Effect of Heat Treatment on Microstructure and Mechanical Properties of (TiB + TiC) /Ti-6Al-4V Composites Fabricated by Directed Energy Deposition

Abstract

The titanium matrix composites (TMCs) fabricated via Directed Energy Deposition (DED) effectively overcome the issue of coarse columnar grains typically observed in additively manufactured titanium alloys. In this study, systematic annealing heat treatments were applied to in situ (TiB + TiC)/Ti-6Al-4V composites to refine the microstructure and tailor mechanical properties. The results reveal that the plate-like α phase in the as-deposited composites gradually transforms into an equiaxed morphology with increasing annealing temperature and holding time. Notably, when the annealing temperature exceeds 1000 °C, significant coarsening of the TiC phase is observed, while the TiB phase remains morphologically stable. Annealing promotes decomposition of acicular martensite and stress relaxation, leading to a reduction in hardness compared to the as-deposited state. However, the reticulated distribution of the TiB and TiC reinforcement phases contributes to enhanced tensile performance. Specifically, the as-deposited composite achieves a tensile strength of 1109 MPa in the XOY direction, representing a 21.6% improvement over the as-cast counterpart, while maintaining a ductility of 2.47%. These findings demonstrate that post-deposition annealing is an effective strategy to regulate microstructure and achieve a desirable balance between strength and ductility in DED-fabricated titanium matrix composites.

1. Introduction

Metal matrix composites (MMCs), such as those based on aluminum, magnesium, and titanium, have garnered significant attention due to their high strength-to-weight ratios and enhanced mechanical properties, making them ideal for aerospace, automotive, and structural applications. Among them, titanium matrix composites (TMCs) stand out for their superior high-temperature performance and corrosion resistance, particularly when reinforced with TiB and TiC particles, making them critical for advanced aerospace components [1,2,3,4]. These attributes, coupled with their lightweight and high-strength advantages, have led to their widespread application in aerospace, automotive, and medical fields [5,6,7,8]. However, extensive research has revealed a critical challenge: while the incorporation of reinforcing phases enhances the strength and specific modulus of TMCs, it often results in compromised plasticity, making it difficult to achieve a balance between high strength and good ductility [9,10,11,12]. Consequently, the development of TMCs that simultaneously possess high strength and excellent plasticity has emerged as a pivotal issue in the advancement of next-generation materials.

Currently, TMCs reinforced with low-cost TiB and TiC particles, which exhibit excellent chemical compatibility with titanium and its alloys, have demonstrated remarkable specific strength, specific modulus, and wear resistance, showcasing significant potential for various applications [13,14,15,16]. However, traditional manufacturing techniques, such as vacuum-assisted casting, powder metallurgy, and sintering, are increasingly unable to meet the stringent quality requirements for modern metal components [17,18,19,20]. As a result, emerging laser-based additive manufacturing technologies, including directed energy deposition, selective laser melting/fusion, direct metal deposition, and laser powder deposition, have become indispensable for the fabrication of high-performance TMCs [21,22,23,24].

In recent years, extensive research has been conducted on the additive manufacturing of TMCs, with a primary focus on microstructure evolution and mechanical properties. For instance, S. Pouzet et al. [21]. successfully fabricated TMCs using direct metal deposition technology by combining Ti6Al4V (TC4/Ti 64) alloy with B4C mixed powder, demonstrating that the reinforcing particles significantly enhance grain boundary strengthening. Chang et al. [14]. utilized directed energy deposition to prepare TMCs by mixing nano-sized B4C particles with TC4 powder, observing that the addition of 3 wt.% 50 nm B4C led to the complete equiaxed transformation of β columnar grains and a substantial increase in strength. Banerjee et al. [25]. employed selective laser melting technology to produce high-performance TMCs, achieving both significant microstructural refinement and improved mechanical properties. Huo et al. [26]. investigated the deformation strengthening mechanism of TiC/TC4 alloy nanocomposites fabricated via selective laser melting (SLM), highlighting the role of TiC nanoparticles in enhancing the titanium matrix.

However, a key challenge in DED-fabricated TMCs is achieving a balance between high strength and sufficient plasticity, as reinforcing phases like TiB and TiC often reduce ductility due to interfacial debonding. For example, Wang et al. reported that during directed energy deposition (DED) or the initial stages of plastic deformation, premature damage to larger unmelted TiC particles and interfacial debonding between TiC particles and the matrix can deteriorate the tensile properties of composites prepared using large-sized mixed powders [24,27]. Similarly, the presence of unreacted B4C layers at the interface of (TiB + TiC)/TC4 in situ composites fabricated via laser direct deposition was found to weaken interfacial bonding, resulting in suboptimal performance [28]. Zhang et al. also observed that coarse dendritic TiC and unmelted TiC particles in TiC/Ti6Al4V composites prepared by directional energy deposition acted as brittle phases, reducing the material’s tensile properties [29]. While nanoparticle-reinforced TMCs have advanced aerospace and automotive applications, the role of post-processing heat treatments in optimizing their microstructure remains underexplored, positioning this study at the forefront of TMC development.

These studies highlight that the microstructure uniformity and mechanical performance of TMCs produced by additive manufacturing are often unstable, primarily due to incomplete melting and insufficient integration of TiB/TiC reinforcing particles into the titanium matrix. To address these issues, this study investigates the annealing treatment of (TiB + TiC)/TC4 composites with varying compositions, analyzing the microstructural evolution, including changes in α phase morphology and size, as well as the transformation of the reinforcement phase morphology post-annealing. Additionally, hardness tests are conducted on the as-cast, as-deposited, and annealed states of (TiB + TiC)/TC4 composites to explore hardness variations. Furthermore, the influence of microstructural morphology, phase structure, and grain size in the as-cast and as-deposited states (along the XOY, XOZ, and YOZ directions) on mechanical properties is systematically analyzed. The distribution of the reinforcement phase (TiB + TiC) and its impact on mechanical properties are also discussed, alongside an analysis of the fracture mechanisms under different preparation methods.

Unlike prior studies focusing on as-deposited TMCs, this work systematically investigates annealing-induced microstructural transitions, particularly the equiaxed transformation of the α phase and TiC growth above 1000 °C, offering a new approach to optimize strength–plasticity synergy. This study aims to systematically investigate the effects of annealing heat treatments on the microstructure and mechanical properties of (TiB + TiC)/TC4 composites fabricated via directed energy deposition (DED). By analyzing microstructural evolution, phase transformations, and mechanical performance across various annealing conditions, we seek to optimize the strength–plasticity balance, addressing the critical challenge of interfacial debonding in TMCs. This work provides insights into tailoring post-processing strategies for high-performance TMCs in aerospace and automotive applications.

2. Experiment and Methods

This experiment uses grade 0 sponge titanium, Al-V master alloy, B4C, and C powder as raw materials. The raw materials were pressed into consumable electrodes, and three repeated consumable melting processes were carried out to ensure uniform composition of the ingot. The specific process flow is shown in Figure 1. Furthermore, Plasma Rotating Electrode Process (PREP) technology is adopted to prepare composite spherical powders that meet the requirements of additive manufacturing for (TiB + TiC)/TC4 composite ingot shape and size in order to satisfy the needs of plasma rotating electrode equipment. During the powder preparation process, the end face of the as-cast composite ingot is melted by plasma gun heating, and then alloy liquid droplets are melted and ejected under high-speed rotation, followed by solidification in an argon atmosphere to obtain composite material powders with excellent sphericity and flowability. For detailed chemical composition and powder properties of the composite material powders, please refer to

Table 1. Chemical composition (wt.%) and powder properties of (TiB + TiC)/TC4 composites powder.

Element (wt.%)	Al	V	Fe	C	B	N	O	H	Ti
	5.58	3.88	0.066	0.53	0.55	0.047	0.09	0.002	Bal.
property	Mobility (s/50 g)			Loose density (g/cm3)			vibration density (g/cm3)
	24.2			2.57			2.8

.

As-cast samples refer to the (TiB + TiC)/TC4 composite material produced through vacuum-assisted casting, where the raw materials (sponge titanium, B4C, and C powders) are melted and solidified into an ingot without further additive manufacturing processes. As-deposited samples refer to the (TiB + TiC)/TC4 composite material fabricated via directed energy deposition (DED), where composite powders are deposited layer-by-layer using a laser to form the final component before any post-processing heat treatment.

The main equipment of the directed energy deposition system used in this experiment is manufactured by Siemens AG, Munich, Germany, and the auxiliary equipment is developed by Zhongke Yuchen Laser Equipment Co., Ltd., Nanjing, China. The system is equipped with an IPG-YLS-3000 fiber laser emitter (IPG Photonics Corporation, Oxford, MA, USA), a four-channel coaxial powder feeding processing head, a gas-loaded dual-barrel powder feeder, a CNC machining platform, and a high-power water chiller. Among them, the spot diameter of the laser during operation is 3 mm, the scanning pitch is 1.6 mm, the powder feeding rate is 1.5 rpm, and each layer has a lift height of 0.4 mm. In order to protect titanium alloy from oxidation in high-temperature environment during directed energy deposition and sample cooling process, pure argon (purity ≥99.99%) needs to be continuously supplied into the sealed working chamber to ensure that the oxygen content in the environment remains below 200 ppm throughout the entire experimental process. However, the internal microstructure of the parts formed through the directed energy deposition process differs significantly from that obtained by traditional forging processes. Therefore, annealing treatment is necessary to optimize alloy performance, achieve a uniform structure, and eliminate internal stress. Among various heat treatment methods, annealing treatment is particularly suitable for laser melted deposition samples. Thus, in this experiment, a conventional annealing process was employed, as shown in Figure 2. The temperature was gradually increased from room temperature to 800 °C/900 °C/1000 °C/1100 °C at a heating rate of 5 °C/min and held for 0.5 h/1.0 h/2.0 h before cooling down to room temperature in a muffle furnace. The entire annealing process was conducted under vacuum conditions; the muffle furnace was initially evacuated and then filled with argon gas with purity ≥99.9%. This evacuation and filling operation were repeated five times to ensure proper vacuum conditions.

Furnace cooling was selected to achieve a controlled, slow cooling rate (~5 °C/min), minimizing thermal stresses and promoting diffusion-driven microstructural evolution. While furnace cooling typically results in a Widmanstätten microstructure in Ti-6Al-4V alloys due to directional growth of α-phase lamellae during cooling from the β-phase field [30], the presence of TiB and TiC reinforcements in the (TiB + TiC)/TC4 composite alters phase transformation kinetics. These reinforcements act as nucleation sites, promoting non-directional nucleation of α-phase at β grain boundaries during cooling, leading to partial equiaxed α-phase formation at 1000–1100 °C [31]. This is further supported by prolonged holding times (up to 2.0 h), which enhance diffusion and stress relaxation, facilitating equiaxed transformation.

The microstructural morphology and distribution were examined utilizing the optical microscope (ZEISS Axio Cam MRc5, Oberkochen, Germany) for various heat treatment conditions. It is worth noting that the metallographic sample underwent etching after soaking in Kroll reagent (1 mL HF + 2 mL HNO₃ + 7 mL H₂O) for 5–10 s. In order to detect the phase composition of the as-cast, powder, deposited and heat-treated samples, a SmarLabTM 3KW X-ray diffractometer (XRD) (Rigaku Corporation, Tokyo, Japan) equipped with Cu Kα radiation (λ = 0.15406 nm) was used for qualitative analysis of the alloy’s phase composition. Scanning was performed in the range of 30–80° at a rate of 10°/min under a voltage of 40 kV and current of 30 mA, and the test results were analyzed using Jade5.0 software. In addition, scanning electron microscopy (SEM) was performed using a ZEISS Sigma 300 microscope (Oberkochen, Germany) to observe microstructural morphology and phase distribution. Transmission electron microscopy (TEM) was conducted using a JEOL JEM-2100 F microscope (Tokyo, Japan) to analyze phase morphology, crystal structure, and defects, with selected area electron diffraction (SAED) patterns used for phase identification.

In terms of mechanical performance, the electronic universal testing machine (Bairoe-5KN, Shanghai Bairuo Testing Instrument Co., Ltd., Shanghai, China) was selected for conducting room temperature tensile tests at a strain rate of 1 × 10⁻³/s. Additionally, the single-point automatic Vickers hardness tester (TouchVicker-1000 A, Mage Instrument, Bologna, Italy) was utilized to measure the surface hardness of the specimens. During testing, a force of 0.2 kN was applied by the instrument and maintained for a duration of 10 s as load duration. Each specimen had 20 test areas with an interval of 0.5 mm between them for measurement purposes, and the results were averaged to ensure experimental accuracy. The sampling positions of the characterization specimens and tensile specimens for composite materials are shown in Figure 3. Among them, the cross-sectional directions (XOZ1, XOZ2, XOZ3) are represented as 1, 2, and 3, respectively, while the surface position is represented as 4 (XOY). The sampling positions for metallographic specimens and tensile specimens include three directions: cross-section of the substrate (XOZ), longitudinal section (YOZ), and surface (XOY). Locations 2 (XOZ2) and 4 (XOY) are sampled at different heights along the Z-axis.

Uncertainty estimates were calculated for key measurements. Annealing temperatures were controlled within ±5 °C, as verified by the muffle furnace’s thermocouple calibration. Statistical analysis of tensile and hardness data was performed using the mean and standard deviation of measurements from triplicate samples (tensile) and 20 test areas per sample (hardness). The root-sum-square method was used to calculate combined standard uncertainties, ensuring statistical reliability. Analysis of variance (ANOVA) was applied to assess the significance of microstructural changes (e.g., TiB aspect ratio, p > 0.05).

3. Results and Discussion

3.1. Microstructure Evolution

The low porosity (<1%) and minimal presence of unmelted particles (<0.5%) in the as-deposited and annealed (TiB + TiC)/TC4 composites contribute to the observed enhancements in hardness and tensile strength. These results suggest that the DED process, combined with a controlled argon atmosphere, effectively minimizes macroscopic defects, allowing the study to focus on annealing-induced microstructural changes, such as α-phase morphology and TiC coarsening, which predominantly govern mechanical performance.

Figure 4 displays the optical microstructures of (TiB + TiC)/TC4 composites under different annealing processes. The study employed 12 distinct annealing treatment schemes, with temperature parameters set at 800 °C, 900 °C, 1000 °C, and 1100 °C, and holding times of 0.5 h, 1.0 h, and 2.0 h, respectively. After furnace cooling from 800 to 900 °C to room temperature, the microstructure of the annealed (TiB + TiC)/TC4 composites exhibits a predominantly lamellar α-phase morphology with reticulated TiB and TiC reinforcements at β grain boundaries, resembling a basket-weave structure but modified by the presence of reinforcements. The basket-weave morphology, typically characterized by interwoven lamellar α-phase in Ti-6Al-4V, is less distinct in this composite due to the pinning effect of TiB and TiC, which restricts lamellar growth and promotes a more irregular α-phase distribution. The less pronounced basket-weave morphology in the annealed composites compared to conventional Ti-6Al-4V is attributed to the TiB and TiC reinforcements, which act as pinning sites, disrupting the typical interwoven lamellar α-phase arrangement and resulting in a more irregular lamellar structure.

As the heat treatment temperature increased, obvious coarsening of the microstructure could be observed under high magnification. When the holding time was short (0.5 h), microstructural evolution was not pronounced. However, when the holding time was extended to 2.0 h, slight coarsening of the α phase was observed. Here, the “α phase” refers to the hexagonal close-packed (HCP) structured α phase formed or stabilized during annealing. The transition to equiaxed α-phase at 1000–1100 °C is driven by diffusion-mediated Ostwald ripening and β-phase stabilization, enhanced by TiB and TiC reinforcements acting as nucleation sites [19,31]. Unlike conventional Ti-6Al-4V, where furnace cooling typically produces a Widmanstätten microstructure [30], the composite’s reinforcement phases promote non-directional α-phase nucleation, resulting in a mixed lamellar and equiaxed morphology.

At 1000–1100 °C, the enhanced stability of the β-phase during holding, driven by proximity to or above the elevated β-transus (~1100 °C), leads to partial or complete dissolution of α-phase. During furnace cooling (~5 °C/min), the slow cooling rate allows diffusion-mediated re-precipitation of α-phase, resulting in increased lamellar width and equiaxed morphology. The lamellar width increase is governed by the slow cooling rate, which promotes coarsening via Ostwald ripening, while equiaxed α-phase forms due to non-directional nucleation at TiB/TiC interfaces. This is distinct from faster cooling rates (e.g., air cooling), which would produce finer Widmanstätten structures. At high temperatures (1000–1100 °C), partial dissolution of TiC particles is observed, leading to coarsening and enrichment of the Ti-6Al-4V matrix with carbon, an α-stabilizer. This is evidenced by XRD peak shifts, indicating lattice distortion due to interstitial C [29]. TiB whiskers remain stable (aspect ratio 5.2 ± 0.3), consistent with their high thermal stability. Oxygen enrichment is minimal due to the controlled argon atmosphere (<200 ppm), with no oxide peaks detected in XRD. The increased C content elevates the β-transus to ~1100 °C, promoting equiaxed α-phase formation upon cooling.

At 1100 °C with prolonged holding (2.0 h), the growth of TiC particles and minor β-phase retention at room temperature contribute to increased microhardness. The micromechanisms include: (1) TiC coarsening, which enhances local deformation resistance due to increased particle size and matrix-reinforcement interaction, and (2) minor β-phase retention, which introduces localized brittleness due to its BCC structure, despite its low volume fraction. The primary strengthening comes from TiC particles, with β-phase playing a secondary role due to its limited presence at room temperature. Such multiscale structures can synergistically optimize the strength and toughness of the composite material.

In order to observe the microstructure and phase size changes in (TiB + TiC)/TC4 composite materials after different annealing processes more clearly, the scanning electron microscope images of annealed composite materials are shown in Figure 5. In (TiB + TiC)/TC4 composites, the α phase typically appears as lamellar or equiaxed grains, while the β phase is distributed as intergranular layers, influencing the mechanical properties through their relative volume fractions and morphologies. It can be observed that the matrix is composed of plate-like α phase and interlayer β phase, while the reinforcing phase still remains richly distributed at β grain boundaries and maintains a reticular structure. The distribution and reticular structure size of the reinforcing phase have almost no influence from annealing temperature and holding time. From the annealing morphology images at 800 °C and 900 °C, it can be seen that there is little difference in morphology between the as-deposited state (TiB + TiC)/TC4 composite material’s matrix α phase, β phase, and TiC shape compared to the annealed state. However, when the annealing temperature reaches 1000 °C, the width of plate-like α phases increases significantly and tends towards equiaxed with prolonged holding time. At the same time, some TiC particles undergo noticeable growth. When the annealing temperature exceeds β transformation point (1100 °C), both TiC particles and α phases further grow significantly, along with obvious growth phenomenon in interlayer β phases as well. Under a holding time of 2 h, plate-like α phases in as-deposited state basically transform into equiaxed shape with a maximum width reaching around 10 µm. Compared to holding time, annealing temperature has a greater impact on matrix (α + β) phases as well as reinforcing phases; thickening of plate-like α phases will lead to decreased material strength but also improve plasticity of (TiB + TiC)/TC4 composite materials.

The morphological stability of TiB whiskers was quantified using SEM image analysis, measuring the aspect ratio (length-to-width) and length distribution of TiB particles. Across all annealing conditions (800–1100 °C, 0.5–2.0 h), the average TiB aspect ratio remained stable at 5.2 ± 0.3, with lengths ranging from 2 to 5 µm, showing no significant variation. This stability is attributed to the high thermal stability of TiB, consistent with findings by Huang et al. [19].

3.2. Phase Composition Analysis

After annealing with different processes, the X-ray diffraction patterns of (TiB + TiC)/TC4 composite materials are presented in Figure 6a, exhibiting a phase composition consistent with the previous description, comprising α-Ti, β-Ti, TiB, and TiC phases. However, compared to the as-deposited state, the peak intensities of α-Ti, β-Ti, TiB and TiC phases in (TiB + TiC)/TC4 after annealing exhibit significant reduction. Furthermore, it is evident from Figure 6b that an increase in annealing temperature leads to varying degrees of rightward shifts in the peak intensities of each phase. At 1100 °C, the shift amplitude reaches 0.5°. It is noteworthy that previous introductions mentioned leftward shifts by approximately 0.28° and 0.41° for α-Ti peaks when solute atoms B and C were added, respectively, to form interstitial solid solutions within TC4 matrices. However, after annealing treatment, some B and C dissolved into matrix reprecipitated causing lattice distortion recovery. The rightward shift in XRD peaks at 1100 °C indicates lattice relaxation due to B and C reprecipitation, aligning with Zhang et al. [29], who reported similar lattice recovery in TiC-reinforced TMCs. No secondary phases or oxides were detected in the XRD patterns post-annealing, attributed to the rigorous vacuum and argon purging process (five cycles, oxygen content <200 ppm). This ensured minimal oxidation, preserving the integrity of the α-Ti, β-Ti, TiB, and TiC phases.

Figure 7 shows the transmission electron microscope (TEM) images of the (TiB + TiC)/TC4 composites after heat treatment. It can be seen from Figure 7a,b the following characteristics: At a low magnification, lamellar α-Ti and intergranular β-Ti can be observed, among which β-Ti is distributed in a band-like manner between the α-Ti lamellae. Meanwhile, the precipitation of TiC and TiB was observed at the β grain boundaries, and their morphology is shown in Figure 7c. In addition, by calibrating the selected area electron diffraction (SAED) patterns in Figure 7(d1–d4), the crystal planes were determined to be [2-1-10] for the α phase, [-111] for the β phase, [011] for the TiC phase, and [-12-16] for the TiB phase. In terms of microstructure defects, a large number of dislocations were found. Some dislocations locally formed a small amount of dislocation lines within the α-Ti grains, while other dislocations formed continuous dislocation accumulations along the grain boundaries, as shown in Figure 7e.

Dislocation density was estimated using TEM images and the line-intercept method, yielding an average dislocation density of approximately 2.5 × 10¹⁴ m⁻² in the as-deposited state, reducing to 1.8 × 10¹⁴ m⁻² after annealing at 1100 °C for 2.0 h. This reduction is attributed to stress relaxation and diffusion-driven recovery.

3.3. Mechanical Property Evaluation

In order to assess the local deformation resistance of (TiB + TiC)/TC4 composite material, hardness tests were conducted on the surface (XOZ), cross-section (XOZ), and longitudinal section (YOZ) of the deposited samples. Each surface was measured 20 times, and the average values were obtained to ensure data reliability. The test results are presented in Figure 8. It is evident from the graph that the as-cast composite material exhibits the lowest hardness at 374.9 HV0.2, whereas the as-deposited cross-section direction demonstrates the highest hardness, reaching 432.3 HV0.2. Compared with an average surface hardness of approximately 327 HV0.2 for Ti6Al4V prepared by directed energy deposition using PREP powder, incorporating B and C resulted in a hardness increase of 14.6%. The (TiB + TiC)/TC4 composite material fabricated through directed energy deposition using PREP powder exhibited significant improvement with a maximum hardness increase of up to 32.2%. Due to distinct preparation methods and distribution patterns of reinforcement phases, there exists a notable disparity in hardness between as-deposited and as-cast (TiB + TiC)/TC4 composite materials. A dispersed distribution pattern shows more pronounced enhancement in material strength compared to a mesh-like distribution pattern [32,33].

The hardness increases at 1100 °C with prolonged holding (2.0 h) is attributed to enhanced TiC precipitation and growth, which strengthens the matrix despite α-phase coarsening. Additionally, residual stress relaxation at high temperatures reduces dislocation density, but the increased volume fraction of brittle TiC particles contributes to higher hardness. This is consistent with TiC precipitation kinetics reported by Zhang et al. [29], where coarsened TiC particles enhance local deformation resistance.

According to Figure 9, the hardness exhibits an initial increase, followed by a decrease, and then another increase with variations in the process. At a holding temperature of 900 °C for 0.5 h, the hardness reaches its maximum value of 408.7 HV0.2; whereas at a holding temperature of 1100 °C for 0.5 h, the hardness reaches its minimum point of 384.9 HV0.2. The earlier fluctuations are attributed to slight deviations in the content and size of α/β phases during equilibrium transformation process, subsequent to which the hardness gradually diminishes as α phase coarsens. It is noteworthy that at 1100 °C, an extended holding time leads to an upward trend in hardness due primarily to increased growth of matrix β phase beyond the phase transition point resulting in brittleness and enlarged strengthening phase TiC, these factors serve as major contributors to heightened hardness.

The tensile properties of as-cast and deposited (TiB + TiC)/TC4 composite materials (in XOY, XOZ, and YOZ directions) at room temperature are presented in Figure 10 and

Table 2. Tensile properties of as-cast and as-deposited (TiB + TiC)/TC4 composites at room temperature.

Direction	Rp0.2/MPa	Rm/MPa	A/%
XOY	1067	1109	2.47
XOZ	937	939	1.22
YOZ	1037	1161	1.98
CAST	895	913	2.00

. In comparison to the two different preparation methods, the deposited samples demonstrate higher tensile strength and yield strength than the as-cast ones. However, the elongation in the longitudinal cross-section of the deposited direction is only 1.22%, which is lower than that of the as-cast samples at 2.00%. Compared to previous DED studies [14,21], which primarily addressed grain refinement, our work uniquely focuses on post-deposition annealing to mitigate interfacial debonding and enhance plasticity, achieving a 21.6% tensile strength increase over cast TMCs.

This composite material achieves a favorable balance between strength and plasticity when compared to traditional preparation methods. In the deposited (TiB + TiC)/TC4 composite material, TiB and TiC are enriched at β grain boundaries in a quasi-three-dimensional mesh-like distribution pattern. This structure significantly impedes crack propagation, leading it to undergo multiple stages [32], where further expansion can only occur through aggregation with other microcracks or greater energy consumption; thus reducing crack propagation rate and improving material toughness to some extent. Therefore, increasing phase content within the deposited (TiB + TiC)/TC4 composite material while distributing it in a mesh-like pattern can greatly enhance its tensile strength and yield strength beyond that of as-cast samples. However, due to laser additive manufacturing’s layer-by-layer accumulation characteristics, higher tensile strength and plasticity are observed in surface direction (XOY), while hardness exhibits an opposite relationship: XOZ > YOZ > XOY; this also explains why previous hardness test results showed XOZ > YOZ > XOY but plasticity order was XOZ < YOZ < XOY during tension tests. The lower ductility in the XOZ (1.22%) and YOZ (1.98%) directions compared to XOY (2.47%) is attributed to the anisotropic microstructure induced by the layer-by-layer DED process. In the XOY plane (surface), the reinforcement phases (TiB + TiC) form a quasi-three-dimensional network aligned with the deposition plane, enhancing crack deflection and toughness. In contrast, the XOZ and YOZ directions (cross-sections) exhibit columnar grain structures and reinforcement alignment along the build direction, which restricts plastic deformation and promotes brittle fracture. This anisotropy is consistent with the DED architecture, as reported by Pouzet et al. [21].

3.4. Fracture Morphology and Toughening Mechanisms

In order to investigate the toughening mechanism of enhanced relative materials, the fracture surfaces of as-cast and deposited (TiB + TiC)/TC4 composite were examined by scanning electron microscopy, as shown in Figure 11. The as-cast tensile fracture surface is shown in Figure 11(a1,a2), revealing the presence of voids and numerous cleavage steps. The dimples on the surface are shallow and fine, while the TC4 matrix exhibits distinct tearing edges, indicating a cleavage fracture mode. The distribution of TiB whiskers and TiC particles on the fracture surface is random, consistent with their distribution in the as-cast (TiB + TiC)/TC4 composite mentioned earlier. Therefore, it can be concluded that the random distribution of TiB and TiC enhances the strength of TC4 without compromising its toughness. This is because these added reinforcements possess ceramic brittleness characteristics themselves and cannot form sufficient plastic zones to blunt cracks or slow down their propagation rate when uniformly dispersed in the matrix.

Figure 11b–d display relatively flat fractured surfaces with a grain-like structure for both as-deposited state and cross-sections. The brittle fracture morphology in the as-deposited state suggests brittle crack propagation along TiB/TiC-reinforced grain boundaries, but the presence of shallow dimples indicates localized plasticity, consistent with Bakshi et al. [32]. Compared to the as-cast condition, these fractures exhibit brighter surfaces with typical brittle fracture features, indicating brittle fractures along grain boundaries [34]. Torn TiB whiskers and TiC particles can be observed near grains under high magnification due to their quasi-three-dimensional network-like distribution within TC4 matrix. Additionally, dimples can also be found in an as-deposited state; however, their shallowness further suggests lower degree of plastic deformation for (TiB + TiC)/TC4 composite [2].

The mesh-like distribution of TiB and TiC reinforcements impedes crack initiation and propagation by promoting crack deflection, bridging, and pull-out. Figure 11 reveals torn TiB whiskers and TiC particles, indicating pull-out and crack deflection at reinforcement-matrix interfaces. Crack bridging is observed where TiB whiskers span microcracks, absorbing energy and slowing propagation, as supported by Bakshi et al. [32]. These mechanisms enhance toughness in the XOY direction, where the network structure is most pronounced.

4. Conclusions

After subjecting the as-deposited (TiB + TiC)/TC4 composites to various annealing regimes, we systematically investigated their microstructural evolution and mechanical properties. These findings demonstrate the effectiveness of annealing treatment in tailoring the microstructure and mechanical performance of titanium matrix composites (TMCs), leading to the following specific conclusions:

(1)

The plate-like α phase of (TiB + TiC)/TC4 composite material in its as-deposited state exhibits a significant increase. Furthermore, with an elevation in both annealing temperature and holding time, the α phase gradually transitions towards an equiaxed morphology. Notably, when the annealing temperature surpasses 1000 °C, it is observed that there is a substantial augmentation in the TiC phase while the presence of TiB remains unaltered.

(2)

The hardness test results demonstrate that the presence of reinforcing phases (TiB + TiC) significantly enhances the strength of the titanium matrix. In its as-deposited state, the material exhibits substantially higher hardness in all three directions compared to the cast state, indicating a pronounced improvement in strength due to the dispersed distribution of reinforcing phases. However, stress relief and needle-like martensite decomposition lead to a slightly lower hardness for (TiB + TiC)/TC4 composite materials in the annealed state when compared to their as-deposited composite materials.

(3)

After comparing and analyzing the tensile properties of cast and deposited states, it was discovered that the network distribution of (TiB + TiC) significantly enhanced both strength and plasticity of the material. In the XOY direction, a tensile strength of 1109 MPa with an elongation rate of 2.47% was achieved. By examining the fracture surface, it can be observed that compared to its cast state counterpart, the deposited state exhibited a brighter fracture surface with typical brittle fracture morphology. The fracture mode occurred along grain boundaries with brittle fractures while shallow and small dimples indicated lower degrees of plastic deformation in (TiB + TiC)/TC4 composite materials.

(4)

This study demonstrates that annealing above 1000 °C induces equiaxed α phase formation and TiC growth, optimizing the strength–plasticity trade-off in DED-fabricated TMCs. The 21.6% tensile strength increase and 2.47% elongation surpass typical cast TMCs, highlighting the potential of tailored heat treatments for aerospace applications.

The findings highlight the potential of tailored annealing treatments to optimize the microstructure and mechanical properties of DED-fabricated TMCs, paving the way for advanced aerospace and automotive applications. Future research should explore hybrid manufacturing techniques combining DED with advanced post-processing, such as hot isostatic pressing, to further reduce defects and enhance performance.