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Quasicrystals in AlNi alloys
Materials Science and Engineering, A 134 (1991 ) 947-950
947
Quasicrystals in A1-Ni alloys C. Pohla and P. L. Ryder lnstitut fiir Werkstoffphysik und Strukturforschung, Universitiit Bremen (F.R.G.)
Abstract
AI-Ni alloys with 10 at.% Ni, 15 at.% Ni, 20 at.% Ni and 25 at.% Ni were rapidly quenched by splat cooling and studied by optical metallography, X-ray diffraction and transmission electron microscopy. In cross-sections the splats showed a layer structure consisting of a fine-grained region at each contact surface separated by a sharp boundary from a central, coarse-grained region. In addition to the equilibrium phases (A1, A13Ni, A13Ni2) the splats contained new, so far unidentified, crystal phases and in some cases a decagonal quasicrystalline phase that had nucleated in the outer layer. Variations of the specimen mass from 50 mg to 200 mg, the melt temperature from 1000 °C to 1700 °C and the piston speed from 2.5 m s l to 5.3 m s-~ were found to have only a slight influence on the splat thickness (averaging 50/~m) and the relative intensities of the X-ray lines.
1. Introduction
Since the discovery of the icosahedral phase (iphase) in an A1-Mn alloy by Shechtman et al. [1] many other A1-T alloys (T= transition metal) have been investigated with regard to the occurrence of quasicrystals. There are only two reports of quasicrystals in binary A1-Ni alloys: Dunlap and Dini [2] detected an /-phase in A186Ni14 by X-ray diffraction, and a decagonal phase was found in Als0Ni20 by Li and Kuo [3] by electron diffraction. In addition, quasicrystals have been found in several ternary A1-Ni-X alloys, e.g. AI-Ni-Si [3, 4], A1-Ni-Cr [5] and A1-Ni-Mn
[5,6]. The present paper presents the first results of a systematic investigation of the effects of the alloy composition and the quenching conditions on the microstructures of rapidly quenched AI-Ni alloys with 10-25 at.% Ni, with particular regard to the formation of quasicrystalline and other metastable phases. 2. Experimental methods
A1-Ni alloys with 10, 15, 20 and 25 at.% Ni, prepared by arc melting under argon, were rapidly quenched with a two-piston splat cooling apparatus. In order to determine the optimum conditions, experiments were carried out with 0921-5093/91/$3.50
four different specimen masses (50 mg, 100 rag, 150 mg and 200 mg), two piston speeds (2.5 and 5.3 m s -~) and various melt temperatures in the range 1000-1700 °C. The splats were investigated by X-ray diffractometry of as-quenched and pulverized specimens, optical metallography and transmission electron microscopy (TEM), including selected-area diffraction (SAD) and energy-dispersive X-ray microanalysis (EDX), of electrolytically thinned foils.
3. Results
X-ray diffraction of the slowly cooled starting materials showed the equifibrium phases f.c.c. A1 and A13Ni, as expected from the phase diagram [7]. Typical X-ray diffraction diagrams of all four alloys in the as-quenched state are shown in Fig. 1. After grinding the materials to a powder, the diagrams shown in Fig. 2 were obtained. All specimens showed lines of f.c.c. Al (marked A in Figs. 1 and 2) and the orthorhombic phase A13Ni (B). In addition, weak lines of hexagonal A13Ni 2 (C) were found in the alloy with 25 at.% Ni. The unmarked peaks in Figs. 1 and 2 have not yet been identified. A comparison of Figs. 1 and 2 shows that pulverizing the specimens changes the relative intensities of the lines in two ways: the intensities of the © Elsevier Sequoia/Printed in The Netherlands
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i
i
i
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30°
40°
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Fig. 3. Metallographic cross-sections of splats from (a) A185NiI5 and (b) AI75Ni25.
,.
Fig. 1. X-ray diffraction of the as-quenched splats. A= AI (f.c.c.), B=AI3Ni (orthorhombic), and C=AI3Ni2 (hexagonal). The unmarked peaks have not yet been identified. i
i
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20 ~,Fig. 2. X-ray diffraction of the pulverized splats. A=A1 (f.c.c.), B=AI3Ni (orthorhombic), and C=AI3Ni2 (hexagonal). The unmarked peaks have not yet been identified.
A13Ni peaks increase relative to the aluminium peaks, and in the alloys with 10 at.% Ni and 15 at.% Ni the unidentified peaks become weaker by a factor of four to five. These effects indicate that
the various phases are not distributed homogeneously throughout the splat. Variation of the specimen mass and the piston velocity had no detectable effect on the X-ray spectra and influenced the resulting splat thickness (mean value approximately 50 pm) only slightly. Increasing the specimen mass from 50 mg to 200 mg or reducing the piston speed from 5.3 m s- 1 to 2.5 m s- 1 gave rise to an increase of about 10/~m in the specimen thickness. The temperature of the melt prior to quenching was found to affect the relative intensities of some of the X-ray peaks, especially the unidentified peaks in the alloys with 10 at.% Ni and 15 at.% Ni. The splat thickness was unaffected over a wide temperature range, but increased slightly below 1200 °C. The metallographic cross-sections (see Fig. 3) show a fine-grained microstructure at each contact surface, separated by a sharp boundary from an inner region with a coarser grain structure. The thickness of the inner layer generally increased from the centre to the edge of the splat. TEM investigation of the outer layer generally showed a grain size of a few nm (Figs. 4(a) and 4(b)). EDX analysis gave the same composition as the alloy, and electron diffraction ring diagrams gave the same lattice plane spacings as X-ray diffraction. The inner layer (Figs. 4(c) and (d)) consists of "cauliflower-shaped" crystallites with a diameter of 1 - 2 / t m . EDX analysis showed that
949
Fig. 4. TEM bright-field micrographs of splats: (a) AI~sNi~5, outer layer; (b) AIv~Ni25, outer layer; (c) AI~sNi~5, inner layer: and (d) AIvsNi2~, inner layer.
the nickel content of these crystals was 5-10 at.% higher than that of the surrounding matrix. Electron diffraction showed no sign of quasicrystalline phases in this layer. Some of the diffraction patterns can be identified as the orthorhombic phase AI3Ni. Others may be indexed as a face-centred cubic phase with a = 0.573 nm, but with additional weak spots at the positions 1{224}, ~{224} etc. Further analysis of these patterns is in progress. In some specimens a few coarse-grained regions were found within the outer layer, with two different morphologies: (a) equiaxed grains with diameters up to 2 p m and (b) elongated grains with a length of about 2 p m and a width of around 0.3 #m. Examples are shown in Fig. 5 together with typical diffraction patterns. By electron diffraction this phase was identified as a
decagonal quasicrystal with a period of 0.4 nm along the tenfold axis. 4. Discussion and conclusions
The occurrence of the decagonal phase in A1-Ni alloys appears to be relatively seldom and sporadic, as found also by Li and Kuo [3]. No evidence was found for the existence of the iphase reported by Dunlap and Dini [2]. On the other hand, the X-ray and electron diffraction results indicate that the A1-Ni splats contain hitherto unknown, presumably metastable phases. Further diffraction and E D X investigations are needed to characterize these phases. The layer structure of the splats has also been observed in other materials [8] and seems to be typical of all splat-cooled materials that solidify in crystalline or quasicrystalline form. A possible
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Fig. 5. TEM of decagonal phase in Al75Ni25. (a) Bright-field, equiaxed grains; (b) bright field, elongated grains; (c) diffraction pattern, electron beam parallel to tenfold axis; (d) diffraction pattern, electron beam parallel to diad axis.
explanation for this microstructure is solidification at the contact surfaces followed by homogeneous nucleation in the remaining melt. This would also explain why the outer layer is thicker in the centre of the splat, which has the longest contact time. References 1 D. Shechtman, I. Blech, D. Gratias and J. W. Cahn, Phys. Rev. Lett., 53(1984) 1951.
2 R.A. Dunlap and K. Dini, J. Phys. F: Met. Phys., 16 (1986) 11. 3 X.Z. Li and K. H. Kuo, Phil Mag. Lett., 58(1988) 167. 4 K.H. Kuo, Mater. Sci. Forum, 22-24(1987) 131. 5 W.L. Zhou, X. Z. Li and K. H. Kuo, Scr. Metall., 23 (1989) 1571. 6 G. van Tendeloo, J. van Landuyt, S. Amelinckx and S. Ranganathan, J. Microsc., 149 (1988) 1. 7 M. Hansen and K. Anderko, Constitution of Binary Alloys, 2nd edn, McGraw-Hill, New York, 1958. 8 H. Selke and E L. Ryder, Mater. Sci. Eng., A 128 ( 1991 ).
947
Quasicrystals in A1-Ni alloys C. Pohla and P. L. Ryder lnstitut fiir Werkstoffphysik und Strukturforschung, Universitiit Bremen (F.R.G.)
Abstract
AI-Ni alloys with 10 at.% Ni, 15 at.% Ni, 20 at.% Ni and 25 at.% Ni were rapidly quenched by splat cooling and studied by optical metallography, X-ray diffraction and transmission electron microscopy. In cross-sections the splats showed a layer structure consisting of a fine-grained region at each contact surface separated by a sharp boundary from a central, coarse-grained region. In addition to the equilibrium phases (A1, A13Ni, A13Ni2) the splats contained new, so far unidentified, crystal phases and in some cases a decagonal quasicrystalline phase that had nucleated in the outer layer. Variations of the specimen mass from 50 mg to 200 mg, the melt temperature from 1000 °C to 1700 °C and the piston speed from 2.5 m s l to 5.3 m s-~ were found to have only a slight influence on the splat thickness (averaging 50/~m) and the relative intensities of the X-ray lines.
1. Introduction
Since the discovery of the icosahedral phase (iphase) in an A1-Mn alloy by Shechtman et al. [1] many other A1-T alloys (T= transition metal) have been investigated with regard to the occurrence of quasicrystals. There are only two reports of quasicrystals in binary A1-Ni alloys: Dunlap and Dini [2] detected an /-phase in A186Ni14 by X-ray diffraction, and a decagonal phase was found in Als0Ni20 by Li and Kuo [3] by electron diffraction. In addition, quasicrystals have been found in several ternary A1-Ni-X alloys, e.g. AI-Ni-Si [3, 4], A1-Ni-Cr [5] and A1-Ni-Mn
[5,6]. The present paper presents the first results of a systematic investigation of the effects of the alloy composition and the quenching conditions on the microstructures of rapidly quenched AI-Ni alloys with 10-25 at.% Ni, with particular regard to the formation of quasicrystalline and other metastable phases. 2. Experimental methods
A1-Ni alloys with 10, 15, 20 and 25 at.% Ni, prepared by arc melting under argon, were rapidly quenched with a two-piston splat cooling apparatus. In order to determine the optimum conditions, experiments were carried out with 0921-5093/91/$3.50
four different specimen masses (50 mg, 100 rag, 150 mg and 200 mg), two piston speeds (2.5 and 5.3 m s -~) and various melt temperatures in the range 1000-1700 °C. The splats were investigated by X-ray diffractometry of as-quenched and pulverized specimens, optical metallography and transmission electron microscopy (TEM), including selected-area diffraction (SAD) and energy-dispersive X-ray microanalysis (EDX), of electrolytically thinned foils.
3. Results
X-ray diffraction of the slowly cooled starting materials showed the equifibrium phases f.c.c. A1 and A13Ni, as expected from the phase diagram [7]. Typical X-ray diffraction diagrams of all four alloys in the as-quenched state are shown in Fig. 1. After grinding the materials to a powder, the diagrams shown in Fig. 2 were obtained. All specimens showed lines of f.c.c. Al (marked A in Figs. 1 and 2) and the orthorhombic phase A13Ni (B). In addition, weak lines of hexagonal A13Ni 2 (C) were found in the alloy with 25 at.% Ni. The unmarked peaks in Figs. 1 and 2 have not yet been identified. A comparison of Figs. 1 and 2 shows that pulverizing the specimens changes the relative intensities of the lines in two ways: the intensities of the © Elsevier Sequoia/Printed in The Netherlands
948 i
B,C
/IJTSM25
,
kla0Ni20
A
e
7 C
L
t~
L.J
>.
B
g A,B
C
AIssNII5
A
A,B
Bg~ g B
Ai~OHilO I
i
i
i
20°
30°
40°
50 o
20
Fig. 3. Metallographic cross-sections of splats from (a) A185NiI5 and (b) AI75Ni25.
,.
Fig. 1. X-ray diffraction of the as-quenched splats. A= AI (f.c.c.), B=AI3Ni (orthorhombic), and C=AI3Ni2 (hexagonal). The unmarked peaks have not yet been identified. i
i
i
i
B
A.B
l
c
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BBB
A.B B
i'~
6
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iI I-I
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)~ +2-
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C
0
~-J
AlasNlls
A A,g
B BB BBB
Al@oHilo i
i
t
i
20 °
30°
40o
50 °
20 ~,Fig. 2. X-ray diffraction of the pulverized splats. A=A1 (f.c.c.), B=AI3Ni (orthorhombic), and C=AI3Ni2 (hexagonal). The unmarked peaks have not yet been identified.
A13Ni peaks increase relative to the aluminium peaks, and in the alloys with 10 at.% Ni and 15 at.% Ni the unidentified peaks become weaker by a factor of four to five. These effects indicate that
the various phases are not distributed homogeneously throughout the splat. Variation of the specimen mass and the piston velocity had no detectable effect on the X-ray spectra and influenced the resulting splat thickness (mean value approximately 50 pm) only slightly. Increasing the specimen mass from 50 mg to 200 mg or reducing the piston speed from 5.3 m s- 1 to 2.5 m s- 1 gave rise to an increase of about 10/~m in the specimen thickness. The temperature of the melt prior to quenching was found to affect the relative intensities of some of the X-ray peaks, especially the unidentified peaks in the alloys with 10 at.% Ni and 15 at.% Ni. The splat thickness was unaffected over a wide temperature range, but increased slightly below 1200 °C. The metallographic cross-sections (see Fig. 3) show a fine-grained microstructure at each contact surface, separated by a sharp boundary from an inner region with a coarser grain structure. The thickness of the inner layer generally increased from the centre to the edge of the splat. TEM investigation of the outer layer generally showed a grain size of a few nm (Figs. 4(a) and 4(b)). EDX analysis gave the same composition as the alloy, and electron diffraction ring diagrams gave the same lattice plane spacings as X-ray diffraction. The inner layer (Figs. 4(c) and (d)) consists of "cauliflower-shaped" crystallites with a diameter of 1 - 2 / t m . EDX analysis showed that
949
Fig. 4. TEM bright-field micrographs of splats: (a) AI~sNi~5, outer layer; (b) AIv~Ni25, outer layer; (c) AI~sNi~5, inner layer: and (d) AIvsNi2~, inner layer.
the nickel content of these crystals was 5-10 at.% higher than that of the surrounding matrix. Electron diffraction showed no sign of quasicrystalline phases in this layer. Some of the diffraction patterns can be identified as the orthorhombic phase AI3Ni. Others may be indexed as a face-centred cubic phase with a = 0.573 nm, but with additional weak spots at the positions 1{224}, ~{224} etc. Further analysis of these patterns is in progress. In some specimens a few coarse-grained regions were found within the outer layer, with two different morphologies: (a) equiaxed grains with diameters up to 2 p m and (b) elongated grains with a length of about 2 p m and a width of around 0.3 #m. Examples are shown in Fig. 5 together with typical diffraction patterns. By electron diffraction this phase was identified as a
decagonal quasicrystal with a period of 0.4 nm along the tenfold axis. 4. Discussion and conclusions
The occurrence of the decagonal phase in A1-Ni alloys appears to be relatively seldom and sporadic, as found also by Li and Kuo [3]. No evidence was found for the existence of the iphase reported by Dunlap and Dini [2]. On the other hand, the X-ray and electron diffraction results indicate that the A1-Ni splats contain hitherto unknown, presumably metastable phases. Further diffraction and E D X investigations are needed to characterize these phases. The layer structure of the splats has also been observed in other materials [8] and seems to be typical of all splat-cooled materials that solidify in crystalline or quasicrystalline form. A possible
950
e
$
'
t
+
o
++
+
,+
O
"
++
0
'
O
|+~.
o.+
. +. +
+ * 0
Fig. 5. TEM of decagonal phase in Al75Ni25. (a) Bright-field, equiaxed grains; (b) bright field, elongated grains; (c) diffraction pattern, electron beam parallel to tenfold axis; (d) diffraction pattern, electron beam parallel to diad axis.
explanation for this microstructure is solidification at the contact surfaces followed by homogeneous nucleation in the remaining melt. This would also explain why the outer layer is thicker in the centre of the splat, which has the longest contact time. References 1 D. Shechtman, I. Blech, D. Gratias and J. W. Cahn, Phys. Rev. Lett., 53(1984) 1951.
2 R.A. Dunlap and K. Dini, J. Phys. F: Met. Phys., 16 (1986) 11. 3 X.Z. Li and K. H. Kuo, Phil Mag. Lett., 58(1988) 167. 4 K.H. Kuo, Mater. Sci. Forum, 22-24(1987) 131. 5 W.L. Zhou, X. Z. Li and K. H. Kuo, Scr. Metall., 23 (1989) 1571. 6 G. van Tendeloo, J. van Landuyt, S. Amelinckx and S. Ranganathan, J. Microsc., 149 (1988) 1. 7 M. Hansen and K. Anderko, Constitution of Binary Alloys, 2nd edn, McGraw-Hill, New York, 1958. 8 H. Selke and E L. Ryder, Mater. Sci. Eng., A 128 ( 1991 ).