Effect of solid waste materials on properties of magnesium matrix composites-A systematic review

Fatih Aydin

TOBB Technical Sciences Vocational School,Karabuk University,Turkey

Abstract The need for light and high specific strength materials in many fields such as automotive and aerospace is increasing day by day.Magnesium(Mg)-based materials have become attractive for many industries thanks to their high specific strength,good vibration damping ability,and recyclability.However,Mg’s low strength and wear resistance are important barriers limiting its industrial use.Researchers are developing Mg matrix composites using various reinforcements and expanding the use of Mg-based materials.The conventional reinforcements are Al2O3,SiC,B4C,TiB2,CNT,and GNPs for the production of Mg matrix composites.Researchers have been trying to reduce the cost of Mg matrix composites in recent years by using cheaper and environmentally friendly reinforcements.These reinforcements are solid wastes such as eggshell,fly ash,red mud,and waste glass,and their use in composite systems is becoming more common day by day.This review focuses on the Mg matrix composites reinforced with solid waste particles and the changes in wear,mechanical,corrosion,and thermal properties with the addition of these reinforcements.

Keywords:Mg matrix composites;Solid waste reinforcements;Wear;Corrosion;Mechanical properties.

1.Introduction

Reducing fuel consumption and energy-saving requirements have increased the need for lightweight and highspecific strength materials[1–2].These needs have forced researchers to discover new materials with superior properties.Magnesium(Mg)has become one of the most important materials for many industries,such as aviation,automotive and aerospace,thanks to its low density,high specific strength,good machinability,and good vibration damping ability[3–6].However,the low strength,poor wear performance and weak elevated temperature properties of Mg alloys limit the use of monolithic Mg for practical engineering applications[7–9].One of the most important ways to improve the properties of Mg and Mg alloys,such as strength,wear and creep,is to reinforce them with stronger phases.Researchers produce Mg matrix composites with superior properties using different reinforcement elements[10,11].From the literature studies,it is well known that carbides(SiC[12–15],B4C[16–19],TiC[20–22]),borides(TiB2[23–25]),nitrides(BN[26–28]),oxides(Al2O3[29–31],TiO2[32,33],ZrO2[34,35])and carbonaceous(Graphite[36,37],carbon nanotube CNT[38,39],graphene nanoplatelets GNPs[40–41])reinforcements are widely used for the production of Mg matrix composites.

Although conventional reinforcement elements improve the hardness,tribological performance,and mechanical properties of Mg-based materials,they also significantly increase the final product cost.Nowadays,researchers use low-cost solid waste materials as reinforcements for composite production to solve this problem and reduce the final product cost.Eggshell,fly ash,red mud,and waste glass have been used as reinforcement materials for composite production in recent years.The use of these reinforcements in composite systems not only improves the different properties of monolithic metals but also reduces environmental pollution[42,43].Other advantages of using waste materials in composite systems are economic benefits due to lower material production cost and reduced composite weight[44].

In the literature,there are many studies on Al matrix composites reinforced by solid wastes.[44,45].When the literature studies are examined,it is seen that rice husk ash,coconut shell ash,eggshell,and bamboo leaf ash are widely used for the production of Al matrix composites.It is also well reported that the microstructure,wear and mechanical behaviour of Al matrix composites are reinforced by waste materials[43].The importance of using different waste reinforcements in producing Mg matrix composites is increasing day by day.In recent years,different waste materials such as fly ash,eggshell,and rice husk have been used for Mg matrix composites[46,47,48].However,there are still limited studies on the use of these reinforcements in Mg-based composite systems.For this reason,it is essential to explore the utilization of these waste materials for the production of Mg matrix composites.In this paper,a literature review on Mg matrix composites using solid wastes as reinforcement materials is presented.The paper also reveals the effect of these reinforcements on different properties such as mechanical,wear,corrosion,thermal,and damping behaviour of the Mg matrix composites.While producing these materials,the homogeneous distribution of the reinforcement elements is critical,and the selection of the production method has critical importance in achieving this.For this reason,the production methods for Mg/waste reinforcement composites were also reviewed.At the end of this review,the reinforcement type,content,particle size and production method which gives the best mechanical,corrosion and wear performances were determined for Mg matrix composites.

2.Solid waste reinforcement materials for Mg matrix composites

In this section,some information is given about the properties of solid waste reinforcements used in the production of Mg matrix composites.These materials are fly ash,eggshell,red mud,cathode ray tube,rice husk,and granite.

2.1.Fly ash

Fly ash(FA)is formed by the combustion of coal in thermal power plants and is captured before exiting the chimney.Since releasing this waste into nature causes environmental pollution,it is essential to collect it in facilities and use it in composite systems.This supplement is also one of the cheapest solid waste materials.It has a high percentage of SiO2and Al2O3in its composition.The chemical composition of FA is given in Table 1.Dinaharan et al.[46]investigated the microstructure and sliding wear performance of AZ31/FA(10 vol.%)composite manufactured by stir casting(SC)and friction stir processing(FSP).From the wear tests,it is reported that the FSP composite showed higher wear performance compared to stir casted composite.Different researchers have stated that FA reinforcement causes an increase in hardness and wear resistance for Mg matrix composites[49–50].

2.2.Red mud

Red mud(RM)is a solid waste formed by the caustic digestion of bauxite ores in the production of alumina.It contains a significant amount of Al2O3,Fe2O3and CaO in its composition.The use of RM particles for the production of Mg matrix composites was reported by Rajamani et al.[51].The chemical composition of RM is given in Table 1.

2.3.Eggshell

Eggshell(ES)is one of the most used waste reinforcements in recent years;it is released after chicken farming and contains approximately 95 wt.% CaCO3in its composition.In addition,the essential advantages of ES are that it is very cheap,low density,easy to find and non-toxic[47].Several researchers used the ES as a reinforcement element for the Mg matrix composites and reported the wear,corrosion,and mechanical behaviour of the composites[47,52].

2.4.Cathode ray tube glass

The cathode ray tube(CRT)is a material that is mostly glass,which makes up a large part of the E-waste and is present in TV and PC monitors[53].The chemical composition of CRT glass is given in Table 2.However,in recent years,with the emergence of flat-screen technologies such as LED,CRT production has stopped,and the recycling of existing CRTs has become a critical issue.The CRT was used as a reinforcement material for Mg hybrid composites and reported the tribological performance by Gopal et al.[54].

3.Fabrication of Mg matrix composites reinforced by waste materials

The Mg matrix composites reinforced by solid waste particles are produced by two routes,liquid and solid methods.For liquid methods,stir casting(SC)and disintegrated melt deposition(DMD)are widely used.Powder metallurgy(PM)and friction stir processing(FSP)are extensively used as a solid route.The reasons for choosing these methods,their advantages,and the features they provide are explained in detail below.

3.1.Stir casting

Stir casting(SC)is based on incorporating powder reinforcement into liquid Mg alloys by mechanical stirring[55].The method’s advantages include its inexpensiveness,flexibility and simplicity[43].Fig.1 shows the schematic representation of the SC method.The uniform distribution of reinforcement particles is the most important challenge for SC method.Besides the significant advantages of SC,thereare also some disadvantages.These are agglomeration of particles,formation of unwanted phases,and inclusions[56].Researchers minimize particle agglomeration by optimizing different parameters such as mixing speed and rotation direction of the stirrer[57].In SC,wettability between the Mg matrix and reinforcement particles can be improved by coating the reinforcement materials[58].To prevent undesirable interfacial reactions,it is recommended to choose reinforcing elements that prevent interfacial reactions.Several researchers have used the SC method to produce Mg/solid waste composites(Table 3).

Table 1Chemical composition of waste reinforcement materials(wt.%).

Table 2Chemical composition of Cathode Ray Tube glass(wt.%).

Fig.1.Schematic representation of SC technique.

3.2.Disintegrated melt deposition

The disintegrated melt deposition(DMD)method combines the conventional casting and spray process.The DMD method uses high superheat temperatures and low striking gas jet speed to produce Mg matrix composites.Fig.2 shows the schematic diagram of the DMD method.In this method,firstly,the materials in the crucible are superheated in a resistance furnace.The composite melt is mechanically stirred to achieve uniform distribution of reinforcement particles.After that the melt is released through a hole at the base of the crucible.The composite melt is disintegrated by argon jets to the melt stream.As a result,the disintegrated composite melt is deposited on the substrate[59].In the literature,there are a limited number of studies in which this method is used in the production of Mg matrix solid waste reinforced composites[47].The fine equiaxed grains,low porosity and uniform distribution can be obtained by DMD technique[1].

Fig.2.Schematic diagram of DMD.

3.3.Friction stir processing

Friction stir processing(FSP)modifies the surfaces and produces surface composites with homogenous microstructure and fine grain size.Fig.3 shows the FSP method.The FSP method has advantages over PM and SC.It is reported that the PM and SC increase the strength of the composites but decrease toughness and ductility.The FSP produces composite with higher surface hardness compared to other techniques.Also,the FSP leads to a homogenous distribution of reinforcement particles.In recent years,this method has been widely used to produce composite surfaces on metallic materials[60].Several researchers preferred the FSP to produce Mg matrix composites reinforced by solid waste particles(Table 3).

Table 3(continued)

Table 3Mg matrix composites reinforced by waste materials.

Fig.3.Schematic representation of the FSP method.

Fig.4.Schematic diagram of the PM.

3.4.Powder metallurgy

In conventional powder metallurgy(PM),the matrix and reinforcement powders are mixed,pressed and then sintered in a controlled atmosphere.The advantages of PM are the minimum reaction between the matrix and the reinforcement,the use of the desired alloys and reinforcement elements,efficient use of the raw materials,and the homogeneous distribution of the particles[43,61].One of the most important disadvantages of the method is that the method is more expensive than other methods due to the high cost of raw materials[1].Fig.4 shows the process setup of the PM.There are different methods used to mix the matrix and reinforcement elements.Vacuum distillation system can be used to produce Mg matrix composites by nano-reinforcement particles[62].Vacuum distillation system was not preferred because the size of solid waste reinforcement particles used in the literature is in micron size.Generally,secondary treatments such as extrusion,rolling,and forging are required to consolidate and shape the composite[1].Gupta et al.[63]produced the Mg2.5Zn/ES composites by conventional PM and applied extrusion to samples.Gupta et al.also used the hybrid microwave sintering.In hybrid microwave sintering,microwave SiC particles/rods susceptors are used to reduce the thermal gradient during the sintering[59].

3.4.1.Mechanical alloying

One of the commonly used mixing methods before pressing is mechanical alloying(MA).MA is a solid-state method which includes repeated cold welding and fracture of particles by high-energy balls[64].MA produces Mg matrix composites with a high content of reinforcements and leads to phase transformation by application of mechanical energy[65].To prevent oxidation in the production method of Mg matrix composites,a protective atmosphere should be used during the process.Aydin et al.[52]used MA as a mixing method for the production of Mg/ES composites.

3.4.2.Hot pressing

In recent years,nano-reinforced and micron-reinforced Mg matrix composites have been produced using the hot pressing(HP)method.The advantage of the HP over conventional PM is that it applies temperature and pressure simultaneously and does not require post-production sintering.Since the resistance of the powders to plastic deformation decreases at high temperatures,production can be carried out under lower pressures in HP.In this method,generally,graphite moulds are used,and productions are carried out under an argon atmosphere[66].Aydin et al.[52]successfully produced Mg/ES composites by hot pressing.

4.Features of waste material reinforced Mg matrix composites

The reinforcement type,composition,applied tests,and the production methods of Mg/waste materials composites are given in Table 3.It can be seen that the commonly used waste reinforcements for the production of Mg matrix composites are fly ash,red mud,eggshell,rice husk,granite,bagasse ash particle,and cathode ray tube.Also,the matrix materials are pure Mg,AZ31,AZ61,AZ91 and Mg-2.5Zn alloy.To produce these composites,researchers chose different productionmethods such as stir casting,friction stir processing,powder metallurgy,and disintegrated melt deposition and examined the properties of the produced materials such as microstructure,hardness,mechanical,wear,corrosion,thermal,and machining.The effect of waste material reinforcements on the microstructure,mechanical,tribological,corrosion and other properties of Mg matrix composites are discussed in the following sections.

Fig.5.SEM images of the samples a)SC AZ31/FA and b)FSP AZ31/FA[46].

4.1.Microstructure of waste reinforced Mg matrix composites

In this sub-section,the effect of waste materials on the microstructure of Mg matrix composites was investigated.In addition,phases formed during production and intermetallic compounds have been reported.Microstructure analysis is of critical importance in composites,as the distribution of the reinforcement particles,possible reactions between the reinforcement and the matrix,the good bonding between the matrix and the reinforcement,and the change in grain size affect many properties such as mechanical,wear and corrosion.In this section,the effects of different production methods on the microstructure of Mg/solid waste composites were also investigated.

Dinaharan et al.[46]performed the microstructural characterization of AZ31/FA composites by SC and FSP(Fig.5).The presence of FA particles amongst coarse grain structure can be discernible in Fig.5a.It can be said that the incorporation of FA is successful by the SC.However,the dispersion of the particles is heterogeneous.They claimed that the presence of reinforcement particles causes heterogeneous nucleation of grains.The difference in density and particle size results in the movement of particles in the time between incorporation and pouring.In addition,it was reported that the FA particles seen in Fig.5a lose their shape and decompose due to interfacial reactions during contact with liquid Mg.The high temperature starts the reaction between molten Mg and FA particles.For Fig.5b(FSP),the reduction in the size of FA particles can be noticeable.They stated that the significant plastic strain by FSP is the main reason for this situation.The average grain size of the SCed and FSPed composites was calculated as 145 and 4 μm,respectively.It was reported that the broken FA particles have a pinning effect which leads to the formation of sub-grain boundaries.In the composite produced by the FSP,no particle-enclosed reactive layer was observed between the FA particles and the Mg matrix.This is due to the fact that the process temperature is not high enough to initiate interfacial reactions[46].

Fig.6.Optical micrograph of AZ31/FA[50].

Dinaharan et al.[50]also conducted the microstructural examination of the AZ31/FA composite produced by FSP(Fig.6).From Fig.6,it is well seen that the FA particles are homogenously distributed across the Mg matrix.They reported that the homogenous distribution of particles could be related to the FSP method,which has no solidification.The lack of free movement of FA particles,which may occur due to the density difference in the plasticized material,prevents segregation.It is well known that the homogenous distribution of the particles is crucial to achieve isotropic properties.It can also be seen that there is no agglomeration of FA particlesand area of the matrix with particle-free regions.The homogeneous distribution of the particles requires effective mixing of the rotating tool,and the tool rotation speed and traverse speed must be well chosen[76].In the FSP,the material is exposed to large strain due to excessive plastic deformation.The metal matrix deforms plastically,whereas reinforcement particles cannot absorb strain.As a result,particle breakage,change in shape and size are observed[77].

Fig.7.SEM images of Mg/CRT/BN composites(a)10 μm,(b)30 μm and,(c)50 μm CRT[54].

Gopal et al.[54]investigated the particle size(10,30 and 50 μm)of CRT on the microstructure of Mg/CRT/BN composites produced by PM(Fig.7).From the Figure,the homogenous distribution of CRT particles of different sizes can be seen.It is also noticed that the composite materials have no significant defects and macro pores.Contrary,it is known that in the composites produced by PM,there are generally pores in the structure,and the material properties are adversely affected[1].In addition,in the SEM images,it was stated that no different structure was formed between the reinforcement and the Mg matrix.The authors also verify this statement by XRD analysis.

Gupta et al.[47]explored the optical micrographs of Mg-2.5Zn/ES composites manufactured by DMD(Fig.8).They claimed that the particle distribution is near uniform in the matrix.It is well seen that the addition of ES leads to significant grain refinement and the formation of equiaxed grains.The grain size of the Mg2.5Zn,Mg-2.5Zn-3ES,Mg-2.5Zn-5ES,and Mg-2.5Zn-7ES composite was reported as 16.7,10.2,7.9 and 6.9,respectively.The grain refinement was associated with the presence of hard ES particles acting as an area of grain nucleation during production,so it inhibits grain growth.For Fig.8,it can also be seen that the ES particles are located along the grain boundaries.The reinforcement particles are pushed during the solidification by the solid particleliquid melt interface[78].For the XRD analysis,the presence of Mg and CaCO3phases was reported for the structure of composite materials.

Fig.8.Optical micrographs of the samples a)Mg-2.5Zn alloy b)Mg-2.5Zn/3ES,c)Mg-2.5Zn/5ES d)Mg-2.5Zn/7ES[47].

Gupta et al.[67]investigated the microstructure of Mg/ES composites by PM(Fig.9).From the images,no macroscopic defects are present,and the grain size significantly decreases with increasing ES content.The addition of ES particles leads to the formation of equiaxed grains.The segregation of ES particles can also be seen at the grain boundaries.It is stated that the regions marked with red in the images are grains.The grain size of the Mg2.5Zn,Mg2.5Zn-3ES,Mg2.5Zn-5ES and Mg2.5Zn-7ES was noted to be 20,7,8 and 8 μm,respectively.The grain refinement was attributed to the presence of ES particles pinning the grain boundaries and activating the Hall-Petch mechanism.For the XRD analysis,they reported that CaCO3phase appeared from the 5 wt.% ES content.The authors stated that no other phases and intermetallic compounds are present.This is proof that there is no interfacial reaction between the reinforcement and the matrix during production.

Aydin et al.[52]revealed the microstructure of pure Mg/ES composites produced by MA and HP(Fig.10).It is seen from the microstructures that there are no macroscopic surface defects and the grain size decreases with the addition of ES.The grain size of the Mg,Mg-2.5ES,Mg-5ES,and Mg-10ES was noted to be 86.4,83.1,80.3 and 70.2,respectively.The grain refinement due to reinforcement particles was attributed to the pinning effect of particles[67].It seems that the reinforcement particles are concentrated at the grain boundaries,and the segregation increases as the amount of ES increases.The authors also reported the presence of Mg and CaCO3phases as a result of XRD analysis.

Srivastava et al.[72]investigated the microstructure of AZ31/ES composites(2,4 and 6 wt.%)by FSP(Fig.11).From the microstructure images,it is seen that the particles are generally homogeneously dispersed,and there are no different defects such as porosity and microcracks.However,the agglomeration of ES was reported for the 6 wt.% ES content.The diffraction peaks of the CaCO3were verified by the XRD analysis.For the phase analysis of AZ31 alloy,MgO,Mg2Al4,Mg2SiO4,and Mg were detected.

Singaiah et al.[74]investigated the microstructure of AZ91/Granite-FA hybrid composite by SC.They reported the presence of homogenous distribution of reinforcement particles(Fig.12).They claimed that the uniform distribution was related to continuous stirring during the production stage.It is reported that some cavities(shown with white arrows)are present due to the removed reinforcement particles during metallographic preparation.They also reported the presence of Mg,Mg17Al12,calcite and SiO2phases by XRD analysis.

Fig.9.Optical micrographs of the samples a)Mg-2.5Zn alloy b)Mg-2.5Zn/3ES,c)Mg-2.5Zn/5ES d)Mg-2.5Zn/7ES[67].

Saranu et al.[75]explored the microstructure of the AZ91/SiC/FA hybrid composites by SC.They claimed the presence of homogenous distribution of reinforcement particles in the AZ91 matrix(Fig.13).They also stated that there are no defects and voids,and there is a good interfacial bonding between the matrix and reinforcement.According to XRD analysis,the presence of Mg,Al12Mg17,Mg2Si,MgO and SiC phases was reported.

4.2.Mechanical behaviour of waste reinforced Mg matrix composites

Several researchers investigated the effect of waste materials on the hardness and mechanical properties such as compression,tensile,and impact of Mg matrix composites.In this sub-section,the reason for the increase and/or decrease in hardness,strengthening mechanisms,and mechanisms of damage was examined in detail.Also,the optimum reinforcement content was determined to obtain the best mechanical properties.Fig.14 shows the hardness of different Mg matrix composites in the literature.Generally,it can be seen that the hardness of the composite increases with the increasing reinforcement content.However,it was reported in some studies that increasing the reinforcement content first causes an increase in hardness and then a decrease[48,67].In the literature,the reasons for the increase in hardness with the addition of reinforcement elements are attributed to different reasons:1)grain refinement due to reinforcement particles,2)resistance to motion of dislocations,3)resistance to plastic deformation during indentation,4)good interfacial bonding[49-51,70,79,80].In Fig.14,a different situation regarding hardness is seen in the Ref[67].The maximum hardness was obtained for the 3 wt.% ES content,and above these value,the hardness tends to decrease.The reason for the decrease in hardness was not reported by the authors[67].In another study,the maximum hardness was obtained for the 7.5 wt.% reinforcement content;above this value the hardness decreased.The authors explained the decrease in hardness with the high porosity formed in the structure[48].

The mechanical properties of some waste-reinforced Mg matrix composites are given in Table 4.Fig.15 also shows the compression and tensile test results of some studies in the literature.Gupta et al.[47]investigated the compression behaviour of the Mg2.5Zn/ES composites by the DMD method.They reported that the 0.2CYS of the samples increases with increasing ES content.The UCS value first decreased for the addition of 3wt.% ES content.Above 3wt.% ES content,the UCS of the samples significantly increases.Failure strain value rises up to 5% ES content and decreases after this value.The increase in CYS was attributed to good inter-facial bonding and non-basal slip mode activation.They also stated that the uniform distribution of ES particles which act as obtacle to dislocation movement aiding in Orowan stregnthening.Crack propagation under compression load occurs along the soft matrix as the matrix and reinforcement particles show good interfacial integrity.Cracks deform the matrix and lead to crack bridging mechanism.This causes crack closure and crack propagation,resulting in higher failure strains.The presence of micron-sized reinforcement particles causes the formation of a residual dislocation network around the particles and activates the Orowan strengthening mechanism.For higher reinforcement content,the sharp ES particles act as stress concentration sites and can be harmful to ductility[81].

Table 4Mechanical properties of waste reinforced Mg matrix composites.

Fig.10.SEM images of the a)Mg,b)Mg/2.5ES,c)Mg/5ES and d)Mg/10ES[52](Yellow arrows show the ES particles).

Fig.11.SEM images of a)AZ31B/2ES(b)AZ31B/4ES(c)AZ31B/6ES[72].

Gupta et al.also[67]studied the mechanical properties of Mg2.5Zn/ES composites by PM.The maximum CYS and UCS were reported for the 3 wt.%ES content.Above this content,the mechanical properties tend to decrease.The increase in strength was attributed to the effect of deformation twinning and texture randomizations due to the presence of ES leads to barriers and decrease the slip path in the untwinned region.As a result,the strength of the composites increases.For higher reinforcement content,the decreae in strength was attributed to:1)increase in grain boundary segregation,and 2)high level of porosity.

Anandajothi et al.[48]reported the increase in yield and tensile strength of AZ91/SiO2-HA hybrid composites up to 7.5 wt.% reinforcement content.Chandradass et al.[70]reported that the tensile strength decreases for the 5 wt.% BA content;above this value,the tensile strength enhances.They also noted that the impact strength of the samples significantly increases with increasing BA content.The increase in impact strength was attributed to the presence of reinforcement particles,which improves the capability of energy-absorbing.Markos et al.[73]also reported the compression,tensile,impact and flexural strength of the AZ31/ES composites.Theyreported the maximum mechanical properties were obtained for the maximum reinforcement content.

Fig.12.SEM image of the AZ91/5 Granite-5 FA composite[74].

The strengthening mechanisms in Mg matrix composites are load bearing effect,generation of geometrically necessary dislocations,Orowan strengthening,and Hall-Petch[81–86].The increase in yield strength due to these four mechanisms can be calculated using a single formula.The improvement in yield strength(YS)can be calculated using the following equation[87].

whereΔσLoadis the load bearing effect,ΔσCTEis the Taylor strengthening mechanism,ΔσOrowanis the Orowan mechanism andΔσHall-Petchis the grain size refinement mechanism.These mechanisms are described in detail below.

4.2.1.Hall-Petch mechanism

Hall-Petch is one of the most crucial strengthening mechanism for Mg matrix composites.Hall-Petch equation calculates the increase in mechanical properties with grain refinement by following the formula[1,88].

Whereσ0andkyare the initial strength and material constant,respectively,dis the grain size.It is reported that thekyis a sign of grain boundary resistance to slip transfer.For the literature,it is well known that the slip systems of the magnesium(HCP metal)is few;therefore,grain refinement has great importance for the mechanical properties of Mg matrix composites.For Mg matrix composites,the reinforcement particles significantly decrease the grain size of the matrix.The grain refinement is related to the heterogenous nucleation of the Mg phase and reinforcement particles.The growth of Mg crystals was restricted by ceramic particles[1].

For Mg matrix composites,the increment in the YS can be calculated by the following equation:

Wheredmcanddmare the grain size of Mg matrix composite and the Mg alloy,respectively.

4.2.2.Taylor strengthening mechanism

This mechanism is also known as thermal and elastic modulus mismatch.Because the difference in thermal expansion coefficient and elastic modulus between the metal matrix and the reinforcement particles is huge,high dislocation density occurs in composite materials[59].The existence of a softer Mg matrix host the nondeforming and stiffer reinforcement particles by the generation of geometrically necessary dislocations[89].

For Mg matrix composites,the increase in YS can be given using the following equation[1,90].WhereMandβare Taylor factor and constant,respectively,Gmandbare shear modulus and Burgers vector,respectively.ρCTEis the density of the geometrically necessary dislocations and can be calculated using the following formula[91].

WhereAis the geometric constant of the particles,Δαis the difference in coefficient of thermal expansion,ΔTis the temperature difference,vPanddPare volume fraction and diameter of the particles,respectively.As a result,the increase in strength can be calculated using the following equation[92].

WhereTpandTtare production temperature and test temperature,respectively.αmandαPshows the coefficient of thermal expansion of the Mg matrix and reinforcement particle,respectively.

4.2.3.Load transfer mechanism

This is an important strengthening mechanism,also known as the load-bearing effect.In this mechanism,the load can be transferred from the matrix to the reinforcement if the interfacial bonding between the reinforcement particles and the Mg matrix is good[1,59].The load transfer mechanism can be explained using the following equation[1].

whereσmis the YS of Mg matrix,landtare the dimension and thickness of the reinforcement particle,respectively.Ais the aspect ratio of the particles.The increase in YS can be expressed using the following equations[93].

Fig.13.Microstructure image of a)Base alloy b)AZ91/2.5 SiC-2.5FA c)AZ91/5 SiC-5FA[75].

Fig.14.The hardness graph of the literature studies.

4.2.4.Orowan strengthening mechanism

The Orowan mechanism is an important strengthening mechanism in nanoparticle reinforced Mg matrix composites.The presence of nanoparticles in the structure prevents the movement of dislocations and increases the strength[90].Since the reinforcement particles used in this review work are micron in size,the effect of this mechanism can be ignored.However,Gupta et al.[47]reported that the presence of ES particles(in micron size)is an obstacle to the movement of dislocation,aiding Orowan strengthening.

In the literature,the decrease in compression and tensile strength for higher reinforcement content was attributed to the following reasons:1)increase in grain boundary segregation,2)agglomeration of reinforcement particles,and 3)high-level porosity[48,67,70].It was also reported that if the reinforcement content reaches a critical value,the particles react with the matrix,and the formation of a new layer occurs.The formed layer deteriorates the bond between the matrix and reinforcement[70].

The mechanical performance of Mg matrix composites is directly related to the interface between the matrix and the reinforcement.The properties of the interface are the strength of bonding and chemical reactions.The porosity in the structure of Mg matrix composites occurs during the production and affects the interfacial reactions to a significant extent.It is known that porosity and inclusions have a negative effect on the mechanical properties of Mg matrix composites,such as yield,tensile and fatigue strength.The inclusion of large quantities of reinforcement particles can cause severe porosity if not degassed before production.The presence of hard particles in the structure inhibits the dislocation motion,so increases the mechanical properties.Also,reinforcement particles form heterogeneous nucleation sites,leading to finer matrix grain size.Another important effect of the particles is to prevent grain growth during deformation at high temperatures.The presence of reinforcement particles in the structure reduces the ductility of Mg matrix composites as it prevents plastic deformation.Conversely,the ductility can be improved due to the grain refinement effect of particles[55,59].

For Mg matrix composites,examining the interfacial behaviour between the matrix and the reinforcement is critical.The interface regions significantly affect the efficiency of load transfer from matrix to reinforcement.The type of reinforcement,its morphology and particle size are critical in the microstructure of the product and morphology of the phases formed at the matrix-reinforcement interface.It is knownthat the interfacial reactions that occur during the production stage improve wetting and the bonding between reinforcement and matrix[55].For example,Gu et al.[94]investigated the interfacial behaviour of ZK60/B4C/SiC hybrid composites.It was reported that B4C was oxidized,and B2O3layer was formed.After that,this layer reacted with Mg and formed MgO and MgB2.The formation of MgB2at the interface has been reported to have a positive effect on increasing the strength of the composite.The factors affecting the interface reactions are the composition of the matrix and the reinforcement,the porosity and the surface cleanliness of the raw materials.As a result,to gain a good properties the interacial ractions should be controlled by choosing a convenient raw materials and production method and process parameters[95].The studies in this review did not provide sufficient information as the researchers did not examine the interactions at matrix-reinforcement interfaces in detail.

Fig.15.Mechanical properties of Mg matrix composites a)Tensile and b)Compression(YS:yield strength,TS:tensile strength,E%:elongation,0.2CYS:compressive yield strength UCS:ultimate compressive strength FS%:fracture strain).

Some researchers investigated the fracture surface images to know the fracture mechanisms.In one of those studies,Gupta et al.[47]explored the compressive fractography of the Mg/ES composites(Fig.16).From the images,the shear band formation is present.The authors claimed that this verifies the composite fracture is matrix driven.Gupta et al.[67]also revealed the fractured image of Mg/ES composites by powder metallurgy(Fig.17).They reported that the failure of the samples is by the shear mode of fracture.They also reported that the surface roughness was minimum for the 3 wt.% ES content.This was attributed to the difference in particle distribution in the matrix.

4.3.Tribological behaviour of waste reinforced Mg matrix composites

Several researchers investigated the tribological performance of Mg matrix composites reinforced by waste materials.They used different wear conditions such as load,sliding speed,and sliding distance and reported the wear rate,wear loss and coefficient of friction.Some wear properties of Mg matrix composites reinforced by waste materials are given in Table 5.Some wear results are also shown in Fig.18.In Table 5,the wear mechanism was also investigated in different wear conditions.It is clearly seen that the wear performance of Mg alloys is improved with the addition of solid waste reinforcements.It is also worth saying that the wear rate and wear loss generally decreases with increasing reinforcement content.For example,it was reported that the wear loss of AZ91 alloy and AZ91/FA composite was 6.8 mg and 5.9 mg,respectively[49].To give another example,the wear rate(mm3/m)of the pure Mg,Mg/2.5ES,Mg/5ES and Mg/10ES was reported as 0.034,0.032,0.015 and 0.014,respectively[52].When the friction coefficient is examined,it is seen that the friction coefficient increases with the increase in the reinforcement ratio in general.However,Anandajothi et al.[48]reported that the COF decreases with increasing reinforcement content.From Table 5,it can be seen that the general wear mechanisms are delamination,abrasion,adhesion and oxidation for composite materials.The enhancement of tribological performance of the composite was attributed to the following reasons:1)the reinforcement particles reduce the contact between the matrix and the counterface surface,2)the improved hardness of the composites by the presence of reinforcement particles,3)the formation of a mechanically mixed layer which decreases the friction between tribo-pair,4)the reinforcement particles act as a load-bearing element,and 5)resistance to plastic deformation[46,48,49,52,54,71,96].

Table 5Wear properties of waste reinforced Mg matrix composites.

Fig.16.Compressive fracture surface images a)Mg-2.5Zn;(b)Mg-2.5Zn-3ES;(c)Mg-2.5Zn-5ES and(d)Mg-2.5Zn-7ES composites[47].

Fig.17.Compressive fracture surface images a)Mg-2.5Zn;(b)Mg-2.5Zn-3ES;(c)Mg-2.5Zn-5ES and(d)Mg-2.5Zn-7ES composites[67].

Fig.18.Wear results of some Mg matrix composites.

Some researchers investigated the analysis of the worn surface to explore the dominant wear mechanism for specific wear conditions.It is well known that the worn surface examination has significant importance for tribological studies.Dinaharan et al.[46]investigated the worn surfaces of AZ31/FA composites produced by SC and FSP(Fig.19).The worn surface of the composites has parallel grooves,which verifies the abrasion mechanism[97,98].In addition,it is seen that the worn surface of the sample produced by SC is more damaged.This was attributed to the presence of particle-free regions.The unreinforced regions exposed to frictional heating and severe deformation can be observed in the structure.

Gopal et al.[54]give the worn surface images of Mg/CRT/BN hybrid composites in Fig.20.The grooves and micro-cracks can be visible in Fig.20a.The presence of micro-cracks verifies the delamination mechanism[99].For Fig.20b and Fig.20c,no cracks are visible,and shallow grooves are present.This confirms the lower materialloss and improved wear performance with increasing CRT content.

Fig.19.SEM images of worn surface of AZ31/FA composites a)SC and b)FSP[46].

Fig.20.Worn surface images of Mg/CRT/BN composites a)5CRT,b)10CRT,and c)15CRT[54].

Fig.21.Worn surface images of a-b)Mg and c-d)Mg-2.5ES[52].

Aydin et al.[52]explored the wear mechanism of Mg/ES composites for different loads.(Fig 21-Fig.22).For the pure Mg,deep grooves are present for 5 N,and a delaminated area is present for 20 N.The increased load leads to severe plastic deformation and delamination[100,101].For Mg/2.5ES composite,some scratches and craters are present on the surface under the load of 5 and 20 N,respectively.For Mg/5ES and Mg/10ES composite,undamaged area and smooth surfaces can be seen under load of 5 N.Under a higher load,the wear severity increases.However,the least damaged surface can be present for the maximum reinforcement content(10 wt.%).

4.4.Corrosion behaviour of waste reinforced Mg matrix composites

It is critical to know the corrosion performance as well as the wear and mechanical properties of Mg matrix composites.Several studies studied the corrosion performance of Mg/waste composites for different corrosion media by potentiodynamic and immersion tests.From the Table 6,it can be generally seen that the corrosion current density and corrosion rate of the samples increases with the addition of waste reinforcement.This means the corrosion performance of the composites was negatively affected by the presence of waste materials.For example,the corrosion rate(mm/year)of AZ91,AZ91/2.5ES,and AZ91/5ES is 1.318,7.183 and 11.262,respectively.However,in one study,the addition of ES particles improves the corrosion performance of the composites for 168 h immersion time(except for 7 wt.%ES content).The corrosion rate of Mg2.5Zn,Mg2.5Zn/3ES,Mg2.5Zn/5ES and Mg2.5Zn/7ES was 0.70,0.40,0.31 and 0.71,respectively.Fig.23 also represents the corrosion rate graph of Mg matrix composites.

Table 6Corrosion properties of waste reinforced Mg matrix composites.

Table 7The other properties of the Mg matrix composites reinforced by waste materials.

Dumpala et al.[49]investigated the corrosion behaviour of AZ91 and AZ91/FA composite produced by FSP.The corrosion current density of AZ91 and AZ91/FA was noted to be 1.04×103(μA/cm2)and 13.63×103(μA/cm2),respectively.They reported that the smaller grain size and the presence of secondary phase(reinforcement particles)decrease the corrosion resistance of Mg matrix composites.The grain size of the AZ91 alloy and AZ91/FA composite was noted to be 166.5 μm and 8.77 μm,respectively.Studies have been reported that both increased and decreased corrosion resistance with grain refinement for Mg alloys[102–104].With the grain refinement,more grain boundary and passive filmformation occur,forming a physical barrier layer on the surface and improving the corrosion performance of Mg alloys.However,the presence of faults such as twinning,dislocations or secondary phases in the structure accelerates the local electrochemical reactions and increases the corrosion rate[105,106].Dumpale et al.[49]also reported that the presence of FA particles in the composite structure decreased the ?phase(Mg17Al12)area.It is known that the AZ91 alloy contains a large amount of Mg17Al12intermetallic along the grain boundaries.The Mg17Al12,with its cathodic nature,shows a passive behaviour compared to the Mg matrix.Mg17Al12is inert in the chloride solution relative to the Mg matrix and acts as a corrosion barrier.In addition,the distribution of this phase in the matrix also significantly affects the corrosion resistance.The formation of a thin passive film on the surface of this phase improves the corrosion performance of the Mg alloy[107,108].In light of this information,it can be said that the decrease in the amount of Mg17Al12phase in the structure negatively affects the corrosion performance of AZ91/FA composites[49].It is also reported that stable passive film(Al2O3)formation can be observed in Mg alloys containing high Al content[109].

Fig.22.Worn surface images of a-b)Mg-5ES and c-d)Mg-10ES[52].

Fig.23.Corrosion rate graph of the Mg matrix composites.

Gupta et al.[47]investigated the corrosion performance of Mg/2.5Zn-(3,5,7 wt.%)ES composites by immersion tests in Hanks’solution for immersion times of 24,48,72,96 and 168 h.They reported that the corrosion rate increased till the end of 24 h and decreased afterward till 168 h.The following reactions can be seen during the degradation of Mg-based materials[110].

For immersion studies,it is critical to examine the effect of different immersion times on the corrosion performance of Mg-based materials.Mg turns into Mg2+by anodic dissolution in corrosive solution.The dissolution of Mg leads to an increase in corrosion rates for the early stages.As the immersion time increases,the anodic solution results in increased Mg2+concentration.Consequently,the generation of H2gas bubbles is observed on the surface of the materials.At higher immersion times,the interaction between Mg2+and OH-increases and a protective Mg(OH)2layer is formed.Since this protective layer has a barrier effect,the corrosion rate decreases.The formation of the(Ca,Mg)3(PO4)2layer was also reported as another important mechanism that reduces the corrosion rate.The reinforcement content in composite materials is one of the important parameters that determine corrosion performance.Gupta et al.[47]reported that the best corrosion resistance was obtained for the 5 wt.% ES content.However,the corrosion resistance decreases above 5 wt.% ES content(for 7 wt.% ES content).The reason of decrease in corrosion resistance was attributed to the presence of a high amount of reinforcement particles acting as cathodic sites and accelerating the corrosion by reacting with an anodic Mg matrix[111].

Aydin et al.[52]investigated the effect of the eggshell(2.5,5 and 10 wt.%)on the electrochemical corrosion performance of pure Mg.It was reported that the corrosion rate increases up to 5 wt.% ES content(11.08 mm/year),and the corrosion rate decreases after this content(3.84 mm/year for 10 wt.%ES content).The increase in the corrosion rate with the rise in the reinforcement element up to a certain level(for%5)was associated with the reaction of the reinforcement particles with the anodic Mg matrix.It was also reported that the presence of the higher amount of reinforcement particles acts as a barrier to electron transfer.For this reason,the corrosion rate decreases due to the reduction of physiological reactions[112].In another study for Mg matrix composites,the increase in corrosion rate with the presence of reinforcement elements was explained by the micro galvanic action between the cathodic particles(CNTs)and the anodic Mg matrix[113].

Aydin et al.[71]also investigated the corrosion performance of AZ91/ES composites in 0.1,0.5,and 1 M NaCl solution for 3.5,7.5 and 11.5 pH.They reported that the addition of ES significantly worsens the corrosion performance of the AZ91 alloy.The deterioration of corrosion performance is attributed to matrix/particle interfaces that disrupt the continuity of the matrix.The preferential locations at these sites occurred for corrosion attacks[114].It is well known that the pH and molarity of the corrosive media significantly affect the corrosion performance of the Mg matrix composites.Aydin et al.[71]reported that the corrosion rate of the samples increases with decreasing pH and increasing solution molarity.In acidic solutions(low pH),since layer Mg(OH)2is soluble,a protective barrier layer does not form on the surface.For free-layer surfaces,Mg is oxidized to Mg2+,after that,intermediated constituents react with water to produce H2and Mg2+.In neutral solutions,the corrosion rate is loweras layer Mg(OH)2provides partial protection on the surface[107,115].It was reported that a thick white film of precipitated Mg(OH)2is formed on the upper of the inner film(if pH is bigger 9)[107].It was also reported that the high corrosion rate in high solution molarity was related to the removal of oxide film on the surface and the adsorption of Cl-ions on the surface.Cl-ions participate in the dissolution reaction,and corrosion accelerates[116].

Fig.24.SEM images of the corroded samples for 168 h immersion a)Mg-2.5Zn,b)Mg-2.5Zn/3ES,c)Mg-2.5Zn/5ES,and d)Mg-2.5Zn/7ES[47].

One of the most important parameters affecting the corrosion performance for Mg matrix composites is the micropores in the structure.It was reported that the addition of reinforcement particles increases the porosity of the Mg matrix composites[52,71].The high porosity content increases the actual exposed area and the amount of corrosion per unit area increases.Second,auto-catalytic corrosion cells form within the micropores,leading to localized corrosion damage.If the porosity content on the surface is high,severe local corrosion damage also increases.Also,higher porosity means that the exposed surface is more active and increases corrosion[107].It should be considered that the addition of solid waste reinforcements in the structure at high content can negatively affect the corrosion performance of the composite as it increases the porosity,especially for the PM.

Gupta et al.[47]investigated the corroded surface of Mg/ES composites after 168 h immersion tests(Fig.24).They reported that the corrosion mechanism is pitting corrosion due to the presence of Cl-ions.Some cracks can also be visible due to water depletion and corrosion products due to shrinkage of the hydroxide layer.It can also be seen that the corrosion products were present in the structure of Mg-2.5Zn/3ES and Mg-2.5Zn/5ES.They proved the presence of MgOH2and(Ca,Mg)3(PO4)2phases by XRD analysis.

Aydin et al.[52]investigated the corroded surface of Mg/ES composites after potentiodynamic corrosion tests(Fig.25).The localized corrosion attacks are present on the surfaces.The crack formation is visible in all samples.It can also be seen that the corroded area is the minimum for the pure Mg.However,the highest corroded area with deep cracks is present for the Mg/5ES composite.It is well known that the if the surface has high cracks,the corrosion rate is high[117].The authors also verified the presence of Mg,O and Cl by the EDS analyses.This result confirms the formation of corroded products on the surface.

Turan et al.[71]investigated the corroded surface of AZ91/ES composites for different corrosion conditions(Fig.26).It can be seen that the formation of corrosion products is higher for composite materials compared to unreinforced alloy.The visible cracks are present for the pH 3.5 and 1 M solution.This is related to the increased Cl-ions causing higher crack formation[118].One of the most important reasons for the high corrosion damage in composite materials is the formation of micro galvanic cells between matrix and reinforcement particles[119].

Fig.25.SEM images of the corroded samples a)Mg,b)Mg/2.5ES,c)Mg/5ES,and d)Mg/10ES[52].

4.5.Other properties of Mg matrix composites reinforced by waste materials

Some researchers investigated the other properties such as damping capacity,ignition resistance,and thermal behaviour of the Mg/waste materials composites.The results of these properties are given in Table 7.Gupta et al.[47]investigated the damping capacity of Mg/ES composites and reported that the damping capacity was improved by the addition of ES particles.The best properties were obtained for the Mg/7ES composite.The enhancement in damping capacity by the addition of reinforcement particles is attributed to the followingreasons:a)improved damping at the interface binding the reinforcement particles in the matrix,b)generation of new dislocations due to mismatch in the thermal expansion coefficient of matrix and reinforcement particles[47,120].

Fig.26.Corroded surface images for different conditions(a),(c),(e)pH 11.5,0.1 M and(b)(d),(f)pH 3.5,1 M[71].

Gupta et al.also[67]investigated the ignition temperature and coefficient of thermal expansion of Mg-2.5Zn/ES composites by PM.They reported the ignition temperature increases with increasing ES content.The increase in ignition temperature was attributed to the presence of reinforcement particles which act as an insulator for the Mg matrix;as a result,higher thermal and dimensional stability was obtained.The presence of the ES particles delays the ignition and burning of the matrix[67,121].The addition of ES particles leads to a decrease in CTE for composite materials.The low CTE values of the composites was attributed to the presence of ES particles with lower CTE value[87,122].

Srivastava et al.[72]studied the thermal behaviour of the AZ31/ES composites by FSP.They reported that the thermal conductivity of the samples decreases with increasing ES content.It can also be seen that the thermal conductivitydecreases with increasing test temperature.One of the most important reasons for the decrease in thermal conductivity is that the FSP method reduces the grain size and increases the grain boundaries,creating an obstacle to the movement of free electrons and phonons[72,123].The decrease in thermal conductivity with the addition of ES particles was associated with the different energy levels of electrons and phonons between the matrix and the reinforcement[124].

5.Conclusion

In recent years,the use of solid waste reinforcements in the production of Mg matrix composites both reduces environmental pollution and reduces the cost of the produced composites.This review systematically discussed the effect of different waste materials on the microstructure,mechanical,wear,corrosion,thermal,damping,and ignition properties of Mg matrix composites.Some of the significant results are:

·The most commonly used waste materials are fly ash,eggshell,cathode ray tube,rice husk and red mud for the production of Mg matrix composites.

·amongst all techniques,powder metallurgy has been used broadly.After powder metallurgy,stir casting and friction stir processing are the most widely used methods.The friction stir processing provides superior grain refinement when compared to other methods.

·The solid waste reinforcement particles are generally distributed homogeneously throughout the matrix.Above 5 wt.% reinforcement content,the agglomeration of particles can be visible.The grain size of the composites decreases with the presence of reinforcement particles,which show the pining effect.

·The hardness of the Mg alloys was improved by the addition of waste materials.Generally,the maximum hardness value was reported for the maximum reinforcement content.The increase in hardness was attributed to grain refinement,resistance to plastic deformation and dislocation movement.

·The higher mechanical properties are obtained with the addition of reinforcement elements compared to the alloy.The increase in mechanical properties was attributed to Hall-Petch,load-bearing effect,Taylor strengthening.In some studies,it was reported that increasing the reinforcement ratio above the critical level causes a decrease in mechanical properties.This is the result of the agglomeration of particles.

·The tribological performance of the samples was improved by the addition of waste materials.This is related to the reducing contact between matrix and counterface material,the increased hardness of the surface,and the presence of the load-bearing element.

·For unreinforced alloy,delamination and abrasion mechanisms were the present;for composite materials,mild abrasion mechanism was dominant.

·Generally,the addition of waste materials deteriorates the corrosion performance of the Mg alloys.The low corrosion resistance of the composites was attributed to the presence of secondary phases,the breaking of matrix particle interface and micro-galvanic corrosion.However,there is also a study in which ES reinforcement improves the corrosion resistance of Mg matrix composites.

·The damping capacity,ignition resistance and thermal behaviour were also investigated by some researchers.The addition of ES particles improves the damping capacity and ignition temperature of Mg matrix composites.However,the addition of ES particles negatively affects the thermal conductivity of the Mg alloys.

·This review revealed the potential of using waste materials in the production of Mg matrix composites.It is predicted that these waste materials will replace expensive traditional reinforcement,thanks to their easy accessibility and low cost.