloy for biomedical applications.This study discusses the advantages of fabricating biomedical alloys using powder metallurgy.These include cost-effectiveness and the production of near-net-shaped components with excellent mechanical properties.The effect of YSZ on the mechanical and microstructural properties of sintered titanium alloy was also reported by Tohgo et al. [26].The sintered alloys were reported to exhibit a relative density of 93 % upon the addition of YSZ reinforcement particles.Moreover, pure Ti recorded a relative density of 100 % after sintering.In addition, X-ray diffraction (XRD) analysis revealed the formation of new and beneficial intermetallic phases such as Ti 2 ZrO and Ti 2 O.The Young's modulus and hardness values of the composites exceed the rule-of-mixture predictions.Obadele et al. [27] fabricated and characterized spark plasma sintered nickel and zirconia-reinforced titanium alloy.The microstructure and hardness were influenced by ZrO 2 reinforcement, which provided a pinning effect within the neck zone in the form of dispersoids.This pinning effect reportedly impedes grain growth and dislocation movement within the grain boundary.The microstructural morphology also revealed the presence of evenly distributed nickel-rich globules.Furthermore, the size of the reinforcement particles plays a crucial role in determining the load-transfer effect between the matrix and reinforcement particles.It has been proven that the strength and hardness of metal matrix composites increase with decreasing reinforcement particle size [28,29].
Understanding the effects of hydrogen produced from the interaction between the titanium alloy and its environment and the inherent hydrogen present in the material as a solid solution or hydride is essential [30].In a typical hydrogen-titanium system, the β-phase dissociates into the α + hydride phase at 300 0 C by eutectoid transformation [31].Generally, the absorbed hydrogen is not evenly dispersed within titanium alloys.It is either present in the host lattice or segregated into microstructural and atomic imperfections such as vacancies, dislocations, grain boundaries and second-phase particles.These sites often act as sinks, that hide the hydrogen atoms within the alloy during mechanical loading [32,33].Several studies have shown that hydrogen is biomedically beneficial as it neutralises hydroxyl radicals [34], assists in the treatment of ischemia [35], and reduces inflammation [36].Noda et al. [37] demonstrated the usefulness of hydrogen, in mitochondria respiratory and protection of cardiac allografts.Another investigation by Otha [38] described the presence of molecular hydrogen in the body as an antioxidant that reduces cytoprotective oxidative stress from free radicals such as peroxynitrite in body cells [39].
Since the literature on the overall influence of YSZ and Si 3 N 4 reinforcements on Ti-6Al-4 V alloy is limited, this study presents an indepth and extensive characterization of the microstructural and nanomechanical performance of YSZ and Si 3 N 4 reinforced Ti-6Al-4 V alloys fabricated using the pulse electric current sintering method.In addition, hydrogen desorption characteristics at elevated temperature were investigated.

Morphology and sintering of starting powders
The commercially available Ti-6Al-4 V powder (purity: 99.9 %, average particle size: 25 µm) was supplied by TLS-Technik, Germany and used as the matrix.Silicon nitride powder (purity: 99 %, average particle size: 1 µm) was purchased from Sigma Aldrich (United States) and used as the first reinforcement.YSZ powder (purity: 94.5, average particle size: 5 µm) was supplied by Stanford Advanced Materials (United States) and used as the second reinforcement.The morphologies of these powders were examined using a Zeiss Ultra 55 field emission scanning electron microscope (FE-SEM), and the resulting SEM images are presented in Fig. 1.Fig. 1a shows that the particles of the Ti-6Al-4 V powders were mostly spherical, non-porous and relatively smooth.The attachment of the small spherical powders (satellites) to larger ones confirms the production of the powder via gas atomization process [40].As shown in Fig. 1b, the Si 3 N 4 particles exhibited an elongated, faceted structure known as the crumpled eight-member Si-N ring.The ring is connected to form interconnected plates by bridging Si-N bonds [41,42].A micrograph of the YSZ powder is shown in Fig. 1c, revealing narrow, uniform size and spherically shaped particles.To ensure a homogeneous dispersion, the powders were mixed in a turbula mixer for 8 h at 49 rpm, and the admixed powders were consolidated in a spark plasma sintering machine to produce sintered compacts.A graphite foil was placed between the punch and graphite die (40 mm in diameter) to prevent powder contamination and increase thermal heterogeneity during sintering.The powders were sintered at 1100 0 C and 50 MPa with a holding time and a heating rate of 10 min and 50 0 C/min, respectively.The densities of the sintered compacts were evaluated from the Archimedes' principle.

Microstructural characterization of sintered compacts
Using a rotating anode x-ray goniometer (SmartLab, Rigaku) operating in the symmetric reflection (θ/2θ) mode, the XRD measurements were conducted for the fabricated samples.The x-ray beam was monochromatized by the incident side Ge-220 double-bounce monochromator with a copper K-α 1 wavelength of 1.541 Å.The 2θ range was between 20 • and 150 • , and intensity was measured using a 2D hybrid pixel detector in 1D mode.Samples 10 mm × 10 mm in size were sectioned from the sintered compacts and prepared for optical and Scanning electron microstructural examination using standard metallographic procedures on a Struers Tegramin 20 automatic grinding and S.O.Akinwamide et al. polishing machine until a shiny surface was achieved.This was followed by etching with Kroll's reagent to reveal the grains and their respective boundaries.Prior to preparation, the sectioned samples were mounted in a multifast epoxy resin using a Struers Pronto press-20 hot mounting machine.Optical micrographs were obtained using a Zeiss optical microscope, and SEM images were examined using a Zeiss Ultra 55 FE-SEM.The energy backscattered diffraction (EBSD) specimens were further polished using a Buehler Vibromet 2 vibratory polisher for 18 h until they were free from scratches and internal deformation induced during sectioning and mechanical grinding.The EBSD patterns were captured when an indexing of at least 75 % was obtained, the captured image was further processed using the MTEX open-source tool in MATLAB.Prior to all the microstructural analysis, the surfaces of the samples were cleansed in a Struers Metason 200 HT ultrasonic cleaner to prevent contamination.To further understand the morphology of the samples at the atomic level, the samples were investigated under a Joel JEM 2200FS field emission transmission electron microscope.Transmission electron microscopy (TEM) lamellas were pre-prepared by a focused ion beam (FIB) on a JEOL JIB-4700 F microscope.To prevent the sample surface from gallium (Ga) ion beam, a 1.2 µm thick platinum protective layer was deposited on top of the regions of interest before milling.Ga ion beam energy/probe current with 30 kV/10 nA and 5 kV/ 30pA were applied for coarse and fine polishing, respectively.After fine polishing, the thickness of TEM lamellas was approximately 50 nm, which was further used for the TEM analysis.

Micro and nanomechanical analysis
The resistance of the specimens to micro indentation was investigated using a Duramin 40 Struers microhardness tester.The samples were subjected to a load of 20 g, and the dwell time was maintained at 10 s throughout the test.Indents were made across the matrix and reinforcement phases, as revealed by the microscopes attached to the equipment.Measurements were performed across the phases, and the final Vickers microhardness value was obtained from an average of ten indentations.Furthermore, nanomechanical analyses, including reduced elastic modulus, nanohardness, penetration depth measurement, and load displacement, were evaluated using a CSM ultra nanoindenter equipped with a high-precision three-sided pyramid Berkovich diamond indenter.All measurements were performed in accordance with ASTM E2546 and ISO 14577 standards.A load of 5 mN, with loading and unloading rates of 20 mN/min, was applied throughout the test.The acquisition rate and contact force were also maintained at 10 Hz and 10 mN respectively.A total of 10 indentations were performed across the matrix and reinforcements, with a spacing of 5 µm, to avoid work hardening on the surface of the specimens [43].The schematic illustration of the sample-indenter contact is shown in Fig. 2, where a, b, and c are the specimen surface before, under, and after loading, respectively.Moreover, h max and hp are the maximum and permanent indentation depths, respectively.
The nanohardness (H IT ) and young modulus (E) of the specimens were calculated using Eqs. 1 and 2, respectively.
Where F max = maximum load applied, and Ap = calibrated area of  contact between the specimen surface and the indenter. Furthermore, Where E * is the reduced modulus of the specimen (Eq.3), Ei is the elastic modulus of the indenter (1141 GPa), Vi 2 is Poisson ratio of diamond indenter (0.07), and v is Poisson's ratio of the specimen.This was in accordance to Oliver-Pharr method [44].
It should be noted that: Where S is the contact stiffness, and β is a constant associated with the shape of the indenter.
The indentation creep (C IT ) of the specimens was estimated using Eq. ( 4): where h 1 is the indentation depth at time t 1 , and h 2 is the indentation depth at time t 2 .(t 1 and t 2 depend on applied load).

Hydrogen concentration measurement
The hydrogen concentrations in the reinforced and unreinforced titanium alloys were measured using thermal desorption spectroscopy (TDS) technique.The specimens used for the analysis were sectioned from the as-sintered compact using electrical discharge machine to a specific size of 7 mm × 5 mm × 0.7 mm.To ensure a perfect surface finish, the specimens were mechanically polished using 1200 grits emery paper to eliminate possible oxidised layer formed on the specimen surface during the wire-cutting process.Prior to the TDS measurement, the specimens were cleaned with distilled water, followed by drying under a helium gas flow to prevent the formation of moisture on their surface.The partial pressure measurement of hydrogen was conducted in an Ultra-high vacuum (UHV) chamber at an average starting pressure of 1 × 10 −9 mbar coupled with a mass spectrometer (SRS residual gas analyser).To ensure UHV chamber was maintained at the required pressure with a reduced pumping time before measurement, the sample was placed in an airlock compartment and pumped to an intermediate pressure of 1 × 10 −6 mbar.The specimen was then transported into the UHV chamber, and measurement was initiated.The total time from the specimen preparation, placing in the airlock, and transportation to the furnace in the UHV chamber was maintained within 10 mins [5].All TDS measurements were performed at a heating rate of 8 K/min from room temperature to 930 K. To ensure reproducibility of the results, all measurements were performed three times, and the total hydrogen concentration was calculated by integrating the area below the desorption rate and temperature curve.

Densification of sintered compacts
The density of the sintered compacts is dependent on the sintering parameters adopted for fabrication [45].In this study, the optimum parameter was preselected based on the sintering conditions at which the unreinforced Ti-6Al-4V matrix recorded a density of 99.9 %.Furthermore, the temperature and holding time adopted during sintering play a crucial role in estimating the density of the sintered compacts.Plastic deformation occurs at reduced temperature, which is often accompanied by particle diffusion and is insufficient for the adequate compaction of the powder particles [46].A rapid decline in densification was observed for the unreinforced Ti-6Al-4V alloy as shown in Fig. 3.A relative density value of 95.4 % was observed in the Ti-6Al-4V alloy reinforced with 5 % YSZ and 2 % Si 4 N 3 .The rapid decrease in densification could result from the different sizes of the reinforcement particles, making it difficult to achieve complete compaction during mixing and sintering operations.Sintering at high temperature also results in the formation of intergranular pores, which combine to form closed pores.Reinforcement of different particle sizes conglomerate and form coarse grains, even in the presence of heterogeneously distributed spherical grains [47].However, the relatively enhanced density recorded in the specimen with 2 % YSZ and 2 % Si 4 N 3 can be ascribed to the high diffusivity of the reinforcement particles in the Ti-6Al-4 V matrix.In a recent investigation, Tsai [48] ascribed an improvement in the densification of sintered compacts to reorientation of the particles through pore filling from grain rotation and boundary sliding.At a given density and temperature during sintering, the densification rate depends on the grain size, according to Eq. 5.
Where ρ is the density, c is the temperature-dependent constant, m is the exponent (4 for diffusion of grain boundary and 3 for diffusion of lattice) and Q denotes the grain size.Considering the dependence of densification rate on grain size, grain growth control is required to attain high densification.
The composite with the highest reinforcement addition (5 %YSZ+5 %Si 3 N 4 ) exhibited the lowest relative density, which can be ascribed to the possible presence of pores and voids caused by the reinforcement particles [49].Furthermore, because the matrix and reinforcement powders have different densities, the powders tend to agglomerate during mixing, preventing adequate bonding during sintering [50].

X-ray diffraction analysis
The XRD patterns of the sintered specimens are shown in Fig. 4 the presence of a crystalline Ti 2 N phase in addition to the α and β parent phases in the sintered composites.The formation of Ti 2 N phase could also be promoted by the reaction between the consolidated powders and nitrogen atmosphere, which occurs when sintering is performed at temperatures above 900 0 C [51].Furthermore, most of the peaks were slightly widened and shifted towards the lower 2θ angle in comparison to the Ti-6Al-4V alloy.The shifting of peaks can be attributed to the presence of fine grains with higher lattice parameters within the α and β phases.Peak widening also confirmed the refinement of the grains in the composites, which is dependent on their lattice strain and crystallite size.The latter was calculated from the full width at half maximum intensity (FWHT), using Williamson-Hall [52] equation (Eq.6).From Table 1, the reduction in the crystallite sizes upon the addition of reinforcements confirms the efficiency of the spark plasma sintering technique adopted for fabrication [53].At the same time, the increased lattice strain shows the degree of distortion and crystal imperfection induced in the composites during powder mixing.Lattice strain can be reduced through heat treatment [54] and equal channel angular pressing [55].

Optical micrographs
Optical micrographs of the specimens are shown in Fig. 5.The Ti-6Al-4 V matrix presented in Fig. 5a reveals elongated lamellar structures formed during cooling above the beta transus temperature, causing nucleation and growth of the alpha phase within the boundaries of the beta phase.Fig. 5b shows a Widmanstätten structure, owing to the homogeneous distribution of the YSZ and Si 3 N 4 reinforcement particles within the boundaries of the alpha and beta grains.The Ti-6Al-4 V matrix in the specimen also exhibited a needle-like alpha phase, known as acicular alpha.Upon increasing the Si 4 N 3 proportion, a bimodal transformation of the lamellar structure was observed in Fig. 5c.This new evolution can be ascribed to the increased nitride reinforcement, which also increased the size of the beta grains.From Fig. 5d, the absence of pores and the evidence of reinforcement distribution show adequate metallurgical bonding between the reinforcement particles and the Ti-6Al-4 V matrix.Fig. 5e shows an increased proportion of reinforcement particles within the Ti-6Al-4 V matrix.The presence of pores in the micrograph can be attributed to the inadequate compaction between the powder particles due to their incoherent sizes.This was also confirmed by the lowest density recorded for the specimen, as shown in Fig. 3.

SEM morphology
The SEM micrographs of the specimens are shown in Fig. 6.The reinforcement-rich zones were evident along the boundaries of the Ti-6Al-4 V grains, which reduced the size of the matrix grains.Fig. 6a reveals a typical lamellar structure.The α-Ti, which has a hexagonal closepacked (HCP) structure, transforms to β-Ti grain when the sintering temperature increases above the allotropic transformation temperature of titanium (850 • C) [56,57].However, the presence of vanadium and aluminium, which are respective beta and alpha stabilising elements, prevent a complete reverse reaction (β-Ti→α-Ti) in the Ti-6Al-4 V matrix during cooling.This provides a dual phase to the microstructure, with the coexistence of α and β.The bright zone in the microstructure represents the vanadium-rich β phase, while the presence of Al and Ti regions was evident in the dark region.In Fig. 6b, the α and β grains exhibit different laths owing to the uniform cooling rate of the composite during the fabrication process.However, the length of the α and β laths are affected by the clusters of reinforcement particles formed at the boundaries of the α and β grains.Figs.6c and 6d show the influence of increased Si 3 N 4 reinforcement on the resulting composites.However, these reinforcements reduce grain growth, resulting in more refined equiaxed grains with homogeneously dispersed YSZ and Si 3 N 4 particles [58].Furthermore, sintering at high temperatures can decompose Si 3 N 4 into nitrogen and silicon, as shown in Eq. ( 7) [59]. ) The presence of YSZ particles in the Ti-6Al-4 V (Fig. 6c) matrix indicates some neck growth resulting from the ZrO 2 dispersoids.A similar observation was reported in a study by German [55], to influence grain growth during sintering.It was further reported that small dispersoids with reduced spacing efficiently reduced grain growth.They have also been reported to yield a pinning effect in the grain boundaries, causing them to curve because their movement is slower than that of the boundaries.The pulling of dispersoids at the boundaries during sintering has been reported to cause grain coarsening [56].Fig. 6e shows the micrograph of the Ti-6Al-4 V matrix reinforced with equal proportions (5 %) of YSZ and Si3N4 particles.Irregular particle sizes resulted in the formation of micropores, which may adversely affect the mechanical properties of the composite.The SEM micrographs further confirm spark plasma sintering as a reliable technique for promoting the formation of equiaxed α, for reduced volume fraction of reinforcement particles.This results in balanced material properties compared with an increased reinforcement volume fraction.

TEM and EBSD examination
TEM examination was performed on site-specific specimens   Ti-6Al-4 V alloys confirmed the Burgers relationship between the α and β phases, which was assisted by the spark plasma sintering process adopted for fabrication [61,62].
To further understand the phase distribution of the reinforced titanium alloy, the microstructure of Ti-6Al-4 V reinforced with 2 % YSZ and 2 % Si 3 N 4 was investigated using EBSD.The resulting phase and  8a shows the growth of the cubic Ti phase from the β-phase with a strong <2111 > texture during the sintering process.During cooling, the β-phase transforms to a hexagonal α-phase with different crystallographic variants.Therefore, the crystallographic orientation of the β-phase will influence the resulting microstructural property of the alloy.From Fig. 8(b-d), the orientation relationship between the YSZ and Si 3 N 4 reinforcements on the Ti-6Al-4 V alloy indicates reduced sizes of the β and α grains.This observation confirms the occurrence of dynamic recrystallisation during sintering [63].The SAED results shown in Fig. 8d further confirm that the reinforcement particles were almost intact without significant defects during sintering, which is similar to the results reported in previous studies [64].The corresponding pole-figure maps are also presented.The [2111], [2111], and [2110] planes, evident in the pole figure maps, exhibit a strong concentration towards the centre of the poles.However, the texture of these pole maps depends on several factors, such as the mode of structural deformation during mixing, the shape of the powder after mixing, which results in a preferential arrangement in the sintering die, and the nature of recrystallisation during sintering [65].

Hardness measurement
The surface resistance of the specimens to the indentation on the microscopic scale was evaluated, and the results are presented in Fig. 9.The highest hardness of 694.72 HV was recorded for the specimen reinforced with 5 % YSZ and 5 % Si 4 N 3 , whereas the unreinforced Ti-6Al-4 V alloy exhibited the lowest hardness value of 285.34 HV.The hardness values of all the reinforced Ti-6Al-4 V alloys were higher than that of the Ti-6Al-4 V specimen.Although the latter exhibited the highest relative density, the presence of semi-reacted reinforcement phases had a more significant influence on the hardness than the density.The increased hardness of the specimens can be ascribed to the homogeneous distribution of these reinforcement particles within the Ti-6Al-4 V matrix, which hinders dislocation movement and causes the specimens to bend around the reinforcement particles within the matrix.This phenomenon is known as the Orowan strengthening mechanism.However, additional dislocations were formed owing to the differential deformation and thermal mismatch between the Ti-based alloy matrix and the reinforcement particles.Eq. 8 [66] describes the Orowan strengthening mechanism of composites.
where Sb is the shear modulus, c is the average particle radius of the reinforcement, f is the Burgers vector of the matrix, and k is the volume fraction of the reinforcement particles in the Ti-6Al-4 V matrix.
In addition, the hardness was affected by the average grain size of the composite.Optical and SEM images showed a reduction in the elongated grains upon the dispersion of YSZ and Si 3 N 4 particles, which is in accordance with the Hall-Petch relationship represented by Eq. 9. Previous studies have also shown that the presence of reinforcement particles within the matrix often forms intermetallic phases [8,67], supporting the load-transfer mechanism between the Ti-6Al-4 V matrix and YSZ and Si 3 N 4 particles.The positioning of the indenter and the where C is the Hall-Petch coefficient and B is the average size of the grains in the composite.

Nanomechanical behaviour
The relationship between the applied load and displacement for all the reinforced Ti-6Al-4 V composites is shown in Fig. 11.The highest displacement/penetration depth of 218 nm was recorded for the unreinforced Ti-6Al-4 V alloy, whereas the lowest displacement of 115 nm was observed for the Ti-6Al-4 V alloy reinforced with 5 % YSZ and 5 % Si 3 N 4 .This observation also confirms that the load transfers from the matrix to the reinforcement in the composites owing to the volume fraction of the smaller reinforcement particles incorporated into the Ti-6Al-4 V matrix [68,69].The load-transfer mechanism (Eq.10) is a major strengthening mechanism responsible for enhanced mechanical properties of composite materials.For a particle-reinforced composite, the load can be effectively transferred from the matrix to the reinforcement through the interface [70].The yield strength increased with an increase in the volume fraction of the reinforcement when the reinforcement particles were distributed adequately within the matrix [71].σ load=0.5PvYs(10) where σ load is the yield strength (MPa), P v is the volume fraction of the reinforcement, and Y s is the yield strength of the matrix (MPa).The loading and unloading curves determine the penetration depth; the loading part of the curve describes the response of the specimens to elastic strain deformation, whereas the unloading segment represents the response to elastic recovery.The load-displacement plots revealed a continuous characteristic, regardless of the proportion or type of reinforcement particles dispersed within the Ti-6Al-4 V alloy.The absence of instability and plasticity also indicates that the indenter has a high dislocation density, allowing plastic deformation to proceed without nucleating new dislocations [72].Moreover, the indentation across the α and β interfaces of the Ti-6Al-4 V reinforced composites can restrict the movement of existing dislocations, causing accumulation of dislocations at the reinforcement-matrix interfaces without re-emission and absorption along the closest boundaries.The interaction between the indenter and piled-up dislocations resists the penetration of the indenter, thus reducing its penetration depth at the interface.This reduced displacement in the reinforced Ti-6Al-4 V alloys can be ascribed to the strengthening mechanism of the reinforcement particles at the α-β interface because the penetration depth on the grains is higher than at the grain boundaries [73].The presence of a small pop-in effect in the reinforced Ti-6Al-4 V alloy (which is absent in the unreinforced Ti-6Al-4 V alloy) can be ascribed to strain transfer and dislocation nucleation in the grain boundary regions when the reinforcement particles inhibit the movement of dislocations induced by the indenter [74,75].The load and indenter acted according to the Power law [55] described in Eqs. 10 and 11, which represent the loading and unloading portions of the curves, respectively.
where P is the applied load, k is the indenter displacement, k f is the final indentation depth, and σ 1 , σ, t 1 , and t are empirical fitting constants.
A plot of penetration depth with respect to time is shown in Fig. 12.The maximum depth (hmax) under an applied load (P max ) was calculated using Eq. ( 12): (12) where h e is the elastic surface displacement and h c is the contact depth.
In this study, h max for Ti-6Al-4 V exhibited a maximum penetration depth of 272.57nm, whereas the addition of 2 % YSZ + 2 % Si 3 N 4 , 2 % YSZ + 5 % Si 3 N 4 , 5 % YSZ + 2 % Si 3 N 4 , and 5 % YSZ + 5 % Si 3 N 4 reinforcements reduced the maximum penetration depth by 23.5 %, 38.08 %, 36.82 %, and 34.24 %, respectively.However, this result indicates an enhancement in the stiffness and strength of the Ti-6Al-4 V composites owing to the resistance offered by the reinforcements to the pinning effect and dislocation motion in the matrix [76].Fig. 13 shows the nanohardness plot of the reinforced Ti-6Al-4 V composites.The specimen containing 5 % YSZ and 5 % Si   the highest nanohardness value of 8655.4MPa, whereas the lowest nanohardness value of 3442.5 MPa was observed for the unreinforced Ti-6Al-4 V alloy.General observations showed that all reinforced Ti-6Al-4 V alloys exhibited improved nanohardness.This result validates the microhardness measurements presented in Fig. 9, in which the microhardness of the specimens was attributed to multiple strengthening mechanisms.Fig. 14 shows the elastic moduli exhibited by the specimens under an applied load.The modulus of elasticity is an inherent property of a material and is dependent on the forces between the atomic bonds and crystal morphology of the phases present [77].The elastic moduli of the alloys with different phases were evaluated based on the elastic moduli and volume fractions of the dominant phases.The highest elastic modulus (155.32GPa) was also observed for the composite with 5 % YSZ and 5 % Si 3 N 4 reinforcement.The specimens reinforced with 2 % YSZ + 5 % Si 3 N 4 and 5 % YSZ + 2 % Si 3 N 4 exhibited elastic moduli of 152.54 and 151.62 nm, respectively.The enhanced elastic modulus of these specimens could be attributed to the formation of secondary phases owing to the reaction between the Ti-6Al-4 V alloy and the reinforcement particles.In conclusion, the decomposition of titanium and zirconium hydrides at high temperatures (≈900 K) increased the hydrogen content, whereas silicon nitride was responsible for hydrogen entrapment within the composites.A summary of the results and temperatures exhibited at peaks 1 and 2 is presented in Table 2. From the results obtained, the fabricated titanium-based composites can be used in the development of novel materials that are biocompatible when used as metallic implants and in other clinical applications.

Conclusion
The fabrication, characterization and hydrogen susceptibility of spark plasma sintered Ti-6Al-4 V alloy reinforced with different proportions of YSZ and Si 3 N 4 reinforcement particles was investigated, and the following key findings were noted: ◆ The high diffusivity of the reinforcement particles within this Ti-6Al-4 V matrix played a crucial role in enhancing the relative density when the proportion of the reinforcement particles was reduced.◆ The peak shifts observed in the XRD plot were ascribed to the refinement of grains in the α and β phases.This was confirmed by the lattice strain and crystallite values of the specimens.◆ The optical micrographs and SEM images showed a proper distribution of the reinforcement particles, which further confirmed    the efficiency of the turbula mixing technique adopted for dispersion.◆ TEM and EBSD images corroborated the establishment of Burger orientation relationship between the α and β phases.
◆ The enhancement of micro-and nanomechanical properties of the reinforced Ti-6Al-4 V composites was attributed to the load transfer mechanism between the matrix and the reinforcement phases.◆ The titanium-based composites exhibited increased hydrogen concentrations, making them suitable for biomedical-related applications such as dental implants, bone fixators and artificial joints.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
extracted from the sintered compacts after detailed microstructural information about the specimens was obtained from FIB.The resulting morphologies and their respective selected area diffraction (SAED) patterns are shown in Fig.7.The SAED spots are labelled as "diff" spots in the diffraction images placed beside the TEM images.The unreinforced Ti-6Al-4 V alloy exhibited a typical lamellar α and adjacent β phase microstructure, as shown in Fig.7a.The dislocation lines evident in Fig.7bcould result from adequate bonding between the reinforcement particles and Ti-6Al-4 V matrix.It is noteworthy that the dislocation lines indicate the presence of internal stress between the particles and matrix, which promotes interfacial strengthening.Fig.7cshows stacking faults and needle-like structures formed during crystal growth.The stacking fault was supported by the mechanical mixing of the powders, resulting in lattice distortion.This further demonstrates that high-temperature sintering does not facilitate complete recovery and recrystallisation.Fig.7dalso indicates the presence of cross-slips induced by the increased Si 3 N 4 reinforcement through the movement of screw dislocations from the slip plane to adjacent dislocations.The micrograph also shows a polycrystalline structure coupled with spherical precipitates.However, the SAED patterns of the matrix in this figure confirm the attachment of the reinforcement particles to the β-Ti phase.A similar observation was reported by Cai et al.[60].Fig.7eshows a micrograph of Ti-6Al-4 V reinforced with 5 % Si 3 N 4 and 5 % YSZ.The presence of high-dislocation density lines and pole elongation observed in the SAED patterns suggests the presence of an existing stacking fault.Notably, piled-up dislocations often exhibit improved resistance towards plastic deformation owing to dispersion strengthening.Furthermore, the corresponding diffraction patterns exhibited by all reinforced
measurements across the matrix and reinforcement phases are shown in Figs.10a and 10b, respectively.

3 N 4 recordedFig. 10 .
Fig. 10.(a) Image of the indenter over the specimen surface (b)Identation pattern of measurements taken across the matrix and reinforcement phases.
Fig. 15 shows a plot of the indentation creep extracted from the load-displacement curves during the load-holding time.All specimens exhibited similar creep values, and the reduced creep value of the reinforced Ti-6Al-4 V composites compared to that of the unreinforced Ti-6Al-4 V alloy indicates creep enhancement in the composites.The creep process in engineering materials occurs in three main stages [78]: (i) transient creep, in which the creep strain rate decreases with time; (ii) steady-state creep, in which the creep strain rate is constant; and (iii) tertiary creep, in which the creep strain rate increases with time until fracture.

Table 1
Lattice strain and crystallite sizes of sintered specimens.

Table 2
Summary of hydrogen concentration measurements results for the studied compacts.