Mayerling Martinez

Gwendoline Fleurier

Frantisek Chmelik

Michal Knapek

Bernard Viguier

Eric Hug

Frantisek Chmelfk

Eric Huga

Charles L Fnlvm4

TEM analysis of the deformation microstructure of polycrystalline cobalt plastically strained in tension

Introduction

Deformation mechanisms in hexagonal close packed (hq,) metals are typically characterized by two contributions: dislocation glide and deformation twins. During plastic deformation, different work hard ening stages can be distinguished: at the beginning, hexagonal metals deform by gliding, and then the deformation is assisted by twinning. The dominant slip system in hep metals depends on the unit oells parameters (c/a ratio ) and on the values of the critical resolved shear stress for the different slip systems. Important role is played by the stacking fault energy ass ociated to basal, prismatic, and pyramidal planes, as well as by the elastic constants [START_REF] Legrand | Relation entre la structure electronique et la facilité de glissement dans les métaux hexagonaux compacts[END_REF]. For titanium, beryllium and zinc, the principal slip system is the prismatic { 1010 ) (1210 ) one, while for magnesium and cobalt it is the basal { 0001 ) (1120 ) one. Al though prismatic slip is the primary slip mode in zirconium and tita nium, both pyramidal and basal slip systems have been observed, as well. Basal slip dominates in magnesium, but prismatic system can be observed in single crystals and pyramidal slip systems were reported in literature as activated by elevated temperature or higher stresses. On the other hand, the only slip mode commonly observed in cobah is the basal slip [ 2 6 ] .

Because of the limited number of slip modes in hep lattioe, tw inning represents an important ahernative for plastic deformation. Twinning in the {10!2} planes is the predominant mode at room temperature and is operative in ail hep metals, because the twinning shear magnitude of titis mode is the lowest [START_REF] Partridge | The crystallography and deformation modes of hexagonal close-packed metals[END_REF]. Other twins, such as { l0il} and {10!3} have been observed in Mg or Mg alloys [ 7 9], while { 1122 } and{ltll} twins have been reported for titanium and zirconium [START_REF] Bozzolo | Misorientations induced by deformation twinning in titanium[END_REF][START_REF] Mccabe | Quantitative analysis of deformation twinning in zirconium[END_REF]. The shape of deformation twins is influenced by the twinning shear. When the shear is small, as is the case of { 1012 } twins, the twin has a wide lenticular shape, since the twin boundary can deviate from the twin ning plane without greatly increasing the twin interface energy. When the shear is large, the twins formed are narrow and have essentially parallel boundaries [START_REF] Partridge | The crystallography and deformation modes of hexagonal close-packed metals[END_REF].

Among all hep metals, cobalt and magnesium have the c/a ratio close to the ideal J8ff value. Cobalt has been much Jess studied and can be of particular interest because it exhibits a much higher melting point and very low stacking fault energy when compared with other hep metals [START_REF] Betteridge | The properties of metallic cobalt[END_REF]. literature regarding the general mechanical behavior of polycrystalline cobalt is scaroe and dates typically back to 1960s. Sorne studies were dealing with temperature effect in mechanical properties [START_REF] Hitzenberg | Electron microscopy of H.C.P. cobalt at various temperatures[END_REF] or deformation twinning [ 14 16 ] . Others works conoem the de formation of single crystals [START_REF] Korner | Weak-beam study of glide dislocations in h.c.p. cobalt[END_REF] or thin filins [START_REF] Dille | The structure and mechanical properties of thick cobalt electrodeposits[END_REF] . Studies dealing with straining of polycryst alline cobalt appeared recently, but those are Acoustic Emission (AE) has proven to be a useful diagnostic tool and many metallic materials, in particular magnesium single crystals [START_REF] Drozdenko | Acoustic emission study on the activity of slip and twin mechanisms during compression testing of magnesium single crystals[END_REF] and Mg based polycrystalline samples [START_REF] Vinogradov | Mechanisms of Plastic Deformation and Acoustic Emission in ZK60 Mg Alloy[END_REF], were extensively studied using this technique. As cobalt is crystallographically similar to mag nesium, the AE technique, widely used in Mg studies, was employed in this study to characterize and understand the deformation processes. As far as we know, it is the first study of Co by this technique. The AE response was monitored using a computer controlled PCI 2 device (Physical Acoustic Corporation, PAC) operating in the waveform streaming mode (the AE signal parameters, such as counts, amplitude or duration, as well as a complete, non parameterized AE signal are stored during the recording). The AE signal from the sensor was pre amplified with a gain of 60 dB. A broadband PAC transducer was mounted on the specimen by means of a wooden clip and a silicon grease.

Samples in different hardening stages were examined in a Transmission Electron Microscope (TEM). Conventional bright field images were made with a Jeol 2010 operating at 200 kV. Samples for TEM observations were prepared by mechanical polishing until about 100 μm thick and then electropolished by the twin jet method in a Struers Tenupol5, with an electrolyte consisting of 90% glacial acetic acid and of 10% perchloric acid at a voltage of 50 V.

Results

The initial microstructure of annealed polycrystalline cobalt is characterized by the presence of numerous stacking faults, some iso lated dislocations and dislocations grouped within small tangles (Fig. 2a). In the hcp lattice, three different types of stacking faults are possible, all lying in the basal plane: two intrinsic faults (I1 and I2) and one of extrinsic character (named E) [START_REF] Hitzenberg | Contrast analysis of intrinsic and extrinsic stacking faults in H.C.P. cobalt[END_REF]. The study of polycrystalline cobalt indicates that these defects are present as separated monolithic planar faults or as segmented stacking faults. These defects are present in cobalt also after deformation, as depicted in Fig. 2b,c. The contrast analysis performed on isolated stacking faults such as the ones shown in Fig. 2a indicates that they are I 1 type intrinsic stacking faults. A similar contrast analysis was attempted on segmented stacking faults, as ex emplified on Fig. 2b andc, with a + g/-g imaging of the same defect. These observations reveal that the fringe contrast is reversed upon each mainly related to nanocrystalline samples [START_REF] Barry | Microstructure and mechanical properties of bulk highly faulted fcc/hcp nanostructured cobalt microstructures[END_REF], thin films [START_REF] Marx | Strain-induced phase transformation of a thin Co film on flexible substrates[END_REF], and sometimes are focused exclusively on the study of twinning modes [ 21 25]. So far the only thorough report describing mechanical beha vior of polycrystalline cobalt was published by Sanderson [26]. This author shows by metallographic observations, and using replica tech niques, that basal dislocation glide is a dominant mechanism at the beginning of plastic deformation while for large strain deformation twinning plays a significant role.

The aim of the present study is to provide a global approach of deformation microstructures in polycrystalline cobalt using transmis sion electron microscopy, and to give an overview of plasticity me chanisms in hcp phase during different stages of work hardening. Moreover, acoustic emission was used in the course of the straining process, as it yields information on the dynamic processes, such as dislocation slip or twinning, involved in the plastic deformation. Materials and experimental methods are presented in the second part. Experimental results in the third part focus on the deformation me chanisms in gliding and twinning stages, respectively. A special em phasis is put on the study of twinning, including statistics of the dif ferent twinning modes. Finally, the fourth part of the paper presents a discussion of the microstructural observations and a statistic analysis of the twin mechanisms operative in cobalt.

Experimental

Materials

Rolled sheets of polycrystalline high purity cobalt (99.9 wt%), 0.5 mm thick, were used as a starting material. The as received material was annealed under secondary vacuum at 1100 °C for 1 h. The analysis by Electron Back Scatter Diffraction (EBSD) coupled to a Scanning Electron Microscope (SEM) indicates an average grain size of around 11 μm and presence of about 8.5% of untransformed face centered cubic phase (Fig. 1a). The EBSD analysis also shows the typical basal {0001} texture present in rolled hexagonal metals (Fig. 1b).

Mechanical and Microstructure Investigations

Monotonous tensile tests were performed at room temperature on dog bone shaped samples with a gauge section of 20 mm in length and 10 mm in width at the strain rate of 2 × 10 -4 s -1 . The tensile axis was set parallel to the rolling direction of the sheet. Strain measurements were achieved by a traditional clip on extensometer. The work crossing of the partial dislocations incorporated in the fault ribbon, see black/white outer fringes in A C and B D segments. Similar config uration was reported in [START_REF] Hitzenberg | Contrast analysis of intrinsic and extrinsic stacking faults in H.C.P. cobalt[END_REF], with a single partial crossing the stacking fault and leading to the same fringe contrast. The situation is somehow more complex here with the presence of three partial dislocations on the same fault ribbon, labelled a, b and c in Fig. 2b. Imaging of the defect with different diffraction vectors shows that partials a and c have the same Burgers vector, different from that of partial dislocation b. A detailed analysis of such defect configuration requires further work and the use of TEM image simulation.

Plasticity Mechanisms in Polycrystalline Cobalt

Three distinct stages can be distinguished from the true strain true strain curves (Fig. 3(a)), as shown also in our previous study [START_REF] Fleurier | Size effects and Hall-Petch relation in polycrystalline cobalt[END_REF]. The first one is related to the NonLinear Elasticity (NLE) of cobalt and is not studied here. The others two, labelled A and B in the following, represent the work hardening stages traditionally encountered in hcp structures. The deformation microstructure in these two stages was studied by TEM. Samples were deformed up to different strain levels: up to around 0.8% for samples in the stage A and up to around 4.1% for samples in the stage B. These levels are located in the middle of each deformation stage.

Strain hardening rate of the sample loaded in tension and the AE signal parameters recorded during the test are shown in Fig. 3(b) and (c). The AE signals appear already in the elastic stage of tension, sug gesting that purely elastic behavior is not present and microplasticity takes place already in the beginning of straining. This observation is in line with many studies on Mg based alloys and is typically attributed to a low value of critical resolved shear stress of basal slip in these ma terials [START_REF] Vinogradov | Mechanisms of Plastic Deformation and Acoustic Emission in ZK60 Mg Alloy[END_REF]. The parameters (counts and amplitudes) of the acoustic emission events exhibit their maxima at the onset of stage A of plastic deformation. This is caused by a massive dislocation motion and mul tiplication (which are known to be an efficient source of AE [START_REF] Heiple | Acoustic emission produced by deformation of metals and alloys-a review, part I and II[END_REF]). As a dislocations. Other type of arrangement frequently detected are arrays of dislocations leading to some polygonization of the grain structure (Fig. 4c), and more rarely dislocation dipoles. Dislocations in arrays as in Fig. 4c are not regularly spaced with some discontinuities inside the grain.

Dislocation arrays could be observed over quite long distance of several micrometers. An example of such a configuration 10 μm long is presented in Fig. 5. The array is lying on basal plane and includes three types of dislocations. The micrograph in Fig. 5a shows a part of this array in a quite thin area of the foil. This image shows the presence of two different dislocation systems identified by black and white arrows. The orientation of the grain, with a foil normal lying near a [0111] axis, allows imaging the feature using three different {0111} diffraction vec tors, and makes possible the identification of the three 〈a〉 disloca tions. This is done in a thicker area of the foil along the same array. For each particular {0111} type reflection, one set of 〈a〉 dislocation is out of contrast, as indicated by a black arrow in Fig. 5b d. Interaction is evidenced between two different basal 〈a〉 dislocations, the principal pile up is represented by 1/3 [1120] dislocations (white arrows in Fig. 5a), while the 1/3 [2110] dislocations exhibit a more extended distribution (black arrows in Fig. 5a). The interaction between these two groups of dislocations creates segments of 1/3 [1210] dislocations (Fig. 5c) leading to a more or less locally regular dislocation network.

Other less common glide systems, identified near grain boundaries, are illustrated in Fig. 6. The foil normal of the region examined in Fig. 6a andb is close to the [1120] axis, and the dislocations were again analyzed using the (1101) reflection. Fig. 6a shows two sets of 〈a〉 dislocations: one pile up corresponds to dislocations that glide from bottom right to top left of the image (black arrows, letter A), the other one (identified by white arrows, letter B) is lying in another plane. This latter micrograph was taken with a beam direction about 20°away from the [1120] axis. Fig. 6b represents the same area, the foil is inclined 5°f rom the [1120] axis at almost 90°from the basal plane. The dislocations B in Fig. 6b are extremely bent and their projected length is very short, indicating that they actually lie in the basal plane. On the other hand, dislocations from the pile up A are straight and their projected length consequence of strain hardening (i.e. due to the increasing number of sessile dislocation in the latter stages of deformation), the mean free path of mobile dislocations is reduced, resulting in a continual decrease in the AE activity. Owing to the very low stacking fault energy of cobalt, cross slip of dislocations is difficult and twinning processes most like dominate at higher strains (stage B). In the beginning of the stage B, the strain hardening rate exhibits a transition as it starts to decrease (with increasing stress) much slower compared with the stage A. Such a be havior suggests a change in the deformation mechanism from disloca tion glide to twinning. Moreover, the AE response in the stage B de creases significantly in terms of counts per event, whereas the amplitudes of the signals remain relatively high. This is another clear manifestation of twinning as the AE waveforms corresponding to twinning are known to be burst like short individual AE signals with a relatively high amplitude. A last stage appears just before the fracture, manifested by a wavy character of strain hardening rate in the end of the test. In addition, this stage is accompanied by the occurrence of more random AE events possessing a low number of counts and rela tively high amplitude, which is another signature of a breakdown of the sample.

It should be mentioned that since the deformation as well as the AE activity result from an interplay of different effects (e.g. there is typi cally no well defined point where one deformation mechanism stops and the other arises), we can, in particular, assess only which one dominates.

Deformation Mechanisms in the Stage A: Gliding Process

Microstructure analysis was performed in samples deformed to 0.8%, using different diffracting vectors, via bright field mode in two beam conditions. The Burgers vectors of the dislocations were char acterized using the standard contrast extinction rules. Transmission electron microscopy observations indicate that dislocation glide is the principal deformation mechanism activated in Stage A. Planar dis location structures, i.e. pile ups and single dislocations, were observed (Fig. 4a,b). Mobile dislocations were identified as ⟨a⟩ type on the image increased when compared to Fig. 6a, confirming that they are not lying in the basal plane, but rather in a prismatic plane. Fig. 6c andd show a smaller pile up located near a grain boundary, the foil normal in this sample is close to the [1213] axis. Both images were taken under {0111} type reflection, while foil is inclined at about 47°from the basal plane. In Fig. 6c, dislocations are clearly distinguished in the left part of the image, while in Fig. 6d the dislocations are edge on. A ste reographic analysis indicates that those dislocations may lie in the (1101) pyramidal plane. Such glide systems have not been so far re ported for polycrystalline cobalt. We conjecture that they are observed close to GB due to special stress state in such locations.

Deformation Mechanisms in the Stage B: Characterization of Twin Systems

The results concerning the microstructure of samples deformed to 4.1%, which is within the hardening stage B, reveal the presence of twins in the vast majority of grains observed as well as corresponding high density of dislocations. Twins exhibit variable size, ranging from hundreds of nanometres to a few micrometers (Fig. 7a). Some twins are very long and feature extremely flat interfaces. Others show lenticular shape and are comparatively smaller. Several twin variants are fre quently present in a single grain. Stacking faults are also sometimes depicted inside the twin as revealed by the diffraction patterns streaks. The matrix is highly deformed and presents a high density of slip bands, while twins seem to be superimposed onto such dislocation arrays (Fig. 7b). Plasticity in the stage B is mainly controlled by twinning and further discussion will focus principally on this deformation me chanism.

Twin systems in hcp metals have been reviewed, and it was shown that different twin types may operate depending on loading conditions and c/a ratio [START_REF] Partridge | The crystallography and deformation modes of hexagonal close-packed metals[END_REF]. Those systems are typically labelled by the twinning plane. Depending on the c/a ratio, a given twin system may induce either a contraction or dilatation along the c axis, which is called ten sion or compression twins, respectively [START_REF] Zhang | Deformation twinning in polycrystalline Co during room temperature dynamic plastic deformation[END_REF]. Identification of the dif ferent modes of twinning can be made by measuring the rotation angle with respect to the appropriate zone axis or by identifying the overlap of very precise axis as proposed by Partridge [START_REF] Partridge | The crystallography and deformation modes of hexagonal close-packed metals[END_REF]. As can be seen in Table 1, most of those twins can be identified by a given rotation of diffraction pattern about 〈 〉 1210 zone axis. Twin classification was based on a maximum deviation of ± 5°from the precisely know mis orientation angle given in Table 1. In order to measure the relative importance of each type of twinning we evaluated their occurrence. The analysis was made in around 50 grains, allowing the classification of hundreds of twins.

Fig. 8 shows the appearance of the most common twin type in polycrystalline cobalt in the present conditions, with an occurrence frequency of around 66%. The typical selected diffraction pattern used for the identification is also present. The twin and the matrix are related by an 86°rotation around the 〈 〉 1210 axis (Fig. 8c) so that this can be classified as {1012} twin. When this type of identification was not pos sible, because of the unfavourable orientation of the grain, the overlap of the [0001] T and [1100] M axes in the matrix and the twin was used to ensure the identification (Fig. 8d). The analysis of different grains in dicates that the morphology and size of this type of twin is highly variable. Fig. 8a shows a grain containing a twin of the same type and variant. Such twins are rather wide (1 3 μm), but {1012} twin can be relatively thin (0.1 μm) and of lenticular shape as depicted in Fig. 7b. Fig. 9 shows the morphology of other types of twins, which were always present when analysing the sample in TEM, but to a lesser extent when compared to the {1012} type. The second most frequent twin in cobalt is the {1011} twin, identified by a rotation of 56°about the 〈 〉 1210 axis (Fig. 10a). The morphology of this twin is always the same, they are small (< 1 μm) and narrow, with interfaces almost parallel to each other. Another single twin observed in cobalt is the {1013} twin, which is also very thin and narrow like the {1011} twin. In this case, the twin and the matrix are related by a 64°rotation around the 〈 〉 1210 axis (Fig. 10b). Stacking faults are evidenced in both, the twins and the matrix, but in the former case the density is much more important as revealed by the existence of streaks in the diffraction pattern corre sponding to the twin.

The last single twin that could be identified in the polycrystalline cobalt was the {1121} one (Fig. 9b), having a thickness of around 400 nm. Its interfaces are extremely flat, and it is very long (around 10 μm), sometimes occupying the whole grain. This twin is rotated 34°a round the 〈 〉 1100 axis (Fig. 10c) and is the only twin type encountered with such rotation axis.

The presence of double twinning was also evidenced in the stage B and is depicted in diffraction patterns in Fig. 11. For the sake of clarity, diffraction patterns were not indexed. Double twinning occurs in polycrystalline cobalt once a compressive twin is already formed. Basal plane first rotates by 56°around 〈 〉 1210 axis to form {1011} twin, then rotates again around the same axis by 86°to form the second order {1012} twin, resulting in the misorientation angle between the matrix and the secondary twin of 38°, as depicted in Fig. 11a. Double twinning mode -{1013} {1012} was also identified in the samples deformed in the stage B, with the corresponding diffraction pattern presented in Fig. 10b. Basal plane rotates 65°around the 〈 〉 1210 axis, and then 86°a round the same axis, again. Finally, the misorientation angle between the matrix and the secondary twin is 30°. These twins are thin and small and the interface between primary and secondary twins was not always easy to reveal.

Discussion

The present work gives a general overview of the deformation of pure cobalt using transmission electron microscopy. The discussion focuses on the new results concerning the mechanisms of plasticity in polycrystalline cobalt in each stage of work hardening, supported by a statistical analysis of the twin formation.

Deformation in the stage A is governed by the glide of dislocations, most of them with ⟨a⟩ Burgers vector. The predominant dislocation configuration correspond to ⟨a⟩ dislocations pile ups in the basal plane, some arrays of dislocations leading to polygonization of the grains are observed, as well. Dislocation dipoles are also present but to a lesser extent. Hence their influence on the deformation of polycrystalline cobalt is certainly limited. Other glide systems were also partly iden tified: ⟨a⟩ type dislocations not lying in the basal plane that are mainly located near the grain boundaries. The relative occurrence of prismatic and pyramidal (P1) system could not be quantified.

Deformation twins are the dominant deformation mechanism in the stage B. They are traditionally categorized as being either tension or contraction twins. For hcp cobalt with the c/a ratio below √ 3, an ex tension twin such as {1012} type usually forms when a tension strain is applied along the c axis, while a contraction twin such as {1011} often appears when a compression strain is applied along the c axis [START_REF] Partridge | The crystallography and deformation modes of hexagonal close-packed metals[END_REF].

Table 1

Rotation axis and angles used for identification of twinning and double twinning modes in cobalt.

During the deformation of polycrystalline cobalt, several modes of twinning including compression ({1011} and {1013}), tension ({1012}and{1121})), as well as double twinning were identified inside the same grain. In a great majority of examined grains, the predominant type is the {1012} mode, which is the most common mode in hcp metals, and it can be present together with other twin modes.

In order to make a statistical evaluation, 45 grains were analyzed, allowing for the identification of a hundred of twins. The occurrence frequency was estimated for each type of twin as the ration of the number of a given twin type over the total number of twins observed. The results indicate that tension twins of the {1012} mode are pre dominant (66%). The second most frequently occurring twin mode is A high density of planar faults is detected in {1011}, {1013}, and {1012} twins, as evidenced by streaks in different electron diffraction patterns {1011} type (16%). Different single or double twins are present in similar proportions in the range of (3 6)% (Table 2). After an extensive litera ture search, the only complete analysis of deformation twins in cobalt was published in recent paper [START_REF] Zhang | Deformation twinning in polycrystalline Co during room temperature dynamic plastic deformation[END_REF]. In this work a statistical study was made using a scanning electron microscope coupled with an electron backscattering diffraction system (SEM EBSD), which allowed for a more rigorous study of the samples. When comparing these results with the present observations they were found to be roughly similar. Zhang et al. [START_REF] Zhang | Deformation twinning in polycrystalline Co during room temperature dynamic plastic deformation[END_REF] reported that fraction of tension twins is very high, in particular it is the {1012} mode which reaches 73.1% in the samples obtained by dy namic plastic deformation, and 82.1% in the samples deformed by compression test. Our results complement those of Zhang et al. with a third mode of deformation (monotonous tensile experiments).

The difference in the frequency of occurrence of different twin modes, as seen in Table 2, can be partially explained when we consider the differences in the calculation methods. In the present, study the number of twins in considered rather than the length of twin bound aries. In the analysis of Zhang et al. [START_REF] Zhang | Deformation twinning in polycrystalline Co during room temperature dynamic plastic deformation[END_REF] the length of each twin boundary was automatically measured and compared with the others. In the present work, {1012} and {1121} twins are larger than the other twin modes. If calculation of the ratio is based on the length, the fre quency of occurrence will naturally increase. In the same way, the frequency of smaller twins (compression and double twins) will be re duced. In these cases, results will match better the calculations of Zhang et al. Finally, the {1122} mode was not detected in the present study, most likely because of the very low amount of such twins.

\\One has also to keep in mind that Zhang et al. performed com pressive loading tests. According to the texture orientation of their spe cimens, loading direction was in their case perpendicular to the c di rection of the crystal structure, in agreement with the finding that tension type (related to the c direction) twins are mainly activated. By contrast, in the present study, tensile testing was performed, with a load axis oriented between 50° and 60° off the c direction, according to the measured texture reported in Fig. 1. Despite the different geometry of mechanical loading with respect to crystal orientation, it appears that the relative occurrence of twin types obtained in this study is very similar to Zhang et al. [START_REF] Zhang | Deformation twinning in polycrystalline Co during room temperature dynamic plastic deformation[END_REF] results. This finding suggests that the grain orientation with respect to the applied load has only a limited effect in Co. This fact is supported by the observation of both compression and tension type twins in the same grain, as reported in the present study and in [START_REF] Zhang | Deformation twinning in polycrystalline Co during room temperature dynamic plastic deformation[END_REF]. Similar situation was also reported in other hcp metals [START_REF] Beyerlein | Statistical analyses of deformation twinning in magnesium[END_REF][START_REF] Lou | Non-Schmid-based {10 12} twinning behavior in polycrystalline magnesium alloy[END_REF].

Conclusion

The mechanisms of plastic deformation in hcp cobalt were revealed in this study using transmission electron microscopy (TEM) and acoustic emission measurements. Plastic deformation was found to be driven by dislocation glide in the first stage (A), whereas deformation twinning was the dominant mechanism in the latter stage (B). Microstructures in the two stages of work hardening have been characterized in detail using trans mission electron microscopy, by means of interrupted uniaxial tensile tests. Observed microstructures (i.e. dislocation slip systems and twinning systems) led to the following conclusions: Plasticity during stage A of work hardening is driven by planar and single glide of basal ⟨a⟩ dislocations. Twinning nucleation and growth are the most active deformation mechanisms during the stage B of work hardening. Compression, tension, as well as double twins are frequently observed within the same grain. Several twinning modes, including {1012}, {1011}, --{1013}, {1121}, {1011} {1012}, and {1013} {1012} , take place during deformation in the stage B of work hardening. A statistical analysis shows that {1012} twins are the most common twins in hcp polycrystalline cobalt. used for twin identification. According to Song and Gray [START_REF] Song | Structural interpretation of the nucleation and growth of deformation twins in Zr and Ti-II. Tem study of twin morphology and defect reactions during twinning[END_REF],who studied deformation twinning in Ti and Zr, this kind of stacking faults is formed by a misplacement of atoms in the shearing transformation during twinning. The configuration and structure of the observed stacking faults are different from intrinsic stacking faults, that are formed by dislocation dissociation or vacancy collapse. Zhang et al. [START_REF] Sun | Structural characterization of twin boundaries in deformed cobalt[END_REF][START_REF] Zhang | Non-equilibrium basal stacking faults in hexagonal close-packed metals[END_REF] also reported the presence of high density of stacking faults within {1012} and {1011} twins in cobalt, and related this feature to the migration of in coherent twin boundaries. Besides, atomistic simulation performed to understand the formation of this kind of stacking faults indicates that when the twin boundary is coherent or nearly coherent, no stacking faults are generated [START_REF] Zhang | Non-equilibrium basal stacking faults in hexagonal close-packed metals[END_REF]. Habit planes of twins analyzed in the present work are rather incoherent as explained in the following paragraph.

Wherever possible, the habit planes of {1012} twin were determined. Twin boundaries are rarely aligned along the theoretical crystal lographic plane. The deviation from the habit plane in cobalt and also magnesium [START_REF] Zhang | Twin boundaries showing very large deviations from the twinning plane[END_REF][START_REF] Sun | Structural characterization of {1013} twin boundaries in deformed cobalt[END_REF][START_REF] Sun | Structural characterization of twin boundaries in deformed cobalt[END_REF][START_REF] Tu | Structural characterization of {1012} twin tip in deformed magnesium alloy[END_REF] can be explained by the predominant atomic shuffling, which plays a prevailing role in the {1012} mode in compar ison with the shear processes. However, there is no consensus regarding these deviations. Ostapovets and Molnar [START_REF] Ostapovets | On the relationship between the "shuffling-dominated" and "shear-dominated" mechanisms for twinning in magnesium[END_REF] reported that these de viations can be caused by a deformation inhomogeneity produced by defects such as twinning dislocations (TDs) on the microscopic level. Gong et al. [START_REF] Gong | Interface structures and twinning mechanisms of twins in hexagonal metals[END_REF] analyzed different types and configurations of inter faces, concluding that for {1012} twins only one type of TD is present and the interface has accommodated large stresses, causing the twin to have a lenticular shape. Twinning boundaries for compression {1011} and {1013} modes are also incoherent. Sun et al. [START_REF] Sun | Structural characterization of {1013} twin boundaries in deformed cobalt[END_REF][START_REF] Sun | Structural characterization of twin boundaries in deformed cobalt[END_REF] found that twin boundaries are not straight and that the interfacial structures are composed of different types of facets.

Finally, the present observations also revealed the formation of two double twinning modes {1011} -{1012} and {1013} -{1012}. They are of lenticular shape and small in size. The frequency of their occurrence is low and very similar whatever the mode. Double twinning has been al ready observed in Mg alloys using various electron microscopy methods [7,39 41] and in Co by using the EBSD analysis [START_REF] Zhang | Deformation twinning in polycrystalline Co during room temperature dynamic plastic deformation[END_REF]. According to the analysis of Mg alloys, second generation twins do not follow the Schmid's law and are associated with the onset of the fracture.

Fig. 1 .

 1 Fig. 1. EBSD analysis of polycrystalline cobalt: (a) distribution of hcp and fcc phases and (b) corresponding pole figure showing the basal {0001} texture of the hcp phase.

Fig. 2 .

 2 Fig. 2. Stacking fault microstructure in polycrystalline cobalt: (a) General view showing stacking faults and dislocations in non-deformed sample (after annealing state); (b) and (c) some stacking faults are segmented by the incorporation of partial dislocations which induces a reversing of the fringe contrast, as revealed by the + g/ g imaging (observations made in deformed cobalt).

Fig. 3 .

 3 Fig. 3. (a) Typical work hardening curve for polycrystalline cobalt and corresponding tensile curve in insert. (b) Relative strain hardening rate θ/θ max and counts per event depicted by acoustic emission versus relative stress level σ/σ max . (c) Relative strain hardening rate θ/θ max and amplitude variation of acoustic emission versus relative stress level σ/σ max .

Fig. 4 .

 4 Fig. 4. Microstructure features of polycrystalline cobalt strained at 0.8% in the stage A: (a) dislocation pile-up, (b) single dislocations and (c) polygonization (polygonization walls are indicated by black arrows).

Fig. 5 .

 5 Fig. 5. Interaction between different basal 〈a〉 dislocations in polycrystalline cobalt strained at 0.8% in the stage A: (a) image of two sets of dislocations in a very thin region of the foil; (b), (c) and (d) represent another region of the pile-up showing the interaction between the two sets of dislocation system.

Fig. 6 .

 6 Fig. 6. Others slip systems in the stage A. Both regions where studied under the same type reflection, g = 0111. In (a) and (b) images where taken under the same diffraction vector but are inclined differently about [1120] axis. In (c) and (d) images where taken using different types of g = 0111 reflection.

Fig. 7 .

 7 Fig. 7. General microstructure of polycrystalline cobalt deformed in the Stage B showing: (a) twins of different sizes (b) slip bands.

Fig. 8 .

 8 Fig. 8. (a, b) TEM microstructure representing different size and morphologies of {1012} twin in cobalt; (b, d) typical dif- fraction patterns used for identification. Diffraction pattern of the matrix is represented by continuous line, and for the twin by dotted line.

Fig. 9 .

 9 Fig. 9. TEM image of the morphology of: (a) {1011} and (b) {1121} twins.

Fig. 10 .

 10 Fig. 10. Typical diffraction patters of: (a) {1011} twin, (b) {1013} twin and (c) {1121} twin. Diffraction pattern of the matrix is represented by continous line, and for the twin by dotted line.

Fig. 11 .

 11 Fig. 11. Typical diffraction patterns of double twinning in polycrystalline cobalt deformed at 4.1%. A schematic representation of lattice orientation relation is also present: (a) -{1011} {1021} twin and (b) -{1013} {1021} twin. Matrix is represented by solids lines, first rotation by green line, second rotation by red ones. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

  rate θ = dσ/dε was then extracted from the true stress (σ) true strain (ε) curves allowing the identification of strengthening stages. To analyse deformation mechanisms in different work hard ening domains, mechanical tests were interrupted in the different stages of strengthening.

• Correspondlng author at: Laboratoire CRlSMAT, Enslcaen, Université de Caen, CNRS, 6 Bd Maréchal Juin, 14050 Caen, France. E-mail ad<Fes:s: mayer ling.martinez @ ensicaen.fr (M. Martlnez). http:/ /dx.doi.org/10.1016/ j .matchar.2017 .09.038 hardening

Table 2

 2 Relative fractions (f) of twin number or twin boundary length for three distinct mechanical loadings ( a tension, b dynamic plastic deformation, c compression).

		Twin type	{1012}	{1121}	{1122}	{1011}	{1013}	│ {1011} {1012}	│ {1013} {1012}
	f of twin number (%)	T a (this study)	66	4	-	16	6	5	3
	f of twin boundary length (%)	DPD b [21]	73.1	10.4	0.8	2.3	11.6	0.7	1.1
		C c [21]	82.1	3.8	0.8	1.2	9.2	1.8	1.1


Enora Gabory
email: enora.gabory@gmail.com

Mathieu Chollet
email: mathieu.chollet@imt-atlantique.fr

Investigating the Influence of Sound Design for Inducing Anxiety in Virtual Public Speaking

Keywords: Human-centered computing → HCI public speaking anxiety, virtual reality, sound design, distraction

Virtual reality has demonstrated successful outcomes for treating social anxiety disorders, or helping to improve social skills. Some studies showed that various factors can impact the level of participants' anxiety during public speaking. However, the influence of sound design on this anxiety has been less investigated, and it is necessary to study the possible impacts that it can have. In this paper, we propose a model relating sound design concepts to presence and anxiety during virtual reality interactions, and present a protocol of a future experimental study aimed at investigating how sound design and in particular sound distractions can influence anxiety during public speaking simulations in virtual environments.

INTRODUCTION

Social anxiety can be a debilitating problem for many people in their everyday lives, in a society where social interactions are extremely frequent in a wide variety of contexts. In particular, public speaking is a very common situation both in our personal -such as giving a speech at a birthday -and professional lives -such as giving a presentation to clients. For lots of people, this exercise can be really stressful and may even constitute a consequence of a social anxiety disorder [START_REF] Ayelet Meron Ruscio | Social fears and social phobia in the USA: results from the National Comorbidity Survey Replication[END_REF]. In these cases, psychotherapy has proven to be a viable option to reduce the stress of public speaking tasks, and approaches such as Cognitive Behavioral Therapy (CBT) are potential treatments with demonstrated efficacy [START_REF] James | Cognitive-behavioral treatment of social phobia[END_REF]. In Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from permissions@acm.org. ICMI '20 Companion, October 25-29, 2020, Virtual event, Netherlands © 2020 Association for Computing Machinery. ACM ISBN 978-1-4503-8002-7/20/10. . . $15.00 https://doi.org/10.1145/3395035.3425227 CBT, a particularly useful tool in the treatment of anxiety is the exposure therapy (ET) (i.e. expose participants to stimuli to trigger their anxiety), used for several types of phobias, including public speaking anxiety [START_REF] James | Cognitive-behavioral treatment of social phobia[END_REF]. With the technological developments that have occurred in recent years in the field of virtual environments and virtual reality, developing virtual reality systems for ET -VRET systems -has become feasible [START_REF] Salehi | Virtual Reality Therapy for Social Phobia: A Scoping Review[END_REF]; commercial solutions have been created and a large amount of research has been devoted to assess their efficacy in comparison with traditional ET methods [START_REF] Thomas | Affective outcomes of virtual reality exposure therapy for anxiety and specific phobias: A meta-analysis[END_REF]. In the case of public speaking, whether it's for social anxiety exposure therapy, or simply for training communication skills, virtual reality has shown to be a valuable solution. Indeed, VRET to treat specific phobias was shown to lead to similar results as in vivo exposure therapy, with both solutions seeming to have a better efficiency than traditional methods relying on imagining the anxiety-provoking situations [START_REF] Russell A Mccann | Virtual reality exposure therapy for the treatment of anxiety disorders: an evaluation of research quality[END_REF][START_REF] Thomas | Affective outcomes of virtual reality exposure therapy for anxiety and specific phobias: A meta-analysis[END_REF]. Vanni et al. showed the efficiency of VRET in the particular case of public speaking [START_REF] Vanni | A survey on virtual environment applications to fear of public speaking[END_REF]. To better understand how to mitigate public speaking anxiety with VRET, and to create better adaptive virtual environments for therapy or for training, there is a need to explore in more detail which parameters of such VR simulations have an impact on inducing anxiety in order to fully understand virtual reality's potential for treating it [START_REF] Vanni | A survey on virtual environment applications to fear of public speaking[END_REF].

Immersive experiences activate all of our senses, and while the visual element predominates and has indeed received a lot of attention in the research community [START_REF] James | How immersive is enough? A meta-analysis of the effect of immersive technology on user presence[END_REF], the auditory element of a VR experience should also be considered. Some studies in the literature have investigated the influence of sound design on realism or on the sense of presence felt in virtual reality [START_REF] James | How immersive is enough? A meta-analysis of the effect of immersive technology on user presence[END_REF][START_REF] Larsson | Auditory-induced presence in mixed reality environments and related technology[END_REF][START_REF] Serafin | Sound design to enhance presence in photorealistic virtual reality[END_REF]. However, the impact of sound design on the anxiety triggered by a VR experience has not received much attention, particularly in the case of public speaking. We argue that sound design may have a large influence on the quality of a VR experience, and that therefore the possible impacts of sound design on VR tasks, including VRET, should be considered, for instance, how do unexpected sound distractions influence participants' experience and reduction in social anxiety.

In this paper, we first introduce related works in the use of VR for public speaking, and the influence of sound design on anxiety and performance. We then propose a conceptual model linking different concepts associated to the impact of sound design, and in section 4 we describe the experimental study that we will conduct to validate this model. Finally, we present future work in section 5.

STATE OF THE ART

Public speaking anxiety isn't the only subject in which VRET is used: we can see in the literature that VRET has been studied for various phobias, such as acrophobia [START_REF] Merel Krijn | Treatment of acrophobia in virtual reality: The role of immersion and presence[END_REF], agoraphobia [START_REF] Viaud-Delmon | High sensitivity to multisensory conflicts in agoraphobia exhibited by virtual reality[END_REF], fear of crime [START_REF] Toet | Effects of signals of disorder on fear of crime in real and virtual environments[END_REF], or social phobias [START_REF] Salehi | Virtual Reality Therapy for Social Phobia: A Scoping Review[END_REF].

Public Speaking in Virtual Reality

Early virtual reality public speaking simulations proposed to use virtual audiences in order to alleviate public speaking fears. North et al. showed the efficiency of using virtual audiences to reduce public speaking anxiety [START_REF] North | Virtual reality therapy: an effective treatment for the fear of public speaking[END_REF], and these results were confirmed by Harris et al. [START_REF] Sandra R Harris | Brief virtual reality therapy for public speaking anxiety[END_REF]. A study investigated the influence on anxiety of three different types of virtual audiences: negative, neutral and positive audiences. The authors showed that in three types of audiences conditions, participants felt a significant amount of anxiety [START_REF] Pertaub | An experiment on public speaking anxiety in response to three different types of virtual audience[END_REF].

More recently, some studies explored which parameters of virtual audiences impact the level of anxiety. Mostarejan et al., investigated the effects of the number of audience members. They showed that a size of three audience members, in comparison with audience of six and fifteen members, induced the highest increase in heart rate measures, which they linked to higher levels of anxiety [START_REF] Mostajeran | The Effects of Virtual Audience Size on Social Anxiety during Public Speaking[END_REF]. Another study by Biesman et al. investigated the effects of ingroup bias on the degree of induced anxiety. They didn't find differences in anxiety when comparing audiences designed to be perceived as ingroup or outgroup by the However, they showed that in the ingroup condition resulted to higher feelings of presence compared to the outgroup condition. Additionally, a higher level of emotional intelligence -the ability to recognize and understand emotions -predicted a lower level in public speaking anxiety in the outgroup condition, while initial predispositions to public speaking anxiety predicted the state of anxiety in the ingroup condition [START_REF] Biesmans | The Effects of Ingroup Bias on Public Speaking Anxiety in Virtual Reality[END_REF]. In a related topic of job interview simulations, Kwon et al. showed that better graphical realism, and the use of HMD or LCD screen over laptops, increased the degree of perceived anxiety [START_REF] Huem Kwon | How level of realism influences anxiety in virtual reality environments for a job interview[END_REF].

When considering which parameters impact the level of anxiety, an important factor in the uses of virtual environments is the feeling of presence, defined as the perceptual illusion of "being there" [START_REF] Kwan Min | Presence, explicated[END_REF]. This is particularly relevant for VRET, as it is assumed that high feels of presence would help to induce emotions or anxiety [START_REF] Merel Krijn | Virtual reality exposure therapy of anxiety disorders: A review[END_REF]. Lee divides presence in three psychological states: physical, i.e. experience virtual objects as actual physical objects, social, i.e. experience virtual social actors as actual social actors, and self presence, i.e. experience virtual self as the actual self [START_REF] Kwan Min | Presence, explicated[END_REF]. Cummings and Bailenson present in a meta-analysis the effect size of various parameters on the feeling of spatial presence [START_REF] James | How immersive is enough? A meta-analysis of the effect of immersive technology on user presence[END_REF]. Tracking level, stereoscopy and field of view have a larger effect on user presence, in comparison with others features such as the presence of sounds or the image quality. In our case, because public speaking is first and foremost a social interaction, the presence sub-concept of social presence is arguably the most important to consider. We can link this to results obtained by Kishimoto and Ding [START_REF] Kishimoto | The influences of virtual social feedback on social anxiety disorders[END_REF], who studied the influences of ambiguous and negative virtual social feedback on anxiety and presence. If no significant differences where found on feels of presence, participants with social anxiety disorders presented higher level of anxiety during public speaking in front of an audience displaying ambiguous feedback compared to an audience with negative feedback.

The Influence of Sound on Anxiety and Performance

As seen above, the feeling of presence influences the amount of anxiety felt by users. Cummings et al. showed that using sound in a virtual environment has a medium-sized effect on presence [START_REF] James | How immersive is enough? A meta-analysis of the effect of immersive technology on user presence[END_REF].

Serafin present two types of sound design: ambient sounds, which are background sounds, and sound events, which can be predictable or not, and used for triggering users. They showed that through the sole use of sound design to represent a panoramic viewpoint, with both types of sounds and without any visual information, it is possible to create a feeling of presence [START_REF] Serafin | Sound design to enhance presence in photorealistic virtual reality[END_REF]. In parallel, Larsson et al. showed that consistency in sound design is important: indeed, an inconsistent sound design, i.e. which is not realistic or seems unrelated to the environment, negatively impacts the feeling of presence. For example, sounds that do not match with the visual environment, such as sounds temporally desynchronized from their source, are unrealistic and reduce presence [START_REF] Larsson | Auditory-induced presence in mixed reality environments and related technology[END_REF].

In the field of virtual environments, video games are a valuable field of study; there, sound design represents a crucial part on how emotions can be induced in the player. Toprac and Abdel-Meguid studied the impact of sound, in survival horror games, on feelings of fear, anxiety and suspense. They observed that medium volume sound effects were the most effective way to induce anxiety in games [START_REF] Toprac | Causing fear, suspense, and anxiety using sound design in computer games[END_REF].

On the impact of sound on performance, some studies showed that noise distractions, such as white noise, emitted during doubletasks, which require a high level of attention, negatively impact the performance on the subsidiary task, and with no impacts on the primary task. More specifically, unpredictable distractions impact the performance more than predictable noise distractions [START_REF] Jay | Reappraisal of the relationship between noise and human performance by means of a subsidiary task measure[END_REF]. Equivalent results are observed on subjects' performance on a complex task also requiring a high level of attention [START_REF] Nagar | Affect and Performance on Cognitive Task as a Function of Crowding and Noise[END_REF]. The effects of sound distractions on subsidiary task or complex tasks were also investigated by Hong. While attending to a primary task, participants had to respond to a sound distraction as a subsidiary task. Hong showed that performance on the primary task declined when participants where confronted to sound distractions [START_REF] Hong | TO ERR IS HUMAN: THE EFFECTS OF DISTRACTION ON ERRORS IN SKILLED PERFORMANCE[END_REF].

We can see that sounds have different impacts, particularly on the feeling of presence as well as on the performance on complex tasks. However, it seems that the influence of sound design on participant anxiety in virtual environments hasn't really been explored yet, particularly during public speaking task. Based on the existing research presented above, we propose a model about the influence of sound design on presence and anxiety during public speaking in virtual reality.

CONCEPTUAL MODEL

The main idea behind this model is to map out how various factors of a virtual experience's sound design combine in a way that ends up impacting the level of anxiety. Based on the literature review presented in the previous section, we have outlined this model in Figure ??. In this figure, the relationships between different subjective factors and virtual reality parameters are outlined by arrows: the majority of these links have been investigated in previous studies (although in different contexts than public speaking simulations) and are represented by bold arrows. In contrast, the dotted line between auditory distractions and anxiety corresponds to an hypothesis we formulate and that we will attempt to verify in an experiment (see 4.1).

Because of the impact of distractions on performance when tasks require a lot of attention -and we assume here it is the case of public speaking tasks, where we need to know what to say, remember the order of the presentation, speak and adopt appropriate nonverbal behaviors, all the while adapting to the audience and to the environment -we suppose that distractions will have a negative impact not just on performance but also on anxiety, both being potentially linked (e.g. a loss of performance might trigger anxiety).

EXPERIMENTAL STUDY

In order to validate the above model, we are planning to run an experimental study where we will explore the impact of sound design and in particular sound distractions on the level of anxiety and on the level of presence during public speaking in virtual reality. To that end, we will immerse participants in a virtual reality environment, consisting of a virtual amphitheater populated by a virtual audience, and containing different sound distractions. In the following subsections, we present our research hypotheses, the virtual environment we use and its visual and auditory stimuli, our experimental protocol, and the measures we plan to obtain.

Participants will be recruited among students at our institution, at a particular point in their curriculum where they have to prepare an oral presentation on an internship done during the summer. Participants will be requested to perform a public speaking presentation in front of a virtual audience provides through a HTC Vive Pro Eye head-mounted display (HMD) with an integrated audio headset and eye-tracking system. Different recordings will be obtained, using a variety of sensors: a camera and microphone will be placed in front of the participants to obtain a general view of their performance. Eye-tracking data will be collected using the HMD integrated system, and an Empatica E4 wristband will be used to record heart rate and electrodermal activity.

Our study will follow a mixed experimental design: each participant will have their presentation divided in three parts, corresponding to three experimental conditions of sound distractions, selected in a random order. Additionally, participants will be randomly assigned between two groups, corresponding to one of the two conditions of realistic or unrealistic sound design, the second one obtained by a desynchronization of sounds with visual cues. Also, in preparation of the experiment, a pilot study will be carried out in order to validate the realism of the sound environment.

Hypotheses

We want to see if sound design, and specifically sounds distractions, are a potential factor influencing the level of anxiety and presence during a public speaking task in virtual reality. The future experiment will try to validate these three hypotheses: H1 A continuous sound distraction increases the level of anxiety at the begin of the diffusion of the sound. This level of anxiety decreases after a while due to an habituation effect. H2 Random variations of the volume of a sound creates an increase in the level of anxiety. H3 An unrealistic sound design negatively impacts the feeling of presence. H3b An unrealistic sound design impacts the level of anxiety.

In the case of H3b, we envision two possibilities of the impact that an unrealistic sound design can have on anxiety. The first possible impact is that an unrealistic sound design, by impacting negatively the feeling of presence, will reduce participants' involvement and therefore reduce anxiety. Secondly, because unrealistic sounds might be uncomfortable or disturbing, it might directly increase anxiety. It is difficult to predict which of these two impacts will happen, and if so, whether one effect will dominate.

These hypotheses will confirm the validity of our model specifically in the context of public speaking. We will measure the level of anxiety with physiological and self-reported measurements, and presence with self-reported measurements.

Virtual environment and stimuli

We use in this work the virtual environment (VE) Cicero, developed using the VHToolkit framework based on the Unity 3D engine [START_REF] Chollet | Manipulating the perception of virtual audiences using crowdsourced behaviors[END_REF]. This system was used in a series of studies realised at the USC Institute for Creative Technologies. Cicero contains a virtual amphitheater where a virtual audience can be spawned and configured with various parameters. In particular, the audience can be configured to display a global attitude towards the speaker by varying arousal and valence parameters. The parameters then impact nonverbal behaviors displayed by the audience members. To create this audience behavior model which generates behaviors from various levels of valence and arousal, a crowdsourcing platform was used, were participants had to select behaviors, i.e. head movements, gaze patterns, facial expressions and postures, corresponding to a particular level of valence and arousal, varying between very low, low, neutral, high and very high [START_REF] Chollet | Manipulating the perception of virtual audiences using crowdsourced behaviors[END_REF]. In our study, we will set arousal and valence parameters to fixed values in order for the audience behaviors to stay stable during the experience so that they do not interference on the level of presence and anxiety. To have an audience which looks interested by the presentation, the level of arousal will be high. The valence will be globally neutral but distributed with different values for each audience members, in order to have a bigger variety of behaviors while avoiding the impact of an audience that would be globally perceived as too positive or too negative. We integrate four audience members in the environment, on the basis of the study of Mosterajan et al. [START_REF] Mostajeran | The Effects of Virtual Audience Size on Social Anxiety during Public Speaking[END_REF], and in order to have the same number of women and men in the audience, with different ethnicities, in order to avoid potential ingroup/outgroup biases [START_REF] Biesmans | The Effects of Ingroup Bias on Public Speaking Anxiety in Virtual Reality[END_REF].

In order to conduct our experiment and test our hypotheses, we added sound design in the virtual environment. Ambient sounds, including sighs, interjections and seat creaking noises were added, and associated with different behaviors of audience members. We also chose simple background sounds, played at a low volume, which we assume should only impact the feeling of presence, not anxiety. We created two variations of these various sounds, namely an unrealistic and a realistic condition, altering the particular timing of these sounds with corresponding visual cues. On the basis of the work of Larsson et al. [START_REF] Larsson | Auditory-induced presence in mixed reality environments and related technology[END_REF], we desynchronize these sounds from the corresponding visual informations to obtain the unrealistic condition of the experiment, and keep them synchronized for the realistic condition. Finally, we include a sound distraction in the form of a sound of a group of people chatting, spatialized in a side room in the VE. We consider that this sound can be particularly impactful to study subjects compared to other types of distractions (e.g. alarms, environmental sounds) because of its social dimension, which could be especially disturbing in the case of users suffering from public speaking anxiety. The volume of this group chat sound is managed in different ways depending of the experimental conditions, in order to create continuous or random distraction conditions, in opposition with a condition without any sound distraction.

Protocol

At the beginning of the experiment, the participant will first have to fill in a number of questionnaires, and will then be equipped with the Empatica E4 wristband and the HMD. Before the experiment begins, the participant will first discover the virtual environment in a habituation sequence, consisting of the same virtual environment but without sounds and without the virtual audience. This will allows us to obtain a reference of the initial physiological state of the participant before the public speaking task. The experiment will consist in three steps of public speaking of five minutes, with short breaks between these steps. Each step corresponds randomly to one condition of the sound distraction variable: no distraction, continuous distraction, or random distractions. Additionally, the sound is configured in a unrealistic or realistic condition depending on the group condition to which the participant is assigned. Participants will be asked to prepare a presentation of a duration around twenty minutes -to make sure to have a presentation of at least fifteen minutes -and to give the slideshow of their oral presentation, to integrate it in the VE -the experimentation comes during a period of oral exams for students in which internship presentations are mandatory; we will simply ask students to reuse those presentations. After five minutes of public speaking, we will interrupt the participants, ask them questions related to their previous condition, set the condition for the next presentation step and then continue for the next step of public speaking. These breaks will help us to separate recordings of the different conditions. Once their presentations are finished, participants will fill in another set of questionnaires in order to obtain measures of their subjective level of anxiety during the three steps, their feeling of presence, and a post-test questionnaire related to their perceptions of distractions and their subjective impression of those distractions' impact on their anxiety.

Materials and Measurements

To obtain physiological measurements, we will use an Empatica E4 wristband, which provides electrodermal activity (EDA) and a heart rate (HR) signal from which we can calculate heart rate variability (HRV), and the eye-tracker integrated to the HMD system.

EDA corresponds to changes in electrical properties of the skin, which reflect physiological arousal. HRV is a measurement of the variability in the time intervals between heart beats, and is related to emotional stability. Eye-tracking data indicates where the subject is looking, which will enable us to know if the participant looks at the source of the sound distraction, and when.

Concerning self-reported data, participants will have to fill at the beginning of the experiment, a demographic questionnaire, the Personal Report of Confidence as a Speaker (PRCS) [START_REF] Gordon | Insight vs. desensitization in psychotherapy: An experiment in anxiety reduction[END_REF], which allows us to get a measure of the participants' public speaking anxiety levels, and an immersive tendencies questionnaire (ITQ) [START_REF] Bob | Measuring presence in virtual environments: A presence questionnaire[END_REF]. Between steps of the experiment, they will have to fill the PRCS in order to have intermediate measures of their anxiety. Finally, at the end of the experiments, they will have to fill a presence questionnaire (PQ) [START_REF] Bob | Measuring presence in virtual environments: A presence questionnaire[END_REF], and a post-test questionnaire to have measurements of their subjective perception of sound distraction and the impact of it on their anxiety.

We will use also a camera and microphone, in order to realise an analysis of the participant' non-verbal behavior, in particular postures, gestures, and vocal behavior.

FUTURE WORK

The experiment we have presented is still in preparation, and therefore our most important task is to finalise the study preparation and subsequently to recruit participants to run it. Once the study has been run and data has been collected, our next step will consist in realising statistical analyses, testing for differences between the various presented conditions in terms of physiological features and subjective presence and subjective anxiety scores, in order to validate our hypotheses. This study's main goal is exploratory, as sound design in VRET is still a largely unstudied field. We intend to develop further our model of sound design parameters influences on the level of presence and anxiety during simulated public speaking, with the end goal of improving simulations and hopefully social anxiety treatment or social skills training outcomes. While this study focuses on sound distractions, a next step can be to improve the sound design by considering elements such as the signal-to-noise ratio, the acoustic properties of the room and other sound properties. A potential follow-up study would be more devoted to studying the influence on sounds conveying social cues, such as backchannels or disagreement sounds, and how those can be linked with audience non-verbal behaviors.

ACKNOWLEDGMENTS

This material is based upon work supported by the RFI Atlanstic 2020.