Optimizing mechanical properties of AA7075 Metal Matrix Composites reinforced with ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ particulates

Department of Aeronautical and Automobile Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education (MAHE), Manipal, Karnataka, India
balbir.s@manipal.edu, https://orcid.org/0000-0001-9591-3694

Advancements in the material properties of defense, aerospace, automobile, aviation, marine, and structural industries are the current fields of interest for many researchers and scientists across the globe . Davis et al. [1] briefs about aluminum and its properties. All types of aluminum alloys (cast and wrought alloys) have been discussed in this paper by stating their chemical composition, mechanical properties, and applications. Advanced materials refer to those with enhanced properties that out-master the properties of the present materials. Aluminum is widely known and used for its properties, which include good formability, corrosion resistance, high electrical and thermal conductivity, good finish ability, and non-pyrophoric [2-3].
Aluminum alloys or aluminum matrix composites (AMCs) have a phenomenally high strength-to-weight ratio, opening doors for various engineering applications. Due to its solid mechanical qualities, AA7075, having zinc as the primary alloying element, is employed in multiple industries, including aerospace, automotive, marine, and structural applications. Wallace and Beddoes [2] employed transmission electron microscopy to assess a heat-treatment procedure that enhances stresscorrosion cracking resistance without losing yield strength in AA7075. According to the findings, heat treatment (as retrogression and re-aging) results in enormous grain-boundary and coherent matrix precipitates. AMCs enhance the base metal's properties by adding reinforcements of different compositions. AMCs have good thermal conductivity and machinability and accept all manufacturing techniques.
When added to matrix material as reinforcement, ceramics improve the tribological and mechanical properties according to their compositions. Ceramics are well known for their high creep resistance and have boiling points ranging from $1000{}^{\circ }\mathrm{C}$$1000{}^{\circ }\mathrm{C}$1000^(@)C1000{ }^{\circ} \mathrm{C}$1000{}^{\circ }\mathrm{C}$ to ${3000}^{\circ }\mathrm{C}$${3000}^{\circ }\mathrm{C}$3000^(@)C3000^{\circ} \mathrm{C}${3000}^{\circ }\mathrm{C}$. Ceramic reinforcements are classified into oxides, nitrides, borides, and carbides. Researchers have reported that ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ceramic particles improve the mechanical and tribological properties such as modulus, creep resistance, wear resistance, fatigue properties, hardness, and yield strength of the base metal matrix [3]. Titanium boride ( ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ ) is known for its strength, chemical inertness, and wear-resisting properties. It has a density of $4.52\mathrm{g}/{\mathrm{cm}}^3$$4.52\mathrm{g}/{\mathrm{cm}}^3$4.52g//cm^(3)4.52 \mathrm{~g} / \mathrm{cm}^{3}$4.52\mathrm{g}/{\mathrm{cm}}^3$. Due to its high hardness, it opens applications for ballistic armor, composite materials, and various cutting tools. It exhibits a melting point of ${2970}^{\circ }\mathrm{C}$${2970}^{\circ }\mathrm{C}$2970^(@)C2970^{\circ} \mathrm{C}${2970}^{\circ }\mathrm{C}$ and a hardness of 1800 HV . Meti et al. [4] stated the importance of ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ reinforcement particles in the AA7075 matrix processed through ultrasound casting. By reinforcing ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ particles, the hardness and wear resistance of the matrix were increased. Further, treating the composite with an ultrasound technique increased the mechanical and tribological properties due to refinement in the grain structure and uniform distribution of particles in the metal matrix [5]. Rajan et al. [6] synthesized the $\mathrm{AA}7075/{\mathrm{TiB}}_2$$\mathrm{AA}7075/{\mathrm{TiB}}_2$AA7075//TiB_(2)\mathrm{AA} 7075 / \mathrm{TiB}_{2}$\mathrm{AA}7075/{\mathrm{TiB}}_2$ composite using an in-situ salt-melt reaction technique. ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ reinforcement particles were synthesized using ${\mathrm{K}}_2{\mathrm{TiF}}_6$${\mathrm{K}}_2{\mathrm{TiF}}_6$K_(2)TiF_(6)\mathrm{K}_{2} \mathrm{TiF}_{6}${\mathrm{K}}_2{\mathrm{TiF}}_6$ and ${\mathrm{KBF}}_4$${\mathrm{KBF}}_4$KBF_(4)\mathrm{KBF}_{4}${\mathrm{KBF}}_4$ salts. SEM micrographs indicated a homogeneous distribution of reinforcing particles with a muscular bonding strength and a distinct interface. The addition of ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ reinforcement particles improved the mechanical behaviour of the $\mathrm{AA}7075/{\mathrm{TiB}}_2$$\mathrm{AA}7075/{\mathrm{TiB}}_2$AA7075//TiB_(2)\mathrm{AA} 7075 / \mathrm{TiB}_{2}$\mathrm{AA}7075/{\mathrm{TiB}}_2$ composite.
Zirconium dioxide (zirconia: ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ) ceramic powder is generally used in dental applications. Unlike other ceramics, zirconia has a high hardness, which makes it brittle. Zirconia possesses high strength, fracture toughness, and wear resistance and can hold high temperatures up to ${2400}^{\circ }\mathrm{C}$${2400}^{\circ }\mathrm{C}$2400^(@)C2400^{\circ} \mathrm{C}${2400}^{\circ }\mathrm{C}$. Zirconia is known as 'ceramic steel' as its modulus of elasticity is similar to steel. In a related study, Reddy et al. [7] investigated the effect of varying ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ concentrations on the mechanical and wear behavior of AA7075-based composites. Their scanning electron microscopy (SEM) analysis confirmed a homogeneous distribution of reinforcement particles throughout the matrix. The results revealed notable improvements in both tensile strength and surface hardness, with the AA7075/1.5 wt% ZrO2 composite exhibiting optimal performance metrics among the compositions studied.
Hybrid composites are in high demand worldwide due to the wide range of emerging applications in engineering. The market is for new-generation material, which holds a high strength-to-weight ratio. Hybrid metal matrix composites (HMMCs) are advanced engineered materials composed of two or more distinct reinforcements i.e. metallic, ceramic, or non-metallic which arestrategically integrated within a metal matrix to achieve synergistic improvements in performance. As highlighted by Zhou et al. [8], HMMCs demonstrate superior mechanical robustness and tribological resilience compared to their monolithic or single-reinforcement counterparts. The study also provides a comparative overview of fabrication techniques,such as stir casting, powder metallurgy, and squeeze casting specificallyemployed in synthesizing hybrid MMCs, each influencing the composite's microstructure and resultant properties. Moreover, the inclusion of diverse reinforcements has been shown to significantly impact mechanical strength, electrical conductivity, and thermal stability. Owing to this multi-functional enhancement, hybrid MMCs are increasingly considered promising alternatives to conventional composites for demanding applications across aerospace, automotive, and electronic sectors. Hybrid composites are in high demand worldwide due to wide branches emerging applications in engineering. One of the most important requirements for
engineering applications is high creep resistance, which is not satisfied by an alloy. Composites can stabilize this drawback. Because of their high strength, stiffness, low weight, and creep resistance, composites can be used in a variety of applications, including defense, aerospace, marine, and structural applications. Aluminum hybrid composites are prominent in these applications as they are highly efficient, low-cost, and easy to adapt to various manufacturing techniques.
Manufacturing techniques used to fabricate any composite plays an essential role in determining the final properties of the composites. There are different ways to process AMCs, such as liquid, solid, and deposition. Research denotes that liquidstate and solid-state processing are extensively used for manufacturing AMCs. Stir casting is designated as the most economical method to fabricate AMCs. Here, the reinforcements are directly introduced into the molten metal. The major challenge in stir casting is that the reinforcement particles tend to sink due to their density relative to the molten metal [89]. According to Kumar et al. [9], the stir casting process remains the most cost-effective and scalable technique for fabricating metal matrix composites (MMCs), particularly advantageous for large-scale industrial production. Aluminum matrix composites (AMCs), owing to their exceptional mechanical strength and superior wear resistance, have garnered significant interest in high-performance structural applications. The authors emphasized that stir casting promotes uniform dispersion of reinforcement particulates within the matrix, enhances wettability between the matrix and reinforcements, and effectively minimizes porosity-thereby contributing to improved interfacial bonding and overall composite integrity. Meti et al. [7] suggested different processing techniques to develop AMCs. AMCs containing different compositions of reinforcements showed better results using liquid-state techniques. Good bonding and straightforward interface with uniform distribution of reinforcements were seen through liquid state techniques. Further, when treating the composite with the ultrasound technique, there was an enhancement in the mechanical and tribological characteristics of the matrix material. They stated that ductility and wear rate decrease as the fraction of reinforcements increases. Prakash and Binay Kumar [10] conducted a comprehensive investigation into the influence of zirconium diboride ( ${\mathrm{ZrB}}_2$${\mathrm{ZrB}}_2$ZrB_(2)\mathrm{ZrB}_{2}${\mathrm{ZrB}}_2$ ) at varying concentrations ( $1-5\mathrm{wt}.\mathrm{\%}$$1-5\mathrm{wt}.\mathrm{\%}$1-5wt.%1-5 \mathrm{wt} . \%$1-5\mathrm{wt}.\mathrm{\%}$ ) combined with $2\mathrm{wt}.\mathrm{\%}$$2\mathrm{wt}.\mathrm{\%}$2wt.%2 \mathrm{wt} . \%$2\mathrm{wt}.\mathrm{\%}$ fly ash on the mechanical and tribological performance of AA7075-based hybrid metal matrix composites (HMMCs) subjected to T 6 heat treatment. Their findings revealed that the composite reinforced with $5\mathrm{wt}.\mathrm{\%}{\mathrm{ZrB}}_2$$5\mathrm{wt}.\mathrm{\%}{\mathrm{ZrB}}_2$5wt.%ZrB_(2)5 \mathrm{wt} . \% \mathrm{ZrB}_{2}$5\mathrm{wt}.\mathrm{\%}{\mathrm{ZrB}}_2$ exhibited a remarkable enhancement in tensile strength and hardness, recording improvements of up to $77\mathrm{\%}$$77\mathrm{\%}$77%77 \%$77\mathrm{\%}$ and $15\mathrm{\%}$$15\mathrm{\%}$15%15 \%$15\mathrm{\%}$, respectively, when benchmarked against the unreinforced AA7075 alloy. Additionally, under a 40 N load condition, the same composite demonstrated a significant $46.94\mathrm{\%}$$46.94\mathrm{\%}$46.94%46.94 \%$46.94\mathrm{\%}$ reduction in specific wear rate and a noticeable decrease in the coefficient of friction. These enhancements are attributed to the synergistic effect of hard ceramic reinforcements and the optimized microstructural characteristics induced by the heat treatment process.
This study is primarily focused on the fabrication of hybrid aluminum matrix composites (AMCs) reinforced with titanium diboride ( ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ ) and zirconium dioxide ( ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ) particulates, aiming to elucidate the interfacial interactions between the ceramic reinforcements and the aluminum matrix. The investigation systematically evaluates the mechanical and tribological performance of hybrid composites with varying reinforcement compositions. Addressing common limitations of conventional composites, this work targets enhancements in toughness, fatigue resistance, corrosion stability, and manufacturability, while enabling tailored property combinations that surpass those achievable by monolithic metals, ceramics, or polymers. The specific objectives include: (1) synthesizing ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$-reinforced hybrid AMCs via controlled processing techniques; (2) comprehensive evaluation of their mechanical properties-including tensile strength, hardness, and wear resistance; and (3) detailed microstructural characterization to correlate reinforcement distribution and morphology with the observed performance across different composite formulations.
One of the most important aspects of this paper is its amendment made when compared to the existing research by the authors, reference [17]. The work here introduces several key amendments compared to the previous version [17], reflecting a shift in focus and enhancement in technical detail. One of the most notable changes is the emphasis on the stir casting process as the primary fabrication method for the hybrid metal matrix composites (HMMCs) in detail here, whereas the previous combined stir casting with hot forging and highlighted the microstructural refinement resulting from the forging process. The forging step and its associated grain refinement benefits have been omitted in the new version, signaling a methodological simplification and redirection of the study. Another important amendment is the expansion of the reinforcement composition range. While the previous work [17] investigated ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ at a constant $5\mathrm{wt}\mathrm{\%}$$5\mathrm{wt}\mathrm{\%}$5wt%5 \mathrm{wt} \%$5\mathrm{wt}\mathrm{\%}$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ at $4{\mathrm{wt}}^0\mathrm{\%}$$4{\mathrm{wt}}^0\mathrm{\%}$4wt^(0)%4 \mathrm{wt}^{0} \%$4{\mathrm{wt}}^0\mathrm{\%}$ and $6\mathrm{wt}\mathrm{\%}$$6\mathrm{wt}\mathrm{\%}$6wt%6 \mathrm{wt} \%$6\mathrm{wt}\mathrm{\%}$, the revised version here broadens this scope by including a lower ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ concentration of $2\mathrm{wt}\mathrm{\%}$$2\mathrm{wt}\mathrm{\%}$2wt%2 \mathrm{wt} \%$2\mathrm{wt}\mathrm{\%}$, offering a more comprehensive evaluation of reinforcement variation and its effects on composite properties. The new work here also incorporates quantitative mechanical and wear performance data, which was absent in the earlier version. Specific values are reported for hardness, which increased from 55 Hv to 102.40 Hv (an $85.45\mathrm{\%}$$85.45\mathrm{\%}$85.45%85.45 \%$85.45\mathrm{\%}$ improvement), and yield strength, which rose from 107 MPa to 123 MPa (a 15% increase). Furthermore, the wear rate of the composites is now explicitly measured (155 $\mu \mathrm{m}$$\mu \mathrm{m}$mum\mu \mathrm{m}$\mu \mathrm{m}$ at 10 N load), and a wear mechanism is proposed, stating that the ceramic reinforcements serve as lubricating agents that reduce direct contact between the matrix and the counterface during sliding. This level of analytical depth was not presented in the previous version. In addition to SEM microstructural analysis, the revised version also highlights the EDX elemental
mapping of the optimal $\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5%TiB_(2)//4%ZrO_(2)\mathrm{AA} 7075 / 5 \% \mathrm{TiB}_{2} / 4 \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{\%}{\mathrm{ZrO}}_2$ composition, which confirms the uniform spatial distribution of aluminum, zinc, titanium, boron, and zirconium elements. This mapping validates the successful dispersion of the reinforcement particles within the matrix and supports the observed improvements in mechanical and tribological properties by ensuring consistent phase interaction and load transfer across the composite structure. There is also refinement of the identification of the best-performing composition with more robust justification based on performance metrics.

In this study, AA7075 was utilized as the matrix material, with its chemical composition detailed in $\mathrm{Tab}.1.{\mathrm{TiB}}_2$$\mathrm{Tab}.1.{\mathrm{TiB}}_2$Tab.1.TiB_(2)\mathrm{Tab} .1 . \mathrm{TiB}_{2}$\mathrm{Tab}.1.{\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ Reinforcements were taken according to the design required ( ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ ) was fixed for $5\mathrm{\%}$$5\mathrm{\%}$5%5 \%$5\mathrm{\%}$ in every iteration and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ with variable composition, i.e., $2\mathrm{\%},4\mathrm{\%}$$2\mathrm{\%},4\mathrm{\%}$2%,4%2 \%, 4 \%$2\mathrm{\%},4\mathrm{\%}$, and $6\mathrm{\%}$$6\mathrm{\%}$6%6 \%$6\mathrm{\%}$ ). According to the below-mentioned calculations, a fixed amount of aluminum alloy 7075 (i.e., 300 gms ) was taken in the crucible.
Dimensions of iron mould: $$$$quad\quad$$ length $=170\mathrm{mm}$$=170\mathrm{mm}$=170mm=170 \mathrm{~mm}$=170\mathrm{mm}$, Thickness $=6\mathrm{mm}$$=6\mathrm{mm}$=6mm=6 \mathrm{~mm}$=6\mathrm{mm}$, Width $=80\mathrm{mm}$$=80\mathrm{mm}$=80mm=80 \mathrm{~mm}$=80\mathrm{mm}$
Density = Mass / Volume

Density of AA7075 $=2.81\mathrm{g}/{\mathrm{cm}}^3$$=2.81\mathrm{g}/{\mathrm{cm}}^3$=2.81g//cm^(3)=2.81 \mathrm{~g} / \mathrm{cm}^{3}$=2.81\mathrm{g}/{\mathrm{cm}}^3$, Volume of the mould $=170\ast 80\ast 6\mathrm{mm}$$=170\ast 80\ast 6\mathrm{mm}$=170**80**6mm=170 * 80 * 6 \mathrm{~mm}$=170\ast 80\ast 6\mathrm{mm}$
Using Eqn. $1\to 2.81\mathrm{g}/{\mathrm{cm}}^3=\mathrm{M}(\mathrm{gm})/[170\ast 80\ast 6](\mathrm{mm})$$1\to 2.81\mathrm{g}/{\mathrm{cm}}^3=\mathrm{M}(\mathrm{gm})/[170\ast 80\ast 6](\mathrm{mm})$1rarr2.81g//cm^(3)=M(gm)//[170**80**6](mm)1 \rightarrow 2.81 \mathrm{~g} / \mathrm{cm}^{3}=\mathrm{M}(\mathrm{gm}) /[170 * 80 * 6](\mathrm{mm})$1\to 2.81\mathrm{g}/{\mathrm{cm}}^3=\mathrm{M}(\mathrm{gm})/[170\ast 80\ast 6](\mathrm{mm})$
$\mathrm{M}=230\mathrm{gm}\approx 250\mathrm{gm}$$\mathrm{M}=230\mathrm{gm}\approx 250\mathrm{gm}$M=230gm~~250gm\mathrm{M}=230 \mathrm{gm} \approx 250 \mathrm{gm}$\mathrm{M}=230\mathrm{gm}\approx 250\mathrm{gm}$

Zirconia paste was applied to the mold. Zircon's low wettability allows high precision casting with a good surface finish. It also prevents the melt from sticking to the die and avoids metal penetration into the mold. The melt was poured using the crucible tongs into the pre-heated iron mold and allowed to solidify. Pre-heating of the mold is required for the uniform distribution of the molten melt. After the fabrication, the hybrid composite was subjected to different machining processes to prepare specimens for various testing. Specimens were prepared for microstructural studies, hardness tests, tensile tests, and wear tests according to ASTM standards. It is schematically shown in Figs. 2 and 3.

Microstructural characterization was conducted employing a high-resolution field emission scanning electron microscope (FE-SEM, ThermoFisher Apreo 2S HiVac, USA) following the guidelines stipulated in ASTM E3. This analysis facilitated an in-depth examination of grain morphology, grain boundary delineation, and the spatial distribution and homogeneity of reinforcement particulates within the aluminum matrix composite. A specimen with a dimension of $10\times 5\times 5$$10\times 5\times 5$10 xx5xx510 \times 5 \times 5$10\times 5\times 5$ was cut out from the cast metal using a hexa-blade. Further, the model's surface was polished using SiC (emery paper), having grit sizes $80,120,400,1000$$80,120,400,1000$80,120,400,100080,120,400,1000$80,120,400,1000$, and 1200 .
Specimens were further polished to mirror finish using a polishing machine (Bainpol Metco, Chennai - India) shown in Fig. 4 containing velvet cloth (lapping cloth), smeared with $1-0.5\mu \mathrm{m}$$1-0.5\mu \mathrm{m}$1-0.5 mum1-0.5 \mu \mathrm{~m}$1-0.5\mu \mathrm{m}$ diamond paste. Metallographic specimens of the hybrid composites were chemically etched using Keller's reagent, composed of $2.5\mathrm{\%}$$2.5\mathrm{\%}$2.5%2.5 \%$2.5\mathrm{\%}$ nitric acid $\left({\mathrm{HNO}}_3\right)$$\left({\mathrm{HNO}}_3\right)$(HNO_(3))\left(\mathrm{HNO}_{3}\right)$\left({\mathrm{HNO}}_3\right)$, $1.5\mathrm{\%}$$1.5\mathrm{\%}$1.5%1.5 \%$1.5\mathrm{\%}$ hydrochloric acid ( HCl ), $1\mathrm{\%}$$1\mathrm{\%}$1%1 \%$1\mathrm{\%}$ hydrofluoric acid (HF), and $95\mathrm{\%}$$95\mathrm{\%}$95%95 \%$95\mathrm{\%}$ deionized water, for a controlled duration of 30 seconds. This etching protocol effectively revealed well-defined grain boundaries and microstructural features, which were subsequently examined in detail via high-resolution scanning electron microscopy (SEM) to elucidate the reinforcement morphology and matrixreinforcement interfacial characteristics.

Fig. 5 depicts the optical micrograph of the dendritic samples captured with 100 x magnification. The hardness evaluation of the $\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$AA7075//TiB_(2)//ZrO_(2)\mathrm{AA} 7075 / \mathrm{TiB}_{2} / \mathrm{ZrO}_{2}$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$ hybrid composites was conducted in accordance with the ASTM E92 utilizing a Vickers microhardness tester (Fuel Instruments and Engineers Pvt. Ltd., India), as depicted in Fig. 6. This standardized testing protocol ensured precise quantification of the composite's resistance to localized plastic deformation, facilitating reliable assessment of the reinforcement effects on matrix hardness. Specimens ( $10\times 5\times 5\mathrm{mm}$$10\times 5\times 5\mathrm{mm}$10 xx5xx5mm10 \times 5 \times 5 \mathrm{~mm}$10\times 5\times 5\mathrm{mm}$ ) were placed, and an indentation was made using a diamond indenter. Each indentation was made for 10 sec with a load of 10 kgf . After the indentation, the average diagonal length was examined and noted down. Further, the Vickers hardness number ( ${\mathrm{H}}_{\mathrm{v}}$${\mathrm{H}}_{\mathrm{v}}$H_(v)\mathrm{H}_{\mathrm{v}}${\mathrm{H}}_{\mathrm{v}}$ ) was determined using the formula:

where ' P ' $=$$=$==$=$ Indentation load (kgf) and 'd' = Mean diagonal length (mm)

Tensile test specimens were meticulously fabricated following the ASTM E8M standard, featuring a gauge length of 16 mm and a thickness of 4 mm , as illustrated in Fig. 7. The specimens were precision-machined from the cast hybrid composite using CNC milling to ensure dimensional accuracy. The geometric design maintained a gauge length to the square root of cross-sectional area ratio within the optimal range of 4 to 4.5 , where ${\mathrm{L}}_0$${\mathrm{L}}_0$L_(0)\mathrm{L}_{0}${\mathrm{L}}_0$ denotes the gauge length and ${\mathrm{A}}_0$${\mathrm{A}}_0$A_(0)\mathrm{A}_{0}${\mathrm{A}}_0$ represents the initial cross-sectional area. Mechanical characterization was conducted on a micro universal testing machine (Mecmesin Multitest 10-i, UK), specifically engineered for low-force microstructural evaluation under ambient conditions. Testing proceeded at a controlled crosshead displacement rate of $0.3\mathrm{mm}/\min$$0.3\mathrm{mm}/\min$0.3mm//min0.3 \mathrm{~mm} / \mathrm{min}$0.3\mathrm{mm}/\min$ until specimen failure, with real-time acquisition of load and displacement data. Subsequently, engineering stress-strain curves were generated to analyze the mechanical response and deformation behavior of the composites.

Dry sliding wear and frictional behavior of the hybrid composites were systematically evaluated using a pin-on-disc tribometer (DUCOM, India) in accordance with the ASTM G99, as depicted in Fig. 8. Test specimens, machined to dimensions of $30\times 5\times 5\mathrm{mm}$$30\times 5\times 5\mathrm{mm}$30 xx5xx5mm30 \times 5 \times 5 \mathrm{~mm}$30\times 5\times 5\mathrm{mm}$, served as pins and were slid against an EN-32 hardened steel disc with a hardness of 62 HRC. The disc maintained a constant peripheral velocity of $1.5\mathrm{m}/\mathrm{s}$$1.5\mathrm{m}/\mathrm{s}$1.5m//s1.5 \mathrm{~m} / \mathrm{s}$1.5\mathrm{m}/\mathrm{s}$ with a fixed rolling diameter of 100 mm . Wear tests were conducted under variable normal loads of $10\mathrm{N},20\mathrm{N}$$10\mathrm{N},20\mathrm{N}$10N,20N10 \mathrm{~N}, 20 \mathrm{~N}$10\mathrm{N},20\mathrm{N}$, and 30 N , spanning sliding distances of 2000 m and 3000 m to simulate different tribological conditions. Throughout testing, real-time measurements of frictional force and volumetric wear were recorded, enabling precise calculation of wear rates. Data variability was analyzed, and friction coefficient versus sliding distance graphs were plotted. To ensure statistical reliability, triplicate specimens per composition were tested under each condition, with averaged results reported for comprehensive performance assessment.

Fig. 9 (a) and (b) presents the high-resolution scanning electron microscope (SEM) micrograph of the AA7075 hybrid composite reinforced with $5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$5wt.%TiB_(2)5 \mathrm{wt} . \% \mathrm{TiB}_{2}$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$ and $4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$4wt.%ZrO_(2)4 \mathrm{wt} . \% \mathrm{ZrO}_{2}$4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$. The image reveals detailed microstructural features, including the uniform dispersion of reinforcement phases within the aluminum matrix and the nature of the interfacial bonding, which are critical to understanding the composite's enhanced mechanical and tribological performance. We can see the variation in the number and distribution of reinforcement particles. Ceramic particles appear to have faceted morphology.
Fig. 9 (c) and (d) presents the SEM micrograph of the AA7075 hybrid composite reinforced with $5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$5wt.%TiB_(2)5 \mathrm{wt} . \% \mathrm{TiB}_{2}$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$ and $6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$6wt.%ZrO_(2)6 \mathrm{wt} . \% \mathrm{ZrO}_{2}$6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$. The Inset of Fig. 10 shows that ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ceramic particles exist with an average particle size of $110\pm 0.7\mathrm{nm}$$110\pm 0.7\mathrm{nm}$110+-0.7nm110 \pm 0.7 \mathrm{~nm}$110\pm 0.7\mathrm{nm}$. The microstructure reveals the complete uniform distribution of the reinforcement's particles. The resulting structure typically has a fine equiaxed grained and well-ordered microstructure, which is desirable in most casting [4-13]. Notably, these particles tend to localize predominantly along grain boundaries, displaying pronounced agglomeration within the matrix. Insets in Figs. 9 and 10 further illustrate the heterogeneous distribution of the reinforcement phases, indicating particle clustering that intensifies with the elevated weight fraction of reinforcements. This phenomenon is attributed to the increased surface energy of the ceramic particles, which promotes aggregation and challenges uniform dispersion at higher loadings [14].
AA7075/5% ${\mathrm{TiB}}_2/2\mathrm{\%}{\mathrm{ZrO}}_2$${\mathrm{TiB}}_2/2\mathrm{\%}{\mathrm{ZrO}}_2$TiB_(2)//2%ZrO_(2)\mathrm{TiB}_{2} / 2 \% \mathrm{ZrO}_{2}${\mathrm{TiB}}_2/2\mathrm{\%}{\mathrm{ZrO}}_2$ hybrid composite resulted in the low mechanical properties obtained from the SEM and mechanical tests conducted. The results are not satisfactory when compared with the $4\mathrm{\%}{\mathrm{ZrO}}_2$$4\mathrm{\%}{\mathrm{ZrO}}_2$4%ZrO_(2)4 \% \mathrm{ZrO}_{2}$4\mathrm{\%}{\mathrm{ZrO}}_2$ system, even though there is less agglomeration of reinforced particles. A lower percentage of the metal matrix may have resulted in low mechanical properties. The solidification front significantly impacts the displacement of the reinforcements in the casting process. In the process of solidification, the liquid metal moves towards the cooling front, while the reinforcements experience forces like drag and shear force from the molten metal. The distribution of reinforcing particles in intra and intergranular areas determines the velocity of the solidification front. When the solidification front's rate falls below a critical velocity in the process of attaining an equilibrium state, the system may maintain a stable or orderly solidification, which can be inferred from Fig. 10 (a) for $\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5%TiB_(2)//4%ZrO_(2)\mathrm{AA} 7075 / 5 \% \mathrm{TiB}_{2} / 4 \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{\%}{\mathrm{ZrO}}_2$ hybrid composite system. When the solidification front's velocity is above the critical velocity, the system may transition to an unstable regime. The reinforcement particles migrate towards the mold walls or get trapped in certain regions depending on the cooling rate, nature, and amount of the reinforcements while achieving the equilibrium state, leading to agglomeration or mushy zones as inferred in Fig. 10(b) for AA7075/5% ${\mathrm{TiB}}_2/6\mathrm{\%}{\mathrm{ZrO}}_2$${\mathrm{TiB}}_2/6\mathrm{\%}{\mathrm{ZrO}}_2$TiB_(2)//6%ZrO_(2)\mathrm{TiB}_{2} / 6 \% \mathrm{ZrO}_{2}${\mathrm{TiB}}_2/6\mathrm{\%}{\mathrm{ZrO}}_2$ hybrid composite system.

Fig. 11 presents the elemental distribution maps obtained via scanning electron microscopy (SEM) for the AA7075/5 wt.% ${\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$${\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$TiB_(2)//4wt.%ZrO_(2)\mathrm{TiB}_{2} / 4 \mathrm{wt} . \% \mathrm{ZrO}_{2}${\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ hybrid composite. The elemental mapping elucidates the spatial dispersion of constituent elements within the composite microstructure. Predominantly, aluminum (Al) forms the matrix, while zinc ( Zn ) is observed as the principal
alloying element. The ceramic reinforcements, represented by titanium (Ti), zirconium (Zr), and boron (B), exhibit a homogenous distribution throughout the matrix without notable agglomeration. This uniform dispersion of reinforcements is critical, as it correlates strongly with enhanced load transfer efficiency and resultant mechanical property improvements, including strength and wear resistance [15, 4].

his section will present the results and discuss the mechanical and physical properties of $\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$AA7075//TiB_(2)//ZrO_(2)\mathrm{AA} 7075 / \mathrm{TiB}_{2} / \mathrm{ZrO}_{2}$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$ hybrid composite fabricated through situ casting.

A hardness test was conducted using a Vickers hardness tester. The test specimens were prepared as described in section 2. Three samples, each with a different reinforcement composition, were tested. To ensure accuracy, more than three readings were taken for each specimen, and the average value of 'D' was calculated. Tab. 2 provides a comparison of the hardness between the base matrix AA7075 and the hybrid composites.

It is inferred from Tab. 2 that, first, the hardness increases gradually, and then suddenly, there is a fall in the hardness value. The reinforcements ( ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ) render the inherent property of the matrix material. The literature shows that the best results were obtained for $5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$5wt.%TiB_(2)5 \mathrm{wt} . \% \mathrm{TiB}_{2}$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$ reinforced composite and similarly $4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$4wt.%ZrO_(2)4 \mathrm{wt} . \% \mathrm{ZrO}_{2}$4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ reinforced composite [18]. It was found that adding $5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$$5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$5wt%TiB_(2)5 \mathrm{wt} \% \mathrm{TiB}_{2}$5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$ and $2\mathrm{wt}\mathrm{\%}{\mathrm{ZrO}}_2$$2\mathrm{wt}\mathrm{\%}{\mathrm{ZrO}}_2$2wt%ZrO_(2)2 \mathrm{wt} \% \mathrm{ZrO}_{2}$2\mathrm{wt}\mathrm{\%}{\mathrm{ZrO}}_2$ to the metal matrix increased the hardness value by $80\mathrm{\%}$$80\mathrm{\%}$80%80 \%$80\mathrm{\%}$ more than that of the as-cast alloy. By adding $5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$$5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$5wt%TiB_(2)5 \mathrm{wt} \% \mathrm{TiB}_{2}$5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$ and $4\mathrm{wt}\mathrm{\%}{\mathrm{ZrO}}_2$$4\mathrm{wt}\mathrm{\%}{\mathrm{ZrO}}_2$4wt%ZrO_(2)4 \mathrm{wt} \% \mathrm{ZrO}_{2}$4\mathrm{wt}\mathrm{\%}{\mathrm{ZrO}}_2$ to the metal matrix, the hardness value increased by $85.45\mathrm{\%}$$85.45\mathrm{\%}$85.45%85.45 \%$85.45\mathrm{\%}$ compared to as-cast alloy, and similarly, by adding $5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$5wt.%TiB_(2)5 \mathrm{wt} . \% \mathrm{TiB}_{2}$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$ and $6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$6wt.%ZrO_(2)6 \mathrm{wt} . \% \mathrm{ZrO}_{2}$6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ to the metal matrix, the hardness value increased by $67\mathrm{\%}$$67\mathrm{\%}$67%67 \%$67\mathrm{\%}$ more than that of as-cats alloy. As the total weight percentage of the reinforcements increases from $7\mathrm{\%}(5\mathrm{\%}{\mathrm{TiB}}_2,2\mathrm{\%}{\mathrm{ZrO}}_2)$$7\mathrm{\%}\left(5\mathrm{\%}{\mathrm{TiB}}_2,2\mathrm{\%}{\mathrm{ZrO}}_2\right)$7%(5%TiB_(2),2%ZrO_(2))7 \% \left(5 \% \mathrm{TiB}_{2}, 2 \% \mathrm{ZrO}_{2}\right)$7\mathrm{\%}(5\mathrm{\%}{\mathrm{TiB}}_2,2\mathrm{\%}{\mathrm{ZrO}}_2)$ to $9\mathrm{\%}(5\mathrm{\%}{\mathrm{TiB}}_2,4\mathrm{\%}{\mathrm{ZrO}}_2)$$9\mathrm{\%}\left(5\mathrm{\%}{\mathrm{TiB}}_2,4\mathrm{\%}{\mathrm{ZrO}}_2\right)$9%(5%TiB_(2),4%ZrO_(2))9 \%\left(5 \% \mathrm{TiB}_{2}, 4 \% \mathrm{ZrO}_{2}\right)$9\mathrm{\%}(5\mathrm{\%}{\mathrm{TiB}}_2,4\mathrm{\%}{\mathrm{ZrO}}_2)$, there is an increase in the resistance to the dislocation of the particles against the localized deformation. As soon as the total weight percentage of the reinforcements exceeds $9\mathrm{\%}$$9\mathrm{\%}$9%9 \%$9\mathrm{\%}$, resistance to the dislocation is decreased against the load. As per the Hall-Petch theory, grain size refinement increases the area of grain boundaries, which enhances resistance to particle dislocations, thereby improving the composite's hardness. There is an enhancement in the load transfer effect by adding the reinforcement particles, thus enhancing the hardness. [4, 13, 19]
The enhanced dispersion of reinforcement particles within the metal matrix significantly minimizes particle segregation in the $\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/2\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/2\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5wt.%TiB_(2)//2wt.%ZrO_(2)\mathrm{AA} 7075 / 5 \mathrm{wt} . \% \mathrm{TiB}_{2} / 2 \mathrm{wt} . \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/2\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ and $\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5wt.%TiB_(2)//4wt.%ZrO_(2)\mathrm{AA} 7075 / 5 \mathrm{wt} . \% \mathrm{TiB}_{2} / 4 \mathrm{wt} . \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ hybrid composites. Notably, the AA7075/5 wt.% ${\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$${\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$TiB_(2)//4wt.%ZrO_(2)\mathrm{TiB}_{2} / 4 \mathrm{wt} . \% \mathrm{ZrO}_{2}${\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ composite exhibits a homogeneous distribution of ceramic reinforcements, as evidenced in Fig. 9, with clearly defined equiaxed grains and well-demarcated grain boundaries. This uniform microstructural morphology mitigates particle agglomeration, thereby contributing to a marked enhancement in hardness values. Conversely, the $\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5wt.%TiB_(2)//6wt.%ZrO_(2)\mathrm{AA} 7075 / 5 \mathrm{wt} . \% \mathrm{TiB}_{2} / 6 \mathrm{wt} . \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ hybrid composite demonstrates a decline in hardness, attributed to uneven particle dispersion and pronounced agglomeration phenomena, which adversely affect load transfer efficiency and matrix-reinforcement interfacial bonding [20].

The yield strength (YS), ultimate tensile strength (UTS), and ductility (expressed as percentage elongation) were quantitatively derived from the load-displacement data, subsequently converted into engineering stress-strain parameters. Fig. 12 illustrates the representative stress-strain curves for the $\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$AA7075//TiB_(2)//ZrO_(2)\mathrm{AA} 7075 / \mathrm{TiB}_{2} / \mathrm{ZrO}_{2}$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$ hybrid composites, highlighting the influence of varied reinforcement fractions on the composite's mechanical response. These curves provide critical insights into the elastic-plastic deformation behavior, strain hardening characteristics, and fracture mechanisms inherent to the synthesized hybrid metal matrix composites.
The baseline yield strength of the as-cast AA7075 alloy is approximately 107 MPa [16]. Incorporation of $5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$$5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$5wt%TiB_(2)5 \mathrm{wt} \% \mathrm{TiB}_{2}$5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$ alongside $2\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$2\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$2wt.%ZrO_(2)2 \mathrm{wt} . \% \mathrm{ZrO}_{2}$2\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ into the aluminum matrix resulted in a significant enhancement of yield strength to 113 MPa . Further augmentation of the ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ reinforcement content to $4\mathrm{wt}.\mathrm{\%}$$4\mathrm{wt}.\mathrm{\%}$4wt.%4 \mathrm{wt} . \%$4\mathrm{wt}.\mathrm{\%}$ elevated the yield strength to 123 MPa , indicating an optimal reinforcement threshold for load-bearing capacity. However, a marginal decline to 110 MPa was observed when the ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ content increased to $6\mathrm{wt}\mathrm{\%}$$6\mathrm{wt}\mathrm{\%}$6wt%6 \mathrm{wt} \%$6\mathrm{wt}\mathrm{\%}$, suggesting possible particle agglomeration or stress concentration effects at higher reinforcement fractions.

Fig. 13 illustrates the variation in yield strength, ultimate tensile strength, and percentage elongation for $\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$AA7075//TiB_(2)//ZrO_(2)\mathrm{AA} 7075 / \mathrm{TiB}_{2} / \mathrm{ZrO}_{2}$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$ hybrid composites with varying reinforcement compositions. The composite containing $5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$5wt.%TiB_(2)5 \mathrm{wt} . \% \mathrm{TiB}_{2}$5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2$ and $4{\mathrm{wt}}^{\mathrm{\%}}\mathrm{\%}{\mathrm{ZrO}}_2$$4{\mathrm{wt}}^{\mathrm{\%}}\mathrm{\%}{\mathrm{ZrO}}_2$4wt^(%)%ZrO_(2)4 \mathrm{wt}^{\%} \% \mathrm{ZrO}_{2}$4{\mathrm{wt}}^{\mathrm{\%}}\mathrm{\%}{\mathrm{ZrO}}_2$ exhibits the lowest ductility, with elongation reduced to approximately $11\mathrm{\%}$$11\mathrm{\%}$11%11 \%$11\mathrm{\%}$, attributable to its markedly enhanced hardness relative to the as-cast alloy. Across all hybrid composites, a pronounced reduction in ductility is observed, correlating with significant hardness improvements. This inverse relationship highlights the classical trade-off between strength and ductility in particulate-reinforced metal matrix composites. The decreased ductility is primarily ascribed to the reduced inter-particle spacing, which impedes dislocation motion and limits plastic deformation. Under applied loading, the strong interfacial bonding between ceramic reinforcement particles and the AA7075 matrix effectively obstructs dislocation glide and crack propagation, thereby augmenting tensile strength but concurrently restricting elongation. These microstructural interactions underscore the composite's enhanced mechanical robustness and fracture resistance [4, 13].
The $\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5wt.%TiB_(2)//4wt.%ZrO_(2)\mathrm{AA} 7075 / 5 \mathrm{wt} . \% \mathrm{TiB}_{2} / 4 \mathrm{wt} . \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ hybrid composite exhibits superior yield strength relative to other compositions. This enhancement is primarily attributed to the uniform dispersion of ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ceramic reinforcements within the aluminum matrix and the strong interfacial bonding achieved between the reinforcements and the matrix phase. Across all compositions, the incorporation of ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ significantly enhances the interfacial integrity and mechanical anchorage, thereby improving the composite's resistance to reinforcement-matrix debonding under applied stress.

Dry wear and friction tests were conducted on the wear and friction monitor. The literature states that AMCs are known for superior wear resistance to monolithic aluminum alloys. The mechanical and wear resistance behavior of the hybrid composite increases significantly with the addition of ceramic reinforcements, ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ZrO 2 , as reported in the literature [4,12,15]. Initially, the volume loss is transient against the sliding distance and attains a steady state after a specific time. In the steady-state wear regime, the wear volume loss is constant for an extended duration.
Fig. 14 illustrates the wear rate behavior of $\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$AA7075//TiB_(2)//ZrO_(2)\mathrm{AA} 7075 / \mathrm{TiB}_{2} / \mathrm{ZrO}_{2}$\mathrm{AA}7075/{\mathrm{TiB}}_2/{\mathrm{ZrO}}_2$ hybrid composites under varying normal loads and sliding distances. An increase in applied load leads to a corresponding escalation in wear rate for each reinforcement composition, indicating a load-sensitive wear mechanism. Similarly, as depicted in Figs. 15 and 16, a progressive increase in sliding distance results in a marked rise in wear rate across all specimens. These observations affirm that both normal load and sliding distance exhibit a directly proportional relationship with wear degradation, attributable to enhanced surface interaction, material removal, and potential thermal softening under prolonged frictional exposure.
The incorporation of ceramic reinforcements in varying weight fractions into the AA7075 aluminum matrix significantly enhances its wear resistance by impeding the extent of plastic deformation within the matrix during tribological interactions. The presence of ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ particulates introduces strong barriers to dislocation motion, thereby contributing to improved structural stability and reduced material loss under sliding conditions.
As the sliding distance increases, friction-induced interfacial heating elevates the surface temperature, driving the material closer to a thermally softened, plastically deformable state. This facilitates greater material removal, thereby increasing the wear rate $[4,13]$$[4,13]$[4,13][4,13]$[4,13]$. Among the studied composites, the $\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5wt.%TiB_(2)//4wt.%ZrO_(2)\mathrm{AA} 7075 / 5 \mathrm{wt} . \% \mathrm{TiB}_{2} / 4 \mathrm{wt} . \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ formulation demonstrates superior wear resistance, primarily attributed to the refined dispersion and homogeneity of the reinforcement phase.
In contrast, the $\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5wt.%TiB_(2)//6wt.%ZrO_(2)\mathrm{AA} 7075 / 5 \mathrm{wt} . \% \mathrm{TiB}_{2} / 6 \mathrm{wt} . \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{wt}.\mathrm{\%}{\mathrm{TiB}}_2/6\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ composite exhibits a comparatively higher wear rate, as noted in the inset of Fig. 14. This behavior is linked to non-uniform dispersion and agglomeration of reinforcement particles, as confirmed by the microstructural features in Fig. 17. The clustering of ceramic particulates reduces the effective load transfer efficiency and creates stress concentration zones, thus accelerating wear.
Furthermore, all hybrid composites consistently display lower wear rates compared to the unreinforced AA7075 alloy. This reduction is primarily due to the solid lubricating effect of ceramic particulates under dry sliding conditions, which act as load-bearing constituents and minimize direct metal-to-metal contact. The synergistic strengthening mechanisms-dislocation-particle interactions and robust interfacial bonding-enhance the hardness and tribological stability of the composites, effectively reducing wear-induced damage [21-23].

Figs. 18, 19, and 20 present scanning electron microscope (SEM) micrographs depicting the worn surface morphologies of AA7075 hybrid composites reinforced with varying weight fractions of ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ particles. The intrinsic high hardness of AA7075 alloy imparts substantial resistance against plastic deformation within the
surface layers during wear. Incorporation of ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ reinforcements within the aluminum matrix effectively impedes dislocation mobility and shear deformation mechanisms, thereby significantly enhancing the composite's tribological performance and wear resistance. The micrographs show that the grooves are oriented parallel to the wear direction. Due to resistance to plastic deformation, heat develops due to friction. As the surface starts to wear, the load applied and heat generated act as an aid to cause adhesion, which results in micro-joint formation. The groves are parallel to the direction of the wear, and the pattern seems to be ploughing out the surface from the material, which gives evidence of two-body abrasion. Cracks, cavities, and pits are not visible in $\mathrm{AA}70705/5\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}70705/5\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{\%}{\mathrm{ZrO}}_2$AA70705//5%TiB_(2)//4%ZrO_(2)\mathrm{AA} 70705 / 5 \% \mathrm{TiB}_{2} / 4 \% \mathrm{ZrO}_{2}$\mathrm{AA}70705/5\mathrm{\%}{\mathrm{TiB}}_2/4\mathrm{\%}{\mathrm{ZrO}}_2$ and $\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/6\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/6\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5%TiB_(2)//6%ZrO_(2)\mathrm{AA} 7075 / 5 \% \mathrm{TiB}_{2} / 6 \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/6\mathrm{\%}{\mathrm{ZrO}}_2$ except in the case of. This may be attributed to the processing quality of AA7075 hybrid composite having $5{\mathrm{wt}}^{\mathrm{\%}}{\mathrm{TiB}}_2$$5{\mathrm{wt}}^{\mathrm{\%}}{\mathrm{TiB}}_2$5wt^(%)TiB_(2)5 \mathrm{wt}^{\%} \mathrm{TiB}_{2}$5{\mathrm{wt}}^{\mathrm{\%}}{\mathrm{TiB}}_2$ and $4{\mathrm{wt}}^{\mathrm{\%}}\mathrm{\%}{\mathrm{ZrO}}_2$$4{\mathrm{wt}}^{\mathrm{\%}}\mathrm{\%}{\mathrm{ZrO}}_2$4wt^(%)%ZrO_(2)4 \mathrm{wt}^{\%} \% \mathrm{ZrO}_{2}$4{\mathrm{wt}}^{\mathrm{\%}}\mathrm{\%}{\mathrm{ZrO}}_2$ and $5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$$5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$5wt%TiB_(2)5 \mathrm{wt} \% \mathrm{TiB}_{2}$5\mathrm{wt}\mathrm{\%}{\mathrm{TiB}}_2$ and $4\mathrm{wt}\mathrm{\%}{\mathrm{ZrO}}_2$$4\mathrm{wt}\mathrm{\%}{\mathrm{ZrO}}_2$4wt%ZrO_(2)4 \mathrm{wt} \% \mathrm{ZrO}_{2}$4\mathrm{wt}\mathrm{\%}{\mathrm{ZrO}}_2$ composition. Grooving and scratching appear more severe in $\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/2\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/2\mathrm{\%}{\mathrm{ZrO}}_2$AA7075//5%TiB_(2)//2%ZrO_(2)\mathrm{AA} 7075 / 5 \% \mathrm{TiB}_{2} / 2 \% \mathrm{ZrO}_{2}$\mathrm{AA}7075/5\mathrm{\%}{\mathrm{TiB}}_2/2\mathrm{\%}{\mathrm{ZrO}}_2$, which characterizes the abrasion. Here, the reinforcements ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ act as a thin lubricating film between the sliding surface (metal-to-metal contact). Due to this lubricating film, wear resistance has improved [4, 12-25].

Figs. 21, 22, and 23 illustrate the Energy Dispersive X-ray Spectroscopy (EDS) elemental mapping of the worn surfaces of AA7075 hybrid composites reinforced with varying concentrations of ${\mathrm{TiB}}_2$${\mathrm{TiB}}_2$TiB_(2)\mathrm{TiB}_{2}${\mathrm{TiB}}_2$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ particles. The EDS spectra confirm the elemental composition, revealing the presence of aluminum ( Al ), zinc ( Zn ), iron ( Fe ), oxygen (O), boron (B), zirconium (Zr), and titanium (Ti), as detailed in Tab. 3. Prolonged sliding contact induces severe surface deformation, resulting in delamination and the subsequent development of a protective oxide layer on the worn regions, which plays a crucial role in mitigating further material degradation. There is an increase in the temperature in between the rotating disc and wear specimen due to dislocations of the surface by shear force. The worn surface which has undergone large plastic deformation is further exposed to the environment. The active ions on the metal surface due to high temperature rise leads to oxidation. A protective oxide layer is formed around the material may reduce the rate of corrosion which also acts as a lubricant, reducing the wear rate. The presence of oxygen in the EDS spectrum confirms the development of this oxide layer on the worn surfaces. There is the presence of Fe in the spectrum graph as the result of mechanically mixed layer (MML) formation due to rubbing action between the disc and hybrid composite surface [4, 25].