CommunicationFormation of Small Blocky Al3TiParticles via Direct Reaction BetweenSolid Ti Powders and Liquid Al

Z.W. LIU, Q. HAN, and J.G. LI

The evolution of titanium powders in the pure alumi-num melt at a lower temperature was studied in ourresearch. The process involved some titanium powdersbeing added into the pure aluminum melt at 1003 K(730 �C), and then the melt was cast into an ingot after 5minutes. A reaction layer composed of some loose Al3Tiparticles was formed on the solid Ti surface due to thereactive diffusion between titanium and aluminum.In-situ blocky Al3Ti particles smaller than 5 lm wereproduced in the aluminum matrix. A reaction-peelingmodel was suggested to illustrate the formation mech-anism of Al3Ti particles, and a simple approach forfabricating in-situ Al3Ti/Al-alloy composites was pro-posed as well.

DOI: 10.1007/s11661-012-1326-y� The Minerals, Metals & Materials Society and ASMInternational 2012

In-situ Al3Ti/Al-alloy composites have attracted con-siderable attention recently due to the excellent proper-ties of Al3Ti phase, such as its high melting point, lowdensity, high Young’s modulus, and good wet abilitywith aluminum.[1] Particularly, the Al3Ti phase also hasan excellent resistance to oxidation and corrosion in thefluoride atmosphere above the melting point of alumi-num.[2] Several manufacturing techniques have beendeveloped in the fabrication of in-situ Al3Ti/Al-alloycomposites, such as powder metallurgy,[3] the centrifugalsolid-particle method,[4] mechanical alloying,[5] and thein-situ casting technique.[6,7]

In general, the in-situ casting technique is one ofsimplicity, economy, and high efficiency and is regardedas a prospective technique for commercial production ofaluminum matrix composites.[8] Wang et al.[6] fabricatedin-situ Al3Ti/Al-alloy composites by adding K2TiF6 intothe molten aluminum alloy at 1223 K to 1323 K (950 �Cto 1050 �C). They found that the size and morphology ofAl3Ti were largely affected by the temperature, holdingtime, and composition of the flux. In their research, mostAl3Ti particles were rodlike in morphology and longer

than 20 lm. Yu et al.[7] prepared Al-Al3Ti in-situ com-posites via the direct reaction method by adding themixture of aluminum and titanium powders into themolten pure aluminum at 1173 K to 1273 K (900 �C to1000 �C). They found that the size of Al3Ti particlesincreased with the increase of Al3Ti content. Most Al3Tiparticles synthesized in their research were rodlike inmorphology and were larger than 50 lm when the Al3Ticontent was more than 5 wt pct.In our research, titanium powders were used as the

only additive and were added into the pure aluminummelt at 1003 K (730 �C). The aim of the research is tostudy the evolution of titanium powders in the moltenaluminum at a lower temperature and to explore a simpleapproach of producing in-situ Al composites as well.Pure aluminum (99.5 pct commercial purity) ingot

and titanium powders (99.7 pct purity, 40 lm) wereused as the matrix and additive, respectively. Sometitanium powders wrapped in aluminum foil were addedinto the molten pure molten aluminum at 1003 K(730 �C) in a graphite crucible in an electrical resistancefurnace. Then the aluminum melt was stirred by using agraphite rod to disperse the solid titanium powders inthe aluminum melt, preventing the titanium powdersbeing sintered together. The amount of titanium pow-ders added corresponded to the ration of Al-4 wt pct Ti.In order to reflect the evolution process of the solidtitanium particles in the liquid aluminum, the melt washeld around 5 minutes and then cast into a metal moldto form an ingot.The phase formed during the fabrication was analyzed

by X-ray diffraction (XRD, Bruker D8, Bruker AXS,Karlsruhe, Germany) using CuKa radiation at 40 kV and40mAand a scan rate of 0.015 deg/s. Themicrostructuralfeatures of the materials were examined by scanningelectron microscopy (SEM, JEOL* 6400) equipped with

energy dispersive spectroscopy (EDS). The size of thein-situ formed particles was analyzed by using ImageJsoftware (LECO Corporation, St. Joseph, MI).While adding the titanium powders into the pure

molten aluminum, chemical reaction with the radiancewas observed in the melt. The main exothermic reactionoccurred as follows: 3Alþ Ti ¼ Al3Ti:Figure 1 shows the XRD pattern of the sample

fabricated in our research. The figure illustrates thatthe Al3Ti phase was formed. In addition, some weaktitanium peaks are present, suggesting that titaniumpowders were unable to react with aluminum completelydue to the short holding time. The Al3Ti phase can beformed preferentially in the Al (l)-Ti (s) system due to itslower free energy of formation than AlTi3 and AlTi,[9] aswell as the rich-Al environment in our research.Figure 2(a) shows the typical microstructure of the

sample. Blocklike particles identified as Al3Ti by EDSwere in-situ formed in the aluminum matrix, most ofwhich were smaller than 5 lm in size. Some looselyaggregated Al3Ti particles were observed as well. In

Z.W. LIU, Postdoctoral Candidate, and J.G. LI, Professor, are withthe School of Materials Science and Engineering, Shanghai Jiao TongUniversity, Shanghai 200240, P.R. China. Contact e-mail: lzw_6260@163.com Q. HAN, Professor, is with the Department ofMechanical Engineering Technology, Purdue University, WestLafayette, IN 47906. Contact e-mail: hanq@purdue.edu

Manuscript submitted March 5, 2012.

*JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.

METALLURGICAL AND MATERIALS TRANSACTIONS A

addition, some large titanium powders were unable toreact with aluminum completely due to the short holdingtime. As a result, these titanium particles were covered byloose reaction layers, as shown in Figure 2(b).

A higher magnification of the image in the markedarea in Figure 2(a) was obtained for further study on thereaction layer, as shown in Figure 2(b). It is obviousthat the reaction layer was composed by some Al3Tiparticles, which became looser from the remainingtitanium to the aluminum matrix, and some separatedparticles were distributed outside the reaction layers aswell. Also, some inter-Al3Ti particle spacings existed inthe reaction layer, as shown in Figure 2(b).

Furthermore, ImageJ software was used to analyzethe size distribution of Al3Ti particles existing in thematrix. The result shows that around 90 pct, Al3Tiparticles were smaller than 5 lm in size and theiraverage size was about 3.5 lm, following the Gaussiandistribution well, as shown in Figure 3.

In order to illuminate the formation mechanism of theblocklike Al3Ti particles with size smaller than 5 lm, aschematic illustration about the evolution of titaniumpowders in the liquid aluminum is shown in Figure 4, inwhich a single titanium particle was regarded as a ball.A reactive diffusion can take place at the Al/Ti interfaceas soon as the solid titanium powder contacts the liquidaluminum. In the temperature range of 973 K to 1123 K(700 �C to 850 �C), the Arrhenius equation for thediffusion coefficients for aluminum in solid titanium(DAl/Ti)

[10] and the diffusion coefficients for solid tita-nium in liquid aluminum (DTi/Al)

[11] can be expressed asthe following equations, respectively.

DAl=Ti ¼ 9:58� 10�9 m2=s � exp �114; 600 J=mol

8:31 J/molK � T

� �

DTi=Al ¼ 4:29� 10�7 m2=s � exp �36; 300 J=mol

8:31 J/molK � T

� �

Fig. 1—XRD pattern of the sample fabricated by means of addingthe titanium powders into the molten aluminum at 1003 K (730 �C)for 5 min.

Fig. 2—(a) Typical microstructure of the sample fabricated in our research. (b) Higher magnification of image in the marked area in (a) andEDS analysis.

Fig. 3—Size distribution of Al3Ti particles in the aluminum matrix.

METALLURGICAL AND MATERIALS TRANSACTIONS A

When T = 1003 K (730 �C), the values of DAl/Ti andDTi/Al are 1.029 10�14 and 5.519 10�9 m2/s, respec-tively. This finding illustrates that solid titanium is themain diffusion species in the Al (l)-Ti (s) system, for thevalue ofDTi/Al around 105 times that ofDAl/Ti. The initialdiffusion of the titanium atoms across the Ti/Al interfaceinto the liquid aluminumcanproduce a saturated solutionadjacent to the interface, resulting in the nucleation ofAl3Ti phase on the solid titanium surface at the Ti/Alinterface. The time for the formation of the Al3Ti nucleiwas named t1, as shown in Figure 4(a).

As the time increased from t1 to t2, the Al3Ti phasecontinued to grow due to the further diffusion of

titanium atoms, as shown in Figure 4(b). According tothe Ti-Al phase diagram,[12] the Al3Ti phase and theAl-rich phase could be produced, because the aluminumconcentration is higher than 75 at. pct. An importantfeature is that the solidus temperature is 938 K (665 �C),which is lower than the temperature of the moltenaluminum. Thereby, the Al3Ti phase was in the solidstate and the aluminum-rich phase was in the liquidstate, respectively. The growth of Al3Ti is radial ratherthan planar due to the spherical shape of the Tiparticles. Owing to the growing type of Al3Ti and theAl-rich liquid phase, some Al3Ti particle spacing existedin the reaction layer (Figure 2(b)).

Fig. 4—Schematic illustration showing the formation mechanism of the blocklike Al3Ti particles (t is the holding time, and t1 < t2 < t3):(a) nucleation of Al3Ti, (b) growth of Al3Ti, and (c) rupture of Al3Ti.

METALLURGICAL AND MATERIALS TRANSACTIONS A

The existence of the particle spacing in the Al3Ti layeras well as the brittleness of the Al3Ti compound couldlead to an easier rupture of Al3Ti particles from thereaction layer. On the other hand, Kwark et al.[12]

reported that the position of formation of the Al3Ti andthe fact that its value was greater than the volume oftitanium consumed were considered to give rise tostresses in the reaction layer. During the growth of theAl3Ti phase, tensile stress was developed in the reactionlayer parallel to the titanium surface, resulting in therupture of the Al3Ti phase. Furthermore, the rupture ofthe Al3Ti phase was also attributed to the shear stressproduced by the liquid aluminum flow. In addition,some works[13–15] reported that the fracture strength of ametal can be reduced in a liquid metal, because theliquid metal affects the fracture behavior at the tip of thecrack, reducing the critical stress intensity for fractureand altering the micromechanism of fractures at thecrack tip, which was named liquid metal inducedembrittlement. Liquid aluminum may give rise to asimilar effect on the fracture of the Al3Ti phase. Basedon the preceding analysis, the Al3Ti phase could beeasily migrated away from the reaction layer during thereactive diffusion, as shown in Figure 4(c).

The rupture of the Al3Ti layer enabled the othertitanium to be in intimate contact with liquid aluminum.As a result, more Ti phase would be transformed intoAl3Ti. Then Al3Ti particles could be peeled off from thereaction layer according to the preceding ruptureprocess of Al3Ti. In the meantime, the reaction layerwould move toward the remaining solid titanium. As theholding time increased, the whole titanium powderscould react with liquid aluminum completely to eventu-ally form Al3Ti particles. As far as the evolution of thesolid titanium powders in the liquid aluminum at1003 K (730 �C) for 5 minutes was concerned, somelarge size titanium particles covered by the reactionlayers were kept in the aluminum matrix after thecasting, as shown in Figure 2.

Based on the preceding analysis, Al3Ti particles couldbe peeled off quickly in the liquid aluminum. As a result,in-situ formed Al3Ti particles had a limited time to grow,leading to the blocky morphology and small size.

The evolution of the solid titanium powders in the liquidaluminumwere studied bymeans of adding titanium in themolten pure aluminum at 1003 K (730 �C) for 5 minutes.The following conclusions are drawn.

1. In-situ formed Al3Ti particles are blocky in mor-phology and smaller than 5 lm in size.

2. A reaction-peeling model is suggested to explainthe formation mechanism of small blocky Al3Tiparticles.

3. The method of adding titanium powders into liquidaluminum can be used to fabricate in-situ Al3Ti/Al-alloy composites.

This research was supported by the United StatesDepartment of Energy under Award No. DE-EE0001100,the North American Die Casting Association, and theChina Scholarship Council of the Chinese Ministry ofEducation.

REFERENCES1. J.M. Wu, S.L. Zheng, and Z.Z. Li: Mater. Sci. Eng. A, 2000,

vol. 289, pp. 246–54.2. L. Arnberg, L. Backerud, and H. Klang: Met. Technol., 1982,

vol. 9, pp. 7–13.3. V. Abbasi Chianeh, H.R. Madaah Hosseini, and M. Nofar:

J. Alloys Compd., 2009, vol. 473, pp. 127–32.4. P.D. Sequeira, Y. Watanabe, and Y. Fukui: Scripta Mater., 2005,

vol. 53, pp. 687–92.5. S.H. Wang, P.W. Kao, and C.P. Chang: Scripta Mater., 1999,

vol. 40, pp. 289–95.6. X.M. Wang, A. Jha, and R. Brydson: Mater. Sci. Eng. A, 2004,

vol. 364, pp. 339–45.7. H.S. Yu, H.M. Chen, L.M. Sun, and G.H. Min: Rare Met., 2006,

vol. 25, pp. 32–36.8. J.V. Wood, P. Davies, and J.L.F. Kellie: Mater. Sci. Technol.,

1993, vol. 9, pp. 833–40.9. L.M. Peng, J.H. Wang, H. Li, J.H. Zhao, and L.H. He: Scripta

Mater., 2005, vol. 52, pp. 243–48.10. M. Thuillard, L.T. Tran, and M.A. Nicolet: Thin Solid Films,

1988, vol. 166, pp. 21–27.11. Y. Du, Y.A. Chang, B.Y. Huang, W.P. Gong, Z.P. Jin, H.H. Xu,

Z.H. Yuan, Y. Liu, Y.H. He, and F.Y. Xie: Mater. Sci. Eng. A,2003, vol. 363, pp. 140–51.

12. J.S. Kwark, S.E. Mohney, J.Y. Lin, and R.S. Kern: Semicond. Sci.Technol., 2000, vol. 15, pp. 756–60.

13. S.P. Lynch: Scripta Metall., 1984, vol. 18, pp. 509–13.14. N.S. Stoloff and T.L. Johnston: Acta Metall., 1963, vol. 11,

pp. 251–56.15. R.E. Clegg: Eng. Fract. Mech., 2001, vol. 68, pp. 1777–90.

METALLURGICAL AND MATERIALS TRANSACTIONS A