MARTENSITIC TRANSFORMATIONS AND THEIR EFFECTS ON SUPERCONDUCTIVITY IN A15 SUPERCONDUCTORS

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HAL Id: jpa-00222167

https://hal.archives-ouvertes.fr/jpa-00222167

Submitted on 1 Jan 1982

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MARTENSITIC TRANSFORMATIONS AND THEIR EFFECTS ON SUPERCONDUCTIVITY IN A15

SUPERCONDUCTORS

R. Flükiger

To cite this version:

R. Flükiger. MARTENSITIC TRANSFORMATIONS AND THEIR EFFECTS ON SUPERCON-

DUCTIVITY IN A15 SUPERCONDUCTORS. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-

357-C4-362. �10.1051/jphyscol:1982451�. �jpa-00222167�

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JOURNAL DE PHYSIQUE

CoZZoque C4, suppZdment au n o 12, Tome 43, de'cembre 1982 page C4-357

MARTENSITIC TRANSFORMATIONS AND THEIR EFFECTS ON SUPERCONDUCTIVITY IN

A15

SUPERCONDUCTORS

Kemforschungszentrwn, I n s t i t u t fur Technische Physik, 7500 KarZsmhe, F . R. G.

(Accepted 9 August 1982)

Abstract.- The crystal structure of Nb3Sn filaments in m~ltifilamentar~ wires has been investigated by neutron diffraction in the temperature range 4.2 I T i_

1 8 0 0 K. Two stress-induced modifications of Nb3Sn have been detected: the first one is a tetragonal distortion (T2) occurring below 800 K, the second is a new, still unresolved phase (T ) , occurring below %I00 K. Both modifications are distinctly different from tae well-known tetragonal phase (TI) observed in the bulk, stress-free state. The effect of these modifications on superconductivity is discussed.

Introduction.- Nb Sn multifilamentary wires are used for the construction of super- conducting high field magnets. Typically, a wire of 1 mm diameter contains lo4 fila- 3 ments having a diameter of the order of 3um. The Nb3Sn filaments are surrounded by a Cu-Sn bronze matrix. Due to the differential thermal contraction of the bronze matrix and the Nb3Sn filaments on cooling from the reaction temperature (1000 K) to the operating temperature (4.2 K), the latter are submitted to a compressive stress. The application of an external tensile stress to such a wire has important consequences for the stiperconductjng critical current density, J,, Indeed, Jc increases as a function of the applied stress and reaches a maximum at a strain value &,, which depends on the wire configuration and varies from 0.3 to 0.7 %. The search for the rezsons of this maximum in Jc has led to the detection of two stress-induced modifications of the Nb3Sn phase. In addition, it resulted in a better understanding of the low temperature phase relationships of the Nb3Sn system in the bulk, stress-free state. In this contribution, the complex problem of instabilities in Nb3Sn and their effects on superconductivity will be discussed in the light of very recent results obtained at the Institut fur Technische Physik.

Nb3Sn in the bulk (stress-free) state.- The low temperature phase diagram in this

-

system has been recently established(l),based on bulk alloys prepared by r.f.levita- tion under high argon pressure. It is characterized by a tetragonal phase (TI), which forms at TN = 43 K and is stable in the composition range 24.5 <x <25.2?0.2 at.% Sn

(1,2). According to Vieland et a1.(3), the tetragonal phase T1 forms by a first order phase transformation, suggesting a congruent formation from the A15 solid solution.

It should be noted, however, that the boundaries of the two-phase regions have only a hypothetical meaning, since a non-vanishing two-phase region would require composi- tional changes at T < 43 K. This seems very unlikely, keeping in mind that the lowest temperature at which diffusion processes were observed in A15 compounds is of the order of 850 K(4). It is characteristical for this type of transformation that it has several features suggesting either a first or a second order transformation: in reality, it is not well described by the current transformation criteria. Usually, the problem is circumvented by saying that the transformation is "on the verge between first and second order".

The first observation of a lattice transformation in Nb3Sn has been reported by Mailfert et a1.(5). This lattice instability is correlated with a softening of the acoustic phonon modes, i.e. an anomalous positive dependence of the shear elastic constant, C' = (1/2)(C11 - C12). A low value of C' in cubic materials indicates a low {110)G10> shear resistance, which facilitates the displacement of Nb atoms to new

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982451

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JOURNAL DE PHYSIQUE

positions by means of a shear transformation. The origin of the C' anomaly in the A15 materials Nb3Sn and V3Si has been the object of different theoretical considera- tions. For VjSi, ~ a b b 6 and Friedel(6) showed that a Jahn-Teller band structure effect could account for the lattice softening. Another mechanism consists in an in- creasing ionic repulsion with decreasing temperature, as proposed by Xusovi'c and Warlimont(7). These authors showed on NiAl that the overlap of the Ni d shells at

low temperature leads to a large negative contribution to C'.

It was longtime unclear whether the cubic or the tetragonal Nb Sn modification would exhibit the higher value of Tc. This is mainly due to the shie?ding effects as a consequence of the considerable difficulties in preparing homogeneous, single- phased Nb3Sn samples. Another obstacle was the erroneous assumption that both phases would occur at the same composition, i.e. at 25 at.% Sn. A considerable progress was accomplished with the recent establishment of the low temperature Nb3Sn phase diagram(l), which proves that below TP1 = 43 K, the cubic and the tetragonal phase at equilibrium cannot exist at the same comoosition. The Sn rich cubic phase limit is at 24.5f0.2 at.% Sn, while higher Sn contents, in particular the stoichiometric composition, are tetragonal. The difference in Sn content of 0.3 to 0.5 % between the two phases is the key for understanding their corresponding superconducting properties. As shown in Fig. 1, Tc varies almost linearly between the limits of the cubic A15 phase, i.e. between 18.5 and 24.5 at.% Sn, at a slope of approximatively 2.5 K/at.%. The maximum value of T,, 17.8 K, is reached at the Sn rich limit: if the cubic phase could be stabilized at stoichiometry, values of Tc between

18.5 and 19 K would be expected. However, the stable phase at stoichiometry is the tetragonal one, TI, with the parameters Tc = 18.0 K and (1-c/a) = 0.006. It follows that the effective lowering of Tc due to the phase transformation lies between 0.5 and 1.0 K(1).

Fig. I :

The variation of p , Tc, ( d ~ , ~ / d T ) ~ ~ in Nb-Sn as a function of the Sn content.

(0) data on bulk samples(1)

(A) data on coevaporated films (8) The dotted line at % 24.5 at .% Sn represents the cubic-tetragonal phase boundary at low temperature.

The variation of the residual resistivity,g, and of the initial slope of the upper critical field, g ~ , ~ / d ~ ) ~

,

in bulk Nb3Sn is represented in Fig. 1, based on the data of Devantay et al.

(t)

and Orlando et a1. (8). It is evident that the change in composition from 24.5 to 25 at.%Sn has a much stronger effect on the el- ectrical resistivity and the initial slope than on Tc.

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In this composition range, a dramatical decrease of P from 20 to 6 llncm is observed on polycrystals, while the initial slope decreases from 26 to 17 T K - ~ . Since the initial slope for dirty type I1 superconductors is proportional to the product py, where y is the electronic specific heat, a better correlation between the decrease of p and that of ~H,~/~TIT, would be expected. However, the comparison is incomplete, since the corresponding values have been measured on different samples with different degrees of homogeneity.

The origin of the sharp change of p at the vicinity of the stoichiometric composition is correlated with a very high degree of ordering in Nb3Sn, rather than to the phase transformation. Indeed, X ray diffraction measurements of the Bragg-Williams order parameter, S, on Nb3Sn did not permit to detect a significant departure from S = 1, within the error limits AS = f 9.02(2). Starting with perfectly ordered Nb3Sn, it is plausible that even small deviations from the stoichiometric composition cau- sing the substitution of Sn atoms on the (2a) sites by Yb have a large effect on the electronic mean free path.

The existence of perfectly ordered state in NbjSn is indirectly confirmed by the extremely low values of the residual resistivity reported on stoichiometric singk crystals _{( 9 ) .}As reported earlier (2), there is a marked analogy between the two A15 systems Nb3Sn and V3Si: a) within the error limit, both systems exhibit order para- aeter values of S = 1, b) in both systems, single crystals were found exhibiting re-

sidual resistivities below 5yQcm, by far the lowest values among all known A15 phases (ordinarily above 20 ~Qcm) and c) V3Si and Nb3Sn are the only A15 compounds exhibiting a low temperature cubic-tetragonal transformation.

Necessary conditions for lattice instabilities in A15 compounds. - The question arises about possible criteria for the occurrence of the low temperature phase transformation in A15 type compounds. In particular, the effect of the atomic ordering is of interest. It is known that the phase transformation in V Si can be suppressed by 3 fast neutron irradiation. Since a very high degree of orderlng was measured in this compound prior to irradiation (lo), this would mean that a nearly perfectly ordered lattice is a necessary condition for lattice instabilities at low temperature. A second necessary condition follows from the phase diagram, i.e. only compositions very close to the stoichiometric one lead to a phase transformation. Both conditions have one point in common: the number of Nb atoms cln the (2a) sites should be very small.

In other words, the overlapping of d bands originating from Nb atoms situated on both the (6c) and the (2a) sites should be avoided. It is interesting that this is just a boundary condition of the "linear" model of ~ a b b i and Friedel (6), which leads to a singularity of the electronic density of states at the vicinity of the Fermi energy.

The necessary conditions for a lattice instability in A15 type compounds leading to a martensitic cubic - tetragonal transformation can thus be summarized as follows: a) perfect atomic ordering, b) stoiehiomctry (only small deviations allowed) and c) singularity or sufficiently strong variation of the electronic density of states at the vicinity of the Fermi level.

The formulation of the condition a) depends on the ordering kinetics at high temperature (2), while condition b) is given by the equilibrium A15 phase field at high temperature (1). This leads to the important conclusion that the occurrence of phase instabilities in A15 type compounds not only depends on the electronic structure at low temperature, but also on the thermodynamics at high temperature. For example, a lack of instability in V3Ge is correlated to the fact that the stoichiometric composition is not included in the equilibrium phase field (4). On the other hand, the stability of the cubic phase in V3Ga is correlated to a deviation from perfect order: S = 0.95 to 0.98 (11).

The effect of a stress field on the Nb Sn phase.

-

As a consequence of the differential thermal contraction of the Cu-Sn ;ronre and of the A15 phase, the Nb3Sn filaments are submitted to a compressive stress which was detected on measuring the variation of the superconducting critical current density, J ~ , as a function of the applied uniaxial tensile stress. The observed maximum of J, at a strain value Em ranging between 0.3 and 0.7 % was interpreted as the point where the applied tensile stress compensates the compressive stress exerted by the matrix. The compressive stress is usually called "prestress" or "pre~om~ression". since the _stress acts in the axial direction, attempts were made to describe the precompression effects by a "linear" model (121, assuming uniaxial stresses only.

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C4-360 JOURNAL DE PHYSIQUE

'The tetragonal stress-induced lattice distortion, T2+ - The first direct observation of the effective stress distribution in Nb9Sn multif~lamentarv wires was recentlv realized by means of neutron diffraction egperiments (13). The stress field acting on the filaments was observed by measuring the variation of the Nb3Sn lattice parameter at different orientations with respect to the wire axis. The method is based on the fact that the elastic range of the Nb3Sn phase under precompression is extended up to

Q 1 %. The variation of the diffraction angle, 20

,

of the (400) peak as a function of the temperature has been represented in Fig. 2. The angles a denote the orientation of the wires with respect to the Incident neutron beam: at u = 45 O, the lattice planes are perpendicular, at a = 135

"

parallel to the wire axis. It has to be noted that the data in Fig. 2 only include a small quantity of crystallites, namely those with the

{loo}

orientation parallel to the wire axis. For these grains, thecontrac- tion in axial direction is much stronger than in radial direction. Since the latter is very similar to that observed in bulk Nb3Sn, it follows that for this quantity of grains the stress distribution can essentially be approximated by an uniaxial distribution. For this particular grain orientation, a maximum of (1 - c/a) was observed

(12), while intermediate (1 - cia) values were found for the other orientations. This experiment shows that the stress state acting on the Nb3Sn filaments has in reality to be described by a three-dimensional-stress tensor. The variation of (1

-

^{c/a) for}

the grains with the

{loo}

orientation parallel to the wire axis as a function of T is plotted in Fig. 3. It is seen that (1

-

c/a) starts to increase asymptotically around temperatures of the order of 800 K. Above this temperature, the bronze matrix is too soft to affect the Nb3Sn lattice, which remains cubic. This stress-induced effect does not represent a phase transformation in the thermodynamical sense: a) the formation temperature is not defined, and b) the value of (1

-

c/a) is not constant for

a given composition, but varies with the orientation of the grains. In contrast to a

-

phase transformation, this effect can be called a tetragonal distortion, which will be denoted in this work as T9.

A new stress-induced phase in Nb7Sn: T3. - By means of neutron diffraction measurements on m~ltifilarnentar~ Nb3Sn wires, Fliikiger et al. (13) detected a phase transformation in the temperature range Q 40 < T < ^% 100 K. In the same work (13), it could be proved that this stress-induced phase is not identical with the tetragonal phase T I . So far, the structure of the new phase, called T3 in this paper, has not been re- solved, due to peak overlapping as a consequence of the presence of different phases at low temperature. Assuming a constancy of the crystallographical angles at go0, only two types of lattice are possible, i.e. orthorhombic or tetragonal {but with

(1

-

c/a) < 01. In both cases, the strongest deviation from cubicity is nearly twice as important as for the tetragonal phase T1. The stress-induced phase T3 seems to be very sensitive to the stress field: a) the onset of the transformation varies from

93.00 ("1

2 e 92.50

100 200 3 00

T(K1

The rate of increase of (1

-

c/a) shown in Fig. 3 is strongly influenced by the difference between the Young's moduli of the Cu-Sn bronze and Nb Sn with temperature.

An anomalous behavior of the Young's modulus, E, of the latter with temperature, is 3 correlated with the above mentioned anomalous behavior of C' and was recently observed by Bussisre et al. (14). These authors report that the E value for Nb3Sn at 4.2 K

is approximatively 113 of the value at 300 K.

L

-

N b3Sn wire

14.10-~ K-'

-- ( x = I . ~ o ~ A ) ^{6 .} ^IO-~K-'

0

a= 4S0: (004) 0c=135O: (400,040)

,

^I ^t ^I

Fig.2:

Axial (a = 45') and radial (a =135O) contraction in multifilamentary Nb3Sn rature wires as shown as a by function the (400) of neu- tempe- tron diffraction peak.

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% 80 to ?. 150 R, depending on the experimental conditions (unpublished data) and b) the distance between overlapping peaks also varies for different wires. It seems that the phase T occurs in all cases when Nb3Sn is produced by the bronze route and ink terfaces ~ b j ~ b 3 Sn or Bronze/Nb3sn are present. Earlier X ray diffraction data of Hoard et al. (15) on bronze-processed Nb Sn tapes showing a change of line shape in

the temperature range 40 < T < 80 K can z e interpreted as the first observation of the phase T3.

Fig. 3:

The Variation of (1-c/a) for the Nb3Sn grains with the {loo} orientation parallel to the wire axis, the direction of strongest com- pression.

0 200 LOO COO 800

Phase relationships and superconductivity in Nb2Sn. - The different crystallographical effects known at present in Nb3Sn under the effect of an external stress field are summarized in Fig. 4. This representation is only schematic and was chosen to show the coexistence of the different phase modifications of Nb3Sn. It should not be confounded with an equilibrium phase diagram. Fig. 4 shows that the tetragonal distortion occurs below 800 R , independently from the composition. Below ?. 100 K, a second stress-induced modification, the phase T3, coexists with T2 down to the lowest temperatures. In nonstoichiometric filaments, there is always a mixture T2 + T3, while for filaments with compositions very close to stoichiometry, a third phase is present. This third phase was analyzed after etching away the bronze matrix and was identified as the te-

I

Fig. 4:

Schematical representation of the crystallographical modifications in a multifilamentary Nb3Sn wire.

The compressive stress induces a tetragonal distortion (T2) and a new low temperature phase(T ) The phase TI' stays

3 :

for the correspondlng T l phase detected after etching away the bronze. There is at present no indication proving or dis- proving stress-induced changes from TI to Tll

.

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JOURNAL DE PHYSIQUE

Table I: Observed Structural Modificati3ns In Nb3Sn ~ultifilamentar~ Wires Crystallographi- Symbol (in Lattice Sn content T c cal Modification this paper) Parameters (at.% Sn) (K)

Cubic A15 a=b=c=5.2902 18.5to%24.5 6.0t017.8

(Cubic, extrapolated) (25) (18.5 to 19)

Tetragonal T1 (1-c/a)=0.006 %24.5 to 25 18.0 Tetr. Distortion T2 (1-c/a)=f ( a > rs24.5

I

^~17.8

Stress-induced phase T3 unresolved L(25Z uncertai4 15 to 17 tragonal phase T,. It has, however, to be proved that its symmetry remains unchanged under precompression. The condition of stoichiometric filaments is encountered in ful- ly reacted wires. Indeed, Smathers and ~arbalestier (16) could show that the A15 phase is first formed at compositions well below 25 at

.

^% Sn, but that the Sn content tends to this value after prolonged heat treatments.

All these modifications occur by martensitic processes, where no diffusion occurs. Relatively small deviations from cubicity, characterized by (1

-

c/a) <10.0081 lead to a marked decrease of Tc by several degrees K. This was recently shown by Sue- naga and Welch (17) on monofilamentary Nb3Sn tapes, where Tc values as low as 15 K were observed. Theabsolutevalue of T for the cubic and the tetragonal T modification for the corresponding Sn contekts as well as for the two stress-inAuced modifications are listed in Table I. Due to shielding effects masking phase superposi- tions and the effect of stress gradients, the Tc values for T2 and T in Table I have to be considered as maximum values. In order to obtain more detailed results, 3 measurements of low temperature specific heat and of H c2 VS. T(13) are in progress.

It appears now that the maximum of J with applied stress can be assigned to the gradual crystallographical change ofCboth stress-induced modifications, T2 and T X ray diffraction measurements under stress undertaken in our laboratory are ex- 3' pected to give more precisions.

The author would like to thank A. Hewat at Ill, Grenoble, for his competent help during the neutron experiments, as well as L. Pintschovius and W. Miillner for fruj tful discussions.

References

(I) H. Devantay, J.L. Jorda, M. Decroux, J. Muller and R. Fliikiger J. Mater, Sci., 16

,

2145 (1981)

(2) R. Pliikiger, ~ d v T ~ r ~ o . Eng., Vol 28, 399 (1982) (3) L.J. Vieland, J. Phys. Chem. Sol.,

2,

1449 (1970)

(4) F. Fliikiger, in "Superconductor Faterials Science", Ed. S. Foner and B.B. Schwartz, Plenum Publ. Corp. New York 1981, p. 511

(5) R. Mailfert, B.W. Batterman and J.J. Hanak, Phys. Lett.

&,

315 (1967) (6) J. ~ a b b g and J. Friedel, J. Physique

2,

^303(1966)

(7) N. Rusovi; and H. Warlimont, phys. stat. sol. (a)

g,

609 (1977)

(8) T.P. Orlando, J.A. Alexander, S.J. Bending, J. Kwo, S.J. Poon, R.H. Hammond, E.J. McNiff and S. Foner, IEEE Trans. Magn., MAG - 17, 368 (1981)

(9) A.J. Arko, D.11. Lowndes, F.A. Miiller, L.W. Roeland, J. Wolfrat, H.W. Myron, R.M. Miiller and G.W. Webb, Phys. Rev. Lett., 46, 1590 (1980)

(10) J.L. Staudenmann, Sol. State Phys.,

3,

121 (1977);

6 ,

461 (1978)

( 1 1 ) R. Fliikiger, J.L. Staudenmann and P. Fischer, J. Less-Common Metals,

2,

²⁵³

( 1 976)

(12) H. Ekin, in "~u~erconductor Materials Science", Ed. S. Foner and B.B. Schwartz, Plenum Puvl. Carp., New York, 1981, p. 455

(13) R. Fliikiger, W. Schauer, W. Specking, L. Oddi, L. Pintschovius, W. Miiller and B. Lachal, Adv. Cryo. Eng., Vol. 28, 364 (1982)

(14) J.F. ~ussiGre, D.O. Welch and M. Suenaga, J. Appl. Phys.

51,

1024 (1 980) (15) R.W. Hoard, R.M. Scarlan, G.S. Smithand

,

C.L. Farrell,

Trans. Magn., MAG

-

17, 364 (1981)

(16) D.B. Smathers and D.C. ~arbalestier, in "Filamentary A15 Superconductors", Ed. M. Suenaga and A.F. Clark, Plenum Press, 1980, p. 143

(17) M. Suenaga and D.O. Welch, accepted by .T. Appl. Phys.

MARTENSITIC TRANSFORMATIONS AND THEIR EFFECTS ON SUPERCONDUCTIVITY IN A15 SUPERCONDUCTORS

HAL Id: jpa-00222167

https://hal.archives-ouvertes.fr/jpa-00222167

Submitted on 1 Jan 1982

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.