Advances in Metal Matrix Composites: Structure, Properties and Applications

1. Introduction

Metal matrix composites (MMCs) are attractive materials due to their unique properties that stem from combining a wide range of matrix materials and reinforcements. The possibility of tailoring the mechanical response, thermophysical properties, chemical and electrical behavior, and wear properties makes MMCs incredibly interesting. The development of such materials has been driven by various applications in different fields, including automotive, aerospace, electrical and electronic applications. The challenge of creating a sustainable society lies in the recyclability of such materials.

Given the fast evolution of these materials and the numerous combination possibilities, the Special Issue “Advances in Metal Matrix Composites: Structure, Properties and Applications” [1] aimed to gather studies on the most recent advances in the field. Articles focusing on material modeling, microstructural characterization, wear testing, and mechanical and thermophysical performance at both room and elevated temperatures are welcome. Articles on the sustainability of MMCs, either by recycling the composite or using recycled matrices and reinforcements, are highly encouraged.

2. An Overview of Published Articles

An overview of the articles included in the present Special Issue is provided here, along with a brief description of the focus for each article. Among the review article and eight research articles, a variety of metal matrices and reinforcements are presented. Similarly, there is a wide variety of different applications for which each composite system is particularly suitable and of interest.

Table 1. List of the articles included in this Special Issue.

Article	MMC Matrix	MMC Reinforcement	Investigations
Chen et al. [2]	Copper (Cu)	Diamond particles	Literature review
Ghahremani et al. [3]	Aluminum-silicon (Al-Si)	Silicon oxide nanoparticles (SiO2np)	Microstructure Wear
Golla et al. [4]	Aluminum-iron (Al-Fe)	Titanium carbide particles (TiCp)	Mechanical properties Wear
Du et al. [5]	Aluminum-silicon (Al-Si)	Silicon carbide particles (SiCp)	Microstructure Transfer efficiency
Hirsch et al. [6]	Aluminum-silicon (Al-Si)	Silicon carbide particles (SiCp)	Mechanical properties Heat treatment
Lewis et al. [7]	Aluminum (Al)	Silicon (Si)	Thermophysical properties
Yin et al. [8]	Magnesium-erbium (Mg-Er)	Nickel and zinc phases (Ni, Zn)	Microstructure Heat treatment
Zhuo et al. [9]	Tungsten-copper (W-Cu)	Tungsten fibers (Wf)	Mechanical properties

presents a summary of the main features investigated in the articles.

The review article by Chen et al. [2] focused on copper-based diamond composites, which have been the subject of numerous investigations for applications requiring higher thermal conductivity. However, the natural non-wetting behavior between diamond particles and copper matrix makes it challenging to fabricate copper-based diamond composites with high thermal conductivity. Thus, to promote wettability between copper and diamond particles, the copper/diamond interface must be modified by coating alloying elements on the diamond surface or by adding active alloying elements with carbon in the copper matrix. In this paper, we review the research progress on copper-based diamond composites, including theoretical models for calculating the thermal conductivity and the effect of process parameters on the thermal conductivity of copper-based diamond composites. The factors that affect interfacial thermal conductivity are emphatically analyzed in this review. Finally, the current problems of copper-based diamond composites and future research trends are recommended.

The research article by Ghahremani et al. [3] focuses on metal matrix nanocomposites (MMNCs), which are becoming materials of choice in various engineering and medical applications due to their superior combination of targeted properties. Among different MMNCs, aluminum-based composites are of particular importance. In many applications, relatively inferior wear limits the practical use of this valued metal. However, reinforcing aluminum and its alloys with ceramics, such as carbon allotropes, may largely circumvent these limitations. In the present study, an aluminum alloy A356/SiO₂ nanocomposite is fabricated using a vibration-assisted casting process, in which varying amounts of silica (SiO₂) nanoparticles, ranging from 0.125 to 0.375 wt.%, are added to the molten alloy. The use of ultrasonic power treatment had a significant influence on the microstructure, hardness, and wear properties. Microstructural and XRD analyses were performed on the fabricated monolithic and composite samples. To evaluate wear behavior, a hardness test and a pin-on-disk experiment were conducted on the samples under 60, 80, and 100 N forces at a constant speed of 1 m/s, with the sliding distance varying from 1000 to 2000 m. The abraded surfaces, wear debris, and EDS analysis were used to identify wear mechanisms. The samples with 0.125 wt.% exhibited the highest increase in hardness and the highest reductions in both friction coefficient and wear rate, by 52%, 50%, and 68%, respectively. The primary governing wear mechanism was abrasion, with limited evidence of delamination.

The research article by Golla et al. [4] highlights the development of high-performance materials in the construction materials industry, with a special focus on the production of aluminum matrix composites (AMCs) containing titanium carbide (TiC) particles. The stir casting method with ultrasonic assistance was employed to enhance the mechanical and tribological properties. ASTM standards were used to evaluate the influence of TiC particles on density, hardness (VHN), ultimate tensile strength (UTS), and wear resistance at various TiC weight fraction percentages (0.0 wt.%, 2.0 wt.%, 4.0 wt.%, 6.0 wt.%, and 8.0 wt.%). Field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) analysis were conducted to investigate the microstructural changes and elemental phases present in the synthesized composite. Results revealed that incorporating 8 wt.% TiC reinforcement into the metal matrix composites demonstrated significant improvements compared to the base alloy. A substantial enhancement in hardness by 32%, a notable increase of 68% in UTS, and a significant 80% rise in yield strength were observed when contrasted with the pure aluminum alloy. The tensile fracture analysis of the specimens revealed the presence of dimples, voids, and cracks, suggesting a brittle nature. To assess the wear characteristics of the composites, dry sliding wear experiments were performed using a pin-on-disk wear tester. Incorporation of TiC particles resulted in a lower coefficient of friction than the base alloy, with the lowest friction coefficient being recorded at 0.266 for 6 wt.% TiC, according to the data. FESEM and energy-dispersive X-ray spectroscopy (EDXS) were used to examine the surfaces of the worn pin. Overall, the inclusion of TiC reinforcement particles in the matrix alloy greatly enhanced the wear resistance and friction coefficient of the Al-6TiC composites. Plowing and adhesion occurred at lower loads, while delamination occurred at higher loads, as observed in the wear test.

The research article by Du et al. [5] examines lightweight SiC-particle-reinforced aluminum composites that have the potential to replace cast iron in brake disks, especially for electric vehicles. This study investigates the effect of SiC particle size and matrix alloy composition on the resulting transfer efficiency and particle distribution. The performance of a specially designed stirring head was studied using a water model, and the stirring head conditions were evaluated to understand the mechanisms of particle transfer and dispersion in molten aluminum. The standard practice of thermal pre-treatment promotes the wetting of the reinforcing particles and commonly causes clustering before the particles are added to the melt. This early clustering affects the transfer efficiency and particle dispersion, where their interaction with the melt’s top-surface oxide skin plays a crucial role. Additionally, the transfer efficiency was linked to both the particle size and the chemical composition of the matrix alloy. Smaller particles aggravated the degree of clustering, and the addition of rare earth elements as alloying elements in the matrix alloy affected the particle dispersion. The stirring parameters should be selected to ensure cluster disruption when the carbides are added to the melt.

The research article by Hirsch et al. [6] investigates the solid-state sintering of cast aluminum powders using resistance heating sintering (RHS), also known as spark plasma sintering or the field-assisted sintering technique, which creates a very fine microstructure in the bulk material. This results in high-performance material properties, with improved strength and ductility compared to conventional production routes for the same alloys. In this study, the mechanical behavior of an RHS-sintered age-hardenable A357 (AlSi7Mg0.6) cast alloy and a SiCp/A357 aluminum matrix composite (AMC) was investigated. Aiming for high strength and good wear behavior in tribological applications, the AMC was reinforced with a high particle content (35 vol.%) of a coarse particle fraction (d50 = 21 µm). Afterwards, the separated and combined effects of particle reinforcement and heat treatment were studied under compressive load at both room temperature and 350 °C. At room temperature compression, the strengthening effect of precipitation hardening was about twice as high as that for the particle reinforcement, despite the high particle content. At elevated temperatures, the compressive deformation behavior was characterized by the simultaneous occurrence of temperature-activated recovery, recrystallisation, and precipitation processes. The occurrence and interaction of these processes were significantly affected by the initial material condition. Moreover, a rearrangement of the SiC reinforcement particles was detected after hot deformation. This rearrangement resulted in a homogenized dispersion of the reinforcement phase, with minimal particle fragmentation, offering potential for secondary thermo-mechanical processing of highly reinforced AMCs.

The research article by Lewis et al. [7], given the ever-growing emphasis on global decarbonization and rapid increases in the power densities of electronics equipment in recent years, studies the new methods and lightweight materials developed to manage heat load as well as interfacial stresses associated with coefficient of thermal expansion (CTE) mismatches between components. The Al–Si system offers an attractive combination of CTE performance and high thermal conductivity, while being a lightweight option. Such materials are of interest to industries where thermal management is a key design criterion, such as the aerospace, automotive, consumer electronics, defense, EV, and space sectors. This paper will describe the development and manufacture of a family of high-performance hypereutectic Al–Si alloys (AyontEX™) by a powder metallurgy method. These alloys are of particular interest for structural heat sink applications that require high reliability under thermal cycling (CTE of 17 μm/(m·°C)), as well as reflective optics and instrument assemblies that require good thermal and mechanical stability (CTE of 13 μm/(m·°C)). Critical performance relationships are presented, coupled with the microstructural, physical, and mechanical properties of these Al–Si alloys.

The research article by Yin et al. [8] investigates the influence of transition metals, such as nickel (Ni) and zinc (Zn), on the formation, morphology, and thermal stability of long-period stacking ordered (LPSO) phases in Mg₉₇Er₂Ni₁ and Mg₉₇Er₂Zn₁ alloys. In the as-cast state, both alloys consist of α-Mg and LPSO phases. Heat treatment at 540 °C for 20 h dissolves block-like and lamellar LPSO phases into the α-Mg matrix in the Mg₉₇Er₂Zn₁ alloy, with lamellar LPSO phases reprecipitating during subsequent cooling from 540 °C to 400 °C. A comparative analysis reveals that nickel significantly enhances the thermal stability of the LPSO phase compared to zinc. Nickel favors the formation of block-shaped LPSO phases, while zinc facilitates lamellar LPSO precipitation within the α-Mg matrix. The LPSO phase in the Mg₉₇Er₂Ni₁ alloy exhibits an exceptionally high melting temperature of 605 °C, the highest reported for an LPSO phase. Additionally, heat treatment at 500 °C for 100 h preserves the area fraction of the LPSO phase in the Mg₉₇Er₂Ni₁ alloy. At 540 °C for 100 h, the LPSO grains grow without phase dissolution or structural transformation of their 18R-type configuration. These findings provide valuable insights into the role of alloying transition metal elements in controlling the stability and morphology of LPSO phases, offering pathways for tailoring the morphology of the LPSO phase in the Mg-based alloys.

The research article by Zhuo et al. [9] investigates tungsten fiber (Wf) reinforced tungsten–copper (W-Cu) composites, which have broad application prospects in fields such as electronic packaging due to their excellent comprehensive properties. However, the correlation between the fiber parameters (content, aspect ratio, and orientation) and the mechanical behavior of the materials remains unclear. A combination of numerical simulation and experimental research was employed to construct a three-dimensional microstructural mechanism model and systematically investigate the influence of fiber parameters on the tensile properties and mechanisms of Wf/W-Cu composites. The results show that the critical fiber aspect ratio is 7.6. Below this value, the fiber pullout mechanism dominates, whereas above this value, fiber tensile fracture is the primary failure mechanism. As the fiber content increases from 1% to 6%, the tensile strength of the composite increases by 9.6%, and the yield strength increases by 10.2%, while the elongation after fracture decreases by 18.6%. As the fiber orientation angle increases from 0° to 90°, the material’s strength initially increases and then decreases, whereas its toughness initially decreases and then increases. Short fibers achieve interface toughening through fiber pullout, crack deflection, and fiber bridging, while long fibers enhance the strength and toughness of the composite through load transfer and the fiber bridging effect. The damage evolution mechanism reveals the regulatory impact of fiber parameters on the multiscale mechanical behavior of the material. The research results can guide the optimization design of the composition and structure of Wf/W-Cu composites, providing new insights into the study of high-performance fiber composites and having significant implications for their engineering applications in extreme environments.

3. Summary

The current Special Issue “Advances in Metal Matrix Composites: Structure, Properties and Applications” collects research and review contributions about metal matrix composites. These materials are particularly attractive due to their unique and tailored properties from the combination of a wide range of matrix materials and reinforcements. The review article and eight research articles included in this Special Issue effectively reflect this variety of possibilities, exploring different combinations of metal matrices and reinforcements. Consequently, the articles focus on a wide variety of different applications and industrial sectors for which each composite system is particularly suitable and of interest.