Replacement with Each Other of Ti and Zr in the Intermetallics of Ale(Sie) TieZr Alloys

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1 Available online at SciVerse ScienceDirect J. Mater. Sci. Technol., 2013, 29(3), 291e296 Replacement with Each Other of Ti and Zr in the Intermetallics of Ale(Sie) TieZr Alloys Tong Gao, Xiangfa Liu * Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan , China [Manuscript received May 8, 2012, in revised form July 8, 2012, Available online 4 February 2013] The structures and compositions of Ti and Zr rich phases in ternary AleTieZr and quaternary AleSieTieZr systems were investigated by energy dispersive spectroscopy and X-ray diffraction. The additions of Ti and Zr were changed. It was found that Ti and Zr can replace each other in the Ti and Zr rich phases of Ale(Sie)Tie Zr alloys. Compositions of the phases have been measured as a function of Ti and Zr additions. The content of Ti (Zr) in the phases increases with its addition in the alloys. Besides, the increase of Ti content can result in a decrease of lattice parameters. Microhardness of the phases in Ale18SiexTieyZr alloys changes with composition evolution. Moreover, the microhardness is higher than that of the intermetallics of ternary AleSieTi and AleSieZr alloys, due to the distortion of crystal structure caused by the replacement of Ti and Zr. KEY WORDS: Replacement; Composition; Lattice parameters; Microhardness 1. Introduction Much attention has been paid to investigate the aluminumbased intermetallics TiAl 3 and ZrAl 3 because of their attractive characteristics, such as relatively low density, good oxidation resistance and high melting temperature [1]. According to AleTi and AleZr phase diagrams, the TiAl 3 and ZrAl 3 intermetallics react with liquid aluminum through peritectic reactions [2]. They have similar crystal structures, i.e., D0 22 in TiAl 3 and D0 23 in ZrAl 3. A lot of work has been done to study the crystal structure of MAl 3 (M ¼ Ti, Zr). For instance, transformation of the tetragonal D0 22 into a cubic L1 2 phase in TiAl 3 intermetallic alloyed with Cr, Mn, Fe, Co, Ni, Cu and Zn has been reported [3e8]. Metastable ZrAl 3 (L1 2 ) and TiAl 3 (L1 2 ) phases were observed in AleZr and AleTi alloys after fast quenching from liquid [9,10]. For the ternary AleSieTi (Zr) system, many references are available. For instance, according to Perrot [11], three possible titanium aluminides exist in ternary AleSieTi system, namely TiAl 3, s 1 and s 2. TiAl 3 is also commonly referred as Ti(Al 1 x Si x ) 3, for some Al atoms can be replaced by Si atoms in the crystal structure, while s 1 and s 2 are written as Ti 7 Al 5 Si 12 and * Corresponding author. Prof., Ph.D.; Tel.: þ ; Fax: þ ; address: xfliu@sdu.edu.cn (X. Liu) /$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. Ti(AlSi) 2 in some other papers. However, since s 1 and s 2 have similar crystal structure (they both belong to tetragonal system) and the compositions of them vary over a wide range, they can be referred as Ti(Al x Si 1 x ) 2 instead [12]. In our previous studies [13,14], a phase evolution from M(Al 1 x Si x ) 3 to M(Al x Si 1 x ) 2 (M ¼ Ti, Zr) was detected at about 14 wt% Si in ternary AlexSie2Ti and AlexSie2Zr systems. Ti and Zr are in the same group of the periodic table of elements and exhibit similar characters. For example, Ti (Zr) can be dissolved in the crystal structure of ZrAl 3 (TiAl 3 ), thus, (Ti 1 x Zr x )Al 3 is commonly used to represent the ternary phase [15]. However, the replacement law of Ti and Zr in the intermetallics and the relationship between Ti and Zr additions are still unknown. What s more, no reference is available for investigating the intermetallics in quaternary AleSieTieZr system. In this work, the Ti and Zr rich phases in Ale(Sie)TieZr alloys were investigated. The addition of Ti and Zr in the alloys was changed. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) methods were used to analyze evolution of crystal structure and composition in the intermetallics. Lattice parameters of the intermetallics were calculated and the microhardness was measured. 2. Experimental Ale1Tie2Zr, Ale1.5Tie1.5Zr and Ale2Tie1Zr alloys (Table 1, wt%), Ale12SiexTieyZr, Ale18SiexTieyZr (x þ y ¼ 3, x ¼ 0.75, 1, 1.5, 2, 2.25, 2.4 wt%) alloys (Tables 2

2 292 T. Gao and X. Liu: J. Mater. Sci. Technol., 2013, 29(3), 291e296 Table 1 Compositions, chemical formula and lattice parameters of the intermetallics in AleTieZr alloys Alloys Compositions of AlTiZr phases (at.%) Chemical formula Lattice parameter (nm) Al Ti Zr a c Ale1Tie2Zr (Ti 1 x Zr x )Al Ale1.5Tie1.5Zr (0.22 x 0.59) Ale2Tie1Zr Note: each value is an average of five measurements. and 3) were prepared using commercially pure Al (99.7%), commercially pure crystalline Si (99.9%), Ti and Zr sponge (99.9%). The alloys were melted in a clean graphite crucible by using a high frequency induction furnace at the temperature of 1200 C, and were poured into a cast iron chill. Metallographic specimens were all cut from the same position of the casting samples, then mechanically ground and polished in standard routines. The microstructure analysis was carried out by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). FESEM was carried under a SU-70 scanning electron microscope operated at 15 kv and linked with an energy dispersive spectrometry (EDS) attachment. XRD measurements were conducted through a Rigaku D/max-rB diffractometer using CuK a radiation at 40 kv and 100 ma. A microhardness tester (DHV-1000) was used to measure the microhardness of as-cast specimens. The machine is connected with a metallographic microscope. The minimum detectable scale of the diamond indenter is mm. According to the properties of different intermetallics, the loading force can be chosen from N to 9.80 N. The indentation is then measured under the metallographic microscope. Using the software YT_M004, the values of microhardness can be obtained. In this work, the loading force is N, and the holding time is 10 s. 3. Results and Discussion 3.1. Intermetallics in AleTieZr alloys Fig. 1 shows the microstructures of Ale1Tie2Zr, Ale1.5Tie 1.5Zr and Ale2Tie1Zr alloys. It can be seen that the main intermetallics exhibit flake-like morphology. The XRD patterns of the alloys are shown in Fig. 2, in which the diffraction lines shift from standard peaks of ZrAl 3 phase. Table 1 presents the EDS result of the intermetallics. According to XRD and EDS results, Ti and Zr rich phase (Ti 1 x Zr x )Al 3, can be described as that some Ti atoms replace Zr atoms in ZrAl 3, keeping D0 23 crystal structure. Similar identification and description of the ternary phase can also be referred in previous works [1,15,16]. From Table 1, it can be seen that the sum of Ti and Zr contents in (Ti 1 x Zr x )Al 3 in these three alloys is almost constant, while the content of Ti (Zr) in the phases increases with its addition in the alloys. We use Ti:Zr ratio (at.%) in the alloys as X axis, and Ti:Zr ratio (at.%) in (Ti 1 x Zr x )Al 3 as Y axis, to reveal the replacement law of Ti and Zr in the phases with their additions in the alloys, as shown in Fig. 3. It demonstrates that there is a linear relevance, indicating that the higher Ti addition in the alloys (the higher Ti:Zr ratio), the more Zr atoms are replaced by Ti atoms in the crystal structure of (Ti 1 x Zr x )Al 3. Since the crystal structure of (Ti 1 x Zr x )Al 3 phase (D0 23 ) belongs to tetragonal system, the lattice parameters a and c can be calculated using following formula, 1 d ¼ r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (1) h 2 þ k 2 a 2 þ l2 c 2 where d is the interplanar spacing, h, k and l are indices of crystallographic planes. Using the values of h, k, l and d corresponding to (Ti 1 x Zr x )Al 3 diffraction peaks (Fig. 2), e.g., ð h 1 k 1 l 1 Þ¼ð0 0 8Þ, d 1 ¼ nm and ð h 2 k 2 l 2 Þ¼ð2 0 0Þ, d 2 ¼ nm of the phase in Ale1Tie2Zr alloy, the parameters a and c can be obtained. The lattice parameters of the three alloys are shown in Table 1. It can be seen that the increase of Ti content in the phases results in the decrease of lattice parameters a and c. It is because that the radius of Ti atom is smaller than that of Zr atom, thus, the lattice parameters decrease with the increase of Ti content. Accordingly, the main peaks of (Ti 1 x Zr x )Al 3 phase exhibit continuous shift, as shown in Fig. 2. The lattice parameter a of TiAl 3 is nm, while that of ZrAl 3 is nm [17]. As the calculated result, the lattice parameter a of (Ti 1 x Zr x )Al 3 is between those of TiAl 3 and ZrAl 3 (Table 1). Besides, it has been reported that addition of Ti to AleZr alloy has a negative effect on the refining performance of Table 2 Compositions, chemical formula and lattice parameters of the intermetallics in Ale12SiexTieyZr alloys Alloys Ale12SiexTieyZr Compositions of AlTiZrSi phases (at.%) Chemical formula Lattice parameter (nm) Al Si Ti Zr a c 0.75Tie2.25Zr (Ti 1 x Zr x )(Al 0.81 Si 0.19 ) Tie2Zr (0.10 x 0.62) Tie1.5Zr Tie1Zr Tie0.75Zr Tie0.6Zr Note: each value is an average of five measurements.

3 T. Gao and X. Liu: J. Mater. Sci. Technol., 2013, 29(3), 291e Table 3 Compositions, chemical formula and lattice parameters of the intermetallics in Ale18SiexTieyZr alloys Alloys Ale18SiexTieyZr Compositions of AlTiZrSi phases (at.%) Chemical formula Lattice parameter (nm) Al Si Ti Zr a c 0.75Tie2.25Zr (Ti 1 x Zr x )(Al 0.18 Si 0.82 ) Tie2Zr (0.12 x 0.60) Tie1.5Zr Tie1Zr Tie0.75Zr Tie0.6Zr Note: each value is an average of five measurements. Fig. 1 Microstructures of AleTieZr alloys: (a) Ale1Tie2Zr, (b) Ale1.5Tie1.5Zr, (c) Ale2Tie1Zr. aluminum [18]. It is known that the lattice parameter a of aluminum is nm [19], close to that of ZrAl 3. Thus, ZrAl 3 particles can act as nucleus sites of aluminum. While there are some Ti atoms dissolved in ZrAl 3 crystal structure, i.e., (Ti 1 x Zr x )Al 3 phase forms, the lattice parameters decrease. As a result, the mismatch between parameters of (Ti 1 x Zr x )Al 3 and aluminum is increased, and the refining performance is deteriorated Intermetallics in AleSieTieZr alloys Fig. 4 shows the microstructures of Ale12SiexTieyZr alloys. Ti and Zr additions vary as 0.75Tie2.25Zr, 1Tie2Zr, 1.5Tie 1.5Zr, 2Tie1Zr, 2.25Tie0.75Zr and 2.4Tie0.6Zr. Except for a-al matrix, gray eutectic Si phase and bright flake-like Ti(Zr) rich phase also form there, as marked in this figure. Fig. 5 and Table 2 are the XRD and EDS results of the Ti(Zr) rich phase. According to XRD and EDS, the phase can be written as (Ti 1 x Zr x )(Al 1 y Si y ) 3, regarding as that some Zr (Ti) atoms replace Ti (Zr) atoms and some Si atoms replace Al atoms in MAl 3 -based (M ¼ Ti, Zr) crystal structure. In these Ale12SiexTieyZr alloys, the sum of Al and Si contents in the intermetallics is almost constant, while the content of Ti (Zr) varies with the Ti (Zr) addition in the alloys. The average composition was calculated, and the intermetallics can be identified as (Ti 1 x Zr x )(Al 0.81 Si 0.19 ) 3 (0.10 x 0.62). From Table 2, it can be seen that the content of Ti (Zr) in the phases increases with the increasing of their contents in the alloys. Thus, in AleSieTieZr alloys, Ti and Zr exhibit similar replacement law as in ternary AleTieZr alloys. As mentioned above, Ti:Zr ratio (at.%) in the alloys was also used as X axis, and Ti:Zr ratio (at.%) in the phases as Y axis, to reveal the replacement law of Ti and Zr (Fig. 6). It shows that there is also a linear relevance, indicating that the higher the Ti addition in the alloys, the more the Zr atoms which are replaced by Ti atoms in the MAl 3 -based crystal structure. The crystal structure of the MAl 3 -based (M ¼ Ti, Zr) phase, i.e., (Ti 1 x Zr x )(Al 0.81 Si 0.19 ) 3, also belongs to tetragonal system. Thus, the lattice parameters of (Ti 1 x Zr x )(Al 0.81 Si 0.19 ) 3 can also be calculated by Eq. (1). The lattice parameters of Ale12Sie xtieyzr alloys are shown in Table 2. With the increase of Ti Fig. 2 XRD of AleTieZr alloys: (a) Ale1Tie2Zr, (b) Ale1.5Tie 1.5Zr, (c) Ale2Tie1Zr. Fig. 3 Content ratio of Ti:Zr in the intermetallics as a function of Ti:Zr ratio in AleTieZr alloys.

4 294 T. Gao and X. Liu: J. Mater. Sci. Technol., 2013, 29(3), 291e296 Fig. 4 Microstructures of Ale12SiexTieyZr alloys: (a) 0.75Tie2.25Zr, (b) 1Tie2Zr, (c) 1.5Tie1.5Zr, (d) 2Tie1Zr, (e) 2.25Tie0.75Zr, (f) 2.4Tie0.6Zr. content in the phases, the lattice parameters a and c decrease gradually, and the diffraction peaks exhibit continuous shift (Fig. 5). It has been mentioned above that the decrease of lattice parameters is due to the increase of Ti content in the phases, since the radius of Ti atom is smaller than that of Zr. As mentioned above, a phase evolution from MAl3-based aluminide to MSi2-based aluminide (M ¼ Ti, Zr) with the increase of Si addition in ternary AleSieTi (Zr) alloys was detected in our previous work[13,14]. The content of Si in the alloys plays an important role in promoting the evolution. For Fig. 5 XRD of Ale12SiexTieyZr alloys: (a) 0.75Tie2.25Zr, (b) 1Tie 2Zr, (c) 1.5Tie1.5Zr, (d) 2Tie1Zr, (e) 2.25Tie0.75Zr, (f) 2.4Tie0.6Zr. Fig. 6 Content ratio of Ti:Zr in the intermetallics as a function of Ti:Zr ratio in Ale12SiexTieyZr alloys.

5 T. Gao and X. Liu: J. Mater. Sci. Technol., 2013, 29(3), 291e296 instance, the phase forms as Ti(Al1 xsix)3 in Ale12Sie2Ti alloy, while it forms as Ti(AlxSi1 x)2 when Si addition is 18 wt %[13]. Also, similar evolution was detected in AlexSie2Zr system[14]. Thus, in order to identify whether similar replacement law of Ti and Zr exists in MSi2-based aluminide in quaternary AleSieTieZr alloys, Ale18SiexTieyZr alloys were prepared. Ti and Zr additions also vary as 0.75Tie 2.25Zr, 1Tie2Zr, 1.5Tie1.5Zr, 2Tie1Zr, 2.25Tie0.75Zr and 2.4Tie0.6Zr. Figs. 7 and 8 show the microstructures and XRD results of the alloys. Except for the Al matrix and blocky primary Si particles, the main Ti(Zr) rich phase also exhibits flake-like morphology, as marked in Fig. 7. Table 3 shows the EDS results. Diffraction peaks of the intermetallics were identified and average compositions were calculated. The phase is written as (Ti1 xzrx)(al0.18si0.82)2 (0.12 x 0.60). It can be regarded as that some Zr (Ti) atoms replace Ti (Zr) atoms and some Al atoms replace Si atoms in MSi2 (M ¼ Ti, Zr) crystal structure. Similar replacement law of Ti and Zr in the phases was discovered, i.e., the increase addition of Ti (Zr) in the alloys leads to the increase of Ti content in (Ti1 xzrx)(al0.18si0.82)2 phases, as shown in Fig. 9. The replacement of Ti and Zr also leads to an evolution of lattice parameters (Table 3). 295 Fig. 8 XRD of Ale18SiexTieyZr alloys: (a) 0.75Tie2.25Zr, (b) 1Tie 2Zr, (c) 1.5Tie1.5Zr, (d) 2Tie1Zr, (e) 2.25Tie0.75Zr, (f) 2.4Tie0.6Zr. XRD and EDS measurements demonstrate that Ti and Zr can replace each other in both MAl3-based and MSi2-based aluminides in AleSieTieZr alloys. Since the composition varies over a wide range, the quaternary phase can be simply written as AlTiZrSi, just as the phase formed in AleSieTi (Zr) alloys is commonly referred as TiAlSi (ZrAlSi)[20]. Fig. 7 Microstructures of Ale18SiexTieyZr alloys: (a) 0.75Tie2.25Zr, (b) 1Tie2Zr, (c) 1.5Tie1.5Zr, (d) 2Tie1Zr, (e) 2.25Tie0.75Zr, (f) 2.4Tie 0.6Zr.

6 296 T. Gao and X. Liu: J. Mater. Sci. Technol., 2013, 29(3), 291e296 the distortion level caused by the replacement of Ti and Zr is the highest. As a result, high crystal distortion level leads to high microhardness. 4. Conclusion Fig Microhardness As mentioned above, the content of Ti (Zr) in quaternary AlTiZrSi intermetallics increases with Ti (Zr) addition in Ale 18SiexTieyZr alloys. In order to discern the effect of composition evolution on the physical properties, the microhardness of AlTiZrSi phases in Ale18SiexTieyZr alloys was tested. Fig. 10 shows the graphical representation of microhardness values, which are obtained by averaging five measurements. It can be seen that the microhardness value of quaternary AlTiZrSi evolves with the Ti and Zr additions in the alloys. Besides, all the quaternary AlTiZrSi phases exhibit higher microhardness than ternary Ti(AlSi) 2 (105.1, HV) and Zr(AlSi) 2 (161.9, HV) phases, which has been tested in our previous work [13,14]. Quaternary AlTiZrSi can be considered as the doping of Ti (Zr) in ZrAlSi (TiAlSi) crystal structure. Since the atom radius of Ti and Zr is different, the replacement of Ti and Zr in AlTiZrSi phases will result in crystal distortion which leads to the increase of microhardness, finally. Besides, the AlTiZrSi phase of Ale18Sie1Tie2Zr alloy has the highest microhardness (Fig. 10), after which the microhardness decreases with the increase of Ti:Zr ratio. The average content of Ti in this phase is at.% while that of Zr is at.%. The difference between Ti and Zr contents is smaller than that in the AlTiZrSi phases of other five Ale18SiexTieyZr (0.75Tie2.25Zr, 1.5Tie1.5Zr, 2Tie1Zr, 2.25Tie0.75Zr and 2.4Tie0.6Zr) alloys, as shown in Table 3. It means that the number of Ti atoms and that of Zr atoms in the crystal lattice of AlTiZrSi phase in Ale18Sie1Tie2Zr alloy are similar. Thus, Fig. 10 Content ratio of Ti:Zr in the intermetallics as a function of Ti:Zr ratio in Ale18SiexTieyZr alloys. Graphical representation of microhardness values of AlTiZrSi phases in Ale18SiexTieyZr alloys. The replacement law of Ti and Zr in the intermetallics of AleTieZr and AleSieTieZr alloys was investigated. By XRD and EDS, it can be found that Ti and Zr atoms can replace each other in the crystal structures of ternary AlTiZr and quaternary AlTiZrSi phases. The content of Ti (Zr) in the phases increases with its addition in the alloys. Accordingly, the lattice parameters of AlTiZr and AlTiZrSi decrease with the increase of Ti content, since the radius of Ti atom is smaller than that of Zr atom. The microhardness of quaternary AlTiZrSi is higher than that of ternary TiAlSi and ZrAlSi phases. In addition, the microhardness of AlTiZrSi phases changes with the composition evolution, which is induced by the crystal distortion caused by the replacement of Ti and Zr in the crystal structures. Acknowledgments This research was financially supported by the National Basic Research Program of China (973 Program, No. 2012CB825702) and the National Natural Science Foundation of China (No ). REFERENCES [1] M.V. Karpets, Y.V. Milman, O.M. Barabash, N.P. Korzhova, O.N. Senkov, D.B. Miracle, T.N. Legkaya, I.V. Voskoboynik, Intermetallics 11 (2003) 241e249. [2] L.F. Mondolfo, Metallography of Aluminium Alloys, John Wiley & Sons Inc, New York, 1943, pp. 45e47. [3] N. Durlu, O.T. Inal, J. Mater. Sci. 12 (1992) 3225e3230. [4] C.D. Turner, W.O. Powers, J.A. Wert, Acta Metall. 37 (1989) 2635e2643. [5] S. Zhang, J.P. Nic, D.E. Mikkola, Scripta Metall. Mater. 24 (1990) 57e62. [6] M.B. Winnicka, R.A. Varin, Scripta Metall. Mater. 25 (1991) 2297e2302. [7] I.S. Virk, R.A. Varin, Metall. Trans. A 23 (1992) 617e625. [8] U. Prakash, R.A. Buckley, H. Jones, C.M. Sellars, J. Mater. Sci. 27 (1992) 2001e2004. [9] S. Hori, Y. Unigame, N. Furushiro, H. Tai, J. Japan Inst. Light Met. 32 (1982) 408e412. [10] J. Braun, M. Ellner, B. Predel, Z. Metallkd. 85 (1994) 855e862. [11] P. Perrot, in: G. Petzow, G. Effenberg (Eds.), AluminiumeSilicone Titanium, VCH, New York, 1990, pp. 283e290. [12] A. Raman, K. Schubert, Z. Metallkd. 56 (1965) 44e52 (in German). [13] T. Gao, P.T. Li, Y.G. Li, X.F. Liu, J. Alloys Compd. 509 (2011) 8013e8017. [14] T. Gao, D.K. Li, Z.S.H. Wei, X.F. Liu, Mater. Sci. Eng. A 552 (2012) 523e529. [15] T.V. Atamanenko, D.G. Eskin, M. Sluiter, L. Katgerman, J. Alloys Compd. 509 (2011) 57e60. [16] S.Z. Han, B.S. Rho, H.M. Lee, S.K. Choi, Intermetallics 4 (1996) 245e249. [17] S. Srinivasan, P.B. Desch, R.B. Schwarz, Scripta Metall. 25 (1991) 2513e2516. [18] G.P. Jones, J. Pearson, Metall. Trans. B 7 (1976) 223e234. [19] L. Tretyachenko, in: G. Eftenberg, S. Ilyenko (Eds.), AleTieZr, Springer, Berlin, 2006, pp. 54e67. [20] X.G. Chen, M. Fortier, J. Mater. Process. Technol. 210 (2010) 1780e1786.