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Journal of Alloys and Compounds 486 (2009) 185–190

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journa l homepage: www.e lsev ier .com/ locate / ja l l com

icrostructure and grain refining performance of a new Al–Ti–C–B master alloy

infeng Nie, Xiaoguang Ma, Haimin Ding, Xiangfa Liu ∗

ey Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China

r t i c l e i n f o

rticle history:eceived 23 May 2009eceived in revised form 27 June 2009ccepted 29 June 2009

a b s t r a c t

A kind of Al–Ti–C–B master alloy with a uniform microstructure is prepared using a melt reaction method.It is found that the average grain size of �-Al can be reduced from 3500 to 170 �m by the addition of0.2 wt.% of the prepared Al–5Ti–0.3C–0.2B and the refining efficiency does not fade obviously within60 min. It is considered that the TiCxBy and TiB2−mCn particles found at the grain center are the effective

vailable online 5 July 2009

eywords:luminum alloyl–Ti–C–B master alloyicrostructure

and stable nucleating substrates for �-Al during solidification, which accounts for the good grain refiningperformance.

© 2009 Elsevier B.V. All rights reserved.

rain refinementucleation

. Introduction

As we know, Al–Ti–B master alloys have become the preferredhoice for the grain refinement of wrought aluminum alloys inndustry [1,2]. However, Al–Ti–B master alloys still have some limi-ations in certain circumstances [3–6]. For example, they are notuitable to be used in alloys containing Zr, Cr or V due to theoisoning effect. Otherwise, the coarse insoluble TiB2 particlesgglomerate easily, which not only causes porosity, streaking inoils and internal cracking in extrusion billets, but also damageshe surface of the roller. Afterward, it is found that TiC particles inl–Ti–C masters alloy are less prone to agglomeration than TiB2,

hus Al–Ti–C grain refiners could have superiority in foils produc-ion except for the serious fading behavior due to the intrinsicnstability of TiC particles in the melt [7–9]. Therefore, it is nec-ssary to develop a new kind of grain refiners which could be freerom the problems mentioned above.

In 1990s, Jiang prepared a new kind of Al–Ti–C–B master alloyhich showed a better refining performance than Al–Ti–C andl–Ti–B master alloys, but it was not applied in industrial produc-

ion [10]. So, it is supposed that the refinement could be improvedreatly if they work together properly in the melt, because TiC and

iB2 share some similarities in improving the grain refining effi-iency of Al–Ti remarkably.

The present study aims to find an effective method to combinehe grain refining effect of TiC and TiB2 particles, and prepare a

∗ Corresponding author. Tel.: +86 531 88392006; fax: +86 531 88395414.E-mail address: xfliu@sdu.edu.cn (X. Liu).

925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2009.06.190

master alloy which can overcome the problems associated withborides agglomeration and the carbides instability. In this paper,a new Al–5Ti–0.3C–0.2B master alloy with a uniform microstruc-ture is prepared. Meanwhile, the improvement of grain refiningperformance is analyzed.

2. Experimental procedure

The Al–5Ti–0.3C–0.2B master alloy was prepared by pure Ti, graphite pow-der, Al–3B master alloy and commercial pure Al using a melt reaction method ina medium frequency furnace. The Al–5Ti–0.3C–0.3B was prepared on the basis ofAl–5Ti–0.4C by the same method and TiB2 particles were added in the form ofAl–5Ti–1B rod. In fact, it is a mixture of Al–5Ti–0.4C and Al–5Ti–1B master alloys. TheAl–3Ti–0.15C master alloy used for comparison is also a good grain refiner preparedby a melt reaction method. In addition, the master alloys used in the experiment arepresented in Table 1.

The comparisons of grain refining performance between 0.2%M1, 0.2%M2 and0.4%M3 master alloys were carried out using commercial pure Al (99.7%, all compo-sitions quoted in this work are in wt.% unless otherwise stated) as the base alloyswith the same procedures. At first, the base alloy was melted in an electrical resis-tance furnace at 720 ◦C. After 20 min holding, the grain refiner was added into themelt and stirred thoroughly. Then it was poured at regular intervals from 5 to 60 mininto a pre-heated KBI cylindrical steel mold with size of Ø50 mm × 25 mm on a fireclay brick. After solidification, the bottom surface of the refined sample in contactwith the brick was etched by a reagent (60% HCl + 30% HNO3 + 5% HF + 5% H2O, thecompositions here are in volume fraction). At last, the pictures of macrostructurewere taken for each sample by a high scope video microscope (HSVM, KH-2200),and the average grain sizes were determined using the linear intercept method.

Metallographic specimens were taken from the center of each sample in thetransverse section, then mechanically ground and polished through standard rou-tines. The microstructure characterization and microanalysis of the samples wereinvestigated by X-ray diffraction (XRD, Rigaku D/max-rB) electron probe microanal-ysis (EPMA, JXA8800R) and field emission scanning electron microscope (FESEM,SU-70).

186 J. Nie et al. / Journal of Alloys and Com

Table 1Preparation of the experimental master alloys.

Alloy designation Preparation methodAl–5Ti–0.3C–0.2B M1 Melt reaction methodAl–5Ti–0.3C–0.3B M2 Mixing Al–5Ti–0.4C and Al–5Ti–1BAl–3Ti–0.15C M3 Melt reaction method

3

3m

Tt

wiwdlp

a2Ftis

Fig. 1. X-ray diffraction pattern of M1 master alloy.

. Results and discussion

.1. Microstructure and microanalysis of Al–5Ti–0.3C–0.2Baster alloy

The M1 master alloy mainly contains three kinds of phases —iC, TiB2 and TiAl3 — embedded in the Al matrix, as evidenced byhe XRD pattern (Fig. 1).

Fig. 2a shows the uniform microstructures of M1 master alloyith plate-like TiAl3 and TiC and TiB2 particles in the matrix. As

llustrated in the magnified microstructure (Fig. 2b), the particlesith a mean size of 2.5 �m are disconnected with each other and

ispersed in the Al matrix homogeneously. Many particles are veryarge with a size above 3 �m. It is also found that lots of smallerarticles with sub-micron size surrounding those bigger ones.

The FESEM line-scanning analysis was carried out to identify TiCnd TiB2 particles in the M1 alloy. Two individual particles about

�m in size along the line A–B were analyzed as shown in Fig. 3a.

rom the main diffraction peaks illustrated in Fig. 3b, the first par-icle is TiC and the second one is TiB2. More particles were analyzedn the sample and most of TiC and TiB2 particles are in an irregularhape.

Fig. 2. Microstructures o

pounds 486 (2009) 185–190

In addition, it is suggested from the analysis result that TiC andTiB2 in M1 are different from the common TiC in Al–Ti–C or TiB2 inAl–Ti–B. As illustrated in Fig. 3b, an obvious B peak is correspondingto Ti and C ones in the position of TiC particle, and a C peak iscorresponding to Ti and B ones in the position of TiB2. It revealsthat there is a trace amount of B in TiC and trace C in TiB2 particles.Therefore, two kinds of TiCxBy and TiB2−mCn solid solutions haveformed in the M1 master alloy.

According to the literatures [11,12], the reactions occurred in theM2 master alloy at the present experiment are:

3Ti + C(s) + 2B + 3Al(l) → TiC(s) + TiB2(s) + TiAl3(s) (1)

6Ti + Al4C3(s) + 2B + 2Al(l) → 3TiC(s) + TiB2(s) + 2TiAl3(s) (2)

Ti + C(s) → TiC(s) (3)

Ti + 3Al(l) → TiAl3(s) (4)

C(s) + Al(l) → Al4C3(s) (5)

where Ti and B are solutes and the Al4C3 can be formed throughreaction (5) in the melt at about 890 ◦C [8]. The temperature forreactions (1) and (2) is above 1200 ◦C, so the Ti, C(s) (or Al4C3) andB can react effectively and the relevant phases formed in the matrix.Just for the similarity of C and B in atomic radius, it is possible forthe formation of TiCxBy and TiB2−mCn solid solutions. Because theamount of C in the melt is a little larger than that of B, the reaction(3) also occurs at the same time.

From the above microstructure characterization and formationmechanism analysis, it is known that the distribution, morphologyand composition of carbides and borides coexisted in M1 masteralloy differ from those in the common Al–Ti–C and Al–Ti–B [4,12,13].

3.2. Grain refinement of Al–5Ti–0.3C–0.2B master alloy

Fig. 4 shows the macrostructures of samples refined by the threedifferent master alloys after 5 and 60 min holding. Compared to thegrains refined by M2 and M3 master alloy, those refined by M1master alloy are much finer in a short and long holding time. It isindicated that the grain refining efficiency of M1 master alloy hasbeen significantly improved by adding a small amount of B in thepreparing process.

Fig. 5 displays the average grain size variation as a function ofholding time. It is seen that the average grain size of �-Al refined

by M3 is higher than 200 �m and increases rapidly after 15 min.The finest grain size of �-Al refined by M2 is about 210 �m whichis obtained within 15 min holding and also increases rapidly withprolonged time. However, the average grain size of �-Al refined byM1 is kept below 200 �m within 60 min holding.

f M1 master alloy.

J. Nie et al. / Journal of Alloys and Compounds 486 (2009) 185–190 187

SEI im

scwtttpoiTm

3

to

Fig. 3. Microanalysis of particles in M1: (a)

If the grain refining efficiency is represented by the average grainize refined by different refiners, the improvement of refining effi-iency can be measured by the decrement of grain size refinedith M1 compared to that refined with M2 or M3. Fig. 6 shows

he improvements of refining efficiency by M1 at different holdingime compared to the other two master alloys. It is suggested fromhe curves and column-map that M1 shows a best grain refiningerformance among the three grain refiners at the same additionf C or B, especially a good fading resistance. Besides, the grain refin-

ng difference between M1 and M2 also indicates that the in-situiC and TiB2 particles formed simultaneously in the melt behave auch better refining effectiveness than that formed separately.

.3. Grain refinement mechanism

According to the most literatures [2,12–15], the morphology, dis-ribution and quantity of TiC or TiB2 particles have major impactsn the grain refining efficiency. With increase of the quantity of

age; (b) line-scanning analysis along A–B.

nucleation substrates and the degree of dispersion of TiC and TiB2particles, the grain refining efficiency and the fading problem canboth be improved. As shown in the microstructure of M1, a largenumber of TiC and TiB2 particles are dispersed in the Al matrix. It isconsidered that the improved dispersion of nucleation substratesis a reason for the improved performance of M1 master alloy.

Crystallization nuclei in aluminum grains refined by M1 mas-ter alloy were analyzed and the analysis results shown in Fig. 7.Two particles located at the grain center and a rose-like structurearound them can be seen in Fig. 7a. The EPMA analysis (Fig. 7b)indicates that the bigger particle is TiCxBy and the smaller one is TiCwithout obvious B. At another grain center, it can be seen severalparticles about 2 �m in size surrounded by some much smaller ones

shown in Fig. 7c. It is confirmed from EPMA analysis (Fig. 7d) thatthe particles are enriched with Ti, C and B. Therefore, it is suggestedthat TiCxBy and TiB2−mCn particles can act as nucleating substratesfor �-Al during solidification. However, the particles at the secondgrain center are not as dispersive as that in the master alloy. It is

188 J. Nie et al. / Journal of Alloys and Compounds 486 (2009) 185–190

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ig. 4. Grain macrostructures of �-Al refined with different master alloys: (a) unr.2%M2 holding for 5, 60 min; (f–g) refined with 0.4%M1 holding for 5, 60 min.

upposed that the small particles with sub-micron size are prone todherence to the big ones to nucleate �-Al together. Thus the mor-hology of nuclei appears as diversion, which shows the complexityf nucleation during solidification process.

�-Al grain; (b–c) refined with 0.2%M1 holding for 5, 60 min; (d–e) refined with

At present, there are few literatures about the co-effectivenessof TiC and TiB2 on the grain refinement of �-Al. Naglic [16] sug-gested that Al–3Ti–0.15C was ineffective in the case of both additionof Al–5Ti–1B and Al–3Ti–0.15C grain refiners to Al–Fe alloy, which

J. Nie et al. / Journal of Alloys and Compounds 486 (2009) 185–190 189

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tions were observed when Al–Ti–C and Al–Ti–B grain refiners were

Fn

ig. 5. Grain refining performance of M1, M2 and M3 master alloys in commercialure aluminum.

ndicated that a simple mixture of TiC and TiB2 particles cannot

mprove the refining performance effectively. It has also been pre-ented in the literature [16] that a simple mixing of different grainefining particles can cause the ineffectiveness of any one type of thearticle. Besides, it was showed in Ref. [18] that no adverse interac-

ig. 7. Microanalysis of grain centers of �-Al refined by M1master alloy: (a) SEI image of aucleus; (d) line-scanning analysis along C–D.

Fig. 6. The improvements of grain refining performance of M1 compared to M2, M3master alloy at the same addition of C.

mixed in different rations. However, it is suggested in the presentstudy that the TiC and TiB2 particles can work together effectively ifthey are combined in a proper method. It is evident that M1 masteralloy with in-situ TiC and TiB2 behaves a much better grain refining

grain nucleus; (b) line-scanning analysis along A–B; (c) SEI image of another grain

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90 J. Nie et al. / Journal of Alloys a

erformance than M2 which is a simple mixture of Al–5Ti–0.4C andl–5Ti–1B in fact as shown in Figs. 4 and 5.

As illustrated in Ref. [17], the �-Al nucleates only on TiB2 parti-les when both Al–5Ti–1B and Al–3Ti–0.15C master alloys are addedo the melt. However, it can be found in Fig. 7c that both TiCxBy andiB2−mCn particles exist in the grain center. Therefore, it is indicatedhat both TiCxBy and TiB2−mCn particles in M1 can act as effectiveucleating substrates for �-Al.

Many studies [19–21] pointed that the C/Ti atom ratio in TiC wasot 1:1 and the substoichiometric TiCx particles with a wide x range,pproximately from TiC0.49 to TiC0.98 without the change of crystaltructure, usually existed in the Al–Ti–C system. Refs. [22–24] alsouggested that a large number of carbon-atom vacancies existed iniCx which seriously influenced the structural stability and somether properties. Thus the corresponding grain refinement potencyf TiC may also be weakened. So it is considered that the fadingehavior of the common Al–Ti–C master alloy is not only due tohe intrinsic instability of TiCx particles [7] but also related to theacancies in them.

According to the experimental results shown in Figs. 3 and 7, it isndicated that B or C atoms actually diffuse into the opposite TiC oriB2 particles and then substitute the vacancies in them. As a result,he amount of the carbon-atom vacancy defects in TiCxBy solidolution is decreased, which can make the whole structure moretable [22,23]. More importantly, the crystal structure of TiC is nothanged. It is also indicated in Ref. [23] that a very small decreasef the lattice parameter over a large of stoichiometry, TiC–TiC2, ofhe order of 1% is only found. Thus the lattice change is negligi-le. According to the above analysis, the nucleating capability ofiCxBy may be improved, and thus the excellent fading resistance

s obtained.

. Conclusions

A kind of Al–5Ti–0.3C–0.2B master alloy with a uniformicrostructure is prepared using a melt reaction method. It is con-

rmed by line-scanning analysis that B or C atoms can diffuse intohe opposite TiC or TiB2 particles when they are formed simultane-

usly in the melt.

Compared to Al–3Ti–0.15C and Al–5Ti–0.3C–0.3B master alloy,l–5Ti–0.3C–0.2B master alloy behaves a much better grain refin-

ng performance. The average grain size of �-Al can be reducedrom 3500 to 170 �m by the addition of 0.2 wt.% the prepared

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pounds 486 (2009) 185–190

Al–5Ti–0.3C–0.2B. Especially the improved grain refining efficiencydoes not fade obviously within 60 min.

It is supposed that the TiCxBy and TiB2−mCn particles found atthe grain center are the effective nucleating substrates for �-Al dur-ing solidification. Improved structural stability of TiCxBy due to thecarbon-atom vacancies substituted by B atoms may be an impor-tant reason for the improvement of grain refining efficiency andfading resistance.

Acknowledgments

This work was supported by a grant from National Science Fundfor Distinguished Young Scholars (No. 50625101), Key Project ofScience and Technology Research of Ministry of Education of China(No. 106103) and “Taishan Scholar” Construction Project for finan-cial support of Shandong Province in China.

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