Influence of hybrid nano ${\mathrm{Al}}_2{\mathrm{O}}_3-{\mathrm{ZrO}}_2$${\mathrm{Al}}_2{\mathrm{O}}_3-{\mathrm{ZrO}}_2$Al_(2)O_(3)-ZrO_(2)\mathrm{Al}_{2} \mathrm{O}_{3}-\mathrm{ZrO}_{2}${\mathrm{Al}}_2{\mathrm{O}}_3-{\mathrm{ZrO}}_2$ reinforcements on microstructure, fracture toughness and fractographic behaviour of A16061 alloy

Composites are a significant class of materials that can meet the growing requirements of contemporary technology. They surpass the limitations of traditional monolithic materials by attaining optimal combinations of strength, hardness and density among other attributes. Sustainable nano-composites provide automotive, biotechnology, electronics and aerospace industries new economic and technological opportunities $[1,2]$$[1,2]$[1,2][1,2]$[1,2]$. To enhance the mechanical
performance and wear resistance of Metal Matrix Composites (MMCs), hard ceramic particles are incorporated into the matrix. Hybrid Metal Matrix Composites (HMMCs) demonstrate better wear resistance, specific modulus, damping capacity and superior specific strength when compared to pure alloys [3]. As a result, on-going inquiry is aimed on the development of advanced composites and improvement of current materials to achieve tailored properties for specific applications.
The practical applications of aluminum alloys are limited by their degree of softness and reduced wear resistance. Common applications for A16061 include the automotive, defense, and marine industries. Its remarkable strength, low weight and resistance to corrosion are the main reasons for its extensive use [4, 5]. The addition of hard ceramic reinforcement particles to aluminum and its alloys creates a matrix composite with localized strengthening and nearly homogenous properties [6]. ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ (alumina) is a widely employed reinforcement due to its remarkable hardness and excellent thermal stability. Zirconium oxide possesses different range of properties (excellent fracture toughness, good resistance to crack propagation, high thermal endurance and reduced thermal conductivity) at elevated temperature. These properties make zirconium di oxide very strong like other ceramics [7-8].
Mixing zirconia ( ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ) with alumina ( ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ ) nanoparticles makes composites even stronger. Now a day's most of the researches are going on Hybrid Metal Matrix Composites (HMMCs) due to its better mechanical and tribological properties. HMMCs can be synthesized by squeeze casting, stir casting, spray atomization casting, powder metallurgy, and plasma spraying technique. Among these stir-casting/ liquid stirring technique is widely used especially for synthesizing discontinuous reinforcing aluminium alloy MMCs [9] also this process is economical compare to other techniques as mentioned above. The difficulty in this process is that particles don't mix evenly; they tend to float which leads to wettability issues and segregation of the reinforcing particulates. Hence, to overcome this problem Yong Yang and his team [10] used an ultrasonic cavitation technique to mix nanoparticles into aluminium, while it solidifies and making the particles spread out evenly. For metal matrix composites fracture toughness is an important property which determines their ability against the crack growth and failure due to applied stress.
Past studies revels that nano graphene and zirconium oxide improves the fracture toughness by enhancing the load bearing capacity and bridging in the matrix $[11,12]$$[11,12]$[11,12][11,12]$[11,12]$. Also, increasing the content of ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ in $\mathrm{Al}6061-{\mathrm{ZrO}}_2$$\mathrm{Al}6061-{\mathrm{ZrO}}_2$Al6061-ZrO_(2)\mathrm{Al} 6061-\mathrm{ZrO}_{2}$\mathrm{Al}6061-{\mathrm{ZrO}}_2$ composites improves the fracture toughness by using SENB and FEA test [13]. Research on HMMCs reinforced with nano particles (Alumina and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ )-Al6061 is limited and most prior work focus on single reinforcement systems also lack of systematic fracture toughness optimization at low nano wt.%. Present study shows the use of novel ultrasonic-assisted stir casting is a key contribution, as it minimizes nanoparticle agglomeration and ensures improved dispersion within the aluminum matrix. Meanwhile the study thoroughly optimizes the reinforcement ratio and identifies the optimum composition for maximum fracture resistance for potential engineering applications.

In the present study Al 6061 alloy and nano ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and nano ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ used as matrix and reinforcements respectively. Chemical composition was determined by using Energy Dispersive Spectroscopy (EDS). Tab. 1 depicts the properties of matrix material and reinforcement. SEM and EDAX analysis reveal peaks that align with the base alloy, nano ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$, and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ in Fig (1-6).

Table 2: Composition analysis of Al6061 alloy.

The synthesis of hybrid nano-composite employed Al6061 Alloy as the matrix, with its elemental composition outlined in Tab. 1. Nano-sized alumina ( ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ ) and zirconia ( ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ) particles, with diameters ranging from 50 to 90 nanometers, functioned as reinforcement agents. The $\mathrm{Al}6061/{\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{ZrO}}_2$$\mathrm{Al}6061/{\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{ZrO}}_2$Al6061//Al_(2)O_(3)//ZrO_(2)\mathrm{Al} 6061 / \mathrm{Al}_{2} \mathrm{O}_{3} / \mathrm{ZrO}_{2}$\mathrm{Al}6061/{\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{ZrO}}_2$ composite was synthesized using ultrasonic-assisted stir casting technique by varying wt $\mathrm{\%}$$\mathrm{\%}$%\%$\mathrm{\%}$ of ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ( $0.5,0.75,1$$0.5,0.75,1$0.5,0.75,10.5,0.75,1$0.5,0.75,1$, and $1.25\mathrm{wt}\mathrm{\%}$$1.25\mathrm{wt}\mathrm{\%}$1.25wt%1.25 \mathrm{wt} \%$1.25\mathrm{wt}\mathrm{\%}$ for each reinforcement). Fig. 7 presents an illustration of the experimental apparatus utilized in this method (manufacturer: SwamEquip). The equipment provided with a graphite crucible, an ultrasonic vibrator, stirring system, digital timer, thermocouples, weighing scale and resistance furnace.

Figure 7: Experimental set up of stir casting equipment.

A weighed amount of Al6061 is placed in an electrical resistance furnace and heated to a temperature of ${750}^{\circ }\mathrm{C}$${750}^{\circ }\mathrm{C}$750^(@)C750^{\circ} \mathrm{C}${750}^{\circ }\mathrm{C}$ until it reaches to melting. Once this temperature is attained a solid hexachloroethane are introduced into the melt which act as an effective degasser stirred vigorously until the clear vortex is formed using an ultrasonic stirrer. Meanwhile, preheated $\left({250}^0\mathrm{C}\right)$$\left({250}^0\mathrm{C}\right)$(250^(0)C)\left(250^{0} \mathrm{C}\right)$\left({250}^0\mathrm{C}\right)$ nano-sized ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ and ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ particulates are introduced into the molten alloy, in a two-steps rather than adding at once to improve the dispersion of the particulates in the molten alloy and stirred at a speed of 500 rpm for duration of ten minutes which generate the vortex and helps dispersion of nanoparticles within the melt [14]. Due to high surface are and agglomeration of the reinforcing particulates mechanical stirring is difficult which leads to wettability issues. Hence an ultrasonic-stirring was used to overcome this problem. After 10 minutes of mechanical stirring action is done, an ultrasonic probe is used to disperse ceramic nanoparticles in the metal pool, creating a homogenous combination (Highenergy vibrations from the probe three-quarters deep in the liquid created many cavitation bubbles in the molten pool. Due to pressure differentials, bubbles at high temperatures and pressures burst quickly, disintegrating clusters and distributing nanoparticles evenly in the liquid metal). Later the mixed molten metals are poured $\left({750}^{\circ }\mathrm{C}\right)$$\left({750}^{\circ }\mathrm{C}\right)$(750^(@)C)\left(750^{\circ} \mathrm{C}\right)$\left({750}^{\circ }\mathrm{C}\right)$ into the prepared mould for complete solidification for few minutes and after complete solidification samples are withdrawn from the mold and it was it was taken off for further testing. The composites were subjected to microstructural examination through SEM, in conjunction with an assessment of fracture toughness. To reduce measurement errors in each trial three specimens were assessed and their mean values were documented.

Microstructural analyses were performed utilizing a SEM (Model: Tescan Vega 3) to assess the dispersion of reinforced particulates $({\mathrm{Al}}_2{\mathrm{O}}_3$$\left({\mathrm{Al}}_2{\mathrm{O}}_3\right.$(Al_(2)O_(3):}\left(\mathrm{Al}_{2} \mathrm{O}_{3}\right.$({\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_{2\mathrm{p}}+\mathrm{Al}6061$${\mathrm{ZrO}}_{2\mathrm{p}}+\mathrm{Al}6061$ZrO_(2p)+Al6061\mathrm{ZrO}_{2 \mathrm{p}}+\mathrm{Al} 6061${\mathrm{ZrO}}_{2\mathrm{p}}+\mathrm{Al}6061$ matrix). To evaluate the fracture toughness of the Al 6061 -based hybrid composite reinforced with nano-scale ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ particles, compact tension (CT) specimens were fabricated in accordance with ASTM E399 criteria. [16]. As shown in fig. 8 during the experiments samples were machined to a thickness (B) of 20.0 mm with notches and pre-cracks for controlled crack growth [17,18]. Using servo-hydraulic universal testing machine (BISS model) fitted with a 25 kN load cell and a displacement transducer for measuring Crack Opening Displacement (COD) fracture toughness test was conducted. Preliminary pre-cracking was conducted to attain an $\mathrm{a}/\mathrm{w}$$\mathrm{a}/\mathrm{w}$a//w\mathrm{a} / \mathrm{w}$\mathrm{a}/\mathrm{w}$ ratio of 0.5 . During testing, to calculate fracture toughness $\left({\mathrm{K}}_{\mathrm{IC}}\right)$$\left({\mathrm{K}}_{\mathrm{IC}}\right)$(K_(IC))\left(\mathrm{K}_{\mathrm{IC}}\right)$\left({\mathrm{K}}_{\mathrm{IC}}\right)$ the critical load was used for the prepared samples which are loaded at 0.55 MPa $\sqrt{m}/\mathrm{s}$$\sqrt{m}/\mathrm{s}$sqrtm//s\sqrt{m} / \mathrm{s}$\sqrt{m}/\mathrm{s}$ until failure [19]. SEM fractography was also conducted.

SEM characterization of Al6061- ${\mathrm{Al}}_2{\mathrm{O}}_3-\mathrm{ZrO}$${\mathrm{Al}}_2{\mathrm{O}}_3-\mathrm{ZrO}$Al_(2)O_(3)-ZrO\mathrm{Al}_{2} \mathrm{O}_{3}-\mathrm{ZrO}${\mathrm{Al}}_2{\mathrm{O}}_3-\mathrm{ZrO}$ hybrid nano-composite at different weight fractions

SEM pictures reveal the microstructure of Al 6061 reinforced with different percentages of ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ composites, as depicted in Fig. $10(\mathrm{a}-\mathrm{h})$$10(\mathrm{a}-\mathrm{h})$10(a-h)10(\mathrm{a}-\mathrm{h})$10(\mathrm{a}-\mathrm{h})$. The examination via EDS ensures the chemical composition of the analyzed regions corresponded with the designated sample formulation. Fig. 11 presents the EDS results for Al6061 reinforced with $1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$1%Al_(2)O_(3)1 \% \mathrm{Al}_{2} \mathrm{O}_{3}$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $1\mathrm{\%}{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{ZrO}}_2$1%ZrO_(2)1 \% \mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{ZrO}}_2$, indicating successful integration of the reinforcements inside the aluminum matrix.

Figure 10 (a-d): SEM images of(a) Al6061, (b) Al6061-1% Al2O3-0.5% ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$, (c) $\mathrm{Al}6061-1\mathrm{\%}\mathrm{Al}2\mathrm{O}3-0.75\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{Al}6061-1\mathrm{\%}\mathrm{Al}2\mathrm{O}3-0.75\mathrm{\%}{\mathrm{ZrO}}_2$Al6061-1%Al2O3-0.75%ZrO_(2)\mathrm{Al} 6061-1 \% \mathrm{Al} 2 \mathrm{O} 3-0.75 \% \mathrm{ZrO}_{2}$\mathrm{Al}6061-1\mathrm{\%}\mathrm{Al}2\mathrm{O}3-0.75\mathrm{\%}{\mathrm{ZrO}}_2$, (d)Al6061-1% $\mathrm{Al}2\mathrm{O}3-1\mathrm{\%}{\mathrm{ZrO}}_2$$\mathrm{Al}2\mathrm{O}3-1\mathrm{\%}{\mathrm{ZrO}}_2$Al2O3-1%ZrO_(2)\mathrm{Al} 2 \mathrm{O} 3-1 \% \mathrm{ZrO}_{2}$\mathrm{Al}2\mathrm{O}3-1\mathrm{\%}{\mathrm{ZrO}}_2$

Fig. 10(a) SEM image of as cast Al6061 matrix shows the uniformly distributed grains with smooth surface. Fig. 10 (b) shows the uniform dissemination of the reinforced particulates ( $1\mathrm{wt}.\mathrm{\%}$$1\mathrm{wt}.\mathrm{\%}$1wt.%1 \mathrm{wt} . \%$1\mathrm{wt}.\mathrm{\%}$ of alumina and $0.5\mathrm{wt}.\mathrm{\%}$$0.5\mathrm{wt}.\mathrm{\%}$0.5wt.%0.5 \mathrm{wt} . \%$0.5\mathrm{wt}.\mathrm{\%}$ of ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ) in Al 6061 matrix indicates the incorporation of hard ceramic nano particles contributing to grain refinement. Fig. 10(c) shows the compact arrangements of $1\mathrm{wt}.\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$1\mathrm{wt}.\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$1wt.%Al_(2)O_(3)1 \mathrm{wt} . \% \mathrm{Al}_{2} \mathrm{O}_{3}$1\mathrm{wt}.\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $0.75\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$$0.75\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$0.75wt.%ZrO_(2)0.75 \mathrm{wt} . \% \mathrm{ZrO}_{2}$0.75\mathrm{wt}.\mathrm{\%}{\mathrm{ZrO}}_2$ particles reinforced to Al 6061 . Agglomeration is seen but dissemination of the particles remains effective; though cluster may leads to stress concentration. Fig. 10 (d) composites with $1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3-\mathrm{Al}6061$$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3-\mathrm{Al}6061$1%Al_(2)O_(3)-Al60611 \% \mathrm{Al}_{2} \mathrm{O}_{3}-\mathrm{Al} 6061$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3-\mathrm{Al}6061$ and $1\mathrm{\%}{\mathrm{ZrO}}_2-\mathrm{Al}6061$$1\mathrm{\%}{\mathrm{ZrO}}_2-\mathrm{Al}6061$1%ZrO_(2)-Al60611 \% \mathrm{ZrO}_{2}-\mathrm{Al} 6061$1\mathrm{\%}{\mathrm{ZrO}}_2-\mathrm{Al}6061$ depict the uniform and refined particles dissemination which leads to improve mechanical properties by hindering dislocation movement. Al 6061 reinforced with $1.25\mathrm{\%}{\mathrm{ZrO}}_2$$1.25\mathrm{\%}{\mathrm{ZrO}}_2$1.25%ZrO_(2)1.25 \% \mathrm{ZrO}_{2}$1.25\mathrm{\%}{\mathrm{ZrO}}_2$ and $1.25\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$1.25\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$1.25%Al_(2)O_(3)1.25 \% \mathrm{Al}_{2} \mathrm{O}_{3}$1.25\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ (Fig. 10e) shows the increase in agglomeration of the reinforced particulates, possibly creating stress concentrations which reduces the fracture toughness. Composite reinforced with $0.5\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$0.5\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$0.5%Al_(2)O_(3)0.5 \% \mathrm{Al}_{2} \mathrm{O}_{3}$0.5\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $1\mathrm{\%}{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{ZrO}}_2$1%ZrO_(2)1 \% \mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{ZrO}}_2$ (Fig. 10(f)) demonstrates a moderate particle dissemination with clustering, due to the lower content of alumina which affecting uniform dissemination. Fig. $10(\mathrm{g})$$10(\mathrm{g})$10(g)10(\mathrm{~g})$10(\mathrm{g})$ demonstrates variation in weight fraction of alumina content ( $0.75\mathrm{\%}$$0.75\mathrm{\%}$0.75%0.75 \%$0.75\mathrm{\%}$ ) with of $1\mathrm{\%}{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{ZrO}}_2$1%ZrO_(2)1 \% \mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{ZrO}}_2$ enhances dissemination and decreases the agglomeration and balances the reinforcement. Al6061 reinforced with $1.25\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$1.25\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$1.25%Al_(2)O_(3)1.25 \% \mathrm{Al}_{2} \mathrm{O}_{3}$1.25\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $1\mathrm{\%}{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{ZrO}}_2$1%ZrO_(2)1 \% \mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{ZrO}}_2$ (Fig. 10h) depicts the concentrated reinforcement particles with increased spacing between them. Higher ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ levels lead to more clustering, which results in diminishing wear resistance and fracture toughness. Among the produced composites with different composition, Al6061 reinforced with $1\mathrm{\%}$$1\mathrm{\%}$1%1 \%$1\mathrm{\%}$ alumina and $1\mathrm{\%}$$1\mathrm{\%}$1%1 \%$1\mathrm{\%}$ Zirconium dioxide shows more uniform dissemination of the reinforcing particles with small amount of clustering which suggest better potential for enhanced mechanical properties.

Fig. 11 shows the EDS spectra of $\mathrm{Al}6061-1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$\mathrm{Al}6061-1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$Al6061-1%Al_(2)O_(3)\mathrm{Al} 6061-1 \% \mathrm{Al}_{2} \mathrm{O}_{3}$\mathrm{Al}6061-1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $1\mathrm{\%}{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{ZrO}}_2$1%ZrO_(2)1 \% \mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{ZrO}}_2$ which confirms the existence of elements such as oxygen ( O ), zirconium ( Zr ), silicon ( Si ), magnesium ( Mg ), and chromium ( Cr ) and also depicts the other elements. Presence of O and Zr indicates that the reinforcing phases were well integrated to base matrix Al 6061 , while a summary
table displays the elemental breakdown of all samples for different composition. This method eliminates EDS spectra redundancy by revealing how reinforcements affect composite chemical composition. Elemental analysis for composite samples comparing Al6061 alloy with different reinforcement combinations is shown in Tab. 4. Quantifying elements like $\mathrm{Zr},\mathrm{O},\mathrm{Zn},\mathrm{Mn},\mathrm{Fe},\mathrm{Si},\mathrm{Cr}$$\mathrm{Zr},\mathrm{O},\mathrm{Zn},\mathrm{Mn},\mathrm{Fe},\mathrm{Si},\mathrm{Cr}$Zr,O,Zn,Mn,Fe,Si,Cr\mathrm{Zr}, \mathrm{O}, \mathrm{Zn}, \mathrm{Mn}, \mathrm{Fe}, \mathrm{Si}, \mathrm{Cr}$\mathrm{Zr},\mathrm{O},\mathrm{Zn},\mathrm{Mn},\mathrm{Fe},\mathrm{Si},\mathrm{Cr}$, TI and Mg reveals how ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ reinforcements affect the alloy's composition. The results show that matrix reinforcement type and quantity directly affect oxygen and zirconium levels.

XRD studies were performed to determine the different phases/crystalline formed in the Al6061 matrix and the nanosized reinforcements, namely ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$. For examining the crystallographic structure of materials XRD is a valuable technique which allows the detection of phase transitions, the distribution of reinforcements, and the presence of any secondary phases formed in stir casting process. The Al6061 alloy shows clear diffraction peaks at $2\theta$$2\theta$2theta2 \theta$2\theta$ values of ${38.47}^{\circ }$${38.47}^{\circ }$38.47^(@)38.47^{\circ}${38.47}^{\circ }$ (111), ${44.73}^{\circ }(200),{65.13}^{\circ }(220),{78.22}^{\circ }(311)$${44.73}^{\circ }(200),{65.13}^{\circ }(220),{78.22}^{\circ }(311)$44.73^(@)(200),65.13^(@)(220),78.22^(@)(311)44.73^{\circ}(200), 65.13^{\circ}(220), 78.22^{\circ}(311)${44.73}^{\circ }(200),{65.13}^{\circ }(220),{78.22}^{\circ }(311)$, and ${82.55}^{\circ }(222)$${82.55}^{\circ }(222)$82.55^(@)(222)82.55^{\circ}(222)${82.55}^{\circ }(222)$, each corresponding to the face-centered cubic (FCC) structure of aluminium as per JCPDS NO 04-0787. For $\alpha$$\alpha$alpha\alpha$\alpha$-alumina ( ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ ), reference peaks are observed at ${25.58}^{\circ }(012)$${25.58}^{\circ }(012)$25.58^(@)(012)25.58^{\circ}(012)${25.58}^{\circ }(012)$, ${35.15}^{\circ }(104),{37.77}^{\circ }(110),{43.39}^{\circ }(113),{52.61}^{\circ }(024),{57.50}^{\circ }(116),{66.51}^{\circ }(018),{68.24}^{\circ }(214)$${35.15}^{\circ }(104),{37.77}^{\circ }(110),{43.39}^{\circ }(113),{52.61}^{\circ }(024),{57.50}^{\circ }(116),{66.51}^{\circ }(018),{68.24}^{\circ }(214)$35.15^(@)(104),37.77^(@)(110),43.39^(@)(113),52.61^(@)(024),57.50^(@)(116),66.51^(@)(018),68.24^(@)(214)35.15^{\circ}(104), 37.77^{\circ}(110), 43.39^{\circ}(113), 52.61^{\circ}(024), 57.50^{\circ}(116), 66.51^{\circ}(018), 68.24^{\circ}(214)${35.15}^{\circ }(104),{37.77}^{\circ }(110),{43.39}^{\circ }(113),{52.61}^{\circ }(024),{57.50}^{\circ }(116),{66.51}^{\circ }(018),{68.24}^{\circ }(214)$, and ${77.03}^{\circ }(300)$${77.03}^{\circ }(300)$77.03^(@)(300)77.03^{\circ}(300)${77.03}^{\circ }(300)$ in accordance with JCPDS NO 46-1212. Meanwhile, monoclinic ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ presents characteristic peaks at ${28.19}^{\circ }(011),{31.48}^{\circ }$${28.19}^{\circ }(011),{31.48}^{\circ }$28.19^(@)(011),31.48^(@)28.19^{\circ}(011), 31.48^{\circ}${28.19}^{\circ }(011),{31.48}^{\circ }$ (111), ${34.13}^{\circ }(002),{35.24}^{\circ }(112),{50.31}^{\circ }(022),{59.68}^{\circ }(013),{62.80}^{\circ }(222)$${34.13}^{\circ }(002),{35.24}^{\circ }(112),{50.31}^{\circ }(022),{59.68}^{\circ }(013),{62.80}^{\circ }(222)$34.13^(@)(002),35.24^(@)(112),50.31^(@)(022),59.68^(@)(013),62.80^(@)(222)34.13^{\circ}(002), 35.24^{\circ}(112), 50.31^{\circ}(022), 59.68^{\circ}(013), 62.80^{\circ}(222)${34.13}^{\circ }(002),{35.24}^{\circ }(112),{50.31}^{\circ }(022),{59.68}^{\circ }(013),{62.80}^{\circ }(222)$, and ${74.39}^{\circ }(023)$${74.39}^{\circ }(023)$74.39^(@)(023)74.39^{\circ}(023)${74.39}^{\circ }(023)$ as given by JCPDS NO 37 1484.

XRD study of Al6061-based composites shows the crystalline structure and phase composition of nano alumina $({\mathrm{Al}}_2{\mathrm{O}}_3$$\left({\mathrm{Al}}_2{\mathrm{O}}_3\right.$(Al_(2)O_(3):}\left(\mathrm{Al}_{2} \mathrm{O}_{3}\right.$({\mathrm{Al}}_2{\mathrm{O}}_3$ and zirconia ( ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ ) in various quantities. As-cast Al 6061 alloy has clear peaks at specific $2\theta$$2\theta$2theta2 \theta$2\theta$ points, indicating a crystalline aluminum phase. $1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$1%quadAl_(2)O_(3)1 \% \quad \mathrm{Al}_{2} \mathrm{O}_{3}$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $0.5\mathrm{\%}{\mathrm{ZrO}}_2$$0.5\mathrm{\%}{\mathrm{ZrO}}_2$0.5%quadZrO_(2)0.5 \% \quad \mathrm{ZrO}_{2}$0.5\mathrm{\%}{\mathrm{ZrO}}_2$ add new peaks alongside Al peaks, showing successful incorporation. With $0.75\mathrm{\%}{\mathrm{ZrO}}_2,{\mathrm{ZrO}}_2$$0.75\mathrm{\%}{\mathrm{ZrO}}_2,{\mathrm{ZrO}}_2$0.75%ZrO_(2),ZrO_(2)0.75 \% \mathrm{ZrO}_{2}, \mathrm{ZrO}_{2}$0.75\mathrm{\%}{\mathrm{ZrO}}_2,{\mathrm{ZrO}}_2$ peaks intensify, ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ peaks persist, and Al matrix dominates. Separate peaks for ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ at $1\mathrm{\%}$$1\mathrm{\%}$1%1 \%$1\mathrm{\%}$ each, indicating strong reinforcement presence. With $1.25\mathrm{\%}{\mathrm{ZrO}}_2,{\mathrm{ZrO}}_2$$1.25\mathrm{\%}{\mathrm{ZrO}}_2,{\mathrm{ZrO}}_2$1.25%ZrO_(2),ZrO_(2)1.25 \% \mathrm{ZrO}_{2}, \mathrm{ZrO}_{2}$1.25\mathrm{\%}{\mathrm{ZrO}}_2,{\mathrm{ZrO}}_2$ peaks intensify, ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ peaks stable, and Al matrix dominates. In $0.5\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3+1\mathrm{\%}{\mathrm{ZrO}}_2$$0.5\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3+1\mathrm{\%}{\mathrm{ZrO}}_2$0.5%Al_(2)O_(3)+1%ZrO_(2)0.5 \% \mathrm{Al}_{2} \mathrm{O}_{3}+1 \% \mathrm{ZrO}_{2}$0.5\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3+1\mathrm{\%}{\mathrm{ZrO}}_2$, lower ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ concentration reduces its peak intensity, while ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ peak intensifies. With $0.75\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3+1\mathrm{\%}{\mathrm{ZrO}}_2$$0.75\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3+1\mathrm{\%}{\mathrm{ZrO}}_2$0.75%Al_(2)O_(3)+1%ZrO_(2)0.75 \% \mathrm{Al}_{2} \mathrm{O}_{3}+1 \% \mathrm{ZrO}_{2}$0.75\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3+1\mathrm{\%}{\mathrm{ZrO}}_2$, clear phases and dominant Al peaks coexist. The $1.25\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3+1\mathrm{\%}{\mathrm{ZrO}}_2$$1.25\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3+1\mathrm{\%}{\mathrm{ZrO}}_2$1.25%Al_(2)O_(3)+1%ZrO_(2)1.25 \% \mathrm{Al}_{2} \mathrm{O}_{3}+1 \% \mathrm{ZrO}_{2}$1.25\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3+1\mathrm{\%}{\mathrm{ZrO}}_2$ sample shows stronger ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ pattern and distinct ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ peaks, indicating good nano-reinforcement integration in Al 6061 matrix. Produced composites with different compositions show ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ phases alongside the dominant Al phase, which indicating the Al6061 structure is preserved. A similar trend was observed by Nagaral M et.al. [20] Confirms the incorporation of reinforcing particulates by maintaining the minimal phase change. The obtained result indicates nano reinforcements are well disseminated within the matrix which is expected to enhance the mechanical strength and wear resistance of the produced composites.

Evaluation of the fracture toughness is essential to evaluate the ability of a material to endure the crack initiation and growth, when it is subjected to stress [21]. The study employed the fracture toughness test to assess the influence of ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ reinforcements in the matrix of Al 6061 . It is evident from the analysis of the results that the optimal reinforcement blend of ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ would be obtained, which further enhances the composite's resistance to crack propagation and overall mechanical properties.

Fig. 13 shows the load-crack opening displacement (COD) curve. The graph shows the variation in the fracture toughness of Al6061 with different reinforcement proportions. Also, it is evident that the load required increases with an increase in COD, which is attributed to the resistance of the aluminium composites to crack propagation under the stress. As-cast Al 6061 samples exhibited a lower load capacity and failed well before the ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ reinforced Al 6061 samples. The incorporation of ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ nano particles enhanced the load bearing capacity and it is evident from the curves (Fig. 13). The Al 6061 composite with $1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$1%Al_(2)O_(3)1 \% \mathrm{Al}_{2} \mathrm{O}_{3}$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $1\mathrm{\%}{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{ZrO}}_2$1%ZrO_(2)1 \% \mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{ZrO}}_2$ shows the highest load capacity and greater improvement in the fracture toughness Al 6061 with $0.75\mathrm{\%}$$0.75\mathrm{\%}$0.75%0.75 \%$0.75\mathrm{\%}$ and $1.25\mathrm{\%}{\mathrm{ZrO}}_2$$1.25\mathrm{\%}{\mathrm{ZrO}}_2$1.25%ZrO_(2)1.25 \% \mathrm{ZrO}_{2}$1.25\mathrm{\%}{\mathrm{ZrO}}_2$ with $1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$1%Al_(2)O_(3)1 \% \mathrm{Al}_{2} \mathrm{O}_{3}$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ are envisaged to enhance load tolerance. The reinforcement of nano ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ particles has a significant effect on the fracture toughness. To evaluate the fracture toughness $K_{IC}$$K_{IC}$K_(IC)K_{I C}$K_{IC}$ the following equation is used

The critical load $P_q$$P_q$P_(q)P_{q}$P_q$, the beginning of unstable crack growth, which is prominently identified at the peak, where a significant change in slope is observed. $B$$B$BB$B$ is the thickness of the specimen, $W$$W$WW$W$ is the width of the specimen and $f(a/w)$$f(a/w)$f(a//w)f(a / w)$f(a/w)$ indicates the ratio of crack length to the specimen width. The $95\mathrm{\%}$$95\mathrm{\%}$95%95 \%$95\mathrm{\%}$ secant line method uses a reference line (which is drawn at $95\mathrm{\%}$$95\mathrm{\%}$95%95 \%$95\mathrm{\%}$ of the initial elastic slope) intersecting with the actual load-COD curve mainly to detect the critical transition from linear to nonlinear behaviour and the unstable fracture propagation.

The fracture toughness assessment of the Al6061 alloy indicates that the crack resistance of the alloy is greatly improved with the addition of nano-sized ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ particles. As cast Al 6061 exhibits a fracture toughness of $7.28K_{IC}$$7.28K_{IC}$7.28K_(IC)7.28 K_{I C}$7.28K_{IC}$. With the addition of $1\mathrm{\%}$$1\mathrm{\%}$1%1 \%$1\mathrm{\%}$ alumina and $0.50{\mathrm{ZrO}}_2$$0.50{\mathrm{ZrO}}_2$0.50ZrO_(2)0.50 \mathrm{ZrO}_{2}$0.50{\mathrm{ZrO}}_2$, toughness increased significantly to $9.23K_{IC}$$9.23K_{IC}$9.23K_(IC)9.23 K_{I C}$9.23K_{IC}$, which highlights the beneficial influence of the synergistic reinforcement. The addition of more ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ to 0.75 and keeping the ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ at 1 gave a toughness value of $9.75K_{IC}$$9.75K_{IC}$9.75K_(IC)9.75 K_{I C}$9.75K_{IC}$ and this means that the higher the content of ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$, the higher the resistance that will be offered to the fracture propagation. The highest fracture toughness value of $10.73K_{IC}$$10.73K_{IC}$10.73K_(IC)10.73 K_{I C}$10.73K_{IC}$ was observed in the specimen containing $1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$1%Al_(2)O_(3)1 \% \mathrm{Al}_{2} \mathrm{O}_{3}$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $1\mathrm{\%}{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{ZrO}}_2$1%ZrO_(2)1 \% \mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{ZrO}}_2$, indicating that the combination of the two nano-reinforcements significantly improved the resistance of the alloy to crack propagation. Nevertheless, a slight decrease in fracture toughness to 10.28 $K_{IC}$$K_{IC}$K_(IC)K_{I C}$K_{IC}$ is observed with an increase in ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ content to $1.25\mathrm{\%}$$1.25\mathrm{\%}$1.25%1.25 \%$1.25\mathrm{\%}$. This reduction can be explained by particle clustering or a change in the interaction between the matrix reinforcement at such high concentrations.
Al 6061 with $0.5\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$0.5\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$0.5%Al_(2)O_(3)0.5 \% \mathrm{Al}_{2} \mathrm{O}_{3}$0.5\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $1\mathrm{\%}{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{ZrO}}_2$1%ZrO_(2)1 \% \mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{ZrO}}_2$, and Al 6061 with $0.75\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$$0.75\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$0.75%Al_(2)O_(3)0.75 \% \mathrm{Al}_{2} \mathrm{O}_{3}$0.75\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3$ and $1\mathrm{\%}{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{ZrO}}_2$1%ZrO_(2)1 \% \mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{ZrO}}_2$ have shown the increased values of fracture toughness of $9.61K_{IC}$$9.61K_{IC}$9.61K_(IC)9.61 K_{I C}$9.61K_{IC}$ and $10.01K_{IC}$$10.01K_{IC}$10.01K_(IC)10.01 K_{I C}$10.01K_{IC}$, respectively, which are greater than that of the as cast Al6061 alloy. These data indicate that the $1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3-{\mathrm{ZrO}}_2$$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3-{\mathrm{ZrO}}_2$1%Al_(2)O_(3)-ZrO_(2)1 \% \mathrm{Al}_{2} \mathrm{O}_{3}-\mathrm{ZrO}_{2}$1\mathrm{\%}{\mathrm{Al}}_2{\mathrm{O}}_3-{\mathrm{ZrO}}_2$ mixture results in the most significant increase in fracture toughness. Beyond this reinforcement content level, though, there is no proportional benefit of an increase in the reinforcement content since the gains level off. The same tendencies have been detected by previous studies [22, 23]. It is essential to optimize the ratio of ${\mathrm{Al}}_2{\mathrm{O}}_3$${\mathrm{Al}}_2{\mathrm{O}}_3$Al_(2)O_(3)\mathrm{Al}_{2} \mathrm{O}_{3}${\mathrm{Al}}_2{\mathrm{O}}_3$ to ${\mathrm{ZrO}}_2$${\mathrm{ZrO}}_2$ZrO_(2)\mathrm{ZrO}_{2}${\mathrm{ZrO}}_2$ to maximize the fracture toughness of the alloy.

Fig. 15 depicts the SEM images of the fracture toughness of the tested samples which reveal failure mechanisms in the synthesized composites at different weight fractions. Fig. 15 (a) depicts the fracture surface of the as-cast Al 6061 indicates the ductile nature behaviour which are marked with large and uniform dimples which demonstrate the micro void coalescence.
Fig. 15 presents images of the cracked surface, illustrating the various shapes associated with each composite configuration of the Al6061 alloy, reinforced with differing amounts of nano- Al 2 O 3 and ZrO 2 . The fracture surface of the unreinforced base alloy (Fig. 15a) exhibits large dimples and significant plastic deformation, indicative of ductile fracture. The images illustrate a trend toward smaller dimples and the development of mixed ductile-brittle properties as the amount of reinforcement rises (Fig. 15b-h). Increased levels of reinforcement lead to more distinct river patterns and crack deflection points, signifying a shift towards brittle fracture behavior. The specimen containing $1\mathrm{\%}$$1\mathrm{\%}$1%1 \%$1\mathrm{\%}$ alumina and $1\mathrm{\%}$$1\mathrm{\%}$1%1 \%$1\mathrm{\%}$ zirconium dioxide (Fig. 15d) demonstrates balanced fracture morphology, featuring both micro void formation and transgranular fracture characteristics, which suggests enhanced toughness attributed to the effective dispersion of nanoparticles.
In contrast, samples with excessive reinforcement demonstrate increased particle aggregation and a tendency for brittle fracture, underscoring the necessity to optimize nano-reinforcement levels to improve mechanical performance in the alloy system.