Franck Ferrer

Alain Barbu

Thierry Bretheau

Jérôme Crépin

François Willaime

Daniel Charquet

Jerome Crepin

Franrois Willaime

Daniel Charquet5

The effect of small concentrations of sulfur on the plasticity of zirconium alloys at intermediate temperatures

Keywords: mechanical behavior, sulfur, deformat. ion mechanisms, temperature, dislocation, Zr alloys, oxygen, hardening effect

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The addition of 25 weight ppm sulfur is enough to reduce by a factor of three the creep rate of zirconium alloys at 400°C (Fig. I} as demonstrated at Framatome ANP [ 1 J. Such a strong effect by such a small concentration suggests that sulfUr acts directly on the mecha nisms controlling the deformation kinetics. At the beginning of the present work, these mechanisms were still poorly identified, and they had to be specified first. Sulfur provides a very powerful probe to test the validity of these deformation mechanisms. By considering all the potential controlling mechanisms and comparing them to the measurements and ob servations performed at al I the reachable scales, most mechanisms could be eliminated, and the most probable controlling mechanism was identified.

Experi mental

Materials

Two different alloys were used in the present study: a-zirconium specially supplied in the form of plates and an industrial alloy supplied in the form of tubes by Cezus Framatome ANP. Both materials show a strong rolling texture with the c axis perpendicular either to the rolling direction or to the tube axis (Fig. 2).

a-Zirco11i11m-lt was supplied in the form of hot rolled plates recrystallized at 650-700'C during 4 h. The resulting grain size is 45 µm, and there are few precipitates (Zr3Fe). Different concentrations of sulfur and oxygen have been supplied, as reported in Table I. The materials containing less than 5 ppm sulfur will be referred to as "low sulfur" (LS), and those with 25 ppm sulfur as "high sulfur•• (HS). The same denomination will be used concerning oxygen content. Neither sulfur nor oxygen contents affect the microstructural parameters (grain size and texture).

Zr-1%Nb-0.13%0 Alloy-The material was supplied in the form of tubes with a 9.5-mm outer diameter and 0.57-mm thickness. The tubes are produced by pilgering in five steps, using a process that allows the reduction of the diameter and the thickness in the same operation. After each rolling step, the tubes were degreased, chemically etched, and annealed at a temperature just below the a/(a+(l) equilibrium temperature, giving a total recrystal lization. The aim of lhis procedure was to obtain fine �-Nb precipitates homogeneously distributed in the a-Zr matrix in order to have improved corrosion properties. The mean diameter of these spherical precipitates was in the range 40--60 nm, and their density was very low (about 120 per mm3); the grain size was 4 µm. The chemical composition is given in Table 2; two different sulfur compositions were supplied with no noticeable effect on the microstructural parameters.

Mechanical Tests

In order to characterize the influence of the sulfur content on the mechanical properties of a-zirconium and Zr-1 % Nb-0.13%0 alloy, creep and constant strain rate tes.ts have been performed.

Creep Tests-They were performed on Zr-1%Nb-O. J 3%0 alloy tubes that were internally pressurized. The initially pressurized specimens were introduced in an electr' ic furnace at room temperature and heated. The creep tests were performed in air or in vacuum conditions (better than 10-6 torr to avoid oxidation in order to be able to observe the surface of the sample after creep). After the test, the specimens were cooled inside the furnace. The tem perature was regulated within ± 3°C. The temperature and stress ranges investigated were respectively 350--430°C and 90-150 MPa.

The creep strain is deli ned as the relative variation 6.D/ D of the tube outer diameter, where D is the outer diameter and t:.. D is the change in outer diameter. It was obtained as the mean value of several measurements performed with a laser micrometer: three different sections of the specimen, along six different generating lines. The accuracy of each mea surement is ± I µm. During the primary creep stage. the samples were taken out from the furnace and measured every 72 h, until 240 h, and every 240 lh after that time. The hoop stress in the tube wall is controlled by adjusting the inner pressure of the argon gas. Since e < < D.,, (e = tube thickness and D,. = mean diameter), the radial stress can be safely neglected and the hoop <TH and axial u,,, stresses assumed constant in the thickness: <T"' = 4e where P is the gas pressure inside the tube at the test temperature.

(I)

Co11sfl//lf Strain Rate Tensile Tests-Tensile tests were performed on a-zirconium speci mens between room temperature and S00°C using an MTS machine equipped with a furnace. The temperature was controlled within ± 2°C and the deformation was measured using an extensometer. The tests were carried out using crosshead velocity control but not with real strain rate control. Nevertheless, as soon as the plastic regime was reached, the strain rate became nearly constant. The crosshead velocity was adjusted so that the strain rate in the plastic regime was s x I o-5s I.

The strain rate sensitivity was measured by performing strain rate increment tests at three levels: S x 10-•s 1, 5 x 10 -5s-•, and S x 10-"s-1; some tests were also performed at very low strain rate ( 10 8s 1). All tests were performed on samples with the stress axis aligned with the rolling direction. On Zr-1%Nb-O.I3%0 alloys, the tests were performed on tubes between 270 and 430°C, using an lnstron machine under a strain rate of S x I 0 �s 1 in the plastic regime.

Microscopy

Observations and characteri zations of the deformation mechanisms have been performed using optical microscopy, Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM); these techniques complemented each other.

Surface Preparation-The samples were electro-polished in a solution of 80% acetic acid and 20% perchloric acid at 20 V, at 7°C for I SO s. The resulting surface was suitable for the observation of the microstructure using SEM and optical microscopy.

171i11 Foil Preparation-Str ips (3 x 10 mm2) were cut from the tubes or from the flat specimens. They were mechanically polished with a 1200 grit paper to I SO µm thickness. The 3-mm-diameter disks were punched and electro-polished (70% methanol, 20% mono butylic ether, and 10% perchloric acid. 20 V, 30"C). When suitably thin, the specimens were washed in methanol.

The TEM studies of the thin foils were carried out using a CM30 Philips with a 300 kV accelerating voltage. 3). The ratio of the creep rates for the HS materials to the LS materials is about 3 and constant in the whole temperature and stress ranges.

Characterization of the Sulfur Effects

If the steady-state creep regime is analyzed using the classical creep law:

(2) then both the stress sensitivity (11 2! 4) and the apparent activation energy (Q a 215 kJ/mol) remain insensitive to sulfur concentration (Fig. 4u andb). It should be noted that a difference of only 6 kJ/mol (Q"5 -Qi.s = 6 kJ/mol) could explain the factor of 3 for the steady state creep rates. Since 6 kJ /mol is smaller than the uncertainty in the values of Q. the full effect of sulfur can be understood by postulating the dependence of the apparent activation energy in sulfur concentration. Nevertheless, the apparent activation energy is directly dependent on the controlling mechanism, and a different value of Q would mean a different mechanism.

However, such a small difference in their activa1ion energy for two different mechanisms is not very plausible. Therefore, in the rest of this paper, Q and the nature of the controlling deformation mechanism will be considered to be independent of Lhe sulfur content. The only dependence of the sulfur content is on the kinetics of the controlling deformation mechanism.

Co11sta111 Strai11-Rllle Tensile Tests

Zr-1% Nb -0.13% 0 Alloys-Sulfur increases the value of the yield stress, the flow stress being always higher for HS than for LS materials (Fig. 5). This effect is stronger at 400°C than at 200°C. If the tensile curves are analyzed by means of the classical hardening law:

CT = Ae" (J)
it can be observed that the sulfur content does not affe ct the value of the strain hardening exponent p, but increases A (Table 3). Thus, the sulfur content shows an effect similar to a strain-rate effect. This analogy between sulfur content and strain-rate is clearly shown in Fig. 6. where the stress-strain curve of a HS material for a strain rate of 6 x 10 -�s-•, and the curve of a LS material under 6 x IO-�s-• are superimposed (see also boldfaces in Table 3).

--• --HS: o=90MPa T=430°C --• -• HS : o=l30MPa --• -LS : o=90MPa , " 10
--• - LS : o=l30MPa 5 -" --• - •�-= : _ :_: _ =-_ =-.. • � � � � =-. : � -_ -_ -_ � -_ - _ : � � � � � � -
--.--High sulfur _ ... -----• • 1 T=200°C -------Low sulfur -------.. ---. -- ,- ------ 120 , - --• •- • 1 T=400°C �'
Then. following standard arguments, since p is associated with the strain hardening mech anism. it can be deduced that sulfur docs not affect the nature of this mechanism. The identity between the sulfur effect and a strain rate effect suggests that sulfur modifies the material viscosity; thus, instead of a modification of the deformation controlling mechanism, it seems that sulfur modifies the kinetics of the mechanism. This is consistent with the independence of the parameters (11, Q) in the steady-state creep law on the sulfur concentration.

cr.-::.irco11ium-The sulfur effect is in agreement with what was reported for Zr-1 %Nb-O. I 3%0 alloy here above. Since a-zirconium contains very few precipitates. this result shows that the sulfur effect is independent of precipitates. Nevertheless. it cannot be excluded that sulfur operates through a coupling with elements present in the two al loys as oxygen or iron. 2

The plot of the tlow stress at 2% strain versus temperature shows the classical ••athermal plateau" 12.31 between 250 and 400°C (Fig. 7a). The sulfur effect appears at about IOO°C and increases up to 400°C: it accentuates the phenomenon, giving rise to an increase of the tlow stress between 400 and 500°C.

The strain-rate sensitivity can be expressed as:

111 = ii In CJ l ii In£ r (4)
Consistent with the above observations, the value of m reduced by sulfur to almost zero between 350 and 400"C (Fig. 7b).

The apparent activation volume is:

V = kT 0 Jn£ I a iJCJ T
which can be expressed in terms of m:

kT v =- " 1/1(1 (5)
(6)

The effective activation volume gives indications on the nature of the mechanism controlling the dislocation movement; it can be deduced from the apparent activation volume of the polycrystal by V = MV,,, where M is the Taylor factor (M = 4 12]). V increases between 200 and 400°C:

• T � 200°C: V � 40bJ a low value indicating that dislocations probably interact with short range or very dense obstacles (Peierls valleys, cross slip. concentrated solid solution ... ). • 300 < T < 400°C: 100 < V < 400b� a value too high for dislocation climb as the controlling mechanism.

In some tests, after a certain amount of plastic strain, the specimen was unloaded and held at a fixed temperature for 30 min. When it was reloaded, the stress climbed beyond the previous flow stress, then dropped, and then followed the normal work-hardening curve. This is an age-hardening peak. The plot of the stress increment (Arr) versus temperature shows that sulfur increases the age-hardening peak (Fig. 8).

The athermal plateau, the age-hardening peak, and the minimum in strain rate sensitivity are generally understood in terms of the interaction between dislocations and oxygen atoms based Shoeck-Seeger type models 14.51. Since sulfur amplifies all these effects, the question of a coupling effect between sulfur and oxygen, or of a supplementary dynamical hardening due to sulfur, must be addressed. These points are discussed below.

The Effectively Active Deformation Mechanisms

The potentially active deformation mechanisms in zirconium are documented 16-101. Nev ertheless, for all the potential mechanisms, we have to determine which of them are prin cipally active, where and when they are activated, how their activation depends on the loading and temperature conditions and, above all, how sulfur modifies their activity. Using both SEM and TEM observations, these mechanisms have been characterized in the whole tem perature and stress range and for HS and LS materials.

An essential conclusion is that no effect of sulfur content on the nature, occurrence, or density of the deformation mechanisms could be shown. Therefore, in the following, they are presented without reference to the sulfur content. The conclusions of the study of the deformation mechanisms are as follows.

Whatever the deformation mode, no twins were observed by either SEM or by TEM; the only active deformation mechanism is plastic glide. Two temperature domains must be considered:

• T < 200°C: The only slip systems are of the prismatic type (Fig. 9a) with a very small amount of pyramidal glide. Only long screw <a> type dislocations (proof of a high Peierls friction) can be observed; in most grains, two of the three possible prismatic slip systems were activated (sometimes three) and showed a homogeneous distribution (Fig. 9b). The dislocations of the different slip systems interact; if the interaction is attractive, they form junctions; if it is repulsive, they cross and yield sessile jogs that are responsible for the formation of long dipoles.

• T > 200°C: Prismatic glide is still dominant (80%), but first-type pyramidal glide is more frequent (I 0%) and cross slip, from the prismatic planes towards the pyramidal ones, can be observed (Fig. IOa). The dislocations are bent in pyramidal planes (Fig. IOb) showing that their movement is no more controlled by Peierls friction. Neither pile-ups nor cells are formed, the dislocations being homogeneously distributed inside the grains.6

Orig i n of the Sulfur Effect

The effect of sulfur on the mechanical behavior of a-zirconium and Zr-1%Nb-0.I3%0 alloy is confirmed, but its origin is still not explained. Sulfur does not modify the micro structure (grain size, texture, precipitates, ... ), nor the nature of the deformation controlling mechanism. It seems to modify the kinetics of that mechanism but not the mechanism itself.

Moreover, it must be kept in mind that this strong effect is due to a very small amount of sulfur. In the following, an attempt is made to identify at what scale and through which defects sulfur can affect the deformation kinetics.

Using all the previous observations, a limited list of potential controlling mechanisms was prepared (Table 4). Each of these mechanisms needs the activity of one elementary phenom enon that involves one type of defect. Thus, the influence of sulfur on the deformation kinetics is the consequence of the interaction of sulfur atoms, alone or associated with other solute atoms, with the relevant defects.

In the following, the relevance of these elementary phenomena is analyzed to attempt to shorten the list and point out which sulfur/defect interaction is the most reliable. Using a microgrid technique[/ l], it has been verified that grain boundary sliding was not observed at any temperature or strain rate neither for HS nor LS materials. Thus, grain boundary sliding is not a deformation mechanism. This does not mean that it is not the controlling mechanism, since a very small amount of sliding could relax the intergranular incompatibilities.

In order to check this hypothesis, three materials with different grain sizes have been obtained by critical working and recrystallization of a-zirconium. The grain sizes are 45 µm (original small grain material referred to as SG), 200 µm (medium grain material referred as MG), and 800 µm (large grain material referred to as LG); the texture and composition of the three materials are identical.

Under constant strain-rate tensile test condi�ions, the flow stress at I% strain shows a classical Hall-Petch behavior (Fig. 11 ): [START_REF] Akhtar | Plastic Deformation of Zirconium Single Crystals[END_REF] where clJ is the grain size a. nd cr0 the single crystal flow stress. HS and LS materials exhibit the same k value, whereas a0 is strongly affected. Thus, sulfur only affects the intragranular behavior, which leads to the conclusion that grain boundary sliding is not the controlling mechanism. Therefore, sulfur/grain boundary interaction is not the origin of Slll lfur effect.

Self Diff usion

The Such a model suggests a law of the form [/5]:

(8)
with where A is a constant depending of the microstructure (I <A < 100), µ the shear modulus, er the applied stress, 11 the stress sensitivity factor, Dv is the bulk self-diffusion coefficient,

and Qv the bulk self-diffusion activation energy.

Only A and" depend on the microscopic details of the model. The Zr-1%Nb-0.13%0 alloy steady state creep behavior can be analyzed with 11 = 4 and Q = 215 kJ/ mol. This can suggest a dislocation climb-controlled mechanism. Equation 8suggests the comparison of the apparent activation energy Q for creep and the bulk self-diffusion activation energy

Qv.

There is large scatter in the available Qv values: 90. 217. 306 kJ/mol according to Gruzin [/6), Lyashenko !171,and Hood et al. !18). respectively. Many authors l /9-2/) have sug gested that, for temperatures lower than 850°C, diffusion could occur both by bulk and pipe diffusion. In general. pipe diffusion has an activation energy value about half the value for bulk diffusion and plays a �redominant role at lower temperatures. Thus, diffusion measure-ments at 850°C could be affected by the dislocation density, and probably be underestimated; this could explain the low values reported by Gruzin and Lyashenko. Finally. Hood et al. give a mean value for the bulk self-diffusion coefficient I /BI: D 90 lo -� -3.17 ± 0.04 cV 9) This equation was derived from different experiments, all performed at temperatures higher than 600°C. No value is available for T = 400"C.

, _1 = x -exp m-• s u . kT (
The creep activation energy we measured (215 kJ/mol) is significantly lower than the value given by Hood et al. (306 kJ/mol), leading to the conclusion that bulk self-diffusion is not the creep-controlling mechanism for T < 400°C. Nevertheless, it is risky to select a controlling mechanism on the basis of the activation energies only, because the relation between activation parameters and mechanisms is far from one-to-one [START_REF] Poirier | Plasticite a haute temperature des solides cristallins[END_REF]. In order to evaluate the possibility of a bulk self-diffusion controlled mechanism, we have used Eqs 8 and 9 to estimate the resulting strain rate. At 400°C the diffusion coefficient value is Dv = l.7 x 10 2ll m2s 1• Under a stress of 130 MPa and for A = 10. the calculated strain rate is 10-11s-1, i.e., much lower than the experimental one (1.5 X 10xs-

1

). This seven orders of magnitude cannot be due to the model. The main cause for that discrepancy is the value for the bulk self-diffusion coefficient that is too low.

In conclusion, all these observations allow the discarding of bulk self-diffusion as the controlling mechanism in the temperature range of interest. But mechanisms implicating pipe self-diffusion remain plausible. Nevertheless, the activation volumes measured between 300 and 400"C are always higher than 100b3, thus excluding dislocation climb mechanisms since their activation volume is of the order of b�. The only diffusion-controlled mechanism that cannot be discarded is jog dragging controlled by pipe diffusion.

Ar rhis swxe we ca11 drall' rhe follml'i11x pmrial conclusio11s: .m(fur i11re ra crs wirh dislocwio11s and rhe meclumism comrolli11K rhe defomwrio11 ki-11erics is a disloca1io11 m echa ni sm.

Then !Im quesrions maurally arise:

• does suljilf behave as w1 immobile obswde pi1111inx dislocarions �

• does sulfur form a mobile cloud slowinK dmrn rhe dislocarion 11101•emem?

Su/ f url Dislocarion lmeracrion

Is Su(fur a Fixed Olwade?-Two types of models have been derived to describe the interaction between dislocations and fixed point defects:

• models for bee marerials ll'ilh a larxe Peierls fricrion in ll'hich poim defecrs inrera cr 11"irh double kinks [23,24) • models for fee 111areriC1/s in which point defecrs acr as pi1111inx poi111s 125.26].

In zirconium, when T < 200"C, we have shown that deformation is controlled by the movement of long <a> type screw dislocations subject to a high Peierls stress: then, the first type of model could be applied to this terr .perature range. When 200 < T < 500°C, the observed long curved dislocation suggest that Peierls friction is no longer active: the second type of model would then be pertinent.

All of these models predict a hardening decrease with increasing temperature, whereas in the case of zirconium/sulfur interaction, the effect appears around 100°C and is at a maxi• mum value around 400°C. Moreover, predictions of hardening by these models, on the basis of an overestimated sulfur/dislocation interaction energy (I eV), give values ranging between 0.1 and I MPa, when the experimental value is 25 MPa. This discrepancy is due to the very low sulfur concentration. Therefore, these models cannot explain the observations. \Ve co11clucle that sulf 11r is mobile.

Sulfur amplifies many effects reputed to be due to oxygen: a dynamical strain-aging pla teau, drop in strain rate sensitivity, and static aging peak. Thus, it is justified to compare sulfur and oxygen effects and try to understand whether they are coupled or independent.

Are the Oxy!:ell and Su/ f ur Ef fects lndepe11de111?

Dynamical strain-aging effects express themselves in precise ranges of temperatures and strain rates. In particular, they disappear when the temperature is too high or the strain rate too low [27][START_REF] Adda | Elements de metallurgie physique[END_REF][29].

At a 5 x 10-5s-1 strain-rate, the sulfur effect appears at I00°C and is at a maximum value around 400°C. Very low strain-rate tests (2 x 10 "s 1 ) performed on HS and LS materials (Fig. 12a) show a sulfur hardening of 25 MPa at I% strain, similar to that obtained at 5 x 10 �s 1 •

The same experiment has been performed on HS and LS materials, but with an oxygen content of 300 ppm. The same hardening amount of 25 MPa was observed (Fig. I 2b). Thus, the sulfur effect is independent of decreasing the oxygen concentration: nevertheless, the oxygen concentration remains too high ( 10 times higher than sulfur amount) to be able, from this experiment, to exclude interactions between oxygen and sulfur. Now, if Figs. 12a and b arc superposed (Fig. 13), it appears that the oxygen effect disappears at low strain-rate, whereas the sulfur effect remains unchanged. Therefore, we can conclude that:

• the sulfur and oxygen effects are independent,

• the nature of these two effects is different Thus. the sulfur effect can be considered as intrinsic, since it is not a simple amplification of the oxygen effect. A complex defect (S/O or S/Fe) could be the effectively active defect: tests performed on high purity zirconium with controlled amounts of impurities are necessary to determine the nature of the active defect.

In order to explain the very strong eff ect of a very loll' co11ce11tration of sulf11r persistillR at low strain rates. we are led to formulate a 11ell' hypothesis; that is .. mlfur could segregate at dislocation cores and modifv their structure. [O] =1200 ppn 0.3 ------.

0.6 0.9

E (%) 1.2 1..5
of ox ygen disappears at low strain-rate (low s11lf11r material; T = 40{fC).

Sulfur segregation at dislocation cores could modify this structure, which would affect most of the properties of these dislocations: Peierls friction, cross slip. pipe diffusion. inter action with other impurities, etc ... The persistence of sulfur hardening at low strain-rates can be explained, as well as the "viscous" sulfur effect. This hypothesis is consistent with all the experimental observations. Atomic scale numerical simulations have been performed in order to check the validity of this hypothesis.

Atomic Scale Simulatio11s

Several experimental techniques. a priori pertinent to verify the effectiveness of sulfur segregation inside dislocation cores, have been considered.

High resolution TEM allows the observation of dislocation cores. Nevertheless. a perfect screw dislocation cannot be imaged by that technique. since its Burgers vector is normal to the image plane. A spread screw core could be imaged. due to its nonscrew components, but since they are very weak, the task was quite hazardous. Because of this difficulty. it was not attempted.

The atom probe performs analysis atom by atom; it could give the atomic composition of the dislocation core. But a s1.1lfur atom is half the weight of a zirconium atom and a zirconium ion (Z�+) has a charge twice that of a sulfur ion (S�+ ); thus, the time of flight of both ions is quasi-idemtical. so that the signals would be very difficult to distinguish.

To avoid these difticulties. atomistic simulations were used. They were performed by combining simulations based on semiempirical interatomic potentials [321 and ab i11itio elec tronic structure calculations. performed in the framework of the density functional theory in the local density approximation. For the latter, the PWSCF plane-wave pseudo-potential code [JJ] was used. The details of this work will be reported elsewhere; the results that are most pertinent to the discussion on the hypothesis of a sulfur segregation at dislocation cores are summarized here.

The preferential bulk site for the dissolution of sulfur was first investigated by ab i11itio calculatiOTllS. by comparing the energies of various relaxed configurations. It was determined that sulfur. like oxygen, is an interstitial rather than a substitutional impurity, and that it is most favorable in the octahedral site. Moreover. sulfur atoms are shown to induce important distortions in the zirconium lattice, as attested by the large increase of the volume of the octahedral site (<lV/V = 30%), whereas the lattice is weakly affected by oxygen atoms (�V/ v = 1%).

The dislocation simulations were performed in two steps. First, the structure in a large supercell was relaxed using a semiempirical interatomic potential for zirconium [32]. In agreement with previous calculations 13 I ,34) ir was found that, within this simplified ap proach, the <a> type screw dislocation is more stable when the core is spread in the basal plane, in disagreement with both experiments and more accurate calculations. However, the configuration with the core spread in the prismatic plane and with secondary spreading in the basal plane-as it should-is found to be metastable [34]. which allows calculations on this correct configuration to be carried out. By examining the displacement of such a dis location, it was found that the glide in the prismatic plane is. in fact, a zig-zag glide in first type pyramidal planes (Fig. 14). This glide mechanism could be at the origin of non-Schmidt law effects reported in titanium and suspected in zirconium.

The ah i11itio calculations were then performed by extracting the positions of the 37 atoms closest to the dislocation core (Fig. 15). The atoms at the outer layer were kept fixed. and the other atoms were allowed to relax. The maximum cancellation of errors was ensured by performing bulk and dislocation calculations with the same geometry. At least three sites close to the core with a negative segregation energy of the order of -0.1 eV have been identified. This attractive impurity I dislocation interaction is confirmed by doubling the cell dimension along the dislocation line. These sites are octahedral-like, but they have no equiv alent in the bulk. Sulfur atoms affect the core structure of the dislocation. but our calculations at present do not allow quantifying the consequences of the distortions on dislocation mobility. The value of -0. I eV can be regarded as rather large in absolute value, when compared to the "core" energy, i.e., the energy difference between cells with and without dislocations, which is only 0.5 eV /<a>. The impurity/dislocation interaction is. therefore, expected to be even more attractive if cells with larger dimensions perpendicular to the dislocation line are considered. Finally, finite temperature effects may also be crucial. The present calculations are. therefore, consistent with the hypothesis of sulfur segregation at <a> type dislocation cores.

c 0 ru !O] [1iOO] • • • • • • • • • • • � • \ \ • �• I •� • ' • ).• I I •.I! • • • • • • • • • • • • • • • • FIG.

Conclusion

The hardening effect of some ten ppm sulfur on the mechanical behavior of zirconium alloys has been clearly demonstrated. It appears at about I 00°C and is at a maximum value around 400°C with a stress increment of 25 MPa under constant strain-rate conditions or a strain-rate decrease of a factor of three under constant stress conditions.

The nature of the deformation mechanisms is not affected sinice Q, 11. and slip systems are unchanged and only their kinetic s is modified: the sulfur effect is viscous-like. The sulfur effect has similarities to the oxygen hardening effect. but it has been demonstrated that it occurs by a different mechanism probably involving a modification of dislocation cores.

  FIG. l-Variari1111 of tliametral creep strain 1ri1h time in /111/y recryswllizetl Zr-1%Nb-0.JJ%0 111 bes 1111tler biaxial s1ress (cirrnmferemial stress = I JO M Pa).

  FIG. 2-Typical mlling text11re of the materials 1111der sltu�\".

  Creep Tests All creep tests have been performed on Zr-1%Nb-O.I3%0 alloy tubes. The sulfur effec� was observed in all the temperature and stress ranges used: 350°C s T s 43Q"C. 90 MPa s. u s 130 MPa. Sul fur shortens 1he primary creep stage and lowers the secondary creep rate (Fig.

  FIG. '!.-Creep nuw., for low 1111d high s111f11r Zr-lo/r Nh-0.JJ"lrO alloys pe1fom1ed 1111der imernal pre.mire or T = 4J(fC'.

  160

FIG. 5 -

 5 FIG. 5-Stress•stwi11 rnn•es for "'"' and hil(lr s11/f11r Zr-l o/r Nh-0.13%0 alloys perfon1 1 ed under di/ Jere/I/ te11 1 perut11re co11ditio11s.

FlG. 6 -

 6 similarit\' 11/ A and p l'll l11es Cl/ a gil'e11 te111perc1111rt' for H S 111111eri11/ 111 /1111 • strain rwes 1111<1 LS mmeria/ 111 hiRlr strui11 rotes). T = 27<fC T = J50°C T = 430°C 6 x 10 's I 6X 1Q-•� -I 6 x I{) 's I 6X (0 •s I 6X 10 'S I 6 x (0 �� I A (MPa) HS 485 The viscous-like effect of s11/f11r (Zr-/'N-;Nb-0.13%0: T = 43fJ'C).

  FIG. 7-£/feC'I of the s11(f11r comem 1111der constant strain-rare c1111dirio11s: a) the arhermal plarea11 (srrain-mre = 5 x tO -'s-1), b) the strain-rare se11siriioi1y.

  FIG.8-£.ffect 11/ the s11/f11r conte111 1111 age-lrarde11i11g.

FIG. 9 -

 9 FIG. 9-Defomwticm medumisms 111 T < 2CXJ'C: a) prismatic slip plane traces (SEM), b) 10118 .\'Crew <a> tvpe disloclllio11s (TEM: basal pmje, . 1io11).

FIG. 10 -

 10 FIG. 10-Vefomwrimi 111eclia11is111s arr> 2Uif'C: a) prismatic slip 1>la11e traces 11•irli some pyramidal plane traces .rn1111esril111 cmss-slip evems (SEMJ. b) <a> r_1pe dislocations be111 in pyramidal planes (TEM: /Ja.ml pmj1'ctim1) .

  FIG. I I-Ef f ect of s11/f11r cm the Half-Perch type /Jehm•ior (strain-rate = 5 X I0-5s-1; T = 40(J'C; 0<Zr).

  a) fll' o sulf i11rerC1crio11 �vpes ccm be dismissed: suljilf/xrai11 bo1111c!C1ry and s11/f11r/ VC/CClllCJ b)

  FIG. 12-The .rnlfur ef f ect persists m low stmin-rcite (5 x JO �.1• 1: T = 4()(J'C). as ll"ellfor hmh: a) a hi�h m:\"t:en co111e111 (/200 ppm), b) a /ou• ox_,•gen co111e111 (300 ppm).

FIG

  FIG.13-The hardening f'.

  FIG. 14-Sdiematic representation of the :ix-:ag m111io11 o( an <a> type Jcreu• di.11ocatirm (repre- Jemed by .L l J(lidinx in the pri.rmati< • plane.

TABLE I -

 I Chemical co111positio11 of a•vrco11i11m materi als.

	Element	0		Fe	Cr	H	Others	s
	Weight, ppm	JOO	1200	IJO	JO	10	<50	<5	25

TABLE 2 -

 2 Chemical composition of Zr-1%Nb-0. IJ%0 materials.

	Element Weight. ppm	0 1200	Nb 1.01 x 10"	Fe 108	H <5	Others <50	<.S	s	2.S

TABLE ' J

 ' -/11f/11e11ce of s11/f11r co11te11t 01 1 the p11ra1111'/ers of tire lwrde11i11g /a11• a = A£ 1

	, CNme tire

TABLE 4 -

 4 The 11orenrial conrmlling mechanisms and rhe associated defects.

	Potential Controlling
	Mechanism	Elementary Phenomenon	Defect
	Relaxation of intergranular incompatibilities	Grain boundary sliding	Grain boundary
	Restoration by dislocation climb and annihilation Sessile jog dragging	Bui k self-diffusion Pipe self-diffusion Bui k self-diffusion Pipe self-diffusion	Vacancy Dislocation Vacancy Dislocation
	Dislocation glide	Peierls friction	Dislocation
	Restoration by cross-slip	Cross-slip	Dislocation
	and annihilation
	Grain Boundary Sliding

Senior Researcher, Laboratoire des Solides lrradies, Ecole Polytechnique, 91128 Palaiseau, France.

Senior Researchers. Laboratoire de Mecanique des Sol ides, Ecole Polytechnique, 91128 Palaiseau, France.

Senior Researcher, Service de Recherches de M�tallurgie Physique, CEA/Saclay, 91 191 Gif /Yvette, France. 'Senior Engineer, Cezus Framatome ANP. Centre de Recherche d'Ugine, 73403 Ugine Cedex, France.

The observations reported in Fig.10could appear contradictory since most of the slip lines corre spond to the activity of prismatic glide and the observed dislocations are in pyramidal planes. The slip lines are the traces left by a great number of dislocations that have glided mainly in prismatic planes. The observed dislocations are those remaining after cooling the material under load. Since these two observations must be consistent, they show that above 20C>°C dislocations glide easily in prismatic planes and with more difficulty in pyramidal ones.


Stefan Cihan Aykut

Jean-Baptiste Comby

Hélène Guillemot

CLIMATE CHANGE CONTROVERSIES IN FRENCH MASS MEDIA: 1990-2010

Keywords: climate change, controversies, public problems, French media, Science and Technology Studies

 and the visibility of controversial points of view. We identify four relevant groups -scientists, politicians, journalists and NGOs -as well as certain social processes that help to explain changes in the attention that controversies received from the media. We conclude with the hypothesis of a third phase characterised by a relatively high degree of attention on controversies in French media.

Introduction

When environmental issues access the mass media, they often give rise (and publicity) to controversies over the accurate definition of the problems, their urgency, and appropriate solutions. Such controversies are indicators of a competition between stakeholders over the framing of an issue. While some environmental issues are highly controversial, others remain or become more consensual. Differences can also be observed in the treatment of the same issue in different national contexts. The aim of this article is to shed light on the treatment of such controversies in the French climate change debate. Specific social groups are, through processes of definition, framing, and agenda-setting, involved in the construction of environmental facts as public problems.

Their success or failure in closing debates results in the construction of issues as either consensual matters-of-fact or controversial matters-of-concern [START_REF] Latour | Why Has Critique Run out of Steam? From Matters of Fact to Matters of Concern[END_REF]. Since the definition of environmental issues is highly dependent on the products of scientific work, the classical focus of policy analysis on the mass media and political actors needs to be enlarged to scientists. We therefore propose to combine media studies and Science and Technology Studies (STS) in a sociology of public problems framework.

In order to investigate how the climate change issue became "publicized", our case study builds on 1) content analysis of three French publications over the period 1987-2007 1 and of television news on the two leading French channels (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) 2 , and 2) 1 Three types of newspapers were analysed: a national newspaper, Le Monde (1987Monde ( -2007), a regional newspaper, Sud Ouest (1994Ouest ( -2007)), and a weekly magazine, L'Express (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007). 2 We analysed 663 reports from TF1 and France2 evening news between 97 and 2006 (Source: database of the National Audiovisual Institute) and found very little evidence for controversies. We therefore do interviews with key actors from different domains who have contributed to the framing of the problem. 3The first section explains our theoretical approach and develops a first hypothesis: the visibility that controversies receive in the media varies between the different phases of the career of a public problem. Such a career is determined by changing social configurations among groups that define the issue, such as scientists, experts, NGOs, state agencies, political groups, lobbyists, journalists, and in some cases parliament or the courts. 4 Using the concepts of career [START_REF] Becker | Outsiders. Studies in the Sociology of Deviance[END_REF] and configuration [START_REF] Elias | Was ist Soziologie?[END_REF] aims to strengthen both long-term and relational perspectives in order to overcome the frequent over-interpretation of the impact of specific causes such as single meteorological, political, cultural or economical events. Symbolic modifications in the framing of a problem, we suggest, depend on social transformations within and between stakeholders of a public problem in a given socio-historical context.

We use this approach in the following sections to analyse the public career of the climate-change issue in France: in the second section, we distinguish two phases in the not present a detailed discussion of controversies in Television news. However, the fact that controversies are more or less visible depending on the media that is analysed is in itself an important finding. construction of climate change as a public problem. In the third, we identify four groups of actors involved in the framing process -scientists, politicians, journalists and NGO activists 5 -and four social processes that affected the configuration of these actors between phases one and two. In section four, we look at the visibility of controversies in French mass media throughout the public problem's career. We show that changes in the attention controversies received can be explained by the social processes affecting the actor configurations.

In the final section, we discuss the hypothesis of a third phase since 2009 in which the very fact that climate change has ascended to the top of political, scientific and media agendas has contributed to making controversies more visible. We believe this increased salience has enlarged the range of stakeholders who wish to take part in defining the problem. This in turn intensified the framing contest. This is the reason why, while it is useful to consider controversies in studying the media construction of environmental issues, media studies can also help to improve the analysis of the dynamics of controversies.

Media Analysis, Public Problems, and Climate Change Controversies -toward a

Theoretical Framework

Media scholars have mainly addressed climate controversies by focusing on "climate sceptics" and their attempts to attack climate research and challenge the legitimacy of climate change as a social problem (McCright and Dunlap, 2000, Antilla, 2005, Lahsen, 5 We consider these four actors as central. Industry, for example, didn't actively engage in the framing contest in France, leaving the initiative to others (Comby, 2009).

2005

). The success of these attacks has been invoked to explain political inaction with regard to the issue [START_REF] Mccright | Defeating Kyoto: the conservative movement's impact on US climate policy[END_REF][START_REF] Mooney | The Republican War on Science[END_REF][START_REF] Oreskes | Merchands of doubt: how a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming[END_REF]. While such analyses offer valuable insights into the strategies and networks of dissenting voices, they are less suitable to explain why certain issues become controversial at a specific point in time and space. A larger framework that tries to answer this question has to include structural reasons for the success of actors in accessing the media and imposing their framing of an issue. Three such frameworks have been applied to the climate change debate. [START_REF] Downs | Up and down with ecology. The 'issue-attention cycle[END_REF] postulates the existence of an "issue attention cycle" for public problems and sees controversy as a natural stage in the historical deployment of an issue in the public sphere: the public's realization of the costs and difficulties involved in the resolution of a problem inevitably blurs the formerly clear message of alarm, and the ensuing controversies lead to a decline of public interest. It has been argued that the American climate change debate follows such a pattern [START_REF] Ungar | The rise and (relative) decline of global warming as a social problem[END_REF][START_REF] Trumbo | Constructing climate change: claims and frames in US news coverage of an environmental issue[END_REF].

A key problem with this approach is its inability to account for cultural differences [START_REF] Grundmann | Climate Change and Knowledge Politics[END_REF]. Another account claims that the journalistic norm of "balanced reporting" leads to a "bias" in the media treatment of climate change, as equal media space is allocated to mainstream science and to the tiny minority of scientists challenging climate change [START_REF] Boykoff | Balance as bias: global warming and the US prestige press[END_REF]. However, the explanatory variable of "journalistic norms" is clearly media-centred. In order to understand the full dynamics of how social problems gain access to and stay on the media agenda, we have to take into account journalists' sources and their strategies [START_REF] Schlesinger | Rethinking the Sociology of Journalism: Source Strategies and the Limits of Media-Centrism[END_REF]. Following a third line of thought, controversies result from a dynamic of overdramatization and backlash through which the newsworthiness of the issue is maintained. In a first phase, actors from the journalistic and political fields gain public attention through the "over-dramatization" of the issue, giving the most alarming scientific results high visibility. In a second phase, as other issues inevitably become more salient again, actors engaged in the debate realize the "risks of communication" they face [START_REF] Weingart | Risks of communication: discources on climate change in science, politics and the mass media[END_REF]: scientists' credibility is on the line in public controversies, politicians fear voters' reactions against climate policies and journalists are concerned about the declining interest of their audiences. Such perceived risks produce a more discreet attitude especially among scientists. Some journalists, on the other hand, switch to another dramatic register to maintain public attention ("global warming is a fairy tale"), which allows "sceptics" easier access to the media.

Two comparative studies show that the early French climate discourse did not conform to any of these logics. The first analysis compares coverage of climate change from 1987 to 1997 in the French newspaper Le Monde and in the New York Times [START_REF] Brossard | Are Issue-Cycles Culturally Constructed? A Comparison of French and American Coverage of Global Climate Change[END_REF]. The authors argue that differences in media coverage are related to differences in culture, i.e. and opposition of American-type "investigative journalism" and French "opinion journalism", which favours story-telling and political events (see also [START_REF] Kuhn | The media in France[END_REF]. As a result -and contrary to what an issue-attention account would suggest -climate change was treated in France from the beginning as a partisan issue. Marmont and Dasnoy (1995) confirm this finding of a more controversial and political debate in France in their case study comparing France, Germany, and Belgium. Their explanation does not rely solely on journalistic norms, but also includes sources and their strategies. The authors show how the different structures of the journalistic and scientific fields and the unequal development of environmental pressure groups led to different patterns of media representation of climate change.

Building on these findings, we seek to draw attention to the evolutions in the French media coverage of climate change that become visible when covering a larger time-span than those studies. In order to do so, we propose a framework that takes into account dynamics in the journalistic field and among sources. Before we present this framework, we will use insights from the sociology of public problems and STS6 to clarify two notions that are central to our study: the construction of public problems and controversies.

In his path-breaking account of the construction of drinking-driving as a public problem in the US, [START_REF] Gusfield | The Culture of Public Problems. Drinking-driving and the Symbolic Order[END_REF] shows how groups of actors -so-called "owners" of the problem -compete for the framing of the issue, and ties the emergence of hegemonic framings to specific "ownership configurations". In a similar vein, [START_REF] Hilgartner | The rise and fall of social problems: a public arenas model[END_REF] direct our attention to the competition between issues for the scarce resource of public attention. They suggest that issue arenas (the press, parliament, ministries, etc.) have natural "carrying capacities" that limit the number of issues under active consideration. In order to attract and maintain attention on an issue, promoters have to dramatize it and appeal to culturally entrenched codes and myths that make it intuitively intelligible. Seen through the lens of the sociology of public problems, [START_REF] Mormont | Source strategies and the mediatization of climate change[END_REF] look at the ownership configuration of an issue, and at the success or failure of owners in putting the issue on the media agenda as well as their ability to impose their framing. Evolutions in such ownership configurations, we suggest, constitute the "career" [START_REF] Becker | Outsiders. Studies in the Sociology of Deviance[END_REF] of a public problem.

Insights from research in STS help to deal with the complex issue of climate controversies. The term "controversy," given to debates or polemics on climate issues, is in itself deeply problematic. For climatologists, there is no controversy on climate change, because their opponents' arguments are not validated through the mechanisms of the scientific community; inversely, opponents claim that the absence of scientific controversy reflects the domination of an institution. As STS scholars have shown in a body of literature on "boundary work," the debates on borders and on the legitimacy of actors and their findings are often part of the controversy [START_REF] Gieryn | Boundary-Work and the Demarcation of Science from Non-Science: Strains and Interests in Professional Ideologies of Scientists[END_REF][START_REF] Jasanoff | Contested Boundaries in policy-relevant science[END_REF]. Furthermore, climate change covers a large range of scientific domains and is a high-stake political and economic issue. We decided not to restrict our analysis to controversies confined to the boundaries of disciplinary science, but instead to take into account much more varied, complex disputes. Such "sociotechnical controversies" "overflow" into several arenas, combine different interests, involve different regimes of justification, and mix scientific, political, and ethical arguments [START_REF] Rip | Controversies as informal Technology assessment[END_REF][START_REF] Pestre | Regimes of Knowledge Production in Society: Towards a More Political and Social Reading[END_REF]. We therefore concentrate on all kinds of "dissonant voices" that initiate and participate in disputes, debates, polemics, and controversies about the urgency, impacts, and social significance of climate change, the appropriateness of the methods of climate sciences, the efficacy of climate policies, as well as the merits of the precautionary principle and of what some perceive as a new "green" ideology. This allows us to draw a more fine-grained picture of the trajectory of the public problem, and of the evolution of resistance and critique it has encountered.

The Construction of Climate Change as a Public Problem in France

Although climate change was on the political agenda as early as in the late 1980s in France, its evolution into a full-fledged public problem only occurred in the early 2000s.

We distinguish two periods in the construction of this public problem: a first from 1988 to 2001/2 and a second from 2002/3 to 2008.

Climate change became a political issue in France through international summits like the Toronto Conference on the Changing Atmosphere ( 1988), the Hague Conference on the Environment (1989) and the economic summit of the Group of Seven (1989) in Paris. 7 At about the same time, the success of green parties at municipal and European elections increased pressure on the political system.8 As a consequence, the new government of Prime Minister Michel Rocard ( 1988) appointed an inter-ministerial group to elaborate a French policy response (GIES, later MIES). 9 A conference on climate change was held in parliament, and the Ministry of Research organized a summit "Planet Earth" attended by President Mitterrand. However, no coherent domestic climate policy or reduction target was adopted and the political system's attention to the issue dropped sharply after the Rio conference in 1992. What is more, the attention of major newspapers stayed very modest throughout the nineties. An analysis of three major French newspapers shows that major peaks in news attention resulted from international events like the 1992 Rio and 1997 Kyoto conferences (Figure 1). Indeed, climate change was not considered a major domestic problem, and climate policy was reduced mainly to its international component [START_REF] Brossard | Are Issue-Cycles Culturally Constructed? A Comparison of French and American Coverage of Global Climate Change[END_REF]. The organization of expertise also contributed to the absence of a public debate on climate change: the reports of the GIES were confidential and their public impact limited, and a commissioned study by the National Academy of Science (1990) was too technical to gain widespread attention. We can thus conclude that despite intense activity in the political system at the very beginning of the period, climate change did not reach the top of the political or the media agenda before the 2000s (Aykut, 2011).

[FIGURE 1]

In the second phase, newspapers covered climate change increasingly through domestic meteorological phenomena (winter storms in 1999, heat wave in 2003) and political events (National climate plans in 2000 and 2004). Peaks of media attention still coincided with international events like the Hague conference in 2000 and the release of the Third (2001) and Fourth (2007) IPCC Assessment Reports. However, coverage of the issue remained at a high level -and even continued to rise -between these events. 10Climate Change became a "structuring discourse" in the mass media11 and began to be "institutionalized" in political programmes and organizations. 12 The creation of the National Observatory on the effects of climatic warming in France (ONERC) and the commissioning of a report on the effects of climate change on French geography by the two chambers of parliament (both 2001) illustrate this process of institutionalization. In 2004, an Environmental Charter was added as an amendment to the French Constitution. Furthermore, the dominant framing of the climate issue changed. While in the first phase the issue had been framed as an issue of international relations, its construction as a public problem in the beginning of the 2000s was based on an "interpretative package" [START_REF] Gamson | Media discourse and public opinion on nuclear power: a constructionist approach[END_REF]) that consisted in two main assertions: human activities induce a supplementary greenhouse effect that leads to global warming, and everyone has to fight it primarily at an individual level, through lifestyle changes and energy savings (Comby, 2009).

Changes in the Ownership Configuration: Four Social Transformations

Two extreme weather events are often invoked to explain the increased media attention to the issue: a major storm in the winter of 1999 and a heat wave in 2003. Both cases were widely interpreted by the media as signs of global warming, even though scientists refused to publicly establish such clear links. We argue that these events certainly played a role, but that the increasing newsworthiness [START_REF] Schlesinger | Putting reality together[END_REF] of climate change relied more on social shifts. In fact, the configuration of the owners of the public problem, their access to the media and to the political definition of the issue changed at the beginning of the 2000s, allowing for a different treatment of climate change in the media. We identify transformations in four fields -journalistic, scientific, state and NGO -that explain these changes.

By 2000, journalists dealing with environmental issues had gained a new status in newsrooms. Mormont and Dasnoy (1995: 54) show that the field of environmental journalism was poorly established in France in their period of analysis, leading to a more "political" framing. Comby (2009) argues that this changed as it became a fullfledged specialty by the beginning of the new millennium. In the process of institutionalization of environmental journalism, new journalists entered the field.

Whereas journalists who traditionally dealt with environmental issues tended to interpret them in ways that were close to activists' points of view and promoted political frames, the new generation of journalists was driven by professional more than political motivations and produced information more closely adjusted to the commercial rules of the journalistic field. In order to increase the newsworthiness of the issue, "anecdotal" frames came to be favoured over "thematic" ones [START_REF] Iyengar | Is anyone responsible? How Television Frames Political Issues[END_REF], and especially television news and popular media focused on "awareness raising" instead of providing more structural explanations. This means that the consequences of climate change were presented in terms of concrete effects on people's life-worlds, and options for mitigation were reduced to individual lifestyle changes. By doing so, environmental journalists contributed to shaping the issue as a matter of individual morality. Contrary to the "political" framing in the beginning of the nineties, this new framing leaves less room for controversy.

Social and institutional changes can also be observed in the scientific field. Until the early 1990s, the French climate research was weakly organized, climate change was not a major research issue in France and most climatologists were not mobilized on that issue, which blurred the scientific message. This left the floor to popularisers and more political frames (Mormont and Dasnoy, 1995: 54,55). Guillemot (2007, chap. 10,11) shows in detail how the French climate research community became more organized in the late 1990s. A key step in this process was the creation of the Institut Pierre Simon Laplace (IPSL) that facilitated the creation of a more comprehensive French climate model by coupling an atmospheric and an ocean model13 . Scientists formerly working on isolated topics began to work together, and different laboratory "cultures" were integrated in the modelling effort [START_REF] Edwards | Representing the Global Atmosphere: Computer Models, Data, and Knowledge about Climate Change[END_REF]. The completion of the model in Scientists play a central role in legitimising environmental journalists in their newsrooms. Speaking about difficulties convincing her boss to run news stories about climate change at the end of the 1990s, a TV journalist says in one of our interviews in 2005: "we were struggling and I'm happy to hear that there now is a scientific consensus (…) I was always looking for the scientist who was willing to say 'yes, it is due to humans that…'". Another example of scientific mobilization in defence of the newsworthiness of climate change is a "workshop" for editors-in-chief organized annually by Jean-Marc Jancovici, an expert on energy savings (Comby, 2009).

The State also played a central role in the public promotion of the problem. The activity of GIES, directed by a high-ranking public official (Yves Martin), is characteristic of the state's role in the first phase. This committee, which worked in a technical and confidential manner 14 (Roqueplo, 1993: 204, 205), regularly promoted the idea of a European carbon tax 15 to reduce CO2 emissions (Aykut, 2011). The tax initiative finally failed and the policy window closed in 1997 with the adoption of the Kyoto protocol. In 2000/2001 the political salience of climate change increased suddenly for two reasons: first, U.S. President George W. Bush's decision not to ratify the Kyoto protocol polarized the issue and contributed to keeping it in media headlines for years. Second, France proposed its first comprehensive climate action programme. This new salience, combined with the unwillingness of the French government to push for a carbon tax after the failure of the European initiative, paved the way for a second actor, the National agency for energy efficiency and savings (ADEME), to frame the topic in accordance with its institutional objectives. 16 In 2004, it launched a communication campaign to promote individual energy savings, backing it with the claim that individuals are responsible for 50% of French greenhouse gas emissions.17 NGOs, journalists, the educational system, and some scientists, took part in this "symbolic 14 Reports on the topic were published and written in terms not easily accessible for the lay public. 15 By 1990 nuclear power generation had come to represent over 70% of total electricity production in France, making the country one of the least vulnerable to a carbon tax in the European Community.

16 ADEME is the result of a merger (1992) of three formerly independent agencies including the French agency for energy efficiency created after the oil crisis in the 1970s. After a decade of low oil prices and nuclear over-capacity, climate change gave renewed impetus to the topic of energy savings.

crusade" [START_REF] Gusfield | The Culture of Public Problems. Drinking-driving and the Symbolic Order[END_REF] that consisted in promoting lifestyle-changes, understood as "greener" daily practices.

Finally, NGOs were engaged in multiple activities aimed at increasing the salience of the climate issue. Their activities included lobbying at different levels of government, the organization of conferences and demonstrations, cooperation with schools and cultural institutions, and the supply of expertise [START_REF] Ollitrault | Militer pour la planète: sociologie des écologistes[END_REF]. Up until the end of the 1990s French NGOs were poorly organized with regard to climate change and did not play a decisive role in promoting the issue. They gained major importance in the discussions before the Kyoto conference, as key environmental NGOs formed the French branch of the Climate Action Network in 1996. The subject was also taken up and popularized by a handful of charismatic leaders, like for example Nicolas Hulot.

Host of a very popular television show (Ushuaïa on TF1), he began his career as an environmental activist in the 1990s, when he founded the "Ushuaïa Foundation" (1990).

His political influence grew in the 2000s: he is said to have spawned the idea of the Environmental Charter (2004), and he convinced five candidates to the 2007 French Presidential Elections (including current President Sarkozy) to sign an "ecological pact". Hulot and Yann Arthus-Bertrand, a photographer influential in the French climate debate, also promoted the climate cause through films ("The Titanic Syndrome" and "Home"). Another feature of the activities of NGOs was their cooperation with state agencies. As an example, Hulot's "Earth challenge "18 , supported by ADEME, brought together dozens of celebrities and around 800,000 individuals on the Internet. As a result, NGO activities contributed to a "consensus mobilization" [START_REF] Klandermans | Potentials, Networks, Motivations, and Barriers: Steps Towards Participation in Social Movements[END_REF] around the climate issue in the 2000s.

The conjunction of these social changes explains why climate change rose to the top of the hierarchy of public problems in France in the early 2000s. In the following section, we will show that the transition between different stages of the public problem's career also entailed changes in the visibility of different types of controversies. This should not come as a surprise, as social configurations determine both the place of a public problem in the newsroom and the means that different actors utilise in order to gain media visibility for their positions. We look at the general presentation of climate sciences in the media (consensual or uncertain), as well as the visibility and variety of controversies.

Controversies in the Career of the Public Problem

A qualitative analysis of the headings of the newspaper Le Monde between 1995 and 2006 provides a first insight into the differences between the two periods (Figure 2). In the first phase, internal scientific controversies were translated in the mass media into a language of uncertainty. In accordance with the cautious stance of climatologists, articles stressed the complexity of climatology and of the "climate machine" and warned about drawing hasty conclusions. Despite persisting uncertainties, they nevertheless called for political action. From around 2003, these themes were no longer dominant and article headlines began to focus more on the drastic consequences of climate change. Responses to "climate-sceptics" became commonplace and the tone of the debate more impassioned. Our interviews show that this evolution of journalists' treatment of the issue coincided with a changing attitude of French climatologists. A prominent climatologist for example says about this period that it had become more important to be persuasive about certainties and points of consensus than to stress limitations and uncertainties.

[FIGURE 2]

In order to get a clear picture of the evolution of controversies, we first categorize them according to the disciplines of contesting actors. Our typology distinguishes five types of actors, depending on their distance from the "core science" of climatology (or Working Group 1 of the IPCC). The first group includes climate scientists; actors from the second group are scientists from disciplines involved in impact research (ecology, geography…); the third group consists of people within disciplines concerned with mitigation (mainly economists); actors from the fourth group are scientists and engineers from other areas (mathematics, physics, geophysics…); and actors belonging to the fifth group are intellectuals or social scientists not professionally concerned with climate change (philosophers, writers, journalists...).

In the first phase, different kinds of dissonant voices expressed themselves in the press, and even though they never dominated the media coverage of the issue, they were by and large tolerated. Throughout the two periods, they mainly consist of public figures from the 4 th group who share a specific profile: Haroun Tazieff in the first period, as well as Claude Allègre and Vincent Courtillot in the second, are at once highly recognized as scientists in domains not related to climate science -all three come from the geosciences -and former ministers or high public officials. They thus combine scientific, political, and media capital, have influential networks in the National Science Academy, and are well known to both scientific and political journalists. Yves Lenoir, an engineer and activist against the French nuclear program, is another important actor in the first phase (also group 4). He argued against the urgency of the climate issue in the press, and published a book promising to unveil "the truth about the greenhouse effect" [START_REF] Lenoir | La vérité sur l'effet de serre[END_REF]. Publishing books is a general strategy used by French contesting actors to overcome the problem of scientific legitimacy they face, as they generally lack publications in academic journals.

It is important to understand the intellectual environment in which the debate on climate change took place. In France in the 1990s, the growing political focus on ecology provoked a strong resistance from a part of the political, intellectual and scientific elite against what they perceived as a new "ideology" or "new ecological order" [START_REF] Ferry | Le nouvel ordre écologique[END_REF]. This counter-movement, formed around resistance to the Rio conference and the introduction of the precautionary principle in France and Europe, culminated in 1992 in the Heidelberg appeal, 19 which was initiated by the French physician Michel Salomon and signed by hundreds of high-profile scientists (including 72 Nobel Prize winners).

This had a significant impact on the French press. Contrary to the US context (McCright andDunlap, 2003, Oreskes and[START_REF] Oreskes | Merchands of doubt: how a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming[END_REF], contesting actors in France have usually had little contact with industry and political cleavages have not been played out in the climate field. Contesting actors come instead from the scientific, administrative, and political establishment (often the French Socialist Party) and defend a certain vision of science, as well as of the relationship between science and the state. In the Heidelberg appeal, they presented the values of state-led progress as being endangered by "irrational environmentalism", to which they opposed "scientific environmentalism." While the appeal did not explicitly aim at climate change, it spoke against nascent 19 http://www.his.com/~sepp/policy%20declarations/heidelberg_appeal.html preoccupations with the global environment and the political initiatives launched to respond to the ecological crisis. The preparatory group, called "Reflection committee for a democratic environmentalism", was made up of scientists from different disciplines and "intellectuals against the green lobby" (Le Monde, 19.06.1992). Some of them, such as Haroun Tazieff, Luc Ferry, Michel Onfray, and Pascal Bruckner, later participated in attacks against climate science or policy, or in controversies about its consequences (Hervé Le Bras).

Although "contesting actors" stayed active, the overall visibility of controversies changed in the second phase. Scientists' presence in the press increased, and less public attention was given to dissonant voices. Actors not sharing the consensus about the reality and urgency of anthropogenic climate change have since then been labelled "sceptics"20 and owners of the problem (essentially environmental journalists and scientists) restrict their access to public discourse. This became explicit in an episode in 2006, as journalists, together with climate scientists, condemned (in a letter to the Science Academy, in articles and interviews, etc.) a sceptical article by Claude Allègre published in L'Express [START_REF] Allègre | Le droit au doute scientifique[END_REF].

A content-analysis of Le Monde articles (Figure 3) provides a more fine-grained picture of the evolution of controversies in the transition between phases one and two. The analysis shows a shift in the topic of controversies in this newspaper around the turn of the millennium: while the first phase was characterized by sporadic but persistent attacks on the hypothesis of human-induced climate change and the "new green ideology" expressed in international climate negotiations, such voices received little media attention in the second phase. Interestingly, the fact that climate change was already considered a reality and an issue for public policy opened up some space for controversies of a new kind on the margins of the dominant frame.

[FIGURE 3]

A qualitative analysis shows that controversies on climate change impacts tended to be "soft", meaning they were presented as "normal" scientific controversies without the polemic rhetoric prominent in the first phase. This was also visible in TV news, as some reports mentioned scientific uncertainties in relation to the consequences of climate change while more generalizing statements about the uncertainties of climate science and modelling disappeared. Likewise, economic controversies obtained public visibility without questioning key elements of the dominant frame such as the reality and urgency of climate change. Another new development in the second phase was the explicitly "scientific" challenge by French Academy of Science member Vincent Courtillot.

Concerned with the role of the sun and of geomagnetic fields, he positioned himself as a member of the scientific community engaged in an academic dispute. This positioning, as well as leading climate scientists' refusal to endorse it, can be understood as a form of "boundary work". The fact that legitimacy in this particular scientific field is perceived as an important asset in public discourse, illustrates climate scientists' central position in the ownership configuration.

However, not everything changed between the two phases, as two general observations about specificities of the French climate discourse demonstrate: First, international controversies did not play any role in the period of observation, even though they were very salient in the US discourse in the 1990s (McCright andDunlap, 2000, Boykoff and[START_REF] Boykoff | Balance as bias: global warming and the US prestige press[END_REF], and had an impact elsewhere, for instance in Germany [START_REF] Weingart | Risks of communication: discources on climate change in science, politics and the mass media[END_REF]. The second general observation is that group one actors (climatologists) did not initiate public controversies. Instead, they presented a unified front against dissonant voices.

A more Controversial Third Phase?

By 2009 a third phase may have begun in which controversies of all kinds obtain greater public visibility. Beginning in autumn 2009, two international controversies received substantial media coverage: the so-called "Climategate" resulting from the publication of leaked emails from the University of East Anglia, and the revelation of errors in the fourth IPCC assessment report concerning the melting of Himalayan glaciers. Domestic controversies were also given significant play in the media. These included debates after the publication of a book called The climate imposture by Claude Allègre, and the mobilization of some geographers, culminating in a conference with the provocative title "No, heaven won't fall on our heads" (French Association of Geographers, 16.09.2010). Our theoretical framework allows us to explain these recent developments in social terms and through a historically informed account of the French climate debate. To do so, we will first look at both changes and continuities between this hypothetical third phase and the preceding ones.

A noteworthy change in French climate discourse since 2009 is that new contesting actors and new themes get access to the media. This concerns most notably scientists who contest claims about the consequences of climate change (group 2) and intellectuals (group 5). Furthermore, dissonant voices from group four (such as Courtillot) have more frequent access to a wider range of media spaces -newspapers, magazines, and talk shows. Accordingly, controversies have diversified, ranging from climate change impacts to the science-policy relationship, expertise processes, climate sciences, computer modelling, environmental ideology, and appropriate measures.

Discourses often mix scientific, political, economic, and epistemic arguments. This allows some of these actors, albeit speaking as individuals, to condense several types of disputes or criticisms, thus gaining a wider audience and support. However, from a historical point of view, neither the subjects nor the actors of phase-three controversies are new. Actors from groups two and five expressed their critiques mezzo voce before phase three, in confidential arenas such as web sites and scientific magazines. The new configuration now allows them to be heard by a wider audience.

How can the increased visibility of dissonant voices be explained? Observers frequently invoke the "Climategate" controversy and the failure of the climate conference in Copenhagen in December 2009. We think, however, that changes in the underlying social conditions play a central role. Our hypothesis is that the very fact that the public problem ascended to the top of media-and political agendas contributed to its more controversial framing. Climate change reconfigures many domains -ranging from scientific disciplines to the organization of newsrooms and priorities in public policyand new actors come to be directly concerned with the issue. Additionally, as a highprofile issue, climate change yields media, scientific and political capital. Events like Copenhagen and "Climategate" have merely accelerated an on-going process, as new actors with other priorities, working methods and world-views have emerged and challenge the existing ownership structure. In what follows, we will show the importance of such social evolutions in the journalistic and the scientific fields.

Scientific and environmental journalists are no longer the only ones who shape the way climate change is treated by the media. The climate issue has increasingly become a topic of interest for journalists specialized in political, social, and economic domains.

While this extension of concerned journalists enlarged the carrying capacity of the journalistic field and explains the constancy of high levels of media attention in the second part of the 2000s (Figure 1), the new journalists also have other priorities and are more open to dissonant voices. They worry less about the scientific legitimacy of alternative sources than about their political and social influence, and give more value to the originality of a story.

On the scientists' side, a similar broadening of visible actors can be observed. As the tools and methods of climate sciences become important issues in the debate, climatologists face critics from scientists belonging to other disciplines concerned by climate change (group 2). Climate models impinge on territories where other disciplines claim legitimacy, leading to competition between different epistemic cultures.

Geographers, for example, generally rely on bottom-up approaches to analyse regional climates, whereas climate modellers tend to "downscale" from global circulation models. In the French context, this led to controversies over of how to appropriately characterize the consequences of climate change. 21It should be noted that even in phase three, controversies reach mainly intellectual parts of the media field such as newspapers and debate shows while they remain relatively absent from media platforms with larger audiences. Indeed, controversies have had The main results of our analysis are summarized in the following table, including the hypothesis of a more controversial phase three (Figure 4).

[FIGURE 4]

Conclusion

In this article, we set out to analyse controversies in climate change coverage through a theoretical framework combining the sociology of public problems, media sociology, and science and technology studies. This led us to view them as the expression of struggles over the ownership and the framing of climate change as a public problem.

A first result is that media coverage of climate change in France did not undergo the classical issue-attention cycle. Instead, media attention steadily increased and recently reached unprecedented heights. This can be explained by the extension of the carrying capacity for the issue in the journalistic field, as climate change became a subject for new journalistic specialties.

Secondly, we identify two phases in the public career of climate change. In a nutshell, the first phase is characterized by a poorly established field of environmental journalism, a scattered/dispersed/ (disperse, éparse) scientific field with scientists reluctant to work with the IPCC and to talk to the media, and a political system defining climate change as a problem of international relations, with little salience in domestic politics. Around 2000-2003, the scientific field became more unified and the IPCC was recognized as a major, uniting actor, leading scientists to become more deeply involved in public discourse. At about the same time, the institutionalization of environmental journalism in most national media and of climate policy in political and administrative arenas united actors from these fields and altered their ways of engaging with the issue.

With this change in the ownership of the public problem, the conditions of access of critical voices were modified and scientific controversies and alternative frames were marginalized.

Finally, we suggest that we may have recently entered a third phase. The issue has become a high-profile public problem with exceptional media attention. However, the increase in the carrying capacity of public arenas comes at a price: as new actors enter the field, the dominant ownership configuration is challenged and controversies (re-) emerge.

Further research should broaden both the range of potential owners, including economic actors in particular, and the range of media platforms considered. Indeed, controversies are unequally visible in different public spaces, indicating different ownership configurations. Variations between traditional media and the Internet, debate shows and television news, or opinion columns and news articles offer rich empirical material for a topology of the social space in which controversies develop and gain visibility. Our corpus inclues 1531 Le Monde articles dealing with climate change (kw: "changement(s) climatique(s)", "réchauffement climatique"/"réchauffement de la planète", "effet de serre"). From this sample, we extracted: 1) articles reporting about a socio-technical controversy 2) articles where an actor takes a controversial position on a subject and responses to that article. The graph shows controversial articles in relation to the total number of articles on CC. The years 1990The years -1992The years , 1993The years -1996The years and 1997The years -1999 were taken together for purposes of statistical significance. The vertical axis shows percentages. Four categories of controversies have been distinguished: critics of political ecology and exaggerated reporting on climate risks (a framing in which mostly group 5 actors are involved in France), polemics on measures and economics (group 3), controversies on climate change impacts and consequences (group 2), challenges of the existence of climate change (mostly group 4).

The first result is that throughout the period of observation, controversies represented a small percentage of the whole sample of articles (max. 10%). Controversies about the existence of anthropogenic climate change and critics of environmentalism were relatively common at the beginning of the 1990s and diminish sharply moving into the 2000s. On the contrary, controversies over impacts increased steadily throughout the 1990s, reaching relatively stable visibility in the years 2000. Controversies over economic measures were present throughout the observed periods, reflecting political discussions about eco-taxes (1992,2000), tradable permits (1997) and national policies (2000,2003). Their cyclical pattern suggests some independence with regard to the rest of the climate change debate.

Figure 4 -Summary table on the three phases

  1997 and the growing institutionalization of climate science in France changed the initial attitude of climatologists towards both the IPCC assessment exercise and the mass media. No French modelling results were included in the IPCC simulations for the first two assessment reports (1990/1995) because no coupled model was available at that time; even in the Third Report (2001), when French coupled models were completed, only four simulations were included, as scientists were reluctant to spend considerable amounts of time on activities they did not consider central to their research. The situation changed radically with the Fourth Report (2007): two different models provided simulations for all IPCC scenarios. Communication to the press also became progressively more professionalized (Tristani-Potteaux, 1997), and some climatologists began to invest time in communicating with or educating journalists.

  unequal access to different media spaces throughout the three phases. Thus, in 663 reports on climate change in the two main French channels' evening news between 1997 and 2006, none of 818 interviewed people questioned the scientific consensus, and between July 2009 and July 2010, only two of 117 news reports dedicated to climate change mentioned controversies. Furthermore, framings that promote the climate issue as crucial still prevail and dissonant voices continue to provoke strong collective reactions from the owners of the problem: as a response to Claude Allègre's book, over 600 climate scientists signed a petition asking the Ministry of Research to publicly express its confidence in their work.

Figure 2 -

 2 Figure 2 -Headlines of articles in Le Monde (translated samples).

Figure 3 -

 3 Figure 3 -Controversies in Le Monde and transition between phase 1 and 2

  ).Concerning sea-level rise, a reliable method for calculation is yet to be found(08.11.2001) Europe is experiencing its hottest autumn in five centuries(08.120.2006)

	Phase 1	Phase 2
	Difficulties correspond to scientific uncertainties	Multiplication of extreme events worries climatologists
	(13.03.1995)	(13.09.2003)
	Climatic uncertainties for future cultures	"In 2100, temperatures will have increased by 1.4 to 6°C"
	(04.04.1995)	(20.06.2004)
	Scenarios about expected climate change still vague	The Earth, sick of mankind
	(23.09.1995)	(20.06.2004)
	The computer is an expert, but not a prophet	On Realclimate.org, specialists track scientific fraud and
	(18.02.1997)	errors (02.01.2004)
	"It's too early to draw conclusions"	Warming of more than 6°C is not excluded
	(28.05.1997)	30.01.2005
	The terrestrial "climate machine" challenges	Sea level rise could be higher and quicker than predicted
	science (05.01.2000)	(25.03.2006)
	Uncertainty undermines predictions	Cyclones: more frequent or more violent?
	(03.10.2001)	(06.09.2006)

Our respective PhD theses include a total of 112 interviews of journalists, climate scientists, activists, and state officials. Most media analyses of climate change are based on content analyses. Combining both content analysis and interviews aims at investigating the social conditions of news productions.

This hypothesis was developed at an international conference on climate change controversies in mass media organized by the authors in September 20 and 21, 2010 in Paris, France. We thank the "Groupement d'intérêt scientifique Climat Environnement Société" and the "Institut des sciences de la communication du CNRS" for their financial and logistical support.

A previous analysis of the French controversy over GMOs has demonstrated the fruitfulness of combining these two strands of literature[START_REF] Bonneuil | Disentrenching experiment. The construction of GM-crop field trials as a social problem in France[END_REF].

A notable difference between climate change and other environmental problems is that the alarm was sounded in international scientific circles. The problem is therefore characterized not by a "rise in generality," but by a phenomenon of "downscaling" to national contexts.

The French Greens obtained

9% and 10.6% in two elections in 1989. 9 Founded in 1989, the "Inter-ministerial group on the greenhouse effect" was transformed into a permanent "mission" in 1990.

On the evening news of the two main TV channels between 1997 and 2006, 60% of the reports dealing with climate change are concentrated in the period after 2003.

After 2000, newspaper coverage of global warming averaged one or more articles per day.

Marteen Hajer (1993: 45, 46) draws this distinction between discourse structuration and institutionalization.

Supplementary modules were added later.

This is a statistical trick, as the 50% are mainly due to emissions from transport and construction. It is misleading to identify them as "individual" emissions.

http://www.defipourlaterre.org/

The denomination "sceptic" was imported from the American debate, but other terms like "denialism" or "contrarian scientists" never imposed themselves in the French discourse.

Again, this is not new: geographer-climatologist Marcel Leroux became, through the publication of several articles a prominent figure amongst sceptical publics in the first phase. However, he barely received any attention in mass media.

Figure 1 -Climate Change Coverage in the French press

The vertical axis shows relative frequency of articles dealing with climate change (number of articles divided by editions). Major events covered in the press are indicated above the lines. Keyword search on Europress database : changement(s) climatique(s), effet de serre, réchauffement global, réchauffement de la planète. For more details on the findings of the news analysis, see Aykut (2011).