Tribological behavior of ZK60Gd alloy reinforced by SiC particles after precipitation hardening

Ehsn Momeni ,Hssn Shrifi,* ,Mortez Tyei ,Ahmd Keyvni ,Ermi Aghie ,Yshr Behnmin

a Department of Materials Science and Engineering, University of Shahrekord, Iran

b Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran

c Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada

Abstract In this research,the effect of precipitation hardening on the tribological behavior of the ZK60Gd/SiC composite was studied.For this purpose,ZK60Gd alloy containing with 5 and 10 wt% SiC were produced with stir casting method.The microstructure characterization of the samples showed the wide distributions of Mg7Zn3 and Gd(Mg0.5Zn0.5) precipitates were formed during casting.The results of hardness measurement after precipitation hardening at different temperatures showed that the hardness peck was obtained at 175°C.The wear tests with different loads (10,40,60,90,and 120 N) and velocities (0.1,0.3,0.6,and 0.9 m/s) were performed on the as-cast and heat treated sample at 125,175,and 225 for 12 h.Between the different precipitation hardening conditions,the precipitation hardened samples at 175 °C had the highest hardness values and least wear rate.The sample containing 10% reinforcement had the least wear rate between the unreinforced alloy and the composites.The results showed that abrasive,adhesive,delamination,MML,and fatigue wear mechanisms were the dominant wear mechanisms for the composite samples.In contrast,the dominant wear mechanism for the unreinforced samples was abrasive,adhesive,delamination,MML,and plastic deformation.

Keywords: Precipitation hardening;Composite;Gadolinium;Mg-Zn-Zr-Gd;Wear map.

1.Introduction

In recent years,magnesium alloys and composites have attracted a lot of attention in most fields where the weight reduction is critical[1–4].They have turned into one of the materials which provide high strength to weight ratio [5,6].Between the light metals,magnesium and its alloys have mostly been used for industrial applications due to their low density,high toughness,recyclability,great damping factor and good machinability [7,8].Furthermore,magnesium is one of the abundant elements present in the earth’s crust (the sixth most abundant in the earth crust).It has a density of 1.75 g/cm3which is about 2/3 of the density of aluminum and it is also more than 4 times lighter than the steel [9–11].However,the low resistance of magnesium to wear and corrosion has caused a serious barrier to the development of its alloys[12,13].

The grain refinement and precipitation hardening are two methods used in magnesium alloys in order to increase the strength and hardness as well as obtaining a refined structure and good mechanical properties [14].Overall,the refinement causes improvement in the mechanical properties and uniformity of the structure in most metals and alloys [15,16].Zinc element is added to the alloys to obtain the precipitation hardening effect [17–22].Between the magnesium alloys,the Mg-Zn system is one of the most effective precipitation-hardened alloys [23,24].Due to the small solubility of most of the alloying elements in magnesium,grain refinement is usually obtained by the formation and distribution of precipitates.Another element used for grain refinement of magnesium alloys is zirconium [25–27].The addition of zirconium results in the increased corrosion resistance of the alloy which is the main reason that Mg-Zn-Zr (ZK) alloys are developed [28,29].Furthermore,the most effective element for grain refinement of magnesium alloys is Gd usually used for developing heat resistant magnesium alloys[23,30–35].However,there seems to be a paucity of research on the effects of Gd on the structure and the wear properties of ZK60 alloy [36].By the addition of Gd element,the as-cast microstructure refines slowly.By the addition of Gd,the Mg-Zn-Gd phase is obtained and by obtaining the Mg-Zn-Gd phase,the amount of the MgZn2phase reduces [37].The presented results have shown that the Mg-Zn-Gd phase has a higher thermal stability compared to the MgZn2phase.It can be mentioned that grain refinement along with heat resistant precipitates is the advantage of Gd addition in the microstructure of the ZK60 alloy.

Among magnesium alloys,the ZK type has the highest mechanical strength [38,39].In this alloying system,the ZK60 alloy has attracted the most attention in recent years due to the simultaneous increase in its strength and toughness [40,41],which has many applications in the automotive and aerospace industries,such as steering wheels,seat frames,fuel tank covers,front end,IP beams,steering column,and driver’s airbag housings.The main focus of the research on ZK60 alloy has been the tensile properties [28,42,43],severe plastic deformation [44–46]and the superplastic behavior of ZK60 [47,48].However,it seems that the wear mechanisms of the ZK60 alloy and its composites have not been thoroughly studied and most of the researchers have focused on the wear behavior of other magnesium alloys such as AZ91 [49–51],AZ31[52–55],and AM60 reinforced with Al2O3fibers [56].

The research done on magnesium matrix composites show that the addition of ceramic particles can cause the improvement of mechanical properties such as the wear properties compared to the alloy without reinforcement [57].Among the ceramic particles,SiC is the best option for use as a reinforcement in magnesium-based composites due to its cheapness,availability,high hardness,and high elastic modulus [28].That makes it suitable for wear-resistance applications such as brake shoes,timing,differential,and camshaft casings,as well as a number of minor components [58].The wear behavior of the AM60 magnesium alloy reinforced with Mg2Si particles showed that the addition of Mg2Si significantly could improve the wear properties and the dominant wear mechanism was abrasive which converts to adhesive with increasing the applied load [59].The wear behavior of the AZ91 magnesium composite reinforced with Ti2AlC showed that at different conditions the composites had better wear resistance and self-lubricant ability compared to the AZ91 magnesium alloy [60].

In addition to making a composite,the precipitation hardening process is also used to increase the yield strength of tough materials such as magnesium,aluminum,nickel alloys and particular steel types.Precipitation hardening provides obstacles which restrict the movement of the dislocations;such a restriction could lead to the increase of the strength of the alloy.This method is the most important strengthening process in non-ferrous alloys.During this process,the small precipitates are uniformly distributed in the soft phase by heating for a specific time causing an increase in the strength of the alloy.The characteristic of the precipitation hardened precipitate is much dependent on the precipitation hardening time and temperature.The precipitation hardening process results in the nucleation and growth of the precipitates due to diffusion.These precipitates obtain the maximum strength when the best distribution and size is reached.Increasing the precipitation hardening temperature results in the nucleation and the formation of the precipitates and increases the hardness[61,62].

To produce magnesium matrix composites that are almost near net and half-shaped,several processing methods have been developed,including co-deposition,pressure infiltration,spray atomization,stir casting,and powder metallurgy.Each method has benefits and drawbacks of its own and is thus used in a variety of contexts.The size,type,volume percentage of the reinforcement,component shape,use of composite materials,and material cost are taken into consideration while choosing a technique.Among them stir casting is one of the most economical ways to produce huge amounts of magnesium matrix composite with a suitable matrix/reinforcement interface.Stir casting may be used to process composite more inexpensively,especially when the particle size is larger and the content is lower than 30% [63].

The purpose of this study is to produce wear-resistant magnesium-based composites (Mg-Zn-Zr-Gd/SiC,which was named ZK60Gd/SiC composite) for applications in the automotive and aerospace industries.For this purpose,the wear behavior of composite and magnesium alloy after heat treatment at different temperatures was studied.To achieve such a goal,a magnesium matrix composite reinforced with SiC particles was produced by stir casting method.Subsequently,different heat treatment cycles were applied to the samples to obtain intermetallic compounds with the Gd element and finally,the wear behavior of the as-cast and precipitationhardened samples were studied under different loads.

2.Materials and method

2.1. Material

A magnesium alloy containing Zn,Zr and Gd was used as the composite matrix with the composition shown in Table 1.Furthermore,SiC particles with a mean size of 38 μm (Merck,＞99%) were used as reinforcement.

Table 1Chemical composition of Mg-Zn-Zr-Gd alloy.

2.2. Casting

ZK60 alloy was used in order to prepare the sample without reinforcement.The alloy slab was melted at 780 °C in an induction furnace in order to add the 1 wt% Gd element.The stir casting method was used in order to prepare the composite samples with the addition of SiC particles.These particles were mixed with speed 320 rpm by titanium stir for 10 min and then cast in a metallic mold.The composites were prepared with 5 and 10 wt% SiC.The detail of casting method described in [52].

2.3. Heat treatment

All the samples were homogenized after casting at 450°C for 10 h and then water quenched in order to prepare a homogeneous structure.Furthermore,the precipitation hardening process was applied on the alloy and composite samples at 125,175,and 225 °C.The different precipitation hardening processes are schematically shown in Fig.1.Furthermore,the designation and condition of the samples are brought in Table 2.

Fig.1.Schematic procedure of the homogenization and precipitation hardening cycles.

Table 2The designation and condition of the samples.

2.4. Mechanical tests

2.4.1.Hardness test

The Micro Vickers test was used by applying a 100 g load for 15 s in order to measure the hardness of the precipitation hardened samples using a Leitz machine.The hardness test was applied 5 times on each sample in order to increase the accuracy of the data and the mean data along with the calculated error was reported.

2.4.2.Wear test

The reciprocating wear test method was used in order to study the wear properties of the alloy and composite samples.The test was applied on samples with a dimension of 3 × 13 × 50 mm3using a 5 mm diameter 52,100 steel pin with a hardness of 56 HRC and a spherical surface effect.The wear test was run under different loads of 10,40,60,90,and 120 N with a linear velocity of 0.1,0.3,0.6,and 0.9 m/s and at a distance of 500 m at room temperature.The test was stopped for every 100 m and the samples were weighted in order to measure the weight loss.The wear rate was calculated based on Archard law (Eq.(1)) [52].

where V: wear volume loss,L: sliding distance,K: Archard’s constant,F: load,k: specific wear rate,and H: hardness.The wear volume loss (ΔV) was calculated based on Eq.(2) [64].

where,R is the average radius of the wear track,r is the radius of the abrasive pin,and d is the width of the wear track.

2.5. Characterizations

The X-ray diffraction analysis (XRD) was applied to the samples in order to determine the intermetallic phases formed due to the presence of alloying elements such as Gd.The Xray diffraction test was done using a Philips machine (model PW1800) applying 30 kV and 40 mA.In all of the tests,a Cukα(λ=1.5405 ?A) X-ray with a scan rate of 0.05 and step size of 0.05° was used.The phase determination was done using X’pert High Score software.In order to study the microstructure of the samples and also to determine the wear mechanisms,an LEO(Model:435VP)scanning electron microscope (SEM) was used along with EDS.

3.Results and discussion

3.1. Microstructure

Fig.2a and b show the microstructure of the as-cast samples.As can be seen,wide distributions of precipitates are formed with the addition of Gd element.Bright and dark phases are present in the matrix of the alloy.The dark phase corresponds to the magnesium matrix.In order to determine the composition of the bright phases,EDS point analysis was used.A continuous phase containing elements such as Mg,Zn,and Gd is formed.According to the thermodynamic calculations [65],this could be due to the formation of Gd(Mg0.5Zn0.5) phase.Furthermore,another phase can be seen in the structure where its elemental analysis shows that it consists of Mg and Zn and according to the thermodynamic calculations [66],it is probably a Mg7Zn3intermetallic phase.According to previous study [67],it has been determined that in the Mg-Zn system at a temperature above 325 °C in the Zn supersaturated regions,Mg7Zn3phase is formed in the inter-dendritic regions.It has also been found that upon increasing the temperature again up to 200 °C,Mg7Zn3phase undergoes a eutectoid reaction and decomposes into a poorly defined layered structure (α-Mg+Mg4Zn7) which is also unstable phase and subsequently is replaced by equilibriumβphase (MgZn).As a result,during wear,with the increase in temperature caused by the friction between the abrasive surfaces and the sample,it causes a phase change and forms a stable beta phase in the matrix,and practically Mg7Zn3phase does not have much effect on the mechanical and wear properties.The XRD analysis was performed to confirm the results of the sample as shown in Fig.3.As can be seen,the Mg7Zn3and Gd(Mg0.5Zn0.5) precipitates were formed in the casting procedure.

Fig.2.SEM micrographs and EDS analysis of the as-cast samples: (a and b) unreinforced alloy,(c) ZK60Gd/5%SiC,and (d and e) ZK60Gd/10%SiC samples,and (f) processed image of (e).

Fig.3.XRD patterns of the as-cast samples.

Fig.2c–f depict the SEM micrographs of the microstructure of the 5ZGA and 10ZGA samples after casting.As seen in Fig.2c and d,the matrix of the composites has precipitations similar to the unreinforced sample,and also the Gd(Mg0.5Zn0.5) and Mg7Zn3intermetallic phases are formed in the composites.Fig.2c (marked by a pink box) shows the relatively uniform distribution of reinforcements in the matrix of the 5ZGA composite.This is confirmed by the processed image (marked by a black box),in which the green areas represent the reinforcements in the blue background (matrix).In Fig.2d and c,the SiC particles are seen with a uniform distribution in the microstructure of the 10ZGA composite.It is also confirmed in the processed image in Fig.2f.The homogeneous distribution of reinforcing particles can improve mechanical properties [68].In Fig.2d (marked by a yellow box),the interface of a SiC particle in the 10ZGA sample at high magnification is shown.From the figure,it is clear that at the interface,there is no crack and/or separation.Whereas a strong and continuous interface is formed,which indicates the good wetting of reinforcements with the matrix melt.A strong interface can control the interface strength of the matrix/particles and subsequently improve the wear properties of the sample [69].

3.2. Microhardness

In Fig.4,the microhardness variation versus the precipitation hardening temperature is shown.According to Fig.4,it is obvious that the hardness value of the alloy and composite samples increase with increasing the temperature up to 175 °C while the further increase in the temperature up to 225 °C decreases the hardness.The increase in the hardness with temperature is related to the formation of more homogenous precipitates in the alloy whereas with further increase in the temperature,the hardness decreases due to the growth of the precipitates and prolonged precipitation hardening.By the comparison between the hardness of the different samples at 175 °C,it is obvious that the 10ZGH175 sample has the maximum hardness whereas the 5ZGH175 sample has a higher hardness compared to the unreinforced alloy.Due to the presence of the SiC ceramic particles,the hardness of the composite increases with the increase in the amount of reinforcement.Increased hardness of the samples due to the presence of ceramic particles is caused by direct and indirect strengthening mechanisms.By adding ceramic particles in the soft metal matrix,due to the difference in the coefficient of thermal expansion of the matrix and the reinforcement,mismatch dislocations are formed at the matrix/reinforcement interface,which increases the hardness of the matrix up to the plastic stage [11,39].This increase in hardness increases with increasing ceramic reinforcement content [70,71].The effective load transfer mechanism from the matrix to the reinforcement is also another factor in increasing the strength of the samples.Meanwhile,the hardness of the unreinforced sample is caused by the presence of precipitates in the matrix,but the difference in the coefficient of thermal expansion of the matrix and precipitates is less than that of the matrix and ceramic particles,and as a result,it causes less hardness.On the other hand,the nature of carbide particles has a much higher hardness than precipitates in the matrix of magnesium,and they practically affect less load transfer.However,considering the high hardness of all the composites compared to the unreinforced alloy,it can be understood that the manufacturing process of the composite has been well done.The interface of the matrix/reinforcement determines the properties of the composite and a good interface could result in high strength and hardness [72–74].

Fig.4.Hardness variations according to the precipitation hardening temperature for the different samples.

3.3. Wear

3.3.1.Weight loss

Fig.5a and b illustrate the weight loss variation versus the sliding distance for the as-cast samples.According to the figure,it is clear that under the two applied loads of 40 and 60 N,the maximum weight loss corresponds to the unreinforced sample and then the 5ZGA sample whereas the least weight loss rate was obtained for the 10ZGA sample.According to the samples,more weight loss was obtained at lower amounts of reinforcement.One interpretation is that at the initial distances of the wear test,the composite surface undergoes plastic deformation without much weight loss [75].By comparing the weight loss of the samples in Fig.5a and b,it is clear that for the 40 N loads,the difference between the 5ZGA and 10ZGA samples is negligible up to the first 200 m.By increasing the sliding distance,the difference between the weight loss of the samples increases in such a way that after 500 m,the weight loss percent of the 10ZGA sample reduces 7% and 109% compared to the 5ZGA and ZGA samples,respectively.With the increase in the wear distance and also the effect of the wear pin on the surface of the composite,the metal starts wearing and gathering around together.The ceramic particles are harder than the metal and sustain more loads.The plastic deformation is obtained by continuing the wear trend and results in intensified work hardening in the metal and consequently a brittle layer on the surface.Therefore,at higher distances,it causes fracture and chipping of this layer [76–78].This results in the weight loss of the samples.The conditions seem similar for the 60 N load whereas with the increase in the distance,the difference between the weight loss of the samples increases in such a way that after 500 m,the weight loss percent of the 10ZGA sample reduces 5% and 101% compared to the 5ZGA and ZGA samples,respectively.By comparing the plots in Fig.5a and b for both loads,one may conclude that with the increase in the applied load,the amount of weight loss increases for all the samples.The reason for such behavior could be that due to the nominal load,the roughness of the surfaces that are in contact with each other penetrate in to each other and mechanically lock.Due to the formation of mechanical locks at the surface,the motion of the surfaces on each other need the roughness present on the surface to be worn out which also results in the wear of the surfaces [61,79,80].

Fig.5.The weight loss variations obtained by the sliding distance for samples: (a) as-cast under 30 N,(b) as-cast under 60 N,(c) precipitation hardened at 125 °C under 30 N,(d) precipitation hardened at 125 °C under 60 N,(e) precipitation hardened at 175 °C under 30 N,(f) precipitation hardened at 175 °C under 60 N,(g) precipitation hardened at 225 °C under 30 N,and (h) precipitation hardened at 225 °C under 60 N.

Fig.5c and d show the variation of the weight loss versus the sliding distance for the precipitation hardened samples at 125 °C.According to the figure,it is clear that for both of the applied loads,the maximum weight loss was related to the ZGH125 sample and then to the 5ZGH125 sample.For the 40 N loads,comparing the weight loss of the samples shows that up to the first 100 m,the difference between the samples is negligible.However,with more increase in the sliding distance,the difference between the weight loss of the samples increases in such a way that after 500 m,the weight loss percent of the 10ZGH125 sample reduces 21% compared to the 5ZGH125 and 110% compared to the ZGH125 sample.However,the condition for the applied 60 N load is quite different in such a way that with increasing the distance,the differences between the weight losses of the samples reduce whereas after 500 m,the weight loss percent of the 10ZGH125 sample reduces 8%and 37%compared to the 5ZGH125 and ZGH125 samples,respectively.By comparing the plots in Fig.5c and d for the two different loads,it can be seen that with increasing the applied load,the weight loss has increased for all the samples.By comparing the plots in Fig.5a–d for the two loads,it is obvious that the weight loss for all the participation hardened samples has decreased.The reason of the weight reduction of the precipitation hardened sample compared to the as-cast samples could be the presence of the intermetallic compounds due to the heat treatment and the increase in the hardness of the samples.

Fig.5e and f depict the weight loss variation versus the sliding distance of the precipitation hardened samples at 175°C.According to the figure,it is clear that the differences in both of the applied loads are negligible.The reason for the similar weight loss of the samples at 40 and 60 N could be due to the presence of the intermetallic compounds formed as a result of heat treatment and the resulting increase in the hardness of the alloy related to it.Two factors result in the weight reduction of the samples [61].In the first place,as the reinforcement increases in the metal matrix,the weight loss is lower.Secondly,as the applied load increases,more weight loss is obtained in all the samples.The increase in hardness due to the precipitation hardening of the sample at 175 °C has compensated the amount of weight loss due to the increased load.

Fig.5g and h illustrate the weight loss variations versus the sliding distance for the sample precipitation hardened at 225 °C.The comparison of the weight loss of the samples after 500 m under 40 N load shows that the weight loss percent of the 10ZGH225 sample compared to the 5ZGH225 and ZGH225 samples reduces 28% and 41%,respectively.For the 60 N loads,the weight losses reduction of the ZK60Gd/10%SiC sample compared to the 5ZGH225 and ZGH225 samples are 10% and 55%,respectively.

3.3.2.Friction coefficient

Fig.6a–c show the variation of the friction coefficient versus the sliding distance for the as-cast samples.According to the figures,it is clear that for the unreinforced sample tested under 40 N,the friction coefficient increases with the increase in the sliding distance.Furthermore,it is shown that with the increase in the sliding distance,the range of the friction coefficient increases.A comparison between the variations of the friction coefficient versus distance suggests that at the beginning of the test,the friction coefficient increases due to the roughness of the surface of the sample.By continuing the wear test,the contact of the surfaces with each other eliminates the roughness and the wear particles combine together resulting in the formation of protective layers where the friction coefficient obtains the steady state [78,81,82].According to the figure,the friction coefficient plot for the samples decreases with the increase in the reinforcement.Due to the fact that in the presence of ceramic particles,metal-to-metal contact decreases,and as a result,the friction coefficient decreases.The presence of ceramic particles in the matrix is one of the main factors in reducing the effective surface of the matrix in contact with the abrasive pin,which reduces the friction between the matrix and the pin.And by increasing the reinforcement content,the percentage of reinforcement on the sample surface also increases,which causes a further reduction in the friction coefficient.

Fig.6.Friction coefficient variations by sliding distance for samples: (a) as-cast ZK60Gd,(b) as-cast ZK60Gd/5%SiC,(c) as-cast ZK60Gd/10%SiC,(d)precipitation hardened at 125 °C,ZK60Gd,(e) precipitation hardened at 125 °C,ZK60Gd/5%SiC,(f) precipitation hardened at 125 °C,ZK60Gd/10%SiC,(g)precipitation hardened at 175 °C,ZK60Gd,(h) precipitation hardened at 175 °C,ZK60Gd/5%SiC,(i) precipitation hardened at 175 °C,ZK60Gd/10%SiC,(j)precipitation hardened at 225 °C,ZK60Gd,(k) precipitation hardened at 225 °C,ZK60Gd/5%SiC,and (l) precipitation hardened at 225 °C,ZK60Gd/10%SiC.

Fig.6d–f illustrate the variation of the friction coefficient versus the sliding distance of the precipitation hardened samples at 125 °C.According to the figure,it is clear that for the unreinforced sample tested under 40 N loads,the friction coefficient increases with the increase in the sliding distance.Furthermore,it is clear that the unreinforced sample has a higher friction coefficient compared to the composite samples.For the variation in the friction coefficient,it can be mentioned that with the increase in the reinforcement content,the pin causes less wear and is more involved with the reinforced particles resulting in higher friction coefficient of the alloy sample and composites containing 5 and 10 wt% SiC.This could be due to the increase in the wear path resulting in the smoothening of the path and also the formation of the protecting layers.As a result,a smoother roughness in the matrix and reinforcement of steady state could be observed.

Fig.6g–i depict the variation of the friction coefficient versus the sliding distance for the sample precipitation hardened at 175 °C.According to the figures,it is obvious that the friction coefficient difference reduces between the unreinforced sample and composites.This is due to the formation of more homogeneous precipitations in the unreinforced sample.It should be mentioned that composites have lower friction coefficients compared to the unreinforced sample.With the increase in the reinforcement,the wear pin does not have enough time to apply wear on the matrix and less involvement happens in the matrix.Furthermore,equilibrium is observed in the friction coefficient plot at lower distance probably because the reinforcement has a more coherent state in the matrix.Fig.6j–l show a variation in the friction coefficient versus the sliding distance for the precipitation hardened samples at 225 °C.As can be seen,similar to the precipitation hardened sample at 175 °C,the friction coefficient of the alloy sample increases with a slight slope while the variation range decreases compared with the as-cast sample.It is obvious from the figure that the variation ranges of the friction coefficient decreases compared to the alloy sample.

3.3.3.Wear rate

Fig.7a and b illustrate the wear rate versus the sliding distance of the as-cast samples.As shown,the 10ZGA sample and then the 5ZGA sample have the least wear rate while the ZGA sample has the most wear rate.The wear rate at a specific load shows the role of the presence of reinforcement in the matrix and the improvement of the wear behavior of the samples.The reason of this wear behavior of the composites is due to the presence of ceramic particles which results in the increase in the wear resistance.In addition,it is evident that the wear rate decreases with increasing the reinforcement percentage.The reason is that with the presence of ceramic particles,the reinforcement percent and the number of reinforcements at the surface increase while in contrast the distance between them decreases resulting in the increase in the cohesiveness of the composite against the wearing pin which increases the wear resistance.The composites have a wear rate close to each other and their wear rate differences increase in comparison with the unreinforced alloy.

Fig.7.Wear rate variations versus sliding distance for samples: (a) as-cast under 30 N,(b) as-cast under 60 N,(c) precipitation hardened at 125 °C under 30 N,(d) precipitation hardened at 125 °C under 60 N,(e) precipitation hardened at 175 °C under 30 N,(f) precipitation hardened at 175 °C under 60 N,(g) precipitation hardened at 225 °C under 30 N,and (h) precipitation hardened at 225 °C under 60 N.

Fig.7c and d show the wear rate versus the sliding distance of the precipitation hardened samples at 125 °C.According to the figure,it is clear that under loads of 40 and 60 N,the role of the presence of reinforcement at the matrix for improving the wear rate does not have a big difference and increases with the increase in the sliding distance.However,the 10ZGH125 sample has the least wear rate in contrast with the 5ZGH125 sample.

Fig.7e and f show the wear rate versus the sliding distance for the precipitated samples at 175 °C.According to the figure,it is clear that the unreinforced sample has the most wear rate.However,the differences between the wear rate plots of the unreinforced alloy and composite samples increases suggesting that the composites have a better wear resistance compared to the unreinforced sample.The comparison of the wear rate of the 5ZGH175 and 10ZGH175 samples shows that similar to the precipitation hardened samples under loads of 40 and 60 N,the 5ZGH175 sample approximately has a constant wear rate.

Fig.7g and h depict the wear rate versus the sliding distance for the precipitation hardened samples at 225 °C.In Fig.7,with the increase in the load,the differences of the wear rate of the unreinforced and composite samples increase suggesting that the composites show a better wear resistance at higher loads.By comparing the wear resistance of the 5ZGH225 and 10ZGH225 samples it can be seen that the 10ZGH225 sample approximately has a constant wear rate and shows a stable structure during wear.The reason can be due to the formation of the deformation regions as a result of the applied load on the pin which has resulted in the formation of work hardening and increase in the wear resistance[83].

3.3.4.Worn surfaces

The worn surface of the ZGA sample under 40 N loads is shown in Fig.8a.According to the figure,a vast amount of plastic deformation regions is observable in the wear surface which has occurred by the effect of applying load on the pin.The vast amount of plastic deformation is the result of work hardening in the alloy matrix and has resulted in the reduction of the wear rate.According to the Fig.8b,it is clear that the addition of the SiC reinforcement up to 5% has caused the formation of fewer parallel grooves with less depth compared to the ZGA sample.In the figure with higher magnification,it is clear that in addition to the parallel grooves,the wear surface has several cracks and the failure of the surface is observed in plate form and delamination is seen in vast amount.In contrast to the unreinforced sample,plastic deformation is not seen at the surface which is due to the increase in the strength of the composite matrix and as a result of the addition of ceramic phases which has reduced the ductility of the structure and caused the formation of cracks.As could be observed in Fig.8c,the addition of SiC up to 10% has caused the formation of parallel grooves at the surface.In the figure,it is obvious that in addition to the parallel grooves,the worn surface has several cracks with big depths which are due to scratches caused by the reinforcement particles separated from the surface.The 10ZGA sample has more ceramic particles compared to the 5ZGA sample and therefore the ductility decreases much more resulting in the formation of higher depth cracks or separation of some reinforcement particles.

The worn surface of the ZGH125 sample under 40 N load is shown in Fig.9a.According to the figure,it is obvious that parallel grooves are formed in the direction of the wear.Furthermore,in the figure,a vast amount of plastic deformation along with removed regions of the wear surface can be observed suggesting the presence of abrasive and adhesive wear mechanisms.The worn surface of the 5ZGH125 sample under 40 N loads is shown in Fig.9b.According to the figure,it is obvious that several parallel grooves are formed at the surface where the grooves are much less than the as-cast samples and with less depth.Also,the deformed regions can be seen.Compared to the as-cast sample,the cracks have significantly decreased and the separation surface is not in plate form.Furthermore,delamination is observed at the worn surface.The worn surface of the 10ZGH125 sample under 40 N loads is presented in Fig.9c.An intense decrease in the parallel grooves is observed compared to the as-cast sample.Furthermore,deformed and separated regions are not observable.From the figure,it is clear that the delamination is still present at the surface.

Fig.9.SEM micrographs of worn surfaces of precipitation hardened samples under 60 N for: (a) unreinforced alloy at 125 °C,(b) ZK60Gd/5%SiC at 125 °C,(c) ZK60Gd/10%SiC at 125 °C,(d) unreinforced alloy at 175 °C,(e) ZK60Gd/5%SiC at 175 °C,(f) ZK60Gd/10%SiC at 175 °C,(g) unreinforced alloy at 225 °C,(h) ZK60Gd/5%SiC at 225 °C,and (i) ZK60Gd/10%SiC at 225 °C.

The worn surface of the ZGH175 sample under 40 N load is shown in Fig.9d.As shown,parallel grooves are formed in the sliding direction.In addition,the formed abrasives located at the layered region have caused the separation of the layers from the wear surface of the samples.The worn surface of the 5ZGH175 sample under 60 N loads is shown in Fig.9e.As seen,with the increase in the applied load,the amount and depth of the abrasive wear have significantly increased.In contrast,delamination is not observed with the increase in the load.Fig.9f displays the worn surface of the 10ZGH125 sample under 40 N load.As could be seen,parallel grooves are evidenced at the surface in the wear direction.These parallel grooves are very less with lower depths compared to the as-cast and precipitation hardened sample at 125 °C.It is obvious that in addition to the parallel grooves,the wear mechanism of the surface is adhesive.However,compared to the precipitation hardened sample at 125°C,the formations of cracks are not observed on the surface and the separation of the surface is also not observed as plate form.Furthermore,at the surface of the sample,a wide distribution of precipitates is seen.

The worn surface of the ZGH225 sample under 40 N load is shown in Fig.9g.In addition to parallel grooves,plastic deformation can be obviously seen,in some regions.The separation present in parts of the surface is caused by its adhesion to the wear pin suggesting that the adhesive wear is also active.According to the Fig.9h,the formed grooves in 5ZGH225 sample are much less than the samples precipitation hardened at 125 °C and the as-cast samples.With a detailed study,it was found that in addition to the parallel grooves,adhesion mechanism can be observed on the surface.Furthermore,compared to the precipitation hardened samples at other temperatures,no cracks are observable at the surface and small amounts of deformation regions are present.According to Fig.9,it is clear that the abrasive and adhesive wear mechanisms is dominated.

According to the amount of weight loss,wear rate and SEM micrographs of the worn surfaces,it was found that the precipitation hardened sample at 175 °C had the highest wear resistance.As a result,this group of samples was selected to complete wear studies at different speeds and loads.

3.3.5.Wear map

Fig.10 illustrate wear rate changes in term of load at different velocity for sample precipitation hardened at 175 °C.It is obvious by increasing the load,wear rate increased,which indicate for all velocity.The lowest wear rate for all sliding speeds is related to 10ZGH175 sample,which indicates that the presence of reinforcing particles had a positive effect on wear resistance at different loads and speeds.Based on the Fig.10a and b,at low speeds (0.1 and 0.3 m/s),the trend of the graphs can be considered linear,but with the increase in speed (0.6 and 0.9 m/s),the slope of the high-load area has increased significantly,which indicates a change in the dominant mechanisms during wear (Fig.10c and d).

Fig.10.Wear rate vs.load changes at: (a) 0.1 m/s,(b) 0.3 m/s,(c) 0.7 m/s,and (d) 0.9 m/s.

Fig.11a shows the worn surface of ZGH175 sample at 0.1 m/s under 10 N.As seen,parallel grooves have been created in the direction of sliding,which is caused by the plowing of the matrix by the abrasive pin,and is characteristic of abrasive wear.In Fig.11b,it is clear that the grooves created on the surface have a lower depth than the unreinforced sample,which is caused by the increased strength of the sample due to compositing,and the strengthening mechanisms are caused by the presence of ceramic particles.However,debris are evident on the worn surface,which can be caused by the detachment of ceramic particles interacting with the abrasive pin.The conditions in sample 11c are also the same,so that the depth of the grooves is less than the other two samples,which is due to the increase in strength by increasing reinforcement content.Fig.11d shows the worn surface of ZGH175 sample at 0.9 m/s under 10 N.As it can be seen,there are large dug-out areas on the worn surface,which is indicative of adhesive wear.Also,partial areas of layering are also observed,which is an expression of delamination.It can be said that by increasing the sliding speed to 0.9 m/s,the wear mechanism has changed from abrasive wear to adhesive wear and delamination.As the sliding speed increases,the instantaneous temperature increase on the surface of the sample enhanced.An increase in temperature on the one hand causes oxidation of the sample surface [84]and on the other hand,with the increase in temperature,the strength of the sample decreases and it causes the pin to stick to the surface of the sample and then a part of the sample surface is torn off.According to Fig.11e,it is clear that the areas of adhesive wear are limited,and the areas of delamination are also observed.The reduction of adhesive wear areas is caused by the presence of reinforcement in the matrix.In such a way that most of the applied load of the abrasive pin is borne by the ceramic particles and the applied load is reduced to the matrix.On the other hand,the presence of reinforcement reduces the friction between the matrix and the reinforcement and prevents the sudden increase in temperature,which is the controlling agent of adhesive wear.With the increase of reinforcement,the conditions have changed and adhesive wear and delamination have not occurred (Fig.11f).More distribution of reinforcement has increased the strength and acceptable reduction of friction,which has prevented the occurrence of adhesive wear.However,plowing can be observed on the surface,which is caused by the detachment of SiC particles and the formation of three-body wear.With the increase of reinforcement,the possibility of detachment the SiC particles also increases.

Fig.11.SEM micrographs of worn surfaces of precipitation hardened samples at 175 °C under 10 N for: (a) unreinforced alloy at 0.1 m/s,(b) ZK60Gd/5%SiC 0.1 m/s,(c) ZK60Gd/10%SiC at 0.1 m/s,(d) unreinforced alloy at 0.9 m/s,(e) ZK60Gd/5%SiC at 0.9 m/s,and (f) ZK60Gd/10%SiC at 0.9 m/s.

With the increase of the applied load up to 60 N at the sliding speed of 0.9 m/s,it is clear that the wear mechanisms have changed.As seen in Fig.12a,the worn surface of the ZGH175 sample is covered with plastic deformed region.It can be observed that with the increase in the load,the worn surface has deeper grooves along with the formation of delamination regions.Also,the areas in the figure are marked with a blue box,which indicates the activation of mechanically mixed layer (MML) wear.According to the EDS analysis,it is clear that the elements of iron and chromium have been separated from the abrasive pin.It can be seen that with the increase in the load,the surface has deeper grooves and separated regions have increased.The conditions for the composite samples are different and the worn surface of the samples is mainly covered with shallow parallel grooves along with MML regions,which shows that compositing is a successful factor in increasing the wear resistance in harsh conditions.

Fig.12.SEM micrographs and EDS analysis of worn surfaces of precipitation hardened samples at 175 °C under 60 N at 0.9 m/s for: (a) unreinforced alloy,(b) ZK60Gd/5%SiC,and (c) ZK60Gd/10%SiC.

By increasing the load up to 90 N at low/high sliding speeds,wear mechanisms have caused more damage to the surface of the samples.According to Fig.13a,it is clear that at low speed,the worn surface of the ZGH175 sample has created massive plastic deformation,which actually caused visible protrusions and depressions on the surface of the sample.Based on the Fig.13b deformed areas are not observed in the 5ZGH175 sample,but the high load caused cracks in the sample.Considering that the crack is in the sliding direction,it can be said that it is caused by the growth of the crack due to the reduction of the ductility of the sample due to the presence of reinforcing particles.Also,a visible crack is observed in the 10ZGH175 sample,considering that the crack is perpendicular to the sliding direction,it indicates the activation of fatigue wear and crack growth.Under conditions of high load and high speed,severe plastic deformation occurs in successive sliding on the wear surface.So that severe plastic deformation simultaneously with a high friction coefficient causes the surface to move in the sliding direction.So that the amount of deformed material decreases as it moves away from the surface of the sample and it has the highest intensity on the upper surface of the sample.This increases the strain caused by shearing on the surface of the sample.The strain causes the orientation of dislocations and their pile-up in the subsurface,which in order to create compliance,ultimately causes the fracture of the primary grain structure in the sub-boundary,so that the isolated part is free of dislocations.With the formation of a wear particle in the cell wall perpendicular to the sliding direction,it causes the nucleation and propagation of a crack,which with successive sliding causes separation due to fatigue wear [85].According to Fig.13d–f,it is clear that with the increase of the sliding speed,the wear mechanism of the unreinforced sample and the composite samples did not change,but the severity of its destruction increased and caused the failure of the samples.

Fig.13.SEM micrographs of worn surfaces of precipitation hardened samples at 175 °C under 90 N for: (a) unreinforced alloy at 0.1 m/s,(b) ZK60Gd/5%SiC 0.1 m/s,(c) ZK60Gd/10%SiC at 0.1 m/s,(d) unreinforced alloy at 0.9 m/s,(e) ZK60Gd/5%SiC at 0.9 m/s,and (f) ZK60Gd/10%SiC at 0.9 m/s.

In order to identify the strengthening mechanism of the samples at high loads and high speed,the high magnification figure of the worn surface along with the profile of the worn surfaces for different samples are shown in Fig.14.According to Fig.14a,it is clear that the destruction of the surface of the unreinforced sample continued until the precipitates were identified on the surface.It can be said that the precipitates have increased the wear resistance.Similarly,in the composite samples,the torn areas continued until the reinforcing particles appeared,and after that,the strength of the interface of these particles controls the wear rate and wear resistance of the samples.

Fig.14.SEM micrographs and simulated profile of worn surfaces of precipitation hardened samples at 175 °C under 120 N at 0.9 m/s for: (a and d)unreinforced alloy,(b and e) ZK60Gd/5%SiC,and (c and f) ZK60Gd/10%SiC.

Fig.15 depicts the wear debris of the precipitation hardened samples at 175 °C.As shown the wear debris created under 10 N of the unreinforced sample,which had fine particles with an average particle size of ＜2 μm (Fig.15a).EDS analysis of the wear debris showed a significant amount of iron,possibly indicating wear of the steel abrasive pin.The wear debris created for the composite samples under a load of 10 N also have a dimension similar to that of the unreinforced alloy,with the difference that the amount of iron identified for the composite samples is more than that of the unreinforced sample,which confirms the increase in increase in strength is due to the presence of ceramic particles and the greater ability of the composite sample to chipping from the surface of the steel pin.With the increase of the applied load (90 N),it is clear that the wear debris of the unreinforced sample were plate-like shape,large and metallic with a size of more than 100 μm,which confirms the severe plastic deformation sample during wear test.Fig.15e and f show the wear debris morphology for the composite samples under 90 N,which shows that it is an agglomerated of large and small particles formed as a result of adhesion and separation of the composite surface layer.EDS analysis showed the presence of strong oxygen,iron and magnesium in the debris.Carbon and silicon were also detected,implying that the wear debris may consist of a mixture of magnesium,magnesium/iron oxides,and SiC.Similar mechanisms have been reported in other magnesium composites [69].

Fig.16 illustrates wear maps in terms of different loads and velocities conditions for unreinforced alloy and composites samples.As seen in Fig.16a,in the unreinforced alloy,the abrasive wear mechanism was dominant at low loads and different speeds and at low speeds and medium loads.In the middle region,at medium loads and speeds,the adhesive wear mechanism was activated.This is despite the fact that at high speeds,adhesive and abrasive wears appear simultaneously on the surface of the sample.At high loads and low speeds,plastic deformation has prevailed,and finally,at high loads and speeds,the two mechanisms of delamination and plastic deformation have caused the destruction of the sample.

Fig.16.Wear map of precipitation hardened samples at 175 °C: (a) unreinforced alloy,(b) ZK60Gd/5SiC,and (c) ZK60Gd/10SiC.

According to Fig.16b,it is clear that the addition of ceramic particles has widened the abrasive wear region,which can be caused by the presence of reinforcing particles and their separation,which causes scratches on the surface of the sample in a wide range of speeds and loads.It is clear that the plastic deformation zone is not observed in the composite,which is caused by the increase in the strength of the composite and its resistance to the load applied by the abrasive pin.It is also clear that at high loads and speeds,the fatigue wear mechanism was activated and the delamination region is limited.In Fig.16c,it is clear that the increase in the percentage of reinforcement has caused a wider increase in the region of abrasive wear,and the region of delamination and adhesive wear has become smaller.Meanwhile,the fatigue wear region is slightly larger,which is caused by the decrease in the ductility of the sample with the increase in the reinforcement content.

4.Conclusions

In this research,the effect of precipitation hardening on the wear behavior of the Mg-Zn-Zr-Gd alloy and Mg-Zn-Zr-Gd/SiC composites at different temperatures was studied and the results are as follows:

1.The precipitation hardening results at temperatures of 125,175,and 225 °C showed that for all the samples,the most hardness after precipitation hardening was obtained for the sample treated at 175 °C.Which is due to the formation of Gd(Mg0.5Zn0.5) precipitates.

2.Uniform and homogeneous distribution of SiC reinforcement in the magnesium alloy matrix was obtained using the stir casting method,which increased the wear resistance of the magnesium sample.

3.The wear tests showed that the friction coefficient of the as-cast alloy samples was in the range of 1.2 and had increased by 12% compared to the composite samples.Whereas with applying the precipitation hardening process the friction coefficient was 0.8 and precipitation hardening at the temperature of 175 °C had the least friction coefficient.The results of the weight loss and wear rate plots showed that after casting,the least amount of weight loss corresponded to the 10ZGA sample for the 40 and 60 N loads.Whereas all the precipitation hardened samples had a lower weight loss compared to the as-cast sample.Between the different conditions,the precipitation hardened sample at 175 °C had the least weight loss.Whereas from all the different samples,the composite sample containing 10% reinforcement had the least wear rate.The microscopic study on the worn surface showed that the dominant wear mechanism for unreinforced precipitation hardened sample at 175 °C was abrasive,adhesive,delamination,MML,and plastic deformation.Whereas the abrasive,adhesive,delamination,MML,and fatigue wear mechanisms were dominant for composite samples.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper