Enhanced Mechanical Properties of Ceramic Rod-Reinforced TWIP Steel Composites: Fabrication, Microstructural Analysis, and Heat Treatment Evaluation

Abstract

This study investigates the development and characterization of ceramic rod-reinforced TWIP (twinning-induced plasticity) steel matrix composites, produced using the lost foam casting technique. Mechanical tests revealed a substantial improvement in both flexural strength and ductility, with the composite demonstrating more than double the strength of unreinforced TWIP steel. Furthermore, a simple low-temperature heat treatment further enhanced these properties, increasing the flexural strength of the composite to 1023 MPa while also improving its ductility. The improvement in mechanical performance is attributed to the formation of additional twins in the TWIP steel matrix during deformation following heat treatment, which resulted in further strengthening of the matrix.

1. Introduction

The continuous evolution of material science demands innovations that meet the increasing performance requirements in various industrial applications [1,2,3]. Modern engineering applications require materials not only to possess exceptional mechanical properties, such as high strength and hardness, but also to exhibit substantial toughness and ductility [4,5]. This dual demand poses a significant challenge, as conventional single-phase materials often fail to meet these multifaceted requirements. For instance, materials that are extremely hard often lack sufficient toughness, making them susceptible to brittle fracture. Conversely, materials with high ductility may not offer the necessary strength for demanding applications. To address this complex challenge, ceramic-reinforced steel matrix composites have emerged as a promising field of research [6,7,8,9]. These composites uniquely combine the desirable properties of ceramics and metals, leveraging the inherent hardness and wear resistance of ceramic reinforcements with the ductility and toughness of a metallic steel matrix.

Traditionally, TWIP (twinning-induced plasticity) steel has been recognized for its exceptional mechanical properties, including high strength and remarkable ductility, making it suitable for demanding applications in automotive and structural industries [10,11,12]. However, the inherent limitations of TWIP steel, such as lower strength compared to some advanced composites, necessitate improvements to extend its applicability. Recent studies have demonstrated that reinforcing TWIP steel with ceramic materials can significantly enhance its mechanical performance. Ceramic reinforcements, known for their high hardness and thermal stability, can improve the overall strength and wear resistance of the matrix material [13,14]. Various methods have been explored to integrate ceramics into metallic matrices, including direct metal deposition, powder metallurgy, and casting techniques [15,16]. Among these, the use of ceramic rods as reinforcements presents a novel approach with potential benefits for achieving improved mechanical properties. Despite these advancements, the optimization of ceramic-reinforced TWIP steel composites remains underexplored. Previous research has primarily focused on the general integration of ceramic particles into metallic matrices, with limited attention to large-sized ceramic rods and their effects on the mechanical properties of TWIP steel [17]. Moreover, while heat treatment processes are known to influence the mechanical properties of composites, a detailed evaluation of specific heat treatment parameters, such as temperature and duration, is often lacking.

The development of ceramic-reinforced steel matrix composites with superior mechanical properties primarily focuses on two key research directions: enhancing the content of ceramic reinforcements and improving the performance of the steel matrix. Increasing the volume fraction of ceramic particles typically leads to significant improvements in hardness, wear resistance, and thermal stability, owing to the intrinsic properties of ceramics. For instance, researchers have extensively studied the effects of incorporating a higher percentage of ceramic particulates such as silicon carbide, titanium carbide, or aluminum oxide [18,19]. However, increasing the ceramic content often results in a trade-off whereby the composite’s ductility decreases, potentially leading to brittle behavior. Thus, achieving the optimal balance between hardness and ductility remains a critical challenge in the design of these composites.

Furthermore, conventional ceramic-reinforced steel matrix composites are limited by the maximum achievable ceramic content due to challenges such as increased brittleness and difficulty in processing. To overcome these limitations, an alternative approach involves the direct incorporation of large-sized ceramic components into the steel matrix [20]. This method can exceed the constraints imposed by conventional ceramic reinforcement limits, allowing for a greater volume of ceramic material to be integrated without compromising the composite’s structural integrity. The use of large ceramic reinforcements can provide substantial improvements in wear resistance and thermal stability, making this approach particularly attractive for high-performance applications.

The existing literature provides a foundational understanding of the benefits of ceramic reinforcements in metallic matrices but lacks comprehensive studies on the use of large-sized ceramic rods, specifically within TWIP steel matrices. Furthermore, the optimization of heat treatment processes for such composites is not well documented, presenting a gap in the knowledge regarding how specific parameters affect the mechanical properties and microstructure of these advanced materials. This study aims to bridge the identified research gap by focusing on the fabrication, characterization, and heat treatment evaluation of large-sized ceramic rod-reinforced TWIP steel composites. By addressing the comprehensive integration of ceramic rods and exploring the effects of heat treatment parameters, this research contributes to a deeper understanding of the mechanisms governing the performance of these composites. The findings have implications for enhancing the application of TWIP steel in industries where high mechanical performance is crucial, thus advancing the field of composite material science.

2. Experimental Materials and Methods

2.1. Preparation of Experimental Materials

In this study, we utilized cylindrical Si₃N₄ceramic rods with dimensions of φ5 mm × 95 mm as the reinforcing phase in the composite material. The ceramic reinforcements used in the current study were sourced from Hongyang Fine Ceramics Co., Ltd. (Shenyang, China). The essential physical properties of these ceramic materials are presented in

Table 1. Physical properties of ceramics.

Density (g/cm3)	Modulus Elasticity (GPa)	Flexural Strength (MPa)	Compressive Strength (MPa)	Thermal Expansion Coefficient (10−6/°C)
>3.2	>290	>600	>2500	>3.1

. The choice of a rod-shaped ceramic reinforcement was deliberate, aiming at maximizing the load-bearing capability and enhancing the mechanical properties of the resultant composite. The matrix material used in this research was twinning-induced plasticity (TWIP) steel, a high-manganese austenitic steel recognized for its exceptional combination of strength, ductility, and energy absorption capabilities. The TWIP effect, which involves the formation of mechanical twins during plastic deformation, significantly contributes to the material’s work-hardening rate, making it an excellent choice for composite applications where both toughness and ductility are crucial. The chemical composition of the TWIP steel matrix is provided in

Table 2. Chemical composition of TWIP Steel (wt%).

Element	C	Si	Mn	S	P
%	0.6	0.2	19	<0.002	<0.002

, which lists the primary alloying elements.

2.2. Preparation of Ceramic Rod-Reinforced TWIP Steel Matrix Composites

In this study, we utilized the lost foam casting method to fabricate large-sized ceramic rod-reinforced TWIP steel matrix composites. This innovative technique allows for the integration of ceramic reinforcements into the steel matrix, achieving a composite material with enhanced mechanical properties suitable for high-performance applications.

The process begins with the preparation of a high-density polystyrene foam model, which serves as the template for the TWIP steel matrix. This foam is precisely cut and shaped into the desired geometric configuration, incorporating cavities designed to house the ceramic rods. Following the creation of the foam model, ceramic rods, each with a diameter of 5 mm and a length of 95 mm, are positioned within the foam cavities. To ensure that the ceramic rods are securely held in place, a suitable adhesive is applied, which helps to maintain their alignment and stability throughout the casting process. The next phase involves coating the foam model with a refractory material. Multiple layers of refractory slurry are meticulously applied to the foam model, and each layer is dried at 45 °C to build up a durable and heat-resistant shell. Once the refractory coating is fully cured, the foam model, now reinforced with ceramic rods and encased in a refractory shell, is embedded in a sand casting box. Molten TWIP steel is then carefully poured into the mold cavity. Once the TWIP steel has solidified, the mold is carefully dismantled, and the refractory coating is removed. This reveals the final ceramic rod-reinforced TWIP steel matrix composite.

Once the ceramic rod-reinforced TWIP steel matrix composites were fabricated, a high-performance industrial computed tomography (CT) system, provided by Granpect Company Limited (Beijing, China), was utilized to assess both the fabrication quality and structural integrity of the large-scale composites. The X-ray source was positioned 2.75 m from the sample, while the detector was placed 3.4 m from the source. The experimental setup is depicted in Figure 1.

2.3. Heat Treatment for Ceramic Rod-Reinforced TWIP Steel Matrix Composites

Heat treatment is a common method for enhancing the mechanical properties of materials. A key objective of this study is to investigate whether heat treatment can improve the mechanical performance of ceramic rod-reinforced TWIP steel matrix composites. High-temperature heat treatments or rapid cooling processes, such as solution treatment, could lead to cracking or detachment of the ceramic rods during cooling. Therefore, a low-temperature heat treatment process was selected for this research. To verify the feasibility of enhancing the mechanical properties of ceramic rod-reinforced TWIP steel matrix composites through low-temperature heat treatment, we designed a straightforward and easily implementable heat treatment process. After preparing the composite samples using the lost foam casting method, the specimens were subjected to heat treatment at 500 °C, with a heating rate of 5 °C/min, held for 60 min, and then naturally cooled in ambient air. Due to the relatively small size of the samples in this study, the 60 min holding time was sufficient to ensure that both the surface and core of the samples reached the target temperature. This heat treatment process is simple, cost-effective, and can utilize residual heat from casting, making it practical for large-scale production and broader applications.

The effectiveness of these heat treatment strategies was assessed by preparing specimens of ceramic rod-reinforced TWIP steel matrix composites and conducting bending tests, as illustrated in Figure 2.

These specimens were precisely cut to the designed dimensions of three-point bending test samples using the wire-cut electrical discharge machining method. The cross-sectional dimensions of the three-point bending specimen were 10 mm × 80 mm, with efforts made to ensure that the ceramic rods were positioned as centrally as possible. The three-point bending test was conducted using an electronic universal testing machine provided by NCS TESTING TECHNOLOGY Co., Ltd. (Beijing, China), with a punch diameter of 10 mm and a loading rate of 2 mm/min. The bending tests yielded critical data on the flexural strength, ductility, and overall toughness of the heat-treated composites. These results were compared to those of untreated TWIP steel matrix materials, highlighting the effects of the different heat treatment processes. Similarly, tensile tests were conducted on 5 mm diameter specimens using the same electronic universal testing machine, with a loading rate of 2 mm/min.

3. Results and Discussion

3.1. Microstructure and Properties Analysis of the Matrix Material

As the matrix material for ceramic-reinforced TWIP steel composites, the mechanical properties of TWIP steel directly influence the overall performance of the composite. Therefore, it is essential to analyze the microstructural state and mechanical properties of the TWIP steel matrix, as well as the impact of heat treatment on its mechanical performance. The microstructural features of the matrix material in its as-cast and heat treatment conditions were analyzed using optical microscopy, as shown in Figure 3.

The microstructure in Figure 3 reveals an austenitic phase with granular carbides precipitated along the grain boundaries and, to a lesser extent, within the grains. This morphology is consistent with typical TWIP steel microstructures, where the distribution of carbides tends to be relatively uniform and sparse. It can be observed that the number of carbides in the TWIP steel matrix slightly increased after heat treatment. The increase in precipitates is expected to influence the mechanical properties of the TWIP matrix. During solidification, carbide precipitates primarily form at the austenite grain boundaries, with some precipitates occurring within the grains. During the heat treatment holding process, the increase in temperature enhances atomic diffusion, leading to the precipitation of carbides at grain boundaries, sub-grain boundaries, and other locations, as shown in Figure 3b. This distribution of carbides is significant as it influences the mechanical properties of the material, potentially reducing tensile strength and elongation due to the presence of precipitates, porosity, and residual stresses formed during casting [21,22]. Carbides contribute to the material’s strength by acting as obstacles to dislocation movement, which enhances the hardness and tensile strength of the composite. This is supported by studies indicating that fine, uniformly distributed carbides can significantly increase the strength of steel by impeding the movement of dislocations. While carbides improve strength, their distribution also affects ductility. Excessive carbide formation or uneven distribution can lead to brittleness, as large carbide particles or clusters can act as stress concentration points and initiate cracks. Balancing carbide distribution is thus essential to optimize the material’s ductility and toughness.

Tensile specimens with a diameter of 5 mm were prepared, and their tensile properties were measured, yielding an average tensile strength of 325 MPa and a yield strength of 247 MPa in the initial casting state. The fracture morphology after tensile testing is shown in Figure 4.

The matrix material also demonstrated an average elongation of 5.3%, indicating some capability for plastic deformation. This plasticity is attributed to the austenitic structure of the matrix, known for its capacity for plastic deformation. The fracture surface of the tensile test specimen, shown in Figure 5, exhibits a classic intergranular fracture mode. Some twin boundaries are also visible in the austenite matrix, indicating that twinning plays a role in plastic deformation during tensile loading. In the as-cast state, TWIP steel typically has a coarse microstructure with large austenite grains and relatively weak grain boundaries. The grain boundaries in as-cast TWIP steel often exhibit lower mechanical strength compared to the grain interiors. This weakening can be due to the presence of impurities or inclusions at the grain boundaries, which can act as stress concentration points and initiate fractures along these boundaries. Although these values indicate a relatively low tensile strength, the overall mechanical properties of the TWIP steel matrix were significantly improved after a simple heat treatment.

To enhance the mechanical properties of the matrix material, and thus the overall performance of the ceramic rod-reinforced TWIP steel composite, we examined the effects of heat treatment. The impact of low-temperature heat treatment on the mechanical properties of the ceramic rod-reinforced TWIP steel matrix is shown in Figure 5.

Figure 5 clearly shows that the heat-treated matrix material exhibits significant improvements in mechanical properties compared to the as-cast state. The average yield strength increased by approximately 60 MPa (an improvement of over 20%), while the tensile strength rose by more than 150 MPa (an increase of about 50%). Notably, the elongation and reduction in area doubled, indicating a substantial enhancement in ductility and plasticity. These improvements are attributed to the release of residual stresses and the stabilization of the austenitic structure, which may facilitate the formation of twin boundaries during deformation, thereby enhancing ductility and toughness [23,24].

The fracture surface morphology of the TWIP steel matrix in both the as-cast and heat-treated states is shown in Figure 6.

As shown in Figure 6 (marked by arrows), the heat-treated TWIP steel matrix exhibits twinning and the formation of some dimples during tensile testing. These features are crucial for enhancing the material’s toughness and ductility [25,26]. The microstructural changes observed are consistent with the mechanical property enhancements, underscoring the beneficial effects of the heat treatment process on the matrix material. Following heat treatment, a noticeable change in the fracture mode was observed. The TWIP steel matrix in the composites exhibited a transition from predominantly intergranular to some transgranular fracture mode. The heat treatment process, which involved heating at 500 °C for 60 min, appears to have improved the matrix’s cohesion and reduced the number of casting defects, leading to enhanced ductility and toughness. The fracture surfaces post-heat treatment displayed increased evidence of ductile voids, suggesting improved material toughness and a more favorable interaction between the ceramic rods and the matrix. This treatment, by reducing residual stresses and promoting favorable microstructural features, significantly enhances the overall mechanical performance of the TWIP steel matrix, making it more suitable for high-performance applications in ceramic rod-reinforced composites.

Twinning-induced plasticity (TWIP) effect is a crucial factor contributing to the improvement of mechanical properties in TWIP steel after heat treatment. Optical microscopy was used to examine the microstructural morphology after tensile testing, as shown in Figure 7. The figure clearly demonstrates that under tensile stress, twin structures are formed in the TWIP steel matrix. Additionally, the number of twins observed after heat treatment is significantly higher than those in the as-cast state, which markedly enhances the overall mechanical performance of the TWIP steel.

3.2. Mechanical Properties Analysis of TWIP Steel Matrix Composites

In this study, the φ5 ceramic rods were successfully integrated with TWIP steel using the lost foam casting method to fabricate ceramic rod-reinforced TWIP steel matrix composites. Industrial CT non-destructive testing was performed on the composites, and the results are shown in Figure 8.

As depicted in Figure 8, the industrial CT non-destructive testing results for the ceramic rod-reinforced TWIP steel matrix composite reveal that there are no significant defects such as pores, cracks, or insufficient filling between the ceramic rods and the matrix. The ceramic rods and the matrix exhibit a good combination of interfaces, providing a solid foundation and prerequisite for a subsequent mechanical performance analysis and heat treatment process optimization. The inclusion of ceramic rods, known for their exceptional hardness and strength, is anticipated to significantly enhance the mechanical properties of the resulting TWIP steel matrix composites. Specifically, the introduction of these ceramic reinforcements is expected to markedly increase the material’s strength. For this study, bending specimens were machined from the φ5 ceramic rod-reinforced TWIP steel matrix composite. These tests were performed on both the ceramic rod-reinforced composite and the unreinforced TWIP steel matrix material, allowing for a direct comparison of their flexural properties. The post-test appearance of the specimens under the as-cast condition is shown in Figure 9.

As illustrated in Figure 9, the TWIP steel matrix material exhibited significant cracking at the bottom of the specimen, leading to near-complete fracture. In contrast, the ceramic rod-reinforced TWIP steel composite demonstrated superior flexural resistance. The composite maintained structural integrity during bending, with only minor cracks appearing on the specimen’s side. Even at greater bending angles, the composite did not suffer complete fracture, showcasing a markedly improved bending performance compared to the standalone TWIP steel matrix. This enhancement in bending performance is further supported by the stress-displacement curves obtained during the flexural tests, which provide a clear comparative visualization of the bending capabilities of the ceramic-reinforced composite and the pure TWIP steel matrix, as shown in Figure 10.

From Figure 10, it is evident that the flexural strength of the TWIP steel matrix composite is more than double that of the unreinforced TWIP steel. The TWIP steel began to fail and crack when the punch displacement reached 3.2 mm. Conversely, the ceramic-reinforced composite withstood a punch displacement exceeding 7 mm, more than twice the displacement before fracture compared to the unreinforced matrix. This result indicates a significant improvement in the mechanical properties of TWIP steel after reinforcement.

The superior bending mechanical properties of the TWIP steel matrix composite, compared to the pure TWIP steel, can be attributed to the reinforcing effect of the ceramic rods. The composite can be conceptualized as a type of “fiber-reinforced composite”, where the “fibers” are the ceramic rods embedded in the TWIP steel matrix. According to the “rule of mixtures” for composite materials, the overall strength of the composite is influenced by the strengths of both the reinforcement (ceramic rods) and the matrix (TWIP steel) [27,28]. Given that ceramics typically exhibit higher strength than the metallic matrix, the resultant composite material naturally exhibits enhanced mechanical properties compared to the matrix alone, as confirmed by the experimental data. While ceramics are inherently brittle, the ceramic-reinforced TWIP steel matrix composite displayed superior plastic deformation capabilities compared to the pure TWIP steel matrix. This observation is somewhat unexpected, given the typical reduction in ductility when brittle materials are introduced into a composite system. However, this phenomenon can be explained by the concept of triaxial compressive stress. Under such stress conditions, brittle materials can exhibit plastic deformation behavior [29]. According to principles of plastic deformation and superplasticity, materials subjected to triaxial hydrostatic pressure can show enhanced plastic deformation capabilities, even if they are intrinsically brittle. In the case of the ceramic-reinforced TWIP steel composite, the TWIP steel matrix, during cooling and solidification, exerts a compressive force on the ceramic rods, effectively encasing them in a triaxial compressive stress state. This confinement allows the ceramic to exhibit some degree of plasticity, thereby enabling the composite to demonstrate better plastic deformation ability under bending stresses than the unreinforced TWIP steel.

The observed mechanical performance enhancements highlight the efficacy of ceramic reinforcement in improving not only the strength but also the ductility and toughness of TWIP steel matrix composites.

3.3. Mechanical Properties of Ceramic Rod-Reinforced TWIP Steel Composites after Heat Treatment

To further assess the feasibility of improving the overall mechanical properties of ceramic rod-reinforced TWIP steel matrix composites via heat treatment, bending tests were conducted following a simple heat treatment process. The flexural performance of the heat-treated composite is illustrated in Figure 11.

As shown in Figure 11a, the flexural strength of the ceramic rod-reinforced TWIP steel matrix composite in its as-cast state reached 805 MPa, significantly higher than the 388 MPa of unreinforced TWIP steel. Following heat treatment, the mechanical properties of the ceramic rod-reinforced TWIP steel matrix composite were further enhanced. The flexural strength increased from 805 MPa to 1023 MPa, a rise of over 200 MPa. The significant enhancement in the flexural strength of the ceramic rod-reinforced TWIP steel matrix composite can be attributed to several key mechanisms. The heat treatment at 500 °C for 60 min facilitated a refinement of the microstructure, including the relaxation of residual stresses. Additionally, the reduction in internal stresses reduces the likelihood of crack initiation, which contributes to both increased strength and ductility. TWIP steel is known for its excellent ductility due to the formation of mechanical twins during deformation. The heat treatment enhances the twin formation within the steel matrix, which not only absorbs the applied stress but also delays the onset of necking, leading to improved ductility [30,31]. This twin-induced plasticity, combined with the reinforcement provided by the ceramic rods, creates a synergistic effect that significantly enhances both the strength and ductility of the composite. This improvement not only reflects an increase in strength but also a notable enhancement in ductility. As depicted in Figure 11b, the displacement of the punch corresponding to the maximum bending force increased from 6.8 mm to 9.6 mm post-heat treatment, indicating that the composite can undergo greater deformation and thus has improved plastic deformation capability. In contrast, the displacement for the TWIP steel matrix alone was only 3.1 mm, highlighting the advantages of the composite material. This suggests that the heat treatment effectively improved the flexural mechanical properties of the TWIP steel matrix composite.

The load–displacement curves of ceramic-reinforced TWIP steel composites in different states are shown in Figure 12. This provides a more intuitive understanding of the effects of low-temperature heat treatment on the material’s mechanical properties.

From Figure 12, it can been seen that the bending curve of the TWIP matrix material alone is smooth, whereas the curves for the ceramic rod-reinforced TWIP steel matrix composites, particularly after various heat treatments, exhibit a serrated pattern. This serrated behavior can be attributed to the interaction between the hard ceramic rod and the more ductile TWIP steel matrix. During bending, as the applied force increases, the composite initially shows an increase in flexural strength. However, as the force continues to increase, microcracks may form preferentially in the ceramic due to its lower plasticity compared to the TWIP steel. These microcracks can release some of the applied stress, resulting in a temporary decrease in the bending stress observed in the curve. The presence of the TWIP steel matrix, with its excellent plastic deformation capabilities, can accommodate these stresses by allowing the crack-affected zones in the ceramic to realign or shift. This reorientation restores the reinforcing effect of the ceramic rod, leading to an increase in the bending load capacity, thus resulting in the observed sawtooth pattern in the stress–strain curve. This cycle of stress relief and load recovery continues until the ceramic reinforcement can no longer provide additional strength, marking the failure point of the composite.

The results of these mechanical tests highlight that heat treatment significantly improves the flexural strength and plastic deformation capacity of the TWIP steel matrix composites. These enhancements validate the feasibility and effectiveness of using simple heat treatment as a method to optimize the mechanical properties of ceramic rod-reinforced TWIP steel matrix composites.

4. Conclusions

This study thoroughly investigated the mechanical and microstructural properties of ceramic rod-reinforced TWIP steel matrix composites, focusing on the synthesis methods, material characterization, and the effects of heat treatment on the composite’s performance. The key findings of this research can be summarized as follows:

• The use of the lost foam casting process successfully integrated ceramic rods into the TWIP steel matrix, resulting in a composite material that exhibits an exceptionally strong composite effect.
• The incorporation of ceramic rods significantly enhanced the mechanical properties of the TWIP steel matrix. Notably, the composite exhibited a more than twofold increase in flexural strength compared to the unreinforced TWIP steel. The improved mechanical performance is attributed to the synergistic effect of the hard ceramic phase and the ductile TWIP matrix, which allows for efficient load transfer and crack deflection mechanisms during mechanical deformation.
• The application of a simple heat treatment at 500 °C for 60 min followed by air cooling resulted in further optimization of the composite’s properties. Post-heat treatment, the flexural strength of the composite increased by approximately 27%, reaching 1023 MPa. Additionally, the ductility of the material, as indicated by the displacement corresponding to the maximum bending force, was significantly enhanced.