A
High-Temperature Corrosion of Ti6Al4V and In-situ Reinforced Composites Fabricated by Laser Additive Manufacturing
Hosia
Kgomo
1,2✉
EmailHKgomo@csir.co.za
Bathusile
Masina
1
EmailBMasina@csir.co.za
Ipfi
Mathoh
1,3
EmailIMathoho@csir.co.za
Vusimuzi
Mulaudzi
2
EmailVusimuzi.mulaudzi@ul.ac.za
1
Photonic Center
CSIR
Manufacturing Cluster
South Africa
2
A
Department of Chemistry
University of Limpopo
South Africa
3
Department of Mechanical Engineering Science, Faculty of Engineering and the Built Environment
University of Johannesburg
2006
Johannesburg
South Africa
Hosia Kgomo1,2*, Bathusile Masina1†, Ipfi Mathoh1,3† and Vusimuzi Mulaudzi2†.
1Photonic Center, Manufacturing Cluster, CSIR, South Africa,
2Department of Chemistry, University of Limpopo, South Africa ,
3Department of Mechanical Engineering Science, Faculty of Engineering and the Built Environment, University of Johannesburg, Johannesburg 2006, South Africa
*Corresponding author. Email: HKgomo@csir.co.za
†Contributing authors: BMasina@csir.co.za; IMathoho@csir.co.za; Vusimuzi.mulaudzi@ul.ac.za
Abstract
This study investigates the high-temperature corrosion behaviour of Ti6Al4V (Ti64) alloy and its composites, TiC/Ti64 and in-situ TiBw/Ti64, fabricated using directed energy deposition. Corrosion tests were performed at 300°C, 600°C, and 900°C in oxygen-rich NaCl–Na₂SO₄ environments. Thermogravimetric analysis showed in-situ TiBw/Ti64 had the highest resistance due to stable boron oxides maintaining protective layer integrity. TiC/Ti64 displayed intermediate resistance, with instability of oxide layers at higher temperatures. Ti64 suffered severe material loss from volatile titanium chlorides and sulphates disrupting oxide scales. Microstructural characterization by Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) revealed critical insights into phase distribution and stability. TiC and TiB reinforcements influenced grain boundary stabilization, phase distribution, and oxide adherence. TiC acted as a barrier to defect growth and oxygen diffusion, improving oxide protection. TiB enhanced resistance through boron oxides and high thermal stability, though galvanic interactions posed challenges. Both composites offered significant improvements over Ti64 for aerospace, marine, and power generation applications exposed to saline and high-temperature environments, especially below 600°C. In-
situ TiBw/Ti64 proved most effective at elevated temperatures due to excellent thermal stability and resistance to oxide spallation. TiC/Ti64, while performing well, showed limitations at high temperatures, requiring further optimization. These findings highlight the importance of composite microstructures in tailoring titanium alloys for harsh environments, with in-situ TiBw/Ti64 particularly suitable for high-temperature service.
KEY WORDS:
Additive manufacturing
High temperature corrosion and Titanium-based matrix composites
1 Introduction
Ti6Al4V (commonly referred to as Ti64) is the most commercially significant titanium alloy, often regarded as the “workhorse” of titanium alloys [1]. Comprising 6% aluminium (an α-phase stabilizer) and 4% vanadium (a β-phase stabilizer), Ti64 exhibits a dual-phase microstructure of α and β phases. Its exceptional strength-to-weight ratio, corrosion resistance in harsh environments such as seawater and acidic media, and outstanding biocompatibility have made it indispensable in aerospace, biomedical, and marine applications [2]. Specifically, its use in medical implants (e.g., hip and knee replacements, dental implants, and surgical instruments) and fatigue-critical aerospace components underscores its versatility. Despite these advantages, Ti64 is limited in high-temperature applications to approximately 315°C. Above this temperature, rapid oxidation and the formation of a non-protective oxide scale significantly reduce its durability, often leading to premature failure [3] [4]. To overcome these limitations, various surface modification strategies such as coatings and anodizing, as well as the development of metal matrix composites (MMCs), have been explored to improve their corrosion and oxidation resistance at elevated temperatures [5].
MMCs were originally developed to enhance wear resistance by incorporating hard reinforcement phases within a metallic matrix [5]. However, the corrosion performance of MMCs is complex and highly dependent on the matrix-reinforcement interactions, environmental conditions, and microstructural features [5][6]. The most common types of MMCs include aluminium matrix composites, magnesium matrix composites, copper matrix composites, nickel matrix composites, and titanium matrix composites (TMCs) which are the main focus in this study.
In recent years, researchers have turned their attention to TMCs as a means to improve both the mechanical and corrosion performance of titanium alloys at elevated temperatures. TMCs offer high specific strength, modulus, and stiffness across a broad temperature range, making them suitable for high-temperature aerospace, automotive, and energy applications [7], [8]. Reinforcement phases such as titanium carbide (TiC) and titanium boride (TiB) have shown promise in modifying oxidation kinetics and enhancing corrosion resistance [9].
Although TMCs are emerging as potential candidates for high-temperature corrosion resistance, there is limited information available on the corrosion behaviour of TiC/Ti64 and in- situ TiBw/Ti64 composites. However, some recent studies provide valuable insights. For example, Kang et al. [10] investigated the hot corrosion behaviour of a TiBw/TA15 composite manufactured using sintering process at 800°C in a NaCl-rich environment. The discontinuous distribution of TiB whiskers in a 3D network within the matrix promoted the formation of a dense oxide scale composed primarily of TiO₂ and Al₂O₃. While NaCl accelerated corrosion, the dense oxide barrier reduced the corrosion rate over time. Cracks and pores caused by the volatilization of Ti and Al chlorides and thermal stress were observed, but TiB whiskers helped stabilize the oxide layer due to refined grain size increases the number of nucleation sites for oxide formation, resulting in a denser and more uniform oxide film mainly TiO₂, which acts as a barrier against further corrosion, which was more uniform and thicker (~ 400 µm) than in the unreinforced alloy. Similarly, Li et al.
[11] studied the hot corrosion behaviour of in-situ TiC and TiC + TiB reinforced TB8 composites manufacture via powder metallurgy process. The combination of TiC and TiB resulted in a refined and homogenous microstructure with enhanced corrosion resistance. After 30 hours of exposure, the TiC + TiB composite exhibited a significantly lower weight gain compared to the unreinforced matrix. This improvement was attributed to the formation of a dense oxide film consisting of TiO₂, Al₂O₃, and SiO₂, which limited the ingress of corrosive species. Notably, the oxide layer was thinner (44 µm) yet denser than that of the unreinforced alloy, and the TiB phase played a crucial role by acting as a nucleation site for protective oxide formation and by reducing Ti ion activity [11]. In a related study, Chen et al. [12] examined TiC/IN625 coatings prepared via extreme high- speed laser cladding (EHLA). The incorporation of TiC refined the dendritic structure and enhanced compositional uniformity, resulting in a 57.5% lower mass gain after 60 hours in a Na₂SO₄ + NaCl environment at 900°C. Improved hot corrosion resistance was linked to the finer grains, low-angle grain boundaries, and formation of a Cr₂O₃-rich oxide layer.
Lv et al. [13] demonstrated the positive impact of TiC on hot corrosion resistance in a novel TiC/GTD222 composite produced by selective laser melting (SLM). The composite exhibited a double-layer oxide scale (TiO₂ inner and Cr₂O₃ outer) and experienced less corrosion than the GTD222 superalloy in a 900°C Na₂SO₄ + NaCl environment. The enhanced performance was attributed to a refined microstructure, high γ′ phase content, and improved oxide scale integrity. Lastly, Li et al. [14] assessed the hot corrosion performance of TiC-reinforced IN718 alloys fabricated via laser direct metal deposition. A 1 wt% TiC addition significantly refined the grain structure and reduced oxide layer thickness by 26%, contributing to lower corrosion rates and the formation of a protective Cr₂O₃ layer. These findings suggest that the addition of ceramic reinforcements such as TiC and TiB into titanium can significantly improve hot corrosion resistance by refining microstructures and promoting the formation of dense, protective oxide scales. Such improvements are often attributed to the pinning of grain boundaries, reduction of oxygen diffusion paths, and stabilization of oxide layers at elevated temperatures. However, despite these promising outcomes, the existing body of literature predominantly emphasizes mechanical property enhancements in titanium matrix composites (TMCs), with limited attention to their high-temperature corrosion performance. Liu et al [15] emphasized that although numerous studies have characterized the strength, hardness, and wear resistance of TMCs, particularly those reinforced with in-situ formed ceramic phases, comprehensive studies on their corrosion behaviour under aggressive thermal environments—such as those simulating marine or industrial gas turbine conditions (e.g., NaCl + Na₂SO₄ salt mixtures at elevated temperatures)—remain relatively scarce. This presents a significant knowledge gap, particularly considering the growing application of titanium-based materials in aerospace, energy, and marine industries where hot corrosion is a major concern. Moreover, there is a notable lack of systematic research comparing the oxidation and hot corrosion behaviour of Ti64 reinforced separately with TiC and TiB phases under identical conditions.
This study, therefore, aims to bridge this gap by systematically investigating the high- temperature corrosion mechanisms of in-situ TiC/Ti64 and in-situ TiBw/Ti64 composites in a 50% NaCl + 50% Na₂SO₄ environment at various temperatures. By doing so, this research contributes new insights into how specific reinforcements influence corrosion resistance, thereby informing the design of more durable TMCs for high-temperature applications.
2 Experimental procedures
2.1 Material
Fig. 1
SEM images of the powders, Ti64 (a) and TiB2 (b) and TiC (c)
TiC, TiB₂, and Ti64 grade 5 powders were utilized in the fabrication of titanium matrix composites (TMCs). Ti6Al4V was supplied by TLS Technik GmbH & Co company, while TiC and TiB2 were supplied by Nanografi company. Ti6Al4V showed smooth spherical particles with particles size ranging between 45–100 µm and this is good for flowability. TiB2 showed fine and irregular particles with particle size distribution of 20–100 µm and it was sieved to 45–100 µm as shown in Fig. 1 (a). TiC showed irregular shaped particles with particle size ranging from 45– 100 µm Fig. 1 (b) and (c).
2.2 Manufacturing of samples
Fig. 2
DED manufacturing system
The Ti64 grade 5, TiC/Ti64, and in-situ TiBw/Ti64 samples were fabricated using the Laser Additive Manufacturing (LAM) technique, specifically through a Directed Energy Deposition (DED) process. The sandblasting of Ti64 base plates (50 × 50 mm L × B) was carried out using a Storm Machinery model SM5005. Sand was used as the blasting medium, at a pressure of 6 MPa for a duration of 3 minutes. The DED system used in this study consist of a high-precision KUKA robotic arm model KR80L30 supplied by KUKA company from Germany as shown in Fig. 2, which provided multi-axis movement of the deposition head consisting of two optics (Reflection mirror and lens) and also cover slide for protection.
The system was equipped with a dual-hopper powder feeder, which played a critical role in delivering different powders simultaneously into the melt pool. For the fabrication of the composite samples, Ti64 powder was fed from one hopper, while the second hopper was used to deliver either TiC or TiB₂ powder, depending on the desired composite. This configuration enabled the in-situ formation of TiC/Ti64 and in-situ TiBw/Ti64 composites during the deposition process.
The energy required to melt the powders and maintain a stable melt pool was provided by IPG Photonics YLR-3000 fiber laser (Ytterbium-based, continuous wave, 1073 nm, manufactured in United State of America). A constant laser energy density of 90 J/mm was applied throughout the manufacturing process to ensure consistent melting and bonding of the powders. Argon gas was employed both as the carrier gas, at a controlled flow rate of 1.5 L/min, and as the shielding gas, at a higher flow rate of 15 L/min. The carrier gas helped transport the powders from the hoppers to the melt pool, while the shielding gas protected the molten material from oxidation and atmospheric contamination. For both the TiC/Ti64 and in-situ TiBw/Ti64 composites, the mass ratio was set at 95% Ti64 and 5% of either TiC or TiB₂. This ratio was maintained through precise control of the powder feed rates from the individual hoppers and rectangular prism of 60x35x10 mm (L x W x H) were manufactured. The Ti64 sample, which served as the baseline for comparison, was fabricated using 100% Ti64 powder and a cube of 60x35x10 mm (L x W x H) was manufactured.
Sample Preparation
The samples were cut using a Struers Labotom-5 abrasive cutting machine equipped with a Struers titanium cut-off wheel (20S25). Water cooling was applied throughout the process to prevent thermal and mechanical damage. A total of 9 samples were cut from each manufactured cubes of 60x35x10 mm (L x W x H) of Ti64, TiC/Ti64, and in-situ TiBw/Ti64 resulting in 27 samples overall. These were divided between electrochemical corrosion testing and high- temperature corrosion testing, with three samples used per condition to ensure repeatability and reliable results. Each sample was cut to a size of 5 mm × 5 mm, suitable for the testing setups and to allow uniform exposure during experiments. The samples were mounted using an AMP 50 automatic mounting press machine in order to be grinded and polished. To mount each sample, an Aka Resin Phenolic SEM black conductive resin was utilized. Mounted samples were grinded with Struers Tegrapol-25 grinding and polishing machine in order to be able to analyse them at micro level. For grinding the samples, silicon carbide (SiC) grinding papers with grain sizes ranging from 80 to 320 and 1200 to 4000 were utilized. The samples underwent polishing using a Daipro MD- Mol 3 µm diamond suspension for a duration of 3 minutes, followed by a colloidal silica 0.04 µm OP-S suspension for the same length of time as the final polishing stage [2].
2.3 Characterization of the as-built Ti64-based matrix composites,Ti64 and after the test
Scanning Electron 50 Microscope (SEM) available at CSIR Laser Enabled Manufacturing Joel JSM-6010PLUS/LA was used to analyse the samples' microstructure of Ti64, TiC/Ti64 and in-situ TiBw/Ti64 characteristics for before and after to check the effect of corrosion. Microstructural pictures were captured at various positions to give an overview of the entire samples. The crystallographic structure of the samples was analysed using PANalytical X’Pert PRO X-ray diffraction available at CSIR Nano Center. X-ray diffraction was used to determine the phase composition of the materials before and after the test in order to confirm any phase transformation during the tests. The scan range of 2θ from 0° to 100° was used, which allows for the detection of a wide range of crystalline phases. X-rays are directed at the sample, and the diffracted rays are measured as a function of the angle (2θ).
2.4 High-temperature corrosion test
Nine samples of Ti64, TiC/Ti64, and In-situ TiB/Ti64 were prepared for high-temperature corrosion tests with constant duration and changing temperature. The conventional mass-change method (weighing samples before exposure; applying salt mixture; exposing at target temperature; cooling; cleaning; and re-weighing) was employed [11] [16]. The ultra-furnace was utilized for conducting high-temperature corrosion tests experiments. For each test, three samples of each material were tested per temperature to ensure repeatability [11]. Before testing, the samples were polished, cleaned, and their initial weight and dimensions were recorded. The first test was performed at 300 ºC for 6 h, followed by a second test at 600 ºC for 6 h, and finally a third test at 900 ºC for 6 h. A solution containing a combination of 5 g of NaCl and 5 g of NaSO4 was prepared in a beaker for the purpose of studying corrosion at high temperatures. The solution was moistened with small amounts of distilled water. The mixture of the salts was applied onto the refined surface of the material using a brush and immediately they were put in the furnace. The high temperature corrosion test was conducted on three materials: Ti64, TiC/Ti64, and In-situ TiB/Ti64. The first test was performed at 300 ºC for 6 h, followed by a second test at 600 ºC for 6 h, and a third test at 900 ºC for 6 h and all samples were air cooled, cleaned with ultrasonic bath and post-test weight measurements and surface characterisation were performed.
3 Results and discussion
3.1 High temperature corrosion results
3.1.1 High temperature corrosion kinetics
Figure 3 presents the mass gain per area of Ti64, TiC/Ti64 and in-situ TiBw/Ti64 as the function of temperature. At 300°C, the mass gain per area for all three material Ti64, TiC/Ti64, and in-situ TiBw/Ti64 remains relatively constant, indicating little to no significant corrosion as shown in Fig. 3 [17]. Due to low kinetics at this temperature causing the molecules to collide ineffectively to form products. The literature reported that titanium alloys exhibit low oxidation rates at temperatures below 400°C due to the slow kinetics of oxidation, which limits molecular collisions necessary for oxide formation [17]. Titanium-based matrix composites exhibit superior oxidation resistance compared to actual titanium alloys. This enhanced performance is primarily attributed to the presence of ceramic reinforcements (such as TiC or TiB), which act as protective
barriers that slow down oxygen diffusion and stabilize the oxide layer, thereby improving the
material’s resistance to oxidation at elevated temperatures [11].
Fig. 3
Oxidation kinetics of high temperature corrosion (average mass gain per area vs temperature) graph of Ti64, TiC/Ti64 and in-situ TiBw/Ti64 for temperature of 300°C, 600°C and 900°C.
At 600°C, corrosion products, likely consisting of protective oxides like TiO₂, prevent further interaction between the alloy and the aggressive environment, resulting in an increase in mass as presented in Fig. 3[18]. This is typical in lower temperature ranges where solid-state corrosion occurs, as the protective oxide scale is dense and adherent. However, beyond 600°C, Fig. 3 shows a sharp decline in mass gain, particularly for Ti64, with the steepest drop observed at 900°C. This sharp decrease is attributed to the complex interaction between NaCl and Na₂SO₄, which form molten salt deposits that initiate accelerated corrosion at these higher temperatures. Anuwar et al. [18]studied the hot corrosion behaviour of Ti64 and noted that at intermediate temperatures, a stable oxide layer forms, providing temporary protection. However, as the temperature increases beyond 600°C, a sharp decline in mass gain is observed, particularly for Ti64. The influence of molten NaCl and Na₂SO₄, which disrupt the TiO₂ layer, accelerates corrosion through the formation of volatile species such as TiCl₄. In the case of TiC/Ti64 and in- situ TiBw/Ti64, the decline in mass gain is more gradual compared to Ti64 as shown in Fig. 3. This suggests that reinforcements (TiC and TiB) contribute to better corrosion resistance by stabilizing the oxide layer and reducing its rate of degradation in the molten salt environment [11].
The presence of carbide and boride acts as a barrier to the diffusion of oxygen, chlorine, and sulphur, limiting the extent of corrosion.
At temperatures around 900°C, the molten NaCl and Na₂SO₄ dissolve the protective oxides through fluxing, forming volatile chlorides (such as TiCl₄) that evaporate and expose the underlying material to further attack. In the case of Ti64, once the oxide layer is compromised, the alloy undergoes accelerated internal oxidation, forming brittle oxides and causing rapid material loss. This behaviour has been observed in studies on titanium alloys in molten salt environments, where NaCl and Na₂SO₄ lead to severe corrosion through oxide dissolution and sulfation reaction [11]. Li et al [11]. found that TiC and TiB composites exhibit superior hot corrosion resistance due to their ability to form more stable and protective oxide scales, reducing the impact of chloride and sulphate attack.
3.1.2 Activation energies for high temperature corrosion
The graph in Fig. 4 presents the relationship between the logarithm of the parabolic rate constant (log Kp) and the inverse temperature (1/T) for Ti64, TiC/Ti64, and in-situ TiBw/Ti64 in a high-temperature corrosion environment.
Fig. 4
Shows oxidation kinetics of high temperature corrosion (logarithm of the parabolic rate constant (Log Kp) against the inverse of temperature (1/T)) graph of Ti64, TiC/Ti64 and in-situ TiBw/Ti64 for temperature of 300°C, 600°C and 900°C.
The parabolic rate constant Kp typically indicates the rate of oxidation or corrosion, and the straight-line relationship in this Arrhenius plot suggests that the oxidation process follows an Arrhenius-type behaviour, meaning that the rate of corrosion increases exponentially with temperature [17], [19]. This type of plot is used to determine the activation energy for high- temperature corrosion or oxidation processes. The steeper slope for Ti64 in Fig. 4 indicates a faster corrosion rate at high temperatures compared to TiC/Ti64 and in-situ TiBw/Ti64 composites. With an activation energy of 198 kJ/mol, Ti64 shows a sharp mass loss due to accelerated reactions [7] [17] [19]. The composites TiC/Ti64 and in-situ TiBw/Ti64 show lower Kp values at higher temperatures, which suggests that these reinforcements (TiC and TiB) significantly reduce the rate of oxidation or corrosion compared to pure Ti64. Chemically, the lower rate constants in the composites can be attributed to the stability of TiC and TiB at high temperatures. These reinforcements form more stable oxide layers or act as barriers that prevent the rapid diffusion of corrosive species such as oxygen and chlorine into the Ti64 matrix. For instance, TiB forms a stable boron oxide (B₂O₃), and TiC is known for its high melting point and chemical stability, both of which enhance the overall oxidation resistance of the composite materials.
Additionally, the formation of these stable compounds at elevated temperatures reduces the volatile products like titanium chlorides (TiCl₄) and boron chlorides (BCl₃), thus mitigating the accelerated corrosion that would typically occur in a molten salt environment. The lower Kp values for TiC/Ti64 and in-situ TiBw/Ti64 suggest a higher activation energy for oxidation, meaning more energy is required to initiate the corrosion process. TiC/Ti64 showed activation energy of 276 kJ/mol while in-situ TiBw/Ti64 showed 282 kJ/mol. This is attributed by the moderate decrease of mass of TiC/Ti64 and in-situ TiBw/Ti64 in Fig. 3 .This agrees with findings from Kumar et al. [20] reported that increasing the activation energy leads to reduced oxidation rates in TMCs alloys. Additionally, Anuwar et al. [18] found that in Na₂SO₄-NaCl environments, Ti64 experiences accelerated oxidation due to the formation of volatile chlorides.
3.2 Microstructure analysis for high-temperature corrosion test
3.2.1 Ti6Al4V (Ti64) microstructure analysis
The microstructures shown in Fig. 5 (a), (b), and (c), represent as-built Ti64 samples for the corrosion test that was conducted for 2 hours, 4 hours and 6 hours, respectively.
Fig. 5
Shows as-built microstructure of Ti64 samples for temperatures of, (a) 300°C, (b) 600°C, and (c) 900°C
Figure 5 (a) displays martensite microstructure with a β-phase, present in the spaces between the α-phase [21]. Figure 5 (b) exhibits both fine and coarser regions of martensites, which are composed of alternating α-phase and β-phase. The α-phase appears as elongated, parallel plates, while the β-phase occupies the intermatersites regions Fig. 5 (c) shows a well-developed martensites microstructure, characterized by a fine interlocking pattern of α-phase and β-phase. The α-phase appears as thin elongated laths, while the β-phase occupies the spaces between the α- martensitic. Simonelli et al. [21] studied SLMed Ti64 and found that rapid cooling leads to fine acicular martensitic α' phase. This supports the observations in Fig. 3, where cooling effects determine the morphology of α- and β-phase distribution [21].
Figure 6 presents the microstructure of Ti64 after the high temperature corrosion test for 300°C, 600°C and 900°C.
Fig. 6
Shows microstructure of Ti64 after high temperature corrosion, (a) 300ºC, (b) 600ºC and (b) 900ºC.
Figure 6 (a) shows the microstructure of Ti64 after exposure to 300°C for 6 hours, revealing significant changes compared to the pre-corrosion state, while remnants of the martensitic structure are faintly visible, the surface is now covered with dark spots and localized degradation, likely indicating corrosion pits or areas where the corrosive environment initiated localized attacks [17], [19]. Despite the visible surface damage, no mass gain was detected, suggesting that any corrosion products formed were minimal, thin, or volatile, such as titanium chlorides (TiCl₄) or sulphates (TiSO₄), which may have evaporated or spalled off. At 300°C, NaCl and Na₂SO₄ promote chloride- induced corrosion and the formation of volatile products. The dark spots likely indicate areas where chloride and sulphate ions penetrated the oxide layer, initiating localized corrosion without significant accumulation of corrosion products. The lack of a substantial protective oxide layer contributed to the visible surface damage. Kumar et al. [20], reported that at temperatures below 400°C, weight gain due to oxidation is negligible, and chloride ions primarily cause localized corrosion without significant accumulation of oxides with minimal structural changes.
At 600°C, as shown in Fig. 6 (b), the Ti64 surface exhibits more advanced degradation than observed at 300°C [17], [19]. While minimal mass gain at 300°C is due to thin or volatile corrosion products, here a substantial mass gain indicates the formation of a thicker, non-protective corrosion layer. The once-visible martensitic structure is now obscured by a rough, granular surface, reflecting the accumulation of oxides, chlorides, and sulphates. The corrosive effects of NaCl and Na₂SO₄ become more pronounced, destabilizing the oxide layer and promoting surface
roughening and cracking. Compared to 300°C, the microstructure shows less definition, with corrosion becoming more uniform and widespread due to deeper salt penetration and chemical interaction. Haifei Lu et al. [22], noted that at 500–600°C, Ti64 forms an initial protective oxide (TiO₂), but in molten salt environments (Na₂SO₄ + NaCl), chloride-induced corrosion leads to the formation of non-protective TiCl₄ and TiSO₄ resulting in a roughened, granular corrosion layer.
At 900°C, the degradation becomes even more severe, as seen in Fig. 6 (c) [17], [19]. Unlike the granular corrosion seen at 600°C, the surface transforms into a highly porous, sponge- like structure, indicating extensive material loss through both volatilization and spallation. The original martensitic microstructure is completely destroyed, a stark contrast to the faint traces still visible at lower temperatures. While 600°C corrosion is dominated by scale growth and surface buildup, at 900°C the alloy undergoes bulk degradation, driven by the formation and evaporation of volatile species like TiCl₄. This indicates a shift from surface-level degradation to deep structural failure, with significantly higher corrosion intensity. Anuwar et al. [18] found that at 750°C and above, molten NaCl and Na₂SO₄ induce rapid degradation of Ti64 by forming volatile TiCl₄ and sulphates, leading to severe spallation and forming a porous, unstable corrosion network [18]. Lu et al. [22] that at 900°C, the oxide scale on Ti64 is completely destabilized, leading to catastrophic material loss. Kumar et al. [20] reported that at 900°C, Ti64 undergoes extreme surface roughening and crack formation, leading to complete loss of its original microstructure.
3.2.2 TiC/Ti64 microstructure analysis
The microstructures shown in Fig. 7 (a), (b), and (c), represent as-built TiC/Ti64 samples for the corrosion test that was conducted for 2 hours, 4 hours and 6 hours, respectively.
Fig. 7
Shows as-built microstructure of TiC/Ti64 samples for temperatures of, (a) 300°C, (b) 600°C, and (c) 900°C.
Figure 7 (a) shows a microstructure characterized by the distribution of TiC particles within the Ti64 matrix. The TiC particles appear as bright, elongated, or clustered regions, primarily located along the grain boundaries of the titanium alloy matrix [23] [24]. The Ti64 matrix, consisting of a mixture of α-phase and β-phase. Figure 7 (b) clearly displays TiC particles (bright, scattered regions) dispersed within the Ti64 matrix. The TiC particles are predominantly located along the grain boundaries and within the matrix, forming a continuous network that provides significant reinforcement to the titanium alloy. Figure 7 (c) shows a microstructure where TiC particles are prominently distributed along the grain boundaries of the Ti64 matrix. The TiC particles, visible as bright, irregularly shaped structures, are scattered throughout the titanium alloy matrix. These results align with Bai et al. [25], found that TiC reinforcement leads to a unique honeycomb-like cellular microstructure, where TiC grains interconnect within the Ti64 matrix, forming a strong reinforcing network [25]. Liu et al. [15] further demonstrated that nano- and micro-TiC particles refine grain structure, reducing the size of prior β-grains. In this study, the presence of TiC along grain boundaries suggests that TiC acts as a nucleation site, limiting grain growth and enhancing mechanical strength [15].
Figure 8 (a), (d), and (c), present TiC/Ti64 after samples for temperatures of 300°C, 600°C, and 900°C, respectively.
Fig. 8
Shows microstructure of TiC/Ti64 after high temperature corrosion, (a) 300ºC, (b) 600ºC and (b) 900ºC.
The original microstructure is largely obscured, but no mass gain was detected, suggesting that any corrosion products formed were very thin, minimal, or spalled off [18] [19]. The pitted surface implies that some corrosion occurred, creating micro-defects, likely due to local differences in electrochemical potential between the Ti64 matrix and the TiC particles.
At 300°C, NaCl and Na₂SO₄ promote slow corrosion, forming volatile compounds like TiCl₄ and destabilizing products like TiSO₄. The TiC particles may have acted as initiators for localized corrosion, but the overall material integrity remained largely unaffected. Yu et al. [26] found that TiC reinforcements help limit overall oxidation but can create localized electrochemical differences, leading to small-scale pitting around TiC particles.
At 600°C, as shown in Fig. 8 (b), the TiC/Ti64 surface exhibits substantial surface degradation [18] [22],. While mentioned above, the initial corrosion at 300°C was limited and involved thin or spalled-off products, the microstructure at this stage is now covered with a thick, uneven corrosion layer. This layer obscures the original martensitic features and reflects an accumulation of corrosion products, including oxides, chlorides, and sulphates. The galvanic interaction between TiC and the Ti64 matrix likely intensified localized corrosion, forming a non- uniform and less protective corrosion scale. Compared to the earlier stage, the corrosion is more severe and continuous, aligning with increased mass gain and deeper surface attack due to more aggressive salt interactions. Chen et al. [12] reported that TiC-reinforced coatings suffer from localized breakdown due to chloride penetration, leading to an uneven corrosion layer.
At 900°C, Fig. 8 (c) reveals severe structural breakdown, surpassing the damage seen at 600°C. In contrast to the roughened surface at 600°C, the surface at this stage becomes highly porous and sponge-like, with no visible TiC particles or martensitic features remaining [19], [27], [28]. This transformation suggests extensive material loss through the volatilization of corrosion products like TiCl₄. While TiC is generally oxidation-resistant, as noted in paragraph 1, it may have contributed to localized electrochemical instabilities that, under elevated temperatures, worsened corrosion. The transition from a covered surface at 600°C to a porous and structurally compromised one at 900°C highlights the extreme aggressiveness of high-temperature salt corrosion and the limitations of TiC reinforcements in such environments. Lv et al. [13] confirmed
that TiC-reinforced coatings experience significant microstructural breakdown at 900°C due to sulphate-induced oxide destabilization, resulting in deeply porous corrosion layers.
3.2.3 In-situ TiBw/Ti64 microstructure analysis
Figure 9 (a) presents a microstructure characterized by the presence of elongated, needle- like TiB particles dispersed within the Ti64 matrix [29][30].
Fig. 9
As-built microstructure of In-situ TiBw/Ti64 samples for temperatures of, (a) 300°C,
(b) 600°C, and (c) 900°C.
The TiB particles, which appear as bright, acicular features, are distributed along the grain boundaries and within the grains of the titanium alloy. The darker regions represent the Ti64 matrix, consisting of the α-phase and β-phase. Figure 9 (a) reveals a distinctive microstructure where needle-like TiB particles are distributed throughout the Ti64 matrix. These TiB particles appear as bright, elongated features within the darker Ti64 matrix. The TiB particles are strategically positioned along grain boundaries and dispersed within the grains. Figure 9 (c) shows a distinctive microstructure where elongated, needle-like TiB (titanium boride) particles are dispersed throughout the Ti64 matrix. These TiB particles appear as bright, acicular structures within the darker titanium alloy matrix. The uniform distribution of these TiB reinforcements shows that they are well-integrated into the matrix. Ren et al. [29] found that TiB pinning effects limit grain growth, leading to finer microstructures in powder metallurgy-fabricated TiB/Ti64 composite [29]. Similarly, Lekoadi et al. [30] reported that TiB whiskers in LMD-processed in- situ TiBw/Ti64 composites reduce β-grain size, contributing to increased strength. This matches observations of elongated TiB particles distributed along grain boundaries, confirming their role in grain refinement [30].
Figure 10 (a) shows the microstructure of in-situ TiBw/Ti64 after oxidation at 300°C for 6 hours in a 50% NaCl and 50% Na₂SO₄ environment, revealing notable changes.
Fig. 10
Shows microstructure of TiC/Ti64 after high temperature corrosion, (a) 300ºC, (b) 600ºC and (b) 900ºC.
The previously well-defined martensitic structure is no longer visible, and the surface appears roughened with numerous dark pits or voids, likely caused by surface corrosion from the corrosive salts [17], [19], [27], [28]. The lack of mass gain suggests minimal oxidation or scaling, with any corrosion products being thin or having spalled off. The dark pits indicate localized corrosion initiated by NaCl and Na₂SO₄, which promote the formation of less protective compounds like titanium chlorides (TiCl₄) and titanium sulphates (TiSO₄), potentially leading to evaporation or spallation of corrosion products. Kang et al. [10], reported that at lower temperatures, TiB-reinforced titanium alloys maintain a relatively intact structure due to the limited activity of Na₂SO₄ and NaCl in initiating corrosion. Similarly, Li et al. [11] found that TiB- reinforced titanium alloys exhibit high stability at temperatures below 500°C due to the slow oxidation kinetics and the protective effects of TiB on the alloy surface. Figure 10 (b) highlights the effects of high temperature corrosion at 600°C, where the in-situ TiBw/Ti64 surface shows significant morphological changes compared to the relatively intact structure at lower temperatures. The surface appears rough and granular, with rounded clusters of corrosion products unevenly distributed across the alloy [19], [27], [28]. This suggests localized corrosion and the formation of thicker, less protective corrosion layers composed of oxides, chlorides, and sulphates. As mentioned earlier, the combined effects of NaCl and Na₂SO₄ at this temperature lead to the formation of volatile and non-adherent compounds like TiCl₄ and TiSO₄. Although TiB contributes to slowing overall degradation, its galvanic interaction with the Ti64 matrix may intensify
localized corrosion. Kang et al. [10], who reported that at 600°C, Na₂SO₄ induces the formation of a porous and non-protective oxide scale on in-situ TiBw/Ti64 composites, increasing the likelihood of localized attack [10]. Additionally, Li et al. [11] found that TiB-containing composites exhibit some degradation at 600°C due to chloride penetration and sulphate interactions, though TiB still helps slow down overall corrosion.
Figure 10 (c) shows the microstructure after oxidation at 900°C, which displays even more severe degradation. The surface becomes porous and sponge-like, and both the martensitic structure and TiB particles are no longer distinguishable, indicating advanced material breakdown [19], [27], [28]. Compared to the 300°C and 600°C condition, the corrosion here is more aggressive, with evidence of mass loss due to the volatilization of corrosion products like TiCl₄. The degradation of both the matrix and TiB reinforcements further supports the idea that TiB’s protective effect diminishes at higher temperatures.. Qingxin et al. [31], who reported that at 800°C, in-situ TiBw/Ti64 composites undergo severe hot corrosion, leading to porous and cracked corrosion scales. Similarly, Shuaidi et al. [11] found that at high temperatures, TiC + TiB composites develop a thick corrosion layer with significant mass loss, suggesting that TiB does not offer complete protection against high-temperature degradation.
3.3 Reaction Mechanism for high temperature corrosion of Ti64, TiC/Ti64 and in-situ TiBw/Ti64
At elevated temperatures, titanium present in Ti-6Al-4V (Ti64) and its reinforced composites, such as TiC/Ti64 and in-situ TiBw/Ti64, undergoes oxidation, leading to the formation of a thermally grown oxide scale primarily composed of titanium dioxide (TiO₂). This oxide layer acts as a protective barrier that slows down further oxygen diffusion into the underlying alloy, thereby enhancing its oxidation resistance to some extent. However, in environments containing mixed salts such as 50% sodium chloride (NaCl) and 50% sodium sulfate (Na₂SO₄), the protective behavior of TiO₂ can be compromised. The molten salts can react with the oxide layer and the underlying metal, forming complex compounds such as sodium titanates (Na₂TiO₃) and promoting the dissolution of the protective oxide film. These reactions accelerate the degradation process through mechanisms like fluxing, spallation, and increased oxygen transport to the metal–oxide interface. Consequently, while TiO₂ provides initial protection under dry oxidation conditions, the
presence of aggressive molten salt mixtures significantly alters the oxidation kinetics and reduces the overall corrosion resistance of Ti64, TiC/Ti64, and in-situ TiBw/Ti64 alloys at high temperatures.
3.3.1 Reactions of Ti64
At elevated temperatures, titanium in Ti64 reacts with oxygen to form a protective TiO₂ layer:
Ti + O₂ → TiO₂ (1)
However, molten NaCl and Na₂SO₄ can degrade this layer:
TiO₂ + NaCl + O₂ → TiCl₄ (g) + Na₂O (2) Ti + Cl₂ → TiCl₄ (g) (3)
Ti + SO₃ → TiSO₄ (4)
TiO₂ + Na₂SO₄ → Na₂TiO₃ + SO₃ (5)
Vanadium and aluminium also oxidize:
V + O₂ → V₂O₅ (promotes further oxidation) (6)
Al + O₂ → Al₂O₃ (some protection, but degrades in salts) (7) Fluxing reactions dissolve protective oxides:
TiO₂ + NaCl → Na₂TiO₃ + Cl₂ (8)
Al₂O₃ + Na₂SO₄ → NaAlO₂ + SO₃ (9) Volatile metal chlorides may form: Al + Cl₂ → AlCl₃ (g) (10)
V₂O₅ + NaCl → NaVO₃ + Cl₂ (11)
3.3.2 Reactions of TiC
TiC is more stable but still reactive:
TiC + O₂ → TiO₂ + CO₂ (12)
TiC + NaCl + O₂ → TiCl₄ + CO + Na₂O (13) Other degradation paths include:
TiC + Na₂SO₄ → Na₂TiO₃ + CO + SO₃ (14)
TiO₂ + NaCl → Na₂TiO₃ + Cl₂ (15)
Al₂O₃ + NaCl → AlCl₃ (g) + Na₂O (16) Carbon may oxidize further:
C + O₂ → CO₂ (17)
3.3.3 Reactions of TiB2
TiB₂ is more resistant but can still degrade:
TiB₂ + NaCl → NaBO₂ + TiCl₄ (g) (18)
B₂O₃ + NaCl → BCl₃ (g) + Na₂O (19)
3.4 X-ray diffraction (XRD) analysis
3.4.1 Ti6Al4V (Ti64) XRD analysis
Figure 11 shows the XRD graph of Ti64 subjected to high-temperature exposure in a corrosive environment (50% NaCl + 50% Na₂SO₄) under an oxygen-rich atmosphere reveals significant phase transformations due to oxidation and hot corrosion.
Fig. 11
XRD of Ti64 as built, and after 300°C, 600°C, and 900°C for 6 h of exposure to a high- temperature corrosive environment (50% NaCl + 50% Na₂SO₄)
The as-built Ti64 sample primarily exhibits peaks corresponding to α-Ti (hcp) and β-Ti (bcc), characteristic of its microstructure [20]. At 300°C, broadening of peaks begins to appear, indicating the onset of surface oxidation. However, no major phase transformation is evident at this stage, suggesting that the initial oxide layers formed are thin and primarily protective.As the temperature increases to 600°C, new peaks corresponding to titanium oxides (TiO₂ - rutile and anatase) emerge, signifying the progressive oxidation of the Ti64 alloy [22]. The presence of a broad hump in the XRD pattern suggests the formation of an amorphous or poorly crystallized oxide layer, a phenomenon commonly reported in literature when Ti64 is exposed to aggressive high-temperature environments, as observed by Lu et al. [22]. The molten NaCl-Na₂SO₄ environment accelerates the oxidation process by dissolving protective oxide scales, exposing fresh material to further attack. At 900°C, the XRD pattern exhibits an even greater intensity of oxide peaks, indicating severe material degradation [22]. The increased presence of TiO₂ (rutile phase) and possible formation of sodium titanates (Na₂TiO2, Na₂Al2O3) confirm the extensive reaction between Ti64, oxygen, and the molten salts. This aligns with findings from Lu et al. [22], where Ti64, subjected to similar conditions exhibited rapid oxidation due to the presence of molten salts acting as a flux to facilitate ion transport.
3.4.2 TiC/Ti64 XRD analysis
The XRD patterns presented in Fig. 12, illustrate the phase evolution of TiC-reinforced Ti64 alloy subjected to high-temperature exposure in a corrosive environment (50% NaCl + 50% Na₂SO₄) for 6 hours under an oxygen-rich atmosphere.
Fig. 12
XRD of TiC/Ti64 as built, and after 300°C, 600°C, and 900°C for 6 h of exposure to a high-temperature corrosive environment (50% NaCl + 50% Na₂SO₄)
The as-built TiC/Ti64 sample exhibits peaks corresponding primarily to α-Ti (hcp) and β- Ti (bcc), with the presence of TiC peaks, indicating that the reinforcement phase remains stable in the initial microstructure [26]. Upon heating to 300°C, no significant phase transformations are observed, as the XRD peaks remain largely similar to the as-built condition, signifying the stability of TiC at lower temperatures However, at 600°C, additional peaks emerge, suggesting the onset of oxidation and corrosion reactions. The presence of new diffraction peaks likely corresponds to titanium oxides (TiO₂ - rutile and anatase phases), which are common oxidation products of Ti64 in high-temperature corrosive environments. The interaction between NaCl and Na₂SO₄ at this temperature likely accelerates oxidation, leading to the formation of complex oxides such as of NaTiO2 or and other sodium titanates, which have been reported in literature to form in Ti-based alloys exposed to molten salts [12]. At 900°C, the XRD pattern exhibits an increased intensity of
oxide peaks, with the presence of more pronounced TiO₂ peaks, suggesting extensive oxidation [13]. Additionally, possible peaks corresponding to NaTiO2 and other alkali-titanium compounds become more evident, indicative of the severe reaction between the Ti matrix and the corrosive molten salts. Literature studies, such as the one by Yuting Lv et al. [13] demonstrated that the addition of TiC to Ti-based alloys can influence oxidation behaviour, either by acting as a diffusion barrier to oxygen ingress or by participating in the reaction to form TiC-depleted zones. However, at higher temperatures, TiC itself can oxidize to form TiO₂ and carbon-based residues, further contributing to the degradation process.
3.4.3 In-situ TiBw/Ti64 XRD analysis
Figure 13 shows the XRD graph of in-situ TiBw/Ti64 alloy subjected to high-temperature oxidation in a 50% NaCl + 50% Na₂SO₄ environment under oxygen-rich conditions provides key insights into phase stability and degradation mechanisms.
Fig. 13
XRD of in-situ TiBw/Ti64 as built, and after 300°C, 600°C, and 900°C for 6 h of exposure to a high-temperature corrosive environment (50% NaCl + 50% Na₂SO₄)
The as-built in-situ TiBw/Ti64 sample exhibits strong peaks corresponding to α-Ti (hcp), β- Ti (bcc), and TiB, indicating the successful reinforcement of Ti64 with TiB phases [10]. At 300°C, the XRD pattern remains largely unchanged, suggesting that oxidation has not significantly altered
the phase composition at this temperature, and TiB remains stable within the Ti64 matrix. At 600°C, new peaks emerge, primarily corresponding to titanium oxides (TiO₂ - rutile and anatase), which indicate the onset of oxidation [11]. Compared to unreinforced Ti64, the oxidation rate appears to be lower, possibly due to the presence of TiB, which has been reported in the literature to enhance the oxidation resistance of Ti64 by acting as a diffusion barrier to oxygen ingress However, TiB itself can react at elevated temperatures, leading to the formation of boron oxides (B₂O₃) and other borate compounds, though these are less stable in molten salt environments. At 900°C, significant phase transformations occur, as evidenced by the intensified TiO₂ peaks and the possible formation of NaTiO2 or NaAl2O3, which result from the interaction between the titanium matrix and the molten salt [10]. The broadening of peaks and the increase in background intensity suggest severe oxidation and corrosion, leading to the formation of a thick, possibly porous, oxide scale. Similar findings have been reported by Qingxin et al. [10], who found that TiB reinforcement provides initial oxidation resistance but eventually undergoes degradation in high-temperature molten salt environments. The presence of molten NaCl-Na₂SO₄ accelerates the oxidation by dissolving protective oxides, thereby exposing fresh material to further attack.
4 Conclusions
High-temperature corrosion studies revealed notable differences among the three materials. In- situ TiBw/Ti64 exhibited the highest corrosion resistance, particularly at 900°C, due to the formation of stable boron-containing oxides, which maintained the integrity of the protective oxide layer. TiC/Ti64, while demonstrating intermediate oxidation resistance, showed susceptibility to oxide layer instability at elevated temperatures, leading to increased mass gain and material degradation. Ti64 experienced the most significant material loss across all temperatures due to the formation of volatile titanium chlorides and sulphates, which disrupted the protective oxide scale and exposed the underlying material to further oxidative attack.
Microstructural analyses revealed critical insights into the role of reinforcing phases. TiC particles enhanced the stability and adherence of the oxide layer, acting as barriers to defect propagation and oxygen diffusion. TiB, on the other hand, provided superior thermal stability and contributed to oxidation resistance through the formation of boron oxides. However, the
TiB phase also introduced challenges related to localized galvanic corrosion, underscoring the need for optimization of particle distribution and interface bonding.
These findings highlight the significant potential of TiC/Ti64 and in-situ TiBw/Ti64 for high- performance applications in aerospace, marine, and energy industries, where materials are exposed to aggressive saline and high-temperature environments. The study also emphasizes the importance of tailoring microstructures and optimizing fabrication processes to maximize the performance benefits of these composites.
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ACKNOWLEDGMENTS
I would like to acknowledge my colleagues from Laser Enable Manufacturing with assistance in the laboratory and the University of Limpopo Department of Chemistry.
5 DECLARATIONS
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Funding
This research was funded by the Department of Science and Innovation (DSI) of South Africa (SA) through the collaborative Program in Additive Manufacturing (CPAM)
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Competing InterestsThe authors have no relevant financial or non-financial interests to disclose.
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Author Contributions
The study was conceived and designed by all authors. Experimental work, data acquisition, and analysis were carried out collaboratively by Hosia Kgomo, Bathusile Masina, Ipfi Mathoho and Vusimuzi Mulaudzi. The initial version of the manuscript was prepared by Hosia Kgomo, with all authors contributing through review and revisions. All authors read and approved the final manuscript.
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