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Al9Ni2 precipitates formed in an Al-Ni dilute alloy
Scripta Materialia, Vol. 37, No. 11, pp. 1721-1725,1997 Elsevier Science Ltd Copyright 0 1997 Acta Metallurgica Inc. hinted in the USA. All rights reserved 1359~6462/97 $17.00 + .OO
Pergamon
PII S1359-6462(97)00329-l
AlpNi2 PRECIPITATES FORMED IN AN Al-Ni DILUTE ALLOY A. Yanxunoto and H. Tsubakino Department of Materials Science and Engineering, Faculty of Engineering, Himeji Institute of Technology, Shosha 2167, Himeji, Hyogo 671-22, Japan (Received April 21,1997) (Accepted July 22, 1997) Introduction Because of low solid solubility of nickel in aluminum, there are few reports on precipitation phenomena in aluminum rich Al-Ni alloys, that is, Fink and Willey studied by hardness measurements and optical microscopic observations (1) and the authors investigated the precipitation in an Al-Ni dilute alloy mainly by resistivity measurements (2). According to the phase diagram of Al-Ni binary alloy system, the stable precipitate formed during aging was expected to be AlsNi. However, the precipitates were not examined in detail in the above mentioned studies. Therefore, in the present study, the transmission electmn microscopic (TEM) observations have been carried out in order to identify the precipitates. It wi:ll be shown that a metastable precipitate, A19Niz,is newly found out. Experimental Procedures The specimen was Al-250 mass ppm Ni alloy. High purity aluminum (99.995%) and nickel (99.99%) were used for .preparing the alloy. Chemical composition of the alloy is listed in Table 1. Specimens ‘were solution heat treated at 903 K for 3.6 ks under argon atmosphere. Aging temperatures were 673,793 and 823 K. For 673 K aging, the specimen was quenched into iced water after the solution heat treatment and then aged for 432 ks in a salt bath. While, for 793 and 823 K aging, the specimens were furnace cooled after the solution heat treatment to each temperature and then kept for 274 and 1210 ks, respectively, in the furnace. The specimen aged at 673 K was thinned down to about 0.1 mm by electropolishing using HCIO,:CHsOH = 1:9 solution and then foil specimens for TEM observations were prepared by twin jet method using HNOXI-&OH = 1:3 solution. Crystal structure of the precipitate and orientation relationship between the precipitate and matrix were examined on the foil specimens. Because of low density of precipitates, the selective potentiostatic etching by electrolytic dissolution (SPEED) metlhod (3) was applied to the specimens aged at 793 and 823 K using a mixture of 0.5 1 of methyl salicylate, 0.1 kg of tetramethyl-ammonium chloride and 4.5 1 of ethyl alcohol, which enables one to dissolve the matrix remaining precipitates. The precipitates were picked out using an acetylcellulose film and then carbon replica films were prepared by a conventional method. 1721
1722
AlgNiz PRECIPITATE
Vol. 37, No. 11
TABLE 1
ChemicalCompositionof the Alloy (mass ppm) Ni
Si
Fe
Cu
Ga
Ti
Al
250
4
3
1
1
0.5
bal.
The TEM observations were carried out by JEM-200CX and JEM-2010 operating at 200 kV in accelerating voltage. Results and Discussion Figure 1 shows the precipitates in the Al-250 mass ppm Ni alloy aged at 673 K for 432 ks. It seems that there are two kinds of precipitates, that is, needle-like type (A) and plate-like type (B). However, it was confirmed by tilting experiments that these are the same kind of precipitates which are formed on (00 1) planes of the matrix, a, that is, the precipitates A in Fig. 1 are parallel to (01 O), or (loo), and those of B are parallel to (OOl), plane. Selected area diffraction pattern taken from one of the precipitates parallel to (OOl),, such as the precipitate B in Fig. 1, is shown in Fig. 2. The direction of the incident beam did not accurately coincide with [OOl], direction but inclined about 5 degree, so as to excite the diffraction from the precipitate. Lattice spacing corresponding to the diffraction spot A is about 0.62 nm, and that for B is the same. Reciprocal lattice vectors go and go are crossed at right angle. The crystal structure of AlsNi was described as orthorhombic system with lattice constants of a0 = 0.65982, bo = 0.73515 and co = 0.48021 mn by Bradley and Tayler (4). The lattice spacings for the spots A and B (=0.62 nm) in Fig. 2 are close to the lattice constant a0 of AbNi, however, the facts that g,, and go are crossed at right angle with the same lattice spacings can not be interpreted by the orthorhombic crystal structure mentioned above. Thus the spots A and B are tentatively indexed as (010) and (loo), respectively, which means that the lattice constants are assumed to be a0 = bo = 0.62 nm, and the resultant key diagram is shown in Fig. 3 with matrix (a) spots. Orientation relationships between a and the precipitate is as follows:
Figure 1. Precipitates in the AI-250 mass ppm Ni alloy aged at 673 K for 432 ks
Vol. 37, No. 11
AlgNiz PRECIPITATE
1723
-. 0 Figure 2. Selected area diffraction nattem of the precipitate parallel to (OOl), like as B in Fig. 1.
.:a,
-5
_ _ 6200 -a
l:Precipitate
Figure 3. Key diagramfor Fig. 2 assuming the spots A and B in Fig. 2 as 014 and 100,. Linrs A-A’,EJ-B’ andC<_ show the intersections between the (OOl), plane and (3 IO), (1 IO), and (1 2 0), planes.
Figure 4. Selected. area$fractio_n patterns ofthe precipitate perpendicular to (OOl), like as A in Fig. 1. Directions of incident beam are parallel to [3 1 0],, [l 1 01, and [I 201, in (a), (b) and (c), respectively.
1724
AlgNi2PRECIPITATE
Vol. 37, No. 11
In order to understand reciprocal lattice points in the directions containing the c-axis of the precipitate, SADPs were also taken on the precipitate such as A in Fig. 1. Although the habit plane is parallel to (loo), or (OlO), in this case, the same indices as Fig. 3 for the matrix are applied to describe the direction in order to make easy understanding of orientation relationship. The SADPs in the [3 TO],, [1 TO], and [l ?O],directions are shown in Fig. 4. These SADPs represent the reciprocal lattice patterns perpendicular to (OOl), with lines of intersection A-A’, B-B’ and C-C’ shown in Fig. 3, respectively. The lattice spacing corresponding to the spot A in Fig. 4 (a) is about 0.85 nm, which appears in Figs. 4 (b) and (c) with the same lattice spacing. The lattice spacings for the spots B, C and D in Fig. 4 are 0.62, 0.27 and 0.43 nm, respectively. Indices of the planes for the spots B, C and D can be deduced from the assumed lattice constants mentioned above as (0 IO), (120) and (1 IO), respectively, and that for A is considered to be (OOl), which means that the lattice constant co = 0.85 nm. The angles between g A and ge (Fig. 4(a)), i A and & @I, and go and gn (c) are about 90, 83 and 85 degree, respectively, from which a crystal structure of the precipitate is deduced to be a monoclinic system with the angle p = 95 degree. An extracted precipitate from the specimen aged at 793 K for 274 ks is shown in Fig. 5, which is large enough for the energy dispersive X-ray (EDX) analysis. Shape of the precipitate is not proper one, because it was broken during the extraction process. The SADP inserted in Fig. 5 is the same to that in Fig. 2. Conpositional analysis by means of EDX analyzer showed that the precipitate consisted of Al and Ni and the concentrations of Al and Ni were 82.4 and 17.6 at.%, respectively. The ratio of Al&Ii is 4.68 which is close to 4.5, that is, the precipitate would be A19Niz. In other Al alloys involving transitional elements, there exists intermetallic phases having a monoclinic crystal structure and stoichiometric composition of A&X2, that is, Al&o2 (a = 0.8556, b = 0.6290, c = 0.6213 nm, p = 94.76 degree) (5) A19Fe2 (a = 0.869, b = 0.635, c = 0.632 nm, p = 94.76 degree) (6) and A&FeNi (a = 0.8598, b = 0.6271, c = 0.6207 run, fi = 94.66 degree) (7). The crystal structure of the precipitate determined in the present study (a = b = 0.62, c = 0.85 nm, p = 95 degree) is very close to those one, although the lattice constants for a,, and COare replaced each other. Similarity in lattice parameters among these phases seems to assure the proposed composition, i.e., Al$?iz. Extracted precipitates from the specimen aged at 823 K for 1210 ks are shown in Fig. 6. Selected area diffraction pattern of the precipitate indicated with the arrow in (a) is shown in (b). Lattice spacings corresponding to A, B and C in (b) are 0.72, 0.34 and 0.38 nm, respectively. The angle between
Figure 5. Extracted precipitate in the specimen aged at 793 K for 274 ks.
Figure 6. Extracted precipitates in the specimens aged at 823 K for 1210 ks:(a), SADP for the precipitate indicated by the arrow in (a):(b) and key diagram for (b):(c).
Vol. 37, No. 11
AlgNi2PRECIPITATE
1725
go and ga is about 62 degree, and that for ga and jj c is about 28 degree. The lattice spacing close to 0.72 nm is absent in the monoclinic crystal structure of A19Niz,while those of 0.34 and 0.38 nm are close to 0.35 (( 012) plane) or 0.34 nm ((102) plane) and 0.38 nm ((111) plane) in A19Ni2.However, the angle between ( 012) and (111) in AlgNiz is about 94 degree and that between (102) and (111) is about 41 degree, which shows that the precipitate in Fig. 6 (a) is not an AbNiz. In AhNi, on the other hand, there an: the lattice spacings of 0.735, 0.343 and 0.383 nm for (OlO), (111) and (101) planes, respectively, and the angle between (111) and (101) planes is 27.9 degree, which agrees with the measured value (28 degree). The key diagram indexed as AlsNi is shown in (c). That is, the precipitate in Fig. 6 (a) is not the AlgNizbut the stable phase, AbNi. It is considered that the A19Ni2is an metastable phase with high thermal stability. Summary 1) In an Al-rich Al-Ni alloy, precipitation sequence is as follows: Supersaturated solid solution + metastable phase, A19Niz+ stable phase, AhNi. 2) The metastable A19Niz has a monoclinic crystal structure with the lattice parameters of a = b = 0.62, I: = 0.85 nm and j3= 95 degree. 3) Orientation relationships between the A19Nizand the matrix as follows: (ooi),//(ooi),, [3 iol~/[ioo],. 4) The habit planes of the precipitate are (00 1},. Acknowledgements The authors wish to thank Professor Emeritus, H.I. Aaronson of Carnegie-Mellon University in the U.S.A. and Professor K. Chattopadhyay of Indian Institute of Science for their helpful suggestions and discussions. References 1. 2. 3. 4. 5. 6. 7.
W.L.Fink and L.A. Willey, Trans. AIMS. 111,293 (1934). H. Tsubakino, U. Ishihara and A. Yamamoto, Mater. Sci. Forum, 217-222,901 (1996). F. Kurosawa, I. Taguchi and R. Matsumoto, J. Jpn. Inst. Met. 43, 1068 (1979). A.J. Bradley and A. Taylor, Philos. Msg. 23, 1049 (1937). A.M. Douglas, ActaCrystallogr. 3,19 (1950). C.J. Simensen and R. Vellasamy, 2. Metallkd. 68,426 (1977). M. Khaidar, C.H. Allibert and J. Driole, ibid. 73,433 (1982).
Pergamon
PII S1359-6462(97)00329-l
AlpNi2 PRECIPITATES FORMED IN AN Al-Ni DILUTE ALLOY A. Yanxunoto and H. Tsubakino Department of Materials Science and Engineering, Faculty of Engineering, Himeji Institute of Technology, Shosha 2167, Himeji, Hyogo 671-22, Japan (Received April 21,1997) (Accepted July 22, 1997) Introduction Because of low solid solubility of nickel in aluminum, there are few reports on precipitation phenomena in aluminum rich Al-Ni alloys, that is, Fink and Willey studied by hardness measurements and optical microscopic observations (1) and the authors investigated the precipitation in an Al-Ni dilute alloy mainly by resistivity measurements (2). According to the phase diagram of Al-Ni binary alloy system, the stable precipitate formed during aging was expected to be AlsNi. However, the precipitates were not examined in detail in the above mentioned studies. Therefore, in the present study, the transmission electmn microscopic (TEM) observations have been carried out in order to identify the precipitates. It wi:ll be shown that a metastable precipitate, A19Niz,is newly found out. Experimental Procedures The specimen was Al-250 mass ppm Ni alloy. High purity aluminum (99.995%) and nickel (99.99%) were used for .preparing the alloy. Chemical composition of the alloy is listed in Table 1. Specimens ‘were solution heat treated at 903 K for 3.6 ks under argon atmosphere. Aging temperatures were 673,793 and 823 K. For 673 K aging, the specimen was quenched into iced water after the solution heat treatment and then aged for 432 ks in a salt bath. While, for 793 and 823 K aging, the specimens were furnace cooled after the solution heat treatment to each temperature and then kept for 274 and 1210 ks, respectively, in the furnace. The specimen aged at 673 K was thinned down to about 0.1 mm by electropolishing using HCIO,:CHsOH = 1:9 solution and then foil specimens for TEM observations were prepared by twin jet method using HNOXI-&OH = 1:3 solution. Crystal structure of the precipitate and orientation relationship between the precipitate and matrix were examined on the foil specimens. Because of low density of precipitates, the selective potentiostatic etching by electrolytic dissolution (SPEED) metlhod (3) was applied to the specimens aged at 793 and 823 K using a mixture of 0.5 1 of methyl salicylate, 0.1 kg of tetramethyl-ammonium chloride and 4.5 1 of ethyl alcohol, which enables one to dissolve the matrix remaining precipitates. The precipitates were picked out using an acetylcellulose film and then carbon replica films were prepared by a conventional method. 1721
1722
AlgNiz PRECIPITATE
Vol. 37, No. 11
TABLE 1
ChemicalCompositionof the Alloy (mass ppm) Ni
Si
Fe
Cu
Ga
Ti
Al
250
4
3
1
1
0.5
bal.
The TEM observations were carried out by JEM-200CX and JEM-2010 operating at 200 kV in accelerating voltage. Results and Discussion Figure 1 shows the precipitates in the Al-250 mass ppm Ni alloy aged at 673 K for 432 ks. It seems that there are two kinds of precipitates, that is, needle-like type (A) and plate-like type (B). However, it was confirmed by tilting experiments that these are the same kind of precipitates which are formed on (00 1) planes of the matrix, a, that is, the precipitates A in Fig. 1 are parallel to (01 O), or (loo), and those of B are parallel to (OOl), plane. Selected area diffraction pattern taken from one of the precipitates parallel to (OOl),, such as the precipitate B in Fig. 1, is shown in Fig. 2. The direction of the incident beam did not accurately coincide with [OOl], direction but inclined about 5 degree, so as to excite the diffraction from the precipitate. Lattice spacing corresponding to the diffraction spot A is about 0.62 nm, and that for B is the same. Reciprocal lattice vectors go and go are crossed at right angle. The crystal structure of AlsNi was described as orthorhombic system with lattice constants of a0 = 0.65982, bo = 0.73515 and co = 0.48021 mn by Bradley and Tayler (4). The lattice spacings for the spots A and B (=0.62 nm) in Fig. 2 are close to the lattice constant a0 of AbNi, however, the facts that g,, and go are crossed at right angle with the same lattice spacings can not be interpreted by the orthorhombic crystal structure mentioned above. Thus the spots A and B are tentatively indexed as (010) and (loo), respectively, which means that the lattice constants are assumed to be a0 = bo = 0.62 nm, and the resultant key diagram is shown in Fig. 3 with matrix (a) spots. Orientation relationships between a and the precipitate is as follows:
Figure 1. Precipitates in the AI-250 mass ppm Ni alloy aged at 673 K for 432 ks
Vol. 37, No. 11
AlgNiz PRECIPITATE
1723
-. 0 Figure 2. Selected area diffraction nattem of the precipitate parallel to (OOl), like as B in Fig. 1.
.:a,
-5
_ _ 6200 -a
l:Precipitate
Figure 3. Key diagramfor Fig. 2 assuming the spots A and B in Fig. 2 as 014 and 100,. Linrs A-A’,EJ-B’ andC<_ show the intersections between the (OOl), plane and (3 IO), (1 IO), and (1 2 0), planes.
Figure 4. Selected. area$fractio_n patterns ofthe precipitate perpendicular to (OOl), like as A in Fig. 1. Directions of incident beam are parallel to [3 1 0],, [l 1 01, and [I 201, in (a), (b) and (c), respectively.
1724
AlgNi2PRECIPITATE
Vol. 37, No. 11
In order to understand reciprocal lattice points in the directions containing the c-axis of the precipitate, SADPs were also taken on the precipitate such as A in Fig. 1. Although the habit plane is parallel to (loo), or (OlO), in this case, the same indices as Fig. 3 for the matrix are applied to describe the direction in order to make easy understanding of orientation relationship. The SADPs in the [3 TO],, [1 TO], and [l ?O],directions are shown in Fig. 4. These SADPs represent the reciprocal lattice patterns perpendicular to (OOl), with lines of intersection A-A’, B-B’ and C-C’ shown in Fig. 3, respectively. The lattice spacing corresponding to the spot A in Fig. 4 (a) is about 0.85 nm, which appears in Figs. 4 (b) and (c) with the same lattice spacing. The lattice spacings for the spots B, C and D in Fig. 4 are 0.62, 0.27 and 0.43 nm, respectively. Indices of the planes for the spots B, C and D can be deduced from the assumed lattice constants mentioned above as (0 IO), (120) and (1 IO), respectively, and that for A is considered to be (OOl), which means that the lattice constant co = 0.85 nm. The angles between g A and ge (Fig. 4(a)), i A and & @I, and go and gn (c) are about 90, 83 and 85 degree, respectively, from which a crystal structure of the precipitate is deduced to be a monoclinic system with the angle p = 95 degree. An extracted precipitate from the specimen aged at 793 K for 274 ks is shown in Fig. 5, which is large enough for the energy dispersive X-ray (EDX) analysis. Shape of the precipitate is not proper one, because it was broken during the extraction process. The SADP inserted in Fig. 5 is the same to that in Fig. 2. Conpositional analysis by means of EDX analyzer showed that the precipitate consisted of Al and Ni and the concentrations of Al and Ni were 82.4 and 17.6 at.%, respectively. The ratio of Al&Ii is 4.68 which is close to 4.5, that is, the precipitate would be A19Niz. In other Al alloys involving transitional elements, there exists intermetallic phases having a monoclinic crystal structure and stoichiometric composition of A&X2, that is, Al&o2 (a = 0.8556, b = 0.6290, c = 0.6213 nm, p = 94.76 degree) (5) A19Fe2 (a = 0.869, b = 0.635, c = 0.632 nm, p = 94.76 degree) (6) and A&FeNi (a = 0.8598, b = 0.6271, c = 0.6207 run, fi = 94.66 degree) (7). The crystal structure of the precipitate determined in the present study (a = b = 0.62, c = 0.85 nm, p = 95 degree) is very close to those one, although the lattice constants for a,, and COare replaced each other. Similarity in lattice parameters among these phases seems to assure the proposed composition, i.e., Al$?iz. Extracted precipitates from the specimen aged at 823 K for 1210 ks are shown in Fig. 6. Selected area diffraction pattern of the precipitate indicated with the arrow in (a) is shown in (b). Lattice spacings corresponding to A, B and C in (b) are 0.72, 0.34 and 0.38 nm, respectively. The angle between
Figure 5. Extracted precipitate in the specimen aged at 793 K for 274 ks.
Figure 6. Extracted precipitates in the specimens aged at 823 K for 1210 ks:(a), SADP for the precipitate indicated by the arrow in (a):(b) and key diagram for (b):(c).
Vol. 37, No. 11
AlgNi2PRECIPITATE
1725
go and ga is about 62 degree, and that for ga and jj c is about 28 degree. The lattice spacing close to 0.72 nm is absent in the monoclinic crystal structure of A19Niz,while those of 0.34 and 0.38 nm are close to 0.35 (( 012) plane) or 0.34 nm ((102) plane) and 0.38 nm ((111) plane) in A19Ni2.However, the angle between ( 012) and (111) in AlgNiz is about 94 degree and that between (102) and (111) is about 41 degree, which shows that the precipitate in Fig. 6 (a) is not an AbNiz. In AhNi, on the other hand, there an: the lattice spacings of 0.735, 0.343 and 0.383 nm for (OlO), (111) and (101) planes, respectively, and the angle between (111) and (101) planes is 27.9 degree, which agrees with the measured value (28 degree). The key diagram indexed as AlsNi is shown in (c). That is, the precipitate in Fig. 6 (a) is not the AlgNizbut the stable phase, AbNi. It is considered that the A19Ni2is an metastable phase with high thermal stability. Summary 1) In an Al-rich Al-Ni alloy, precipitation sequence is as follows: Supersaturated solid solution + metastable phase, A19Niz+ stable phase, AhNi. 2) The metastable A19Niz has a monoclinic crystal structure with the lattice parameters of a = b = 0.62, I: = 0.85 nm and j3= 95 degree. 3) Orientation relationships between the A19Nizand the matrix as follows: (ooi),//(ooi),, [3 iol~/[ioo],. 4) The habit planes of the precipitate are (00 1},. Acknowledgements The authors wish to thank Professor Emeritus, H.I. Aaronson of Carnegie-Mellon University in the U.S.A. and Professor K. Chattopadhyay of Indian Institute of Science for their helpful suggestions and discussions. References 1. 2. 3. 4. 5. 6. 7.
W.L.Fink and L.A. Willey, Trans. AIMS. 111,293 (1934). H. Tsubakino, U. Ishihara and A. Yamamoto, Mater. Sci. Forum, 217-222,901 (1996). F. Kurosawa, I. Taguchi and R. Matsumoto, J. Jpn. Inst. Met. 43, 1068 (1979). A.J. Bradley and A. Taylor, Philos. Msg. 23, 1049 (1937). A.M. Douglas, ActaCrystallogr. 3,19 (1950). C.J. Simensen and R. Vellasamy, 2. Metallkd. 68,426 (1977). M. Khaidar, C.H. Allibert and J. Driole, ibid. 73,433 (1982).