Corrosion Behavior of AZ91/TiC/Al2O3 Hybrid Composites

In the current research, the corrosion behaviors of AZ91 Mg-hybrid composites amalgamated with titanium carbide and alumina reinforcing particles have been investigated. AZ91 Mg-hybrid composites have been developed by using the bottom pouring vacuum-based stir casting process to attain the homogeneous distribution of reinforcing particles in the AZ91-Mg matrix. The corrosion behavior of AZ91-hybrid composites has been evaluated by using corrosion potential (OCP) and potential dynamic polarization scans, as well as friction coefficient, have been deduced to examine the wear behavior of hybrid composites in sodium chloride (3.5%) solution. However, before and after each corrosion and wear test of AZ91-hybrid composites samples, the metallographic structures have been examined by using a scanning microscopic setup. The corrosive results reveal that the corrosion rate of the AZ91-Mg matrix has been greater than AZ91/TiC/Al2O3 composites. The AZ91/TiC/Al2O3 composites result also confirms that the corrosion resistance increases with an increase in wt% of TiC and Alumina. The prime reason for to increase in the corrosion rate is due to the increase in the number of matrix/reinforcement interfaces. However, the friction coefficient and wear rate of AZ91 Mg-hybrid composites rose with the increase in TiC wt%. The type of wear mechanism of AZ91 Mg-hybrid composites has been demonstrated that initially abrasive wear has been observed but later it predominantly transforms to both (i.e., adhesive and abrasive wear) with an increase in TiC wt%.


Introduction
In recent years, the usages of AZ-series magnesium alloys are commonly employed in industrial applications. Especially, AZ91 alloy has been used for various applications such as engine head covers, car body parts, airplane wings, etc. [1,2]. In developed countries, over the course of time, AZ91 Mg alloys replace traditional materials such as bronze and iron-based materials and show advantageous applications. Generally, researchers preferred magnesium metal matrix composites (Mg-MCs) for light-weight applications which enhance the strengthening properties under high temperatures conditions and good durability as well as high corrosion resistance [3]. Therefore, Mg-MCs are reinforced with hard ceramic particles such as SiC, B 4 C, graphite, alumina, etc. ensuring improved mechanical and tribological properties [4].
Hybrid metal matrix composites are very significantly considered in the industry especially in airspace and automotive due to their astonishing properties when compared with monolithic materials. However, non-homogeneous distribution and contraction of particle size in the matrix composites can somehow degrade the properties of the hybrid composites illustrating dislocation generation and agglomerate particles between the matrix and reinforcing interfaces. Moreover, when Mg-matrix is reinforced with 2 or more reinforcing particles then the formation of structural matrix composites is called magnesium-matrix hybrid composites. Universally, many studies have been performed on Mg-MCs to determine their usage in industrial applications having high strength and wear properties. When TiC reinforcement amalgamate with Mg-base matrix, it shows enhanced tensile strength and wear resistance properties of Mg composites [5]. The processing of AZ31 (WC and ZrO 2 ) hybrid matrix composites with hard WC particulates describes improved strengthening properties of AZ31-hybrid composites [6]. The inclusion of alumina reveals enhanced wear resistance of AZ31 hybrid composites because of their high hardness and heat resistance properties [7]. Moreover, the inclusion of chromium and boron carbide particles in Mg-matrix was studied rarely. Such Mg-hybrid composites determine improved hardness and tensile properties in contrast with the base matrix and portray a crucial role in the up-gradation of strengthening properties of Mg-hybrid composites [8].
The Mg-matrix composites fabricated by stir casting are globally adapted processes even if various techniques have been involved for the fabrication of Mg-composite materials. The stir casting process contributes several advantages like its uniform distribution, simplicity, and low cost as compared to other manufacturing techniques. In this technique, with the passage of time, modified versions have been introduced by including bottom pouring and squeeze pressure arrangement in the setup. This modified setup is known as the bottom pouring vacuum-based squeezed stir casting method [9]. In this method, the reinforcing particles are uniformly distributed in the matrix material producing minimal casting defects. Through the literature study, it is reported that uniform distribution is not enough to obtain the required properties of the Mg-composites but high wettability is also considered. High wettability is another important factor to obtain high-quality magnesium matrix composites [10]. However, this method (bottom pouring vacuum-based squeezed stir casting method) is the best-suited method to overcome both factors. Besides, in this method, reinforcing particles are amalgamated with the matrix material in a muffle furnace to produce homogeneous monolithic/hybrid magnesium-matrix composites.
Some researchers have also studied the corrosion behavior of magnesium-based composites by considering its influence majorly depending on the inclusion of reinforcing the content of the Mg-matrix. Singh et al. [11] fabricated Mg-based composites reinforced with chromium by a vacuum-based squeeze stir casting process. Their investigation indicated that the corrosion resistance increased with the addition of Cr reinforcement in the Mg-matrix while dipped in NaCl solution for 48, 72, and 96 h. Oza et al. [12] investigated the corrosion rate of Al/SiC/Graphite composites fabricated by the powder metallurgy process. The results reveal that the corrosion rate of Al-hybrid composites was uniform when dipped in NaCl solution initially for 24 h (nosignificant effect shown) but after a short period, the corrosion rate increased periodically due to the inclusion of SiC and graphite particles in the Al-matrix as compared to the base matrix. Neubert et al. [13] studied the corrosion rate of Mg-reinforced hybrid composites reinforced with alumina particles fabricated by the squeeze casting process. The study demonstrated the corrosion behavior in a 35 g/l NaCl environment for both 5 wt% and 7 wt% reinforced composites reveals superior corrosion resistance as compared to the base matrix. In previous studies, the strengthening, tribological, and corrosion properties of the Mg-hybrid composites with varying reinforcing particles have been investigated.
However, in the literature review, the effect of reinforcing particles on the corrosion behavior of AZ91Mg-hybrid composites fabricated by two-step stir casting has been rarely investigated [14,15].
For this purpose, current research work has been formulated accordingly. This research study includes the examination of the corrosion behavior of reinforcing particles (titanium carbide and alumina) with the AZ91 Mg-hybrid composites by using the bottom pouring vacuum-based squeezed stir casting method.

Materials
AZ91 Mg alloy has been used as a matrix material to produce Mg-hybrid composites using the vacuum-based squeezed stir casting method (Swamp Equip Pvt. Ltd., Chennai) as shown in Fig. 1. The matrix material in ingot form has been procured from local vendors i.e., India-mart Intermesh Ltd., Noida, UP, India, whereas for the current study of base material, i.e., cast with a similar method. However, titanium carbide and alumina particles with an average particle size of 100-500 um have been used as reinforcing particles procured from Sigma Aldrich Chemicals Pvt. Ltd, Bangalore and Chemical Drug House Pvt. Ltd, Delhi, respectively. The chemical distribution of the base matrix, i.e., AZ91 Mg alloy, is analyzed through literature and metallographic study is given in Table 1 and Fig. 2a [16]. Moreover, the microscopic images of the matrix and reinforcing particles are shown in Fig. 2a-c.

Experimental Procedure
Initially, to remove the moisture contents of reinforcing particles, TiC and Al 2 O 3 powders have been allocated in an electric oven at 300 °C individually. Then under constant stirring, AZ91 matrix ingots have been placed in a vacuumpreheated furnace setup equipped with Ar + SF 6 cover gas to snuff out the fire. Then reinforcements (TiC + Al 2 O 3 ) have been released into AZ91-Mg melt under constant stirring. About 800-850 °C of stirring temperature and a fixed stirring speed of 455 rpm have been provided to the fabricating setup to attain uniform distribution. Further, the compositions have been set by the design of the experiment as mentioned in Table 2. After 15 min of stirring, the hybridized melt has been released to the MS mold using the bottom pouring passage arrangement. The bottom pouring arrangement is a pre-heater having a narrow inclined track (250 °C) integrated at the bottom of the furnace to preserve the temperature of hybridized compo-melt. Then the compo-melt is dropped to MS mold and immediately squeezed pressure (250 MPa) is enforced by the hydraulic press for 10 min to eliminate the residual deformities. After solidification under atmospheric temperature conditions, Mg-based hybrid fabricated specimens have been moved out from the MS mold and then slash out as per ASTM experimenting dimensions.

Corrosion Tests
The corrosion test has been conducted by using the potential dynamic (PD) polarization electrochemical technique. The experimentation has been performed on Princeton Versa Stat400 Potentiostat setup integrated with Studio chemical software (Spectro Analytics Lab Pvt. Ltd, Greater Noida). The test setup also includes a 3-electrode corrosion cell having a working electrode, saturated AgCl reference electrode, and a counter electrode made up of platinum. However, operating electrodes have been prepared by wrapped with insulated copper wire on the face (one of them) of the specimen by using Al-insulating tape and fixed in resin. The Mg-sample surfaces have been wet grounded with SiC emery papers from 220 to 600 grit. Moreover, as per ASTM standards, grounded Mg specimens have been degreased with acetone and then washed with dry air as well as with distilled water [17]. The corrosion behaviors of Mg specimens have been also demonstrated in 3.5% NaCl immersion solution at 25 °C of ambient temperature. In addition, OCP (open circuit corrosion potential) measurements have been performed for 30 h in an isolated cell. Then, potentiodynamic polarization measurement values have been measured under a scan of 1.6 mV/s with the potential ranges in-between − 300 and 1600 mV. After conducting, each experiment, electrolyte has been restored and Mg specimens are polished and then rinsed with distilled water. Mg specimens are also cleaned with acetone to examine the accurate measurement on the composite surface. On average, three repeated tests have been performed with Mg-composite specimens so that the actual value has been evaluated.

Rockwell Hardness and Dry-Sliding Test
The Rockwell hardness of the Mg-hybrid composites has been investigated by using Rockwell Hardness Tester with the MS steel ball of 1/8' inch in size from Mechanical Testing Lab, ME Department, Jamia Millia Islamia, Delhi. An average of five readings has been considered to get the actual value of the hardness of each Mg-hybrid composite. A load of 100 kgf has been applied to the AZ91 Mg-composites and then the indented hardness value is measured. However, the dry-sliding behavior of AZ91 Mg and AZ91-hybrid composite specimens has been tested by using a pin-on-disc Tribotester machine setup from KOM lab, Mechanical Engineering Department, Delhi Technical University, Delhi. A load of 50 N has been applied for 10 min, at a sliding speed of 5 m/s under ambient conditions.

Scanning Microscopic Analysis
The microscopic images of fabricated AZ91 Mg alloy and AZ91 Mg-hybrid composite specimens are shown in Fig. 3a-d. The microscopic results of Mg-hybrid composites show the homogeneous distribution of matrix and reinforcing particles having Mg 17 Al 12 sub-surfaces as well as the uniform amalgamation of Mg 17 Al 12 and reinforcing interfaces. However, their diffraction patterns also illustrate in Fig. 4a & b to validate their composition distributions from Nano-technology Lab, Jamia Millia Islamia, Delhi [18]. The diffraction peaks of Mg-hybrid composites confirm the presence of Mg and Mg 17 Al 12 as the highest peaks and other small peaks of reinforcing elements. Then after that for the corrosion immersion test, Mg-hybrid specimens have been immersed in a 3.5% of NaCl solution for 96 h and compared with each other. After the immersion test, the microscopic data values have been recorded by using Jeol high resolution. Figure 5A & b compares the influence of the inclusion of reinforcing particles (TiC and Alumina) through the OCP stabilization plotting curves of the AZ91 Mg alloy and AZ91 Mg-hybrid composite specimens dipped in 3.5% NaCl solution. The Mg-hybrid composite specimens generally show fluctuating OCP values. The curves depict that both specimen B and D hybrid composites are less for specimen A (AZ91Mg/3TiC/12Al 2 O 3 ). This fluctuating behavior of Mg-hybrid specimens can attribute to a simultaneous breakdown and corrosion product formation. However, specimen A illustrates the relative stability of OCP followed by specimen C (AZ91Mg/9TiC/6Al 2 O 3 ). Moreover, within the measurement period, it is demonstrated that specimen D displays the highest OCP values as well as represents the feasibility of thermodynamic stability in comparison with other Mg-hybrid composites. Further, Mg specimen C exhibits the minimum OCP value having high thermodynamic drift toward corrosion in a 3.5% NaCl immersion solution medium for 96 h. Table 3 summarizes the results of corroding potentials and the current densities of AZ91 Mg and AZ91 hybrid composite specimens. These results have been obtained through a potentiodynamic polarization test. The corrosion potentials pursue a similar trend to the results of the potential value obtained from OCP measurements that depict in Fig. 5b. When evaluated corrosion potential values of Mg-hybrid specimens A and B are compared with obtained OCP measurements shows lower potential and high fluctuating values.

Corrosion Behavior
It is observed that the OCP result values are less than those of Mg specimens A between 1500 and 1800th minutes of evaluated measurement values (Fig. 5b). This evidences less corrosion potential and lowers thermodynamic stability with the rise in the period. From Figure 5a, it is seen that the AZ91 Mg-hybrid composites show analogous polarization plots representing passivation initially then effective corrosion initiates. However, when the corrosion    [19]. Moreover, the outcome results of Table 3 also depict that the corrosion resistance decreases with increasing in wt% of TiC and increases with an increase in wt% of Alumina. AZ91 Mg-composite which has the highest wt% of alumina reveals the maximum corrosion resistance value. This is confirmed by the minimal corrosion current density i.e., 2.793 A/cm 2 . Commonly, the corrosion rate of metal matrix composites (MMCs) gets initiated through chemical or physical heterogeneity nature such as deformities, mechanically damaged regions, matrix-reinforcement interfaces, dislocations, or intermetallic regions [20]. In AZ91/TiC/Alumina reinforced composites, the type of corrosion mechanism has been observed due to the origination of corrosion deformities in TiC/Mg and Alumina/Mg interfaces [21]. This is generated due to the formation of micro-crevices or pits in the Mg-base matrix around the reinforcing-matrix interfacial regions observed in the form of droplets of a particle. However, in case of AZ91/TiC/Alumina hybrid composites, it shows an increment in corrosion tendency with the rise in wt% of titanium carbide. This is due to the inclusion of hard titanium carbide particles having a high density in comparison to alumina reinforcement. Thus, Mg-hybrid composites exhibit densifying composite particles which lead to the formation of a large number of matrix/reinforcement interfaces [22]. After the performance of the electrochemical test, the microscopic images represent the surface morphology of Mg-hybrid composites shown in Fig. 6a-e. The morphological images of Mg-hybrid composites show deep cracks, hair-line cracks, and grooves as a type of inter-crystalline corrosion. From Fig. 6a, it has been observed that more dissolution of anodic Mg-matrix occurred as compared to cathodic TiC/Alumina particles in the form of deep cracks. Table 3 represents the Rockwell hardness results of the Mghybrid composites. The results show that the Rockwell hardness value increments marginally with raise in the wt% of TiC particles in the AZ91 Mg-hybrid composites. This is because of the inclusion of TiC which is higher in density as compared to the AZ91 matrix and TiC reinforcement. However, alumina has a low hardness level as compared to Fig. 6 Corroded microscopic morphologies of a SP1, b SP2, c SP3, d SP4, and e SP5 titanium carbide. Moreover, Table 3 also derives the porosity levels of AZ91 Mg alloy and AZ91 Mg-hybrid composites. Porosity levels depict that AZ91 Mg-hybrid composites have high porosity levels as compared to base matrix i.e., AZ91 Mg alloy due to the inclusion of both reinforcing particles [23]. Thus, marginal increments in the Rockwell hardness values have been represented in Mg-hybrid composites as compared to mono-composites i.e., AZ91 Mg-matrix. Figure 7 illustrates the variation of friction coefficient values with an increase in the period. Initially, during commencing of the wear test, observations have been performed for 120 s to examine the variation in friction coefficient values. During experimentation, it is due to the result of a hard counter ball in localized regions causing adhesion to the surface of Mg-composites.

Wear Behavior
In addition to that, the friction coefficient value of AZ91/9TiC/6Al 2 O 3 (C) and AZ91/6TiC/9Al 2 O 3 (B) hybrid composites depicts increments in wear rate with an increase in time. This is because of an increment in the proportion of AZ91 Mg-hybrid composite wear debris which sticks to the composite surface with the rise in coefficient friction value with an increase in time of contact. As per the integral assessment, Fig. 7 depicts that TiC particles have the highest percentage having the highest friction coefficient value in comparison to other Mg-hybrid composites (Specimen D). This implies that AZ91/12TiC/3Al 2 O 3 hybrid composite exhibits the highest wear rate as compared to other AZ91/ TiC/Al 2 O 3 Mg-hybrid composites. AZ91/12TiC/3Al 2 O 3 composite depicts that with the increase in the reinforcing percentage of TiC, a decrement in wear resistance has been observed. However, Specimen A exhibits the least friction coefficient. This indicates that during the testing period, AZ91/3TiC/12Al 2 O 3 composite demonstrates a relatively low wear rate value or constant friction coefficient value in comparison to other AZ91 Mg-hybrid composites. Predominantly, the low friction coefficient value signifies an abrasive wear mechanism due to the existence of a hard carbide ball (tungsten carbide) rubbing over the soft AZ91/3TiC/12Al 2 O 3 surface. This reason is quite significant to support the hardness value of AZ91 Mg-hybrid composite which shows the hardest fabricated composite as compared to AZ91 Mg-mono-composites [24]. These hybrid composites display minimum plastic deformation and low friction coefficient value.
Further, Fig. 8 represents the worn track pattern surfaces of AZ91 Mg-hybrid-composite. As per a detailed examination, the mono-composite mainly depicts an abrasive wear mechanism. The proportion of adhesive is slightly lower than abrasive wear which depicts minimal worn-out debris elements sticking to the sub-surface of the specimen of AZ91 Mg alloy [25]. However, in the case of 9 wt% of alumina containing AZ91Mg/6TiC/9Al 2 O 3 composite, it can be shown that the friction coefficient value increases steadily for 700 s, and after that significant drop in friction coefficient value. During the comparison of friction coefficient values, it is summarized that the preliminary wear mechanism for AZ91/TiC/Al 2 O 3 hybrid composites is adhesive, which later transforms into an abrasive wear mechanism. The validation of adhesive and abrasive wear mechanisms of AZ91 hybrid composites through scanning microscopic images is shown in Fig. 8a-c. Moreover, the confirmation of abrasive wear evidences through the existence of ploughing grooves within the wear tracks. Whereas, the representation of the adhesive wear mechanism is confirmed as the removal of patches from the Mg-hybrid surfaces that stick to the wornout surfaces. The removal of patches of the AZ91-hybrid composites shows local welding at the interfaces and the subsequent ruptures with the formation of grooves. However, 12 wt% of alumina in Mg-hybrid composites depicts a predominantly adhesive wear mechanism with an increment in the coefficient of friction value with time. Figure 8c represents the accumulation of large wear debris on the AZ91/12TiC/3Al 2 O 3 composite specimen which behaves like fragmentary welding of wear debris over the composite surface.

Conclusions
The corrosion and wear behavior of AZ91/TiC/Alumina hybrid composites exhibit 3, 6, 9, and 12 wt% of TiC and alumina as reinforcement has been fabricated and investigated successfully. The results concluded that 1. The corrosion resistance of hybrid composite i.e., AZ91/3TiC/12Al 2 O 3 has been superior to that of other hybrid composites and AZ91 Mg alloy when dipped in 3.5% NaCl immersion solution for 96 h. 2. The corrosion rate of AZ91 Mg-hybrid composites has increased with an increase in wt% of titanium carbide. 3. The increase in the number of matrix/reinforcement interfaces has been observed in the AZ91 Mg-hybrid composites with the increase in wt% of TiC and decrease in wt% of alumina. This is the prime reason observed for the increase in corrosion resistance of AZ91 Mg-hybrid composites with the increase in wt% of alumina. 4. Consequently, the friction coefficient and wear rate of AZ91 Mg-hybrid composites are affected significantly by the increase in wt% of titanium carbide. 5. Predominant transformation of abrasive to adhesive wear mechanisms has been observed in AZ91 Mg-hybrid composites with the increase in wt% of TiC and alumina. Funding For this research, authors have received no external or institutional funding.

Data Availability
The data that support the findings of this study are available to the corresponding author.

Conflicts of interest
The author declares no conflict of interest.
Informed Consent Not applicable.