The creep behavior of Mg-9Al-1Si-1SiC composite at elevated temperature

Pengwen Zhou,Shoxiong Zhng,Ming Li,b,Hongxi Wng,?,Weili Cheng,?,Lifei Wng,Hng Li,Wei Ling,Yiming Liu

a Shanxi Key Laboratory of Advanced Magnesium based Materials,College of Materials Science and Engineering,Taiyuan University of Technology,Taiyuan 030024,China

b Southwest Technology and Engineering Research Institute,Chongqing 400039,China

Received 29 April 2019;received in revised form 20 June 2019;accepted 21 June 2019 Available online 10 June 2020

Abstract The creep properties of as-cast Mg-9Al-1Si alloy and Mg-9Al-1Si-1SiC composite were compared.The results show that Mg-9Al-1Si-1SiC composite performs a better creep resistance than that of Mg-9Al-1Si alloy at constant temperature and stress(473K,70MPa).Besides,the creep behavior of Mg-9Al-1Si-1SiC composite at various temperature from 448K to 498K and under stresses of 70-90MPa were systematically investigated.The Mg-9Al-1Si-1SiC composite exhibited a stress exponent from 5.5 to 6.9 and the creep activation energy fell within the range of 86-111kJ/mol.The results showed that the creep mechanism of Mg-9Al-1Si-1SiC composite was mainly attributed to the effects of secondary phase strengthening mechanism and dislocation climb mechanism.? 2020 Published by Elsevier B.V.on behalf of Chongqing University.This is an open access article under the CC BY-NC-ND license.(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Microstructure;Stress exponent n;Creep activation energy Q;Mg-9Al-1Si-1SiC composite.

1.Introduction

Due to the weight reduction objective of automotive industries,high temperature-resistant magnesium alloys have been attracted increased interests for decades[1-3].Recently,researchers have paid more and more attention on the high temperature properties and microstructures of Mg-Al based alloys[4-7].It has been reported that the strength,ductility,hardness and creep resistance of Mg-Al based alloys at high temperature could be enhanced by combined addition of Si elements[8].Because of the Mg2Si with high melting point and modulus,the improvement of mechanical properties of Mg-Al alloy by Si elements is favorable.However,the Mg2Si usually with coarse Chinese-script-shaped in Mg-Al alloy,it cannot improve mechanical properties in the nanometer scale.Furthermore,previous reports revealed that there is remarkable reinforced effect to Mg alloy by ceramic with thermally stable,such as the Al2O3to magnesium alloy AM60[9],the TiB2and Y2O3to pure Mg[10,11],TiO2to ZM5 and the SiCp to Mg-8Sn-1Zn alloy[12,13],these all led to a signifi cant improvement in mechanical properties.Generally,the addition of SiCp particles in the magnesium matrix is regarded as one of the most effective methods due to its high thermal stability and relatively low cost.In fact,Sourav et al.[14].found that all the nanocomposites exhibited superior creep resistance than the unreinforced 2.0Ca+0.3Sb(wt%)added AZ91 alloy.F.Labib et al.[15]reported that grain refine ment strengthening,coefficien of thermal expansion(CTE)mismatch strengthening mechanism and load transfer mechanism were affected by the content of SiC for pure magnesium.In addition,Mondal et al.[16].have reported that various creep temperatures led to the different creep behaviors on AE42-SiC composites.Viswanath et al.[17]investigated the creep behavior and mechanical properties of AZ91-SiCp composites.The results revealed that different creep characteristics were attributed to reinforcement content and applied load.

However,studies about the creep behavior of n-SiCp/Mg-9Al-1Si composite have not been studied yet.Therefore,the effect of grain size and distribution of secondary phase on the creep properties of as-cast Mg-9Al-1Si alloy and Mg-9Al-1Si-1SiC composite were investigated at constant temperature and stress(473K,70MPa).In addition,operative creep mechanism for as-cast Mg-9Al-1Si-1SiC composite was probed thoroughly.

2.Experimental procedures

Both Mg-9Al-1Si alloy and Mg-9Al-1Si-1SiC composite was melt in a steel crucible under a protective atmosphere of CO2and SF6[18].SiC nanoparticles(n-SiCp)was used to serve as a reinforcement with an average size about 60nm.Due to the existence of interference between reinforcement and matrix,the n-SiCp was pre-heated at 773K to remove water vapor,organics and absorbed gases adhering to the particles surface[19].Pure Mg and pure Al were added into the crucible preheated at 623K.Then,the master alloy of Al-30% Si(wt%)were added to the crucible when the temperature raised to 993K and kept for 15min.The melt was raised to 1003K and held for 20min.The melt of Mg-9Al-1Si alloy was casted into a preheated mold with a diameter of 40mm and length of 100mm.On the other hand,the melt cooled down to 863K to add SiC(1wt%)nanoparticles,which were quickly added into the semisolid slurry with stirred(100 r/min).Then the temperature of melt was raised at 973K to make sure that the slurring convert into liquid state.The stirring process stopped until the temperature reached 973K.Then,the ultrasonic probe was dipped into the melt for ultrasonic treatment(1.6kW and 20kHz)for 20min.Finally,the n-SiCp/Mg-9Al-1Si composite melt was poured into a preheated mold.

The Leica 2700M optical microscope(OM,Leica Microsystem GmbH,Wetzlar,Germany)and a MIRA3 scanning electron microscope(SEM,TESCAN Ltd,Brno-Kohoutovice,Czech Republic)were used to analyze microstructures.The composition of second phases was determined by energy dispersive spectrometer(EDS).The transmission electron microscopy(TEM)using a JEM-2100F(JEOL Ltd,Tokyo,Japan)was used to certifie n-SiCp.The tensile creep test was conducted in a RDL100 creep machine.

3.Results and discussions

3.1.Microstructural characteristics

Fig.1 shows the micrographs of as-cast Mg-9Al-1Si and Mg-9Al-1Si-1SiC alloys.As indicated in Fig.1(a)and(b),some Chinese-script-shaped and black massive-like phases could be observed along the dendrite boundaries.The EDS results validated that these phases are Mg2Si and Mg17Al12,respectively.It can be observed that the particle size of Mg2Si reduce from almost 50μm to 25μm,as show in Fig.1(c)and(d),and the size of Mg17Al12have be refine too.The grain size of Mg-9Al-1Si-1SiC composites is smaller than that of Mg-9Al-1Si alloy,which can be attributed to the influenc of n-SiCp by the heterogeneous nucleation mechanism onα-Mg.Besides,the restricted effect to the grain boundary ofα-Mg by n-SiCp particles during solidificatio plays an important role in the modificatio of microstructures.[20,21].In addition,the size of Mg2Si and Mg17Al12phase are great refine and the distribution becomes homogeneous after the addition of n-SiCp.It is probably due to the nucleation sites for Mg2Si and Mg17Al12phase provided by the n-SiCpwhich results in the size of both phases reducing greatly.

Furthermore,the n-SiCp distributes uniformly in Mg-9Al-1Si-1SiC composites,as illustrated in Fig.1(e),the result of electron diffraction patterns(SAD)in region A(Fig.1(f))indicates that the particles are n-SiCp.As shown in Fig.2,the n-SiCp have a predominant effect on the modificatio of microstructures of Mg-9Al-1Si alloy[22].

Creep curves of as-cast Mg-9Al-1Si and as-cast Mg-9Al-1Si-1SiC composites were observed clearly in Fig.3(a).It is apparent that the curve of as-cast Mg-9Al-1Si-1SiC composites shows typical three-stages creep characteristics:a more obvious primary creep stage than Mg-9Al-1Si followed frst by steady-creep state and then tertiary stage.Detailed creep properties of as-cast composites at 473K and 70MPa are displayed in Table 1.As indicated,the stable creep rate of as-cast Mg-9Al-1Si-1SiC composites dropped compared to as-cast Mg-9Al-1Si alloy,which reached to 6.94 E?7 s?1.It is attributed to the improvement of homogeneity of microstructure by n-SiCp in composites.For Mg-9Al-1Si-1SiC composites,it is possible that the refine Mg17Al12and Mg2Si phases decrease the chance of crack extend with a creep life achieved 50.59h.Fig.3(b)and(c)displays the side surfaces of as-cast Mg-9Al-1Si alloy and Mg-9Al-1Si-1SiC composites at 473K and 70MPa.It indicates that both Mg-9Al-1Si alloy and Mg-9Al-1Si-1SiC composites appear cracks along the Mg17Al12phases,however,the cracks in the Mg-9Al-1Si-1SiC are more scatter.Besides,some microcracks are adjacent to the Mg17Al12phases and Mg2Si phases.It can be seen that microcracks are generated at grain boundary,where distributed coarse and low melting pointβ-Mg17Al12(as shown in Fig.3(b)).As we all know,the existence of coarseβ-Mg17Al12phases aggravates the spreading of microcracks,which results in the formation of holes in Mg-9Al-1Si composite.As shown in Fig.3(c),β-Mg17Al12phases are refine due to the addition of n-SiCp and microcracks are still generated at the grain boundary in Mg-9Al-1Si-1SiC composite.It's remarkable that the range of microcracks is not enlarged and the size of cavities becomes smaller.

Previous studies[22]reported that the addition of n-SiCp led to the significan refinemen and uniform distribution of stable Mg2Si phase at elevated temperature,which facilitated to hinder the sliding of grain boundary.At an elevated temperature,as we all know,the mechanism of grain boundary sliding plays a vital role in the whole of tensile process[23].Meanwhile,the dispersed distributed n-SiCp in grains interiors could hinder the motivation of dislocation[24,25].In addition,n-SiCp distributed at grain boundary could also prevent grain boundary from sliding by pinning effect,which contributes to the enhancement of the creep properties of Mg-9Al-1Si-1SiC composites.

Fig.1.OM,SEM and TEM micrographs of as-cast samples:(a,c)Mg-9Al-1Si;(b,d)Mg-9Al-1Si-1SiC;(e)the distribution of SiC nanoparticles and selected area diffraction(SAD)patterns of n-SiCp in Mg-9Al-1Si-1SiC composite.

Table 1Creep properties of as-cast composites at 473K and 70MPa.

In order to investigate the properties of creep resistance and mechanism of as-cast Mg-9Al-1Si-1SiC composites,a series of creep tests at different temperatures(448K,473K and 498K)and stresses(70MPa,80MPa and 90MPa)are conducted.A set of representative creep curves are presented in Fig.4.The detailed statistics are displayed in Table 2.As shown in Fig.4,the tendency of creep curves of Mg-9Al-1Si-1SiC composite at different temperatures and stresses are almost similar.Note that,a very well-define primary stage,second stage and tertiary stage appear in all of curves.It is noteworthy that the time of stable creep becomes shorter with stress increasing at a constant temperature or at constant stress with temperature increasing.Consequently,the creep life reaches up to 112.77h at 448K/70MPa,which indicates the property of creep resistance is the most favorable(Table 2).

Fig.2.The effect of n-SiCp on Mg-9Al-1Si alloy.

Table 2Creep properties of Mg-9Al-1Si-1SiC composite at different stresses and temperatures.

In fact,there is a power-law equation can be used to describe the minimum creep rate(·ε)of metals and the external stressσas follows[26-29]:

whereQis the activation energy for creep,Ais the material constant,nis the stress exponent,Ris the gas constant(R=8.31J/(mol·K)),andTis the absolute temperature andσis the applied stress,respectively.The relationship between n andσcould be derived at the constant temperature:

The relationship betweenQandTcould be derived at a constant stress:

Fig.3.Creep curves and Side surfaces of Mg-9Al-1Si and Mg-9Al-1Si-1SiC composites at 473K/70MPa creep test:(a)Creep curves,(b)Mg-9Al-1Si,(c)Mg-9Al-1Si-1SiC.

Fig.4.Creep curves of Mg-9Al-1Si-1SiC composite at different stresses and temperatures.

Fig.5.The linear relationship between ln·εand lnσat different temperatures(a),ln·εand T?1 at different stresses(b)of Mg-9Al-1Si-1SiC composite.

Table 3The creep stress exponent and creep activation energy of Mg-9Al-1Si-1SiC composite at different temperatures and stresses.

Fig.5(a)shows the relationship between ln·εand lnσon as-cast Mg-9Al-1Si-1SiC composites at various temperatures.It can be they approximately display linear relation.It obviously obverse that it approximately forms a line with a slope of approximately 6 by data points from various conditions,illustrating that Mg-9Al-1Si-1SiC composite exhibits a typical feature of dislocation climbing controlled creep[26].The detailed stress exponent n is also listed in Table 3.It is clearly observed that the stress exponent n increases gradually with temperature increases.In fact,the high temperature Mg-9Al-1Si-1SiC composite possess large grain size and a faster sliding of grain boundary comparing the low temperature.It is noteworthy that the Gifkins'core-mantle theory suggests that an increased fin grain size results in the creep exponent decreasing[30].For Mg-9Al-1Si-1SiC composite,the higher of the temperature,the more significan effect of the stress on the properties of creep resistance will be.According to stress exponent n,creep mechanism of composites is induced.n=4-6,dislocation climb is the main creep mechanism is;n≥6,creep mechanism is induced by secondary phases strengthening effect[31,32].The research above indicates that the stress exponent n is related to the microstructure,temperature as well as stress.When the temperature of creep increases from 448K to 498K,the stress exponent n of as-cast Mg-9Al-1Si-1SiC composite increases from 5.5 to 6.9 revealing the major creep mechanism of Mg-9Al-1Si-1SiC composite transforms from dislocation climb at 448K into secondary phases strengthening effect at 498K.Besides,the creep mechanism at 473K can be regarded as a transition between the two temperatures.In fact,the particle of n-SiCp distributing in grain impede dislocation climb at all temperatures[25].However,there are different morphology of Mg17Al12phase at different temperature in Mg-9Al-1Si-1SiC composite,As shown in Fig.6(c,f,i),with increasing temperature of creep,the distribution of the Mg17Al12phase was extremely dispersed and the size was remarkably decreased to become much fine particles.Meanwhile,the n-SiCp particle promotes the creep mechanism to transform to secondary phases strengthening effect.Thus,the creep performance was dominated by the secondary phases.

Fig.5(b)shows the linear relationship between ln·εandT?1of Mg-9Al-1Si-1SiC composite at different stresses.The results of creep activation energyQare listed in detail in Table 3.The creep activation energyQcan reflec the ways of dislocations overcoming obstacles so that creep mechanism of composites can be obtained indirectly.It is reported that lower creep activation energyQresults in dislocations overcoming obstacles by cross-slip.While the higher creep activation energyQresults in dislocations overcoming obstacles by dislocation climb,which is probably attributed to diffusion of vacancies and atoms.Previous study[32,33]has shown that creep mechanism is dominated by grain boundary diffusion when creep activation energyQreaches to 92 KJ/mol,and creep mechanism is dominated by lattice diffusion whenQreaches to 135 KJ/mol.In the present study,the creep activation energy Q of Mg-9Al-1Si-1SiC composite increased from 86 to 111 KJ/mol which is close to grain boundary diffusion activation energy of Mg[26].It can be concluded that the value of activation energyQin 70MPa and 90MPa are symmetry about the 92 KJ/mol.However,comparing with the creep in 70MPa and 90MPa,there is a higher creep activation energyQwhen creep stress in 80MPa,which indicates that the major creep mechanism is probably dislocation climbing for Mg-9Al-1Si-1SiC composite.And the value of activation energy suggests that creep mechanism of as-cast Mg-9Al-1Si-1SiC composite also consists by grain boundary diffusion and the dislocation cross-slip because it reaches 111 KJ/mol[30].The stress of creep may influenc the movement of dislocation:there is a little influenc at low stress such as 70MPa because of the hinder effect of n-SiCp,which distributing in the interior of grain and grain boundary.The mechanism of dislocation cross-slip and dislocation climb appearance with the increase on a stress elevate at 80MPa.Besides,there is not enough time for dislocation climb to be happened for Mg-9Al-1Si-1SiC composite creep in 90MPa,the dislocation of cross-slip and grain boundary diffusion act as two mechanisms.Based on the results of creep stress exponent n and creep activation energyQ,it can be concluded that the dominant operative creep mechanism of Mg-9Al-1Si-1SiC composite is dislocation climb and secondary phase strengthening effect at(448-498K)/(70-90MPa).

Fig.6.Side surfaces of Mg-9Al-1Si-1SiC composite at different stresses and temperature creep test.

In order to exploration the creep fracture mechanism of Mg-9Al-1Si-1SiC composite,the side surfaces of Mg-9Al-1Si-1SiC composite at different stress and temperature creep test are shown in Fig.6.As shown in Fig.6(a,b,c),the size of cavities is getting bigger with the stress increasing at a constant temperature.It is similar with the temperature increasing under the constant stress condition,Generally,thermally creep of metals should void grain growth at grain boundaries.There are cavities existence in the vicinity of a macroscopic crack tip which leads to the formation of microcracks and ultimate fracture[34].During creeping,dislocation pile-ups at grain boundary interfaces,which results in the stress concentration.However,stress concentration accelerates crack initiation,meanwhile,cracks develop into cavities with grain boundary sliding[35].Then,cavities spread along grain boundary which is perpendicular to the direction of stress with temperature and stress increasing,as presented in Fig.6(d,e,f,g,h,i).It can be concluded that the cavities are grew up and gathered which lead to the increase of stable creep rate of the composite and the decrease of creep resistance[36].Therefore,Mg-9Al-1Si-1SiC composite exhibits lower performance in creep resistance under high temperature and high stress.

4.Conclusions

(1)The distribution of secondary phases is more uniform in as-cast Mg-9Al-1Si-1SiC composite than that in ascast Mg-9Al-1Si composite,which indicates the restricted effect to the grain boundary ofα-Mg by n-SiCp particles during solidificatio plays an important role in the modificatio of microstructures.

(2)The stable creep rate of as-cast Mg-9Al-1Si composite is 1.17 E?6 s?1and the creep life is 37.34h at 473K/70MPa.The stable creep rate of as-cast Mg-9Al-1Si-1SiC composite decreases by one order of magnitudes and reaches up to 6.94 E?7 s?1.Owing to the modificatio effect by n-SiCp particles of microstructures,the creep life prolongs to 50.59h.

(3)The stress exponent n of as-cast Mg-9Al-1Si-1SiC composite ranges from 5.5 to 6.9 and the creep activation energy of it is from 86 to 111 KJ/mol.Creep mechanism of as-cast Mg-9Al-1Si-1SiC composite is possibly major controlled by the co-effect of climbing of dislocation and secondary phase particles strengthening effect when the creep test is conducted under(448-498K)/(70-90MPa)conditions.

Acknowledgments

This work was supported by Shanxi provice scientifi facilities and instruments shared service platform of magnesium-based matierals electric impulse aided forming(201805D141005);National Natural Science Foundation of China(51404166,51704209,U1810208);Science and Technology Major Project of Shanxi province(20191102008,20191102007,20181101008);Natural Science Foundation of Shanxi Province(201701D121045);Shanxi Province Science Foundation for Youths(2016021063);The Projects of International Cooperation in Shanxi(201803D421086).