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Defect structures of VB2 and NbB2 single crystals
Journal of the fess-Common
DEFECT
Metals, 82 (1981) 309
STRUCTURES
- 316
309
OF VB, AND NbB, SINGLE
CRYSTALS*
K. NAKANO Government
industrial
Research
Institute Nagoya, Nagoya ~~apan~
M. DOI, K. KUWAYAMA
and T. IMURA
Department
Faculty of Engineering,
of Metallurgy,
Nagoya
University, Nagoya (Japan}
Summary Defect structures of VB, and NbB, single crystals grown by the floating zone process were examined by chemical etching and X-ray topography. In both diborides the etched sections of single crystals had numerous etch pits along the (1120) directions in the (0001) plane, and etch pits and line patterns along the [OOOl] direction in the flO%O} and {ll%Ofplanes respectively. Etch pit arrays and line patterns along the (1210) and (2113) directions were observed in the (lOil}planes of NbB, single crystals. Lang X-ray topographs of thin sections of both diborides showed pronounced dislocation images along the [OOOl]direction. It is concluded from contrast experiments that these dislocations have a pure edge character and that their Burgers vectors lie in the (0001) plane.
1. Introduction VB, and NbB, have high strengths at high temperatures, high melting points and high resistances to scaling, and they are of considerable interest for use in heat-resistant alloys. These materials have a hexagonal C32 crystal structure as have many other diborides [l]. Papers in which the defect structures of single crystals of this crystal structure have been reported are few, except for TiB, single crystals grown by the modified Verneuil process [Z]. In this paper we describe the defect structures of VB, and NbB, single crystals grown by the floating zone method [3].
* Paper presented at the 7th International Symposium Compounds, Uppsala, Sweden, June 9 - 12, 1981. c
on Boron, Borides and Related
Elsevier Sequ~ia/Printed
in The Netherlands
Fig. 1. Etched sections of a VB, single crystal (the large arrows are 30 pm long; the small arrows show precipitate-like patterns): (a) (0001); (b) (loio); (4 (1120): (d) (1010).
311
2. Experimental
details
VB, and NbB, single crystals were grown by the floating zone method (growth rate, 12 - 14 mm h-’ ; atmosphere, argon at a pressure of 10 - 14 atm ; total metal impurity, less than 0.9 wt.%; B/metal, 1.9 - 2.0) [3]. Planes parallel to (OOOl), (lOTO), (1120) and (1071) (NbB, only) were cut from as-grown VB, and NbB, single crystals, polished with diamond pastes (9, 6, 3 and 1 pm in diameter) and etched (Table 1). The etched surfaces were examined with an optical microscope. TABLE
1
Etching conditions
for VB, and NbB, single crystals
Crystal
Etchant.(by
W
HF:HNO,:H,O HF:HNO,:H,O
NbB,
volume) = 4:1:4 = 4:1:4
Temperature (“C)
Etching time (min)
20 20
2-3 5 - 30
- 25 - 25
Specimens whose surfaces were nearly parallel to the (lOi0) plane were prepared from the crystals. Specimen thicknesses t were 0.15 mm (VB,; pAgKortz 0.8) and 0.05 mm (NbB,; pLAgKat z 1.8). X-ray topographs were taken with a Lang camera (Rigaku Denki, model A3) attached to a Rigaku Denki rotor unit X-ray generator, model RU-3H. A silver rotating anode was used to obtain a strong X-ray beam. Experimental conditions were as follows: diameter of X-ray source, 100 pm; operating voltage, 40 kV; operating current, 4 mA; exposure time, 11 - 30 h (Ilford L4 nuclear plate; 50 pm).
3. Results Etched sections parallel to the (OOOl), (1010) and (1120) planes of VB, single crystals are illustrated in Figs. l(a), l(b) and l(c) respectively. Numerous etch pits (with a density of the order of lo7 cm-2) along (1120) directions were found in the (0001) plane. Numerous straight-line patterns (etching grooves) along the [OOOl]direction were observed in the (lOi0) and (1120) planes. Several etch pits were also found in these prismatic planes. As well as these line patterns, needle-like patterns along the [OOOl]direction were occasionally observed in the prismatic planes as shown in Fig. l(d) (arrows). Etched sections of (OOOl), (lOiO), (1120), (lOi1) and (lOi1) planes of an NbB, single crystal are shown in Figs. 2(a), 2(b), 2(c), 2(d) and 2(e) respectively. In the (0001) plane, numerous etch pits (with a density of the order of lo6 cme2) along (1120) directions were observed as for VB, single crystals. Numerous straight-line patterns and etch pits along the [OOOl] direction were found in the {lOiO} and (1120) planes. Etch pit arrays (an etch pit density of the order of lo6 cmW2) and line patterns along the (1210) and (2113) directions were seen in the {lOil} planes.
312
313
Fig, 2. Etched sections of al NbB, single crystal (the arrows are 30 pm long): (a) (0001); (b) (1010); (c) (1120); (d), (e) (1011).
Lang X-ray topographs of a VB, single-crystal thin section parallel to the (1010) plane are given in Fig. 3. In Figs. 3(a) and 3(b) the topographs -were taken with the 1210 reflection. The white parts in the topographs correspond to the regions where the Bragg condition is not satisfied. Straight-line patterns parallel to the [OOOl] direction were found predominantly. Figure 3(c) is a topograph taken with the 0001 reflection. In this topograph the line patterns had disappeared. Lang topographs of an NbB, single-crystal thin section parallel to the (lOi0) plane are given in Fig. 4. In Fig. 4(a) the topograph was taken with the 1210 reflection, and the line patterns parallel to the [OOOl] direction were observed as for the VB, single crystal. Figure 4(b) is a topograph taken with the 0001 reflection; the line patterns had disappeared although some macroscopic strain might remain.
b
c Fig. 3. Lang topographs of a VB, single-crystal section parallel to the (1010) plane (the directions of the diffraction vectors are indicated by the arrows and are 1.0 mm on the tomographs):
(a), (b) g/MO;
(c) s/0001.
Fig. 4. Lang topographs of an NbB, single-crystal section parallel to the (lOi0) plane (the directions of the diffraction vectors are indicated by the arrows and are 1.0 mm on the topographs): (a) g/1%0; (b) Gaul.
315
4. Discussion Single crystals grown from the melt under high temperature gradients using techniques such as the floating zone method and the modified Verneuil method usually have several defects (precipitates, lattice defects, grain boundaries etc.) [Z, 41. Precipitates such as Widmanstdtten structures found in TiB, [2] and ZrB, [4] were not observed in the present two types of crystal, although precipitate-like patterns were sometimes found in the etched sections of the prismatic planes of VB, single crystals. The fact that typical Widmanstatten structures were not observed indicates that the second phase, with a composition slightly different from those of the VB, and NbB, matrices, was difficult to precipitate owing to the relatively wide (VB,) or wide (NbB,) homogeneity ranges of the VB, and NbB, phases [5]. Grain boundaries often appeared in both kinds of crystals, as seen in Figs. 3(a) and 3(b). In the etched sections of the (0001) plane of VB, and NbB, single crystals, high density etch pits (lo6 - 10’ cmm2) along the (1120) directions were observed. In the {lOil} planes of the NbB, single crystal, etch pit arrays (lo6 cmP2) were also observed along the (1210) and (2113) directions. These etch pit densities lead to line densities of the order of lo3 cm-‘. In contrast, the line densities of the line patterns observed on the prismatic planes of VB, and NbB, single crystals were estimated to be of the order of lo3 cm-‘. Both have values of the same order of magnitude. Electron probe microanalysis along the directions parallel and perpendicular to the line patterns showed few changes in compositions. Consequently it appears that most of the etch pits observed in the (0001) and {lOil} planes correspond to dislocation terminals and that the line patterns in the prismatic planes are dislocation traces. Dislocations must exist in the {lOiO} planes if we consider the observations of the directions of etch pit arrays. Low, Jr., and Guard [6] observed similar dislocation traces in Fe-3.2%Si single crystals. It seems reasonable to assume that the straight-line patterns parallel to the [OOOl] direction in the topographs taken with 1210 (VB,) or 1210 (NbB,) reflections must be dislocation images if we take into account the above discussions about etching figures and the image disappearance in the topographs taken with the 0001 reflection. We can speculate about the type of Burgers vector b from the criterion g.b = 0 where g is the diffraction vector. It can be concluded that b for the dislocations in question should be perpendicular to the [OOOl]direction and of the (l/3)(1120) type if these dislocations are perfect. These dislocations have a pure edge character. The dislocations are considered to be generated by {lO?O}prismatic slip due to the high thermal stresses induced in the growth process. Acknowledgments The authors wish to thank Dr. Y. Kumashiro trotechnical Laboratory, for the crystal growth.
and E. Sakuma, Elec-
316
References 1 2 3
B. Aronsson, T. Lund&ram and S. Rundqvist, Berides, Silicides and Phosphides, Methuen, London, 1965, p. 46. S. A. Mersol, C. T. Lynch and F. W. Vahldiek, Anisotropy in Single Crystal Refractory Compounds, Vol. 2, Plenum, New York, 1968, p. 41. K. Nakano, K. Nakamura, Y. Kumashiro and E. Sakuma, J. Cryst. Growth,
52 (1981) 602.
4 5 6
J. S. Haggerty, J. L. O’Brien and J. F. Wenckus, J. Cryst. Growth, 3 - 4 (1968) 291. R. P. Elliott, Constitution of Binary AZZoys,McGraw-Hill, New York, 1st Suppl., 1965, pp. 113, 142. J. R. Low, Jr., and R. W. Guard, Acta Metall., 7 (1959) 171.
DEFECT
Metals, 82 (1981) 309
STRUCTURES
- 316
309
OF VB, AND NbB, SINGLE
CRYSTALS*
K. NAKANO Government
industrial
Research
Institute Nagoya, Nagoya ~~apan~
M. DOI, K. KUWAYAMA
and T. IMURA
Department
Faculty of Engineering,
of Metallurgy,
Nagoya
University, Nagoya (Japan}
Summary Defect structures of VB, and NbB, single crystals grown by the floating zone process were examined by chemical etching and X-ray topography. In both diborides the etched sections of single crystals had numerous etch pits along the (1120) directions in the (0001) plane, and etch pits and line patterns along the [OOOl] direction in the flO%O} and {ll%Ofplanes respectively. Etch pit arrays and line patterns along the (1210) and (2113) directions were observed in the (lOil}planes of NbB, single crystals. Lang X-ray topographs of thin sections of both diborides showed pronounced dislocation images along the [OOOl]direction. It is concluded from contrast experiments that these dislocations have a pure edge character and that their Burgers vectors lie in the (0001) plane.
1. Introduction VB, and NbB, have high strengths at high temperatures, high melting points and high resistances to scaling, and they are of considerable interest for use in heat-resistant alloys. These materials have a hexagonal C32 crystal structure as have many other diborides [l]. Papers in which the defect structures of single crystals of this crystal structure have been reported are few, except for TiB, single crystals grown by the modified Verneuil process [Z]. In this paper we describe the defect structures of VB, and NbB, single crystals grown by the floating zone method [3].
* Paper presented at the 7th International Symposium Compounds, Uppsala, Sweden, June 9 - 12, 1981. c
on Boron, Borides and Related
Elsevier Sequ~ia/Printed
in The Netherlands
Fig. 1. Etched sections of a VB, single crystal (the large arrows are 30 pm long; the small arrows show precipitate-like patterns): (a) (0001); (b) (loio); (4 (1120): (d) (1010).
311
2. Experimental
details
VB, and NbB, single crystals were grown by the floating zone method (growth rate, 12 - 14 mm h-’ ; atmosphere, argon at a pressure of 10 - 14 atm ; total metal impurity, less than 0.9 wt.%; B/metal, 1.9 - 2.0) [3]. Planes parallel to (OOOl), (lOTO), (1120) and (1071) (NbB, only) were cut from as-grown VB, and NbB, single crystals, polished with diamond pastes (9, 6, 3 and 1 pm in diameter) and etched (Table 1). The etched surfaces were examined with an optical microscope. TABLE
1
Etching conditions
for VB, and NbB, single crystals
Crystal
Etchant.(by
W
HF:HNO,:H,O HF:HNO,:H,O
NbB,
volume) = 4:1:4 = 4:1:4
Temperature (“C)
Etching time (min)
20 20
2-3 5 - 30
- 25 - 25
Specimens whose surfaces were nearly parallel to the (lOi0) plane were prepared from the crystals. Specimen thicknesses t were 0.15 mm (VB,; pAgKortz 0.8) and 0.05 mm (NbB,; pLAgKat z 1.8). X-ray topographs were taken with a Lang camera (Rigaku Denki, model A3) attached to a Rigaku Denki rotor unit X-ray generator, model RU-3H. A silver rotating anode was used to obtain a strong X-ray beam. Experimental conditions were as follows: diameter of X-ray source, 100 pm; operating voltage, 40 kV; operating current, 4 mA; exposure time, 11 - 30 h (Ilford L4 nuclear plate; 50 pm).
3. Results Etched sections parallel to the (OOOl), (1010) and (1120) planes of VB, single crystals are illustrated in Figs. l(a), l(b) and l(c) respectively. Numerous etch pits (with a density of the order of lo7 cm-2) along (1120) directions were found in the (0001) plane. Numerous straight-line patterns (etching grooves) along the [OOOl]direction were observed in the (lOi0) and (1120) planes. Several etch pits were also found in these prismatic planes. As well as these line patterns, needle-like patterns along the [OOOl]direction were occasionally observed in the prismatic planes as shown in Fig. l(d) (arrows). Etched sections of (OOOl), (lOiO), (1120), (lOi1) and (lOi1) planes of an NbB, single crystal are shown in Figs. 2(a), 2(b), 2(c), 2(d) and 2(e) respectively. In the (0001) plane, numerous etch pits (with a density of the order of lo6 cme2) along (1120) directions were observed as for VB, single crystals. Numerous straight-line patterns and etch pits along the [OOOl] direction were found in the {lOiO} and (1120) planes. Etch pit arrays (an etch pit density of the order of lo6 cmW2) and line patterns along the (1210) and (2113) directions were seen in the {lOil} planes.
312
313
Fig, 2. Etched sections of al NbB, single crystal (the arrows are 30 pm long): (a) (0001); (b) (1010); (c) (1120); (d), (e) (1011).
Lang X-ray topographs of a VB, single-crystal thin section parallel to the (1010) plane are given in Fig. 3. In Figs. 3(a) and 3(b) the topographs -were taken with the 1210 reflection. The white parts in the topographs correspond to the regions where the Bragg condition is not satisfied. Straight-line patterns parallel to the [OOOl] direction were found predominantly. Figure 3(c) is a topograph taken with the 0001 reflection. In this topograph the line patterns had disappeared. Lang topographs of an NbB, single-crystal thin section parallel to the (lOi0) plane are given in Fig. 4. In Fig. 4(a) the topograph was taken with the 1210 reflection, and the line patterns parallel to the [OOOl] direction were observed as for the VB, single crystal. Figure 4(b) is a topograph taken with the 0001 reflection; the line patterns had disappeared although some macroscopic strain might remain.
b
c Fig. 3. Lang topographs of a VB, single-crystal section parallel to the (1010) plane (the directions of the diffraction vectors are indicated by the arrows and are 1.0 mm on the tomographs):
(a), (b) g/MO;
(c) s/0001.
Fig. 4. Lang topographs of an NbB, single-crystal section parallel to the (lOi0) plane (the directions of the diffraction vectors are indicated by the arrows and are 1.0 mm on the topographs): (a) g/1%0; (b) Gaul.
315
4. Discussion Single crystals grown from the melt under high temperature gradients using techniques such as the floating zone method and the modified Verneuil method usually have several defects (precipitates, lattice defects, grain boundaries etc.) [Z, 41. Precipitates such as Widmanstdtten structures found in TiB, [2] and ZrB, [4] were not observed in the present two types of crystal, although precipitate-like patterns were sometimes found in the etched sections of the prismatic planes of VB, single crystals. The fact that typical Widmanstatten structures were not observed indicates that the second phase, with a composition slightly different from those of the VB, and NbB, matrices, was difficult to precipitate owing to the relatively wide (VB,) or wide (NbB,) homogeneity ranges of the VB, and NbB, phases [5]. Grain boundaries often appeared in both kinds of crystals, as seen in Figs. 3(a) and 3(b). In the etched sections of the (0001) plane of VB, and NbB, single crystals, high density etch pits (lo6 - 10’ cmm2) along the (1120) directions were observed. In the {lOil} planes of the NbB, single crystal, etch pit arrays (lo6 cmP2) were also observed along the (1210) and (2113) directions. These etch pit densities lead to line densities of the order of lo3 cm-‘. In contrast, the line densities of the line patterns observed on the prismatic planes of VB, and NbB, single crystals were estimated to be of the order of lo3 cm-‘. Both have values of the same order of magnitude. Electron probe microanalysis along the directions parallel and perpendicular to the line patterns showed few changes in compositions. Consequently it appears that most of the etch pits observed in the (0001) and {lOil} planes correspond to dislocation terminals and that the line patterns in the prismatic planes are dislocation traces. Dislocations must exist in the {lOiO} planes if we consider the observations of the directions of etch pit arrays. Low, Jr., and Guard [6] observed similar dislocation traces in Fe-3.2%Si single crystals. It seems reasonable to assume that the straight-line patterns parallel to the [OOOl] direction in the topographs taken with 1210 (VB,) or 1210 (NbB,) reflections must be dislocation images if we take into account the above discussions about etching figures and the image disappearance in the topographs taken with the 0001 reflection. We can speculate about the type of Burgers vector b from the criterion g.b = 0 where g is the diffraction vector. It can be concluded that b for the dislocations in question should be perpendicular to the [OOOl]direction and of the (l/3)(1120) type if these dislocations are perfect. These dislocations have a pure edge character. The dislocations are considered to be generated by {lO?O}prismatic slip due to the high thermal stresses induced in the growth process. Acknowledgments The authors wish to thank Dr. Y. Kumashiro trotechnical Laboratory, for the crystal growth.
and E. Sakuma, Elec-
316
References 1 2 3
B. Aronsson, T. Lund&ram and S. Rundqvist, Berides, Silicides and Phosphides, Methuen, London, 1965, p. 46. S. A. Mersol, C. T. Lynch and F. W. Vahldiek, Anisotropy in Single Crystal Refractory Compounds, Vol. 2, Plenum, New York, 1968, p. 41. K. Nakano, K. Nakamura, Y. Kumashiro and E. Sakuma, J. Cryst. Growth,
52 (1981) 602.
4 5 6
J. S. Haggerty, J. L. O’Brien and J. F. Wenckus, J. Cryst. Growth, 3 - 4 (1968) 291. R. P. Elliott, Constitution of Binary AZZoys,McGraw-Hill, New York, 1st Suppl., 1965, pp. 113, 142. J. R. Low, Jr., and R. W. Guard, Acta Metall., 7 (1959) 171.