铸造Al-50Si合金组织和性能变化

Evolution of microstructure and mechanical properties

of as-cast Al-50Si alloy due to heat treatment and P modi?er

content

Fuyang 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 ,?

a

School of Materials Science and Engineering,Harbin Institute of Technology,Harbin 150001,China b

IFW Dresden,Institute for Complex Materials,P.O.Box 270116,D-01171Dresden,Germany c

School of Materials Science and Engineering,Inner Mongolia University of Technology,Hohhot 010051,China d

Hubei Key Laboratory of Advanced Technology of Automobile Parts,School of Automotive Engineering,Wuhan University of Technology,430070,China e

TU Dresden,Institut für Werkstoffwissenschaft,D-01062Dresden,Germany

a r t i c l e i n f o Article history:

Received 12September 2014Revised 4March 2015Accepted 7March 2015

Available online 9March 2015Keywords:Al–50Si alloy

Superheat treatment Microstructure

Mechanical property

a b s t r a c t

The effects of superheat temperature,content of modi?er (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 unmodi?ed 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.Introduction

Electronic 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 coef?cient 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 re?ned Si phase with bene?cial shape and distribution [10–12].For example,Liu et al.[13]have investigated the modi?cation 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-?ed 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 modi?cation 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 re?ne-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 modi?cations on the mechanical properties of the material.

/10.1016/j.matdes.2015.03.008

0261-3069/ó2015Elsevier Ltd.All rights reserved.

?Corresponding author.

E-mail address:jfsun@ (J.Sun).

F.Cao et al./Materials and Design74(2015)150–156151

2.Experimental details equipped with an energy dispersive X-ray(EDX)setup.

Differential scanning calorimetry(DSC)measurements were

of 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 1

The O,N,H contents in the samples fabricated at different superheat temperatures SEM images of Al–50Si with different concentration of the modi?er:(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)150–156

of300K 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-re?ned 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 unmodi?ed 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 unmodi?ed alloy.

AlP particles can be accordingly formed

the Al–P master alloy into the hypereutectic

useful for re?ning the primary Si phase.

the fact that both the crystal structure and

AlP(cubic structure,a=5.45?)is similar to

ture,a=5.431?),and hence,the AlP particles

nucleation sites for primary Si[20–22].The

and changes the morphology from particle shape

when the P addition is1.3wt.%(Fig.2(f)).This

tribution and morphology of AlP would

nucleation sites in the melt for the primary

then cause the increasing size of primary

A typical backscattered electron image of

P is shown in Fig.3along with the corresponding

maps.It can be also observed that the particle

mary Si phase is rich in Al and P,which can

AlP according to the elemental scanning measurements

in Fig.3(b)–(d).

Fig.4shows the cooling curves of the Al–50Si

of the P concentration.The nucleation temperature of the eutectic Si at about843K does not change signi?cantly 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 unmodi?ed 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-i?cation,AlP can nucleate easily owing to a suf?cient 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 the?rstly-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 ?rstly-formed primary Si[26].

SEM image of the primary Si in the alloy modi?ed 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 unmodi?ed alloy.Due to the smaller amount of P atoms in the melt with 0.5wt.%P,it is dif?cult 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 in?uence 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)in

rial,as shown in Fig.5(c).However,comparing

Fig.5(d)reveals that after heat treatment the

Si are round and smooth,whereas large amounts

solves into the Al matrix and the eutectic Si

shape.The morphology variation in both primary

may have led to reduction of stress concentration

may be bene?cial for enhancing the mechanical

alloy.

3.2.Mechanical properties

The ultimate tensile strength(UTS)of

different P contents is presented in Fig.6.

alloys is higher compared to the unmodi?ed

properties of the hypereutectic Al–Si cast

largely dependent on the primary Si characteristics morphology and distribution)[13].Accordingly,the P addition may positively in?uence the tensile properties of the as-cast alloys due to the re?nement and modi?cation 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 unmodi?ed 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 solidi?cation[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,as

Fig.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 different

conditions.

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 unmodi?ed,modi?ed and heat-treated samples after room tem-perature tensile tests.The fracture surface of the unmodi?ed 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 unmodi?ed alloy,and some dimples indicative of ductile fracture are also observed.

4.Conclusions

The 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 its

average 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 phase

varies from plate-like morphology to blocky shape and a minimum size of38l m is obtained with the addition of

0.5wt.%P.

(3)The primary Si nucleation temperature decreases by about

55K 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 unmodi?ed 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 modi?er and T6heat treatment are

bene?cial 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. Acknowledgments

This project was supported by National Natural Science Foundation of China(Grant No.51375110),the National973Plan Project of China(Grant No.2010CB631205),the Fundamental Research Funds for the Central Universities of China(WUT:2013-IV-076),and Jingshun Liu acknowledges the support from Scienti?c Research Foundation of Inner Mongolia University of Technology of China(Grant No.ZD201405).

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