铸造Al-50Si合金组织和性能变化

Evolution of microstructure and mechanical propertiesof as-cast Al-50Si alloy due to heat treatment and P modifiercontentFuyang Cao a ,Yandong Jia a ,b ,Konda Gokuldoss Prashanth b ,Pan Ma a ,b ,Jingshun Liu a ,c ,Sergio Scudino b ,Feng Huang a ,d ,Jürgen Eckert b ,e ,Jianfei Sun a ,⇑aSchool of Materials Science and Engineering,Harbin Institute of Technology,Harbin 150001,China bIFW Dresden,Institute for Complex Materials,P.O.Box 270116,D-01171Dresden,Germany cSchool of Materials Science and Engineering,Inner Mongolia University of Technology,Hohhot 010051,China dHubei Key Laboratory of Advanced Technology of Automobile Parts,School of Automotive Engineering,Wuhan University of Technology,430070,China eTU Dresden,Institut für Werkstoffwissenschaft,D-01062Dresden,Germanya r t i c l e i n f o Article history:Received 12September 2014Revised 4March 2015Accepted 7March 2015Available online 9March 2015Keywords:Al–50Si alloySuperheat treatment MicrostructureMechanical propertya b s t r a c tThe effects of superheat temperature,content of modifier (P)and T6heat treatment on the microstruc-ture and mechanical properties of the Al–50Si alloy have been investigated systematically by scanning electron microscopy (SEM)and differential scanning calorimetry (DSC).The results indicate that the pri-mary Si exhibits a plate-like morphology,with average size decreasing with increasing of the superheat temperature for the unmodified alloy.The morphology of primary Si changes to small blocky shape at an optimal P content of 0.5wt.%,and the nucleation temperature increases for the alloy with 1.3wt.%P because of the ease of formation of the AlP phase.The nucleation temperature is lower for 0.5wt.%P due to lack of P atoms at relatively higher temperature.The ultimate tensile strength was enhanced by the addition of P followed by the T6heat treatment,and the maximum ultimate tensile strength ($160MPa)was observed for the sample containing 0.5wt.%P.Ó2015Elsevier Ltd.All rights reserved.1.IntroductionElectronic packaging materials are required to protect the elec-tronic components from physical damage,mechanical forces,atmospheric chemical contamination,etc.[1].As the electronic packaging requires increasingly smaller size,lighter weight and higher integration,new packaging materials have to be developed to improve the performance of electronic components.However,the properties of traditional packaging materials can no longer sat-isfy the practical requirements [2–4].Hypereutectic Al–Si alloys with high Si content (50–70wt.%)are one of the ideal candidates for electronic packaging application as a result of the positive combination of properties,such as relatively low coefficient of thermal expansion (CTE),which closely matches that of GaAs or Si semiconductor materials,high thermal conductivity,low density and superior strength [5].However,the main limitation of this type of material is the presence of the coarse,irregular,and brittle primary Si phase that can act as soft spots for premature crack initiation,deteriorating the overall mechanical properties of these materials [6,7].Therefore,it is essential to modify the microstructure of hypereutectic Al–Si alloy to optimize the mor-phology and distribution of the primary and eutectic Si [8,9].Efforts have been made to modify the microstructure of hypereutectic Al–Si cast alloys in order to achieve a refined Si phase with beneficial shape and distribution [10–12].For example,Liu et al.[13]have investigated the modification of hypereutectic Al–24%Si alloys with Si–P,which leads to the formation of primary Si with size of 19l m.Choi et al.[14]have reported that the mor-phology of primary Si in hypereutectic Al–20%Si alloy can be modi-fied from star-like to the polygon or blocky shape by the addition of c -Al 2O 3nanoparticles.Moreover,the spray forming technology was also used to prepare the Al–35%Si alloy with size of the Si phase less than10l m [15].However,only little attention has been paid to the modification of Al–Si alloys with high Si contents (e.g.50wt.%).The present study analyzes this mentioned above aspect by examining the potential of different superheat temperatures and phosphorus contents as modifying agents for simultaneous refine-ment of both the size and morphology of primary Si phase in the as-cast Al–50Si alloy.Additionally,the work investigates the effect of the induced microstructural modifications on the mechanical properties of the material./10.1016/j.matdes.2015.03.0080261-3069/Ó2015Elsevier Ltd.All rights reserved.⇑Corresponding author.E-mail address:jfsun@ (J.Sun).F.Cao et al./Materials and Design74(2015)150–1561512.Experimental details equipped with an energy dispersive X-ray(EDX)setup.Differential scanning calorimetry(DSC)measurements wereof Al–50Si alloy processed using different superheat temperatures(a)100,(b)200,(c)300,(d)400and(e)500K,(f)temperature.the nucleation and growth of primary Si [17].A higher superheat temperature is necessary to promote the dissolution and to Table 1The O,N,H contents in the samples fabricated at different superheat temperatures SEM images of Al–50Si with different concentration of the modifier:(a)0,(b)0.1,(c)0.3,(d)0.5,(e)0.7and (f)1.3152 F.Cao et al./Materials and Design 74(2015)150–156of300K is used in order to evaluate the effects of P on the microstructure and properties of the Al–50Si alloy.The microstructures of the Al–50Si alloy with different P con-tents are shown in Fig.2.The micrographs indicate substantial microstructural differences in the morphology,size and distribution of the primary Si with respect to the un-refined material(Fig.2(a)). The morphology of primary Si is still plate-like,but its average size is reduced to62.2±1.5l m for the sample with0.1wt.%P(Fig.2(b)) compared to the unmodified alloy(117±2.5l m).With increasing the P content to0.5wt.%,the average size of the primary Si decreases further to38.0±2.1l m.In addition,the morphology is no longer plate-like but blocky in shape as displayed in Fig.2(d). The average size of the primary Si grows as the P content increases to0.7wt.%and1.3wt.%,reaching values of about39.4±1.3l m and 41.2±2.3l m,respectively(Fig.2(e)and(f));yet,the size of the pri-mary Si is smaller than the unmodified alloy.AlP particles can be accordingly formedthe Al–P master alloy into the hypereutecticuseful for refining the primary Si phase.the fact that both the crystal structure andAlP(cubic structure,a=5.45Å)is similar toture,a=5.431Å),and hence,the AlP particlesnucleation sites for primary Si[20–22].Theand changes the morphology from particle shapewhen the P addition is1.3wt.%(Fig.2(f)).Thistribution and morphology of AlP wouldnucleation sites in the melt for the primarythen cause the increasing size of primaryA typical backscattered electron image ofP is shown in Fig.3along with the correspondingmaps.It can be also observed that the particlemary Si phase is rich in Al and P,which canAlP according to the elemental scanning measurementsin Fig.3(b)–(d).Fig.4shows the cooling curves of the Al–50Siof the P concentration.The nucleation temperature of the eutectic Si at about843K does not change significantly with the addition of P.On the other hand,the primary Si nucleation temperature changes with increasing P content.The nucleation temperature of primary Si is about1334K for the unmodified alloy,and the tem-perature increases by$22K with the addition of1.3wt.%P.It is reported that dissolved Si atoms may aggregate on the surface of the AlP substrates to nucleate with the cube–cube orientation relationship to have similar lattice parameters[22].During solid-ification,AlP can nucleate easily owing to a sufficient amount of P atoms in the melt with1.3wt.%P.The Si crystals tend to grow by the aggregation of Si atoms on the pre-formed primary Si crystal surfaces in accordance with the twinning plane re-entrant edge (TPRE)mechanism during the growth stage[23].Compared to the smooth surfaces of thefirstly-formed primary Si[24],the AlP parti-cles possess additional twinning planes[25].Thus the Si atoms are more inclined to aggregate on the surface of AlP rather than on the firstly-formed primary Si[26].SEM image of the primary Si in the alloy modified with0.5wt.%P and corresponding their EDX images for(b)Al,(c) Fig.4.DSC curves of the Al–50Si alloy as a function of P addition.The nucleation temperature of the primary Si decreases by about 55K in the sample with0.5wt.%P compared with the unmodified alloy.Due to the smaller amount of P atoms in the melt with 0.5wt.%P,it is difficult for the AlP phase to nucleate and a larger degree of supercooling is needed.As a result of the similar electronic structure between the Si and P atoms,the nucleation of primary Si is restricted.Therefore,primary Si nucleates after the nucleation of the AlP phase nucleation at a relatively lower temperature.In order to understand the influence of the heat treatment on the microstructure,the alloy with0.5wt.%P with ST=300K is used for the following investigation.The microstructures of Al–50Si alloy before and after heat treatment are shown in Fig.5. Compared with the as-cast alloy(Fig.5(a)),the size of primary Si increases slightly(about42.2±1.5l m)inrial,as shown in Fig.5(c).However,comparingFig.5(d)reveals that after heat treatment theSi are round and smooth,whereas large amountssolves into the Al matrix and the eutectic Sishape.The morphology variation in both primarymay have led to reduction of stress concentrationmay be beneficial for enhancing the mechanicalalloy.3.2.Mechanical propertiesThe ultimate tensile strength(UTS)ofdifferent P contents is presented in Fig.6.alloys is higher compared to the unmodifiedproperties of the hypereutectic Al–Si castlargely dependent on the primary Si characteristics morphology and distribution)[13].Accordingly,the P addition may positively influence the tensile properties of the as-cast alloys due to the refinement and modification of the primary Si.The ulti-mate tensile strength of the alloys increases with increasing the P content,reaching131MPa for the sample with0.5wt.%P,which is four times higher than that of the unmodified alloy.After T6heat treatment,the UTS of the alloy with0.5wt.%P reaches160MPa,which is$22%higher than the same alloy without heat treatment and is only32MPa less than the same alloy fabricated by rapid solidification[29].This result is consistent with the microstructural evolution reported in Fig.5showing that the Al–Si alloy can be precipitation-strengthened after T6heat treat-ment.Consequently,the T6heat treatment plays a crucial role during the fragmentation and spheroidization process of eutectic Si[30].It has been reported[31]that coarse and elongated Si par-ticles tend to fracture more frequently than spherical particles,asFig.5.SEM microstructures of Al–50Si alloy(a)and(b)before,and(c)and(d)after heat treatment.Fig. 6.Ultimate tensile strength for the Al–50Si alloys processed at differentconditions.coarse Si particles can induce high levels of stress concentration and consequently result in the reduction of UTS.Fig.7shows the morphology of the fracture surface of the unmodified,modified and heat-treated samples after room tem-perature tensile tests.The fracture surface of the unmodified alloy exhibits a brittle morphology along with a large number of sec-ondary cracks(Fig.7(a)).Figs.7(b)and(c)display the fracture sur-face of the alloy with0.5wt.%P addition and after T6heat treatment.It can be also seen that the specimen presents the typical brittle fracture,but the amount of secondary cracks decreases as compared with the unmodified alloy,and some dimples indicative of ductile fracture are also observed.4.ConclusionsThe microstructural evolution and the tensile properties of the Al–50Si alloy has been investigated in detail and the following con-clusions can be drawn as follows:(1)Primary Si phase exhibits a plate-like morphology,and itsaverage size decreases from171l m(for ST=100K)to about 103l m(for ST=500K).However,the optimal ST is observed to be300K.(2)Due to the formation of AlP particles,the primary Si phasevaries from plate-like morphology to blocky shape and a minimum size of38l m is obtained with the addition of0.5wt.%P.(3)The primary Si nucleation temperature decreases by about55K with the addition of0.5wt.%P,with further increase of the P content to 1.3wt.%,the nucleation temperature increases by about22K compared with the unmodified Al–50Si alloy.(4)The edges of the primary Si becomes round and smooth,whereas the eutectic Si transforms to spherical shape after the proper heat-treatment.(5)Both the addition of P modifier and T6heat treatment arebeneficial for the enhancement of the mechanical property of the Al–50Si alloy.The largest UTS of160MPa is obtained with the addition of0.5wt.%P and after T6heat treatment. 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