Fabrication, Microstructure and Mechanical Properties of Boron Carbide (B4Cp) Reinforced Aluminum Metal Matrix Composite - A Review

This review paper surmised the work on Al/B4C composite, different methods of processing, microstructure and mechanical properties of different size and quantity of B4C with different grade of aluminum alloy. Since B4Cphave high hardness, good chemical resistance, good nuclear properties and low density it is widely used by the researcher among all other reinforcements like TiC, SiC, Al2O3 in MMC. Stir casting method is simple and economical with some disadvantages like porosity, agglomeration and inter-metallic compounds. However these are minimized in powder metallurgy methods of fabrication of MMC.


Introduction
The origination of metal matrix composite is the addition of hard, high strength, high modulus ceramic reinforcements to a softer metals (matrix)to produces a material whose mechanical properties are between the matrix alloy and ceramic reinforcement.Generally Aluminum, Magnesium, Copper, Zinc, Lead, Iron, and Nickel are used as matrix materials and SiC, TiC, Al 2 O 3 , B 4 C are used as reinforcement.As boron carbide is one of the hardest material (3700 Hv), Boron carbide exhibits high degree of chemical stability, very good thermal properties, low density (2.52 g cm -3 ) even lower than Al 2 O 3 (3.20 g cm -3 ) and SiC (3.96 g cm -3 ) with an high elastic modulus of 427 GPa [1, 2 and 3].Another distinguished characteristic of boron carbide is its ability to absorb neutron in a nuclear reaction which makes it suitable for nuclear shielding applications [4].A detailed review of Al/B 4 C metal matrix composite considering all the various aspects such as possible fabrication methods, their microstructure and mechanical properties have been explored.

Fabrication of Al-B 4 C Metal Matrix Composite
Al-B 4 C composites can be fabricated by many different methods but two methods are mostly used such as: molten method (such as stir casting), solid state method (such as powder metallurgy).

Stir Casting
A stir casting technique is a liquid metallurgical route of casting metal matrix composite.In stir casting Technique the MMC can be procedured through both in-situ and ex-situ methods.In Ex-situ methods, reinforcements are added externally into moltain matrix followed by proper stirring and then poured into the molds of desired shape.The major challenges confronted in the fabricating the MMCs by stir casting

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International Conference on Mechanical, Materials and Renewable Energy IOP Publishing IOP Conf.Series: Materials Science and Engineering 377 (2018) 012092 doi:10.1088/1757-899X/377/1/012092methods are to obtain a homogeneous distribution of reinforcements, its wettability into matrix, porosity and unwanted chemical reactions.Non uniform distribution of particles generally leads to three type of defects namely clustering, agglomeration and segregation [3,4].Incorporation of B 4 C particles into the aluminum matrix melt is very challenging below 1100 0 C.There are several methods have been suggested by many authors to improve wettability of B 4 C p into the moltain matrix.Fluxs such as K 2 TiF 6 , KCl-KF, etc are added to enhance wettability of particle into moltain matrix.The use of K 2 TiF 6 have duel benefit as Ti acts as a grain refiner and makes a strong bond with aluminum metal and potassium and fluoride prevents the gas entrapment in to the melt [6,7].Apart from fluxs Mg,Ti, Zr and Sc, can also be used together or individually to improve wettability of B 4 C into the moltain matrix [8,9 and 16].
Figure 1 shows the typical stir casting setup, in this casting technique the optimum process parameters choosen by various authors are as melt temperature 800-920 0 C, stirring speed 250-500 rpm, mould preheat 400-500 0 C under innert atmosphere by providing argon gas into the melt or the use of hexacloroethane to remove the hydrogen gas from the melt [5,6,7,12 and 19].

Powder Metallurgy
Powder Metallurgy is solid state methods of producing Aluminium metal matrix composite.Powder Metallurgy composites having very high mechanical properties, because in these methods reinforced particles are uniform distributed in the matrix.Therefore, Al/B 4 C composites produced by these methods are generally isotropic.Another important fact about powder metallurgy process is that the unwanted interfacial compounds between the matrix material and reinforcement particles are less and minimum segregation of reinforcement particles.The unwanted interfacial compounds usually decorate the mechanical properties.Matrix and reinforced particles need to be pretreated, some of the researchers have reported that the matrix particles are atomized by water, argon gas and stearic acid while mixing whereas the boron carbide particles are pre-oxidized to get B 2 O 3 oxide film for proper interfacial bonding.It has also been reported that in powder metallurgy method, high volume up to 20-30 % of boron can be incorporated significantly into the matrix material.The average size of matrix particles are between 10-100µm for good mixing and average size of boron carbide particles are between 2-23 µm have been reported for proper bonding between matrix and reinforcements.Since mixing is required in this method, different mixers are used such as turbula mixer , shaker mixture and planetary ball , and it is dry milled with ZrO 2 ball with diameter 4-8 mm and Chromium steel ball with diameter 6 mm.The compacting pressure generally reported is between 300-600 MPa.Since the powder metallurgical composites under goes secondary processing such as cold pressed, hot pressed, die pressed, sometime both cold and hot

Microstructures Al-B 4 C Composite Produced By Various Techniques
Microstructures of any composite reveal its physical properties.Microstructures of different processing methods are discussed in following sections.

Stir Casting Process
Microstructure of aluminum metal matrix composite fabricated by stir casting method are characterized by XRD, Optical Microscope, SEM,FESEM etc., Microstructure of composite give a clear description about distribution of particle, bonding between matrix and B 4 C , inter-metallic phases.Among all these, solidification mechanism of the melt composite also plays significant role in describing its microstructures.Due to particle pushing phenomena B 4 C particles are rejected by solid front and trapped in inter-dendrite zone and leaving the dendrite branches as particle-free regions in the materials [12 and 13].Other factors also influences the microstructures are size of reinforcement particles and its quantity.It has been observed that the reinforcements having size more than 20 µm shows uniform dispersion as shown in figure 2, whereas particles less than 10µm lead to agglomerations [6,7].(d, e, f) within the matrix [7] Several research confirm that the optimum quantity of reinforcement should up to 10% either by wt. or by vol%, less quantity of boron carbide leads to several particle free region where as higher quantity leads to Porosity and increasing sites for heterogeneous pore nucleation i.e. the particles are surrounded by surface gas layers and the hindered liquid metal flow due to more particle clustering [20].It is also evident that Ti compound reaction layer is found around B 4 C reinforcement when adding K 2 TiF 6 flux to the melt.The number of particles transferred to the melt depends upon the level of Ti present in the flux, a higher Ti level resulting in increased particle transfer.Incorporation is facilitated by exothermic reaction between the B 4 C particles and Ti Present in the flux to produce Ti compound around boron carbide particles which intern prevent B 4 C particle to decompose into matrix, hence increase mechanical properties [5,17].

Powder Metallurgy
In powder Metallurgy process during extrusion many structural changes occurs, namely change in grain structure, particle orientation along the direction of extrusion.In the parallel direction of extrusion microstructure shows anisotropic property as particle have tendency to rotate and align along the extrusion Direction shown in figure 3

Mechanical Properties of Al-B 4 C Composite Produced By Various Techniques
Reinforcement particles are the load bearing elements in the composite material hence homogeneous distribution leads to higher mechanical properties.

Stir Casting
The Mechanical properties of the Al-B 4 C composites like hardness, tensile strength, compressive strength and wear behavior mainly depend upon the distribution of reinforcement particles in the matrix, proper incorporation of reinforcements in the matrix and amount of Reinforcements (wt% or vol %) actually dispersed into the matrix.Reinforcement particles are the load bearing elements in the composite materials, hence for higher tensile strength of composite materials the reinforcements should be uniformly distributed and properly incorporated.The tensile strength also increases with increase weight percentage of reinforcements as it offers strength to matrix alloy by offering more resistance to induced tensile stresses.Since the variation in coefficient of thermal expansion between the matrix and the boron carbide particle is very high, this causes higher dislocation density in the matrix and load bearing capacity of the hard particles increases hence the strength of composite increases.The hard B 4 C particles prevents the crack to propagate, rather it acts as barrier for crack propagation.Also the compressive strength of The Al-B 4 C composite as B 4 C particles offers more restriction to the plastic flow during deformation [5, 20].The

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International Conference on Mechanical, Materials and Renewable Energy IOP Publishing IOP Conf.Series: Materials Science and Engineering 377 (2018) 012092 doi:10.1088/1757-899X/377/1/012092yield stress increases with decreasing inter-particle distance.It has been recommended from researcher that the inter-particle distances (λ) values should be within 10 μm.Boron carbide and aluminum reaction forms several inter-metallic phases at higher temperature like AlB 2 , Al 3 BC, Al 4 C 3 .These phase increases the flow stress in the composite consequently decreases the ductility of composite.Therefore fabrication of composite at higher temperature leads to the formation of more brittleness phases [19].From all these conclusion it is concluded that Boron carbide is most promissing reinforcement and it can be used for superior mechanical properties of the composite.

Figure 2 ,
Figure 2, bigger B 4 C particulates (a, b, c,) and agglomeration of smaller B 4 C particulates(d, e, f) within the matrix[7] . Al-B 4 C cryomilled followed by hot pressing produces clean and strong interface between Al and B 4 C.Various phases are identified by XRD analysis, apart from Al and boron carbide small amount ofAlB 2 ,Al 3 BC and Al 4 C 3 phases are present with the Al and B 4 C [11].

Figure 3
Figure 3 OM micrographs of the B 4 C/6061Al with different contents of (a) 10%, (b) 20%, (c) 30% and (d) 40%[11] Tensile strength of Aluminium boroncarbide composite fabricated by powder metallurgy process increases with increasing B 4 C but at higher content of B 4 C tensile strength decrease.Also the tensile strength increases with increasing secondary processing temperature (Hot rolling).The strengthening mechanism of Al-B 4 C attributed to the following factors: (a) grain refinement and dislocation strengthening mechanism,Dislocation strengthening can play an important role.When The composites are prepared through two stages: Primary Processing (vacuum hot pressing) and secondary processing (hot rolling) the thermal mismatch is developed due to difference in coefficient of thermal expansion between B 4 C and Aluminium alloy this thermal mismatch lead to the dislocation density and it increases around the B 4 C particles.(b)Load transfer effect that is the presence of B 4 C particles can transfer load from matrix to the B 4 C Particles (c) Orowan strengthening.[23,24] .Bie B et al; [22] reported that the higher particle content leads to a higher yield strength but lower ductility, he has reported that 15-B 4 C/Al composite indicating plasticity and ductile fracture whereas 30-B 4 C/Al is reported of brittle fracture as shown in figure 4. Q shen et al;[25] have fabricated Al-7075/ B 4 C composite fabricated through plasma activated sintering and he has observed that hardness, bending strength and compressive strength increases with increasing B 4 C content but at higher B 4 C content the these properties decreases due to algomeration of B 4 C particle .The Mechanical properties like compressive fracture and yeild strength increases by about 18% on increasing the sintering temperature from 450 C to 530 C.

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International Conference on Mechanical, Materials and Renewable Energy IOP Publishing IOP Conf.Series: Materials Science and Engineering 377 (2018) 012092 doi:10.1088/1757-899X/377/1/012092pressare used respectively to get desired properties at different pressure and temperature.Due to secondary processing the void contents are very lass and the composite attains the relative density up to 99.9%[8,