Aymen Amine Assadi
email: aymen.assadi@ensc-rennes.fr

Abdelkrim Bouzaza

Isabelle Soutrel

Philippe Petit

Karim Medimagh

Dominique Wolbert

Amine

Bouzaza Abdelkrim

Petit Philippe

Medimagh Karim

Wolbert Dominique

A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: From pilot to industrial scale

Keywords: Photocatalysis, Plasma treatment, combined process, pilot scale, industrial application

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

Introduction

Many studies have shown that Volatile Organic Compounds (VOC) emissions are not negligible and their impacts on the environment and human health are serious and threatening [START_REF] Dominique | Introduction au traitement de l'air[END_REF][START_REF]International Programme on Chemical Safety (IPCS), OECD screening information databases 3-methyl butanal[END_REF]. Therefore, there is currently an important deal of interest in developing processes to treat air pollutants, such as photocatalytic (UV/TiO2) oxidation in the gas phase. UV/TiO2 process is an attractive technique, as heterogeneous photocatalysis for environmental cleanup. It operates at ambient temperature and the by-products are usually CO2, H2O and nonharmful mineral acids [START_REF] Assadi | Photocatalytic oxidation of Trimethylamine and Isovaleraldehyde in an annular reactor: Influence of the Mass Transfer and the relative humidity[END_REF][START_REF] Pichat | Some views about indoor air photocatalytic treatment using TiO2: Conceptualization of humidity effects, active oxygen species, problem of C1-C3 carbonyl pollutants[END_REF][START_REF] Ochiai | Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification[END_REF]. Also, non-thermal plasma (NTP) is one of the most frequently used oxidation processes to control air pollution [START_REF] Ochiai | Environmental and Medical Applications of TiO2 Photocatalysts and Borondoped Diamond Electrodes[END_REF]. It consists of generating an electrical discharge that treats gas present between electrodes separated by a dielectric material.

However, application of non-thermal plasma leads to low the mineralization [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF].

Photocatalysis, due to its adsorption step, tends to keep the by-products in the adsorbed state in order to reach their complete oxidation, achieving, thus, a higher mineralization value [START_REF] Assadi | Photocatalytic oxidation of Trimethylamine and Isovaleraldehyde in an annular reactor: Influence of the Mass Transfer and the relative humidity[END_REF][START_REF] Pichat | Some views about indoor air photocatalytic treatment using TiO2: Conceptualization of humidity effects, active oxygen species, problem of C1-C3 carbonyl pollutants[END_REF][START_REF] Ochiai | Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification[END_REF]. Therefore, it is interesting to try enhancing NTP's activity by adding TiO2 with external UV [START_REF] Taranto | Combining Cold Plasma and TiO2 Photocatalysis To Purify Gaseous Effluents: A Preliminary Study Using Methanol-Contaminated Air[END_REF]. Moreover, many researchers found that combining plasma and photocatalysis improved the global performance of the process [START_REF] Ochiai | An effective method for a separation of smoking area by using novel photocatalysis-plasma synergistic air-cleaner[END_REF][START_REF] Maciuca | Synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] Kletta | Acetaldehyde removal using an atmospheric non-thermal plasmacombined with a packed bed: Role of the adsorption process[END_REF]. Indeed, a synergetic effect can be expected between these technologies [START_REF] Ochiai | An effective method for a separation of smoking area by using novel photocatalysis-plasma synergistic air-cleaner[END_REF][START_REF] Maciuca | Synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] Kletta | Acetaldehyde removal using an atmospheric non-thermal plasmacombined with a packed bed: Role of the adsorption process[END_REF]. Although many studies were carried out on the efficiency of associated process at lab scale, the extrapolation of this process at industrial scale has not been extensively explored. Here, our methodology was to extend previous research [START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF][START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination -synergetic effect and byproducts identification[END_REF] by proposing a new investigation about plasma, photocatalysis and their association in-situ at pilot and industrial scales. Firstly, a special attention is given to the comparison of the obtained results at pilot scale with those obtained at lab-scale, which is innovative in comparison to latter studies [START_REF] Guillerm | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF]. The second innovation of this research is about the investigation of the real effluent treatment with an industrial reactor.

At pilot scale study, isovaleraldehyde was chosen as a test molecule because it is significantly present in exhaust gases from animal quartering [START_REF]Pollutions olfactives : origine, législation, analyse, traitement[END_REF]. Furthermore, it is listed as one of the surrogates for the six major classes of indoor VOCs (aromatic, aldehyde, alkane, ketone, alcohol, and chlorocarbon) by the French Environment and Energy Management Agency (ADEME) [START_REF]Pollutions olfactives : origine, législation, analyse, traitement[END_REF].

In this investigation, a new photocatalysis-surface plasma hybrid system is incorporated in a pilot reactor and the influence of some operating parameters on the removal of exhaust gases from animal quartering centers is investigated. On the other hand, at the same scale, Ochiai and co-workers investigated and developed a photocatalysis-plasma hybrid air-purification reactor using a titanium mesh sheet modified with TiO2 (TMiP TM) and a surface dischargeinduced plasma (SPCP) unit for treat a gas of smoking area [START_REF] Ochiai | An effective method for a separation of smoking area by using novel photocatalysis-plasma synergistic air-cleaner[END_REF].

Here, we test the combined system with a novel photocatalyst of glass fiber tissue combined with surface discharge plasma which is established in a continuous reactor to investigate the influence of some operating parameters on the removal in exhaust gases from animal quartering centers.

Materials and methods

Materials and methods at pilot scale

The experimental part will include two major elements: the reactor description and the gas sampling and analysis.

Reactor description

The experimental set-up is an air handling unit manufactured by CIAT (Compagnie Industrielle d'Applications Thermiques-France) with a flow rate capacity up to 5000 m 3 .h -1 (Fig. 1). It contains a pre-filtration box, a cooling bank, a vapor humidifier and an electrical heater. The second box contains the pollutant injection zone. Two boxes (A & B) permit an upstream and a downstream concentration pollution sampling. Photocatalytic and plasma treatment system is installed between these boxes (A & B), and finally an activated carbon filter is maintained at the exit in order to adsorb the residual pollution. A booster is used to generate the air flow rate. Its quantification is done thanks to a diaphragm by measuring a pressure drop. A "Testo 625" probe is used to measure the temperature and the relative humidity. This pilot was better described in our previous study concerning an investigation with photocatalysis alone [START_REF] Assadi | Isovaleraldehyde elimination by UV/TiO2 photocatalysis: comparative study of the process at different reactors configurations and scales[END_REF].

Fig.1: A functional diagram of the used pilot unit

The high voltage electrode was an array of grid wires placed at approximately 4 mm above the ground electrode. The grid had a thickness of 2 mm and the distance between adjacent wires was 20 mm. The ground electrode was a copper tape placed into a plat of glass. The photocatalytic zone had a cross-section area (0.30 m x 0.30 m). It contains the pleated photocatalytic media, which is maintained by grid wire into glass plate, offering a surface of 2.16 m 2 . This area was irradiated by 36 Philips UVA lamps (PL-L 24W/827/4P). The used material was a coated Glass Fiber Tissue (GFT) with 6.5 g.m -2 of colloidal silica to ensure the fixation of 6.5 g.m -2 of titanium dioxide nanoparticles. It is supplied by Ahlstrom Research and Services. This photocatalytic material had been described in detail in our previous works Plasma was obtained by submitting the electrodes to a sinusoidal high voltage ranging from 0 to 30 kV at a 50 Hz frequency. The outer electrode was connected to the ground through a 50 nF for collecting the charges transferred from the reactor. The applied voltage (Ua) and high capacitance voltage (Um) were measured by LeCroy high voltage probes and recorded by a Lecroy oscilloscope (Wave Surfer 24 Xs, 200 MHz).

Gas sampling and analysis

The preparation of synthetic polluted air stream was largely described in previous studies [START_REF] Assadi | Photocatalytic oxidation of Trimethylamine and Isovaleraldehyde in an annular reactor: Influence of the Mass Transfer and the relative humidity[END_REF][START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF][START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination -synergetic effect and byproducts identification[END_REF][START_REF] Guillerm | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF]. Briefly, the air-isovaleraldehyde gas mixture was prepared by passing synthetic air (Air Liquide) through liquid isovaleraldehyde (Sigma-Aldrich, 97%). Indeed, pollutant liquid was heated, vaporized and mixed with a zero-air flow in an especially designed Bronkhorst vaporization/mixing chamber (CEM). In these conditions, the tested concentrations ranged from 2 to 10 mg.m -3 . Gas samples were analyzed by gas chromatography with flame ionization detection (Chrompact FFAP-CB column). A standard iodometric titration method was used to estimate the formation of the downstream ozone.

CO concentrations were measured by CO ZRE gas analyzer. To monitor the progression of pollutant mineralization, CO2 was analyzed by a Cosma Beryl analyzer reference 100 equipped by spectrophotometer Fourier Transform Infrared (FTIR) brand Environnement SA.

Materials and methods at industrial application

Several materials were used in order to better investigate and analyze the performance of combined system

Analysis system

The industrial emissions were measured by a FISONS Gas-phase Chromatograph (GC) using a Flame Ionisation Detector (FID). Indeed, different Carbotrap cartridges have been used in order to concentrate the pollution: Carbopack Y, allows the selective retention of heavy compounds from C12 to C20. Another Carbopack B traps the compounds C5 to C12, while the light compounds from C2 to C5were adsorbed with Carboxen 1003. Thus, pollution was firstly concentrated on different Carbotrap (Supelco); the pumping was done by a Gillian LFS-113 pump under a flow rate of 50 mL.min -1 . Secondly, Turbomatrix Perkin Elmer unit was used to desorb thermally the concentrated compounds, which will be analyzed through a transfer line to the GC/MS Perkin Elmer Clarus 500. A detailed description of the experimental protocol of analysis was published elsewhere [START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination -synergetic effect and byproducts identification[END_REF]. In addition, TRS MEDOR analyzer (Chromatotec, France) was used to analyze the sulfur compounds concentrations. In fact, the sample under a 100 mL min -1 flowrate was continuously swept to an analyzer which was equipped by a capillary column and an electrochemical detection [START_REF] Guillerm | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF].

Along with the concentration on Carbotrap, a series of analysis were done in-situ using the equipment provided by Explorair Company (France). A thermal desorption unit coupled to Fast Gas Chromatograph-Mass spectrometer (TD/FASTGC/MS) was used to analyze concentration at different sampling points A, B, C and D (Fig. 2). This TD/FAST GC/MS gives new possibilities for continuous analysis by offering excellent performances represented by the speed, flexibility, sensitivity and repeatability of the analysis. An RTX column (20 m length 0.18 mm external diameter and 1 µm thickness), which is especially adapted for volatile fatty acids, was used. Helium gas constituted the mobile phase. A Micro Gas Chromatograph-Mass spectrometer (µGC/MS) was used to analyze mercaptans.

Fig.2: photography of industrial pilot at Saria with different sampling

In order to analyze ammonia after pumping and bubbling it through an HCl solution of 40 L.h - 1 of downstream, a spectrophotometric method using Nessler reagent was used [14].CO2, CO, and NOx were analyzed with the same equipment that was used at pilot scale investigation.

Ozone was monitored on-line by an ozone analyzer (Environnement SA_O342M).

Industrial reactor description

A second prototype (Pr2) was built for the industrial application. This prototype had the same configuration that Pr1 with only some technical improvements over the distance between UV light and electrodes. Pr1 and Pr2 were inserted and connected in series in the industrial reactor which had the same configuration as the pilot reactor. Indeed, it was designed with a 316L stainless steel to be able to withstand corrosion induced by the chemical compounds. It comprised a pre-filtration box, an upstream pollution measurement box, two boxes for prototypes (Pr1&2), and a downstream concentration measurement box. A ventilation box consisted of a medium-pressure centrifugal fan is added. This scroll-free fan was coupled directly to a 3 kW motor. The assembly was capable of generating an air flow rate ranging from 0 to 1000 m 3 .h -1 .

In order to generate the plasma in both prototypes, the same system as for pilot scale investigation was used. The second plasma generating system came from the Institut de Thus, this photocatalytic medium was maintained for industrial test.

Results and discussion

Investigation at pilot scale

In order to monitor the performance of the process, some operational conditions were studied.

Process efficiency was evaluated thanks to the following parameters:  Specific energy (SE; J.L -1 ) = 3600*(P (W) / Q(m 3 .h -1 ))/1000

(1

)
where P is input power adjusted by changing the applied voltage (Ua) and Q is the flowrate  CO2 and CO selectivity's are calculated according to the following equations:

𝑆 𝐶𝑂 2 = Δ𝐶𝑂 2 n (C|COV) .([COV] 0 -[COV] s )
(2)

𝑆 𝐶𝑂 = Δ𝐶𝑂 n (C|COV) .([COV] 0 -[COV] s ) (3)
where [COV]0 and [COV]s are the inlet and outlet concentrations of Isoval respectively (ppmv), n(C|COV) is the stoichiometric coefficient of the removal reaction. In our case it is equal to 5.

Ozone production is investigated by several researchers and correlated with numerical simulation [START_REF] Capitelli | Plasma kinetics in atmospheric gases[END_REF]. Indeed, non-thermal plasma leads to ozone formation through the production of oxygen atoms by electron collisions on O2:

O2 + e -→ 2O • + e -

followed by an addition reaction:

O • + O2 +M → O3 +M, with M = N2 or O2 (2)
where O • refers to atomic oxygen which was generated by O2 dissociation due to impact with plasma discharge. In our case, the novelty is that this synergetic effect was maintained at high flow rates (25 times higher than that at the lab-scale). Indeed, Isoval oxidation efficiency with the associated system was 8 % higher than the sum of the oxidation efficiencies recorded, under the same conditions when plasma and photocatalysis were used separately. This result is similar to that obtained by Ochiai and co-workers. In fact, they investigated and developed a photocatalysis-plasma hybrid air-purification reactor using a titanium mesh sheet modified with TiO2 (TMiP TM) and a surface discharge-induced plasma

Removal efficiency

By-products formation

Table 1 gives the list of the by-products detected. At pilot scale, we note an improvement of mineralization when plasma was associated with photocatalysis. In fact, the mineralization by photocatalytic process is about 75 % and it is equal to 27 % when plasma was taken alone.

Nevertheless, with the associated process, CO2 selectivity exceeds 55 %. This enhancement is attributed to the formation of more reactive species due to the presence of TiO2 in plasma zone [9, 13]. On the other hand, CO selectivity doesn't exceed 10 % with combined process.

It is interesting to note that the combined system leads to better CO2 selectivity than for plasma. Thus CO can be decomposed with plasma because recent reports by Ochiai et al.

indicate that although using combined system, CO cannot decompose successfully [START_REF] Ochiai | Field performance test of an Air-Cleaner with photocatalysis-plasma synergistic reactors for practical and long-term use[END_REF][START_REF] Ochiai | Development of a Coil-Shape Photocatalysis-Plasma Synergistic Reactor for a Practical and Long-Term Usable Air-Cleaner[END_REF].

Moroever, many research works confirm that such mineralization improvements are related to the porosity of the medium: porosity induces a longer residence time of gas intermediates byproducts during diffusion through the solid pore system [START_REF] Guillerm | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF]. This behavior can enhance the converting of intermediates by-products into CO2 leading to the little amount of CO.

These behaviors of selectivity are similar to that was observed at lab-scale experiments. In fact, this trend is attributed to the role of TiO2 in the discharge zone which leads to the formation of more reactive species. Moreover, external UV light is able to activate ozone on the surface of TiO2 in order to improve the byproducts mineralization. Consequently, intermediates by-products have been reduced due to various other species (O • , N, OH • , O2• -, O3, NO2, NOx, etc.) [START_REF] Ochiai | Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification[END_REF].

In the same way, the combined process performance was enhanced in term of residual ozone.

Indeed, the results show that residual ozone formed by plasma alone is higher than that formed with the associate system. This can be explained by the decomposition of ozone due to the presence of external UV [START_REF] Taranto | Combining Cold Plasma and TiO2 Photocatalysis To Purify Gaseous Effluents: A Preliminary Study Using Methanol-Contaminated Air[END_REF][START_REF] Maciuca | Synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF]. Similar laboratory trends were also observed at pilot scale under various operating parameters [13].

Industrial application

SARIA Industries is an animal quartering centers. It collects from farmers and meat industry the waste which can represent a risk to human health and environment. This waste is then subject to specific treatments within dedicated industrial units. These treatments generate a polluted gas effluent loaded with VOCs and ammonia.

Our treatment plant was installed on SARIA factory on mid-June 2012. The tests began during the second half of June under high temperature (32.4 °C) and relative humidity (56%).

Before turning on the fan, a filter for dust and grease retention was positioned upstream of the pilot plant. The inlet concentration of pollutants (primarily aldehydes) was not stable and depends on the plant quartering centers. This forces us to alternate systematically analysis upstream and downstream of the industrial pilot.

Photocatalysis alone

The results of VOC's oxidation are represented in fig. 4. After a stabilization period of several hours, the oxidation efficiency with photocatalysis alone was hovering around 21-24% for the three aldehydes (isobutyraldehyde, isovaleraldehyde, and 2-methylbutyraldehyde).

Additionally, oxidation efficiency of ammonia was around 25 % with inlet concentration about 20 mg.m -3 . On the other hand, we also note the formation of byproducts, mainly acetic acid, propionic acid and butanoic acid. However, the values of removal efficiencies in the industrial application were less than the pilot study. In fact, the high relative humidity observed for the real effluent has a significant effect. This behavior arises because water vapor can deposit on the surface of the photocatalyst and occupy its active sites [START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination -synergetic effect and byproducts identification[END_REF]. If the number of active sites is considered constant, it becomes evident that the competitive effect increases with increasing effluent humidity.

DBD plasma alone

The oxidation efficiencies of some aldehydes and mercaptans with plasma surface discharge alone are summarized in Fig. 5. Except isovaleraldehyde, the oxidation efficiencies were around 30-35%. The removal efficiency of dimethyl disulfide (DMDS) with low inlet concentration (< 10 mg/m 3 ) doesn't exceed 40 %.This low degradation rate is usually observed for sulfur compounds [START_REF] Guillerm | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF].

In fact, with hydrogen sulfide, at similar value of plasma injected energy, the conversion efficiency does not exceed 25% [START_REF] Guillerm | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF].

Additionally, the same behavior has been observed with the aldehydes. The increase of water vapor content reduces the electron density and quenches the active chemical species [START_REF] Pichat | Some views about indoor air photocatalytic treatment using TiO2: Conceptualization of humidity effects, active oxygen species, problem of C1-C3 carbonyl pollutants[END_REF][START_REF] Ochiai | Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification[END_REF].

Combined DBD plasma/photocatalysis

As shown in Fig. 6, it is clear that the combination of plasma and photocatalysis improve the performance of the industrial reactor. In fact, the oxidation efficiencies were around 60-75%

for the three aldehydes.

Here, the synergetic effect was not clear because operating parameters like the temperature, the inlet concentration of pollutant and relative humidity were not stable during all the experiments. This may be the most serious problem for providing and confirming a synergetic effect at industrial scale.

Moreover, the high value of RH can affect also the synergistic effect in combined DBD plasma/photocatalysis. Indeed, the removal efficiency due to the combined process is not alltime better than the value corresponding to the sum of the removal efficiencies of each process taken separately.

On one hand, with combined DBD plasma/photocatalysis, water vapor content may enhances the formation of reactive species as in the equation [13]:

H2O + e - H  + HO 

On the other hand, at higher levels of RH, water vapor content limits the electron density and quenches the active chemical species [4, 5].

Thus, the reactor at the pilot scale in this study is more efficient than the industrial system.

Fig. 6: The removal efficiencies of some pollutants present in the effluent of SARIA using combined system (Total pollution = 30 mg.m -3 , T= 30°C, RH=53 %, Q=350 m 3 .h -1 ).

Fig. 7 shows inorganic by-products concentrations. As we can see, with combined system, COx , NOx and ozone were detected. Since the combined process produces higher quantities of electrons and reactive species (such as O • and HO • ), this latter oxidize then the pollutants' molecules into CO2 and CO [24]. These results were in agreement with lab works treating the removal of some VOCs [START_REF] Maciuca | Synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF]. Ozone was detected downstream the reactor at a lower concentration than that obtained at lab-scale study. This is probably due to the elevated temperature and the higher relative humidity in the air. In fact, an elevated temperature leads to a higher decomposition of ozone and an oxidation of NO molecules to NO2 [START_REF] Dubus | Contribution à l'étude thermique d'un réacteur à décharge à barrière diélectrique[END_REF].

Moreover, the previous result at lab-scale showed that ozone is strongly reduced when RH increases. Its formation is probably inhibited [START_REF] Thevenet | Influence of water vapour on plasma/photocatalytic oxidation efficiency of acetylene[END_REF][START_REF] Chen | Effect of relative humidity on electron distribution and ozone production by DC coronas in air[END_REF]. Indeed, the consumption of ozone in the presence of radicals (H • + HO • ) due to electronic impact on water can be explained by these two reactions:

O3 + OH •  O2 + HO2 • (4) O3 + H •  O2 + HO • (5)

Conclusion

The main results of combined plasma photocatalytic treatment were:

(1) The effect of associated treatment is higher than the sum of individual steps. This synergy is partly confirmed at pilot scale with high values of flowrate.

(2) Plasma-assisted photocatalytic process can provide a suitable alternative for the pollution classic treatments.

(3) At pilot scale, the associated process leads towards total oxidation. The by-products formed during plasma treatment are oxidized to CO2.

Globally, the transition to an industrial experiment was studied and discussed using industrial prototypes. It brought many interesting facts about the viability of the process in complex environments. The scale-up feasibility of the process seems to be possible for the treatment of an industrial and real gas effluent.

[ 3 ,

 3 12-14].

  Chimie des Milieux et Materiaux de Poitiers (IC2MP). This second high voltage power amplifier (Trek 30/20A) driven by a function generator (FLUKE_271 DDS) was used to generate a sine waveform [10]. The high voltage applied (Ua) and the voltage across the capacity (Um) were measured by LeCroy high voltage probes and the signals were visualized with a digital oscilloscope (Lecroy_model LT342, 500 MHz) [10]. Results at lab and pilot scales showed that the used material (Glass Fiber Tissue (GFT) can resist to the discharge.

Fig. 3

 3 Fig.3shows the variation of Isoval oxidation efficiency with plasma, photocatalysis, and

(Fig. 3 :

 3 Fig. 3: Oxidation efficiency of three tested processes at pilot scale (Einj = 6 J.L -1 ,

Fig. 4 :

 4 Fig. 4: The removal efficiencies of some pollutants of SARIA effluent using

Fig. 5 :

 5 Fig.5: Removal efficiencies of pollutants present in SARIA effluent using surface

Fig. 7 :

 7 Fig. 7: Temporal monitoring of inorganic by-products concentrations at downstream of

FiguresFig. 1 :Fig. 2 :Fig. 3 :(Fig. 4 : 3 ,Fig. 5 : 3 ,Fig. 6 : 3 ,Fig. 7 : 3 ,

 12343536373 Figures

Table 1 : CO and CO2 selectivities and residual ozone with different processes at pilot scale experiments

 1

Table 1 :

 1 CO and CO2 selectivities and residual ozone with different processes using pilot

	Tables:
	scale
		Selectivity of CO2 (%) Selectivity of CO (%)	Ozone (ppm)
	Plasma	27	14	17
	Photocatalysis	75	. < 1	. < 1
	Combined process	55	8	12

Acknowledgment

We gratefully acknowledge the financial support provided by the French National Research Agency (ANR) for this research work. We would like also to thank SARIA group for assistance provided within the scope of this industrial application and VALLET C., TATIBOUET J. M. and BATIOT-DUPEYRAT C. for their assistances


P Castany

F Pettinari-Sturmel

J Douin

A Coujou

TEM quantitative characterization of short-range order and its effects on the deformation micromechanims in a Ti-6Al-4V alloy

Keywords: Dislocations, Short-range order (SRO), Titanium alloys, Transmission electron microscopy (TEM), In situ straining

TEM quantitative characterization of short-range order and its effects on the deformation micromechanims in a Ti-6Al-4V alloy

Philippe Castany, Florence Pettinari-Sturmel, Joël Douin, Armand Coujou

To cite this version:

Philippe Castany, Florence Pettinari-Sturmel, Joël Douin, Armand Coujou. TEM quantitative characterization of short-range order and its effects on the deformation micromechanims in a Ti-6Al-4V alloy. Materials Science and Engineering: A, 2017, 680, pp.85-91. [START_REF] Savage | Anisotropy in the room-temperature deformation of α-β colonies in titanium alloys : role of the α/β interface[END_REF].1016/j.msea.2016.10.020. hal-

Introduction

Titanium alloys are used in a wide range of domains requiring high mechanical strength and low density. The Ti-6Al-4V alloy is the most used composition of these alloys for aerospace and aeronautic applications. Ti-6Al-4V alloys are two-phase / alloys whose microstructure can be fully nodular, fully lamellar or duplex depending on thermo-mechanical treatments.

Mechanical properties of these alloys are mainly due to the majority volume fraction phase that is the h.c.p.  phase. The predominant slip systems in  phase consist of a-type Burgers vector dislocations gliding in the basal (0001) or prismatic planes [START_REF] Bridier | Analysis of the different slip systems activated by tension in a α/β titanium alloy in relation with local crystallographic orientation[END_REF][START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF][START_REF] Zaefferer | A study of active deformation systems in titanium alloys: dependence on alloy composition and correlation with deformation texture[END_REF]. As in pure -titanium, the main parameter responsible for the mechanical strength of these alloys was shown to be the three-dimensionally spread core structure of a-type screw dislocations that reduces their mobility [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF][START_REF] Naka | The plasticity of titanium at low and medium temperatures[END_REF][START_REF] Farenc | An in situ study of prismatic glide in α titanium at low temperatures[END_REF][START_REF] Bacon | Atomic-Scale Modeling of Dislocations and Related Properties in the hexagonal-Close-Packed Metals[END_REF][START_REF] Aoki | Atom-based bond-order potentials for modelling mechanical properties of metals[END_REF][START_REF] Douin | Direct measurement of the variation in the energy of a dislocation locked in specific orientations[END_REF]. When a stress is applied, the core of screw dislocations has to recombine in a different metastable and glissile core structure in order to allow dislocations to slip. The transition between these two core configurations leads thus to a high lattice friction stress. On the other hand, the / interfaces in lamellar colonies have generally a minor role in the strengthening in comparison with the core structure of screw dislocations [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF][START_REF] Suri | Room temperature deformation and mechanisms of slip transmission in oriented single-colony crystals of an α/β titanium alloy[END_REF][START_REF] Savage | Anisotropy in the room-temperature deformation of α-β colonies in titanium alloys : role of the α/β interface[END_REF][START_REF] Castany | In situ transmission electron microscopy deformation of the titanium alloy Ti-6Al-4V: Interface behaviour[END_REF]. This weak effect is due to the orientation relationship between the  and  phases that allow easily dislocations to cross these / interfaces. Short-range order (SRO) is also another possible source of the mechanical strength of Ti-6Al-4V alloys, but very few studies deals with this point in industrial alloys. SRO was specifically evidenced to have an effect on 112 0 1 1 00

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mechanical strength of binary Ti-Al model alloys with similar compositions of the  phase of Ti-6Al-4V. For example, the existence of SRO provides a significant strengthening in a Ti-6Al (at.%) during creep tests at room temperature [START_REF] Neeraj | Short-range order (SRO) and its effect on the primary creep behavior of a Ti-6wt.%Al alloy[END_REF]. SRO was evidenced from neutron diffraction in this alloy [START_REF] Neeraj | Low temperature creep of titanium alloys: microstructure, deformation mechanisms and modeling[END_REF], but can also be detected from the dislocation arrangement after deformation by Transmission Electron Microscopy (TEM): indeed, when there is SRO, the deformation becomes more heterogeneous with dislocations pile-ups with sometimes paired dislocations at the head of pile-ups as previously observed in Ti-6Al [START_REF] Neeraj | Short-range order (SRO) and its effect on the primary creep behavior of a Ti-6wt.%Al alloy[END_REF][START_REF] Neeraj | Phenomenological and microstructural analysis of room temperature creep in titanium alloys[END_REF] or Ti-5Al [START_REF] Xiao | Planar dislocation bands formed in Ti-5at.%Al single crystals deforming by double prism slips[END_REF] binary titanium alloys (at.%). In other -titanium alloys, observation of pile-ups is also sometimes attributed to the presence of SRO even if this feature is just mentioned and not investigated [START_REF] Ambard | Role of interphases in the deformation mechanisms of an α/β titanium alloy at 20 K[END_REF][START_REF] Odegard | Low Temperature Creep of Ti-6Al-4V[END_REF][START_REF] Truax | Plastic behaviour of titanium-aluminium alloys[END_REF]. Else, the presence of ordered  2 (Ti 3 Al) nanoprecipitates in Ti-alloys has been mentioned in the literature and attributed to solute partitioning in primary  grains, as well as their detrimental effects on fracture toughness and low cycle fatigue [START_REF] Rosenberg | Titanium alloying in theory and practice[END_REF][START_REF] Hardt | High-temperature fatigue damage mechanisms in nearα titanium alloy IMI 834[END_REF][START_REF] Kumar | Evolution of damage in near α IMI-834 titanium alloy under monotonic loading condition: A continuum damage mechanics approach[END_REF].

Recently, this precipitation has been investigated in details by Radecka et al. using TEM and atom probe tomography. In their case, superlattice diffraction spots are clearly visible in the  phase [START_REF] Radecka | Precipitation of the ordered α 2 phase in a near-α titanium alloy[END_REF].

Contrarily to SRO in -titanium alloys, SRO was extensively studied in Ni-based superalloys.

In theses alloys, the SRO is described as nano-sized diffuse domains that lead to a high friction impeding the glide of isolated dislocations [START_REF] Pettinari | Local order in industrial and model γ phases of superalloys[END_REF]. Generally, a pair of dislocations propagates at the head of pile-ups [START_REF] Pettinari | Local order in industrial and model γ phases of superalloys[END_REF][START_REF] Pettinari-Sturmel | Characterisation of short-range order using dislocations[END_REF]. A strong difference between SRO and long rang order (LRO) precipitates, on a "dislocation point of view", is that LRO precipitates need several paired dislocations to be sheared and, when they are totally sheared, individual dislocations can glide in the plane that contained theses precipitates [START_REF] Glas | Order and mechanical properties of the γ matrix of superalloys[END_REF]. Several pairs of dislocations gliding on the same plane are thus a signature of ordered precipitates, whereas a unique pair at the head of pile-ups corresponds to SRO. In addition, in the electron diffraction patterns, the presence of LRO is associated with superlattice spots whereas SRO not.

The aim of this paper is to contribute quantitatively in the understanding of SRO in Ti-6Al-4V alloys and its effects on the mechanical strengthening and dislocations arrangement via in situ TEM straining experiments.

Experimental

The material of this study is an industrial Ti-6Al-4V alloy with a duplex microstructure composed of primary alpha nodules  P and lamellar colonies  S /(Fig. 1). The nodules and lamellar colonies have roughly the same size of about 10 µm and similar volume fractions.

The b.c.c  phase is found mainly is lamellar colonies with a total volume fraction of about 3% [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF][START_REF] Castany | In situ transmission electron microscopy deformation of the titanium alloy Ti-6Al-4V: Interface behaviour[END_REF]. Because no effect of short-range order was evidenced in secondary alpha plates S in lamellar colonies, this paper focus on primary alpha nodules P .

Specimens for in situ TEM straining experiments were first mechanically polished and then thinned down by twin-jet electropolishing at a temperature of -15°C. The electrolyte used was the A3 solution commercialized by STRUERS. TEM post mortem observations were also performed in order to compare in situ results with the microstructure of samples deformed macroscopically in tension.

In situ experiments were performed at room temperature with a Gatan straining holder in a 200 kV JEOL 2010 TEM equipped with a SIS CCD camera for video-rate recording.

Burgers vectors of dislocations were determined by the g.b=0 invisibility criterion and slip planes were characterized by the analysis of slip traces left on the surfaces of specimens during in situ straining experiments. All measurements of distances between dislocations were corrected to take in account the tilt of the slip planes into which dislocations are gliding. As theses measurements were done during in situ experiments on moving dislocations, the accuracy of measurement is lower than for conventional TEM observations. Depending on the measurement, the accuracy is then of about 10-20%.

Results

All moving dislocations observed during in situ straining experiments have a-type Burgers vectors and glide mainly in prismatic or basal planes depending on the crystallographic orientation of the primary  P nodule observed. Cross-slip is rarely observed contrarily to previous experiments performed in the  phase of lamellar colonies [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF]. An example of the typical motion of dislocations in a  P nodule is shown in Fig. 2. The projection of their Burgers vector is indicated by the line labelled "b". Dislocations appear clearly to glide paired: two dislocations, numbered 1 and 2, glide together in a correlated way across the grain. Screw segments remain roughly straight and move together, as well as curved nonscrew segments. Non-screw segments have a relatively steady motion, whereas straight screw segments have a more jerky motion as observed previously in the  phase of lamellar colonies of the same alloys [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF]. All dislocations in this grain glide in the same set of prismatic planes whose trace is indicated in Fig. 2b. Slip traces are rigorously straight indicating that cross-slip does not occur. Fig. 3 shows the same grain under different diffracting conditions after in situ straining: almost all dislocations remain paired even when the stress is removed. Paired dislocations are also observed for basal slip. For example, two pairs of dislocations, numbered 1 and 2 in Fig. 4, has a correlated motion for their screw segments as well as non-screw ones.

The distance separating both paired dislocations varies during their motion during in situ experiments: an example is given in Fig. 5 wherein the spacing between paired dislocations is reported as a function of the average position of a pair of dislocations gliding in prismatic (Fig. 5a) or basal plane (Fig. 5b). The average values taken from several pairs are summarized in Table 1: this spacing is almost independent of the slip plane and is larger for edge segments than for screw segments. The values measured after in situ straining experiments are also reported in Table 1 with a relative uncertainty of about 20% as the measurements have been performed on dynamic sequences. This shows that the spacing remains constant after the stress is removed for screw segments and slightly decreases for edge segments.

The measurement of velocities of paired dislocations moving under stress during in situ straining shows that the screw segments move almost twice slower than edge segments (Table 2). In the prismatic plane, both edge and screw segments moves faster than in basal plane.

This difference is most probably due to a different stress depending on the experiment. But, as the ratio between the velocities of edge and screw segments is exactly the same whatever the experiment, one can deduce that the velocity is independent of the slip plane.

In some  P nodules, dislocations form large pile-ups instead of single pairs of dislocations.

Such pile-ups are due to Frank-Read sources operating inside the grains. An example is given in Fig. 6, where a mobile segment rotates around an anchoring point (arrowed in the second frame). At t=0s, four screw segments are numbered 1, 2, 3 and 4. The segment numbered 4 is mobile and rotates around the anchoring point of the source. On the next frame of the recorded video, at t=0,04s, the segment numbered 4 has rotated and is moving quickly toward the right (that is why it is invisible in this frame). After this rotation, the new segment rotating around the anchoring point is numbered 5. In the last image, the segment numbered 4 is visible and the segment numbered 5 has made another rotation leaving a new screw segment toward the left and the right of the source. At the head of this pile-up, the two first dislocations are clearly paired whereas further dislocations not (Fig. 7). Contrarily to the paired dislocations presented in Fig. 2 or Fig. 4, only screw segments can be observed here because non-screw segments move almost immediately out of the specimen. The slip traces in Fig. 6 correspond to a prismatic plane. After this source has generated several tens of dislocations, some dislocations cross-slip to a first-order pyramidal plane, that is highlighted by a change in the orientation of the slip traces (Figure 8).

Observations of specimens after macroscopic tensile tests up to 0.3% of plastic strain are reported in Fig. 9. Dislocations arrangements are very similar to the previous in situ observations: in some nodules, dislocations appear to be paired (pairs numbered 1, 2 and 3 in Fig. 9a) whereas in other nodules, dislocations form large pile-ups (Fig. 9b). Dislocations in Fig. 9 have a-type Burgers vectors. In situ observations are thus consistent with the behaviour of dislocations during the macroscopic deformation.

Discussion

In all nodules observed, moving dislocations have a-type Burgers vectors and glide in basal or prismatic planes that are the main slip systems in -titanium alloys [START_REF] Bridier | Analysis of the different slip systems activated by tension in a α/β titanium alloy in relation with local crystallographic orientation[END_REF][START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF][START_REF] Zaefferer | A study of active deformation systems in titanium alloys: dependence on alloy composition and correlation with deformation texture[END_REF][START_REF] Savage | Deformation mechanisms and microtensile behavior of single colony Ti-6242Si[END_REF]. The activated slip systems during in situ straining experiments follow the Schmid law according to previous studies [START_REF] Bridier | Analysis of the different slip systems activated by tension in a α/β titanium alloy in relation with local crystallographic orientation[END_REF], i.e.:

-basal slip is only observed when prismatic slip systems have lower Schmid Factors (example in Fig. 4); -prismatic slip is activated even if basal or pyramidal slip systems have similar Schmid factors (examples in Fig. 2 and Fig. 6).

Cross-slip is only observed in large pile-ups when several tens of dislocations are piled-up (Fig. 8). In this specific configuration, the Schmid factors of both prismatic and pyramidal planes are the same, allowing cross-slip. However, cross-slip begin only when dislocations are impeded to glide in their initial prismatic plane because of the relatively high density of previous dislocations in this plane. This observation is in contrast with previous observations in the  phase in lamellar colonies of the same alloy wherein multiple cross-slip frequently occurs leading to wavy slip traces [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF]. Moreover, in nodules, cross-slip is never observed when dislocations are paired and the slip traces are straight for paired dislocations and pileups. Cross-slip is thus not as frequent in  P nodules than in lamellar colonies. In -titanium alloys, inhibition of cross-slip is frequently attributed to the presence of short-range order (SRO) that leads to a heterogeneous deformation constituted of dislocations pile-ups [START_REF] Neeraj | Short-range order (SRO) and its effect on the primary creep behavior of a Ti-6wt.%Al alloy[END_REF][START_REF] Neeraj | Phenomenological and microstructural analysis of room temperature creep in titanium alloys[END_REF][START_REF] Xiao | Planar dislocation bands formed in Ti-5at.%Al single crystals deforming by double prism slips[END_REF].

Classically, at least one pair of dislocations has to glide at the head of pile-ups in the presence of SRO because of the influence of the chemical fault that formed after the passage of a first dislocation. In alloys wherein SRO is clearly identified such as Ni-based superalloys, pairs of dislocations lie at the head of pile-ups, which mainly contain numerous dislocations [START_REF] Pettinari-Sturmel | Characterisation of short-range order using dislocations[END_REF][START_REF] Olfe | Dislocation groups, multipoles, and friction stresses in alpha-CuZn alloys[END_REF][START_REF] Gerold | On the origin of planar slip in f.c.c. alloys[END_REF][START_REF] Schwander | Configurational energy change caused by slip in short-range ordered Ni-Mo[END_REF][START_REF] Plessing | Short-range order and the mode of slip in concentrated Cu-based alloys[END_REF]. In titanium alloys, a few studies have reported the presence of such pairs in a binary Ti-6Al alloy [START_REF] Neeraj | Short-range order (SRO) and its effect on the primary creep behavior of a Ti-6wt.%Al alloy[END_REF][START_REF] Neeraj | Phenomenological and microstructural analysis of room temperature creep in titanium alloys[END_REF] wherein SRO was evidenced from neutron diffraction [START_REF] Neeraj | Low temperature creep of titanium alloys: microstructure, deformation mechanisms and modeling[END_REF].

As a unique pair of dislocations at the head of pile-ups has been observed during our experiments (Fig. 7), and no superlattice diffraction spot has been identified, it can be unambiguously attributed to the presence of SRO in  nodules. After the glide of the leading pair of dislocations, the SRO is sufficiently destroyed to allow single dislocations to glide in the same slip plane. The slip plane is then softened and further dislocations can glide more easily in this plane (without being paired) than in another slip planes leading to a heterogeneous deformation. If an intragranular source is activated, several dislocations can thus glide in this softened plane and lead to the formation of large pile-ups (Fig. 6). However, if no intragranular source is activated, no pile-ups are formed (Fig. 2 and Fig. 4) in spite of the presence of SRO. Thus, only a unique pair of dislocations is observed to glide in a correlated way. The presence of such pairs is also due to the presence of SRO because, if there is no SRO, only isolated dislocations should be observed. This unique feature, never observed previously, occurs when intragranular sources are not activated in nodules that deformed by dislocations emitted from / interfaces. Emission from / interfaces was frequently observed in lamellar colonies of the same alloy [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF], but only single dislocations are emitted in lamellar colonies instead of pairs like in nodules. This difference in dislocation behaviour between the primary nodules and the  secondary phases may be attributed to the difference in SRO (presence or not), which is directly connected with a slight difference in Al content.

An attempt was made to identify any difference in chemical composition between the primary nodules and the  secondary ones using Electron Energy Loss Spectroscopy, but the difference has not been detectable, meaning this difference is very low. But there could be one as the precipitation of the  phases appear at different step during the thermomechanical elaboration process.

In pile-ups, dislocations are only of screw character that is due to their specific core structure that lowers the mobility of screw segments in comparison with non-screw ones [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF][START_REF] Naka | The plasticity of titanium at low and medium temperatures[END_REF][START_REF] Farenc | An in situ study of prismatic glide in α titanium at low temperatures[END_REF][START_REF] Bacon | Atomic-Scale Modeling of Dislocations and Related Properties in the hexagonal-Close-Packed Metals[END_REF][START_REF] Aoki | Atom-based bond-order potentials for modelling mechanical properties of metals[END_REF][START_REF] Douin | Direct measurement of the variation in the energy of a dislocation locked in specific orientations[END_REF]. A consequence is that non-screw segments move quickly out of the specimen during in situ TEM straining [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF][START_REF] Farenc | An in situ study of prismatic glide in α titanium at low temperatures[END_REF]. In lamellar colonies of the same alloy, non-screw segments was estimated to glide at least 100 times faster than screw segments [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF]. A similar feature is observed for dislocations forming pile-ups in  P nodules (Fig. 6) that suggests the presence of SRO in these nodules does not affect the velocity and the behaviour of dislocations in comparison with lamellar colonies except the formation of pile-ups. However, in nodules that deform with single pairs of dislocations (Fig. 2 and Fig. 4), the situation differs because nonscrew segments are measured to glide only about 2 times faster than screw segments (Table 2). In these nodules, the presence of SRO affects significantly the motion of non-screw segments. On the other hand, the motion of screw dislocations seems not greatly influenced by the presence of SRO and their motion is still mainly controlled by their core structure. This result suggests that there is some fluctuation of SRO depending on each nodule. Such micrometer-scale fluctuations of SRO were already observed in Ni-based alloys [START_REF] Pettinari-Sturmel | Characterisation of short-range order using dislocations[END_REF]. This difference of microstructure of deformation (pile-ups or single pairs of dislocations) is also observed in macroscopically deformed specimens (Fig. 9).

In this Ti-6Al-4V alloy, there is strong differences in the behaviour of dislocations depending on whether they glide in  P nodules or in  phase of lamellar colonies (see Ref. [START_REF] Castany | Experimental study of dislocation mobility in a Ti-6Al-4V alloy[END_REF] for results in lamellar colonies):

-pile-ups or pairs of dislocations in nodules instead of single dislocations in lamellar colonies;

-very rare cross-slip in nodules (straight slip traces) instead of extensive cross-slip in lamellar colonies (wavy slip traces);

-ratio between edge and screw segment velocities of about 2 in nodules (with dislocations pairs) instead of more than 100 in lamellar colonies.

These characteristics show unambiguously that the presence of SRO is only significant in nodules. In the  phase of lamellar colonies, there is most probably no SRO or, if there is, its magnitude is too weak to have any effect on the behaviour of dislocations. This difference in SRO can only be due to a slight difference in the chemical composition of the  phase in lamellar colonies and in nodules. Indeed, it is well known that in such duplex microstructure, an alloying element partitioning can take place leading to a enrichment of  stabilizing elements in nodules to the detriment of lamellar colonies [START_REF] Lütjering | Titanium[END_REF]. In Ti-6Al-4V alloys,  stabilizing elements are aluminium and oxygen that are both able to order: in binary Ti-Al alloys, SRO is due to the ordering of aluminium atoms [START_REF] Neeraj | Short-range order (SRO) and its effect on the primary creep behavior of a Ti-6wt.%Al alloy[END_REF][START_REF] Neeraj | Phenomenological and microstructural analysis of room temperature creep in titanium alloys[END_REF][START_REF] Lienert | In situ single-grain peak profile measurements on Ti-7Al during tensile deformation[END_REF] whereas ordering of oxygen atoms is also reported in pure titanium with high oxygen content [START_REF] Williams | The influence of oxygen concentration on the internal stress and dislocation arrangements in α titanium[END_REF]. Moreover, a combined effect of ordering of both aluminium and oxygen (as impurity content) is also observed in some alloys [START_REF] Lütjering | Titanium[END_REF][START_REF] Welsch | Deformation modes of the alpha-phase of Ti-6Al-4V as a function of oxygen concentration and aging temperature Metal[END_REF]. In  P nodules, the oxygen and aluminium concentration is thus sufficiently higher than in lamellar colonies in order to promote the formation of SRO.

In systems wherein SRO has been deeply investigated such as Ni-based superalloys, quantitative analyses have been carried out from the position of moving dislocations during in situ TEM straining experiments, allowing to evaluate the energies associated with SRO [START_REF] Pettinari-Sturmel | Characterisation of short-range order using dislocations[END_REF][START_REF] Saada | Pile-ups in thin foils: application to transmission electron microscopy analysis of short-range-order[END_REF][START_REF] Pettinari-Sturmel | The fluctuation of short-range order evidenced by mobile dislocations in the γ-phase of a nickel-based superalloy[END_REF]. A similar approach can be used for pairs of dislocations in Ti-6Al-4V like those in Fig. 2 and Fig. 4. All the forces exerted on each dislocation of a pair are summarized in the Fig. 10. Some of these forces are the same for both dislocations of the pair:

-the applied stress b a , where b is the Burgers vector and  a the applied stress;

-the friction stress b SS due to the solid solution friction of alloying elements (this force is assumed to be the same for each dislocation of the pair);

-the elastic interaction force A/X between both dislocations of the pair, where A is a constant depending on the character of the dislocation and X the spacing between both dislocations.

In addition, the glide of the first dislocation of the pair (labelled 1 in Fig. 10) destroys partially the SRO and creates behind a planar fault known as "diffuse antiphase boundary" (DAPB) [START_REF] Schwander | Configurational energy change caused by slip in short-range ordered Ni-Mo[END_REF]. An energy  1 is thus associated with this fault for the first dislocation. The glide of the second dislocation of the pair (labelled 2 in Fig. 10) destroys partially the remaining SRO and creates a new fault with the associated energy  2 .

The equilibrium of each dislocation can thus be given by the following equations:

( ) (1)
( ) (2)
The equations ( 1) and ( 2) corresponds to the first and the second dislocation respectively. It is assumed that dislocations are in quasi-static equilibrium and that they are straight and infinitely long, i.e. the elastic interactions between each segment of the same dislocation are neglected. As the structure of the head of a pile-up has been demonstrated to be not significantly affected by the free surfaces of the thin foil [START_REF] Saada | Pile-ups in thin foils: application to transmission electron microscopy analysis of short-range-order[END_REF], this aspect is also neglected in this paper in order to simplify the problem. These equations are valid for screw and edge segments with the condition to use the correct value of the constant A. The values of this constant are denoted A S for screw segments and A E for edge segments and are expressed as:

(3)

( )   (4)
Where  is the shear modulus,  is the Poisson's ratio with a value of about 0,33 and b is the Burgers vector with a value of 0.292 nm. As a consequence, the ratio between the spacing of screw segments and edge ones can be calculated to be only a function of :

From the data of Table 1, the average experimental values of this ratio under stress are calculated to be 0.64 in the prismatic plane and 0.57 in the basal plane. These values are sligthly lower than the theoretical value due to the effect of the core structure of screw segments that is not taken in account in the model used. Indeed, the core structure of screw segments acts as an intrinsic anchoring that impedes their motion and in turn increases the spacing between both screw segments of the pair. However, the theoretical and experimental values are quite close that validates the simple model used to represent the dislocation pair.

For pile-ups in Ni-based alloys, it was previously calculated that during an in situ TEM straining experiment, the resolved applied stress in the slip plane  a is nearly equal to the friction stress  SS . Consequently, the difference ( a - SS ) is about zero or has a very low value that can be neglected [START_REF] Saada | Pile-ups in thin foils: application to transmission electron microscopy analysis of short-range-order[END_REF]. The DAPB energy of each dislocation of the pile-up can thus be calculated from the elastic interaction forces due to all dislocations of the pile-up [START_REF] Saada | Pile-ups in thin foils: application to transmission electron microscopy analysis of short-range-order[END_REF]. In the specific case when the pile-up is only constituted of a unique pair of dislocations (Fig. 2 and Fig. 4 for example), the relationship ( a - SS ) = 0 can not be established because, in pile-ups, this relationship is calculated from the last dislocation of the pile-up that is assumed to move freely in the slip plane. However, in some rare cases, a third dislocation is observed to glide freely in the same plane of a pair with a distance until several micrometers from the pair. In turn, the difference ( a - SS ) can also be assumed to be near zero when the pile-up is constituted of a sole pair of dislocations. The equations ( 1) and ( 2) lead thus to conclude that  1 is almost equal to the elastic interaction forces between both dislocations of the pair and that  2 is equal to zero or has a very low value in comparison with  1 . In other words, that means SRO does not change any longer after the glide of two dislocations. As the value of  2 is sufficiently low, no further dislocations are needed in the pile-up to allow the first pair of dislocations to glide: single pairs of dislocations can thus be observed instead of pile-ups of several dislocations. Using the value of the spacing of edge dislocations in Table 2 (that is the same for both prismatic and basal planes), the value of  1 can be calculated as 11.6 mJ.m -2 , which corresponds to a stress  1 /b equivalent to 40 MPa. As the core structure of dislocations has a strong influence on the dislocation screw segments, it may affect their position and thus the determination of the value of  1 for screw dislocation. Indeed, the high lattice friction stress due to this core structure is not taken in account in the model used. To avoid this core structure effect, the value of  1 obtained only from edge dislocations has thus to be considered as the accurate value.

In Ni-based alloys, the value of  1 was measured to be about 30 mJ.m -2 [START_REF] Pettinari-Sturmel | Characterisation of short-range order using dislocations[END_REF][START_REF] Saada | Pile-ups in thin foils: application to transmission electron microscopy analysis of short-range-order[END_REF], i.e. more than twice the value in nodules of the Ti-6Al-4V. That is most probably the reason why single pairs can be observed in the Ti-6Al-4V alloy whereas only pile-ups with one pair at their head are observed in Ni-based alloys. This indicates also a low degree of SRO, and may explain why SRO is present in nodules and not in lamellar phases: the difference in chemical composition is very slight and induces a weak SRO.

Conclusion

As a summary, the presence of SRO in  P nodules of a Ti-6Al-4V alloy is evidenced by the formation of pile-ups or pairs of dislocations during in situ TEM straining experiments. It has been quantitatively evaluated to be weak and has been distinguished from  2 nanoprecipitates.

SRO is not present in lamellar colonies due to a probable partitioning of alloying elements that enriches the nodules in  stabilizing elements. These elements, aluminium and oxygen, promote the formation of SRO.

Dislocations are observed to be preferentially elongated along their screw direction as in lamellar colonies because their mobility is mainly controlled by their core structure, even with SRO. However, SRO affects significantly the behaviour of dislocations by:

-inhibiting the cross-slip;

-promoting the formation of pile-ups or pairs of dislocations (depending on the type of dislocation source operating in the nodule);

-reducing strongly the velocity of edge segments of paired dislocations.

From the measurement of the spacing between paired dislocations, the diffuse antiphase boundary (DAPB) energy  1 is evaluated to 11.6 mJ.m -2 . This DAPB energy is due to the fault created when a single dislocation glide in the short-range ordered lattice. The resistance caused by this fault is high enough to inhibit the motion of single dislocations: in turn, at least Table 1. Average spacing between paired dislocations, for screw and edge segments respectively, in prismatic and basal planes during in situ straining (with stress) and post mortem observations (without stress); the standard deviation is also indicated for each measurement.

During

Fig. 1 .

 1 Fig. 1. Low magnification TEM micrograph of the duplex microstructure consisting of primary alpha nodules  P and lamellar colonies  S /.

Fig. 2 .

 2 Fig. 2. Paired dislocations gliding in prismatic planes during in situ straining; a slip trace at the surface of the specimen is indicated by (tr P).

Fig. 3 .Fig. 4 .Fig. 5 .

 345 Fig. 3. Same grain as in Fig. 2 observed after in situ straining experiments when the stress is

Fig. 6 .

 6 Fig. 6. Planar prismatic slip during in situ straining with a dislocation source composed of a mobile dislocation segment rotating around an anchoring point (straight arrow); the circular arrow indicates the next displacement of the mobile dislocation.

Fig. 7 .

 7 Fig. 7. The two first dislocations of the pile-up of the Fig. 6 are paired.

Fig. 8 .

 8 Fig.8. Cross-slip from prismatic to first-order pyramidal plane; the trace of the initial prismatic slip plane is indicated by a dashed line and labelled "trace P"; the trace of the firstorder pyramidal plane is labelled "trace 1".

Fig. 9 .Fig. 10 .

 910 Fig. 9. Microstructure of macroscopically deformed specimens showing paired dislocations (a) and planar slip (b) in prismatic planes.

Fig. 1 .

 1 Fig. 1. Low magnification TEM micrograph of the duplex microstructure consisting of primary alpha nodules  P and lamellar colonies  S /.

Fig. 2 .

 2 Fig. 2. Paired dislocations gliding in prismatic planes during in situ straining; a slip trace at the surface of the specimen is indicated by (tr P).

Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .

 3456 Fig. 3. Same grain than in Fig. 2 observed after in situ straining experiments when the stress is removed.

Fig. 7 .

 7 Fig. 7. The two first dislocations of the pile-up of the Fig. 6 are paired.

Fig. 8 .

 8 Fig.8. Cross-slip from prismatic to first-order pyramidal plane; the trace of the initial prismatic slip plane is indicated by a dashed line and labelled "trace P"; the trace of the firstorder pyramidal plane is labelled "trace 1".

Fig. 9 .

 9 Fig. 9. Microstructure of macroscopically deformed specimens showing paired dislocations (a) and planar slip (b) in prismatic planes.

Fig. 10 .

 10 Fig. 10. Schematic representation of the forces exerted on both dislocations of a pair under an applied stress (the dislocations move from left to right).

Table 2 .

 2 in situ straining Post mortem observations Average velocities of paired dislocations and ratio between edge and screw segment velocities in both prismatic and basal planes.

		Average spacing	Standard deviation	Average spacing	Standard deviation
		(nm)	(nm)	(nm)	(nm)
	screw	45	21	45	15
	Prismatic plane
	edge	70	24	60	12
	screw	40	15	40	18
	Basal plane
	edge	70	19	60	23
		Edge velocity (nm.s -1 )	Screw velocity (nm.s-1)	Ratio
	Prismatic plane	5.5	2.4	2.3
	Basal plane	2.3	1.0	2.3

two dislocations, moving in a correlated way, are needed to overcome the resistance of this fault. SRO is thus identified to be an additional cause of the strength of Ti-6Al-4V alloys.


urbanization, expansion, animation of a territory

As a musician and an ethnologist, I am teaching in the University of Bordeaux. With a particular interest in Latin America, I started in 1992 a research on the popular singers who improvise their songs. That took me to the land of Nordeste, the northeastern region of Brazil. There, I found amateurs of improvised poetry : the cantadores and their admirers, gathered in nocturnal sessions called cantorias.

The passion for the word expressed in accordance with the rules of art is a cultural element shared throughout Latin America. Singing poetry and the tournament between poets are appreciated. But in Nordeste, people give supreme importance to the total improvisation of the sung verses. That is the art of the repente (the word means sudden), a local tradition whose vivacity is remarkable. Its actors are much more numerous than before.

They are called repentistas, cantadores or violeiros. Repentistas because they always renew their verses ; cantadores, they are popular artists capable of singing any subject proposed by the public ; violeiros, they accompany themselves with a viola, a kind of guitar emblematic of their profession.

Urbanization has boosted the evolution of the tradition of the repente. It appeared in the 19th century in a semi-arid and rural area of the interior of Nordeste, called Sertão. Droughts caused periodic exodus and the Nordestinos constituted the main contingent of migrant workers in the country. For half a century, they have participated in the industrialization and the construction of large cities. This rural exodus has depopulated the Sertão, whose itinerant poets of old were changed into emigrant poets, following their public towards the cities and adapting their art to the modernity.

The urban project needs social adjustment and cultural adaptation. I will expose the tradition of the repente, art of the word whose ethics is spontaneity and adaptation ; then, how improvising poets have conquered new production spaces, diversifying their activities ; finally, the reason for the development of a popular expression in the cities where cultural diversity and the pressure of modernity are real-life : the improvisers practice a form of animation combining creation and mediation.

Improvisation: a mobile tradition

The emergence of the improvised verse is stimulated by the encounter between poets and admirers in a night session. A protocol rules traditional singing : two singers play viola, sitting side by side at the foot of some wall, in front of the audience that often asks them for themes or styles. The poems are always produced by two authors alternating the verses. The poetic dialogue, its emulation and some rules (for example the obligation to retake in the first line the rhyme that the other has left at the end of his verse, rhyme that can not be predicted), everything guarantees the authenticity of improvisation. It is the essential value, before other formal criteria. In this way, the repentistas are distinguished from other poets of the Nordeste, the coquistas. These ones play a drum called pandeiro and usually mount a stanza mixing memorized elements and improvised passages. In addition to this genre called coco de embolada, I mention here other genres of oral poetry that are more local, such as : the coco de roda, that unites dance, percussions, choir and poetry in the coastal zones ; the maracatu rural, from Nazaré da Mata, State of Pernambuco, where improvised verses alternate with very fast music ; the aboio, linked to the breeding of cattle, whose poets sing without accompaniment, with some Arab or Turkish influence.

Compared to these very varied genres, the repente is reputed to carry the highest degree of poetic improvisation. Now I will characterize its music.

The music

Unlike the verses, the melody that sustains them is not improvised. The singers learn by ear many traditional tunes, and they use one of these in singing, with some personal variation. Thus, the melodies evolve gradually. New tunes are also created by derivation from the old ones. The poet who opens the song chooses in this vast repertoire the air that seems more convenient to sustain the theme to be developed. The poet who responds to him must repeat this air bearing a particular feeling. It is the constant support of the poem. Certain tunes are always associated with a peculiar poetic form, including a certain refrain. Sometimes, the singer outlines the chosen tune preluding on his viola.

The melodies are composed to sing a note per syllable, except the last one that can be decorated with vibrato or melisma. They are functional to sing a metrical poetry where the lines measure 7, 10 or 11 syllables. The minor mode is exceptional in the melodies, that have a strong modal character. The most frequent modes are our major scale and alterations of this one (the forth degree of the scale can be pitched a half up, or the seventh pitched a half down, or a combination of both). Often, the notes are not sung in a temperate way, that means there is a slight difference of tone between the sung and the corresponding note in the arm of the viola, divided into twelve equal halves as in a guitar. That is more noticeable when the singer comes from a rural area, since urbanization tends to normalize the modes. Anyways, the modes are very variable, but the structure remains : the tunes have to be a good support for the poetic forms, which also suffer constant evolution.

The viola playing is the indispensable element to improvisation : the repentistas say that their inspiration can not do without the sound of the strings. They carefully tune the two instruments, which have 7 to 10 strings of metal, some double or triple, grouped in 5 choirs. However, with the exception of some prelude or contrechant, the viola music is extremely monotonous. The whole session is accompanied by the swing of the fingers on a single major chord, barely adorned with some other notes. The constant character of harmony and rhythm is the unifying element, it allows the creation of the unpredictable by the improviser. At the same time, he has to memorize some of what the colleague has said, and think about what he is going to say in the next stanza, mounted in a few seconds. The instrument is the poet's tool in performance, because its music reconciles attention and concentration.

To illustrate that, I will talk about the songs that are not improvised and that mark a pause in the protocol of the improvisation session. At some point, a listener may request a composed song, usually nostalgic. It will be interpreted by one of the poets who will be accompanied then with diverse harmonies, majors and minors chords (three to six chords for an ordinary song). In short : for a well known song, the versatility of chords ; for the suspense of the poetic tournament, the monolithic character of harmony.

The musical aspects are important for the listeners, who have their preferences according to the vocal, rhythmic or scenic characteristics of the singer. However, for a singer, these aspects are not criteria determining his professional quality, which depends only on his mastery of poetry and language. Regarding the language, it is the Portuguese in use in Brazil, with high demands on the richness of vocabulary and the sound quality of the rhymes. Poets sing according to the pronunciation of their region of origin and can integrate regionalisms that are often indigenous words. But I am not aware of any improviser producing in any indigenous language of Brazil. Now we must talk about poetry.

The poetry

The singers perform their oral performances following what they call modalities. That's the structure that links text to music. They have almost fifty different modalities, in constant evolution : some fall into disuse, new ones emerge, generally invented in anonymity. The modality determines the metric, the length of the verses, the structure of the rhymes and sometimes a refrain or a forced melody. The most used is the sextilha, six-lines verse of seven syllables, with rhyme between the even lines. The décima is also widely used and considered as a masterpiece in poetic construction. There are two types of décima : one has ten lines of seven syllables, one ten lines of ten. To build his verse, the improviser has to find four different rhymes and organize them according to a complex structure, ABBAACCDDC. This is what Spaniards call décima espinela, from the name of Vicente Espinel, an Andalusian poet of the sixteenth century. It is still in use in improvised poetry by the Chilean, Argentine and Uruguayan payadores, or by Cuban trovadores. That means it is one poetic form common to Latin American countries. An important precision :when in Spanish they count 8 syllables in a line, in Portuguese they count 7 because the syllables are counted only until the last stressed one (which is necessarily followed by an unstressed one, or a silence). Likewise, the decasilábicos lines of Brazil correspond to the lines of 11 syllables of the Spanish speakers.

Often, the décima ends in a mote, a refrain of one or two lines that someone proposes to the two poets before they begin to improvise. One must memorize the mote to develop its theme, retake its rhymes in lines 6, 7 and 8, and repeat it at the end of the verse. Another modality with the same exigency is the galope whose peculiarity is a line of 11 syllables. It is used to sing bucolic themes. Other modalities present different demands, when the two improvisers must alternate the lines instead of the verses. For example, in a modality called quadrão perguntado, they must respond in a single line to what was asked in the previous line. In the modalities o cantador de você and gabinete, the singers can show their virtuosity repeating a long refrain.

The number of forms that can be heard during a cantoria session has no equivalent in other genres of oral poetry. To express themselves, poets undergo a demanding tradition that they move forwards, inventing new forms, with an objective of excellence. This care of diversification, in addition to an aesthetic particularity, is a test of flexibility for the cantador -a thought capable of moving within any frame is also capable of mobilizing in any circumstance. Thus the critical spirit is trained, which is the cantador's ethical requirement.

The topics

That ethic is revealed by listening to what is sung in the sessions. What is said before the public and how to say it is the major criterion for judging an improvisation. This criterion is called oração, the others being formal, called rima and métrica : a poet has to produce regular lines, without breaking the rhythm ; he also has to find many good rhymes. In a session, the listeners propose a subject, aloud or writing it on a piece of paper deposited on a tray that also collects their contribution. In the same way, a fan of poetry can propose a versified topic that will be the mote ending the stanzas. The singers have the obligation to satisfy the requests. They fulfill their role as popular artists by staging the listeners' lives. Each session is an exchange time : ideas circulate and feelings are shared. There is a permanent dialogue between the poets, and between them and their listeners. Thus, the topics are always renewed, through the public in the varied places that singers travel.

The subject field is very wide. It embraces : poetic art itself, singing the value of poets and the grief of the disappeared ; fun themes, the most appreciated being a challenge among singers ; love, happy or unhappy ; bucolic subjects ; existential or philosophical themes ; those that we can call ethical, moralistic or edifying ; lastly, social and political issues. A repentista must know how to sing any subject. That is possible by the preparation of improvisers. They refrain from using verses from memory, but learn many words to fecundate their imagination. They read books, dictionaries, newspapers or the Internet, listen to radio and television, and especially attend the performances of other repentistas. Of necessity, they learn of the social, political and newsworthy currentness, because they have to comment it. They are the scholars of popular culture.

Their knowledge legitimizes their satirical role. At a playful level, they can exchange insults in the cantoria, which is a theatre of rivalry. From a moral point of view, they have complete freedom of speech to speak of power and denounce its abuses and imperfections, in harsh words. The listeners take advantage of that to propose the most palpitating subjects, that the singers will treat with critical spirit. Several times I heard this opinion: "The singers say what the people would say if it could." Many poets consider themselves defenders of the people, at the forefront of social concerns. Many conceive of their art as a popular education movement and cantorias as a moment of emancipation of the spirits.

A conquering movement: new production spaces, new actors

This description of the cantoria was based on a traditional meeting said pé de parede (at the foot of a wall), as they usually organize in private house for some party, or in bar or restaurant with sharing of benefits between patron and singers. It would be very incomplete without mentioning the new production spaces. Since 1950, contests are organized by cultural associations where poets and apologists act to promote their art. These events take place in prestigious places of culture such as theatres, welcoming hundreds of people -a larger audience than in family, private or country sessions. Contests are competitions between duos : two collaborating singers are standing on the stage for a thirty minutes presentation, the viola slung over shoulders, next to a jury that imposes the issues and notes the duo, only according to the three criteria rima, métrica and oração. In front of the scene, the spectators encourage their favorites. The winners win trophies, a good cachet and increased fame in the repente community, which is highly competitive. The contests are very successful, every week happens at least one in some city of Brazil. Thus, opportunities to put the titles into play are frequent. These competitions, invented by the singers, contributed to the promotion of their art and to professionalize them -now the cachet payment tends to replace the collection on the tray, contracts are substituted for oral commitments. Tradition evolves in this way, thanks to the rivalry between poets who invent unprecedented circumstances to improvise.

This occurs on radio programs devoted to improvisation, produced and animated by poets. During the broadcast, they promote the upcoming events and improvise, despite the physical absence of the public that sends the requests by phone or mail. In the areas most affected by the rural exodus, broadcasting is a regular occasion to listen to repentistas. Several have learned the basics of their art in this way, before frequenting their first sessions.

Since the last years of the last century and the development of tourism on the coast, beaches are a new space for more humble poets. Without request, they sing a few stanzas addressing the tourists. They are criticized by their more famous colleagues, for walking away from tradition : they sing by day and do not satisfy demands. In these circumstances, in contrast to traditional sessions, there is little interaction, because on the beach they merely beg for money. But the said « beach poets » implanted an unprecedented form of improvisation in inauspicious spaces. These « poetas de praia » contribute to the expansion and visibility of their practice.

It is also worth mentioning the recent festivals in public square, in a context of amplified music scene and eclectic programming. There the singers have the opportunity to sing for thousands of people.

The diversification of the actors appears after an opening opportunity. About forty years ago, female poets have emerged in an exclusively male professional environment. The evolution towards the mix of social practices in Brazil was used by these women to speak up in assembly. They equate men in terms of artistic ability and critical spirit, and conform to their vocation as a popular poet saying everything. But it was not easy to emancipate themselves from the reserved attitude traditionally assigned to women. That may explain the small number of active women poets. On the other hand, the audience of singers is mixed ; there are women passionate about poetry who accompany the poets on their tours or do not miss a contest. However, during the research I have met only two female poets : they were witnessing to the difficulty that exists to overcome the prejudices, and were proud of the current evolution.

Record production is an important aspect of the activity of today's singers, who use their C.D. like a card to look for contracts. What is recorded is an oral work of a different nature, because it is difficult to improvise in a studio, without inspiration stimulated by requests of listeners. Then, it is not repente, but elaborate poems. Instead of writing the poems, the authors repeat them until they reach the best expression. Topics are chosen by the constituted duo and alternate with composite songs. Like a written and published poem, a C.D. leaves a trail, unlike the evanescence of an authentically improvised verse. Fans use this medium to hear sung poetry and compensate for the scarcity of events where they live.

Hence, today's singers estimate that they are thousands and well more numerous than their predecessors. The dynamism of the tradition accompanies a remarkable qualitative evolution : the majority of the cantadores of old were illiterate, lived in precarious conditions and cared little for musical, scenic or corporate criteria -that does not reduce anything their quality of great poets whose memory is alive in the tradition. The epigones work to rise socially, studying, promoting good shows, organizing their profession through associations, hand in hand with other defenders of popular culture.

The activism of the cantadores and their admirers, associated in the coproduction of contexts in order to fulfill and perpetuate the repente, leads to an appropriation of the spaces where they take the floor to mobilize the consciences.

Mobility, urbanization and expansion: animation of a territory

Without stopping, the cantadores go to meet new audiences, who are also very mobile by the characteristic emigration of the northeastern region. The climate of the semiarid Sertão, alternating a dry season and another favorable to the crop, causes seasonal migrations. There are also exoduses due to exceptional droughts. There are other demographic, economic and political reasons : the Nordeste has the worst social indexes in the country, and a negative migration balance with all other regions. A crowd of Nordestinos participated in the exploitation of rubber trees in Amazonia, in the industrialization of the south of the country and in the construction of its capitals, particularly São Paulo, the largest city in South America. They followed the general urbanization, major phenomenon of the last half century : in 1960, when the new futuristic capital Brasilia was built, for one Brazilian living in the city, four were living in the countryside. A few years ago the proportion was reversed. That led to a huge rural exodus in the Sertão, birthplace of the tradition.

The cantadores always moved a lot, on foot or mounted in case of the oldest ones that I have interviewed, born during the first world war. Nowadays, the mobility of the present ones is increasing, due to sociodemographic mutations. By necessity, followed their public, and for the most part urbanized. They took advantage of the modern means of transport, of communication, of information ; conquered the new spaces of expression already mentioned ; raised their school level and their knowledge of the world ; had to attend to more diversified requests from listeners confronting modernity. These new conditions of production stimulated the cantadores, since the capacity of adaptation is the spring of an improviser.

In a favorable context of rapid mutations, the poets say that they live a wonderful evolution, being more educated and organized than before. Thousands of poets today live from their popular art to the confines of the vast Brazil, wherever they can magnify the Portuguese language in their poems, wherever emigrants feel the need to organize a session where the culture of the northeastern terroir will be sung.

The social dynamics shown by the poets have no equivalent in the other genres of oral poetry in effect in Brazil. According to my hypothesis, these genres do not give as much importance to improvisation, its actors reiterate the forms instead of to recreate them, and therefore do not propel necessary adaptations to the tradition.

In my research I became interested in other categories of popular poets. For example to allow comparison, the coquistas testify that their number is declining, with the opportunities to sing being diminished. Yet, they sing coco, a funnier genre than the repente, characterized by fast tempo, catchy choruses and entertaining street shows. In their duels, coquistas use a very appreciated coarse langage and in the embolada, they exchange insults… But they improvise partially, memorizing picaresque verses that skillfully arm according to the context and the reactions. Often, they act animating open spaces (streets, squares, fairs), which are not conducive to talk sense. The listeners do not participate in the subject, because coquistas do not take orders. They deliver little informative or critical content, unlike what is heard in cantorias.

The two genres have similarities : it is a poetry sung in a popular context. But they differ in terms of meaning. When there is more improvisation, a space can be created where free and deep words can be exchanged. That is typical of the repentistas. I have shown that mobility of thinking is related to spatial mobility. It is also related to the desire for social transformation that a repentista have, wanting to be a spokesman for his Brazilian people.

In conclusion, the songs improvisers animate a territory in which they divulge cultural elements typical of the region where their art appeared. However, this territory where they update the tradition is without limit, due to the expansion of the repente by the roads of the northeastern emigration. Improvisers are mediators who promote adaptation strategies. They constantly produce and transform the elements of the culture of the people who listen to them.

Bibliography

  LODDO Daniel, « Cururu e Siriri, chants et musique des fêtes religieuses du Mato Grosso », in Musiques d'Amérique Latine. Cordes : CORDAE/LaTalvera, 1998, pp. 47-64 LODDO Daniel et ROUGIER Thierry, « Cantadores du Nordeste du Brésil : un art en devenir », in L'art des chansonniers : actes du colloque de Gaillac des 28, 29, 30 novembre 2003. Toulouse : Conservatoire Occitan, Centre des Musiques et Danses Traditionnelles Toulouse-Midi-Pyrénées ROUGIER Thierry, Les cantadores, poètes improvisateurs de la cantoria : une tradition en mouvement dans le Nordeste brésilien. PhD thesis in ethnology, under the direction of Sory Camara, Bordeaux 2, 2006, 333p Discography Repentistas nordestinos -troubadours actuels du Nordeste du Brésil. Cordes : CORDAE / La Talvera, 2006, 2 C.D. with an illustrated booklet of 96 p redacted by Daniel Loddo and Thierry Rougier Payadores de Chile -Poètes improvisateurs du Chili, Langon : Daquí, label des Nuits Atypiques. 2 C.D. with illustrated booklet -translation of poems and interviews, presentation text: Thierry Rougier. Thanks The article is based on an ethnomusicological survey conducted in Brazil by the association CORDAE CORDAE (Centre Occitan de Recherche, de Documentation et d'Animation Ethnographiques) BP 40, 23 Grand rue de l'Horloge, 81170 Cordes-sur-ciel, France tel : (33) 05 56 62 19 17 mail : talvera@talvera.org site : www.talvera.org director : Daniel LODDO


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