- Email: [email protected]
C-type dislocations emitted from cracks introduced in a thin foil of LaNi5
Journal of Alloys and Compounds 269 (1998) 294–296
L
C-type dislocations emitted from cracks introduced in a thin foil of LaNi 5 a, a b a H. Inui *, T. Yamamoto , Zhang Di , M. Yamaguchi a
b
Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606 -8501, Japan State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, China Received 5 January 1998; received in revised form 10 January 1998
Abstract The nature of dislocations emitted from cracks introduced in a thin foil of LaNi 5 has been characterized by means of transmission ¯ electron microscopy. Dislocations emitted from cracks are determined to be of the c-type (b (Burgers vector)5[0001]) moving on h1010j ¯ prism planes. Although grown-in dislocations of the a-type (b51 / 3k1120l) locate near cracks where the emission of c-type dislocations occurred, these a-type dislocations are not observed in motion, indicating the difficulty in the motion of a-type dislocations. The implications of these results for controlling the extent of cracking during cycles of hydriding–dehydriding of alloys based on LaNi 5 are discussed. 1998 Elsevier Science S.A. Keywords: LaNi 5 ; Powdering; Dislocation; Slip system; Transmission electron microscopy
1. Introduction Alloys based on the intermetallic phase, LaNi 5 (the hexagonal CaCu 5 -type structure; Fig. 1) have been used as negative electrode materials of rechargeable nickel-metal
Fig. 1. The crystal structure of LaNi 5 and some possible Burgers vectors of dislocations. *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 98 )00164-9
hydride (Ni-MH) batteries because of their fast activation, high storage-capacity, long life-cycle and excellent electrochemical charge / discharge kinetics [1–4]. Powdering of these alloys after cycles of hydriding–dehydriding is one of the properties to be improved to make the life of Ni-MH batteries longer. The ease of powdering is a direct consequence of the brittleness of these alloys. Brittle cracking occurs to relieve the stress accumulated during cycles of hydriding–dehydriding, resulting in powdering of these alloys. However, if dislocations can occur and move easily in these alloys, the accumulated stress may be relieved by the motion of dislocations. Thus, knowledge of dislocation properties is indispensable for improving the life of Ni-MH batteries. However, since there have been only a limited number of studies on dislocations in LaNi 5 -based alloys [5,6], the nature of dislocations such as Burgers vectors, slip planes and the stress needed to set them in motion have remained largely unknown. In the present paper, we present the results of our transmission electron microscopy (TEM) study on the nature of dislocations in LaNi 5 . Dislocations analyzed include not only grown-in ones, but also those introduced accidentally in a thin foil TEM specimen.
2. Experimental procedures A LaNi 5 alloy was prepared by Ar arc-melting of
H. Inui et al. / Journal of Alloys and Compounds 269 (1998) 294 – 296
295
3. Results
Fig. 2. Dislocations emitted from cracks in a TEM foil of LaNi 5 . The crystallographic directions are indicated in the figure.
high-purity La (4N-grade) and Ni (4N-grade). The alloy was annealed at 9308C for 48 h in vacuum. The standard double-jet polishing method with a solution of perchloric acid and methanol was employed to perforate TEM foils. Dislocations were accidentally introduced in a thin foil specimen when the specimen was set onto a TEM holder. TEM observations were made with a JEM-2000FX electron microscope operated at 200 kV. Burgers vector (b) determinations were made according to the standard g?b criteria (g: reflection vector) [7].
In Fig. 2 the dislocations introduced in a TEM foil of LaNi 5 are shown. As is evident from the curvature of dislocations imaged, they are emitted from cracks intro¯ duced by mishandling. The foil normal is close to [1010] and the crystallographic directions are indicated in the figure. Most of the dislocations are in the form of trains on slip planes whose trace is parallel to the [0001] direction. Dislocations seen in Fig. 2 have a Burgers vector with a c-component, since they are visible with g50002. The result of contrast analysis made to determine the nature of these dislocations in Fig. 2 is presented in Fig. 3. Dislocations emitted from cracks are visible when imaged with g50002 (Fig. 3(a)–(c)), but invisible when imaged with ¯ ¯ (Fig. 3(e)), yielding the g50220, (Fig. 3(d)) and g52420 Burgers vector b5[0001] (c-type dislocations). On tilting the specimen about [0001], the projection width of the slip ¯¯ planes is wider when imaged along [1120] (Fig. 3(b)) than ¯ when imaged along [1010] (Fig. 3(a)), and they are seen ¯ end-on when imaged along [2110] (Fig. 3(c)), indicating ¯ that the slip planes are (0110). Thus, the slip system of dislocations emitted from cracks is determined to be ¯ [0001]. Weak-beam imaging of dislocations emitted (0110) from cracks reveals no appreciable dissociation, as shown in Fig. 4. This indicates the high stacking fault energy on prism planes. We observed some grown-in dislocations. An example of grown-in dislocations is seen in Fig. 3. Dislocations
¯ ¯¯ ¯ Fig. 3. Contrast analysis of dislocations shown in Fig. 2; (a) g50002 / B (beam direction)5[1010], (b) g50002 / B5[1120], (c) g50002 / B5[2110], (d) ¯ / B5[2110], ¯ ¯ / B5[1010]. ¯ g50220 (e) g52420
296
H. Inui et al. / Journal of Alloys and Compounds 269 (1998) 294 – 296
deed, numerous a-type dislocations are observed to be introduced as misfit dislocations at interfaces between the matrix and hydrides after hydriding treatments [5,6]. However, the present result indicates that the motion of a-type dislocations is more difficult than that of c-type dislocations. This may be one of the most plausible reasons why cracking occurs so easily in LaNi, resulting in the ease of powdering. We believe that a strategy to make a slip (slip via a-type dislocations) in LaNi 5 easier is needed to be devised in order to control the extent of cracking, thereby making the life of Ni-MH batteries last longer.
5. Conclusions Fig. 4. Weak-beam image of dislocations emitted from cracks. The image ¯ was recorded with g50002 near the [1010] zone-axis orientation.
marked A in Fig. 3(d) and (e) are invisible when imaged with g50002 (Fig. 3(a)–(c)), indicating that they are a-type ¯ ¯ dislocations with b51 / 3k1120l (either b51 / 3[1120] or ¯ b51 / 3[1210]). We also observed grown-in dislocations ¯ with b5[0001]. Grown-in dislocations with b51 / 3k1120l and b5[0001] seem to occur in almost equal fractions.
4. Discussion From the crystallography point of view, we expect ¯ basal several possible slip systems such as (0001)k1120l ¯ ¯ prism slip in the hexagonal LaNi 5 slip and h1100jk1120l ¯ lattice (Fig. 1). The slip system of h1010j[0001] presently identified has been rarely observed in crystals with hexagonal symmetry. Even though a-type grown-in dislocations locate near cracks where the emission of c-type dislocations is observed (Fig. 3(d) and(e)), their motion was not observed. This may indicate that c-type dislocations can move on prism planes more easily than a-type dislocations can do on either basal or prism planes, although it is very difficult to evaluate what stress was resolved for these possible slip systems when the crack was introduced in the thin foil. Determining operative slip systems and their critical resolved shear stresses in LaNi 5 is currently in progress in our group using single crystals. Strains accumulated during cycles of hydriding–dehydriding have been reported to be anisotropic from measurements of X-ray line-broadening [8–10]; the extent of line-broadening for hhki0j reflections increases with an increase in the number of cycles of hydriding–dehydriding, while that for h000lj reflections stays almost constant. This may indicate that only a-type dislocations are accumulated during cycles of hydriding–dehydriding. In-
The nature of dislocations emitted from cracks introduced in a thin foil of LaNi 5 is determined to be of the ¯ prism planes. Both a- and c-type c-type moving on h1010j grown-in dislocations occur almost in equal fractions. The motion of a-type dislocations seems to be more difficult than that of c-type dislocations. Since a-type dislocations have been reported to be introduced as misfit dislocations during cycles of hydriding–dehydriding, a strategy to make the motion of a-type dislocations easier needs to be devised to improve the life of Ni-MH batteries.
Acknowledgements This work was supported by Grant-in-Aid for Scientific Research (No. 08555162) from the Ministry of Education, Science and Culture, Japan.
References [1] J.H.N. van Vucht, F.A. Kuijpers, H.C.A.M. Bruning, Philips Res. Rep. 25 (1970) 133. [2] J.J.G. Willems, K.H.J. Buschow, J. Less-Common Metals 129 (1987) 13. [3] P.H.L. Notten, R.E.F. Einerhand, Adv. Mater. 3 (1991) 343. [4] E.H. Kisi, C.E. Buckley, E.M. Gray, J. Alloys Comp. 185 (1992) 369. [5] G.-H. Kim, C.-H. Chun, S.-G. Lee, J.Y. Lee, Acta Metall. Mater. 42 (1994) 3157. [6] G.-H. Kim, S.-G. Lee, K.-Y. Lee, C.-H. Chun, J.Y. Lee, Acta Metall. Mater. 43 (1995) 2233. [7] P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley, M.J. Whelan, Electron Microscopy of Thin Crystals, Butterworths, London, 1967. [8] A. Percheron-Guegan, C. Lartigue, J.C. Achard, P. Germi, F. Tasset, J. Less-Common Metals 74 (1980) 1. [9] J.F. Lynch, J.J. Reilly, J. Less-Common Metals 87 (1982) 225. [10] R.L. Cohen, K.W. West, J. Less-Common Metals 95 (1983) 17.
L
C-type dislocations emitted from cracks introduced in a thin foil of LaNi 5 a, a b a H. Inui *, T. Yamamoto , Zhang Di , M. Yamaguchi a
b
Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606 -8501, Japan State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, China Received 5 January 1998; received in revised form 10 January 1998
Abstract The nature of dislocations emitted from cracks introduced in a thin foil of LaNi 5 has been characterized by means of transmission ¯ electron microscopy. Dislocations emitted from cracks are determined to be of the c-type (b (Burgers vector)5[0001]) moving on h1010j ¯ prism planes. Although grown-in dislocations of the a-type (b51 / 3k1120l) locate near cracks where the emission of c-type dislocations occurred, these a-type dislocations are not observed in motion, indicating the difficulty in the motion of a-type dislocations. The implications of these results for controlling the extent of cracking during cycles of hydriding–dehydriding of alloys based on LaNi 5 are discussed. 1998 Elsevier Science S.A. Keywords: LaNi 5 ; Powdering; Dislocation; Slip system; Transmission electron microscopy
1. Introduction Alloys based on the intermetallic phase, LaNi 5 (the hexagonal CaCu 5 -type structure; Fig. 1) have been used as negative electrode materials of rechargeable nickel-metal
Fig. 1. The crystal structure of LaNi 5 and some possible Burgers vectors of dislocations. *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 98 )00164-9
hydride (Ni-MH) batteries because of their fast activation, high storage-capacity, long life-cycle and excellent electrochemical charge / discharge kinetics [1–4]. Powdering of these alloys after cycles of hydriding–dehydriding is one of the properties to be improved to make the life of Ni-MH batteries longer. The ease of powdering is a direct consequence of the brittleness of these alloys. Brittle cracking occurs to relieve the stress accumulated during cycles of hydriding–dehydriding, resulting in powdering of these alloys. However, if dislocations can occur and move easily in these alloys, the accumulated stress may be relieved by the motion of dislocations. Thus, knowledge of dislocation properties is indispensable for improving the life of Ni-MH batteries. However, since there have been only a limited number of studies on dislocations in LaNi 5 -based alloys [5,6], the nature of dislocations such as Burgers vectors, slip planes and the stress needed to set them in motion have remained largely unknown. In the present paper, we present the results of our transmission electron microscopy (TEM) study on the nature of dislocations in LaNi 5 . Dislocations analyzed include not only grown-in ones, but also those introduced accidentally in a thin foil TEM specimen.
2. Experimental procedures A LaNi 5 alloy was prepared by Ar arc-melting of
H. Inui et al. / Journal of Alloys and Compounds 269 (1998) 294 – 296
295
3. Results
Fig. 2. Dislocations emitted from cracks in a TEM foil of LaNi 5 . The crystallographic directions are indicated in the figure.
high-purity La (4N-grade) and Ni (4N-grade). The alloy was annealed at 9308C for 48 h in vacuum. The standard double-jet polishing method with a solution of perchloric acid and methanol was employed to perforate TEM foils. Dislocations were accidentally introduced in a thin foil specimen when the specimen was set onto a TEM holder. TEM observations were made with a JEM-2000FX electron microscope operated at 200 kV. Burgers vector (b) determinations were made according to the standard g?b criteria (g: reflection vector) [7].
In Fig. 2 the dislocations introduced in a TEM foil of LaNi 5 are shown. As is evident from the curvature of dislocations imaged, they are emitted from cracks intro¯ duced by mishandling. The foil normal is close to [1010] and the crystallographic directions are indicated in the figure. Most of the dislocations are in the form of trains on slip planes whose trace is parallel to the [0001] direction. Dislocations seen in Fig. 2 have a Burgers vector with a c-component, since they are visible with g50002. The result of contrast analysis made to determine the nature of these dislocations in Fig. 2 is presented in Fig. 3. Dislocations emitted from cracks are visible when imaged with g50002 (Fig. 3(a)–(c)), but invisible when imaged with ¯ ¯ (Fig. 3(e)), yielding the g50220, (Fig. 3(d)) and g52420 Burgers vector b5[0001] (c-type dislocations). On tilting the specimen about [0001], the projection width of the slip ¯¯ planes is wider when imaged along [1120] (Fig. 3(b)) than ¯ when imaged along [1010] (Fig. 3(a)), and they are seen ¯ end-on when imaged along [2110] (Fig. 3(c)), indicating ¯ that the slip planes are (0110). Thus, the slip system of dislocations emitted from cracks is determined to be ¯ [0001]. Weak-beam imaging of dislocations emitted (0110) from cracks reveals no appreciable dissociation, as shown in Fig. 4. This indicates the high stacking fault energy on prism planes. We observed some grown-in dislocations. An example of grown-in dislocations is seen in Fig. 3. Dislocations
¯ ¯¯ ¯ Fig. 3. Contrast analysis of dislocations shown in Fig. 2; (a) g50002 / B (beam direction)5[1010], (b) g50002 / B5[1120], (c) g50002 / B5[2110], (d) ¯ / B5[2110], ¯ ¯ / B5[1010]. ¯ g50220 (e) g52420
296
H. Inui et al. / Journal of Alloys and Compounds 269 (1998) 294 – 296
deed, numerous a-type dislocations are observed to be introduced as misfit dislocations at interfaces between the matrix and hydrides after hydriding treatments [5,6]. However, the present result indicates that the motion of a-type dislocations is more difficult than that of c-type dislocations. This may be one of the most plausible reasons why cracking occurs so easily in LaNi, resulting in the ease of powdering. We believe that a strategy to make a slip (slip via a-type dislocations) in LaNi 5 easier is needed to be devised in order to control the extent of cracking, thereby making the life of Ni-MH batteries last longer.
5. Conclusions Fig. 4. Weak-beam image of dislocations emitted from cracks. The image ¯ was recorded with g50002 near the [1010] zone-axis orientation.
marked A in Fig. 3(d) and (e) are invisible when imaged with g50002 (Fig. 3(a)–(c)), indicating that they are a-type ¯ ¯ dislocations with b51 / 3k1120l (either b51 / 3[1120] or ¯ b51 / 3[1210]). We also observed grown-in dislocations ¯ with b5[0001]. Grown-in dislocations with b51 / 3k1120l and b5[0001] seem to occur in almost equal fractions.
4. Discussion From the crystallography point of view, we expect ¯ basal several possible slip systems such as (0001)k1120l ¯ ¯ prism slip in the hexagonal LaNi 5 slip and h1100jk1120l ¯ lattice (Fig. 1). The slip system of h1010j[0001] presently identified has been rarely observed in crystals with hexagonal symmetry. Even though a-type grown-in dislocations locate near cracks where the emission of c-type dislocations is observed (Fig. 3(d) and(e)), their motion was not observed. This may indicate that c-type dislocations can move on prism planes more easily than a-type dislocations can do on either basal or prism planes, although it is very difficult to evaluate what stress was resolved for these possible slip systems when the crack was introduced in the thin foil. Determining operative slip systems and their critical resolved shear stresses in LaNi 5 is currently in progress in our group using single crystals. Strains accumulated during cycles of hydriding–dehydriding have been reported to be anisotropic from measurements of X-ray line-broadening [8–10]; the extent of line-broadening for hhki0j reflections increases with an increase in the number of cycles of hydriding–dehydriding, while that for h000lj reflections stays almost constant. This may indicate that only a-type dislocations are accumulated during cycles of hydriding–dehydriding. In-
The nature of dislocations emitted from cracks introduced in a thin foil of LaNi 5 is determined to be of the ¯ prism planes. Both a- and c-type c-type moving on h1010j grown-in dislocations occur almost in equal fractions. The motion of a-type dislocations seems to be more difficult than that of c-type dislocations. Since a-type dislocations have been reported to be introduced as misfit dislocations during cycles of hydriding–dehydriding, a strategy to make the motion of a-type dislocations easier needs to be devised to improve the life of Ni-MH batteries.
Acknowledgements This work was supported by Grant-in-Aid for Scientific Research (No. 08555162) from the Ministry of Education, Science and Culture, Japan.
References [1] J.H.N. van Vucht, F.A. Kuijpers, H.C.A.M. Bruning, Philips Res. Rep. 25 (1970) 133. [2] J.J.G. Willems, K.H.J. Buschow, J. Less-Common Metals 129 (1987) 13. [3] P.H.L. Notten, R.E.F. Einerhand, Adv. Mater. 3 (1991) 343. [4] E.H. Kisi, C.E. Buckley, E.M. Gray, J. Alloys Comp. 185 (1992) 369. [5] G.-H. Kim, C.-H. Chun, S.-G. Lee, J.Y. Lee, Acta Metall. Mater. 42 (1994) 3157. [6] G.-H. Kim, S.-G. Lee, K.-Y. Lee, C.-H. Chun, J.Y. Lee, Acta Metall. Mater. 43 (1995) 2233. [7] P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley, M.J. Whelan, Electron Microscopy of Thin Crystals, Butterworths, London, 1967. [8] A. Percheron-Guegan, C. Lartigue, J.C. Achard, P. Germi, F. Tasset, J. Less-Common Metals 74 (1980) 1. [9] J.F. Lynch, J.J. Reilly, J. Less-Common Metals 87 (1982) 225. [10] R.L. Cohen, K.W. West, J. Less-Common Metals 95 (1983) 17.