# Mechanical Properties of Titanium Diboride Particles Reinforced Aluminum Alloy Matrix Composites: A Comprehensive Review

Authors: V. Mohanavel; M. Ravichandran; V. Anandakrishnan; Alokesh Pramanik; M. Meignanamoorthy; Alagar Karthick; M. Muhibbullah
Journal: Advances in Materials Science and Engineering
 (2021)

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## Abstract

Aluminum alloys with silicon, magnesium, and copper were extensively used alloying elements in various applications because of their excellent properties. In recent decades, aluminum matrix composites (AMCs) are an advanced engineering material widely utilized in diverse engineering applications, including aircraft, automobile, marine, and shipbuilding, owing to their low density, lightweight, good stiffness, superior strength, and good tribological properties. Aluminum is abundant and its use is as vast as the ocean. It is also the most used matrix material in the composite arena. Therefore, incorporating a ceramic particle into a relatively soft aluminum matrix improves hardness, strength, stiffness, creep, fatigue, and wear properties instead of the conventional materials. This article is an assay to review and spotlight some recent works on the mechanical behaviors of aluminum-based titanium diboride reinforced metal matrix composite. This review article concentrates on the mechanical properties and the fabrication processes of Al-TiB2 composites to provide a valuable reference to nurture future research precisely.


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## Body

## 1. Introduction

In the past few decades, aluminum matrix composite has acted an essential role in material science, especially in aircraft, marine, automobile, transportation, and defense sectors [1]. Several investigations have reported that the inclusion of ceramic filler contents to the matrix improves the mechanical, physical, and tribological properties [2]. Aluminum matrices that are incorporated with hard ceramic filler contents expose the augmented mechanical properties as compared to the plain alloy materials [3]. Figure 1 exhibits the list of wrought aluminum alloy. Due to their high strength-to-weight ratio, high thermal conductivity, good corrosion resistance, and improved mechanical properties, aluminum metal matrix composites (AMCs) are increasingly used as structural materials. Composite materials are becoming more popular due to their unique properties and high strength-to-weight ratio. Ceramic particles provide exceptional strength and wear resistance to AMCs [4–6]. Figure 2 reveals the classification of aluminum composites fabrication process. A large range of filler particles such as SiC, Si3N4, ZrN, TiN, TiB2, Al2O3, BN, WC, and SiO2 has used the reinforcements for the manufacture of composites. Amid the other filler materials, titanium diboride (TiB2) is a promising candidate filler material for aluminum-based composites. It exhibits an enticing combination of mechanical and physical properties, superior strength, perfect hardness, high melting point, excellent corrosion resistance, and excellent wear protection [7–10]. Figure 3 illustrates the advantages and disadvantages of different techniques for composites.Figure 1
List of wrought aluminum alloy.Figure 2
Classification of aluminum composites fabrication process.Figure 3
Advantages and disadvantages of different techniques for composites.Titanium diboride (TiB2) particle does not react with molten aluminum and cannot form reaction products at the intergap between matrix and reinforcement [11]. Compared with the other filler contents, TiB2 is a desirable strengthening agent for aluminum-based metal matrix composites [12]. Titanium diboride based aluminum matrix composites were recently employed in the manufacturing of automobile piston, vehicle drive shaft, cylinder liners, cutting tools, crank shaft, brake drum, and bicycle frames and were also employed in aerospace, marine, and automotive industries because of their good stiffness, superior strength, high temperature stability, and lightweight [13]. AMCs have been fabricated using a variety of methods like compocasting, melt stirring casting, powder metallurgy, in situ casting, squeeze casting, and spray forming and mechanical alloying methods. Figures 4(a)–4(c) reveal the schematic diagram of stir casting, powder metallurgy, and hot extrusion process.Figure 4
Schematic diagram of fabrication process: (a) stir casting, (b) powder metallurgy, and (c) hot extrusion.
(a)(b)(c)Liquid state processing method contains incorporation of ceramic particles externally or formed inside the molten metal. The former is known as ex situ (stir casting technique) while the latter is called as in situ (direct melt reaction technique or exothermic salt-metal reaction technique) fabrication. Liquid state casting techniques have shown some incomparable benefits like constant dissemination of filler particles in the matrix and strong interfacial attachment between the matrix and the filler particle [14]. Figure 5 reveals the classification of fabrication method of AMCs.Figure 5
Classification of fabrication method of AMCs.Figure6 exhibits the influence of various parameters on hardness. A number of researchers have produced titanium diboride reinforced AMCs using various techniques. This scientific review article provides an aerial view of research efforts that are focused on mechanical properties and synthesizing techniques of aluminum-based titanium diboride composites. Table 1 reveals the physical and mechanical properties of several ceramics reinforcements. The SEM image of the TiB2 particles is depicted in Figure 7.Figure 6
Influence of various parameters on hardness.Table 1
Physical and mechanical properties of several ceramics reinforcements.
Sl. noPropertiesTiCB4CZrB2TiB21Melting point (°C)28302780304032252Density (g cm−3)4.772.526.084.523Hardness (GPa)22–2418–203525–354Thermal conductivity (Wm−1 K−1)31.832.524.2760–1205Molar mass (g/mol)59.8952.255112.8569.4896Crystal structureCubicRhombohedralHexagonalHexagonal6Elastic modulus (GPa)450472451560Figure 7
SEM image of TiB2 particles.


## 2. Fabrication Techniques of Al-TiB2 Composites

Among the several manufacturing techniques, the two techniques that are being used quite often are in situ casting and stir casting. Figure8 reveals the process parameters influence the production of composites through the melt stirring route.Figure 8
Process parameters influence the production of composites through the melt stirring route.


### 2.1. Stir Casting

The processing route is the most significant consideration in the fabrication of AMCs. In 1968, S. Ray dispersed alumina (Al2O3) ceramic filler materials into the Al melt, and in this process the incorporated filler material is blended with a molten state alloy by means of mechanical stirring [15]. For the processing of discontinuous rein-forced AMCs, several researchers prefer to employ stir casting route. The foremost objective of melt stirring is that it is trouble-free, flexible, unproblematic, reasonable, and appropriate for bulk production [16, 17]. Mohanavel et al. [18] utilized melt stirring to manufacture AA6351/SiC AMCs. SEM images of the resultant AA6351/SiC AMCs comprising 4%, 8%, and 12% SiC, respectively, are revealed in Figures 9(a)–9(c). The SEM images demonstrate a nearly homogeneous dispersion of the SiC in the AA6351 alloy. Moreover, the stir casting is affordable and provides an efficient stirring movement in the melts due to the sound particle-matrix association in the filler material. The cast-on route for the development of inhomogeneous filler in integrated MMCs is the most widely used liquid state casting process. Figure 10 reveals the experimental structure for the fabrication of composites through melt stirring route.Figure 9
SEM images of (a) Al6351/4 wt% SiC AMCs, (b) Al6351/8 wt% SiC AMCs, and (c) Al6351/12 wt% SiC AMCs [18].
(a)(b)(c)Figure 10
Experimental structure for the fabrication of composites through melt stirring route.


### 2.2. In Situ Casting

The in situ technique has been an attractive processing route for producing AMCs. In situ synthesizing technique was started in the early 1990s. In this process in situ reaction between the halide salts and molten metal takes place to form reinforcement particles. Exothermic method is more effective than the stir casting process [19, 20]. In situ formed reinforcement exhibits homogeneous dissemination of fine sized filler materials. It is effortlessly accomplished without the need for incorporation of wetting agent. The in situ reactions, as provided in the following equations, resulted in the generation of TiB2 particles. Consider the following:(1)K2TiF6+132Al⟶Al3Ti+43AlF3+2KF2KBF4+3Al⟶AlB2+2AlF3+2KFAl3Ti+AlB2⟶TiB2+4AlThe formation pattern of TiB2 can be classified as follows. (a) The incorporation of K2TiF6 (potassium hexafluorotitanate) and KBF4 (potassium tetrafluoroborate) to molten aluminum generates intermetallic compounds, specifically Al3Ti and AlB2, which serve as for Ti and B atoms. (b) Boron atoms travel in the direction of particles from Al3Ti. (c) The reaction occurred between atoms Ti and B in a gap from the surface of Al3Ti to form TiB2. (d) Boron atoms begin diffusing into TiB2 particles because of the smaller scale. (e) Dissolution of Al3Ti particle owing to normal cracking and fragmentation of Al3Ti particles, which contribute to enriched TiB2 generation rate. (f) Generation of TiB2 particles, after the entire reaction. Moreover, the KBF4 inorganic salt was incorporated slightly in excess of the stoichiometric ratio to avoid the formation of titanium trialuminide (Al3Ti). Figure 11 reveals the manufacturing of in situ composite employing inorganic salts and aluminum matrix alloy.Figure 11
Manufacturing of in situ composite employing inorganic salts and aluminum matrix alloy.In preparing AMCs, “in situ” techniques offer significant advantages over the conventional processing routes and in situ casting route is more economical. In situ type of processing is now in commercial use for TiB2 particle reinforced aluminum matrix composites [21–23]. Moreover, the TiB2 reinforcement particles in the composite inhibit dislocations, resulting in higher tensile strength. This interface allows for an efficient load transfer between the matrix alloy and the reinforcement. As the TiB2 particle content increases, this composite has higher mechanical properties than pure aluminum alloy [22]. Figure 12 displays the experimental arrangement of in situ casting. In [23], the dry sliding wear parameters on LM4/TiB2 composites were analyzed using the Taguchi method. Particulates reinforced aluminum matrix composite has better tribological properties compared to unreinforced aluminum matrix composite. Mohanavel et al. [21] employed in situ casting to manufacture AA6351/TiB2 AMCs. Microhardness and strength increase with the enhancement of the weight percentages of TiB2 particles. SEM micrographs of the resultant AA6351/TiB2 AMCs comprising 0%, 4%, and 8% TiB2, respectively, are revealed in Figures 13(a)–13(e). The SEM micrographs demonstrate a homogeneous dispersion of the TiB2 in the AA6351 alloy.Figure 12
Experimental arrangement of Al-TiB2 in situ composite.Figure 13
SEM micrograph of AA6351/TiB2 AMCs containing: (a) 4% TiB2, (b) 4% TiB2, (c) 8% TiB2, (d) 8% TiB2, and (e) XRD patterns of AA6351/TiB2 in situ composites [21].


## 2.1. Stir Casting

The processing route is the most significant consideration in the fabrication of AMCs. In 1968, S. Ray dispersed alumina (Al2O3) ceramic filler materials into the Al melt, and in this process the incorporated filler material is blended with a molten state alloy by means of mechanical stirring [15]. For the processing of discontinuous rein-forced AMCs, several researchers prefer to employ stir casting route. The foremost objective of melt stirring is that it is trouble-free, flexible, unproblematic, reasonable, and appropriate for bulk production [16, 17]. Mohanavel et al. [18] utilized melt stirring to manufacture AA6351/SiC AMCs. SEM images of the resultant AA6351/SiC AMCs comprising 4%, 8%, and 12% SiC, respectively, are revealed in Figures 9(a)–9(c). The SEM images demonstrate a nearly homogeneous dispersion of the SiC in the AA6351 alloy. Moreover, the stir casting is affordable and provides an efficient stirring movement in the melts due to the sound particle-matrix association in the filler material. The cast-on route for the development of inhomogeneous filler in integrated MMCs is the most widely used liquid state casting process. Figure 10 reveals the experimental structure for the fabrication of composites through melt stirring route.Figure 9
SEM images of (a) Al6351/4 wt% SiC AMCs, (b) Al6351/8 wt% SiC AMCs, and (c) Al6351/12 wt% SiC AMCs [18].
(a)(b)(c)Figure 10
Experimental structure for the fabrication of composites through melt stirring route.


## 2.2. In Situ Casting

The in situ technique has been an attractive processing route for producing AMCs. In situ synthesizing technique was started in the early 1990s. In this process in situ reaction between the halide salts and molten metal takes place to form reinforcement particles. Exothermic method is more effective than the stir casting process [19, 20]. In situ formed reinforcement exhibits homogeneous dissemination of fine sized filler materials. It is effortlessly accomplished without the need for incorporation of wetting agent. The in situ reactions, as provided in the following equations, resulted in the generation of TiB2 particles. Consider the following:(1)K2TiF6+132Al⟶Al3Ti+43AlF3+2KF2KBF4+3Al⟶AlB2+2AlF3+2KFAl3Ti+AlB2⟶TiB2+4AlThe formation pattern of TiB2 can be classified as follows. (a) The incorporation of K2TiF6 (potassium hexafluorotitanate) and KBF4 (potassium tetrafluoroborate) to molten aluminum generates intermetallic compounds, specifically Al3Ti and AlB2, which serve as for Ti and B atoms. (b) Boron atoms travel in the direction of particles from Al3Ti. (c) The reaction occurred between atoms Ti and B in a gap from the surface of Al3Ti to form TiB2. (d) Boron atoms begin diffusing into TiB2 particles because of the smaller scale. (e) Dissolution of Al3Ti particle owing to normal cracking and fragmentation of Al3Ti particles, which contribute to enriched TiB2 generation rate. (f) Generation of TiB2 particles, after the entire reaction. Moreover, the KBF4 inorganic salt was incorporated slightly in excess of the stoichiometric ratio to avoid the formation of titanium trialuminide (Al3Ti). Figure 11 reveals the manufacturing of in situ composite employing inorganic salts and aluminum matrix alloy.Figure 11
Manufacturing of in situ composite employing inorganic salts and aluminum matrix alloy.In preparing AMCs, “in situ” techniques offer significant advantages over the conventional processing routes and in situ casting route is more economical. In situ type of processing is now in commercial use for TiB2 particle reinforced aluminum matrix composites [21–23]. Moreover, the TiB2 reinforcement particles in the composite inhibit dislocations, resulting in higher tensile strength. This interface allows for an efficient load transfer between the matrix alloy and the reinforcement. As the TiB2 particle content increases, this composite has higher mechanical properties than pure aluminum alloy [22]. Figure 12 displays the experimental arrangement of in situ casting. In [23], the dry sliding wear parameters on LM4/TiB2 composites were analyzed using the Taguchi method. Particulates reinforced aluminum matrix composite has better tribological properties compared to unreinforced aluminum matrix composite. Mohanavel et al. [21] employed in situ casting to manufacture AA6351/TiB2 AMCs. Microhardness and strength increase with the enhancement of the weight percentages of TiB2 particles. SEM micrographs of the resultant AA6351/TiB2 AMCs comprising 0%, 4%, and 8% TiB2, respectively, are revealed in Figures 13(a)–13(e). The SEM micrographs demonstrate a homogeneous dispersion of the TiB2 in the AA6351 alloy.Figure 12
Experimental arrangement of Al-TiB2 in situ composite.Figure 13
SEM micrograph of AA6351/TiB2 AMCs containing: (a) 4% TiB2, (b) 4% TiB2, (c) 8% TiB2, (d) 8% TiB2, and (e) XRD patterns of AA6351/TiB2 in situ composites [21].


## 3. Comparison of In Situ versus Stir Casting

Table2 explicates results obtained by various researchers where TiB2 is the reinforcement and the processes involved mostly are in situ and stir casting and the percentages at which superior mechanical properties emerge are spotted in Table 3. Both in situ and stir casting method can fabricate Al-TiB2 composites with strong mechanical and wear properties. Most of the explorations have been done on titanium diboride filler particle for aluminum based in situ AMCs. Titanium diboride reinforced aluminum matrix composites are fabricated using halide salt reaction technique and it has proven to be cost effective [24]. Al-TiB2 in situ composites are usually synthesized by the incorporation of inorganic salts like K2TiF6 and KBF4 which react with molten aluminum and form TiB2 in the Al melt [25–30]. The Al-TiB2 in situ AMCs reveal superior mechanical and tribological properties compared with the base matrix alloy. In situ technique offers high mechanical bonding, homogeneous dissemination of small size of fillers in the matrix, thermodynamic stability, and clear interfacing between the solid and liquid in contrast to stir casting [31, 32]. TiB2 involved composites have utilized stir casting, squeeze casting, centrifugal casting, and compocasting rarely. It is also more economical when compared to in situ casting. Mechanical and physical properties like density, porosity, flexural strength, and compression strength are analyzed by a few researchers only, where TiB2 plays a vital role as reinforcement.Table 2
Titanium diboride reinforced composites properties.
MMC within (wt %)Manufacturing processMechanical propertiesRemarksReferenceAl 6061- TiB2 (0, 4.98, 9.29, 13.62 wt %)In situ castingMicrohardness, ultimate tensile strengthHardness and tensile strength of the composite enhanced as compared to nonreinforced plain alloy matrix.[33]AA6061-TiB2 (0, 12 wt %)In situ castingBrinell hardness, UTS, % elongationAll these mechanical characteristics of the AMCs were superior to those of pure Al matrix alloy. The incorporation of TiB2 filler contents into the AA6061 alloy has diminished the elongation of the composite.[34]Al-TiB2 (1, 4, 7 wt %)Salt-metal reaction routeHardness, UTSAll these mechanical properties of the AMCs were superior to those of plain aluminum.[35]A356-TiB2 (0, 2.12, 4.66, 8.37 wt %)In situ reaction processUTS, YS, elongationMechanical behaviors of the AMCs were greater than those of plain alloy but superior testing results are obtained at 8.37 vol% of TiB2 reinforced composites.[36]AA2219-TiB2 (0, 5, 10 wt %)In situ routeHardness, UTS, YS, and elongationBoth hardness and UTS are being increased when the mass concentration of TiB2 filler contents reached up to 10%. Addition of TiB2 reduced the elongation of the plain matrix alloy.[37]Al 6063- TiB2 (0, 4.29, 9.15, 13.12 wt %)In situ castingHardness and tensile strengthHardness and tensile strength properties are improved with the augmented content of hard titanium boride reinforcement.[38]Al 6061- TiB2 (0, 2, 4, 6, 8, 10 wt %)High energy metal stirring routeHardness, UTSHardness and UTS of the developed AMCs were superior to those of base matrix but better results are obtained at the maximum percentage of TiB2 reinforced composite.[39]AA7075-TiB2 (0, 3, 6, 9 wt %)Exothermic reaction processMicrohardness, UTS, % elongationBoth the UTS and the microhardness were enriched as compared to pure Al matrix. Incorporation of hard TiB2 reduced the elongation of the base matrix.[40]Al 6063/TiB2 (0, 5, 10 wt %)Salt base exothermic reaction processMicrohardnessMicrohardness of the produced composite increased steadily as hard TiB2 reinforcement content increased.[41]A356-TiB2 (0, 12.5 wt %)In situ casting processUltimate tensile strength, yield strength.Yield strength and tensile strength of the aluminum metal matrix composites were higher than those of unreinforced plain matrix alloy and then increased with the increase in the content of TiB2 particle.[42]AA6061-TiB2 (0, 4, 8, 12 wt %)Stir castingHardness, tensile strengthThe tensile strength and the microhardness of the prepared composite tend to increase with the increase in TiB2 content.[43]Al-6Cu-0.2Mg-Mn-TiB2 (0, 1, 3, 5 wt %)In situ castingMicrohardnessMicrohardness was greater than that of base matrix alloy. The variation in the presence of reinforcement particles is visible with increased content of TiB2.[44]AA6061-TiB2 (10 wt %)Stir castingHardness and tensile strengthAA6061/10 wt%TiB2 AMC exhibits 58HV of microhardness and 195 MPa of tensile strength. These AMCs are fabricated in liquid state by stir casting method.[45]A390-TiB2In situ castingHardness, UTS, and % elongationUltimate tensile strength, ductility, and hardness of the produced composite were greater than those of nonreinforced matrix alloy but highest results are obtained at the largest percentage of TiB2 reinforced composite.[46]AA2219/TiB2 (0, 5, 10 wt %)In situ reaction processUTS, yield strength, ductilityTensile and yield strength were greater than those of parent alloy and they raised with the raise in reinforcement, in all composites. Incorporation of TiB2 reduced the elongation of the pure Al matrix.[47]LM 25/TiB2 (0, 2.5, 5, 7.5)Mixed salt methodUTS, yield strength, % elongationUTS and yield strength were superior to those of base aluminum and they were enriched with the augmented mass proportion of reinforcement while the mechanical characteristics of the composite increases owing to the presence of TiB2 filler materials.[48]Al/TiB2 (0, 1.5, 2.5, 3.5, 5 and 10 wt% TiB2)Powder metallurgyCompressive strengthMechanical property was greater than that of basic alloy.[49]A356/TiB2 (0, 0.5, 1.5, 3, and 5 vol %)Stir castingHardnessUse of hard TiB2 has a remarkable effect in increasing microhardness and UTS of the Al composite.[50]A356/TiB2 (0, 3, 5.6, 7.8 vol%)In situ castingUTS, yield strength, and fracture toughnessThe produced composite with 7.8 vol% TiB2 shows the greatest improvement in mechanical performance when compared to the base metal.[51]AA6061/TiB2 (0, 5, 7 wt%)In situ castingMicrohardness, UTS, elongationIn all the composites microhardness and UTS were greater than those of unreinforced base matrix and they increased with increase in reinforcement content. The addition of TiB2 particulates to the AA6061 matrix has led to reduced ductility of the AMC.[52]AA7075/TiB2 (0, 5, 10 wt%)In situ casting (mixing salt route)Bending strengthBending characteristics are enhanced with the increased content of filler materials.[53]AA6061- TiB2 (0, 3, 6, 9 wt %)In situ reaction processMicrohardness (HV)Microhardness of the experimental AMCs was greater than that of plain alloy but highest results are achieved at the superior percentage of TiB2 reinforced composite.[54]AA1100-TiB2 (0, Al4.5%Cu-15 vol%TiB2, Al4.5%Cu3 %C-15 vol%TiB2)In situ methodTensile strength, elongationTensile strength of the composite was greater than that of unreinforced plain alloy and all these characteristics were enriched when augmented amount of filler content is found. Addition of TiB2 reduced the ductility of the matrix alloy.[55]Commercial pure (CP) Al-TiB2 (0, 5, 10, 15, 20 vol %)Powder metallurgyUTS, yield strengthUTS and yield strength were superior to those of base alloy in both processes and were enhanced with increase in filler material, in all composites.[56]LM 25-TiB2 (0, 2.5, 5, 7.5 wt %)In situ methodBrinell hardness, UTS, yield strength, elongationThe improved hardness and the reduction in the ductility of LM25-TiB2 AMCs are observed when TiB2 content is increased in the AMCs.[57]Al-4Cu-TiB2 (0, 2.5, 5, 7.5, 10 wt %)In situ methodHardness (HV)The microhardness of the fabricated composite tends to augment with the rise in TiB2 content.[58]Al-2.5% TiB2 (25%, 120%, 140% KBF4 excess than stoichiometry) 2.5% TiB2In situ methodMicrohardness (HV)Microhardness of the AMCs increased as compared to nonreinforced alloy matrix.[59]AA6061-TiB2 (0, 3, 6, 9, 12 wt %)Stir castingHardness, UTS, yield strength,Yield strength, hardness, and tensile strength were superior to those of nonreinforced base matrix alloy.[60]AA6061-TiB2 (0, 2, 4, 6, 8, 10, 12 wt %)In situ and equal channel angular pressing (ECAP)Hardness, UTS, elongation,Both the tensile strength and the hardness were enhanced as compared to pure plain matrix alloy. The elongation of the AMCs was found to be somewhat lower than that of the base alloy.[61]Al-B4C-TiB2 (10, 20, 30, 40 wt %TiB2)Vacuum infiltrationHardness (HRA) and flexural strengthThe hardness and flexural strength of the specimens tend to decrease during the increment of the reinforcement content.[62]AA6061, AA6061-5 wt% TiB2, AA7015, AA7015-5 wt% TiB2Hot extrusionHardnessHardness of the AMCs was superior to that of plain alloy and it is enhanced with the rising content of hard TiB2 reinforcement.[63]AA6061, AA6061-10% SiC- 2.5% TiB2, 5% TiB2Stir castingHardness (HV)Hardness of the Al-10%SiC-2.5%TiB2 AMCs increased as compared to basic alloy matrix.[64]Al6061-TiB2 (0, 6, 8, 10)In situ casting routeHardness, UTS, elongationHardness and UTS of the prepared AMCs were enriched linearly as TiB2 content increased. The incorporation of hard TiB2 filler materials into the AA6061 matrix has led to diminished elongation of the AMC.[65]Commercial pure aluminium (CP)-TiB2 (0, 2.5, 5 wt% TiB2)In situ casting processHardness, UTSHardness, fracture, and tensile strength improved with enhanced titanium diboride particle content.[66]AA7178—0, 3, 6, 9 wt%TiB2)In situ casting methodHardness, compression strength, and tensile strengthMicrohardness, compression, and tensile strength are being improved when the weight fraction of TiB2 particles reached up to 9%.[67]A1100-TiB2, AlCu TiB2 (15Vf% TiB2)Exothermic reaction processUTS, yield strengthMechanical properties of the AMMCs increased as compared to unreinforced plain matrix alloy.[68]Al-7Si/TiB2 (0, 5, 10 wt% TiB2)In situHardness, UTS, yield strengthMaximum hardness, yield, and tensile strength of the AMCs are obtained where the TiB2 filler material reached 10%.[69]Al-4%Cu-TiB2 (chemical reaction time-15, 25, 35 min)In situ castingHardness, UTS, yield strength, elongationMechanical properties of the developed aluminum matrix AMCs were superior to those of unreinforced matrix.[70]A356-TiB2 (0, 0.5, 1.5, 3.5-micron and nano TiB2) (casting temperature 750, 800, 900°C)Melt stirring castingUTS and yield strengthYield and tensile strength were augmented compared to those of basic alloy. A356-1.5 wt%TiB2-900°C AMC exhibits higher mechanical properties.[71]AA6061/TiB2/Gr 0%, 5%, 10%, 20%TiB2 + 2%Gr, 5%, 10%, 20% TiB2,Melt stirring methodUTS and hardnessUTS and hardness of the AMCs were augmented compared to those of base alloy and were then boosted with the augmented content of filler material.[72]AA6063/TiB2 (0, 5, 10)In situHardness (HV)Bulk hardness of the composite was superior to that of base alloy and it was enriched with the increasing content of filler material.[73]Al6063- TiB2 (2.8, 6.7, 10 wt%)In situMicrohardness (HV)Microhardness was 27.25% times superior to that of base aluminum while hardness increased with increased content of reinforcement.[74]A356- TiB2 (2, 3, 4, 5, 6 wt% TiB2) reaction time (20, 25, 30, 35, 40 min) temperature (800, 850, 900, 950, 1000°C)In situHardness (HV), UTSUse of TiB2 has a significant influence in enriching the hardness and tensile strength of the AMCs.[75]AA6061-10%SiC-(0, 2.5, 5 wt % TiB2)Stir castingHardness, tensile strengthHigher hardness was achieved at the percentage of Al-10%SiC-2.5TiB2 AMC and maximum tensile strength obtained at the percentage of Al-10%SiC-0%TiB2 AMC.[76]AA6061-5%, 10%TiB2-1, 2, 3, 4% GrStir castingHardness, compressive strength, tensile strengthHardness and UTS of the aluminum composites were superior to those of parent matrix alloy and it is enriched by the increasing content of 10 wt%TiB2 and 2 wt%Gr.[77]Al2014-TiB2 (0, 5 wt% TiB2 (5 wt%TiB2 + 0.5% Ceo2)In situ castingHardness, tensile strength, yield strengthHardness, yield, and tensile strength were superior to those of base alloy and they were enriched by augmenting the amount of filler content (TiB2 + CeO2).[78]AA2219-TiB2/ZrB2 (0, 3, 6%)In situ castingMicrohardness (HV)Microhardness of the composite was superior to that of basic alloy.[79]Commercial pure aluminum (CP)-TiB2 (0, 5 vol% TiB2, 5 vol%TiB2 + 0.5 wt%CeO2)In situ castingUTS, YS, elastic modulusMechanical properties of the experimental composites were superior to those of parent matrix alloy.[80]A356-TiB2 (0, 2.5, 5, 7.5, 10%) At T6 treatedIn situ compositeVickers hardness (HV)Microhardness was improved as compared to pure base matrix aluminum alloy.[81]Al-4 wt%Cu-5, 10, 15, 20%TiB2)Hot isostatic processingHardness, yield strength, UTSAll these properties of the composite were superior to those of base material.[82]Al6061-TiB2 (10, 11, 12, 13, 14 wt% TiB2)In situ casting processHardness, UTSPeak UTS and hardness of the produced AMCs are obtained where the TiB2 content reached 14%.[83]A356-TiB2 (0, 2.5, 5, 7.5, 10 wt %)Salt metal reaction processHardness, UTS, % elongationMechanical behaviors of the produced aluminum-based metal matrix composites were superior to those of nonreinforced alloy.[84]AA7075-TiB2 (0, 9 wt% TiB2)In situ methodMicrohardness and tensile strengthUTS and hardness of the AA7075-9 wt% titanium boride AMCs increased as compared to nonreinforced alloy matrix.[85]AA2009-TiB2 (8 wt% TiB2) solution temperature (498, 508, 520, 530°C)Exothermic reaction processHardness and UTSThe manufactured AMCs reveal the maximum hardness after solution was treated at 530°C and the superior tensile strength after solution was treated at 520°C.[86]AA2024-TiB2 (0, 7 wt% TiB2)Stir castingMicrohardnessMicrohardness of the developed composite was higher than that of unreinforced monolithic alloy.[87]AA7075-TiB2 (0, 6, 9, 12 wt% TiB2)In situ methodMicrohardness and tensile strengthMechanical properties of the manufactured composites were superior to those of monolithic alloy. Superior UTS and hardness of the proposed AMCs is achieved where the TiB2 content reached 12%.[88]Table 3
Superior mechanical properties of titanium diboride based aluminum matrix composites.
MMCsManufacturing processMacrohardness (BHN)Microhardness (HV)UTS (MPa)YS (MPa)ReferenceAA7050/6 wt% TiB2In situ casting technique——746701[25]AA6061/12 wt% TiB2Exothermic reaction process88.6—173.694.2[34]Al/7 wt% TiB2Salt-metal reaction route30—136.6—[35]A356/8.37 vol% TiB2In situ reaction process——258.5217.6[36]AA2219/10 wt% TiB2In situ casting technique—96234205[37]AA6063/13.12 wt% TiB2In situ casting—59.25140.9—[38]AA6061/10 wt% TiB2High energy stir casting—73.93171—[39]AA7075/9 wt% TiB2In situ method—128285—[40, 85]AA6063/10 wt% TiB2In situ casting route—110——[41, 73]A356/12.5 wt% TiB2Exothermic reaction process——382324[42]AA6061/12 wt% TiB2Stir casting—72.46137.86—[43]Al-6Cu-0.2Mg-1Mn/5 wt% TiB2In situ casting—84——[44]AA6061/10 wt% TiB2Melt stirring process—58195—[45]AA2219/10 wt% TiB2In situ reaction method——426.97301.75[47]LM/7.5 wt% TiB2Mixed salts method——202164[48]A356/0.5 wt% TiB2Stir casting—118——[50]AA356/7.8 vol% TiB2In situ casting——382328[51]AA6061/7 wt% TiB2Mixed salt route—47145—[52]AA6061/9 wt% TiB2Exothermic reaction process—123——[54]Al-4.5%Cu-3%C/15 vol% TiB2In situ method——257.6—[55]Al(CP)/20 vol% TiB2Powder metallurgy—-—191115[56]LM25/7.5 wt% TiB2In situ reaction casting81—202164[57]Al-4Cu/10 wt% TiB2Liquid state in situ casting—138——[58]AA6061/12 wt% TiB2Stir casting—77.93140—[60]AA6061/2.5 wt% TiB2/10% SiCLiquid state stir casting—71.46——[64]AA6061/10 wt% TiB2In situ method—74.83165.74—[65]Al(CP)/wt% TiB2In situ method——18096[66]AA7178/9 wt% TiB2In situ casting route—94211—[67]Al-7Si/13.12 wt% TiB2In situ reaction process—102209152[69]AA6061/20 wt% TiB2/2GrStir casting route—91.4170—[72]AA6061/2.5 wt% TiB2/10 wt% SiCStir casting—75——[76]AA6061/10 wt% TiB2/2 wt% grStir casting—79.04——[77]AA2219—6 wt%TiB2/ZrB2In situ casting—155.6——[79]A356/2.5 wt% TiB2In situ technique—92——[81, 84]AA6061/14 wt% TiB2In situ technique—128334—[83]AA2009/8 wt% TiB2 (solution temperature 520°C)In situ method——538364[86]AA2024/7 wt% TiB2Stir casting—116——[87]AA7075/12 wt% TiB2In situ technique—141287.95—[88]


## 4. Conclusions

This review article examines the impact of titanium diboride (TiB2) on aluminum matrix composites (AMCs). As seen in the previous section, various authors have discussed the mechanical properties of fabricated Al/TiB2 composites, such as hardness, tensile strength, compressive strength, and yield strength. It shall be opined that the stir casting technique is the rare one in making Al-TiB2 composite, whereas the in situ technique is the often-used technique due to its formation of more uniform TiB2 particles in the matrix. The TiB2 particles involved composites have rarely utilized squeeze casting, centrifugal casting, stir casting, and compocasting. Al alloys containing TiB2 reinforcements are to reveal enriched mechanical properties compared to monolithic alloys. Reported works also indicate the linearly augmented mechanical properties with the incorporation of TiB2. Several research works recommended that tensile strength and hardness of the AMCs were enriched with the inclusion of a rise in mass fraction of TiB2 particle contents.



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