Enhancing Structural, Mechanical, and Radiation-Shielding Properties of Al-B₄C Hybrid Composites

Abstract

In this study, novel Al₆₀₆₁-(30-x)B₄C-xSm₂O₃ (x = 0, 1, 3, 5, 7, and 9 wt%) composites were fabricated using high-energy ball milling followed by cold pressing and sintering. The aim was to improve both the mechanical performance and radiation-shielding capabilities by integrating Sm₂O₃ as a reinforcement phase. Microstructural analyses via XRD and SEM-EDX revealed that the addition of Sm₂O₃ significantly enhanced phase uniformity, reduced porosity, and improved interfacial bonding, especially by mitigating the inherent poor wettability between Al₆₀₆₁ and B₄C. As a result, the relative density, hardness, and wear resistance were considerably improved with an increasing Sm₂O₃ content. Monte Carlo simulations (MCNP6.2) demonstrated that while thermal neutron shielding showed a slight decline due to the reduced boron content, fast neutron and gamma-ray attenuation were substantially enhanced owing to the high atomic number and density of Sm₂O_3. The results demonstrate that the mechanical performance and superior neutron-shielding properties contribute to new visions in material design and applications and have the potential to provide safer and more effective radiation-protection solutions that are environmentally sustainable.

1. Introduction

Radiation plays a vital role in nuclear energy, healthcare, agriculture, and various industrial fields. However, ionizing radiation poses significant health and environmental risks if not properly managed [1,2,3,4]. To mitigate these effects, the development of lightweight, durable, and environmentally sustainable shielding materials has become a crucial research area [5,6]. In this context, metal matrix composites (MMCs) reinforced with neutron-absorbing and high-density phases offer promising solutions. Particularly, the use of boron carbide (B₄C) for thermal neutron absorption and rare earth oxides such as samarium oxide (Sm₂O₃) for fast neutron and gamma-ray attenuation has attracted increasing attention. This study aims to design and evaluate Sm₂O₃-reinforced Al-B₄C hybrid composites to enhance both the structural and radiation-shielding performance, contributing to material sustainability and personnel safety in radiation-exposed environments [7,8,9,10].

B₄C/Al metal matrix composites have remarkable neutron-absorption capacity due to their low density, high mechanical strength, and superior thermal and chemical stability. These properties are based on the high thermal neutron cross-section (about 3840 barns) of the boron-10 isotope naturally present in B₄C, which provides an effective neutron-shielding function within the Al matrix. With a density of approximately 2.5 g/cm³, these composites offer advantages in terms of both being lightweight and their strength, making them suitable solutions for high-demand areas such as mechanical engineering, aerospace technologies, and the nuclear industry. In addition, the homogeneous B₄C distribution achieved through manufacturing techniques such as powder metallurgy, hot extrusion, and rolling enables these composites to achieve both a high strength-to-weight ratio and effective neutron-shielding performance [11,12,13,14,15,16]. However, increasing the B₄C content in Al-B₄C composites negatively affects the machinability and corrosion resistance of the composite, limiting its application areas. In addition, high levels of B₄C reinforcement make homogeneous distribution within the matrix difficult and have undesirable effects on the density and microstructural integrity [17,18,19,20]. This limits the ability of Al-B₄C composites to achieve the desired level of the thermal neutron macroscopic absorption rate. Therefore, gadolinium (Gd), samarium (Sm), and their oxides (Gd₂O₃ and Sm₂O₃), which have high thermal neutron cross-sections, are widely investigated as additional reinforcement phases in composite matrices. These oxide reinforcements not only enhance neutron-shielding performance but also contribute to the development of stable and multifunctional material systems by maintaining mechanical stability in Al matrix composites [21,22,23,24].

Samarium is among the light rare earth elements and is characterized by its high thermal stability, difficult processability, and mechanical strength. Global samarium reserves are estimated to be approximately 9.6 megatons, with about 35% (approximately 3.5 megatons) located in China. The remaining reserves are scattered across various countries, primarily in the United States, India, Brazil, Sri Lanka, and Australia. This distribution increases the strategic importance of samarium on a global scale, making the security of supply a crucial issue [6,25,26,27]. This offers a significant advantage in both industrial production and sustainability. Sm₂O₃ is a reinforcing element with a high price/performance balance. The fact that it can be supplied from various regions increases its logistical importance and contributes to its broader use in production processes [28,29,30,31].

In addition, it has a very high neutron cross-section compared to elements such as samarium, boron, and cadmium, making Sm₂O₃ an effective neutron-shielding material. This makes Sm₂O₃ a reliable and functional reinforcement for nuclear engineering, radiation shielding, and other advanced engineering applications. Furthermore, Sm₂O₃ is envisaged to form a layer in the Al/Sm₂O₃ composite system surrounding the Al powders, which could favorably modify the low wettability present in the Al/B₄C composite system [32].

Al6061 is a suitable matrix material for metal matrix composites (AMCs) due to its high machinability, good wear resistance, and corrosion resistance [33,34]. B₄C is a preferred ceramic reinforcement for AMCs due to its high hardness, low density, and superior neutron-absorption capacity [35,36,37]. However, poor wettability and interfacial reactions between Al and B₄C weaken particle bonding, reducing the mechanical integrity and neutron-shielding efficiency [38,39]. This fundamental incompatibility leads to a loss of performance in Al matrix composites, and addressing this issue with binder phases or innovative material design will fill an important gap in the literature [40,41,42].

This study aims to produce Al₆₀₆₁-(30-x)B₄C-(x)Sm₂O₃ (x = 0, 1, 3, 5, 7, and 9) hybrid composites by incorporating Sm₂O₃ as a supplementary reinforcement into the Al-B₄C composite system. The primary objective is to explore whether the addition of Sm₂O₃ can improve the microstructural, mechanical, and radiation-shielding characteristics of the composites. Particular attention is given to evaluating the influence of Sm₂O₃ on the density, hardness, wear resistance, corrosion behavior, and attenuation of thermal neutrons, fast neutrons, and gamma rays. The broader aim is to assess the potential of these composites for use in radiation-prone environments, to enhance performance, and to contribute to environmentally sustainable shielding solutions.

2. Experimental Procedures

2.1. Preparation of the Al-B₄C-Sm₂O₃ Hybrid Composites

Commercially available Al₆₀₆₁ (15–53 µm), B₄C (44 µm), and Sm₂O₃ (43 µm) powders (Nanografi Nano Teknoloji AŞ, Ankara, Türkiye) of approximately 99.9% purity were used in this study. The powders were precisely weighed and placed in vial containers and milled in a shaker-type mill (Chisun Tech, Shenzhen, China) at room temperature for 5 h at a frequency of 50 Hz. During the grinding process, hardened steel balls with a diameter of 15 mm and a steel vial with a volume of 80 mL were used, with a ball-to-powder weight ratio of 5:1. To prevent overheating during grinding, a 2 min rest period was provided every 5 min of operation.

2.2. Compaction and Sintering

The powder mixtures obtained from the grinding process were cold pressed uniaxially for 1 min under a pressure of 720 MPa and formed into 10 mm-diameter compacts. The green compacts were sintered at 600 °C for 3 h in an argon environment using a Protherm-brand vacuum furnace (Alser Teknik, Istanbul, Türkiye).

2.3. Density Measurements

The density measurements of the sintered composite specimens were conducted using the WSA-224 setup based on Archimedes’ principle, following the ASTM D792 standard procedure [43]. The theoretical densities of the composites were calculated using Equation (10).

2.4. Structural and Microstructural Characterization

Phase analysis was carried out using a PANalytical Empyrean X-ray diffractometer (PANalytical, Almelo, The Netherlands) equipped with a Cu-Kα radiation source (λ = 1.5406 Å) operating at 40 kV and 40 mA. The XRD scans were conducted over a 2θ range of 10–90° with a step size of 0.02° and a scanning rate of 1°/min, which enabled accurate identification of the crystalline phases. The surface microstructure and elemental distribution of the composites were analyzed using a FEI QUANTA 450 (FEI, Brno, Czech Republic) field-emission scanning electron microscope (FE-SEM) integrated with an Oxford X-MaxN 80 (Oxford Instruments, Oxfordshire, UK) mm² energy-dispersive X-ray spectroscopy (EDX) detector. The SEM images were taken under high-vacuum conditions with an acceleration voltage of 15–20 kV, providing detailed insights into the particle distribution, agglomeration behavior, and interfacial bonding within the composite matrix.

2.5. Microhardness and Wear Testing

Microhardness values were determined using a Shimadzu HMV-G21 tester (Shimadzu, Kyoto, Japan) with a 0.98 N load applied for 15 s. The recorded hardness was calculated as the average of ten individual indentations per sample.

Wear behavior was evaluated using a TRIBOtechnic-TRIBOtester (TRIBOtechnic, Clichy, France) under a load of 12 N, with a total sliding distance of 200 m and a velocity of 12 m/s. The wear track dimensions were analyzed using a Taylor Hobson 2D (Taylor Hobson, Leicester, UK) profilometer. All tests were conducted in ambient conditions with a relative humidity of 42–49% and a temperature between 25 and 29 °C. The wear volume and wear rate were determined using Equations (1) and (2), based on profilometric measurements of the abrasion tracks [44,45].

$$V=A·l$$

(1)

$$WR={\displaystyle \frac{V}{S}}$$

(2)

where

• V is the wear volume (mm³),
• S is the sliding distance (m),
• A represents the cross-sectional area of the wear track (mm²),
• L is the length of the wear path (mm),
• WR denotes the wear rate (mm³/m).

2.6. Electrochemical Testing

Electrochemical corrosion tests were performed in a 3.5 wt.% NaCl solution at 25 ± 2 °C using a Gamry Reference 1010E potentiostat (Gamry Instruments, Warminster, PA, USA) with a standard three-electrode setup consisting of a silver/silver chloride (Ag/AgCl) reference electrode, platinum counter electrode, and the sample as the working electrode. The samples were stabilized at open circuit potential (OCP) for 30 min before measurement. Polarization curves were recorded in the range of ±250 mV vs. OCP at a scan rate of 1 mV/s. Corrosion current density (I_corr) values were determined using the Tafel extrapolation method, and corrosion rates were calculated based on these values. For accurate calculations, the actual density values determined based on the Archimedes principle were used. A comprehensive overview of the experimental process is illustrated in the schematic flow diagram shown in Figure 1. The corrosion rate (CR) was determined using Equation (3), which is expressed as follows:

$$CR=k{\displaystyle \frac{i_{corr }EW}{\rho }}$$

(3)

where k = 3.27 × 10⁻³ is a unit conversion constant, I_corr denotes the corrosion current density in (µA.cm⁻²), EW is the equivalent weight in (g.eq⁻¹), and $\rho$ = experimental measured density in (g.cm⁻³).

2.7. Monte Carlo Simulations

MCNP6.2 is an advanced Monte Carlo-based software developed by Los Alamos National Laboratory that simulates the interactions of particles (e.g., photons, neutrons, and electrons) with matter using statistical methods [46]. In this study, the linear attenuation coefficient (LAC) of Al-B₄C-Sm₂O₃ composites against gamma rays and the macroscopic cross-sections against thermal neutrons were calculated using the MCNP6.2 code, and the radiation-shielding properties of the materials were evaluated [47,48,49,50,51,52].

The simulation geometry, shown in Figure 2, consists of a neutron source placed in a vacuum, a target material, and an F4 tally-type detector measuring the average flux per cm² per source. Energy values of 0.48 MeV (characteristic energy due to the n + B interaction) were used for gamma rays, with 2.53 × 10⁻⁸ MeV for thermal neutrons and 2 MeV for fast neutrons. The purpose of selecting these specific energy levels over a broad energy spectrum is to conduct a more precise and targeted analysis by focusing on the fundamental interactions between neutrons and gamma radiation.

Before each simulation, the reference free flux (I₀) for the case without target material and the target flux (I_x) calculated with Al-(30-x)B₄C-(x)Sm₂O₃ (x = 0, 1, 3, 5, 7, and 9) composites were determined. Thus, the attenuation performances of different composites were comparatively analyzed. To ensure statistical accuracy, 10⁸ neutron histories were simulated in each instance, and all results were obtained with a statistical error rate of less than 1%. Attenuation ratios for gamma, thermal neutrons, and fast neutrons were calculated using Equation (4). In contrast, the linear attenuation coefficient (LAC) and macroscopic cross-sections were obtained using Equations (5) and (6) based on the Beer–Lambert law. This method provides a reliable basis for numerically evaluating the radiation shielding effectiveness of materials [53].

$$\mathit{N}\mathit{e}\mathit{u}\mathit{t}\mathit{r}\mathit{o}\mathit{n} \mathit{A}\mathit{b}\mathit{s}\mathit{o}\mathit{r}\mathit{p}\mathit{t}\mathit{i}\mathit{o}\mathit{n} \mathit{R}\mathit{a}\mathit{t}\mathit{e}(\mathit{\%})=\left(1-{\displaystyle \frac{{\mathit{I}}_{\mathit{x}}}{{\mathit{I}}_0}}\right)\times \mathbf{100}$$

(4)

$${{\mathit{I}}_{\mathit{x}}={\mathit{I}}_{\mathit{o}}\mathit{e}}^{{-\sum }_{\mathit{t}}\mathit{x}}$$

(5)

$${{\mathit{I}}_{\mathit{x}}={\mathit{I}}_{\mathit{o}}\mathit{e}}^{-\mathit{\mu }\mathit{x}}$$

(6)

In this expression, $I_o$ and $I_x$ represent the intensities of the incident and transmitted neutrons or photons, respectively. The term x denotes the thickness of the shielding material in centimeters. ${\sum }_t$ corresponds to the macroscopic cross-section for neutron interactions, while $\mu$ indicates the linear attenuation coefficient (LAC) for gamma rays in the absorbing medium. Furthermore, the half-value layer (HVL), mass attenuation coefficient (MAC, μ_m), and mean free path (MFP) were calculated. The (MAC) (${\mu }_m$) (Equation (7)) is a quantity that describes the probability of interactions between gamma photons and the mass per unit area for a given medium and can be calculated from the Beer–Lambert law with the computed value of μ as follows [54]:

$${\mathit{\mu }}_{\mathit{m}}=\left({\displaystyle \frac{\mathit{\mu }}{\mathit{\rho }}}\right)$$

(7)

$\rho$ is the experimental density of the shield for which the MAC value is calculated. The half-value layer (HVL) refers to the thickness of material necessary to reduce the incident radiation intensity by 50%. It is derived from the linear attenuation coefficient ($\mu$) as indicated in Equation (8). Meanwhile, the mean free path (MFP) represents the average distance a photon travels before interacting with the material, and it can also be calculated using Equation (9).

$$\mathit{H}\mathit{V}\mathit{L}={\displaystyle \frac{\mathbf{0.693}}{\mathit{\mu }}}$$

(8)

$$\mathit{M}\mathit{F}\mathit{P}={\displaystyle \frac{\mathbf{1}}{\mathit{\mu }}}$$

(9)

The MCNP6.2 Monte Carlo code was employed to simulate gamma-ray and neutron-shielding behavior. The simulation model was developed using actual composite densities (measured via Archimedes’ method) and SEM-EDX-based structural data. Although experimental transmission measurements were not conducted, this simulation strategy allowed us to evaluate shielding performance under realistic conditions and offer predictive insights that complement the experimental results.

3. Results and Discussion

Table 1. Chemical compositions of the Al-B₄C-Sm₂O₃ composites.

Materials	Chemical Composition (wt.%)
	Al6061	B4C	Sm2O3
Al-30B4C	70	30	0
Al-29B4C-1Sm2O3	70	29	1
Al-27B4C-3Sm2O3	70	27	3
Al-25B4C-5Sm2O3	70	25	5
Al-23B4C-7Sm2O3	70	23	7
Al-21B4C-9Sm2O3	70	21	9

lists different weight fractions of Al₆₀₆₁-B₄C-Sm₂O₃ hybrid composites that hold promise for engineering uses. These composites were produced through mechanical milling and classified based on their Sm₂O₃ reinforcement. The Al₆₀₆₁-(30-x)B₄C-(x)Sm₂O₃ (x = 0, 1, 3, 5, 7, and 9) composites were named None, 1Sm₂O₃, 3Sm₂O₃, 5Sm₂O₃, 7Sm₂O₃, and 9Sm₂O₃, respectively. This research thoroughly explores the mechanical, physical, and radiation-shielding behaviors of these hybrid composites with different Sm₂O₃ ratios, addressing the needs of engineering applications.

Table 2. Densities of the Al-B₄C-Sm₂O₃ composites.

Materials Code	Theoretical Density (g/cm3)	Experimental Density (g/cm3)	Relative Density (%)
None	2.64	2.25	85.24
1Sm2O3	2.66	2.30	86.37
3Sm2O3	2.70	2.43	90.16
5Sm2O3	2.74	2.48	90.38
7Sm2O3	2.78	2.52	90.58
9Sm2O3	2.83	2.58	91.24

presents the density measurements of the Al-B₄C-Sm₂O₃ composites. Data analysis reveals that as the Sm₂O₃ content increases in the Al-B₄C-Sm₂O₃ composites, the theoretical density (Equation (10)), relative density (Equation (11)), and experimental density values show a corresponding increase. Sm₂O₃ acts as an effective binder between the Al and B₄C powders, improving sinterability by mitigating the poor wettability between the particles. This binding effect yields a more homogeneous and dense composite structure, resulting in a linear increase in relative density by reducing the porosity.

Specifically, the relative density rises from 85.24% to 91.24% as the Sm₂O₃ reinforcement increases from 1% to 9%, while the measured density grows from 2.25 g/cm³ to 2.58 g/cm³. These results highlight that incorporating Sm₂O₃ into the Al/B₄C system plays a key role in improving the composite’s mechanical and physical properties, acting as an essential binder.

$${\rho }_{theo }={\displaystyle \frac{1}{\left(\frac{{\mathrm{m}}_{\mathrm{A}\mathrm{l}}}{{\mathrm{m}}_{total} · {\mathsf{\rho }}_{\mathrm{A}\mathrm{l}}}+\frac{{\mathrm{m}}_{{\mathrm{S}\mathrm{m}}_2{\mathrm{O}}_3}}{{\mathrm{m}}_{total} · {\mathsf{\rho }}_{{\mathrm{S}\mathrm{m}}_2{\mathrm{O}}_3} }+\frac{{\mathrm{m}}_{{\mathrm{B}}_4\mathrm{C}}}{{\mathrm{m}}_{total} · {\mathsf{\rho }}_{{\mathrm{B}}_4\mathrm{C}}}\right) }}$$

(10)

$${\mathsf{\rho }}_{rel}(\mathrm{\%})=\left({\displaystyle \frac{{\rho }_{exp}}{{\rho }_{theo}}}\right)\times 100$$

(11)

3.1. XRD Analysis

The phase distributions and phases present in Al-B₄C-Sm₂O₃ composites were determined via X-ray diffraction (XRD) analysis and are shown in Figure 3. X-ray analysis revealed the presence of Al, B₄C, and Sm₂O₃ phases in the composite structure [16,55,56,57]. The results obtained indicate that these components remain chemically stable throughout the sintering process, and no undesirable interphase formation occurs between the matrix and reinforcement phases. This is a positive indicator for maintaining structural integrity and achieving the targeted material properties [58].

With increasing Sm₂O₃ reinforcement, the intensity of the peaks belonging to the Sm₂O₃ phase in the XRD patterns increased significantly. Simultaneously, the peak intensity associated with the B₄C phase showed a marked reduction, whereas the peaks corresponding to the Al phase remained largely unchanged. In composites containing less than 5% Sm₂O₃, the peak intensity of this phase was very low, making it difficult to detect via XRD. However, when the Sm₂O₃ content was increased to 7% and 9%, the peaks corresponding to the respective phase became visible. This can be attributed to the fact that the increasing Sm₂O₃ reinforcement, along with decreasing B₄C content, creates a new phase distribution and morphology in the composite matrix. This structural change is reflected in the XRD data by affecting the distribution of phases in the matrix.

XRD data highlighted that a greater amount of Sm₂O₃ made its presence in the composite matrix more noticeable. The SEM images in Figure 4 further verified this by depicting a more homogeneous spread of Sm₂O₃ particles and a smoother phase distribution in the microstructure. This indicates that Sm₂O₃ is effectively integrated into the composite structure, contributing to its structural stability. Together, these results provide important insights into how Sm₂O₃ reinforcement plays a vital role in the composite, significantly altering the phase distribution as its content increases.

3.2. SEM-EDX Analysis

Scanning electron microscopy (SEM) was employed to closely analyze the microstructural homogeneity and phase distribution of Al-B₄C-Sm₂O₃ composite powders fabricated in varying proportions using the mechanical milling process. The micrographs presented in Figure 4 show the overall phase distribution of the composites in the left column and local details of the surface morphology in the right column. An analysis of the images revealed that B₄C particles, in particular, are clustered in certain regions. This is attributed to the limited wettability of the Al₆₀₆₁ matrix with B₄C and points to dispersion problems often encountered in composites with high B₄C reinforcement. The results show that Sm₂O₃ reinforcement contributes to the formation of a more homogeneous microstructure by reducing the heterogeneous dispersion [38,39].

However, increasing the Sm₂O₃ reinforcement in the matrix reduces the agglomerations and leads to a more homogeneous appearance. This suggests that Sm₂O₃ reinforcement acts as a binder, addressing the low wettability issue in the Al/B₄C system and contributing to a more even distribution of B₄C particles in the microstructure [32]. As a result, as shown in Figure 4b,c increasing the Sm₂O₃ reinforcement leads to a more homogeneous and compact structure with significantly reduced agglomeration. This improvement is primarily due to Sm₂O₃’s ability to occupy interstitial voids in the matrix, thereby reducing porosity. Additionally, Sm₂O₃ exhibits better wettability and interfacial bonding with the Al matrix compared to B₄C. This facilitates improved densification during sintering and enhances the overall mechanical integrity by minimizing pore formation. These findings emphasize the key role of Sm₂O₃ in bonding behavior and microstructural refinement, which ultimately improves both the mechanical and physical properties. Furthermore, Sm₂O₃ addition ensures a more uniform distribution of B₄C particles, contributing to enhanced fast neutron and gamma-ray-shielding capabilities. However, due to the decrease in B content with the increasing Sm₂O₃ addition, a partial reduction in thermal neutron-shielding efficiency was observed, as boron is the primary contributor to thermal neutron absorption. The microstructural improvements observed in the SEM analysis are consistent with the density measurements, confirming the correlation between the structural compactness and increased density. Therefore, it can be concluded that the enhanced shielding performance is attributed not only to the intrinsic properties of Sm₂O₃ but also to its positive effect on reducing porosity and improving the uniformity of B₄C particle dispersion within the matrix.

Figure 4. SEM micrographs of (a,a’) None, (b,b’) 5Sm₂O₃, and (c,c’) 9Sm₂O₃ composites.

Figure 4. SEM micrographs of (a,a’) None, (b,b’) 5Sm₂O₃, and (c,c’) 9Sm₂O₃ composites.

SEM-EDX analysis was conducted on the None, 5Sm₂O₃, and 9Sm₂O₃ composites, with the results shown in Figure 5, Figure 6 and Figure 7. The SEM images, along with the mapping data, demonstrate that Sm₂O₃ enhances the microstructure by ensuring a more homogeneous material distribution. Sm₂O₃ particles surround the B₄C particles, effectively eliminating the poor wettability between Al₆₀₆₁ and B₄C. The initial powders were confirmed via EDX analysis. However, light elements like boron are complex to analyze for EDX [59].

These elements’ low atomic number and weak X-ray emission impede accurate detection at low concentrations. Light elements, such as lithium and beryllium, also encounter difficulties similar to those experienced in EDX quantification. Due to these limitations, alternative analytical methods might be required for an accurate quantitative analysis of light elements. This limitation is likely responsible for the discrepancy between the initial composition ratios and the EDX findings.

3.3. Density and Microhardness Measurements

Figure 6a illustrates the relative density and hardness values of the Al-B₄C-Sm₂O₃ composites. Each sample was measured at least ten times, and the average values are reported. The findings reveal that incorporating Sm₂O₃ reinforcement markedly enhances hardness. For example, the hardness of the composite without Sm₂O₃ (47.47 HV) rises to 81.69 HV with the addition of 9% Sm₂O₃. This improvement can be attributed to Sm₂O₃ filling the voids in the composite matrix, leading to a more compact and homogeneous structure.

The literature cites numerous sources highlighting the difficulty in achieving good wettability between Al₆₀₆₁ and B₄C [34,40,41,42]. Problems related to poor wettability have often been encountered in the production of Al-B₄C composites. Nevertheless, in line with the density measurements, the hardness results indicate that Sm₂O₃ serves as an effective binder, helping to overcome the challenges caused by poor wettability. Despite B₄C being a rigid material, the increased hardness with a higher Sm₂O₃ content reinforces its critical role as a binder in enhancing the composite’s microstructure.

SEM observations and density measurements support the increase observed in hardness values. Adding Sm₂O₃ reduces the particle agglomeration of B₄C, encouraging a more even distribution throughout the matrix. As a result, the microstructure improves, raising the composite’s mechanical performance and overall durability. Moreover, the observed decrease in the hardness measurement standard deviation in composites with 7% and 9% Sm₂O₃ implies that the material properties become more homogeneous at these levels of reinforcement. This finding highlights the effectiveness of Sm₂O₃ in achieving a more homogeneous structure, contributing to improved mechanical properties and consistency within the composite material.

The wear rate and resistance values of Al-B₄C-Sm₂O₃ composites (X = 0–9) are presented in Figure 6b. These data show that increasing Sm₂O₃ reinforcement significantly decreases the wear rate and improves wear resistance. In particular, the sample containing 9% Sm₂O₃ had the highest wear resistance, while the unreinforced None composite exhibited the lowest wear resistance.

The scanning electron microscopy (SEM) images presented in Figure 7 reveal the microstructural features of the worn surfaces in detail. In the None composite without Sm₂O₃, particle breakage and irregular wear scars were observed due to insufficient coalescence. This can be attributed to the weak interfacial bonds between the matrix and the reinforcement phase. With an increase in the Sm₂O₃ ratio, a more homogeneous and compact microstructure was formed in the composites. This homogeneity contributes to the improvement of wettability and strengthening of interfacial bonds between the matrix and reinforcement phase. As a result, increasing the Sm₂O₃ content allowed the wear process to be more stable and controlled.

These findings suggest that Sm₂O₃ acts as a binder, strengthening the matrix and enhancing wear resistance by improving the mechanical properties, including the hardness and density. In particular, strengthening the matrix-reinforcement interface and homogenizing the microstructure are critical in improving wear resistance.

3.4. Electrochemical Corrosion Test

Potentiodynamic polarization curves were used to assess the electrochemical performance of Al-B₄C-Sm₂O₃ composites and to examine how different Sm₂O₃ ratios influence their corrosion behavior. Figure 8a,b display these curves along with the related data. The corrosion current density (I_corr) and corrosion potential (E_corr) values were obtained by extrapolating the cathodic polarization curves to their intersection with the E_corr horizontal line.

The data reveal that Sm₂O₃ reinforcement leads to a positive shift in E_corr values within the Al-B₄C matrix, pointing to a slight enhancement in corrosion performance. Despite this, a significant rise in both I_corr and the corrosion rate occurred with an increasing Sm₂O₃ content. For instance, in the None composite, the I_corr was measured at 72.63 µA/cm²; however, with the addition of Sm₂O₃, this value increased to 274.21 µA/cm² in the 9Sm₂O₃ composite. Correspondingly, the corrosion rate increased from 0.646 mm/year to 2.184 mm/year.

These findings suggest that while the addition of Sm₂O₃ stabilizes the corrosion potential to some extent, it disrupts the continuity of the protective oxide films on the aluminum matrix, significantly increasing the corrosion rate [60,61,62,63]. The discontinuity introduced by Sm₂O₃ particles likely impairs the passive oxide layer, which typically shields the composite from corrosion. Additionally, the addition of Sm₂O₃ induced a micro-galvanic effect by creating micro-cathodic regions with varying electrochemical potentials within the material, resulting in an increased corrosion rate [64,65]. Consequently, although the addition of Sm₂O₃ may enhance some mechanical properties, it negatively impacts the corrosion resistance of Al-B₄C composites by increasing their vulnerability to corrosive environments. In conclusion, the study reveals that increasing the Sm₂O₃ content, although beneficial for mechanical strength, compromises the corrosion behavior of these composites, suggesting a trade-off between enhanced structural properties and reduced corrosion resistance.

3.5. Neutron and Gamma-Radiation-Shielding Properties

The macroscopic cross-section and attenuation rate of thermal neutrons in 2 mm-thick Al-B₄C-Sm₂O₃ composites are presented in Figure 9a. Initially, the thermal neutron macroscopic cross-section of the None composite was measured at 23.14 cm⁻¹. However, as the Sm₂O₃ content increased, this value gradually decreased to 22.55 cm⁻¹, 22.42 cm⁻¹, 21.29 cm⁻¹, 20.19 cm⁻¹, and 18.97 cm⁻¹ with 1%, 3%, 5%, 7%, and 9% Sm₂O₃ additions, respectively.

The decrease in the thermal neutron macroscopic cross-section had a direct effect on the neutron-attenuation rate of the composites. While the composite without Sm₂O₃ achieved a 99.02% attenuation rate at a 2 mm thickness, this value declined to 98.9%, 98.87%, 98.58%, 98.24%, and 97.75% as the Sm₂O₃ reinforcement rose from 1% to 9%.

The reduction observed in the macroscopic thermal neutron cross-section and attenuation rate, even though samarium possesses a higher inherent thermal neutron cross-section than boron, is likely due to variations in the equivalent content of the two elements. Specifically, Sm’s neutron-absorption ability is lower on a per-volume basis than B, given that Sm has an equivalent B value of 0.559. Consequently, substituting B₄C with Sm₂O₃ in the composite leads to a relative decrease in neutron-absorption performance as the Sm₂O₃ content increases [66].

The fast neutron macroscopic cross-section and attenuation rates for 2 cm Al-B₄C-XSm₂O₃ composites, with X ranging between 0% and 9% Sm₂O₃, are shown in Figure 9b. The fast neutron cross-section of the pure Al-B₄C composite was measured at 0.18 cm⁻¹. With the addition of 1% Sm₂O₃, this value slightly decreased to 0.18 cm⁻¹. However, as the Sm₂O₃ content increased, the fast neutron cross-section rose, reaching a maximum of 0.19 cm⁻¹ with 9% Sm₂O₃ addition.

Regarding fast neutron-attenuation rates, the highest attenuation was observed in the Al-B₄C composite containing 9% Sm₂O₃, where the attenuation rate reached a peak of 32.15%. This observation suggests that increasing the Sm₂O₃ reinforcement up to 9% enhances the attenuation of fast neutrons, consistent with the results observed in the thermal neutron macroscopic cross-section. Nevertheless, exceeding this concentration results in a diminished rate of improvement in fast neutron attenuation, implying the existence of an optimal Sm₂O₃ reinforcement level for effective neutron absorption.

In addition to the importance of the compositional elements, composites with a high relative density and low porosity exhibit superior neutron-attenuation and gamma-ray-absorption properties. The relative density is directly related to the neutron- and gamma-ray-shielding performance; specifically, an increased relative density enhances fast neutron attenuation, as well as the linear attenuation coefficient (LAC) for gamma rays. In this context, sintering parameters play a critical role in optimizing the shielding capacity by ensuring an appropriate density and structural integrity.

As shown in Figure 9c, the linear attenuation coefficient (LAC) and photon attenuation ratios of Al-B₄C-Sm₂O₃ composites were evaluated at a photon energy of 0.478 MeV. The variable X represents the varying Sm₂O₃ reinforcement, with the unreinforced (None) composite exhibiting an initial LAC value of 0.19 cm⁻¹. This value increased significantly with the addition of Sm₂O₃ and reached a maximum of 0.22 cm⁻¹ at a 7% Sm₂O₃ reinforcement. Similarly, in terms of the photon-attenuation rate, the None composite initially had an attenuation rate of 32.28%, which increased by 12.55% to 36.33% with the addition of 7% Sm₂O₃. These findings indicate that Sm₂O₃ reinforcement significantly enhances the photon-attenuation capacity of the composite, thereby providing advantages for applications that require enhanced radiation shielding.

Table 3. Calculated half-value layer (HVL), mean free path (MFP), and mass attenuation coefficient (MAC) results for the Al-B₄C-Sm₂O₃ composites.

Materials Code	MAC (cm2/g)	MFP (cm)	HVL (cm)
None	0.085	5.131	3.556
1Sm2O3	0.085	5.093	3.529
3Sm2O3	0.085	4.776	3.310
5Sm2O3	0.086	4.666	3.233
7Sm2O3	0.086	4.557	3.158
9Sm2O3	0.087	4.427	3.068

shows the MAC, MFP, and HVL values of the Al-B₄C-Sm₂O₃ composite. Since the density of the composite increases with Sm₂O₃ addition, it was calculated that the LAC value of the composite increased by 15.8% and the MAC value increased by 0.84% with 9%Sm₂O₃ reinforcement; 9% Sm₂O₃ addition decreased the MFP by 13.71%, and the HVL decreased from 3.55 cm to 3.07 cm.

Summarily, the enhanced gamma-ray and fast neutron attenuation in Sm₂O₃-reinforced composites is primarily attributed to the higher atomic number (Z = 62) and density of Sm₂O₃, which increase the probability of photon interactions (e.g., Compton scattering) and neutron inelastic scattering. Additionally, the reduced porosity achieved through improved microstructural densification further enhances the shielding by limiting radiation pathways and scattering losses. Conversely, the slight decrease in thermal neutron attenuation with increasing Sm₂O₃ reinforcement results from the reduced boron concentration, since boron-10 has a superior thermal neutron-absorption cross-section. This interplay between the composition and microstructure highlights the importance of balancing boron and Sm₂O₃ reinforcement to achieve effective multi-spectrum radiation shielding.

These simulation outcomes were strongly supported by experimental observations of microstructure and density, which informed the model design and validated its predictive capability.

4. Conclusions

For the first time, Al-B₄C-(30-x)Sm₂O₃ (x = 0, 1, 3, 5, 7, and 9) composites were successfully designed and fabricated. The impact of Sm₂O₃ reinforcement on the hardness, density, wear resistance, corrosion resistance, and effectiveness of the materials as gamma and neutron shields was evaluated.

Mechanical tests revealed that the relative density of the composites increased from 85.24% to 91.24%, while the actual measured density rose from 2.25 g/cm³ to 2.58 g/cm³ with increasing Sm₂O₃ reinforcement. Similarly, the hardness improved from 47.47 HV (no Sm₂O₃) to 81.69 HV (with 9 wt.% Sm₂O₃). This enhancement is attributed to Sm₂O₃’s role in filling voids within the matrix, leading to a denser and more homogeneous microstructure. Wear testing showed a clear trend: higher Sm₂O₃ reinforcement reduced the wear rate and significantly increased wear resistance. The 9 wt.% Sm₂O₃ sample demonstrated the best performance in this regard.

Microstructural characterizations using XRD confirmed the presence of Al, B₄C, and Sm₂O₃ phases, while SEM analysis illustrated that Sm₂O₃ was homogeneously distributed. This uniform distribution improved the microstructural compactness and mitigated the poor wettability between Al and B₄C, contributing to enhanced wear behavior and mechanical stability.

Electrochemical corrosion testing revealed a nuanced behavior: while the corrosion potential improved slightly, the corrosion rate increased at higher Sm₂O₃ reinforcement. This complex interaction between Sm₂O₃ and the corrosive medium highlights the need for additional investigation and presents novel insight into the corrosion-reinforcement interplay in such hybrid systems.

Radiation shielding simulations using the MCNP6.2 code demonstrated that while the thermal neutron macroscopic cross-section slightly decreased due to the reduced boron content, the addition of Sm₂O₃ significantly enhanced the fast neutron-shielding and gamma-ray-attenuation capabilities. This dual benefit is primarily due to Sm₂O₃’s high atomic number and density, which boost radiation interaction probabilities.

In summary, the results confirm that Sm₂O₃ reinforcement improves the mechanical strength, wear resistance, and fast neutron/gamma-shielding performance, making these composites promising candidates for advanced structural and radiation-protection applications with sustainable design considerations.