Characterization and strengthening mechanism of CNT/TiB2 particulates added AZ91D composites

In the current work, magnesium (AZ91D) matrix composites reinforced with different weight fractions (5, 10, and 15%) of titanium diboride (TiB2) and 1.5 wt% carbon nanotubes (CNTs) are fabricated using stir casting. The improvements in mechanical, wear and corrosion resistance properties are evaluated as per ASTM guidelines. The synergistic strengthening effect of TiB2 and CNT is also studied. It was discovered that the AZ91D/(1.5CNT-10TiB2) composite outperformed other magnesium matrix composites in terms of strength and ductility. Experimental characterization and quantity analysis revealed that the load transfer process of CNT, thermal mismatch, and grain refinement are the primary factors leading to the composite’s increased tensile strength. Porosity tends to increase due to variance in the thermal expansion coefficient of particles and matrix material; Orowan strengthening mechanism plays a prominent role in enhancing tensile strength. Because of the influence of synergistic strengthening, microparticles TiB2 increased the proportion of load transmission mechanisms, and thermal mismatch facilitated the homogenous distribution of CNTs. Wear resistance and corrosion resistance increase with the inclusion of CNTs and TiB2 content. An abrasive-type wear mechanism is seen in the SEM image, and the wear craters are also seen in all the SEM images. Adding TiB2 significantly improves the cast composites’ resistance to corrosion because of grain refinement. Higher addition of TiB2 influences higher pitting corrosion due to poor grain refinement.


Introduction
Magnesium alloys effectively compete with aluminium and alloy steels as they possess lower density and weight and higher strength (1.75-1.85 g cm −3 ) [1]. Magnesium alloys possess greater damping capacity, dimensional stability and good heat dissipation [2]. The disadvantages of monolithic magnesium and magnesium alloys, such as lower creep strength at high temperatures and comparatively poor corrosion resistance due to high chemical activity and subsequent lower equilibrium potential prone to galvanic corrosion, are overcome through the formation of magnesium-based composites [3]. The corrosion resistivity and creep strength can be enhanced by adequately adding particulates as reinforcements in the magnesium matrix. The selection of matrix material (magnesium and its alloys) depends on the end-user applications and operational conditions [4]. Because of their high modulus of elasticity and tensile strength with low density, carbon nanotubes (CNTs) reinforcements are considered in Mg composite grids. The CNTs must be scattered well and distributed uniformly in the matrix of composites to achieve better properties. The added CNTs must be lower, as higher reinforcements lead to agglomeration [5].
Carbon nanotubes (CNTs) are allotropic carbon forms most commonly manufactured through chemical vapour deposition and cylindrically shaped. For example, nanotubes form networks inside composite materials to boost rigidity and material damping [6]. CNTs exhibit exceptional heat conductivity, electrical properties, and mechanical characteristics. CNTs, besides graphite, are firmly chemically inert and counteract practically any chemical consequence except if subjected to more excellent oxygen and temperatures at the same time, making them highly resistant to corrosion [7]. Titanium diboride (TiB 2 ) is a ceramic with extremely high strength and endurance, evidenced by its high hardness, higher ratio of strength to density, melting point (3225°C), improved thermal conductivities, and abrasion resistance. TiB 2 is a material that is used in gaskets, wear components, and cutting instruments. Because of its toughness, it is employed to create ballistic shields and wear-resistant coatings in molten metal crucibles [8].
Sun et al [9] developed novel ex situ TiB 2 micro-particles added Mg-10Li-3Al composite through stir casting. Intensive grain refinement and CTE mismatch effects of TiB 2 improve the strength of the composites, whereas TiB 2 pre-treatment lessens cluster formations and imperfections in composites. The robust grain refining consequence of TiB 2 was primarily responsible for the composites' strong interaction. Xiao et al [10] 2.5 wt% nanoparticles of TiB 2 particles were incorporated into Al-TiB 2 alloy fabricated by reaction of an Al-K 2 TiF 6 -KBF 4 in molten AZ91. The strength in yield, tensile, and elongation at fracture of nanocomposite is enhanced due to deformation and Orowan strengthening. Aydin and Durgut [11] explored the wear behaviour of AZ91 and discovered that for all sliding distances and several sliding speeds, the loss of volume in wear rises as the load increases. The sliding distance highly contributes to the wear, and the worn surface shows oxidation and abrasive wear mechanisms. The results of artificial neural network (ANN) were compared with support vector regressor and random forest method, and promising output was obtained from ANN models. Tsukamoto [12] used spark plasma sintering to create CNT-added magnesium composites. It was asserted that obtaining composites with superior properties was challenging due to low dispersibility and developing accumulations of CNT, as well as the existence of void spaces and fractures in the composites. It was proposed that introducing Nano ZrO 2 fragments enhanced dispersibility and defences of CNT agglomeration creation, while the other is stuffing voids and fractures with low melting metals like Sn.
Upadhyay et al [13] reviewed different fabrication methodologies to make Mg-CNT composites and their potential stipulations and applications. It was discovered that inter-particle interaction of the matrix and CNTs, homogeneous scattering of the CNTs in the matrix, and aspect ratio are the primary determining aspects of manufactured composite attributes. Furthermore, homogeneous CNT distribution, wt% of CNTs, its orientation and in-between bonding strength may be held to account for enhancing mechanical properties. Pressure infiltration was used to create TiB 2 and graphene-based AZ91 nanocomposite [14]. When likened to pure alloy, the composite containing just TiB 2 , the ratio of porosity was higher, with higher ratios of graphene, the measured compressive strength was 415 MPa, and for 0.25 vol% graphene added composite, it was 140.6 HB. Including 0.25 vol% graphene resulted in the most significant abrasion rebellion, the mechanism of abrasive and adhesion was observed based on loads. Meher and Mahapatra [15] fabricated and investigated the tribological behaviour of RZ5/10 wt%TiB 2 . The introduction of TiB 2 resulted in a substantial enhancement in wear resistance. As the load increases, so makes the wear loss, with a subsequent reduction in the friction coefficient. Furthermore, as the sliding distance increased, so made the loss of wear and friction coefficient. Mustu et al [16] adopted the hot pressing method to fabricate ZK60 alloy, ZK60/15 wt%TiB 2 , and ZK60/15 wt%TiB 2 + 0.5GNPs composites and investigated the tribological and compressive properties. The ideal wear efficiency was acquired with ZK60/15 wt%TiB 2 and 15 wt%TiB 2 + 0.5GNPs specimens below 40 and 10 N loads. The dominant mechanism of wear was found to be abrasion at minimal loads and delamination at elevated loads. Anas et al [17] blended fabricated Al, Cu, Mg and 0-2.5 wt% CNT powders in a high-energy ball mill and subsequently forged and hot extruded. With CNT inclusions, the milled powder particle size decreased; however, there is hardly any noticeable impact on crystallite size. Al alloy-CNT Nano composite's material strength has improved because of ultrafine-sized grains, finer precipitation of secondary phase, and homogeneous CNT scattering. Disintegrated melt deposition integrated with hot extrusion was used to fabricate AZ31 + 1vol% CNT composite [18], and found that the composite had a longer failure time and a smaller area than its monolithic equivalents. The manufactured magnesium nanocomposite demonstrated a duplex crystal structure with light alpha (α) grain areas and black eutectic (interfacial Mg 17 Al 12 ) beta (β) areas. Zhao et al [19] developed CNT reinforced AZ91D composite using ultrasonic processing. The introduction of 0.5 wt% CNTs    [20] made a CNT + AZ61 composite; a pre-dispersion-based ultrasonic vibration method was adopted. Pre-dispersion might decompose the CNT groupings and dissipate the CNTs through a slurry mixture. When likened to Mg-6Zn alloy, the CNTs/AZ61 has higher ultimate tensile, yield strength, and elongation. CNTs on fractography are extracted straightforwardly from the matrix. From the performed literature, it was discovered that adding CNTs to a metal matrix promotes the refinement of grains, and with the appropriate methodology, the clustering of CNTs can be avoided. These characteristics of CNTs impart higher strength, and hence are chosen as reinforcement. Titanium diboride (TiB 2 ) ceramic is considered as reinforcement in the AZ91D matrix that can impart higher mechanical strength. Hence hybrid composite is considered in this work, which can be fabricated via the economic stir casting route.

AZ91D Alloy
In this research, AZ91D is selected as the matrix material, an alloy of magnesium. The AZ91D elemental composition is shown in table 1. AZ91D containing zinc (1%) and aluminium (9%), is a popular alloy in the Mg-Al-Zn series. This cast alloy is a widely used alloy with higher purity, superior corrosion opposition, castability, and strength [21]. The density of the alloy is 1.81 g cm −3 . The tensile strength of AZ91D is 240 MPa, 156 MPa strength in yield, hardness of 70 HV, 2.7 J impact strength and 26 μm m −1°C−1 thermal expansion coefficient [22]. Figure 1 shows the AZ91D SEM and EDX.
CNT-reinforced composites provide high abrasion resistance and breaking resilience, as well as antiwear characteristics and low weight. Composite made from CNTs could reduce spacecraft and aircraft mass by approximately 30% [23]. The single-walled CNTs were purchased from Sigma-Aldrich, having 99% carbon nanotubes with an average diameter of 0.84 nm. Figure 2 depicts the micrograph of CNTs used in this study. TiB 2 is utilized to make composites in which the existence of the particles enhances the matrix's toughness and rupture strength [24]. Southern India Scientific Corporation, Chennai, supplied TiB 2 with 99% purity and a particle size of 75 μm. Figure 3 micrographs of the TiB 2 utilized.

Fabrication of CNT, TiB 2 reinforced AZ91D composites
As reinforcement, CNT with a 50 nm mean diameter was used. A two-step stir-casting technique was used to fabricate. Stir casting has several benefits over other fabrication techniques, including its flexibility, simplicity, low cost, and ability to handle larger quantities. The melting of the matrix was done in N 2 presence to prevent the burning and to evaporate of magnesium alloy. At 720°C, melting of AZ91D was done and subsequently cooled at 560°C for further processing in the semi-solid state [25]. The preheated CNTs and TiB 2 particles were fed into the semi-solid Mg alloy. After 15 min of stirring the semi-solid melt at 500 rpm speed, the needed composites were cast using the preheated die into the required shape and dimension. Different weight fractions of TiB 2 (5, 10, and 15%) were added with 1.5 wt% CNT and reinforced in the matrix of AZ91D. Figure 4 presents the experimental setup for fabricating the hybrid magnesium nanocomposites (HMNCs). The manufactured magnesium composites are employed in transfer cases, radiator supports, driveline parts, orthopaedic and biomedical implants, and aerospace elements such as wheels, engine gearboxes, structural objects, garage doors, and edge flaps. While pouring molten charge in stir casting, the die is preheated to avoid thermal shock, thermal fatigue and chilling effect.

Characterization of Fabricated HMNCs
Numerous discontinuities in materials, like inclusions, are distinguished using microscopes in samples that are as-polished [26]. Such material shortcomings serve as failure-initiating locations; thus, assessment of ambiguity in size and shape and dissemination is required to establish a connection between engineering reliability and mechanical characteristics via microstructural assessment [27]. The composites density is calculated theoretically based on the rule of mixtures provided in equation (1), considering the weight fractions of TiB 2 and CNT and matrix material. Based on ASTM procedure D792, the experimental density of fabricated HMNCs is determined based on Archimedes' principal with distilled water as suspending medium. The porosity percentage is calculated using equation (2) [28].  material's performance to a load applied suddenly and the required energy to rupture the sample; the experiment follows the ASTM E23 criterion [30]. Assessing the hardness of materials is a highly validated and beneficial strategy for quantifying and equating a material's structural properties so that reasonable precautions are engrossed during real-world implementation. The composite Vickers hardness (HV) is measured using the   ASTM E92 criterion, and a diamond indenter with pyramid-shaped (136°angle) with an applied load of 10 kg was used [31]. A rectangular-shaped specimen was made per the ASTM D790 benchmark for the bending test.
The specimen was positioned on the universal testing machine, and a vertical load was imposed till the breakeven stage was reached. A flexure test is typically used to assess the modulus and strength in the flexural of a material, which is especially important if the part being manufactured will be exposed to flexural loading during service [32].
The Pin-On-Disc (PoD) test is widely used to analyze material wear effectiveness and categorization. It is regarded as a universal test used to evaluate a material's sliding wear behaviour by the ASTM G99 process [33]. For 10 min, specimens were exposed to a 20 N load on an EN31 disc with a 60 mm track diameter and 300 rpm speed. Corrosion is described as the deterioration of materials caused by chemical interactions on the surface due to prevailing aggressive conditions. An AC supply is used as an input to the system in the electrochemical impedance spectroscopy (EIS) approach to establish the material's corrosion resistance. The EIS method collects impedance data for the imposed signal using frequency at a static potential under the ASTM G106 benchmark [34].

Result and discussion
The casted specimens are subjected to microstructural analysis to identify the CNTs and TiB2 distribution pattern in the AZ91D matrix. The agglomeration problem associated with CNTs inclusion is avoided by  selecting proper casting parameters. Uniformity in reinforcement distribution is seen in the base matrix; with higher loadings of TiB 2 , higher grey particles are visible [35]. 15% reinforcement of TiB 2 leads to agglomeration due to faster solidification related to the reinforcements and matrix thermal expansion coefficients [36]. Figure 5 presents the optical micrographs of 5, 10 and 15% TiB 2 included casted specimens.
The number of interlayer structures was considerably higher around TiB 2 particles than in other areas, meaning that TiB 2 microparticles were primarily responsible for increased CNT dispersion and grain refining [37]. The AZ91D/(1.5CNT-10TiB 2 ) composite, on the other hand, had thick interlayers and higher discontinuities than the AZ91D/(1.5CNT-15TiB 2 ) composite. Clusters of CNTs were absorbed simultaneously in AZ91D/(1.5CNT-5TiB 2 ) composite, implying that well-dispersed CNTs were incorporated into the AZ91D/(1.5CNT-10TiB 2 ) composite's interlayer structures. TiB 2 microparticles have been found to have a high potential for increasing CNT homogenous dispersion. These findings suggest that CNTs and the AZ91D matrix have robust interfacial interactions. The external stress on the metal matrix was transmitted to the rigid reinforcements via interface adhesion among the reinforcements and matrix as per the theory of shear-lag. The matrix-transferred applied load was effectively sustained in situ because of higher interface adhesion among CNTs and matrix [38]. Grain refinement occurs with the addition of reinforcements and CNTs, which pave the way for improved strength. Dispersion seems to be homogeneous due to the effectiveness of the stir-casting procedure. Higher incorporation of TiB 2 favours clustering of particulates that may lower the mechanical strength of the composites due to increased brittleness and improper adhesion of reinforcements and matrix.
Archimedes principle-based density is calculated experimentally and is compared with theoretical densities, and the calculated % porosity of the HMNCs is presented in table 2, figures 6 and 7. Observation from the experimental density and % porosity values presents that TiB 2 and CNTs reinforce inclusion, and porosity tends to increase due to variance in the thermal expansion coefficient of particles and matrix material [39].
The porosity % tends to increase with higher reinforcement loadings in the matrix material. The experimental density closely matches the theoretical density because of the higher efficiency of the stir casting procedure and subsequent solidification and casting. Higher inclusion of TiB 2 particles substantially increases the density of the HMNCs [40].
The tensile strength of the cast specimens is presented in figure 8, along with the stress-strain curve. With the inclusion of 1.5% CNTs in AZ91D, ultimate tensile strength (UTS) increases, and with further inclusion of TiB 2 secondary particles, a considerable improvement in UTS is absorbed until 10% addition [41]. Further addition of TiB 2 to 15% lowers the UTS of the HMNCs due to improper distribution of particulates leading to reduced strength. The bonding of particulates with the AZ91D matrix is reduced due to agglomeration. The ductility of the AZ91D + 1.5%CNT composite is higher than the unreinforced AZ91D as the added CNT improves the strength. But with the inclusion of ceramic TiB 2 , the toughness of the HMNCs is improved, which leads to lower elongation. The ceramic reinforcements restrict the dislocation movement and the Orowan strengthening mechanism plays a prominent role towards the enhancement of the tensile strength of the HMNCs [42]. The stress-strain graph shows that with the inclusion of TiB2, there is a considerable reduction in strain even though the stress is increased due to the restriction of dislocation movement. Figure 9 presents the cast composites' strength in yield and hardness values. Inclusion of CNTs, the nanoparticle's surface area is higher, which is the essential factor for improved strength and hardness [43]. With the inclusion of TiB 2 , hardness, strength, and yield improved as the reinforced TiB 2 are harder than the matrix. But with a 15% addition of TiB 2, there is a decrease in strength due to agglomerated TiB 2 that leads to poor bonding. The added TiB 2 in the grain boundaries inhibits dislocations and increases the strength [44].
To completely understand CNTs and TiB 2 particles have a synergic strengthening effect on the mechanical activity of the AZ91D/(1.5CNT-10TiB 2 ) composites, the key strengthening processes identified are reinforcement of grain, load transmission, thermal disparity, precipitate strengthening and Orowan strengthening [45]. TiB 2 is also thermodynamically stable in the composites (CNT-TiB 2 )/AZ91D. In the case of Orowan strengthening, the reinforcements were considered extremely small and had a grain distribution of uniform size and dimension. CNTs and TiB 2 microparticles may contribute to the dual-reinforced composite's mechanical strength [46]. As a result, load transfer, grain refinement of TiB 2 microparticles and CNTs, and  hybrid reinforcement thermal mismatch all played a role in the impact of hybrid reinforcements on AZ91D matrix composites [47]. Figure 10 depicts the normalized fracture surface morphologies of composites AZ91D/(CNT-TiB 2 ) produced by various combination proportions tensile tests at room temperature. Figure 10(a) describes the fracture surface of the composite AZ91D/(1.5CNT-5TiB 2 ) has well-scattered and cracked TiB 2 microparticles. Some of the TiB 2 microparticles apparently fractured when tensioned ( figure 10(b)). Based on these micrographs, the fracture of the composite was ascribed by the rupture of TiB 2 , which resulted in the development and propagation of cracks in the AZ91D matrix. During the tensile load, the locally available stresses tend to accumulate on interfacial connection and then shift to brittle TiB 2 particles ( figure 10(c)). Local stresses developed around the microcracks of TiB 2 due to an increase in load, causing the initiation and propagation of matrix cracks which explains how the ductility of the composite is lowered by TiB 2 particles. Figure 10(d) shows fissures and fractured TiB 2 particles in the composite AZ91D/(1.5CNT-15TiB2). Figures 10(e) and (f) demonstrate the CNTs morphology on the fractured surface. The CNTs acted as a bridge, joining the damaged matrix and inhibiting fracture propagation, which explains why composite AZ91D/(1.5CNT-10TiB 2 ) had higher plasticity. On the fracture surface, pull-out and peeling of CNTs were also detected ( figure 10(f)). An improved bonding at interfaces of matrix and CNTs is responsible for the successful reinforcement of the CNT load transfer into the composite. When the load applied shifts from matrix to CNTs, fracturing from the outermost layer results in CNT peeling and effective load transmission. It also reduced the transport capacity of TiB 2 micro-particles, preventing the fracturing of particles and creating matrix cracks Mg. Significant fractures occurred on the surface of the composite ( figure 10(g)). The CNT is combined and aggregated without direct contact with the AZ91D matrix, which allowed the presence of a high number of voids and faults in the CNT clusters (figure 10(i)) to be verified, which were the sources of cracks that hampered the flexibility and strength of the material during tensile testing.
For flexural strength determination on the cast HMNCs, a 3-point bending procedure is adopted. Observation shows that the inclusion of CNTs and TiB 2 has a role in increasing the flexural strength of the HMNCs as they lower the ductility and flexibility of the composite by increasing its stiffness and toughness [48], as seen in figure 11. With the inclusion of 1.5 wt% of CNTs in AZ91D, the flexural strength is increased by 3.93% compared to unreinforced AZ91D. Similarly, adding TiB 2 by 15 wt% leads to an increase in flexural strength by 18.70% compared with AZ91D + 1.5CNT composite.
Based on G99 ASTM guidelines, the pin-on-disc (PoD) experiments are performed with an axial load of 20 N, 300 rpm disc speed and a testing time of 10 min. The PoD equipment is attached to a data acquisition system (DAQ) to track the friction parameters. Cylindrical specimens with 15 mm and 9 mm diameter lengths are  prepared for testing [49]. The pin and disc surface roughness is maintained at 5 microns before testing. Before commencing the trial, the pins are weighed in an electronic balance having a precision of 0.001 g. After the trial, the pins are weighed again, and the difference in weight is calculated, which is the wear loss. Figure 12 presents the wear loss, and CoF obtained from the wear test. With the inclusion of CNTs and TiB 2 , wear loss is considerably decreased due to hard ceramic particulates and the lubricating nature of CNTs [50]. Concerning CoF, it is observed that with the inclusion of 1.5 wt% CNTs, a lower CoF is observed due to the increased surface area and lubrication offered by the nanoparticles. With further incorporation of hard TiB 2 particles, the hardness of the HMNCs increases, which subsequently increases the CoF values [51,52].
The variation of CoF recorded using DAQ is plotted over time and is presented in figure 13. Initially, the CoF tends to increase, and after some time, it tends to settle down. The variation in CoF during the test may be due to the hindrance offered by the rigid ceramic reinforcements [53]. The highest CoF is sensed for AZ91D + 1.5CNT Figure 17. Corrosion structure of as cast and HMNCs. + 15TiB 2 HMNCs, and the lowest CoF is observed for AZ91D + 1.5CNT composite. Figure 14 presents the worn surface SEM micrographs for all the cast specimens.
Rough surface is sensed for AZ91D and TiB 2 reinforced HMNCs and a smoother surface after the wear test is observed for AZ91D + 1.5CNT composite. The smoother surface is because of the nanoparticle's higher surface area, which is also influenced by the lubricating nature of the CNTs. The reinforced TiB 2 is exposed in some places during the wear test, and minor surface cracks are also visible after the wear test. In as-cast AZ91D alloy, the wear track is deep, whereas with the addition of 1.5% CNT, the smooth surface deliberates the lubrication nature of CNTs. With TiB 2 addition, the hardness increases and subsequently, the lubrication nature lowers, and the surface becomes rough with grooves. Ploughing-type wear is observed. With higher reinforcements, the fusion of reinforcements is seen on the surface due to a higher friction coefficient and friction force. Abrasive type wear mechanism is seen in the SEM image, and the wear craters are also seen in all the SEM images [54].
The EIS corrosion test is performed on the cast composites under a 3.5% NaCl environment, and the corrosion rate obtained is presented in figure 15. It is observed that the inclusion of CNTs increases resistance to corrosion by lowering the current density and improving potential [55]. Among the secondary addition to magnesium composites, CNTS are, without hesitation, an excellent choice [56]. CNTs possess poor water wettability and sturdy corrosion resistance; galvanic corrosion is observed between the magnesium matrix and CNTs [57]. Adding TiB 2 considerably improves the cast composites' resistance to corrosion because of grain refinement [58]. For increased corrosion resistance, the Icorr value has to be lower. In this work, the 10% addition of TiB 2 provided better resistance. With the higher addition of TiB 2 , the resistance to corrosion decreases [59].
The association between the density of current (log) and overpotential are represented by a Tafel plot [60]. The Tafel curves depicted in figure 16 shows that CNTs and TiB 2 -incorporated NHMCs shows a reduced rate of corrosion than unreinforced alloy. The polarisation graph of HMNCs shows lower currents, denoting that they are incredibly resistant to corrosion [61,62]. An elevated Tafel slope indicates a greater energy bandgap, which leads to increased overpotential and consumes more energy to activate the system. As a result, the sample containing 10% TiB 2 has improved corrosion tolerance. Higher addition of TiB 2 influences higher pitting corrosion due to poor grain refinement [63].
The surface of the specimens subjected to corrosion tests is evaluated for their surface texture. The pitting corrosion has occurred at the grain boundaries [64]. CNTs added AZ91D composite has a lower corroded surface whereas 15%TiB 2 added HMNCs have a rough surface due to galvanic corrosion and pitting corrosion [65]. Figure 17 presents the corrosion structure obtained for the as-cast and HMNCs. The corrosion occurs on the grain boundaries, visible in cast alloy and on other HMNCs. With the inclusion of TiB 2 , corrosion is inhibited as the reinforcement restricts the impact of corrosion reagents.

Conclusion
In this study, the stir casting procedure is adopted for fabricating different weight fractions of TiB 2 reinforced 1.5 wt%CNTs added AZ91D magnesium alloy composite. The observations made from characterization and microstructural analysis are: 1. Homogeneous distribution of TiB 2 and CNTs is achieved with the stir casting technique. 15 wt% addition of TiB 2 in AZ91D + 1.5CNT composite produces agglomeration of TiB 2 particulates. Grain refinement happens with the inclusion of CNTs.
2. The addition of TiB 2 enhanced the structures of interlayers containing CNTs implying a better capacity for dispersing CNTs and a more significant contribution of CNTs to thermal mismatch and load transfer evolutions.
3. Including CNTs and TiB 2 increases porosity because of the coefficient of linear thermal expansion difference in particles and matrix material. The porosity % tends to increase with higher reinforcement loadings in the matrix material. The higher inclusion of TiB 2 particles substantially increases the density of the HMNCs.
4. With the inclusion of 1.5% CNTs in AZ91D, UTS increases, and with further inclusion of secondary reinforcement of TiB 2 particles, a considerable improvement in UTS is absorbed until 10% addition. Further addition of TiB 2 to 15% lowers the UTS due to agglomeration producing lower strength. The key strengthening processes identified are grain refinement, load transmission, thermal mismatch, precipitate strengthening and Orowan strengthening.

5.
Hardness and flexural strength increase with the addition of CNTs and TiB2 on the grain boundaries, hindering the dislocation movement and lowering the ductility and increasing stiffness and toughness.
Including 1.5 wt% of CNTs in AZ91D, the flexural strength is increased by 3.93% compared to unreinforced AZ91D. Similarly, adding TiB2 by 15 wt% leads to an increase in flexural strength by 18.70% compared with AZ91D + 1.5CNT composite.
6. Combining CNTs and TiB2 lowers wear loss due to hard ceramic reinforcements and the lubricating nature of CNTs. Lower CoF is observed due to the nanoparticles' increased surface area and lubrication. With further incorporation of hard TiB 2 particles, the hardness of the HMNCs increases, which subsequently increases the CoF values. Abrasive type wear mechanism is seen in the SEM image, and the wear craters are seen in all the SEM images.
7. CNTs and TiB 2 -incorporated HMNCs possess a lower rate of corrosion than AZ91D. The polarization curve of HMNCs shows lower anodic, cathodic, and Icorr current flow, implying that it is incredibly resistant to corrosion. HMNCs with 10 wt% TiB 2 inclusion delivers improved resistance to corrosion. Higher addition of TiB 2 influences higher pitting corrosion due to poor grain refinement. Galvanic and pitting-type corrosion mechanism is observed for the cast composites.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).