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Investigation on the icosahedral quasicrystal phase in Mg70.8Zn28Nd1.2 alloy
JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009, p. 264
Investigation on the icosahedral quasicrystal phase in Mg70.8Zn28Nd1.2 alloy ZHANG Jinshan (张金山), YAN Jie (严 杰), HAN Fuyin (韩富银), LIANG Wei (梁 伟), XU Chunxiang (许春香), ZHOU Cuilan (周翠兰) (College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China) Received 22 February 2008; revised 11 November 2008
Abstract: Mg70.8Zn28Nd1.2 (mole fraction) alloy containing icosahedral quasicrystal phase (I-phase) was prepared under conventional metal casting conditions. The microstructure, phase constitution and phase structure of the alloy were investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM). The results showed that the spherical phase in Mg70.8Zn28Nd1.2 alloy was a simple icosahedral quasicrystal with stoichiometric composition of Mg40Zn55Nd5 and quasi-lattice of 0.525 nm. In this research, the as-cast microstructure of Mg70.8Zn28Nd1.2 alloy mainly consisted of Mg40Zn55Nd5 icosahedral quasicrystal phase and Mg7Zn3 columnar crystal matrix. In the growing process of Mg40Zn55Nd5 icosahedral quasicrystal phase, the growth morphology mainly depended on interface energy, adsorption effect of Nd and cooling rate. Keywords: Mg-Zn-Nd; icosahedral quasicrystal phase; microstructure; growth morphology; adsorption; rare earths
Since the first discovery of stable Zn60Mg30Y10 icosahedral quasicrystal phase (I-phase) in Zn-Mg-RE (Y) alloys by Luo et al. in Beijing Institute of Aeronautical Materials[1], icosahedral quasicrystal phases were found in many Zn-rich Zn-Mg-RE alloy systems in which RE=Y and rare earth elements Gd to Er in Periodic Table[2–4]. This type of Zn-Mg-RE quasicrystal has caught great interest of many condensed matter physicists and great efforts have been made on the investigation of its formation mechanism, structure and properties[5–9]. Differing from the Al-based quasicrystals, the Zn-Mg-RE quasicrystal does not include Al and transition metals but rare earth elements; it can be obtained through rapid solidification as well as slow cooling under normal metal casting conditions[3]. However, in many previous reports, more attention has been focused on Zn-Mg-Y quasicrystal alloy because I-phase and α-Mg coexist near Mg-rich corner of Zn-Mg-Y ternary system and the I-phase can be in direct equilibrium with Mg solid solution phase. According to literature[4], Zn-rich Zn55Mg40Nd5 ternary alloy can form a simple icosahedral quasicrystal (P-type) with previous composition through rapid solidification process. However, to our best knowledge, microstructure details of this quasicrystal alloy were rarely reported and almost no reports about obtaining I-phase in Mg-rich Mg-Zn-Nd alloy have been found yet. Therefore, the purpose of this study
was to prepare Mg-rich Mg70.8Zn28Nd1.2 quasicrystal alloy through conventional metal casting method. The microstructure of as-cast alloy, morphology evolvement and growth mechanism of quasicrystal phase were specially investigated in the paper.
1 Experimental The Mg70.8Zn28Nd1.2 (mole fraction) alloy was obtained by melting 99.99% Mg, 99.99% Zn and 99.99% Nd in medium frequency furnace under Ar atmosphere. After melting the raw materials at 1073 K, the melt was cooled down to 983 K. Holding for 10 min at 983 K, the melt was poured into a metal mold with a diameter of 70 mm and height of 180 mm preheated at 473 K and then cooled down at room temperature. The diameter of the cylindrical alloy samples is 20 mm. To explore the influence of cooling rate on quasicrystal phase growth process, part of the molten alloy was poured into a ladder metal mold preheated at 473 K. The dimension of different cross sections of the ladder metal mold is Φ 30 mm×30 mm, Φ 20 mm×20 mm and Φ 10 mm×10 mm with corresponding solidification modulus (M) of 5, 3.3 and 1.67, respectively. The microstructure and composition of different phases in the alloy were investigated by scanning electron microscopy (SEM, JSU-6700F)
Foundation item: Project supported by National Natural Science Foundation of China (50571073), Natural Science Foundation of Shanxi Province (2009011028-3, 2007011067, 20051052) and High-School Student Project of Taiyuan City (07010713) Corresponding author: ZHANG Jinshan (E-mail: [email protected]; Tel.: +86-351-6018208) DOI: 10.1016/S1002-0721(08)60232-X
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ZHANG J S et al., Investigation on the icosahedral quasicrystal phase in Mg70.8Zn28Nd1.2 alloy
equipped with electron dispersive spectroscopy (EDS). Phase identification and lattice parameter were analyzed by X-ray diffraction (XRD, Y-2000) using monochromatic Cu Kα radiation. Phase structures were identified by transmission electron microscopy (TEM, JEM-2010). Alloy samples for TEM analysis were prepared by ion beam thinning.
2 Results and discussion 2.1 Microstructure of as-cast Mg70.8Zn28Nd1.2 alloy X-ray diffraction patterns of the as-cast Mg70.8Zn28Nd1.2 alloy are shown in Fig.1. Except peaks of Mg7Zn3 phase, other diffraction peaks with relatively high intensities can be assigned to the icosahedral quasicrystal phase, which indicates that the as-cast solidification microstructure of Mg70.8Zn28Nd1.2 alloy mainly consists of icosahedral quasicrystal phase and Mg7Zn3 crystal phase. Indexing of the quasicrystal phase was accomplished by using Elser’s method[10] and the quasi-lattice parameter was calculated to be aR=0.525nm from the diffraction peak of 211 111. Typical SEM micrograph of as-cast Mg70.8Zn28Nd1.2 alloy is displayed in Fig.2(a), showing the spherical phase homogeneously distributed in the matrix. The matrix was identified as Mg7Zn3 phase while the spherical phase was identi-
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fied as an icosahedral quasicrystal phase. The Mg7Zn3 matrix exhibits the characteristics of columnar crystals. Besides, white strip-like structures were also observed along the grain boundaries. EDS confirmed that it belonged to (Mg, Zn)99.5 Nd0.5 ternary intermetallic compound. From Fig.2(a), it is clear that the average diameter of all the spherical particles with high roundness is no larger than 15 μm. Fig.2(b) is the magnified morphology of spherical phase in Fig.2(a). At a macro scale, the spherical phase does not show characteristics of crystal plane. EDS standard analysis was also used to verify the composition of the spherical phase. The spherical phase has approximately 40.54at.% Mg, 54.68at.% Zn, 4.78at.% Nd and its average composition can be estimated to be Mg40Zn55Nd5. The selected area diffraction patterns in Figs.3(a–c) obtained from the Mg-Zn-Nd spherical phase corresponded to the electron diffraction patterns of icosahedral structure with five-, three- and two-fold symmetry zone axes, respectively. From cluster structure points of view, the icosahedral quasicrystal can simply be classified into two groups. One is Mackay-type icosahedral quasicrystal which consists of the Mackay-icosahedral clusters and another is Frank-Kasper type icosahedral quasicrystal which consists of rhombic-triacontahedron clusters. Each group has two types of structures called P-type (simple icosahedral) and FCI-type (face-centered icosahedral)[11]. In the present study, the arrangement of diffraction spots follow τ3–inflation instead of τ–inflation along the 5-fold direction in the 2-fold pattern (Fig.3(c)), where τ denotes the golden mean. This confirms that the structure of spherical I-phase obtained in as-cast Mg70.8Zn28Nd1.2 alloy belongs to the simple icosahedral quasicrystal (P-type) structure. Moreover, combined with XRD analysis results, the most intense peak (221001) of I-phase reflects that Mg40Zn55Nd5 quasicrystal belongs to FrankKasper type I-phase. 2.2 Formation mechanism of spherical quasicrystal phase
Fig.1 X-ray diffraction pattern of as-cast Mg70.8Zn28Nd1.2 alloy
In the research processes of quasicrystal, all the slowly grown equilibrium quasicrystals show perfect polyhedron
Fig.2 Typical micrograph of as-cast Mg70.8Zn28Nd1.2 alloy
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morphology and the growth morphology often reflects the point group symmetry of the quasicrystals[2,12,13]. The quasicrystal morphology like pentagonal dodecahedron was observed in the above-mentioned literatures. In such situation, the shape also reflects the slowest growing planes which have the highest atomic density. Quasicrystals with petal- like morphology were commonly observed in quasicrystal alloys. For the formation mechanism of petal-like quasicrystal with five-fold symmetry, Kim et al. attributed the growth mechanism to intrinsical quasi-structure of quasicrystal in the research processes of Al-Mn petal-like quasicrystals[14]. The formation process of petal-like quasicrystals can be divided into two important stages. At the first stage, close to the quasicrystal nucleation stage, quasicrystal nucleus exhibits typical pentagonal dodecahedral growth morphology. In succession, the secondary important stage resulting in the petal-like quasicrystal is that quasicrystal grows along its preferred growth orientations of five-fold symmetric axes. According to atomic cluster theory, abundant icosahedral atomic clusters (IAC) exist in simple metallic melt near the liquidus and with great degree of undercooling, which was proposed by Frank[15] in terms of researching structures of molten alloy and amorphous alloy. The reason is that the energy of IAC is 8% lower than atomic clusters with hexagonal close-packed (hcp) and face-centered cubic (fcc) structure[15]. Recent research results have shown that IAC structure with five-fold rotational symmetry exists in some amorphous alloy and even in single atom metallic liquid. Moreover, IAC can develop a lot with increasing undercooling in alloy melt. A larger volume fraction of icosahedral short-range order structure is favorable to the formation of I-phase in the melt[16-19]. This kind of I-phase retains icosahedral local short-range order structure from the alloy melt. Therefore, there exists obvious structural heredity between alloy melt and I-phase. In the present experiment conditions, when Mg-Zn-Nd based alloy melt was cooled down to the vicinity of liquidus temperature (983 K) and held at that
JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009
temperature, Mg40Zn55Nd5 IAC short-range ordered structure would exist widely in the alloy melt. The nucleation process of Mg40Zn55Nd5 spherical quasicrystal should coincide with petal-like quasicrystals proposed by Kim et al.[14] At this important stage, quasicrystal nuclei, which were directly evolved from icosahedral atomic clusters in the undercooled alloy melt, would nucleate in the equilibrium shape of pentagonal dodecahedral to sustain its stability and lowest surface energy. Macroscopically, the morphology of Mg40Zn55Nd5 icosahedral quasicrystal phase does not show characteristics of crystal plane, that is to say, the whole phase interface exhibits a smooth round ball (as shown in Fig.2(a)). Thus, it can be deduced that interfacial energy plays a very important role in the growth morphology of Mg40Zn55Nd5 I-phase. In the Nd-rich tiny region formed by Mg-Zn-Nd IAC, surplus Nd would diffuse from the interior part of quasicrystal solid phase to solid/liquid interface front. Meanwhile, because almost all the Nd atoms have functioned during the growth of quasicrystal phase, Nd atoms also diffuse from farther interior part of liquid phase to solid/liquid interface. The as-mentioned bilateral diffusion caused the enrichment of Nd element at the front of solid/liquid interface. There exist distribution differences of the enrichment of Nd atoms on every crystal plane of formed tiny quasicrystal grains. This phenomenon should be attributed to the adsorption effect of Nd surface active element. According to adsorption theory, the adsorption capacity of Nd varies with different crystal planes. Normally, more Nd atoms would be absorbed on crystal planes with larger interfacial tension; its adsorption of Nd brings about reduced interface tension and slow growth rate. While for crystal plane with smaller interfacial tension, the adsorption content of Nd is small, and its adsorption of Nd results in invisible change of interfacial tension as well as inconspicuous variation of growth rate. As a result, interfacial energy from different growing planes of quasicrystal phase would turn into uniformity and their growth rate become
Fig.3 Typical SAD patterns of the primary I-phase (a) Five-fold axes; (b) Three-fold axes; (c) Two-fold axes. The 5-fold and 2-fold directions in the 2-fold SAD patterns are pointed out by arrows. The spot marked by ‘O’ indicates the 211111 reflection with a lattice spacing d = 0.2497 nm
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ZHANG J S et al., Investigation on the icosahedral quasicrystal phase in Mg70.8Zn28Nd1.2 alloy
uniform as well. At the same time, the tendency of its preferred growth orientations became inconspicuous and the final growth morphology of I-phase changed. According to Wulff principle, for a given crystal volume, the equilibrium condition is that the interfacial energy of the crystal must be minimal[20]. That is,
∑σ
K
AK → min
(1)
The adsorption content of Nd for K crystal plane of I-phase follows Gibbs adsorption formulation: Γ
K
C dσ
=−
K
(2)
RT d C
where C is the concentration of Nd, R is gas constant and T is thermodynamic temperature in Kelvin. After a large amount of Nd is adsorbed on K crystal plane, its interfacial tension can be calculated by integration: C
σ C = σ 0 − RT ∫ K
K
Γ
K
dC (3) C where σ0K denotes interfacial tension of K crystal plane before adsorption and σcK denotes interfacial tension of K crystal plane after adsorption. 0
Therefore, the total interfacial energy of the whole I-phase is: K C Γ K ∑ σ C AK = ∑ σ 0 − RT ∫ C d C AK K K 0 K
(4)
Obviously, the following expression should come into existence so as to ensure minimal free interfacial energy of the I-phase: K
∑∫
Γ
K
(5) d CAK → max C It can be seen that micro-adsorption not only causes growth rate at different directions to be uniform but also reduces the total free energy, suggesting that the adsorption process is spontaneous. Therefore, in the initial nucleation K
0
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stage, I-phase nucleates in equilibrium morphology; in the subsequent growth process, adsorption effect promotes equality of interfacial energy along different growing directions, which favors for the growth of Mg40Zn55Nd5 quasicrystal into spherical morphology. According to the theory of interfacial stability, under conventional metal casting conditions, the macro-solidification morphology of the primary I-phase was influenced by adsorption effect as well as interfacial stability. Whether I-phase can keep spherical morphology or not also depends on the stability of spherical solid/liquid interface as it moves toward liquid phase. If the interface becomes unstable, spherical quasicrystal will evolve into elliptical or dendritic morphology[21]. In this research, in order to obtain the critical technical parameters of Mg40Zn55Nd5 spherical quasicrystal phase, the evolution law of its morphology was discussed under different cooling rates. Typical optical microstructures of Mg70.8Zn28Nd1.2 alloy from bottom, middle and top parts of ladder metal mold are shown in Figs.4(a–c) respectively. From bottom to top, the cooling rate becomes slower and the corresponding solidification modulus is 1.67, 3.3, and 5, respectively. The microstructure of the bottom alloy mainly consists of spherical quasicrystal phase and Mg7Zn3 matrix. With lowering cooling rate, the spherical quasicrystal has decreased a lot, some of which have been replaced by some morphology-changed quasicrystal and developed into polyhedron or dendritic quasicrystal phase. Meanwhile, the size of quasicrystal phase has increased a lot. It can be concluded that the decrease of cooling rate and prolonged solidification process have made the growth of Mg40Zn55Nd5 I-phase more efficiently. According to the Mg-Nd binary phase diagram[22], the equilibrium partition coefficient k0 is less than 1 within the Nd content range of Mg70.8Zn28Nd1.2 alloy. The Nd solute atoms would congregate at growing interfacial front during quasicrystal growth process, causing constitutional supercooling at the front of
Fig.4 Microstructures of as-cast Mg70.8Zn28Nd1.2 alloy from different parts of the ladder metal mold (a) Bottom part; (b) Middle part; (c) Top part
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growing interface. The appearance of constitutional supercooling would make the spherical growing interface become unstable. At last it would cause quasicrystal morphology to transform from spherical to polyhedron or even to dendritic shape. The indispensable Nd atoms would diffuse more efficiently from liquid phase to solid/liquid interface with slower cooling rate, which makes the enrichment of Nd atoms more aggravating. The larger the constitutional supercooling, the larger the probability of spherical interface transforming into polyhedron or dendritic shape. The quasicrystal morphologies noted by arrows in Figs.4(b, c) have well elucidated the growth morphology evolution process of Mg40Zn55Nd5 I-phase. In the microstructure of alloy with the slowest cooling rate, there exists no spherical quasicrystal phase but only polyhedron, dendritic quasicrystal and Mg7Zn3 matrix (shown in Fig.4(c)). According to Mullins et al.[23]: the relationship between stable-growing critical radius Ra and undercooling ΔT can be deduced from the following equation: Ra =
2Tm Γ 1 ks 1 + (l + 2) 1 + l (1 + ) ∆T 2 kL
(6)
where kS is the thermal conductivity of the solid phase, kL the thermal conductivity of the liquid phase, Γ the ratio of interfacial energy to latent heat of solid phase per unit volume, l the rank of spherical harmonic function, Tm the melting point of the alloy, and ΔT the degree of undercooling in the melt. The stable-growing critical radius Ra of spherical quasicrystal decreases with increasing ΔT, which makes spherical interface become more unstable. In a word, only under favorable growth conditions can Mg40Zn55Nd5 I-phase grow with a relatively stable spherical interface and obtain morphology of high roundness. In the present study, the critical radius Ra for stable-growth of the spherical quasicrystal is about 15 μm.
3 Conclusion The as-cast microstructure of Mg70.8Zn28Nd1.2 alloy mainly consisted of icosahedral quasicrystal phase and Mg7Zn3 phase. I-phase in Mg70.8Zn28Nd1.2 alloy could be confirmed as a typical simple icosahedral quasicrystal with a stoichiometric composition of Mg40Zn55Nd5 and quasi-lattice of 0.525 nm. I-phase had a perfect spherical morphology and distributed homogeneously in the matrix of Mg7Zn3. The growth morphology of Mg40Zn55Nd5 I-phase mainly depended on Nd adsorption effect, interfacial energy and cooling rate. Nd adsorption effect made the interfacial energy become even along every growth direction, which was favorable for growing into high-roundness spherical quasicrystal. Decrease of cooling rate in the molten alloy would
make the I-phase grow more efficiently. The constitutional supercooling caused by solute atoms made the interface unstable, which led to morphological transformation from spherical to polyhedron or even to dendritic shape. In the research, the critical radius Ra for stable-growth of Mg40Zn55Nd5 spherical quasicrystal was no larger than 15 μm.
References: [1] Luo Z P, Zhang S Q, Tang Y L, Zhao D S. Quasicrystals in ascast Mg-Zn-RE alloys. Scripta Metallurgica et Materialia, 1993, 28(11): 1513. [2] Niikura A, Tsai A P, Inoue A, Masumoto T. Stable Zn-Mgrare-earth face-centred icosahedral alloys with pentagonal dodecahedral solidification morphology. Philosophical Magazine Letters, 1994, 69(6): 351. [3] Tsai A P, Niikura A, Inoue A, Masumoto T. Highly ordered structure of icosahedral quasicrystals in Zn-Mg-RE (RE=rare earth metals) systems. Philosophical Magazine Letters, 1994, 70(3): 169. [4] Niikura A, Tsai A P, Inoue A, Masumoto T. New class of amorphous and icosahedral phases in Zn-Mg-rare-earth metal alloys. Japan Journal of Applied Physics, 1994, 33: L1538. [5] Luo Z P, Zhang S Q, Tang Y L, Zhao D S. On the stable quasicrystals in slowly cooled Mg-Zn-Y alloys. Scripta Metallurgica et Materialia, 1995, 32(9): 1411. [6] Saito H, Fukamichi K, Goto T, Tsai A P, Inoue A, Masumoto T. Concentration dependence of the magnetic properties of melt-quenched P-type Mg30GdxZn70–x quasicrystals. Journal of Alloys and Compounds, 1997, 252: 6. [7] Chattopadhyay K, Ravishankar N, Goswami R. Shapes of quasicrystals. Progress in Crystal Growth and Characterization of Materials, 1997, 34: 237. [8] Abe E, Sato T J, Tsai A P. Structure and phase transformation of the Zn-Mg-rare-earth quasicrystals. Materials Science and Engineering A, 2000, 294-296: 29. [9] Tsai A P. A test of Hume-Rothery rules for stable quasicrystals. Journal of Non-Crystalline Solids, 2004, 334&335: 317. [10] Elser V. Indexing problems in quasicrystal diffraction. Physical Review B, 1985, 32: 4892. [11] Ebalard S, Spaepen F. The body-centered-cubic-type icosahedral reciprocal lattice of Al-Cu-Fe quasicrystal. Journal of Materials Research, 1989, 4(1): 39. [12] Dubost B, Lang J M, Tanaka M, Sainfort P, Audier M. Large Al-Cu-Li single quasicrystal with triacontahedral solidification morphology. Nature, 1986, 324: 48. [13] Tsai A P, Inoue A, Masumoto T. Preparation of a new Al-CuFe quasicrystal with large grain sizes by rapid solidification. Journal of Materials Science Letters, 1987, 6: 1403. [14] Kim D H, Cantor B. Growth morphology of the icosahedral phase in rapidly solidified Al-5at.%Mn. Scripta Metallurgica, 1989, 23: 1859. [15] Frank F C. Supercooling of liquids. Proceedings of the Royal Society of London A, 1952, 215: 43.
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ZHANG J S et al., Investigation on the icosahedral quasicrystal phase in Mg70.8Zn28Nd1.2 alloy [16] Peichert H, Klein O, Dosch H, Denk M, Honkimaki V, Lippmann T, Reiter G. Observation of five-fold local symmetry in liquid lead. Nature, 2000, 408: 839. [17] Spaepen F. Five-fold symmetry in liquids. Nature, 2000, 408: 781. [18] Xing L Q, Eckert J, Loser W, Schultz L. Effect of cooling rate on the precipitation of quasicrystals from the Zr-Cu-Al-Ni-Ti amorphous alloy. Applied Physics Letters, 1998, 73(15): 2110. [19] Kelton K F. Crystallization of liquids and glasses to quasicrystals. Journal of Non-Crystalline Solids, 2004, 334&335: 253. [20] Shi L. Alloy Thermodynamics. Beijing: Mechanical Industrial
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Press, 1992. 456. [21] Zhang J S, Du H W, Liang W, Xu C X, Lu B F. Effect of Mn on the formation of Mg-based spherical icosahedral quasicrystal phase. Journal of Alloys and Compounds, 2007, 427: 244. [22] Gorssea S, Hutchinsonb C R, Chevaliera B, Nieb J F. A thermodynamic assessment of the Mg-Nd binary system using random solution and associate models for the liquid phase. Journal of Alloys and Compounds, 2005, 392: 253. [23] Mullins W W, Sekerka R F. Morphological stability of a particle growing by diffusion or heat flow. Journal of Applied Physics, 1963, 34: 323.
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Investigation on the icosahedral quasicrystal phase in Mg70.8Zn28Nd1.2 alloy ZHANG Jinshan (张金山), YAN Jie (严 杰), HAN Fuyin (韩富银), LIANG Wei (梁 伟), XU Chunxiang (许春香), ZHOU Cuilan (周翠兰) (College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China) Received 22 February 2008; revised 11 November 2008
Abstract: Mg70.8Zn28Nd1.2 (mole fraction) alloy containing icosahedral quasicrystal phase (I-phase) was prepared under conventional metal casting conditions. The microstructure, phase constitution and phase structure of the alloy were investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM). The results showed that the spherical phase in Mg70.8Zn28Nd1.2 alloy was a simple icosahedral quasicrystal with stoichiometric composition of Mg40Zn55Nd5 and quasi-lattice of 0.525 nm. In this research, the as-cast microstructure of Mg70.8Zn28Nd1.2 alloy mainly consisted of Mg40Zn55Nd5 icosahedral quasicrystal phase and Mg7Zn3 columnar crystal matrix. In the growing process of Mg40Zn55Nd5 icosahedral quasicrystal phase, the growth morphology mainly depended on interface energy, adsorption effect of Nd and cooling rate. Keywords: Mg-Zn-Nd; icosahedral quasicrystal phase; microstructure; growth morphology; adsorption; rare earths
Since the first discovery of stable Zn60Mg30Y10 icosahedral quasicrystal phase (I-phase) in Zn-Mg-RE (Y) alloys by Luo et al. in Beijing Institute of Aeronautical Materials[1], icosahedral quasicrystal phases were found in many Zn-rich Zn-Mg-RE alloy systems in which RE=Y and rare earth elements Gd to Er in Periodic Table[2–4]. This type of Zn-Mg-RE quasicrystal has caught great interest of many condensed matter physicists and great efforts have been made on the investigation of its formation mechanism, structure and properties[5–9]. Differing from the Al-based quasicrystals, the Zn-Mg-RE quasicrystal does not include Al and transition metals but rare earth elements; it can be obtained through rapid solidification as well as slow cooling under normal metal casting conditions[3]. However, in many previous reports, more attention has been focused on Zn-Mg-Y quasicrystal alloy because I-phase and α-Mg coexist near Mg-rich corner of Zn-Mg-Y ternary system and the I-phase can be in direct equilibrium with Mg solid solution phase. According to literature[4], Zn-rich Zn55Mg40Nd5 ternary alloy can form a simple icosahedral quasicrystal (P-type) with previous composition through rapid solidification process. However, to our best knowledge, microstructure details of this quasicrystal alloy were rarely reported and almost no reports about obtaining I-phase in Mg-rich Mg-Zn-Nd alloy have been found yet. Therefore, the purpose of this study
was to prepare Mg-rich Mg70.8Zn28Nd1.2 quasicrystal alloy through conventional metal casting method. The microstructure of as-cast alloy, morphology evolvement and growth mechanism of quasicrystal phase were specially investigated in the paper.
1 Experimental The Mg70.8Zn28Nd1.2 (mole fraction) alloy was obtained by melting 99.99% Mg, 99.99% Zn and 99.99% Nd in medium frequency furnace under Ar atmosphere. After melting the raw materials at 1073 K, the melt was cooled down to 983 K. Holding for 10 min at 983 K, the melt was poured into a metal mold with a diameter of 70 mm and height of 180 mm preheated at 473 K and then cooled down at room temperature. The diameter of the cylindrical alloy samples is 20 mm. To explore the influence of cooling rate on quasicrystal phase growth process, part of the molten alloy was poured into a ladder metal mold preheated at 473 K. The dimension of different cross sections of the ladder metal mold is Φ 30 mm×30 mm, Φ 20 mm×20 mm and Φ 10 mm×10 mm with corresponding solidification modulus (M) of 5, 3.3 and 1.67, respectively. The microstructure and composition of different phases in the alloy were investigated by scanning electron microscopy (SEM, JSU-6700F)
Foundation item: Project supported by National Natural Science Foundation of China (50571073), Natural Science Foundation of Shanxi Province (2009011028-3, 2007011067, 20051052) and High-School Student Project of Taiyuan City (07010713) Corresponding author: ZHANG Jinshan (E-mail: [email protected]; Tel.: +86-351-6018208) DOI: 10.1016/S1002-0721(08)60232-X
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ZHANG J S et al., Investigation on the icosahedral quasicrystal phase in Mg70.8Zn28Nd1.2 alloy
equipped with electron dispersive spectroscopy (EDS). Phase identification and lattice parameter were analyzed by X-ray diffraction (XRD, Y-2000) using monochromatic Cu Kα radiation. Phase structures were identified by transmission electron microscopy (TEM, JEM-2010). Alloy samples for TEM analysis were prepared by ion beam thinning.
2 Results and discussion 2.1 Microstructure of as-cast Mg70.8Zn28Nd1.2 alloy X-ray diffraction patterns of the as-cast Mg70.8Zn28Nd1.2 alloy are shown in Fig.1. Except peaks of Mg7Zn3 phase, other diffraction peaks with relatively high intensities can be assigned to the icosahedral quasicrystal phase, which indicates that the as-cast solidification microstructure of Mg70.8Zn28Nd1.2 alloy mainly consists of icosahedral quasicrystal phase and Mg7Zn3 crystal phase. Indexing of the quasicrystal phase was accomplished by using Elser’s method[10] and the quasi-lattice parameter was calculated to be aR=0.525nm from the diffraction peak of 211 111. Typical SEM micrograph of as-cast Mg70.8Zn28Nd1.2 alloy is displayed in Fig.2(a), showing the spherical phase homogeneously distributed in the matrix. The matrix was identified as Mg7Zn3 phase while the spherical phase was identi-
265
fied as an icosahedral quasicrystal phase. The Mg7Zn3 matrix exhibits the characteristics of columnar crystals. Besides, white strip-like structures were also observed along the grain boundaries. EDS confirmed that it belonged to (Mg, Zn)99.5 Nd0.5 ternary intermetallic compound. From Fig.2(a), it is clear that the average diameter of all the spherical particles with high roundness is no larger than 15 μm. Fig.2(b) is the magnified morphology of spherical phase in Fig.2(a). At a macro scale, the spherical phase does not show characteristics of crystal plane. EDS standard analysis was also used to verify the composition of the spherical phase. The spherical phase has approximately 40.54at.% Mg, 54.68at.% Zn, 4.78at.% Nd and its average composition can be estimated to be Mg40Zn55Nd5. The selected area diffraction patterns in Figs.3(a–c) obtained from the Mg-Zn-Nd spherical phase corresponded to the electron diffraction patterns of icosahedral structure with five-, three- and two-fold symmetry zone axes, respectively. From cluster structure points of view, the icosahedral quasicrystal can simply be classified into two groups. One is Mackay-type icosahedral quasicrystal which consists of the Mackay-icosahedral clusters and another is Frank-Kasper type icosahedral quasicrystal which consists of rhombic-triacontahedron clusters. Each group has two types of structures called P-type (simple icosahedral) and FCI-type (face-centered icosahedral)[11]. In the present study, the arrangement of diffraction spots follow τ3–inflation instead of τ–inflation along the 5-fold direction in the 2-fold pattern (Fig.3(c)), where τ denotes the golden mean. This confirms that the structure of spherical I-phase obtained in as-cast Mg70.8Zn28Nd1.2 alloy belongs to the simple icosahedral quasicrystal (P-type) structure. Moreover, combined with XRD analysis results, the most intense peak (221001) of I-phase reflects that Mg40Zn55Nd5 quasicrystal belongs to FrankKasper type I-phase. 2.2 Formation mechanism of spherical quasicrystal phase
Fig.1 X-ray diffraction pattern of as-cast Mg70.8Zn28Nd1.2 alloy
In the research processes of quasicrystal, all the slowly grown equilibrium quasicrystals show perfect polyhedron
Fig.2 Typical micrograph of as-cast Mg70.8Zn28Nd1.2 alloy
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morphology and the growth morphology often reflects the point group symmetry of the quasicrystals[2,12,13]. The quasicrystal morphology like pentagonal dodecahedron was observed in the above-mentioned literatures. In such situation, the shape also reflects the slowest growing planes which have the highest atomic density. Quasicrystals with petal- like morphology were commonly observed in quasicrystal alloys. For the formation mechanism of petal-like quasicrystal with five-fold symmetry, Kim et al. attributed the growth mechanism to intrinsical quasi-structure of quasicrystal in the research processes of Al-Mn petal-like quasicrystals[14]. The formation process of petal-like quasicrystals can be divided into two important stages. At the first stage, close to the quasicrystal nucleation stage, quasicrystal nucleus exhibits typical pentagonal dodecahedral growth morphology. In succession, the secondary important stage resulting in the petal-like quasicrystal is that quasicrystal grows along its preferred growth orientations of five-fold symmetric axes. According to atomic cluster theory, abundant icosahedral atomic clusters (IAC) exist in simple metallic melt near the liquidus and with great degree of undercooling, which was proposed by Frank[15] in terms of researching structures of molten alloy and amorphous alloy. The reason is that the energy of IAC is 8% lower than atomic clusters with hexagonal close-packed (hcp) and face-centered cubic (fcc) structure[15]. Recent research results have shown that IAC structure with five-fold rotational symmetry exists in some amorphous alloy and even in single atom metallic liquid. Moreover, IAC can develop a lot with increasing undercooling in alloy melt. A larger volume fraction of icosahedral short-range order structure is favorable to the formation of I-phase in the melt[16-19]. This kind of I-phase retains icosahedral local short-range order structure from the alloy melt. Therefore, there exists obvious structural heredity between alloy melt and I-phase. In the present experiment conditions, when Mg-Zn-Nd based alloy melt was cooled down to the vicinity of liquidus temperature (983 K) and held at that
JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009
temperature, Mg40Zn55Nd5 IAC short-range ordered structure would exist widely in the alloy melt. The nucleation process of Mg40Zn55Nd5 spherical quasicrystal should coincide with petal-like quasicrystals proposed by Kim et al.[14] At this important stage, quasicrystal nuclei, which were directly evolved from icosahedral atomic clusters in the undercooled alloy melt, would nucleate in the equilibrium shape of pentagonal dodecahedral to sustain its stability and lowest surface energy. Macroscopically, the morphology of Mg40Zn55Nd5 icosahedral quasicrystal phase does not show characteristics of crystal plane, that is to say, the whole phase interface exhibits a smooth round ball (as shown in Fig.2(a)). Thus, it can be deduced that interfacial energy plays a very important role in the growth morphology of Mg40Zn55Nd5 I-phase. In the Nd-rich tiny region formed by Mg-Zn-Nd IAC, surplus Nd would diffuse from the interior part of quasicrystal solid phase to solid/liquid interface front. Meanwhile, because almost all the Nd atoms have functioned during the growth of quasicrystal phase, Nd atoms also diffuse from farther interior part of liquid phase to solid/liquid interface. The as-mentioned bilateral diffusion caused the enrichment of Nd element at the front of solid/liquid interface. There exist distribution differences of the enrichment of Nd atoms on every crystal plane of formed tiny quasicrystal grains. This phenomenon should be attributed to the adsorption effect of Nd surface active element. According to adsorption theory, the adsorption capacity of Nd varies with different crystal planes. Normally, more Nd atoms would be absorbed on crystal planes with larger interfacial tension; its adsorption of Nd brings about reduced interface tension and slow growth rate. While for crystal plane with smaller interfacial tension, the adsorption content of Nd is small, and its adsorption of Nd results in invisible change of interfacial tension as well as inconspicuous variation of growth rate. As a result, interfacial energy from different growing planes of quasicrystal phase would turn into uniformity and their growth rate become
Fig.3 Typical SAD patterns of the primary I-phase (a) Five-fold axes; (b) Three-fold axes; (c) Two-fold axes. The 5-fold and 2-fold directions in the 2-fold SAD patterns are pointed out by arrows. The spot marked by ‘O’ indicates the 211111 reflection with a lattice spacing d = 0.2497 nm
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ZHANG J S et al., Investigation on the icosahedral quasicrystal phase in Mg70.8Zn28Nd1.2 alloy
uniform as well. At the same time, the tendency of its preferred growth orientations became inconspicuous and the final growth morphology of I-phase changed. According to Wulff principle, for a given crystal volume, the equilibrium condition is that the interfacial energy of the crystal must be minimal[20]. That is,
∑σ
K
AK → min
(1)
The adsorption content of Nd for K crystal plane of I-phase follows Gibbs adsorption formulation: Γ
K
C dσ
=−
K
(2)
RT d C
where C is the concentration of Nd, R is gas constant and T is thermodynamic temperature in Kelvin. After a large amount of Nd is adsorbed on K crystal plane, its interfacial tension can be calculated by integration: C
σ C = σ 0 − RT ∫ K
K
Γ
K
dC (3) C where σ0K denotes interfacial tension of K crystal plane before adsorption and σcK denotes interfacial tension of K crystal plane after adsorption. 0
Therefore, the total interfacial energy of the whole I-phase is: K C Γ K ∑ σ C AK = ∑ σ 0 − RT ∫ C d C AK K K 0 K
(4)
Obviously, the following expression should come into existence so as to ensure minimal free interfacial energy of the I-phase: K
∑∫
Γ
K
(5) d CAK → max C It can be seen that micro-adsorption not only causes growth rate at different directions to be uniform but also reduces the total free energy, suggesting that the adsorption process is spontaneous. Therefore, in the initial nucleation K
0
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stage, I-phase nucleates in equilibrium morphology; in the subsequent growth process, adsorption effect promotes equality of interfacial energy along different growing directions, which favors for the growth of Mg40Zn55Nd5 quasicrystal into spherical morphology. According to the theory of interfacial stability, under conventional metal casting conditions, the macro-solidification morphology of the primary I-phase was influenced by adsorption effect as well as interfacial stability. Whether I-phase can keep spherical morphology or not also depends on the stability of spherical solid/liquid interface as it moves toward liquid phase. If the interface becomes unstable, spherical quasicrystal will evolve into elliptical or dendritic morphology[21]. In this research, in order to obtain the critical technical parameters of Mg40Zn55Nd5 spherical quasicrystal phase, the evolution law of its morphology was discussed under different cooling rates. Typical optical microstructures of Mg70.8Zn28Nd1.2 alloy from bottom, middle and top parts of ladder metal mold are shown in Figs.4(a–c) respectively. From bottom to top, the cooling rate becomes slower and the corresponding solidification modulus is 1.67, 3.3, and 5, respectively. The microstructure of the bottom alloy mainly consists of spherical quasicrystal phase and Mg7Zn3 matrix. With lowering cooling rate, the spherical quasicrystal has decreased a lot, some of which have been replaced by some morphology-changed quasicrystal and developed into polyhedron or dendritic quasicrystal phase. Meanwhile, the size of quasicrystal phase has increased a lot. It can be concluded that the decrease of cooling rate and prolonged solidification process have made the growth of Mg40Zn55Nd5 I-phase more efficiently. According to the Mg-Nd binary phase diagram[22], the equilibrium partition coefficient k0 is less than 1 within the Nd content range of Mg70.8Zn28Nd1.2 alloy. The Nd solute atoms would congregate at growing interfacial front during quasicrystal growth process, causing constitutional supercooling at the front of
Fig.4 Microstructures of as-cast Mg70.8Zn28Nd1.2 alloy from different parts of the ladder metal mold (a) Bottom part; (b) Middle part; (c) Top part
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JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009
growing interface. The appearance of constitutional supercooling would make the spherical growing interface become unstable. At last it would cause quasicrystal morphology to transform from spherical to polyhedron or even to dendritic shape. The indispensable Nd atoms would diffuse more efficiently from liquid phase to solid/liquid interface with slower cooling rate, which makes the enrichment of Nd atoms more aggravating. The larger the constitutional supercooling, the larger the probability of spherical interface transforming into polyhedron or dendritic shape. The quasicrystal morphologies noted by arrows in Figs.4(b, c) have well elucidated the growth morphology evolution process of Mg40Zn55Nd5 I-phase. In the microstructure of alloy with the slowest cooling rate, there exists no spherical quasicrystal phase but only polyhedron, dendritic quasicrystal and Mg7Zn3 matrix (shown in Fig.4(c)). According to Mullins et al.[23]: the relationship between stable-growing critical radius Ra and undercooling ΔT can be deduced from the following equation: Ra =
2Tm Γ 1 ks 1 + (l + 2) 1 + l (1 + ) ∆T 2 kL
(6)
where kS is the thermal conductivity of the solid phase, kL the thermal conductivity of the liquid phase, Γ the ratio of interfacial energy to latent heat of solid phase per unit volume, l the rank of spherical harmonic function, Tm the melting point of the alloy, and ΔT the degree of undercooling in the melt. The stable-growing critical radius Ra of spherical quasicrystal decreases with increasing ΔT, which makes spherical interface become more unstable. In a word, only under favorable growth conditions can Mg40Zn55Nd5 I-phase grow with a relatively stable spherical interface and obtain morphology of high roundness. In the present study, the critical radius Ra for stable-growth of the spherical quasicrystal is about 15 μm.
3 Conclusion The as-cast microstructure of Mg70.8Zn28Nd1.2 alloy mainly consisted of icosahedral quasicrystal phase and Mg7Zn3 phase. I-phase in Mg70.8Zn28Nd1.2 alloy could be confirmed as a typical simple icosahedral quasicrystal with a stoichiometric composition of Mg40Zn55Nd5 and quasi-lattice of 0.525 nm. I-phase had a perfect spherical morphology and distributed homogeneously in the matrix of Mg7Zn3. The growth morphology of Mg40Zn55Nd5 I-phase mainly depended on Nd adsorption effect, interfacial energy and cooling rate. Nd adsorption effect made the interfacial energy become even along every growth direction, which was favorable for growing into high-roundness spherical quasicrystal. Decrease of cooling rate in the molten alloy would
make the I-phase grow more efficiently. The constitutional supercooling caused by solute atoms made the interface unstable, which led to morphological transformation from spherical to polyhedron or even to dendritic shape. In the research, the critical radius Ra for stable-growth of Mg40Zn55Nd5 spherical quasicrystal was no larger than 15 μm.
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