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A TEM study of long-period stacking in a ZrCr2 Laves phase
M E T A L L O G R A P H Y 18:183-192 (1985)
I g3
SHORT COMMUNICATIONS A TEM Study of Long-Period Stacking in a ZrCr2 Laves Phase
XIAN YING MEN(}* AND DEREK (). NORTHWO()D
Department of Engineering, Materials, University of Windsor. Wind.~or, Ontario. Canada N9B 3P4
Introduction ZrCr2 is an intermetallic phase, the structure of which depends on the electron concentration. ZrCr2 can form one of two structure types, namely, the cubic CI5-MgCu2 prototype Laves structure [1] or the hexagonal C14-MgZn2 structure [2]. ZrCr2 and associated pseudobinaries are presently being investigated as hydrogen storage materials [3, 4]. The ZrCr~ binary alloy absorbs large quantities of hydrogen, approximately 1.3 atoms per formula unit [5, 6]. However, because of its low plateau pressure (-0.1 kPa), ZrCr2 is of little use as an energy storage material. By partial substitution of Fe for Cr in ZrCr2 equilibrium pressures can be raised to more acceptable levels [3, 7]. Structure of Zr(FexCrl-.x)2 intermetallic Laves phases with x = 0-1.0 are being investigated as part of a larger study into their suitability for hydrogen (energy) storage application. ZrCr~ and Zr(M,Cr~ ~), Laves phases also are being investigated as the precipitates in Zircaloy-2 and Zircaloy-4. Their structure, morphology, and distribution can have a large effect on the structure and properties of the alloys. The present note reports some preliminary studies of the ZrCr2 binary using electron diffraction and high resolution electron microscopy.
*Presently on leave from Beijing Center of Physical and Chemical Analysis. Beijing, China.
© Elsevier Science Publishing Co. inc., 1985 52 Vanderbilt Ave., New York. NY 10017
0026-0800/85/$03.30
184
X. Y. Meng and D. O. Northwood
Experimental Details The ZrCr2 alloy was prepared by melting components in an arc furnace under a high purity argon atmosphere. High purity elements were utilize.d, i.e., 99.9% pure zirconium and 99.99% pure chromium. The alloy was melted four times to ensure compositional uniformity. The ingot was very brittle, falling into a powder when cut using a low speed diamond saw. This powder was ground lightly using a pestle and mortar to produce a finer powder. A suspension of the powder was made with alcohol and drops of the suspension were placed on a " h o l e y " Formvar film [8]. Specimens were then examined using a transmission electron microscope equipped with the double tilt specimen holder. An operating voltage of 100 KV was used. The composition of the powder was checked using x-ray energy dispersive spectroscopy on an SEM.
Results and Discussion Figure l a shows the general size and shape of the ZrCr2 particles (powder). Many of the particles are hexagonal in shape. Figure lb is an electron
I
(a) FI(~. 1. (a) Bright-field electron micrograph showing shape of ZrCr2 powders; (b) corresponding diffraction pattern of [001] zone axis; (c) dislocation distribution along the (100) type directions in a ZrCr2 particle.
Long-Period Stacking in a ZrCr2 Laves Phase
185
(b)
~m 1
(c)
186
X. Y. Meng and D. O. Northwood
diffraction pattern of the hexagonal shape particle taken down the [001] zone axis. The particles themselves contain a number of dislocations (Fig. lc), which appear to be distributed along the (100) type directions. The general angles included between two dislocations are 60 ° The sample was tilted around gl,o using the double tilt holder, and systematic electron diffraction patterns were taken for [001], [114], [113], [112], [223], [221], [114], [113], [112], [223], and [332] zone axes. Representative electron diffraction patterns are given in Fig. 2.
(a)
(b)
(c)
(d)
FI~. 2. Electron diffraction patterns including (HI~IO) * diffraction spots array from a C14-prototype ZrCr2 cyrstal. (a) [113], (b) [112], (c) [223], and (d) [221] zone axis.
Long-Period Stacking in a ZrCr: Laves Pkase
187
The tilt angle of the crystal. 0, was calculated using the formula: tan20 = tan2v ~- tan2y + tan2x tanZv where x and ), are the tilt angles for the two axes in the double tilt holder. The projection of the reciprocal lattice down [110]* was constructed using the systematic ~ and the tilt angle [9], and is shown in Fig. 3. The figure shows that the c axis of the crystal is perpendicular to [100]. Using the information contained in the diffraction patterns given in Figs. l b. 2, and 3, and the [010] zone axis pattern (Fig. 4b). ZrCr2 is deduced to have the C14 hexagonal Laves phase structure with oo - 0.5079 nm and co = 0.8367 nm. Lattice plane images obtained by high resolution electron microscopy and the associated diffraction patterns provided evidence for the presence of long-period stacking variants in the C14 structure. Figure 4 shows heavily faulted stacking in the Laves phase in as-cast ZrCre. For a close-packed hexagonal structure, extinction of the diffraction spots will occur when H - K = 3n (rt is an integer) and k is odd. Only the diffraction spots for which L is an even number will appear in 00L. Usually the presence and intensity of diffraction spots in + 10k and _+20L are used to analyze ~he I,tyers in the ,mit cells of long-period structures. The streaked electron diffraction pattern (Fig. 4b) indicates that heavy faulting exists. A 20-layer structure has been found by enlarging the diffraction pattern. The region labelled A in Fig. 4a is the typical 20layer structure. For the close-packed hexagonal lattice, a 20-layer structure is identified by the symbol 20H using the Remsdell notation. Figure 5 shows a 12H type long-period stacking with the long-period distance equal to 4.9201 nm. The long-period distance was calculated C~
114
004 003
,
7"
002
/113 9
112""
"
I
oo~,j
"222_
o 110
220
&
113 -
5~-
-
221-
~ - 1 1 7)
oo,~,
...." 2 2 3
111 _---'-
111.. 002.
o----
114
334
.o
_ -
.~__ ~-;~-~
FIG. 3.
224
,9
..... 9 333
'
-~--'9-332
-
331
o.--
.....
_
o--
330
221
-331
-- ~
~-- 22'~
-~
223" o
224
>--o-.
33 "~
-~6--
332 o--
334.
Projection of reciprocal lattice down ! 1 I[]]~ for hexagonal lattice ZrCr: phase
tk
!i!iiii,iiiiiii!ill ii~iiii!~ (a~r~::iiiii
l S nm
iiii~::ii!i~
'
F~G. 4. (a) [010] lattice plane image of ZrCr2 Laves phase showing multiple fault stacking and the 20H structure; (b) the associated diffraction pattern. 188
Long-Period Stacking in a ZrCr2 Laves Phase
189
using the information contained in the diffraction pattern given in Fig. 5c. F r o m the two diffraction patterns of 12H structure, it can be seen that each diffraction pattern c o r r e s p o n d s to a mixture structure. Figure 5b shows a mixture of a 12-layer type structure and an 8-layer type structure. The 8-layer structure is indicated in the 20L diffraction spots. Figure 5c shows clearly a mixture of a 12-layer type structure and a 16-layer type structure. The 16-layer structure exists b e t w e e n (103)* and (106)*, (1015)* and (1018)*, and (10b)* and (108)* of the 12-layer structure orderly. The lattice images show the 12-layer structure together with an n × 12-layer structure. L o n g - p e r i o d stacking structures have been found in other L a v e s phases, such as the Mg-based hexagonal structures [10-13]. The ZrCr2 phase contains defects other than f r o m stacking sequences and these can be seen in the lattice plane images. Areas marked A or B in Fig. 5a indicate the presence of either a dislocation (A) or dislocation loop (B) within the structure. The orientation and nature of the dislocation
(a) FIG. 5. (a) [010] lattice plane image of 12H along with n x 12H type structures. (b) The associated diffraction pattern. (c) An electron diffraction pattern showing a mixture of a 12layer type structure and a 16-layer type structure.
190
Long-Period Stacking in a ZrCr2 Laves Phase
191
indicated by the lattice images agree with those deduced from Fig. lc,
where diffraction contrast is used to image the dislocation.
Conclusions l. ZrCr2 in the as-cast condition has a CI4-MgZn2 hexagonal type Laves structure with ao = 0.5079 nm and co = 0.8367 nm 2. The ZrCr2 Laves phase has many lattice defects. 12H, n × 12H, and 20H long-period stacking structures were found. Mixtures of either the 12-layer type structure and the 8-layer type structure or the 12-layer type structure and the 16-layer type structure were also found. The long-period unit cell parameters for the 12-layer structure were ao = 0.5079 nm and co = 4.9201 nm
The authors are grateful for the financial assistance of the Natural Sciences and Engineering Research Council of Canada (grant A4391). Sincere thanks are due to Professor D. F. Watt for stimulating discussions.
REFERENCES I. J. B. Friauf, The crystal structures of two intermetallic compounds, J. Amer. Chem. Soc. 49:3107-3114 (1927). 2. J. B. Friauf, The crystal structure of magnesium di-zincide, Phys. Rev. 29:34-40 11927). 3. D. G. lvey and D. O. Northwood, Storing energy in metal hydrides: A review of the physical metallurgy, J. Mater. Sci. 18:321-347 (1983). 4. D. G. Ivey and D. O. Northwood, Hydrogen storage characteristics of Zr(B~BI ~)_~, B = Fe, Co, B' = Cr, Mn, and x = 0.4, 0.5, 0.6, J. Mat. for Energy Systems 4:222228 (1983). 5. A. Pebler and E. A. Gulbrangen, Equilibrium studies on the systems ZrCrz-H2, ZrVzHz, and ZrMo2-H2 between 0° and 900°C, Trans. A1ME 239:1593-1600 11967). 6. D. Fruchart, A. Rouautt, C. B. Shoemaker, and D. P. Shoemaker. Neutron diffraction studies of the cubic ZrCrzDx and ZrVzDx(Hx) phases. J. ~ f the Less-Common Metals 73:363-368 11980). 7. D. G. Ivey and D. O. Northwood, Hydrogen absorption-desorption characteristics of Zr(FexCr~ x)z, in Hydrogen Energy progress V, Proceedings of the 5th World Hydrogen Energy Conference, Vol. 3 (T. N. Veziroglu, J. B. Taylor, eds.), International Association for Hydrogen Energy, New York (1984), Chap. 22, pp. 1395-1404. 8. D. Van Dyck, Direct structure imaging in electron microscopy, in Diffraction and lmaging Techniques in Material Science (S. Amelinckx, R. Gevers and J. Van Landuyt, eds.), North-Holland, Amsterdam (1978), Vol. 1, pp. 355-396. 9. J. A. Gard (ed.), The Electron Optical Investigation o f Clays, Mineralogical Society, London (1971), Chap. 2, pp. 27-45. 10. Y. Komura and K. Tokunaga, Structure studies of stacking variants in Mg-base FriaufLaves phase, Acta Co'st. B36:1548-1554 11980). 11. Y. Komura and Y. Kitano, Long-period stacking variants and their electron-concen-
192
X. Y. M e n g and D. O. N o r t h w o o d
tration dependence in the Mg-base Friauf-Laves phases, Acta Cryst. B33:2496-2501 (1977). 12. Y. Kitano, Y. Komura, and H. Kajiwara, Two-dimensional lattice images of the Mgbase Friauf-Laves phase and a new type defect, Acta Cryst. A36:16-21 (1980). 13. Y. Kitano, Y. Komura, and H. Kajiwara, Electron microscope observation of FriaufLaves phase Mg (Cul xAlx): with x = 0.465, Trans. Japan Inst. Met. 18:39-45 (1977).
I g3
SHORT COMMUNICATIONS A TEM Study of Long-Period Stacking in a ZrCr2 Laves Phase
XIAN YING MEN(}* AND DEREK (). NORTHWO()D
Department of Engineering, Materials, University of Windsor. Wind.~or, Ontario. Canada N9B 3P4
Introduction ZrCr2 is an intermetallic phase, the structure of which depends on the electron concentration. ZrCr2 can form one of two structure types, namely, the cubic CI5-MgCu2 prototype Laves structure [1] or the hexagonal C14-MgZn2 structure [2]. ZrCr2 and associated pseudobinaries are presently being investigated as hydrogen storage materials [3, 4]. The ZrCr~ binary alloy absorbs large quantities of hydrogen, approximately 1.3 atoms per formula unit [5, 6]. However, because of its low plateau pressure (-0.1 kPa), ZrCr2 is of little use as an energy storage material. By partial substitution of Fe for Cr in ZrCr2 equilibrium pressures can be raised to more acceptable levels [3, 7]. Structure of Zr(FexCrl-.x)2 intermetallic Laves phases with x = 0-1.0 are being investigated as part of a larger study into their suitability for hydrogen (energy) storage application. ZrCr~ and Zr(M,Cr~ ~), Laves phases also are being investigated as the precipitates in Zircaloy-2 and Zircaloy-4. Their structure, morphology, and distribution can have a large effect on the structure and properties of the alloys. The present note reports some preliminary studies of the ZrCr2 binary using electron diffraction and high resolution electron microscopy.
*Presently on leave from Beijing Center of Physical and Chemical Analysis. Beijing, China.
© Elsevier Science Publishing Co. inc., 1985 52 Vanderbilt Ave., New York. NY 10017
0026-0800/85/$03.30
184
X. Y. Meng and D. O. Northwood
Experimental Details The ZrCr2 alloy was prepared by melting components in an arc furnace under a high purity argon atmosphere. High purity elements were utilize.d, i.e., 99.9% pure zirconium and 99.99% pure chromium. The alloy was melted four times to ensure compositional uniformity. The ingot was very brittle, falling into a powder when cut using a low speed diamond saw. This powder was ground lightly using a pestle and mortar to produce a finer powder. A suspension of the powder was made with alcohol and drops of the suspension were placed on a " h o l e y " Formvar film [8]. Specimens were then examined using a transmission electron microscope equipped with the double tilt specimen holder. An operating voltage of 100 KV was used. The composition of the powder was checked using x-ray energy dispersive spectroscopy on an SEM.
Results and Discussion Figure l a shows the general size and shape of the ZrCr2 particles (powder). Many of the particles are hexagonal in shape. Figure lb is an electron
I
(a) FI(~. 1. (a) Bright-field electron micrograph showing shape of ZrCr2 powders; (b) corresponding diffraction pattern of [001] zone axis; (c) dislocation distribution along the (100) type directions in a ZrCr2 particle.
Long-Period Stacking in a ZrCr2 Laves Phase
185
(b)
~m 1
(c)
186
X. Y. Meng and D. O. Northwood
diffraction pattern of the hexagonal shape particle taken down the [001] zone axis. The particles themselves contain a number of dislocations (Fig. lc), which appear to be distributed along the (100) type directions. The general angles included between two dislocations are 60 ° The sample was tilted around gl,o using the double tilt holder, and systematic electron diffraction patterns were taken for [001], [114], [113], [112], [223], [221], [114], [113], [112], [223], and [332] zone axes. Representative electron diffraction patterns are given in Fig. 2.
(a)
(b)
(c)
(d)
FI~. 2. Electron diffraction patterns including (HI~IO) * diffraction spots array from a C14-prototype ZrCr2 cyrstal. (a) [113], (b) [112], (c) [223], and (d) [221] zone axis.
Long-Period Stacking in a ZrCr: Laves Pkase
187
The tilt angle of the crystal. 0, was calculated using the formula: tan20 = tan2v ~- tan2y + tan2x tanZv where x and ), are the tilt angles for the two axes in the double tilt holder. The projection of the reciprocal lattice down [110]* was constructed using the systematic ~ and the tilt angle [9], and is shown in Fig. 3. The figure shows that the c axis of the crystal is perpendicular to [100]. Using the information contained in the diffraction patterns given in Figs. l b. 2, and 3, and the [010] zone axis pattern (Fig. 4b). ZrCr2 is deduced to have the C14 hexagonal Laves phase structure with oo - 0.5079 nm and co = 0.8367 nm. Lattice plane images obtained by high resolution electron microscopy and the associated diffraction patterns provided evidence for the presence of long-period stacking variants in the C14 structure. Figure 4 shows heavily faulted stacking in the Laves phase in as-cast ZrCre. For a close-packed hexagonal structure, extinction of the diffraction spots will occur when H - K = 3n (rt is an integer) and k is odd. Only the diffraction spots for which L is an even number will appear in 00L. Usually the presence and intensity of diffraction spots in + 10k and _+20L are used to analyze ~he I,tyers in the ,mit cells of long-period structures. The streaked electron diffraction pattern (Fig. 4b) indicates that heavy faulting exists. A 20-layer structure has been found by enlarging the diffraction pattern. The region labelled A in Fig. 4a is the typical 20layer structure. For the close-packed hexagonal lattice, a 20-layer structure is identified by the symbol 20H using the Remsdell notation. Figure 5 shows a 12H type long-period stacking with the long-period distance equal to 4.9201 nm. The long-period distance was calculated C~
114
004 003
,
7"
002
/113 9
112""
"
I
oo~,j
"222_
o 110
220
&
113 -
5~-
-
221-
~ - 1 1 7)
oo,~,
...." 2 2 3
111 _---'-
111.. 002.
o----
114
334
.o
_ -
.~__ ~-;~-~
FIG. 3.
224
,9
..... 9 333
'
-~--'9-332
-
331
o.--
.....
_
o--
330
221
-331
-- ~
~-- 22'~
-~
223" o
224
>--o-.
33 "~
-~6--
332 o--
334.
Projection of reciprocal lattice down ! 1 I[]]~ for hexagonal lattice ZrCr: phase
tk
!i!iiii,iiiiiii!ill ii~iiii!~ (a~r~::iiiii
l S nm
iiii~::ii!i~
'
F~G. 4. (a) [010] lattice plane image of ZrCr2 Laves phase showing multiple fault stacking and the 20H structure; (b) the associated diffraction pattern. 188
Long-Period Stacking in a ZrCr2 Laves Phase
189
using the information contained in the diffraction pattern given in Fig. 5c. F r o m the two diffraction patterns of 12H structure, it can be seen that each diffraction pattern c o r r e s p o n d s to a mixture structure. Figure 5b shows a mixture of a 12-layer type structure and an 8-layer type structure. The 8-layer structure is indicated in the 20L diffraction spots. Figure 5c shows clearly a mixture of a 12-layer type structure and a 16-layer type structure. The 16-layer structure exists b e t w e e n (103)* and (106)*, (1015)* and (1018)*, and (10b)* and (108)* of the 12-layer structure orderly. The lattice images show the 12-layer structure together with an n × 12-layer structure. L o n g - p e r i o d stacking structures have been found in other L a v e s phases, such as the Mg-based hexagonal structures [10-13]. The ZrCr2 phase contains defects other than f r o m stacking sequences and these can be seen in the lattice plane images. Areas marked A or B in Fig. 5a indicate the presence of either a dislocation (A) or dislocation loop (B) within the structure. The orientation and nature of the dislocation
(a) FIG. 5. (a) [010] lattice plane image of 12H along with n x 12H type structures. (b) The associated diffraction pattern. (c) An electron diffraction pattern showing a mixture of a 12layer type structure and a 16-layer type structure.
190
Long-Period Stacking in a ZrCr2 Laves Phase
191
indicated by the lattice images agree with those deduced from Fig. lc,
where diffraction contrast is used to image the dislocation.
Conclusions l. ZrCr2 in the as-cast condition has a CI4-MgZn2 hexagonal type Laves structure with ao = 0.5079 nm and co = 0.8367 nm 2. The ZrCr2 Laves phase has many lattice defects. 12H, n × 12H, and 20H long-period stacking structures were found. Mixtures of either the 12-layer type structure and the 8-layer type structure or the 12-layer type structure and the 16-layer type structure were also found. The long-period unit cell parameters for the 12-layer structure were ao = 0.5079 nm and co = 4.9201 nm
The authors are grateful for the financial assistance of the Natural Sciences and Engineering Research Council of Canada (grant A4391). Sincere thanks are due to Professor D. F. Watt for stimulating discussions.
REFERENCES I. J. B. Friauf, The crystal structures of two intermetallic compounds, J. Amer. Chem. Soc. 49:3107-3114 (1927). 2. J. B. Friauf, The crystal structure of magnesium di-zincide, Phys. Rev. 29:34-40 11927). 3. D. G. lvey and D. O. Northwood, Storing energy in metal hydrides: A review of the physical metallurgy, J. Mater. Sci. 18:321-347 (1983). 4. D. G. Ivey and D. O. Northwood, Hydrogen storage characteristics of Zr(B~BI ~)_~, B = Fe, Co, B' = Cr, Mn, and x = 0.4, 0.5, 0.6, J. Mat. for Energy Systems 4:222228 (1983). 5. A. Pebler and E. A. Gulbrangen, Equilibrium studies on the systems ZrCrz-H2, ZrVzHz, and ZrMo2-H2 between 0° and 900°C, Trans. A1ME 239:1593-1600 11967). 6. D. Fruchart, A. Rouautt, C. B. Shoemaker, and D. P. Shoemaker. Neutron diffraction studies of the cubic ZrCrzDx and ZrVzDx(Hx) phases. J. ~ f the Less-Common Metals 73:363-368 11980). 7. D. G. Ivey and D. O. Northwood, Hydrogen absorption-desorption characteristics of Zr(FexCr~ x)z, in Hydrogen Energy progress V, Proceedings of the 5th World Hydrogen Energy Conference, Vol. 3 (T. N. Veziroglu, J. B. Taylor, eds.), International Association for Hydrogen Energy, New York (1984), Chap. 22, pp. 1395-1404. 8. D. Van Dyck, Direct structure imaging in electron microscopy, in Diffraction and lmaging Techniques in Material Science (S. Amelinckx, R. Gevers and J. Van Landuyt, eds.), North-Holland, Amsterdam (1978), Vol. 1, pp. 355-396. 9. J. A. Gard (ed.), The Electron Optical Investigation o f Clays, Mineralogical Society, London (1971), Chap. 2, pp. 27-45. 10. Y. Komura and K. Tokunaga, Structure studies of stacking variants in Mg-base FriaufLaves phase, Acta Co'st. B36:1548-1554 11980). 11. Y. Komura and Y. Kitano, Long-period stacking variants and their electron-concen-
192
X. Y. M e n g and D. O. N o r t h w o o d
tration dependence in the Mg-base Friauf-Laves phases, Acta Cryst. B33:2496-2501 (1977). 12. Y. Kitano, Y. Komura, and H. Kajiwara, Two-dimensional lattice images of the Mgbase Friauf-Laves phase and a new type defect, Acta Cryst. A36:16-21 (1980). 13. Y. Kitano, Y. Komura, and H. Kajiwara, Electron microscope observation of FriaufLaves phase Mg (Cul xAlx): with x = 0.465, Trans. Japan Inst. Met. 18:39-45 (1977).