Influence on the properties of TMCs of ceramic and intermetallic composite reinforcements (B₄C, TiₓAly and TiₓSiy) fabricated by inductive hot pressing

Research article Influence on the properties of TMCs of ceramic and intermetallic composite reinforcements (B 4 C, Ti x Al y and Ti x Si y ) fabricated by inductive hot pressing Cristina M. Ar´ evalo a , Isabel Montealegre-Mel´ endez a,* , Erich Neubauer b , Michael Kitzmantel b , Sheila Lascano c , Eva M. P´ erez-Soriano a a Departamento de Ingeniería y Ciencia de los Materiales y del Transporte, Escuela Polit´ ecnica Superior, Universidad de Sevilla, Calle Virgen de ´ Africa 7, Sevilla, 41011, Spain b RHP-Technology GmbH, Forschungs- und Technologiezentrum, 2444, Seibersdorf, Austria c Departamento de Ingeniería Mec´ anica, Universidad T´ ecnica Federico Santa María, Avenida Vicu˜ na Mackenna 3939, Santiago, 8940572, Chile ARTICLE INFO Keywords: Titanium matrix composites Intermetallics Powder metallurgy Inductive hot pressing Microstructure ABSTRACT Ambitious and competitive, the aerospace industry continuously demonstrates to be one of the leading engineering sectors either at exigence and new technologies development. As lightning the weight of aircrafts is one of the main targets, the spotlight is usually on material research by which new ones may be produced to pursue this aim and still offer the necessary performances. The combination of the properties of titanium and other materials as reinforcements provides really interesting results as titanium matrix composite materials, also known as TMCs. Various samples of titanium matrix composite materials with different reinforcements have been under study to determine the influence of the reinforcements and their respective proportions on the properties of the material. These samples composed of grade 1 commercially-pure titanium as matrix and B 4 C, Ti x Al y and Ti x Si y as reinforcements, have been manufactured through powder metallurgy in the same conditions of temperature and pressure via Inductive Hot Pressing (IHP). A total of eight composite materials have been arranged in several different groups to confront their compositions. Thus, this analysis reports results for the influence of the powder size of the matrix and the ceramic reinforcement, the effect of varying the volumetric composition of B 4 C, and the selection of different intermetallic reinforcements. These tests and the obtained information serve for a project in which the main goal is to determine which compositions of the studied composite materials reach a high enough specific stiffness for a suitable application in the aerospace industry. 1. Introduction The aerospace engineering sector is well known for its high investment and effort put into research and the development of vanguard technology [1,2]. Besides multiple achievements that have made aircrafts safer and more efficient over the last decades, there will always be room for improvement in terms of crafting materials [3–6]. Although fulfilling the design and performance * Corresponding author. E-mail addresses: [email protected] (C.M. Ar´ evalo), [email protected] (I. Montealegre-Mel´ endez), [email protected] (E. Neubauer), [email protected] (M. Kitzmantel), [email protected] (S. Lascano), [email protected] (E.M. P´ erez-Soriano). Contents lists available at ScienceDirect Heliyon journal homepage: www.cell.com/heliyon https://doi.org/10.1016/j.heliyon.2024.e37682 Received 9 May 2024; Received in revised form 27 July 2024; Accepted 8 September 2024 Heliyon 10 (2024) e37682 Available online 12 September 2024 2405-8440/© 2024 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). requirements while reducing the total weight is not an easy task, great progress has been made due to being a key target of study in this sector [7,8]. Titanium settled as a fundamental metal for aeronautics in the ’60s and its alloys are the materials which most aircrafts are made of nowadays [9,10]. Its specific physical properties and other interesting qualities such as high corrosion resistance and good thermal resistance and conductivity have granted titanium its position [11,12]. Even so, this never-satisfied industry is always moving forward, and the current focus is on composite materials [13]. Composite materials offer high-performance properties and lightened structure weight by combining different nature materials and applying diverse techniques [14]. Among these various types of composites, this study concentrates on titanium-matrix composite materials (also known as TMCs) with concrete ceramic and intermetallic particles as reinforcements [15]. The reactivity of the Ti-matrix is crucial. The chosen ceramic reinforcement is B 4 C, as it has already been proven to be one of the most suitable options for TMCs. B 4 C acts as a source of carbon and boron, leading to the formation of TiC and TiB as secondary phases within the matrix. These compounds have been studied for their stability within the Ti-matrix and their impact on enhancing properties as in the Young modulus and tribological behaviour whilst not increasing their density [16–20]. The interface zone between the matrix and B 4 C particles is of particular interest. The formation of in-situ secondary phases (TiB and TiC) favours the strengthening of the titanium matrix, being their good interfacial bonding among matrix and reinforcement a feature to be highlighted [21–25]. The primary advantage of these in situ phases lies in the strong interfacial bonding between the reinforcement and the matrix. Additionally, the ceramic reinforcements formed during in situ processing are finer, more thermodynamically stable, and uniformly distributed in the metal matrix. Aside from these ceramics, TiAl, Ti +Si and Ti +Al powders are added too in order to enhance the final properties by generating intermetallic reinforcements, although this may limit the formation of secondary phases of the previous ceramics. Ti x Al y intermetallics often grant improvements to mechanical properties, such as tensile, fatigue and deformation strengths, as well as thermal stability [26, 27]. For its part, Ti x Si y intermetallics can also enhance mechanical properties; though it is often of special interest because of other benefits, these possibly being resistance to oxidation and other types of corrosion, high-temperature strength and better dimensional stability [28,29]. Additionally, it is also of interest to study the effects of using non-alloyed powders such as Ti +Al and Ti +Si instead of their alloyed versions. This variation can influence the formation of in-situ intermetallics, which are crucial for the final properties of the material. Regarding the manufacturing method for these titanium-matrix composite materials, powder metallurgy presents several advantages in comparison to other methods. Capability of reproducing complex-geometry parts, repeatability of parts manufacturing, better homogeneity and reduced material waste are some of them [30–32]. Inductive Hot Pressing (IHP) sets to be the powder-compaction applied technique for the studied materials; being often a great option because of its rapid heating cycles [33,34]. The samples employed in the tests of this research have been fabricated in collaboration with RHP-Technology GmbH. The aim of this analysis is to determine the variations in the final properties of the manufactured materials concerning the different blends they are obtained from. The evaluated properties are density, hardness, Young modulus, microstructure and an X-Ray Diffraction (XRD) analysis. Thereby, the experiment results provide conclusions regarding the size of the metallic powder of the matrix and the ceramic reinforcement, the influence in the variance of the volumetric composition of B 4 C as ceramic reinforcement and the intermetallic reinforcement choice among the introduced candidates. As commented first, obtaining new materials that are capable of performing in exigent scenarios and allowing for a reduction in the total weight of the aircraft is of great importance in the aerospace industry. The tests carried out and presented here belong to a titanium matrix composite material study in which the main target was to obtain the greatest specific stiffness. Thus, besides stating the results of each studied comparison, a final goal is to determine the most suitable TMC for the requested property. 2. Materials and methods 2.1. Materials This section provides further details about the involved starting materials, such as the results of granulometry tests, morphology, and the volumetric compositions for the composite materials. The features of the powders of the studied materials are expected to play an important role in their final properties. As such, a granulometry study needs to be made to determine the distribution of the size of Table 1 Granulometry data. Material Powder d 10 [ μ m] d 50 (Average Size) [ μ m] d 90 [ μ m] Ti (fine-grained) 12.72 28.75 58.13 Ti (coarse-grained) 76.33 109.05 156.42 B 4 C 39.48 62.07 96.67 B 4 C (fine-grained) 15.47 23.31 34.95 TiAl 53.77 74.65 104.39 Ti +Al 6.84 16.16 ~100 (Ti), ~12 (Al) 131.18 Ti +Si 3.79 31.46 ~50 (Ti), ~6 (Si) 86.89 C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 2 the powders and their morphology. The equipment employed to accomplish this task has been a Mastersizer 2000 (Malvern Instruments, Malvern, UK); therefore, the average particle size can be verified. Starting off with the metallic matrix, two sizes of titanium (grade 1) powder have been considered for the matrixes of the samples in order to study their influence. Titanium powders (TLS) were requested with a first particle diameter size lower than 45 μ m and a second size between 75 μ m and 180 μ m. Several reinforcements of ceramic and metallic natures are part of the studied composite materials. Thus, the starting blends contain reinforcement powders of B 4 C (supplied by H.C. Starck), TiAl and Ti +Al (supplied by TLS), and Ti +Si in different proportions and combinations, conforming up to eight TMCs. The granulometry results for better accuracy are shown in Table 1. It is important to point out that for Ti +Al and Ti +Si powders, these results represent the average between the titanium powder and the aluminium or silicon powder, as said sizes are quite different and two peaks appear in their particle size distribution graphics. Images in Figs. 1 and 2 are Secondary Electron Scanning Electron Microscopies (SE SEM) carried out to visualize the morphology of each type of powder, having utilized the FEI Teneo scanning-electron microscope. While morphologies of both matrix powders are the same being spherical, in Fig. 1 it is appreciable the difference in diameter size between them: Fig. 1a corresponds to the titanium powder of fine grain and Fig. 1b to the coarse grain one. In the case of reinforcements, various morphologies are distinguishable due to their different natures. B 4 C powders (Fig. 2a) show a faceted morphology and different particle sizes. Ceramic powders in Fig. 2b are of the same nature as the previous one, though in a smaller size in comparison to the previous one. In turn, alloyed TiAl powder (Fig. 2c) resembles the matrix powders because of being spherical too. Regarding the last two reinforcements, titanium spheres can be seen in both images: larger titanium spheres and aluminium as small and irregular chips are present in Ti +Al powder (Fig. 2d), while irregular silicon grains fill the interspace among titanium fine grains in Ti +Si powder (Fig. 2e). The conjunction of matrix and reinforcement powders and the powder-metallurgy process (detailed further) results in the studied composite materials. From this point on, in order to ease the citation of these materials, each composition will receive a denomination according to the following rules. The coding consists of three pairs of letters and numbers. In general terms, the letter refers to the nature of the material, and the number describes the size or quantity. The first pair is related to the matrix powder, meaning T1 finegrained titanium and T2 coarse-grained titanium. The second pair refers to Reinforcement A, which can be A0 (no intermetallic reinforcement), A1 (Ti +Al), A2 (TiAl), or S1 (Ti +Si). The last pair is for Reinforcement B, which can be B20 (B 4 C added in volumetric 20 %), B30 (B 4 C added in volumetric 30 %), or B30F (finer grain B 4 C added in volumetric 30 %). The reinforcement particles employed were similar in size to those used in previous studies, in particular, for B 4 C particles. This was done in order to assess potential variations based on manufacturing parameters, rather than the sizes of the reinforcement particles themselves. Nevertheless, specimens were also fabricated with fine B 4 C, and it was indeed confirmed that the final properties were not superior. The studied samples can be denominated as it is gathered in Table 2. The samples were produced based on the authors’ previous research. The results for samples with 10 %, 20 %, and 30 % B 4 C indicated that specimens with higher B 4 C content demonstrated enhanced specific properties. The maximum reinforcement level evaluated was 30 % vol., as higher levels, such as 50 % B 4 C, resulted in matrix saturation and the development of cracks at the matrixreinforcement interfaces, thus higher contents were not utilized [35,36]. 2.2. Experimental procedure Once the compositions of the available blends have been introduced, the steps of the manufacturing procedure carried out by RHP- Technology GmbH and their characterization for later analysis are stated in this section. 2.2.1. Fabrication of the samples Firstly, comes the preparation of the powders, where the components of the blends are weighed in a high-precision scale. The Fig. 1. SE−SEM images of the titanium (grade 1) powders used for the matrixes: a) fine-grained powder (d 50 =28.75 μ m), and b) coarse-grained powder (d 50 =109.05 μ m). C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 3 necessary mass of powders can be worked out using their density and the dimensions of the sample to be fabricated. This sample consists of a cylindric disc of 20 mm in diameter and 5 mm in thickness. The next stage is the mixing of the powders, for which a tubular machine is used. All the components of each blend are deposited in the same recipient, and it is left making a swinging movement for one or 2 h. After this, the green parts are prepared to confer the samples with their final geometry. The die consists of a cylinder with a hole of 20 mm in diameter. A thin and flexible rectangle plate of graphite with the measurements of the inner surface of the whole and two 20 mm diameter cylinders is required. Before using them, these pieces are sprayed with boron nitride. After having dried, the graphite rectangle is rolled up and introduced into the die. Then the cylinder that will work as the base is partially introduced in the die, slightly lower than the middle height. A graphite disk is introduced over this cylinder to avoid compacted powder to directly stick with it. The whole system is taken to a precision balance and the necessary powders are added into the die. The next step is putting another graphite disk, a short metallic cylinder that prevents direct contact between pieces and the other cylinder (which works as the punch) to close the die. Then, the powders inside are compacted by a manual piston to prepare the system for the pressing phase. The powder compaction method chosen is IHP, which presents a fast heating-rate of 100–200 K/min. The isostatic pressing takes Fig. 2. SE−SEM images of the selected reinforcement powders: a) B 4 C, b) B 4 C (fine-grained), c) TiAl, d) Ti +Al, and e) Ti +Si. Table 2 Blend details of each sample. TMC Volumetric Composition Matrix Reinforcement A Reinforcement B T1A0B30 Ti+30%B 4 C Ti (fine) –B 4 C (coarse) T1A0B30F Ti+30%B 4 C Ti (fine) –B 4 C (fine) T2A2B20 Ti+20%TiAl+20%B 4 C Ti (coarse) TiAl B 4 C (coarse) T2A2B30 Ti+20%TiAl+30%B 4 C Ti (coarse) TiAl B 4 C (coarse) T1A2B30 Ti+20%TiAl+30%B 4 C Ti (fine) TiAl B 4 C (coarse) T1A1B30 Ti+20%TiAl+30%B 4 C Ti (fine) Ti +Al B 4 C (coarse) T1S1B20 Ti+20%TiSi+20%B 4 C Ti (fine) Ti +Si B 4 C (coarse) T1S1B30 Ti+20%TiSi+30%B 4 C Ti (fine) Ti +Si B 4 C (coarse) Fig. 3. IHP manufacturing process scheme. C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 4 place inside the hot-pressing machine, which is self-made by RHP-Technology GmbH. The manufacturing process scheme can be seen in Fig. 3. A high-frequency electromagnetic current produced by an induction coil is used in the process of applying heat to the cast. At the same time, pressure is applied by the action of two pistons. Once the heating cycle is completed and the wait for the part to cool down is over, the cast can be taken out of the machine and extracted the part. Concerning the analysed samples in this project, all of them have been manufactured under the same conditions: a heating cycle that reaches 1000 ◦C with a holding time of 15 min at a pressure of 50 MPa. The last step is the cleansing of the samples. Here the traces of boron nitride and graphite are eliminated by using a sandblast machine. The action of sandblasting consists of applying a high-pressure fluid and a centrifugal force with abrasive capabilities, in order to remove traces of other products used in the process from the final sample and smoothing the surface. Additionally, the suitable action prior to the specimen characterization is their proper preparation following the standard protocol [37]. 2.2.2. Specimen characterization The last stage regarding the experimental procedure is the characterization of the samples. Several tests have been carried out in order to determine the mechanical properties and microstructure. Starting off with the studied physical and mechanical properties, the considered ones are density, hardness, and elastic modulus. As density stands as a truly relevant magnitude in the comparison among materials in this experiment, it has been calculated in three different ways: theoretically (the forecasted by the rule of mixtures), geometrically (setting relation between the samples volume and their weight) and experimentally according to the Archimedes’ method [38]. Additionally, a relative density is calculated as a percentage of the ratio between the experimental and the theoretical densities. Vickers hardness tests were made to determine the hardness of the pieces, for which the Duramin-A300 (Struers, Copenhagen, Denmark) was utilized. This machine was employed in a scale of HV10 that applies a 10 kg load for 40 s, testing each sample eight times to obtain more values and increase accuracy. Concerning the Young modulus, it can be determined using the density of the material along with the longitudinal and transverse speeds of ultrasound propagation in various directions of the sample, as described by equations (1)–(3). The employed instrument for this measurement has been the Krautkramer USM 25 (Krautkramer, GE Sensing & Inspection Technologies, Massachusetts, US). Velocity =Thickness Round trip transit time / 2 (1) Poissonʹs ratio ( ν ) = 1−2⋅(VT/VL)2 2−2⋅(VT/VL)2(2) Youngʹs Modulus (E) = VL2⋅δ⋅(1+ ν )⋅(1−2 ν ) 1− ν (3) where δ is the density, υ the Poisson’s ratio, and V L and V T the longitudinal and transverse velocities respectively. The microstructures of these materials have been studied through three different methods. To obtain a first approach to the microstructures, concerning morphology, grain distinction, phases and porosity; optical microscopy has been made with the Nikon Model Epiphot 2000 (Nikon, Tokyo, Japan). SEM images must be taken to appreciate its microstructure in greater detail and clearly distinguish the secondary phases; for which task, the FEI-Teneo scanning-electron microscope (Thermo Fisher Scientific, Massachusetts, US) was employed. Finally, an XRD analysis was made to identify each crystalline phase and its presence through semiquantitative methods via the powder diffractometer D8I-90 (Bruker Corporation, Massachusetts, US) and the special software Diffrac EVA (Bruker Corporation, Massachusetts, US). In order to consider which materials could meet the requirements for their application in the aerospace industry, their specific stiffnesses must be evaluated. This has been worked out by dividing the Young modulus of each material by their Archimedes’ density. A value around 50 GPa cm 3 /g could be settled as a desirable result for candidate materials. Table 3 Results for the Vickers Hardness tests (HV10). TMC Relative density [%] Vickers Hardness Young modulus [GPa] T1A0B30 91.86 372 ±13 184 T1A0B30F 90.38 367 ±11 164 T2A2B20 87.44 266 ±3 141 T2A2B30 86.15 345 ±20 188 T1A2B30 87.92 334 ±11 162 T1A1B30 95.07 468 ±25 181 T1S1B20 83.73 405 ±12 138 T1S1B30 87.87 402 ±21 129 C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 5 3. Results and discussion 3.1. Physical and mechanical properties Having the manufacturing and specimens’ preparation being detailed, it is possible to proceed to analyse the test results, beginning with the densities calculations. As mentioned in the previous section, density is a magnitude of special interest when researching lighter composite materials and trying to determine the TMC with higher specific stiffness. Thus, density has been calculated theoretically (rule of mixtures) and applying Archimedes’ method (presented in Table 3, along with the values of relative density). The calculated densities are contained in a relatively short range, but the differences are appreciable. As intended, all densities are considerably lower than the density of titanium (~4.50 g/cm 3 ), an indispensable requirement when working on developing these lighter materials. When working out the relative density as well, it is desirable to obtain values above 90 %. In previous research, where ceramic reinforcements were employed, it was observed that when reinforcing Ti, the porosity increased with the content of the ceramic particles [39]. These phenomena are attributed to the porosity associated with individual particles and particle agglomerations; as seen in T1A0B30F. Nevertheless, the crucial issue is to achieve good mechanical properties, both hardness and Young modulus, which indicates good consolidation of the powders. Among the results in Table 3, the high densification of T1A1B30 (the material containing non-alloyed Ti +Al powder) should be highlighted, especially against similar materials with similar compositions, such as T2A2B30 and T1A2B30 (whose aluminium is present in form of TiAl). The properties ascertained from the measurement of the elastic modulus demonstrate higher achieved values. In prior studies, the incorporation of TiB into titanium matrices at a similar content (30 vol%) yielded values around 160 GPa [40]. Nevertheless, the addition of coarse B 4 C (T1A0B30) resulted in values reaching 184 GPa. This observation underscores the role of TiC in enhancing the matrix strength. Furthermore, with a lower amount of B 4 C, the Young’s modulus exhibits values below 140 GPa [41]. Therefore, the aim is to investigate the effect of adding higher quantities of this type of reinforcement. The results for Vickers hardness (tested eight times on a scale of HV10 for each sample) are also shown in Table 3, where the average values and the deviations are presented. Regarding these values, the increased hardness of the Ti x Si y composites with respect to the rest, and T1A1B30 being the hardest should be highlighted. In this case, the size of the powder does not seem to play a major role in the hardness results. On the other hand, the variation in the volumetric composition of B 4 C shows different effects. In previous studies where the Fig. 4. Optical microscopies of the studied TMCs: a) T1A0B30, b) T1A0B30F, c) T2A2B20, d) T2A2B30, e) T1A2B30, f) T1S1B20, g) T1S1B30, and h) T1A1B30. C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 6 reinforcement content was 10 wt% B 4 C in Ti grade 5, the hardness did not exceed 360 H V [39]. Furthermore, less volumetric composition of this ceramic reinforcement lowers severely the hardness in Ti x Al y composites, while the hardness is minimally increased Ti x Si y composites. It should be mentioned that when measuring hardness in this way, measurement error and other factors, such as the exact incision site, must be taken into account. Although an attempt is always made to incise the matrix, if this is done in regions close to the formation of intermetallics such as grain boundaries, the hardness can vary considerably. For example, in the case of the TMCs reinforced with Ti +Si, there is a large proliferation of Ti x Si y intermetallics in their microstructures, as commented on later. This means that the incision point is likely to be close to these phases and therefore less flexible, which explains their increased Vickers hardness value. Finishing with mechanical properties, the Young modulus of the samples has been measured via ultrasound propagation. The data results are again presented in Table 3. The first commentary to make is that all the tested materials present a higher Young modulus than titanium grade 1 (110 GPa), which is fundamental in the research of lighter materials that resist higher stresses. A remarkable result is that the TMCs reinforced with Ti x Si y compounds perform lower values than the rest in this test. While a lower quantity of B 4 C lowers the Young modulus in Ti x Al y composites, it has a contrary behaviour in Ti x Si y composites. These results show that using nonalloyed Ti +Al powder turns into a tougher material than ones that use TiAl, although stiffness is slightly higher in the analogue material without intermetallic reinforcement. Another observation of interest is that, despite having the highest densification and being the hardest in the Vickers tests, T1A1B30 is still slightly less stiff than T2A2B30. 3.2. Microstructural study Through optical and Back Scattering Electron SEMs (BSE-SEM) microscopies, with XRD analysis, it is possible to observe the differences in microstructures in these materials varying on their starting powders, compositions, and phases. For a general microstructure description, optical microscopies are displayed in Fig. 4, while BSE-SEM are in Fig. 5. The first two microscopies (Fig. 4a and b) represent the TMCs without intermetallic reinforcement. The ceramic reinforcement is clearly distinguishable from the titanium matrix and some porosity can be appreciated in both optical microscopies. These images being put side to side at the same scale evidences the difference in the size of the ceramic reinforcement shards, being notoriously bigger in T1A0B30 (Fig. 4a) than in T1A0B30F (Fig. 4b). In their corresponding BSE-SEM images (Fig. 5a and b), the in-situ secondary phases of B 4 C can be seen in little formations near the ceramic fragments, in major quantities in T1A0B30F. Fig. 4c–. d, and Fig. 4e show T2A2B20, T2A2B30, and T1A2B30 respectively; materials that share TiAl as their intermetallic reinforcement. The three of them present visual similarities, although their composition differences can easily be observed. B 4 C and titanium presence is common to the previous materials, while Ti x Al y phases are found in the slightly darker spots within the matrix. When comparing T2A2B20 (Fig. 4c) to T2A2B30 (Fig. 4d), the volumetric composition of B 4 C increased by 10 % in this latter is Fig. 5. BSE-SEM images of the studied TMCs: a) T1A0B30, b) T1A0B30F, c) T2A2B20, d) T2A2B30, e) T1A2B30, f) T1S1B20, g) T1S1B30, and h) T1A1B30. C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 7 appreciable. This grants Ti x Al y phases more regions to settle. However, when comparing these TMCs in the BSE-SEM images (Fig. 5c and d) the amount of ceramic reinforcement in T2A2B30 is being lowered at the same time, with less formation of in-situ secondary phases as a consequence. T1A2B30 (Fig. 4e) looks really similar to T2A2B30, as the only difference in composition between them is that the titanium powder in T1A2B30 is finer. Fig. 5e shows the TiB and TiC also present in T1A2B30 but in slightly minor abundance. Fig. 4e presents an increased quantity of TiAl spots and reduced porosity compared to Fig. 4c and d. T1S1B20 (Fig. 4f) and T1S1B30 (Fig. 4g) are quite different visually from the rest of the images as these are the only TMCs with Ti +Si powders as intermetallic reinforcement. They differ from each other in the volumetric composition of B 4 C by 10 %, being higher in T1S1B30, as can be denoted from the images. These samples present notorious porosity; however, T1S1B30 still has the third-highest densification. Apart from the B 4 C reinforcements, the formation of diverse Ti x Si y phases can be seen at grain boundaries in Fig. 5f and g. The optical microscopy of T1A1B30 is shown in Fig. 4h, which is the material with Ti +Al powders as intermetallic reinforcement, instead of pre-alloyed TiAl. The microstructure of this sample resembles the one of T1A2B30, its target comparison in terms of intermetallic reinforcement. However, in Fig. 5h, this TMC that uses non-alloyed powders presents no visible in-situ intermetallic formations, as well as less porosity; this is translated into the highest densification percentage among all the studied materials. Observing the BSE-SEM images, it is possible to analyse the microstructures with further detail. An Energy Dispersive Spectroscopy (EDS) has been carried out on some samples in order to predict the present phases in them and the rest of the TMCs. These results in conjunction with the XRD analysis, displayed later in this section, have made it possible to determine the nature of the phases. Fig. 6 shows the BSE-SEM images of T1A2B30, while Table 4 contains the numeric results of an EDS that allows us to associate each marked test spot to the predicted phase. These results are also valid for other TMCs, like the ones with no intermetallic reinforcements and the ones reinforced with Ti x Al y intermetallics. Regarding the microstructure of T1A2B30, at the magnification of Fig. 6, it is possible to clearly distinguish the present phases. The black shards correspond to B 4 C, which are commonly accompanied by the secondary phases formed in-situ: TiB appearing in whiskers, and also some smaller regions of TiC nearby, which appear in the form of dendrites [21]. The formation of concentric Ti 3 Al formations takes place around the TiAl regions, appearing during the sintering process when the decomposed TiAl and aluminium bind to the titanium of the matrix. T1A0B30 (Fig. 7) presents a microstructure that consists of the titanium matrix, the B 4 C fragments and its related secondary phases. The decomposition of boron particles from the B 4 C reinforcement originates the in-situ TiB whiskers, which remain really close to the ceramic shards they come from. The formation of TiC dendrites is also clearly visible in nearby regions as well but slightly more distanced from B 4 C. The accumulation of these decomposed particles near the B 4 C is known as the reaction layer, which can be seen surrounding the ceramic particles in Fig. 7b. In these BSE-SEM images of T1A0B30F (Fig. 8), the in-situ phases originating from the ceramic reinforcement are both quite abundant in their habitual source regions. Taking into account that carbon has a higher diffusivity in titanium than boron, the fact of having used a finer grain of B 4 C has allowed the particles to flow more smoothly through the titanium matrix and has favoured greatly the formation of TiC in this composite material. The reaction layer in this material is thicker than in the previous, being especially visible in both Fig. 8a and b. The BSE-SEM images of T2A2B20 (Fig. 9) are more akin to T1A2B30 due to the presence of the Ti x Al y phases. The volumetric composition of the ceramic reinforcement in T2A2B20 is 10 % lower than in the other TMCs reinforced with TiAl, allowing these phases to form more freely at the cost of a reduced formation of in-situ secondary phases of B 4 C. TiB whiskers are still visible near the B 4 C fragments, although in a much minor presence. TiC can hardly be spotted in these images as the composition of this secondary phase is drastically low. Being this TMC the one with the lowest Young modulus among those reinforced with TiAl and the least hard in general tells much about the relevance of the in-situ secondary phases of B 4 C on the mechanical properties. The microstructure of T2A2B30 seen in Fig. 10 is quite similar to the previous one in Fig. 9. As it has been described in its composition and seen in the optical microscopies, B 4 C fragments are more abundant in this TMC. This favours the apparition of TiB and TiC, which greatly enhances the mechanical properties of this composite material and makes its Young modulus the highest of all the studied materials. Ti x Al y phases are still present and visible but in a slightly lower quantity as seen previously in Fig. 7. The microstructure of T1A1B30 can be seen in the BSE-SEM images in Fig. 11. Its appearance is overall similar to the material which shares components, T1A2B30. However, the usage of Ti +Al powders instead of alloyed TiAl leads to a different distribution and formation of compounds. A higher presence of the in-situ secondary phases of B 4 C can be observed and confirmed by the later XRD Fig. 6. BSE-SEM images of T1A2B30 and marks of the EDS spots. C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 8 analysis results. This strengthens the material endowing it with the highest Vickers Hardness and the third highest Young modulus. On the other hand, the TiAl regions are no longer present in Fig. 11. In T1A1B30, the Ti +Al spreads composing Ti 3 Al and minorly TiAl 3 in the titanium grain boundaries rather than in the form of TiAl. The finer grain of the titanium matrix powders and the usage of nonalloyed Ti +Al powders have eased the evolution of these Ti x Al y phases during the sintering process. This could be the factor for which T1A1B30 has tested the highest densification results among all studied materials, being close to 10 % higher than T1A2B30. Regarding the TMCs reinforced with Ti x Si y intermetallics, the BSE-SEM of T1S1B30 in Fig. 12 shows the test spots for the EDS on this material. The numeric results displayed in Table 5 and the corresponding XRD analysis later justify the prediction of phases in this Table 4 EDS analysis results for T1A2B30. Spot 1 Spot 2 Spot 3 Element B C Al Ti C Al Ti C Al Ti Weight [%] 3.74 3.35 0.38 92.53 3.60 17.79 78.61 3.61 32.09 64.30 Atomic [%] 13.45 10.85 0.55 75.15 11.65 25.63 62.72 10.78 42.62 46.61 Predicted Phase Ti Ti 3 Al TiAl Spot 4 Spot 5 Spot 6 Element B C B C Ti B C Ti Weight [%] 77.24 22.76 8.06 9.56 82.38 4.03 13.01 82.96 Atomic [%] 79.04 20.96 22.86 24.41 52.74 11.68 33.98 54.34 Predicted Phase B 4 C TiB TiC Fig. 7. BSE-SEM images of T1A0B30. Fig. 8. BSE-SEM images of T1A0B30F. Fig. 9. BSE-SEM images of T2A2B20. C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 9 ⁃ When focusing on the specific stiffness, the target property, the best material is T2A2B30 (coarse-grain titanium matrix, TiAl intermetallic reinforcement and B 4 C in a volumetric composition of 30 %), with a specific Young modulus of 57.49 GPa cm 3 /g. It is recommended for those applications of TMCs that require the most exigent performance regarding stresses and weight reduction. ⁃ Additionally, even though T1A1B30 has a specific Young modulus of 50.28 GPa cm 3 /g (12.55 % lower than T2A2B30), it should be also highly considered because it offers an excellent low porosity and the highest Vickers Hardness. Depending on the necessities, it could result in a more convenient application. Funding This work was supported by the Agencia Nacional de Investigaci´ on y Desarrollo (ANID) of Chile government [FONDECYT 1231834]. CRediT authorship contribution statement Cristina M. Ar´ evalo: Writing – review & editing, Methodology. Isabel Montealegre-Mel´ endez: Writing – original draft, Formal analysis. Erich Neubauer: Resources. Michael Kitzmantel: Resources. Sheila Lascano: Methodology, Data curation. Eva M. P´ erez- Soriano: Methodology, Investigation. 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. Acknowledgements The authors would like to thank Lorenzo Costanzo Maciel and Manuel Fal-Conde P´ erez, students of the Bachelor’s Degree in the University of Seville. Some figures have been designed using resources from Freepik.com (pch.vector). The authors want to thank the Universidad de Sevilla for the use of experimental facilitates at CITIUS, Microscopy and X-Ray Laboratory Services (PPIT-2022-I.5 ISABEL MONTEALEGRE MEL´ ENDEZ and PPIT-2023-I.5 CRISTINA M. AR´ EVALO MORA). References [1] A. Lorenzo-Espejo, M.L. Mu˜ noz-Díaz, J. Mu˜ nuzuri, B. Ribeiro, Aerospace sector innovation in Portugal and Andalusia: a search for cross-border collaboration opportunities, Eur. Plann. Stud. (2022), https://doi.org/10.1080/09654313.2022.2101879. [2] C.O. Aigbavboa, A. Ebekozien, N. Mkhize, An assessment of South African airlines’ growth in the era of Fourth Industrial Revolution technologies: the unexplored dimension, J. Facil. Manag. (2023), https://doi.org/10.1108/JFM-07-2022-0076. [3] S. Kilimtzidis, V. Kostopoulos, Static aeroelastic optimization of high-aspect-ratio composite aircraft wings via surrogate modeling, Aerospace 10 (2023) 251, https://doi.org/10.3390/aerospace10030251. [4] B. Shahriari, K. Shojaei, Stress analysis of the rotating carbon-carbon composite disk of an axial compressor, Mech Adv Compos Struct 10 (2) (2023) 271–282, https://doi.org/10.22075/MACS.2023.26800.1390. [5] B. Genc Oztoprak, O. Solentas, Evaluation of the efficiency of laser usage to prevent bird strikes at airports, Int. J. Environ. Sci. Technol. 20 (2023) 7403–7416, https://doi.org/10.1007/s13762-023-04879-9. [6] C. Xiao-li, Z. Chao, B. Cheng-an, A review of electric propulsion for air vehicle research, in: Chinese Society of Aeronautics and Astronautics (Eds) Proceedings of the 10th Chinese Society of Aeronautics and Astronautics Youth Forum. CASTYSF 2022. Lect Notes Electr Eng, vol. 972, Springer, Singapore, 2023, https://doi. org/10.1007/978-981-19-7652-0_15. [7] N.-P. Xue, Q. Wu, Y. Zhang, B.-H. Li, Y.-D. Zhang, S. Yang, Y. Zhu, J. Guo, H.-J. Gao, Review on research progress and comparison of different residual stress strengthening methods for titanium alloys, Eng. Fail. Anal. 144 (2023) 106937, https://doi.org/10.1016/j.engfailanal.2022.106937. [8] Z. Feng, J. Li, Y. Tian, J. Shi, J. Zhang, L. Xu, L. Pan, Research progress of advanced composite materials for aerospace applications, Fuhe Cailiao Xuebao/Acta Materiae Compositae Sinica 39 (9) (2022) 4187–4195, https://doi.org/10.13801/j.cnki.fhclxb.20220926.003. [9] Titanium Processing Center, History, development and applications, in: https://www.titaniumprocessingcenter.com/titanium-history-developments-and- applications. (Accessed 4 December 2023). [10] D. Blanco, E.M. Rubio, R.M. Lorente-Pedreille, M.A. S´ aenz-Nu˜ no, Lightweight structural materials in open access: latest trends, Materials 14 (2021) 6577, https://doi.org/10.3390/ma14216577. [11] C. Hu, L. Leotoing, P. Castany, D. Guines, D. Gloriant, Effect of microstructure on the formability of Ti21S alloy. ESAFORM 2021 – 24th International Conference on Material Forming, 2021, https://doi.org/10.25518/esaform21.1630. [12] P. Kalita, K.R. Cochrane, M.D. Knudson, T. Ao, C. Blada, J. Jackson, J. Gluth, H. Hanshaw, E. Scoglietti, S.D. Crockett, Ti-6Al-4V to over 1.2 TPa: shock Hugoniot experiments, ab initio calculations, and a broad-range multiphase equation of state, Phys. Rev. B 107 (9) (2023) 094101, https://doi.org/10.1103/ PhysRevB.107.094101. [13] J.M. J´ aquez-Mu˜ noz, C. Gaona-Tiburcio, J.A. Cabral, M. Lara-Banda, F.H. Estupi˜ n´ an-L´ opez, P.R. Zambrano, F. Almeraya-Calder´ on, Corrosion behaviour of titanium alloys using electrochemical noise. Paper presented at the ECS Trans 101 (1) (2021) 167–172, https://doi.org/10.1149/10101.0167ecst. [14] J.K. Katiyar, J. Al Hammad, A.S. Mohammed, Tribological properties of light metal matrix composites, Encyclopedia of Materials: Composites 1 (2021) 389–401, https://doi.org/10.1016/B978-0-12-819724-0.00104-X. [15] D. Sharma, V. Kumar, S. Singh, Parametric study of the spark plasma sintering process on the mechanical properties of multi-layer graphene reinforced Ti6Al4V nanocomposites, Trans. Indian Inst. Met. 76 (2023) 1015–1025, https://doi.org/10.1007/s12666-022-02811-2. [16] H. Pan, L. Ding, Y. Xu, L. Li, J. Chen, S. Wang, P. Jiang, Microstructure, mechanical and thermal properties of titanium matrix composites with different reinforcements, Mater. Res. Express 9 (9) (2022) 096512, https://doi.org/10.1088/2053-1591/ac8f1f. [17] Z. Wang, X. Bai, M. Que, X. Zhou, Wire arc additive manufacturing of network microstructure (TiB+TiC)/Ti64Al4V composites using flux-cored wires, Ceram. Int. 43 (9) (2023) 4168–4176, https://doi.org/10.1016/j.ceramint.2022.09.299. [18] E. Fereiduni, A. Ghasemi, M. Elbestawi, Microstructural characterization and mechanical properties of nano-scale/sub-micron TiB-reinforced titanium matrix composites fabricated by laser powder bed fusion, J. Alloys Compd. 896 (2022) 163054, https://doi.org/10.1016/j.jallcom.2021.163054. C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 16 [19] S. Charukesh, K. Gobivel, K. Gowtham, E. Saikiran, V. Kailash Niveshith, Understanding the process parameters on milling of Ti/B 4 C metal matrix composite, Mater. Today Proc. (2023), https://doi.org/10.1016/j.matpr.2023.02.248. [20] Z. Xiu, B. Ju, J. Zhan, N. Zhang, P. Wang, K. Zhao, M. Liu, A. Yin, W. Chen, Y. Jiao, H. Wang, S. Li, X. Zhu, P. Wu, W. Yang, Microstructure and mechanical properties of core-shell B 4 C-reinforced Ti matrix composites, Materials 16 (2023) 1166, https://doi.org/10.3390/ma16031166. [21] I. Montealegre-Mel´ endez, C. Ar´ evalo, A.M. Beltr´ an, M. Kitzmantel, E. Neubauer, E.M. P´ erez-Soriano, Microstructural and XRD analysis and study of the properties of the system Ti-TiAl-B 4 C processed under different operational conditions, Metals 8 (5) (2018) 367, https://doi.org/10.3390/met8050367. [22] R. Li, H. Yue, S. Luo, F. Zhang, B. Sun, Microstructure and mechanical properties of in situ synthesized (TiB+TiC)-reinforced Ti6Al4V composites produced by directed energy deposition of Ti and B 4 C powders, Mater. Sci. Eng., A 864 (2023) 144466, https://doi.org/10.1016/j.msea.2022.144466. [23] A.A. Golyshev, A.G. Malikov, V.M. Fomin, et al., Microstructure and mechanical properties of a (TiB+TiB 2 +TiC)/Ti–6Al–4V composite material formed in the process of in situ synthesis in selective laser melting, J. Eng. Phys. Thermophys. 95 (2022) 1802–1808, https://doi.org/10.1007/s10891-022-02653-1. [24] K. Zyguła, M. Wojtaszek, Processing and characterization of β titanium alloy composite using power metallurgy approach, Materials 15 (2022) 5800, https:// doi.org/10.3390/ma15175800. [25] Z. Asghar, T. Aziz, A.R. Khan, et al., High-hardness high-fracture-toughness B4C-TiB2 composite produced by reactive hot pressing, J. Mater. Eng. Perform. 32 (2023) 7225–7233, https://doi.org/10.1007/s11665-022-07660-0. [26] S. Decker, J. Lindemann, L. Krüger, Metal matrix composites based on Ti-6242 synthesized by spark plasma sintering, Mater. Sci. Eng., A 732 (2018) 35–40, https://doi.org/10.1016/j.msea.2018.06.103. [27] F. Appel, R. Wagner, V. Kumar, Intermetallics: Titanium Aluminides. Reference Module in Materials Science And Materials Engineering, 2017, https://doi.org/ 10.1016/B978-0-12-803581-8.02542-X. [28] I. Zglobicka, R. Zybala, K. Kaszyca, et al., Titanium matrix composites reinforced with biogenic filler, Sci. Rep. 12 (2022) 8700, https://doi.org/10.1038/ s41598-022-12855-5. [29] S. Tkachenko, J. Cizek, R. Muˇ s´ alek, K. Dvoˇ r´ ak, Z. Spotz, E.B. Montufar, T. Chr´ aska, I. Kˇ rupka, L. ˇ Celko, Metal matrix to ceramic matrix transition via feedstock processing of SPS titanium composites alloyed with high silicone content, J. Alloys Compd. 764 (2018) 776–788, https://doi.org/10.1016/j. jallcom.2018.06.086. [30] Y. Wang, M. Zhu, L. Dong, G. Sun, W. Zhang, H. Xue, Y. Fu, A. Elmarakbi, Y. Zhang, In-situ synthesized TiC/Ti-6Al-4V composites by elemental powder mixing and spark plasma sintering: microstructural evolution and mechanical properties, J. Alloys Compd. 947 (2023) 169557, https://doi.org/10.1016/j. jallcom.2023.169557. [31] T. Chen, C. Suryanarayana, C. Yang, Advanced titanium materials processed from titanium hybride powder, Powder Technol. 423 (2023) 118504, https://doi. org/10.1016/j.powtec.2023.118504. [32] Jl Zhou, Jy Yang, Xf Zhang, et al., Research status of tribological properties optimization of high-entropy alloys: a review, J. Mater. Sci. 58 (2023) 4257–4291, https://doi.org/10.1007/s10853-023-08255-3. [33] RHP-Technology GmbH, Inductive hot pressing. https://rhp.at/en/innovations-technology/innovative-sintering-technologies/hot-pressing-and-fastsps/ inductive-hot-pressing. (Accessed 4 December 2023). [34] I. Montealegre-Mel´ endez, C. Ar´ evalo, A.M. Beltr´ an, M. Kitzmantel, E. Neubauer, E.M. P´ erez-Soriano, Reaction layer analysis of in situ reinforced titanium composites: influence of the starting material composition on the mechanical properties, Metals 10 (2) (2020) 265, https://doi.org/10.3390/met10020265. [35] E.M. P´ erez-Soriano, C.M. Ar´ evalo-Mora, I. Montealegre-Mel´ endez, In situ titanium composites: XRD study of secondary phases tied to the processing conditions and starting materials. In Book “Inelastic X-Ray Scattering and X-Ray Powder Diffraction Applications, IntechOpen, London, 2020, https://doi.org/10.5772/ intechopen.88625. [36] I. Montealegre-Mel´ endez, C. Ar´ evalo, E.M. Perez-Soriano, E. Neubauer, M. Kitzmantel, Estudio y evaluaci´ on de la influencia de los par´ ametros de fabricaci´ on y los materiales de partida, en las propiedades finales de materiales compuestos de base titanio, fabricados in situ mediante compactaci´ on en caliente, IV Jorn. Invest. Desarro. Innov. (2017). Seville (Spain). [37] ASTM E3, Standard guide for preparation of metallographic specimens. https://www.astm.org/e0003-11r17.html. (Accessed 4 December 2023). [38] ASTM B962-18, Standard test methods for density of compacted or sintered powder metallurgy (PM) products using Archimedes’ principle. https://www.astm. org/b0962-17.html. (Accessed 4 December 2023). [39] K. Soorya Prakash, P.M. Gopal, D. Anburose, V. Kavimani, Mechanical, corrosion and wear characteristics of powder metallurgy processed Ti-6Al-4V/B 4 C metal matrix composites, Ain Shams Eng. J. 9 (4) (2016) 1489–1496, https://doi.org/10.1016/j.asej.2016.11.003. [40] R.R. Atri, K.S. Ravichandran, S.K. Jha, Elastic properties of in-situ processed Ti–TiB composites measured by impulse excitation of vibration, Mater. Sci. Eng., A 271 (1) (1999) 150–159, https://doi.org/10.1016/S0921-5093(99)00198-7. [41] B.J. Choi, Y.J. Kim, In-Situ (TiB+TiC) particulate reinforced titanium matrix composites: effect of B 4 C size and content, Met. Mater. Int. 19 (2013) 1301–1307, https://doi.org/10.1007/s12540-013-6024-9. C.M. Ar´ evalo et al. Heliyon 10 (2024) e37682 17