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Phase transitions and superconducting properties of binary and Ti, Ta, Ga and H alloyed Nb3Sn
Physica 135B (1985) 359-363 North-Holland, Amsterdam
PHASE TRANSITIONS AND SUPERCONDUCTING PROPERTIES OF BINARY AND Ti, Ta, Ga AND H ALLOYED Nb3Sn W. G O L D A C K E R and R. F L I d K I G E R Kernforschungszentrum Karlsruhe, lnstitut ffir Technische Physik, Postfach 3640, D-7500 Karlsruhe 1, Fed. Rep. Germany
The low temperature phase transformation in Nb3Sn has been analyzed for various samples (bulk and multifilamentary) with the additions Ta, Ti, Ga, H by means of X-ray powder diffractometry. The transformation behavior of Nb3Sn with and without additions was studied on powdered 19-core filaments of bronze-processed wires which were previously characterized by measurements ofJ~, Tc and the eletrical resitivity P0. A lattice transformation was observed for the binary Nb3Sn, for 0.9 at.% Ga and 1.3 at.% Ta, while no transformation was found for an effective Ta, Ti and H content of 4.3, 1.3 and 0.6 at.%, respectively. It is found that the suppression of the lattice instability by additives is not uniquely correlated to lattice parameter variations with respect to binary Nb3Sn. A necessary additional criterion is the chemical nature of the additive, which determines its position on the 6c sites (e.g. Ta, Ti) or on the 2a sites (e.g. Ga, A1, Sb, Ni).
1. Introduction It follows from a series of investigations on the A15 type compound Nb3Sn that the superconducting properties are considerably affected by alloying with small amounts (~<4 at.%) of additives, e.g. H [1], A1 [2, 3], Ga [4], Sb [5, 6], Ta [7, 8], Ti [9], Ni [10] and others. These observations were made independently on the configuration, i.e. bulk, thin film or multifilamentary. The additives cause a substantial increase of the electrical resistivity, P0 [3, 4, 11], thus leading to an enhancement of the upper critical field, Bc2. In the case of multifilamentary Nb3Sn wires, the direct consequence is an enhancement of the critical current density at high fields [3, 4, 7-11]. Most additives cause a slight enhancement of T c, up to a few tenths of degree over that of binary Nb3Sn [2, 6, 8, 9, 11]. In some cases, the amount of the additive corresponding to the maximum of T c is just sufficient to suppress the cubic-tetragonal phase transformation at low temperature [12141 . The criteria for the suppression of the lattice instability by alloying are not well defined. Recently, it was proposed [15] that the occurrence of the cubic-tetragonal phase transformation could be predicted based uniquely on the lattice parameter change with respect to that of the
binary Nb3Sn. In the present paper, it will be shown that an additional criterion has to be taken into account, i.e. the chemical nature of the additive, which also determines its position in the A15 lattice, either at the 6c or at the 2a sites.
2. Experimental The crystal structure analysis was performed by means of a Seifert high resolution X-ray powder diffractometer, using a secondary quartz monochromator. The powdered samples were mounted in a continuous flow cryostat, the accessible temperature range being 5 ~< T ~< 300 K. The samples consisted either of powdered Nb3Sn bulk material or of filaments after etching from bronze-processed 19 core binary and ternary (alloyed) Nb3Sn wires (see table I). In each case, Si powder was added as a standard. In general, the powdering procedure did not lead to observable broadening of the diffraction lines, the specific line shape and width thus reflects the composition gradient across the sample. From the variation of the lattice parameter, a, with the Sn content, i.e. --13 x 10 5 nm per at.% Sn [16], semiquantitative line shape considerations can give detailed information about the global compositional profile of the samples, as will be published elsewhere [17]. The
0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
W. Goldacker and R. Fliikiger / Phase transitions in binary and alloyed Nb~Sn
360
d i f f r a c t o m e t e r resolution was d e m o n s t r a t e d on a very h o m o g e n e o u s polycrystalline Nb3Sn bulk sample ( n u m b e r i in table I) containing 24.5 a t . % Sn, the Sn distribution being c o m p r i s e d within -+0.3 at. % a r o u n d this value (as d e t e r m i n e d f r o m the calorimetric superconducting transition width). T h e partly overlapping reflexes of both, the cubic ( 5 5 % ) and the tetragonal ( 4 5 % ) phase were separated and the t e m p e r a t u r e d e p e n d e n c e of (1 - c/a) could be d e t e r m i n e d . T h e fractions of the different phases were evaluated f r o m the (400) intensity ratios. T h e reaction conditions for the binary and alloyed 19 core Nb3Sn wires were chosen in o r d e r to obtain fully reacted filaments with a small c o m p o s i t i o n gradient, the average composition being as close as possible to stoichiometry. In o r d e r to facilitate the full reaction, the diffusion paths were s h o r t e n e d by rolling the wires of 0 . 6 m m d i a m e t e r to tapes of 0 . 1 m m thickness prior to reaction, the resulting A15 filaments having a thickness of --10 x 10 6 mm. A f t e r det e r m i n a t i o n of the critical current density, Jc, and Be2 on the wires, the b r o n z e was etched away and the filaments were characterized by measurements of P0 (see ref. 11), T c and X - r a y diffraction.
3. Results and discussion
3.1. Binary Nb3Sn T h e results on different samples are s u m m a r ized in table I. F o r alloyed Nb3Sn , the effective contents of the additives in the A15 layer is given. T h e composition o f the bulk sample, 24.5 a t . % Sn, is very close to the Nb-rich tetragonal phase b o u n d a r y [13, 16]. T h e value of (1 - c/a) for the tetragonal fraction of this sample was d e t e r m i n e d to be 0.0043, i.e. considerably smaller than 0.0061, the value for a stoichiometric single crystal m e a s u r e d by Mailfert et al. [18]. It follows that the (1 - c/a) value is r e d u c e d w h e n m o v i n g away f r o m stoichiometry towards the tetragonal phase b o u n d a r y . A t the same time, no significant c h a n g e of T M was observed. For the b r o n z e processed binary Nb3Sn samples, a successive decrease of the reaction t e m p e r ature f r o m 800 to 700°C led to a decreasing tetragonal v o l u m e fraction going f r o m 75% (sample 2) to 50% (sample 4). As follows f r o m a study on position and shape of the X - r a y diffraction lines on these samples [17], this is correlated on one h a n d to a shift of the m e a n c o m p o s i t i o n
Table I Diffractometric results on binary and alloyed Nb3Sn samples in the bulk and filamentary configuration (Aa = --+3x 10 ~rim) No. Effective Composition (at.%)
Reaction cond. (°C/hours)
a30¢,K (nm)
Volume fraction at 10 K: V,(I - c/a): V~
1
Nb~Sn(bulk)
0.52889
0.45(0.0043): 0.55
43
17.8
2 3 4 5 6 7 8
Nb3Sn Nb3Sn Nb3Sn + 0.6 H + 4.3 Ta + 2.8 Ta + 1.7Ta
sint. 1500° (100 atm. Ar) 800/70 750/138 700/290 800/70 800/70 800/70 800/70
0.52892 0.52888 0.52888 0.52934 0.52880 0.52884 0.52888
0.75(0.0052): 0.25 ~0.5/~ 0.5 0.5(0.0057): 0.5 cubic cubic cubic -~0.5(broad): ~0.5
17.9 17.9 17.9 17.2 17.9 18.1 18.2
9 10 11
+ 1.3 Ti + 1 Ni + 0.9 Ga
800/70 800/70 800/72
0.52873 0.52860 0.52876
cubic cubic 0.85(0.0043): 0.15
43 ~43 ~43 ~20 -50 -
T M
t'c
(K)
(K)
~43
18.0 18.0 18.3
361
W. Goldacker and R. Fliikiger / Phase transitions in binary and alloyed Nb3Sn
toward lower Sn contents for the reaction at 700°C and on the other hand to a significant broadening of the composition distribution, i.e. the cubic volume fraction is higher for the sample reacted at 700°C than for that reacted at 800°C. It is interesting that the diffusion reacted sample 2 (slightly closer to stoichiometric composition than the bulk sample 1) has a narrow composition distribution, with a very small overlapping with the cubic phase field.
i
.529z,
.5291 -3transforming binary Nb~Sn
i
o cubic • tetragona[ at T= 10 K
Alloying Elements on 6c sites
o(300K/ [nml
-]-
i
i
~,\\\\\\\\\\ \. ~tetragonal\\\\\\
".,,'~\\\\\\\\"~ooX \\"
T°
.5288.
O
To
o Ti
3.2. A l l o y e d N b 3 S n .5285
The samples investigated in the present work have somewhat higher lattice parameter values than those previously reported by Drost et al. [10]. The difference is due to the fact that the filaments are now fully reacted, leading to considerably narrower X-ray lines with drastically reduced contributions arising from the low Sn contents. Therefore, the average composition is closer to stoichiometry, which explains the higher lattice parameter values. When cooling below room temperature, a very slight line broadening was observed for the Ta and Ti alloyed samples (numbers 6 to 9 in table I), which is attributed to remaining internal stress after etching away the surrounding bronze. Line broadening at low temperatures was even more pronounced for the Ni alloyed samples. The cubic-tetragonal phase transition was suppressed in all Ta and Ti alloyed samples except for that with the lowest investigated Ta content, i.e. 1.7 at.% (number 8). This sample exhibited about 50% of tetragonal phase and is therefore
o(300K [nm] ~ , ~ .5289-
/Kunz
et o1.[20]
.5288- (diffusion reactedl
0
i
½
3
¼ (ItP)oTo
Fig. 1. Lattice p a r a m e t e r variation in the (Nb I xTa~)3Sn as a funtion of the Ta content.
system
a
6
5294
&
Z ot.O/o
i
i
i
i
o cubic • tetragonai at T=10K
on 2a s i t e s
.5291 __L
T
2
Alloying Elements
o(300K] [nm]
transforming binary Nb~Sn
4
,AI. \\\ .oTT-,At ~\~.tefragona I ~ \ \ 1 \
.5288 • ,x..
Mo
.5285
"-
b 0
1
2
3
4 or.%
Fig. 2. Occurrence of tetragaonal phase in Nb3Sn as a function of different additives. (a) Additives lying on the 6c sites (To, Ti). (b) Additives lying on the 2a sites (Ga, AI, Sb, Ni). The lattice parameter values for AI and Sb additives have been extracted from refs. 2 and 6, respectively. just located at the cubic/tetragonal phase boundary. The diffraction lines for the tetragonal phase could not be well separated from the cubic line, in order that T M could not be determined with precision, but lies in the range 20 <~ T M ~< 50 K, thus revealing a broad distribution of (1 c/a) values over the sample. The diffraction pattern thus confirms the low temperature (Nbl_xTax)3Sn phase diagram established earlier [13] on the basis of sintered samples. When comparing the present results on diffusion reacted samples with those on sintered or melted samples,
362
W. Goldacker and R. Fliikiger / Phase transitions in binary and alloyed Nb3Sn
the question arises whether the very different formation temperatures (-800°C compared to 1200 and -2000°C, respectively) affect the equilibrium at compositions close to stoichiometry. There is some evidence that with increasing Ta content the composition of the diffusion reacted samples deviates more and more from stoichiometry, towards Nb richer contents. This is illustrated by fig. 1 where the lattice parameter variation of the present Ta alloyed Nb3Sn samples prepared by diffusion reaction is compared to the corresponding data of Kunz and Saur [20], who investigated the region up to 40 at.% Ta on arc melted samples. The suppression of the phase transition in the system (Nb 1 xTax)3Sn is thus not only due to the increasing Ta content, but also to the increasing deviation from stoichiometry. The shift towards lower Sn contents reported here is in agreement with the observations of Tafto et al. [19]. For Ti alloyed Nb3Sn no significant lattice parameter shift with increasing Ti content was reported [20]. Therefore the reduction of the lattice parameter of sample 9 is mainly attributed to a deviation from stoichiometry, which itself is sufficient to hinder the phase transformation. The deviation from stoichiometry in Ti alloyed Nb3Sn samples confirms previous results of Tachikawa et al. [91. The occurrence of a lattice instability for the additives Ta and Ti substituting Nb on the 6c sites (as recently shown by Tafto et al. [19]) can be summarized by defining a region in a diagram representing the lattice parameter as a function of the additive content (fig. 2a). It follows that the small contents are sufficient to suppress the phase transformation, which necessarily implies that the transforming region is restricted to very small variations of the lattice parameter with respect to that for binary Nb3Sn. The situation is, however, quite different for the additives Ga, A1, Sb and Ni substituting Sn atoms on the 2a sites of the AI5 lattice. The 0.9 at.% Ga alloyed sample shows a nearly complete cubic/tetragonal phase transformation in spite of a markedly reduced lattice parameter. The same behavior was observed for the additions A1 [2] and Sb [5, 6], the latter showing the largest lattice parameter change (the change of sign for ( 1 - c/a) in Nb3A11 xSbx for
x ~ 0 . 1 [5] is here of secondary importance). These data have been plotted in fig. 2b, which shows a much wider transformation region than for Ta and Ti additions. For the additives in fig. 2b, the lattice parameter change is mainly caused by their substitution, rather than by deviations from the stoichiometric Nb content. From the present data it can be concluded that the occurrence of the lattice instability in alloyed Nb3Sn is dominantly influenced by the chemical nature of the additives, which determines their positions in the perfectly ordered [21] Nb3Sn lattice. Besides deviations from stoichiometry encountered particularly for Ta and Ti additions, it can be said that the substitution of additives on the 6c sites has a much stronger effect in suppressing the lattice instability of Nb3Sn than that on the 2a sites. This result can be easily understood in terms of the stronger interaction between chain atoms.
References [1] L.J. Vieland, A.W. Wicklund and J.G. White, Phys. Rev. B l l (1975) 3311. [2] L.J. Vieland and A.W. Wicklund, Phys. Lett. 34A (1971) 43. [3] R. Akihama, K. Yasukochi and T. Ogasawara, IEEE Trans. Magn. MAG-13 (1977) 803. [4] R. Bormann, D.Y. Yu, R.H. Hammond, T.H. Geballe, S. Foner and E.J. Mc Niff, Jr., IEEE Trans. Magn. MAG-21 (1986) 456. [5] Y. Fuji, J.B. Hastings, M. Kaplan, G. Shirane, Y. Inada and N. Kitamura, Phys. Rev. B25 (1982) 364. [6] L.J. Vieland, J. Phys. Chem. Solids 31 (1970) 1449. [7] J.D. Livingston, IEEE Trans. Magn. MAG-14 (1978) 611. T. Luhman, K. Kaiho and M. Suenaga, Adv. Cryog. Eng. 26 (1980) 543. [8] E. Springer, M. Wilhelm, H.J. Weisse and G. Rupp, Adv. Cryo. Eng. 30 (1984) 747. [9] K. Tachikawa, T. Asano and T. Takenchi, Appl. Phys. Lett. 39 (1981) 766. K. Tachikawa, H. Sekine and Y. Iijima, J. Appl. Phys. 53 (1982) 5354. [10] E. Drost, R. F1/ikiger and W. Specking, Cryogenics 24 (1984) 622. [11] E. Drost, W. Specking and R. Fliikiger, IEEE Trans. Magn. MAG-21 (1985) 423. [12] R. Roberge, H. LeHuy and S. Foner, Phys. Lett. 82A (1981) 259. [13] R. Flfikiger, Adv. Cryo. Eng. 28 (1982) 399.
W. Goldacker and R. Fliikiger / Phase transitions in binary and alloyed Nb3Sn [14] W. Goldacker and R. Flfikiger, IEEE Trans. Magn. MAG-21 (1985) 807. [15] R. Roberge, IEEE Trans. Magn. MAG-21 (1985) 811. [16] H. Devantay, J.L. Jorda, M. Decroux, J. Muller and R. Fl/ikiger, J. Mat. Sci. 16 (1981) 2145. [17] W. Goldacker and R. F1/ikiger, to be published.
363
[18] R. Mailfert, B.W. Batterman and J.J. Hanak, Phys. Lett. 24A (1967) 315. [19] J. Tafto, M. Suenaga and D.O. Welch, J. Appl. Phys. 55 (1984) 4330. [20] W. Kunz and E. Saur, Z. Physik 189 (1966) 401. [21] R. Fliikiger, R. Isernhagen, W. Goldacker and W. Specking, Adv. Cryo. Eng. 30 (1984) 851.
PHASE TRANSITIONS AND SUPERCONDUCTING PROPERTIES OF BINARY AND Ti, Ta, Ga AND H ALLOYED Nb3Sn W. G O L D A C K E R and R. F L I d K I G E R Kernforschungszentrum Karlsruhe, lnstitut ffir Technische Physik, Postfach 3640, D-7500 Karlsruhe 1, Fed. Rep. Germany
The low temperature phase transformation in Nb3Sn has been analyzed for various samples (bulk and multifilamentary) with the additions Ta, Ti, Ga, H by means of X-ray powder diffractometry. The transformation behavior of Nb3Sn with and without additions was studied on powdered 19-core filaments of bronze-processed wires which were previously characterized by measurements ofJ~, Tc and the eletrical resitivity P0. A lattice transformation was observed for the binary Nb3Sn, for 0.9 at.% Ga and 1.3 at.% Ta, while no transformation was found for an effective Ta, Ti and H content of 4.3, 1.3 and 0.6 at.%, respectively. It is found that the suppression of the lattice instability by additives is not uniquely correlated to lattice parameter variations with respect to binary Nb3Sn. A necessary additional criterion is the chemical nature of the additive, which determines its position on the 6c sites (e.g. Ta, Ti) or on the 2a sites (e.g. Ga, A1, Sb, Ni).
1. Introduction It follows from a series of investigations on the A15 type compound Nb3Sn that the superconducting properties are considerably affected by alloying with small amounts (~<4 at.%) of additives, e.g. H [1], A1 [2, 3], Ga [4], Sb [5, 6], Ta [7, 8], Ti [9], Ni [10] and others. These observations were made independently on the configuration, i.e. bulk, thin film or multifilamentary. The additives cause a substantial increase of the electrical resistivity, P0 [3, 4, 11], thus leading to an enhancement of the upper critical field, Bc2. In the case of multifilamentary Nb3Sn wires, the direct consequence is an enhancement of the critical current density at high fields [3, 4, 7-11]. Most additives cause a slight enhancement of T c, up to a few tenths of degree over that of binary Nb3Sn [2, 6, 8, 9, 11]. In some cases, the amount of the additive corresponding to the maximum of T c is just sufficient to suppress the cubic-tetragonal phase transformation at low temperature [12141 . The criteria for the suppression of the lattice instability by alloying are not well defined. Recently, it was proposed [15] that the occurrence of the cubic-tetragonal phase transformation could be predicted based uniquely on the lattice parameter change with respect to that of the
binary Nb3Sn. In the present paper, it will be shown that an additional criterion has to be taken into account, i.e. the chemical nature of the additive, which also determines its position in the A15 lattice, either at the 6c or at the 2a sites.
2. Experimental The crystal structure analysis was performed by means of a Seifert high resolution X-ray powder diffractometer, using a secondary quartz monochromator. The powdered samples were mounted in a continuous flow cryostat, the accessible temperature range being 5 ~< T ~< 300 K. The samples consisted either of powdered Nb3Sn bulk material or of filaments after etching from bronze-processed 19 core binary and ternary (alloyed) Nb3Sn wires (see table I). In each case, Si powder was added as a standard. In general, the powdering procedure did not lead to observable broadening of the diffraction lines, the specific line shape and width thus reflects the composition gradient across the sample. From the variation of the lattice parameter, a, with the Sn content, i.e. --13 x 10 5 nm per at.% Sn [16], semiquantitative line shape considerations can give detailed information about the global compositional profile of the samples, as will be published elsewhere [17]. The
0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
W. Goldacker and R. Fliikiger / Phase transitions in binary and alloyed Nb~Sn
360
d i f f r a c t o m e t e r resolution was d e m o n s t r a t e d on a very h o m o g e n e o u s polycrystalline Nb3Sn bulk sample ( n u m b e r i in table I) containing 24.5 a t . % Sn, the Sn distribution being c o m p r i s e d within -+0.3 at. % a r o u n d this value (as d e t e r m i n e d f r o m the calorimetric superconducting transition width). T h e partly overlapping reflexes of both, the cubic ( 5 5 % ) and the tetragonal ( 4 5 % ) phase were separated and the t e m p e r a t u r e d e p e n d e n c e of (1 - c/a) could be d e t e r m i n e d . T h e fractions of the different phases were evaluated f r o m the (400) intensity ratios. T h e reaction conditions for the binary and alloyed 19 core Nb3Sn wires were chosen in o r d e r to obtain fully reacted filaments with a small c o m p o s i t i o n gradient, the average composition being as close as possible to stoichiometry. In o r d e r to facilitate the full reaction, the diffusion paths were s h o r t e n e d by rolling the wires of 0 . 6 m m d i a m e t e r to tapes of 0 . 1 m m thickness prior to reaction, the resulting A15 filaments having a thickness of --10 x 10 6 mm. A f t e r det e r m i n a t i o n of the critical current density, Jc, and Be2 on the wires, the b r o n z e was etched away and the filaments were characterized by measurements of P0 (see ref. 11), T c and X - r a y diffraction.
3. Results and discussion
3.1. Binary Nb3Sn T h e results on different samples are s u m m a r ized in table I. F o r alloyed Nb3Sn , the effective contents of the additives in the A15 layer is given. T h e composition o f the bulk sample, 24.5 a t . % Sn, is very close to the Nb-rich tetragonal phase b o u n d a r y [13, 16]. T h e value of (1 - c/a) for the tetragonal fraction of this sample was d e t e r m i n e d to be 0.0043, i.e. considerably smaller than 0.0061, the value for a stoichiometric single crystal m e a s u r e d by Mailfert et al. [18]. It follows that the (1 - c/a) value is r e d u c e d w h e n m o v i n g away f r o m stoichiometry towards the tetragonal phase b o u n d a r y . A t the same time, no significant c h a n g e of T M was observed. For the b r o n z e processed binary Nb3Sn samples, a successive decrease of the reaction t e m p e r ature f r o m 800 to 700°C led to a decreasing tetragonal v o l u m e fraction going f r o m 75% (sample 2) to 50% (sample 4). As follows f r o m a study on position and shape of the X - r a y diffraction lines on these samples [17], this is correlated on one h a n d to a shift of the m e a n c o m p o s i t i o n
Table I Diffractometric results on binary and alloyed Nb3Sn samples in the bulk and filamentary configuration (Aa = --+3x 10 ~rim) No. Effective Composition (at.%)
Reaction cond. (°C/hours)
a30¢,K (nm)
Volume fraction at 10 K: V,(I - c/a): V~
1
Nb~Sn(bulk)
0.52889
0.45(0.0043): 0.55
43
17.8
2 3 4 5 6 7 8
Nb3Sn Nb3Sn Nb3Sn + 0.6 H + 4.3 Ta + 2.8 Ta + 1.7Ta
sint. 1500° (100 atm. Ar) 800/70 750/138 700/290 800/70 800/70 800/70 800/70
0.52892 0.52888 0.52888 0.52934 0.52880 0.52884 0.52888
0.75(0.0052): 0.25 ~0.5/~ 0.5 0.5(0.0057): 0.5 cubic cubic cubic -~0.5(broad): ~0.5
17.9 17.9 17.9 17.2 17.9 18.1 18.2
9 10 11
+ 1.3 Ti + 1 Ni + 0.9 Ga
800/70 800/70 800/72
0.52873 0.52860 0.52876
cubic cubic 0.85(0.0043): 0.15
43 ~43 ~43 ~20 -50 -
T M
t'c
(K)
(K)
~43
18.0 18.0 18.3
361
W. Goldacker and R. Fliikiger / Phase transitions in binary and alloyed Nb3Sn
toward lower Sn contents for the reaction at 700°C and on the other hand to a significant broadening of the composition distribution, i.e. the cubic volume fraction is higher for the sample reacted at 700°C than for that reacted at 800°C. It is interesting that the diffusion reacted sample 2 (slightly closer to stoichiometric composition than the bulk sample 1) has a narrow composition distribution, with a very small overlapping with the cubic phase field.
i
.529z,
.5291 -3transforming binary Nb~Sn
i
o cubic • tetragona[ at T= 10 K
Alloying Elements on 6c sites
o(300K/ [nml
-]-
i
i
~,\\\\\\\\\\ \. ~tetragonal\\\\\\
".,,'~\\\\\\\\"~ooX \\"
T°
.5288.
O
To
o Ti
3.2. A l l o y e d N b 3 S n .5285
The samples investigated in the present work have somewhat higher lattice parameter values than those previously reported by Drost et al. [10]. The difference is due to the fact that the filaments are now fully reacted, leading to considerably narrower X-ray lines with drastically reduced contributions arising from the low Sn contents. Therefore, the average composition is closer to stoichiometry, which explains the higher lattice parameter values. When cooling below room temperature, a very slight line broadening was observed for the Ta and Ti alloyed samples (numbers 6 to 9 in table I), which is attributed to remaining internal stress after etching away the surrounding bronze. Line broadening at low temperatures was even more pronounced for the Ni alloyed samples. The cubic-tetragonal phase transition was suppressed in all Ta and Ti alloyed samples except for that with the lowest investigated Ta content, i.e. 1.7 at.% (number 8). This sample exhibited about 50% of tetragonal phase and is therefore
o(300K [nm] ~ , ~ .5289-
/Kunz
et o1.[20]
.5288- (diffusion reactedl
0
i
½
3
¼ (ItP)oTo
Fig. 1. Lattice p a r a m e t e r variation in the (Nb I xTa~)3Sn as a funtion of the Ta content.
system
a
6
5294
&
Z ot.O/o
i
i
i
i
o cubic • tetragonai at T=10K
on 2a s i t e s
.5291 __L
T
2
Alloying Elements
o(300K] [nm]
transforming binary Nb~Sn
4
,AI. \\\ .oTT-,At ~\~.tefragona I ~ \ \ 1 \
.5288 • ,x..
Mo
.5285
"-
b 0
1
2
3
4 or.%
Fig. 2. Occurrence of tetragaonal phase in Nb3Sn as a function of different additives. (a) Additives lying on the 6c sites (To, Ti). (b) Additives lying on the 2a sites (Ga, AI, Sb, Ni). The lattice parameter values for AI and Sb additives have been extracted from refs. 2 and 6, respectively. just located at the cubic/tetragonal phase boundary. The diffraction lines for the tetragonal phase could not be well separated from the cubic line, in order that T M could not be determined with precision, but lies in the range 20 <~ T M ~< 50 K, thus revealing a broad distribution of (1 c/a) values over the sample. The diffraction pattern thus confirms the low temperature (Nbl_xTax)3Sn phase diagram established earlier [13] on the basis of sintered samples. When comparing the present results on diffusion reacted samples with those on sintered or melted samples,
362
W. Goldacker and R. Fliikiger / Phase transitions in binary and alloyed Nb3Sn
the question arises whether the very different formation temperatures (-800°C compared to 1200 and -2000°C, respectively) affect the equilibrium at compositions close to stoichiometry. There is some evidence that with increasing Ta content the composition of the diffusion reacted samples deviates more and more from stoichiometry, towards Nb richer contents. This is illustrated by fig. 1 where the lattice parameter variation of the present Ta alloyed Nb3Sn samples prepared by diffusion reaction is compared to the corresponding data of Kunz and Saur [20], who investigated the region up to 40 at.% Ta on arc melted samples. The suppression of the phase transition in the system (Nb 1 xTax)3Sn is thus not only due to the increasing Ta content, but also to the increasing deviation from stoichiometry. The shift towards lower Sn contents reported here is in agreement with the observations of Tafto et al. [19]. For Ti alloyed Nb3Sn no significant lattice parameter shift with increasing Ti content was reported [20]. Therefore the reduction of the lattice parameter of sample 9 is mainly attributed to a deviation from stoichiometry, which itself is sufficient to hinder the phase transformation. The deviation from stoichiometry in Ti alloyed Nb3Sn samples confirms previous results of Tachikawa et al. [91. The occurrence of a lattice instability for the additives Ta and Ti substituting Nb on the 6c sites (as recently shown by Tafto et al. [19]) can be summarized by defining a region in a diagram representing the lattice parameter as a function of the additive content (fig. 2a). It follows that the small contents are sufficient to suppress the phase transformation, which necessarily implies that the transforming region is restricted to very small variations of the lattice parameter with respect to that for binary Nb3Sn. The situation is, however, quite different for the additives Ga, A1, Sb and Ni substituting Sn atoms on the 2a sites of the AI5 lattice. The 0.9 at.% Ga alloyed sample shows a nearly complete cubic/tetragonal phase transformation in spite of a markedly reduced lattice parameter. The same behavior was observed for the additions A1 [2] and Sb [5, 6], the latter showing the largest lattice parameter change (the change of sign for ( 1 - c/a) in Nb3A11 xSbx for
x ~ 0 . 1 [5] is here of secondary importance). These data have been plotted in fig. 2b, which shows a much wider transformation region than for Ta and Ti additions. For the additives in fig. 2b, the lattice parameter change is mainly caused by their substitution, rather than by deviations from the stoichiometric Nb content. From the present data it can be concluded that the occurrence of the lattice instability in alloyed Nb3Sn is dominantly influenced by the chemical nature of the additives, which determines their positions in the perfectly ordered [21] Nb3Sn lattice. Besides deviations from stoichiometry encountered particularly for Ta and Ti additions, it can be said that the substitution of additives on the 6c sites has a much stronger effect in suppressing the lattice instability of Nb3Sn than that on the 2a sites. This result can be easily understood in terms of the stronger interaction between chain atoms.
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W. Goldacker and R. Fliikiger / Phase transitions in binary and alloyed Nb3Sn [14] W. Goldacker and R. Flfikiger, IEEE Trans. Magn. MAG-21 (1985) 807. [15] R. Roberge, IEEE Trans. Magn. MAG-21 (1985) 811. [16] H. Devantay, J.L. Jorda, M. Decroux, J. Muller and R. Fl/ikiger, J. Mat. Sci. 16 (1981) 2145. [17] W. Goldacker and R. F1/ikiger, to be published.
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[18] R. Mailfert, B.W. Batterman and J.J. Hanak, Phys. Lett. 24A (1967) 315. [19] J. Tafto, M. Suenaga and D.O. Welch, J. Appl. Phys. 55 (1984) 4330. [20] W. Kunz and E. Saur, Z. Physik 189 (1966) 401. [21] R. Fliikiger, R. Isernhagen, W. Goldacker and W. Specking, Adv. Cryo. Eng. 30 (1984) 851.