effect of heat-teatment by squeeze cating

Materials Science and Engineering A 506(2009)34–38

Contents lists available at ScienceDirect

Materials Science and Engineering

A

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m s e

a

Effects of heat-treatment on microstructures and mechanical properties of WO 3particle and Al 18B 4O 33whisker hybrid reinforced Al matrix composites by squeeze casting

L.Geng ?,Y.C.Feng,Z.Z.Zheng,J.Zhang,Q.W.Wang

School of Materials Science and Engineering,Harbin Institute of Technology,P.O.Box 433,Harbin 150001,PR China

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

Received 29July 2008

Received in revised form 7November 2008Accepted 7November 2008Keywords:

Aluminum matrix composites Interfacial reaction Mechanical properties Microstructure

a b s t r a c t

In this paper,WO 3p and Al 18B 4O 33w hybrid reinforced aluminum matrix composites were fabricated by squeeze casting.Heat-treatment of the composites was carried out at 500?C for different times.Gibbs free energy calculation indicated that the reaction of WO 3with Al was spontaneous.DTA result showed that the reaction of WO 3with Al took place at 396?C and 487?C.XRD results showed that the interfacial reaction occurred after heat-treatment at 500?C for 5h and 10h and the reaction products were WO 2.90and WO 2.83,which resulted in the formation of micro-voids at the interface between particle and matrix and led to mechanical properties of the composites decreasing.However,mechanical properties of the composites heat-treated for 10h did not decrease obviously compared with that of heat-treated for 5h.The reason may be that the produced aluminum oxide ?lm covered the WO 3p and retarded progress of the chemical reaction.

?2008Elsevier B.V.All rights reserved.

1.Introduction

WO 3possesses good photons absorbability because W is a high atomic number element and can absorb photons radiation effectively.Aluminum borate whisker has high strength and high Young’s modulus and low coef?cient of thermal expansion,which are competitive with those of SiC whisker.But the cost of alu-minum borate whisker (Al 18B 4O 33w,denoted as ABOw)is almost one thirtieth of that of SiC whisker [1].So WO 3particle (denoted as WO 3p)and Al 18B 4O 33whisker reinforced aluminum matrix hybrid composites (denoted as (WO 3p +ABOw)/Al)with good radiation shielding effectiveness and high mechanical properties,will have wide application prospects.

It is well known that interfacial characteristics in metal matrix composites reinforced with ceramic reinforcements play a signif-icant role in determining the mechanical properties,and many researches on the interface reaction and its effect on the mechanical properties of metal matrix composites have been done [2,3].

In the past,the interfacial reaction in ABOw reinforced alu-minum matrix composites (AMCs)has been extensively studied [4–8].The ABOw reacts with Mg contained in aluminum alloy at the temperatures higher than 517.2?C and with pure aluminum at the temperatures higher than 726.3?C [7].

?Corresponding author.Tel.:+8645186418836;fax:+8645186413922.E-mail address:genglingroup@https://www.360docs.net/doc/1817670982.html, (L.Geng).It has been proved that the interfacial reaction will lead to the formation of micro-voids at the interface due to the volume effect of the reaction and the decrease of mechanical proper-ties of the composites fabricated by powder metallurgy [9–11],so it is very important to control the interfacial reaction in AMCs containing the WO 3p.AMCs are usually heat-treated and plas-tic formed at elevated temperature up to 550?C [12–14],so it is also important to investigate the reaction thermodynamics of the interfacial reaction at elevated temperatures.However,the interfa-cial reaction of WO 3p in AMCs prepared by squeeze casting has not been studied to our knowledge.So in this paper,the inter-facial reaction between WO 3p and aluminum was studied,and the effects of heat-treatment on microstructures and mechani-cal properties of the (WO 3p +ABOw)/Al hybrid composite were discussed.

2.Experimental details

The (WO 3p +ABOw)/Al hybrid composites used here are pure aluminum (≥99.6wt.%Al)reinforced with 5vol.%WO 3p and 20vol.%ABOw.The size of WO 3p is around 3?m,the ABOw is with a diameter of 0.5–1?m and a length of 10–30?m.The (WO 3p +ABOw)/Al hybrid composites were fabricated by a squeeze casting technique.In the fabrication processes,the hybrid preform and the mould were preheated to 560?C,and then pure aluminum at 830?C was poured into the preheated mould and then a pressure of 60–80MPa was applied and held for 3min.

0921-5093/$–see front matter ?2008Elsevier B.V.All rights reserved.doi:10.1016/j.msea.2008.11.009

L.Geng et al./Materials Science and Engineering A506(2009)34–38

35

Fig.1.Dimensions of tensile specimen(mm).

The heat-treatment of the hybrid composites was performed at 500?C for5h and10h,respectively,in air furnace and then air-cooled to room temperature.

The phase compositions of as-cast and heat-treated compos-ites were identi?ed using a Philips X’Pert X-ray diffractometer with Cu K?radiation.The thermal analysis was performed using a Met-tler TG/SDTA851e differential thermal analysis apparatus with a heating rate of10?C/min under?owing Ar protection.The ten-sile tests were carried out on an Instron-1186tension machine at room temperature and the tensile rate was0.5mm/min.The sizes and dimensions of tensile specimens are shown in Fig.1.Tensile fractographs of the hybrid composites were investigated by scan-ning electron microscope(SEM)observation.The microstructures of the hybrid composites with and without heat-treatment were examined using a Philips CM-12transmission electron microscope (TEM).The specimens for TEM observation were thinned by ion milling.

3.Results and discussions

3.1.Thermal analysis

The WO3particles,which act as the oxygen sources,are ther-modynamically unstable with respect to matrix aluminum.So displacement reaction of WO3and aluminum occurs and more sta-ble oxide(alumina)is formed.The reaction formula of WO3particle and aluminum is as follows:

2Al+WO3=W+Al2O3(1) The Gibbs free energy of formula(1)can be calculated according to the two reaction formulas as follows[15]:

2

3

W+O2=23WO3(2) G0=?556,472+158.57T(3)

4

3

Al+O2=23Al2O3(4) G0=?1,115,454+209.2T(5) According to formulas(3)and(5),the Gibbs free energy of for-mula(1)is as follows at the temperatures from298K to932K:

G0=?838,473+75.945T(6) Because the Gibbs free energy of formula(1)at the temperatures from298K to932K is negative value,the reaction of WO3particle with aluminum is spontaneous.

In order to determine the reaction temperature of WO3particle with aluminum,the thermal analysis was performed.DTA curve of the as-cast(WO3p+ABOw)/Al hybrid composite is given in Fig.2. It was found that there were two exothermic peaks around the temperatures of396?C and487?C,which indicated that interfacial reactions took place in the hybrid composite.In order to study the effects of heat-treatment on the interfacial reaction of WO3

particle Fig.2.Differential thermal analysis curve of as-cast(WO3p+ABOw)/Al hybrid com-posite.

with aluminum in the composite,the heat-treatment temperature was determined to be500?C according to the DTA curve.

3.2.XRD analysis

XRD patterns of as-cast and heat-treated(WO3p+ABOw)/Al hybrid composites are shown in Fig.3.Only the peaks of Al,ABOw and WO3were found on the pattern of as-cast specimen.On the patterns of heat-treated specimens,the new peaks of WO2.90and WO2.83,namely blue tungsten oxide,were found.It is noticed that there is no peak of W in XRD patterns of all the three samples,indi-cating that the reaction of formula(1)did not occur.The formation of the WO2.90and WO2.83might be resulted from the following reaction:

Al+WO3→WO3?x+Al2O3(7) So far there is no available thermodynamic data of formula(7). However the DTA and XRD results in this study indicated that the reaction of formula(7)could take place.Moreover,the two exother-mic peaks around the temperatures of396?C and488?C were corresponding to the reaction of formation of WO2.90and WO2.83, respectively.This reaction should be controlled by the diffusion of oxygen atom and the WO2.90and WO2.83might be resulted from oxygen slightly reduced from WO3[9].The XRD results

showed

Fig.3.XRD patterns of as-cast and heat-treated(WO3p+ABOw)/Al hybrid compos-ites.

36L.Geng et al./Materials Science and Engineering A 506(2009)

34–38

Fig.4.TEM micrographs of as-cast and heat-treated (WO 3p +ABOw)/Al hybrid composites.(a)As-cast;(b)heat-treatment at 500?C for 5h;(c)heat-treatment at 500?C for 10h.

that interfacial reactions took place during heat-treatment and also proved that the exothermic peak in Fig.2was corresponding to the interfacial reaction temperature.3.3.Microstructures

TEM micrographs of as-cast and heat-treated (WO 3p +ABOw)/Al hybrid composites are shown in Fig.4.From the TEM micrograph of the as-cast specimen,it was found that there was a slight interfacial reaction between WO 3p and aluminum matrix during squeeze casting.However,the reaction product was not identi?ed in XRD pattern because the interfacial reaction during squeeze cast-ing was very slight.Because the reaction products were very ?ne and dispersive,they could not be identi?ed by selected area elec-tron diffraction pattern.From the TEM micrographs of heat-treated specimens,it was found that there were many micro-voids at the interface between WO 3p and aluminum matrix.These micro-voids would be deleterious to the mechanical properties of the compos-ites.

3.4.Tensile behavior

The stress–strain curves of as-cast and heat-treated (WO 3p +ABOw)/Al hybrid composites are given in Fig.5.Detailed tensile properties,the ultimate tensile strength ( b ),yield strength ( 0.2),elongation (?)and elastic modulus (E )are shown in Table 1.We could see that the mechanical properties of the hybrid compos-ites were deteriorated obviously by the https://www.360docs.net/doc/1817670982.html,pared with the as-cast composite,the b , 0.2,?and E of the com-posite heat-treated for 5h decreased by 12.7%,11.2%,56.9%and

4.6%,respectively.However,compared with the composite heat-treated for 5h,the decrement of b , 0.2,?and E of the composite heat-treated for 10h was only 7.4%,1.4%,3.4%and 2.4%,respec-tively.It was also seen from Fig.5that the stress–strain curves of the hybrid composites heat-treated for 5h and 10h were almost the same.These indicated that there was no obvious effect of heat-treatment time on the mechanical properties of the hybrid composite when the heat-treatment time was longer than 5h.Blaz et al.[11]reported that the aluminum oxide ?lms,which

were

Fig.5.Stress–strain curves of as-cast and heat-treated (WO 3p +ABOw)/Al hybrid composites.

L.Geng et al./Materials Science and Engineering A506(2009)34–3837

Table1

Mechanical properties of as-cast and heat-treated(WO3p+ABOw)/Al hybrid composites.

Ultimate tensile strength(MPa)Yield strength(MPa)Elongation(%)Elastic modulus(GPa)

As-cast290.3180.5 1.3787

Heat-treatment for5h253.3160.20.5983

Heat-treatment for10h234.5157.90.5781

formed by the reaction of WO3with aluminum,covered the W-rich particles and retarded the further progress of the chemical reaction and effectively reduced the formation rate of micro-voids around the particles.So the microstructure change of the heat-treated com-posites was not obvious with further increase of heat-treatment time after5h,and so the mechanical properties of the heat-treated composites changed very little.

Fracture morphologies of as-cast and heat-treated(WO3p+ ABOw)/Al hybrid composites are given in Fig.6.All the compos-ites had a brittle fracture characteristic on a macroscopic scale.On the fracture surfaces of all the composites,there were many bared particles or their marks due to the debonding of interface between reinforcements and matrix.For the heat-treated composites,there were many visible cracks between WO3particles and Al matrix. These indicated that the interfacial strength of the heat-treated hybrid composites was lower.A poorly bonded interface gave way to debonding and subsequent crack growth in the matrix[16].

3.5.Discussion

By analyzing thermodynamics of the reaction of WO3p with alu-minum,we knew that the reaction formula(1)was a spontaneous reaction at the temperatures from298K to932K.From the result of DTA analysis,we knew that WO3p could react with aluminum at the temperatures of about396?C and487?C.The XRD results showed the formation of WO2.90and WO2.83in the heat-treated composites,indicating that the reaction of formula(7)took place practically in the composites.

Compared with tungsten,aluminum has a higher af?nity to the oxygen,so the chemical reaction of formula(7)should be sponta-neous in theory.During the reaction,oxygen atoms in the WO3p diffused to the interface and reacted with aluminum,so the reac-tion should be controlled by diffusion of oxygen atom in the WO3p. According to the results of Blaz et al.[11],aluminum oxide?lms were formed by the reaction of WO3with aluminum.However,the aluminum oxide phase was not detected by the XRD result in this study,which might be due to the small amount of the aluminum oxide and the weaker absorbability to X-ray.The TEM observation had shown the existence of the second phases obviously.However, because of the?ne size of the interfacial phases,we could not deter-mine their structure by selected area diffraction in the TEM.It is supposed that the interfacial products might be WO2.90,WO

2.83

and

aluminum oxide.

The produced aluminum oxide?lm covered the WO3p and

retarded the further progress of the chemical reaction,so when

the aluminum oxide?lm was produced,the chemical reaction rate

Fig.6.Fracture morphologies of as-cast and heat-treated(WO3p+ABOw)/Al hybrid composites.(a)As-cast;(b)heat-treatment at500?C for5h;(c)heat-treatment at500?C for10h.

38L.Geng et al./Materials Science and Engineering A506(2009)34–38

became slower.Therefore,even though the hybrid composites were held at500?C for longer than5h,the interfacial reaction could not happen obviously any more,and there was no obvious effect on the microstructures and properties of the composites when the heat-treatment time was longer than5h.

Blaz et al.[11]reported that the reaction of WO3p with alu-minum could lead to a volume reduction of32.6%,which accounted for the formation of micro-voids at the interface between WO3p and aluminum matrix.The produced micro-voids became crack sources under tensile stress and the cracks propagated rapidly along the interface between WO3p and aluminum matrix,which led to the mechanical properties of the composites declining.

4.Conclusions

(1)When the(WO3p+ABOw)/Al hybrid composites were heat-

treated at500?C for5h and10h,an obvious interfacial reaction between WO3p and aluminum took place.The reaction hap-pened at about396?C and above,and the reaction products were WO2.90and WO2.83.

(2)Many micro-voids were formed at the interface due to the inter-

facial reaction,which led to an obvious decrease of mechanical properties of the composites after heat-treatment at500?C for 5h and10h compared with the as-cast composite.

(3)There was no obvious effect of heat-treatment time on the

microstructures and properties of the composites when the heat-treatment time is longer than5h,which might be attributed to the formation of aluminum oxide?lms at the inter-

face between WO3p and aluminum matrix which could inhibit the further progress of the interfacial reaction. Acknowledgements

The?nancial support provided by the key program for Hei-longjiang province Natural Science Foundation(contract number: ZJC0604)and“973”program of China(Project No.2008CB617604) is gratefully acknowledged.

References

[1]K.Suganuma,T.Fujita,N.Suzuki,K.Niihara,J.Mater.Sci.Lett.9(1990)633–635.

[2]M.Y.Zheng,K.Wu,M.Liang,S.Kamado,Y.Kojima,Mater.Sci.Eng.A372(2004)

66–74.

[3]H.Y.Yue,W.D.Fei,L.D.Wang,Mater.Sci.Eng.A472(2008)231–234.

[4]J.Hu,J.X.Zhang,S.W.Tang,W.C.Ren,Mater.Sci.Eng.A433(2006)94–99.

[5]W.G.Chu,W.D.Fei,W.Yang,C.K.Yao,Mater.Chem.Phys.68(2001)56–61.

[6]G.Bi,H.W.Wang,R.J.Wu,D.Zhang,Z.L.Shi,Trans.Nonferrous Met.Soc.China

9(1999)785–790.

[7]L.J.Yao,G.Sasaki,H.Fukunaga,Mater.Sci.Eng.A225(1997)59–68.

[8]J.Hu,W.D.Fei,C.K.Yao,Mater.Sci.Eng.A333(2002)170–175.

[9]A.Torosyan,L.Takacs,J.Mater.Sci.39(2004)5491–5496.

[10]L.Blaz,J.Kaneko,M.Sugamata,Mater.Chem.Phys.81(2003)387–389.

[11]L.Blaz,J.Kaneko,M.Sugamata,R.Kamei,Mater.Sci.Eng.A349(2003)111–119.

[12]P.Cavaliere,E.Evangelista,Compos.Sci.Technol.66(2006)357–362.

[13]C.S.Ramesh,M.Sa?ulla,Wear263(2007)629–635.

[14]S.V.S.Narayana Murty,B.Nageswara Rao,B.P.Kashyap,Compos.Sci.Technol.

63(2003)119–135.

[15]S.K.Wei,Thermodynamics of Metallurgical Process,Shanghai Scienti?c&Tech-

nical Publishers,Shanghai,1980.

[16]S.Ozden,R.Ekici,F.Nair,Composites:Part A38(2007)484–494.