Spark Plasma Sintering and Characterization of Al-TiB 2 Composites

: In this study, Al-TiB 2 compacts fabricated by spark plasma sintering methods at di ﬀ erent temperatures were characterized for densiﬁcation, microstructural development, and mechanical properties. Sintering parameters used were temperatures of 500 ◦ C and 550 ◦ C under the pressure of 30 MPa. A very dense microstructure with uniform phase distribution and porosity was produced in the sample sintered at 550 ◦ C with 2.5 wt% TiB 2 . The same sample exhibited excellent hardness value, and a high-tensile strength attributed to full metallurgical bonding, presence of sub-micron sized grains, and their uniform distribution. These results show that the TiB 2 addition enhanced the composite’s hardness, sintered density, and tensile strength. In all the sintered samples, the fractographs revealed a mixed-mode fracture (ductile and brittle).


Introduction
In the past few decades, metal matrix composites (MMCs) have been extensively used in aviation and automobile industries and defense sectors due to their high toughness, excellent specific strength, and elastic modulus [1]. The properties of MMCs can be enhanced by adding particulate reinforcements [2][3][4]. Aluminum is one of the low-cost and lightweight metals, and the Al-6061 alloy mainly contains magnesium and silicon. Al-6061 is broadly used in multiple engineering applications, experimentation, and commercial applications including frames of light mobility vehicles, construction, aviation and aerospace, etc. because of its corrosion resistance, superior machinability, good surface finish, etc. Its excellent corrosion resistance makes it an appropriate material for marine structural applications [5]. Various causes such as thermal expansion difference, even distribution of reinforcements, and fabrication conditions that distress the mechanical properties have been examined previously [6][7][8]. MMCs strengthened by ceramic fibers, whiskers, and particles are most widely used, which possess the ability to strengthen the performance parameters of manufacturing operations. Ceramic particles have high strength, hardness, low thermal expansion, and high chemical resistance. Reinforcements like ZrSiO 4 , B 4 C, TiB 2 , and SiC are used to ameliorate the hardness and wear resistance of MMCs without considerably changing the density of the material [9][10][11][12][13][14][15][16]. Among these, TiB 2 has emerged as a good reinforcement due to its outstanding features such as thermal stability, oxidation resistance, high melting point (3225 • C), and exceptional hardness (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) GPa Vickers at room temperature). It is chemically stable for a wide range of temperatures, high modulus, and has good electrical conductivity [17]. Thus, the properties of Al-6061 alloys are greatly improved with the addition of TiB 2 particles [18]. Rana Pratap Singh et al. reported that improved the mechanical properties of Al 2014 with the addition of 5%TiB 2 , is fabricated using a powder metallurgy process [19]. Z. Sadeghian et al. stated that Al 20 wt% TiB 2 spark plasma sintered and hot extrude composites have excellent mechanical properties [20]. Another study was done by M. Askarpour et al. using the accumulative roll bonding (ARB) method on Al-TiB 2 composites and reported that the composite had an ultra-high strength and fine grain size [18]. Using a two-step milling method, the Al-TiB 2 nanocomposite was produced [21]. Al-TiB 2 composites are mainly fabricated by stir casting [22] and other different processing techniques are using like friction stir processing [23], salt metal reaction process [24], aluminothermic reaction process [25], etc. to overcome the problems and increase the properties other manufacturing processes are using. Unconventional sintering techniques like pressure less sintering, microwave-assisted hot press, electro sinter forging, spark plasma sintering (SPS), etc. are now common sintering methods. Application of pressure causes a reduction in sintering temperature in pressure-assisted sintering technique, which improves the mechanical properties over conventional sintering [26,27]. Nauri Saheb stated that the spark plasma sintering technique was the best process for the Al 6061 alloy to achieve theoretical density and better mechanical properties [28]. In the current study, Al-TiB 2 composites were fabricated and effectively solid-state sintered at low pressure (30 MPa) using the spark plasma sintering technique at 500 • C and 550 • C sintering temperature. The investigation of the microstructures, Ultimate Tensile Strength (UTS), hardness, and physical properties of Al-TiB 2 composites at 500 • C and 550 • C sintering temperature was carried out.

Materials and Methods
Aluminum alloy 6061 powder synthesized by the gas atomized method with a particle sizẽ 100 µm (from Ampal Inc. Flemington, NJ, USA) was used as a matrix material and TiB 2 with a 10 µm particle size (Krish Met, Chennai, India) and density of 4.52 g/cm 3 was used as reinforcement (Tables 1 and 2). The Scanning Electron Microscopy morphology of the as-received powders is shown in Figure 1. The Al powder and 2.5%, 5%, 7.5%, and 10% wt% TiB 2 was mixed homogenously by ball milling machine without balls for 20 min. Spark plasma sintering (SPS) system (Model: DR. Sinter 21050 SPS furnace) was used for the sintering of the mixed powders. The sintering was carried out in an argon atmosphere at 500 • C and 550 • C with a heating rate of 100 • C/min. During the complete span of heating and sintering, the powder compact was subjected to a uniaxial pressure of 30 MPa. The relative density of the sintered samples was obtained from the relationship between theoretical density (ρ theoretical ) and actual density (ρ actual ). The equation for theoretical density is given as follows: where ρ i is the theoretical density of the ith element and w i is the weight fraction of the ith element.   The Archimedes principle was used to evaluate the actual density of the samples, where w1and w0 are the mass of the compacts in water and air, respectively, and ρ0 is the water density.
For microscopic studies, initially, SiC sheets of various sizes (220, 400, 600, 800, 1000, and 1200) were used for polishing the sintered samples, and later, diamond paste (6 μm particles) was used for fine polishing. Keller's reagent (2 mL HF (48%) + 3 mL HCl + 5 mL HNO3 + 190 mL distilled water) was used for etching on the polished samples to examine clear microstructures in an optical microscope (Leica DMS1000, Wetzlar, Germany) and scanning electron microscope (MODEL: ZEISS EVO180, Oberkochen, Germany). The hardness of sintered Al-TiB2 composites was obtained using the Micro Vickers hardness tester (Leco Micro hardness testing machine LM248AT, St. Joseph, MI USA). The indentation load applied on the sample was 0.5 kgf for a dwell period of ten seconds. The following formula gives the Micro Vickers hardness value. 2 1.854 Vickers micro hardness value = P l (3) where P is the exerted load in grams; and l is the mean length of d1 and d2 (diagonals).
Flat tensile samples, prepared as per the Metal Powder Industries Federation (MPIF) standards The Archimedes principle was used to evaluate the actual density of the samples, where w 1 and w 0 are the mass of the compacts in water and air, respectively, and ρ 0 is the water density.
For microscopic studies, initially, SiC sheets of various sizes (220, 400, 600, 800, 1000, and 1200) were used for polishing the sintered samples, and later, diamond paste (6 µm particles) was used for fine polishing. Keller's reagent (2 mL HF (48%) + 3 mL HCl + 5 mL HNO 3 + 190 mL distilled water) was used for etching on the polished samples to examine clear microstructures in an optical microscope (Leica DMS1000, Wetzlar, Germany) and scanning electron microscope (MODEL: ZEISS EVO180, Oberkochen, Germany). The hardness of sintered Al-TiB 2 composites was obtained using the Micro Vickers hardness tester (Leco Micro hardness testing machine LM248AT, St. Joseph, MI, USA). The indentation load applied on the sample was 0.5 kgf for a dwell period of ten seconds. The following formula gives the Micro Vickers hardness value.
where P is the exerted load in grams; and l is the mean length of d 1 and d 2 (diagonals).  Figure 2 lists the sintered density of Al-TiB 2 composites at 500 • C and 550 • C. As the concentration of TiB 2 in the Al-6061 matrix is increased, the sintered density also increases, and it is highest in 2.5% TiB 2 composite. The sintered density of the Al-6061 alloy was observed to be 85.12% at 500 • C, which increased up to 98.36% with 2.5% TiB 2 addition. Further increase in TiB 2 concentration up to 10% reduced the density to 92.2%. At 550 • C, a similar trend was observed with an increase in sintered density at 2.5% (99.81%), then decreased (92.2%) at 10% of TiB 2 . The presence of TiB 2 particles restricts the expansion of pre-alloyed powder as the sintering takes place at solid-state conditions with a faster heating rate. As a result, the composites with a higher TiB 2 content had the least expansion in the initial stage of sintering, and later densification occurred due to a faster heating rate and the load displaced on the punch. A total of 2.5% TiB 2 powder in the Al matrix increased the density, but above 2.5%, TiB 2 sintered density was gradually decreased. The reason is that at 2.5%, TiB 2 concentration of reinforcement in the matrix uniformly distributed, but as the TiB 2 increased in the matrix, the reinforcement was not evenly distributed, and TiB 2 agglomeration and voids formed in the matrix. Increasing the sintering temperature from 500 • C to 550 • C with a fixed amount of TiB 2 improved the sintered density slightly with correspondingly lowering the porosity. This is due to the benefits of elevated temperature on diffusion rate, sinterability, and matrix reinforcement bonding [17,[29][30][31].

Densification Behavior of Sintered Samples
Metals 2020, 10, x FOR PEER REVIEW 4 of 12 Figure 2 lists the sintered density of Al-TiB2 composites at 500 °C and 550 °C. As the concentration of TiB2 in the Al-6061 matrix is increased, the sintered density also increases, and it is highest in 2.5% TiB2 composite. The sintered density of the Al-6061 alloy was observed to be 85.12% at 500 °C, which increased up to 98.36% with 2.5% TiB2 addition. Further increase in TiB2 concentration up to 10% reduced the density to 92.2%. At 550 °C, a similar trend was observed with an increase in sintered density at 2.5% (99.81%), then decreased (92.2%) at 10% of TiB2. The presence of TiB2 particles restricts the expansion of pre-alloyed powder as the sintering takes place at solid-state conditions with a faster heating rate. As a result, the composites with a higher TiB2 content had the least expansion in the initial stage of sintering, and later densification occurred due to a faster heating rate and the load displaced on the punch. A total of 2.5% TiB2 powder in the Al matrix increased the density, but above 2.5%, TiB2 sintered density was gradually decreased. The reason is that at 2.5%, TiB2 concentration of reinforcement in the matrix uniformly distributed, but as the TiB2 increased in the matrix, the reinforcement was not evenly distributed, and TiB2 agglomeration and voids formed in the matrix. Increasing the sintering temperature from 500 °C to 550 °C with a fixed amount of TiB2 improved the sintered density slightly with correspondingly lowering the porosity. This is due to the benefits of elevated temperature on diffusion rate, sinterability, and matrix reinforcement bonding [17,[29][30][31].

Micro-Structural and SEM Analysis
SEM micrographs of the Al-TiB2 compositions sintered at 500 °C and 550 °C are shown in Figure  3. From the micrographs, it was observed that temperature difference was showing the difference in the microstructure. The 500 °C sintered composite microstructures with a high area of light phase appeared when compared with the 550 °C sintered compacts, where the reason was that the temperature was not sufficient for the aluminum to fully pack around the TiB2 particles. As the TiB2 content increased in the matrix, TiB2 particles restrict the Al-Al contacts and as a result, Al penetration in between the TiB2 particles is restricted, which leads to the formation of the pores and

Micro-Structural and SEM Analysis
SEM micrographs of the Al-TiB 2 compositions sintered at 500 • C and 550 • C are shown in Figure 3. From the micrographs, it was observed that temperature difference was showing the difference in the microstructure. The 500 • C sintered composite microstructures with a high area of light phase appeared when compared with the 550 • C sintered compacts, where the reason was that the temperature was not sufficient for the aluminum to fully pack around the TiB 2 particles. As the TiB 2 content increased in the matrix, TiB 2 particles restrict the Al-Al contacts and as a result, Al penetration in between the TiB 2 particles is restricted, which leads to the formation of the pores and slight reduction in the sintered density. Agglomerations (light phase) and porosity (dark phase) are crucial micro-structural characteristics that can be observed in the SEM images [32]. This agglomeration might be the outcome of the localized distribution of powders, while blending due to the non-identical size and density of TiB 2 and Al-6061 [33]. It was observed from the micrographs that the aggregates of TiB 2 particles were dispersed in the aluminum matrix at grain boundaries forming clusters around the grain. An increase in TiB 2 content caused a decrease in uniformity and homogeneity [34]. This ultimately resulted in increasing agglomerations with an increase in the TiB 2 concentration in the Al-6061 composite. Porosity was decreased while the uniformity was increased by increasing the temperature from 500 • C to 550 • C due to an increase in sintered density. Compared with other (Mahesh Paidpilli [35], V. Jaya Prasad [36], Z.Sadeghian [20]) Al-TiB 2 composite microstructures, TiB 2 particles were not clearly visible in the current work microstructures.

Mechanical Properties
The micro Vickers hardness values of the Al alloy and Al-TiB 2 composites are displayed in Table 2. As Al is a soft metal and TiB 2 , being a ceramic material with high hardness, this resulted in an increase in the hardness values of the metal matrix composites [37]. For Al at 500 • C, the hardness of pure Al-6061 was observed to be 65 HV 0.5 , which increased to 76HV 0.5 with 2.5% TiB 2 addition. Furthermore, with an increase in TiB 2 concentration up to 10%, the hardness value declined to 68HV 0.5 . At 550 • C, a similar trend was observed with an increase in hardness at 2.5% (77 HV 0.5 ) and then decreased up to 10% (72HV 0.5 ). As expected, the Vickers's hardness trends were analogous to that of sintered density. There was a substantial variation in the values of thermal expansion of Al-6061 (23.5 × 10 −6 • C −1 ) and TiB 2 (5.4 × 10 −6 • C −1 ), which causes residual stresses in the specimens during cooling. The strength of the MMC proliferates due to Orowan's mechanism, which allows the TiB 2 particulates to become obstacles to the migration of dislocations [38,39]. During SPS, hard and brittle TiB 2 particles restrict local rearrangement and plastic deformation of aluminum particles [35]. Furthermore, pre-alloyed 6061 Al powders are harder due to solid solution strengthening during processing, and the presence of the TiB 2 reinforcement particles in the matrix increases the hardness [40]. The tensile properties of composites are shown in Table 3. It was observed that the UTS of the composite increased from 112 MPa (Al-6061) to 314 MPa (Al-6061 + 10% TiB 2 ) at 500 • C and from 120 MPa (Al-6061) to 316 MPa (Al-6061 + 10% TiB 2 ) at 550 • C. The interaction between TiB 2 particles and dislocations contributes to the excellent tensile strength of the developed composites under load. The dislocation density increased in the matrix due to the mismatch of the coefficients of the thermal expansion of matrix, and reinforcement phases during the heating and cooling in the spark plasma sintering process [41]. The movement of dislocations under load was stopped by reinforcement particles and a hence substantial increase in tensile strength. As the TiB 2 volume increased in the matrix, the ductility was decreased due to the TiB 2 particles inhibiting the plastic flow [35]. A comparison ( Table 4 shows a comparison of the tensile strength of the current work with other works with the other works (Selvaganesan M [42] Johny James. S [43], S. Suresh [44], V. Jaya Prasad [36] Fei Chen [45]) done on the Al alloy with different reinforcement ceramics with different manufacturing processes at different temperatures, showed that this solid state sintered Al-TiB 2 composite had better mechanical properties.    [40] Al/10 vol% of (TiC + TiB 2 ) Al/20 vol% of (TiC + TiB 2 ) Al/30 vol% of (TiC + TiB 2 ) Al/40 vol% of (TiC + TiB 2 ) The fractography is shown in Figure 4. From the fractography analysis, the mix-mode of fracture can be clearly seen. It is inferred from the images that cup and cone type fractures occurred in the Al matrix, and the ductile mode of fracture occurred due to the coalescence of voids. No chemical bonds formed in between the Al and TiB 2 particles and so for each TiB 2 particle acting as the void.
When we applied a tensile load, the TiB 2 particle did not deform, the Al matrix around the TiB 2 particle was deformed, and a void was formed, which was attributed to the ductile fracture [46]. Whereas in the titanium diboride, a brittle fracture was seen due to it being a ceramic material. Micro-cracks are formed at the interfacial boundaries due to the residual stresses, which play a vital role in reducing the hardness values [47]. Generally, it can be inferred from the magnified view of fractographs that there were more fractured particles observed along the line made, intersecting the crack growth paths more than the debonded particles on the site. Plastic deformations in the sample proliferated while the sample was close to the sintering temperature (550 • C), which led to the dislocations of slip systems in the lattice structure [48].

Conclusions
In this study, composites of Al-6061 and TiB 2 were fabricated by the solid state sintering successfully using the spark plasma sintering (SPS) technique at 500 • C and 550 • C under 30 MPa pressure. The main conclusions are the follows: Sintered density and hardness values have shown similar trends. Sintered density increased bỹ 15.5% at 500 • C and~15.4% at 550 • C with 2.5% TiB 2 . Further increase in the content of TiB 2 decreased the sintered density at both temperatures. Hardness value increased by~16% at 500 • C and there was a~13% rise at 550 • C with 2.5% TiB 2 with respect to the Al-6061 alloy.
There was a significant increase in UTS with the addition of TiB 2 in the composite. The UTS increased gradually with the addition of TiB 2 at 500 • C as well as 550 • C.
From the SEM analysis, it was observed that the rate of agglomeration increased with the gradual increase in TiB 2 content. The degree of clustering of Al-TiB 2 nanoparticles is observed in the Al matrix, which is attributed to the difference in the densities between the matrix phase (2.7 g/cm 3 ) and the reinforcement phase (4.52 g/cm 3 ). The tendency of agglomeration at higher volume fractions of reinforcement arises because of the huge variation in the sizes of Al powder and TiB 2 powder particles. The nano-size powders tend to agglomerate at the interstitial sites between the Al powders during mixing and compaction.
There exist extensive equiaxed dimples on the fracture surface of Al-6061 alloy indicating the occurrence of large plastic deformation precisely before the failure. The development of micro-cracks is initiated under the local three-dimensional state of stress and propagates by increasing tensile load. Finally, these micro-cracks coalesce and reach a critical size, leading to the fracture of the sample.
Using spark plasma sintering, the sintered density of 99.8% at 2.5% addition of TiB 2 was achieved and was also corroborated by a Vickers hardness of 77 HV, which was the maximum.
It was found that even without reaching the liquid phase sintering temperature of the Al-6061 alloy (~600 • C), the samples attained 99.8% density at 550 • C with the addition of 2.5wt% TiB 2.
The unprecedented stiffness (2.5%, before ductile failure) was attained within the solid state sintering temperature.
The decrease in elongation with an increase in TiB 2 reinforcement shows that the dislocation density increased in the matrix and TiB 2 hindered the dislocation motion.

Conflicts of Interest:
The authors declare no conflict of interest.