Richard Kitney

Michael Adeogun

Yoshiyuki Fujishima

Ángel Goñi-Moreno

Richard Johnson

Mary Maxon

Scott Steedman

Sean Ward

David Winickoff

Jim Philp

Enabling the Advanced Bioeconomy through Public Policy Supporting Biofoundries and Engineering Biology

Keywords:

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

The development of more than 50 published national bioeconomy strategies, including those from all G7 nations, reflects significant differences in their approaches. Most, however, stress the importance of biomass as a feedstock in future manufacturing. With some notable exceptions, many of these strategies lack detailed information about the enabling roles of biotechnology in delivering their envisioned bioeconomy. The reasons include both technical and societal challenges. This paper suggests the need for identifying key technical challenges and addressing them in public policy. In particular, it focuses on two key challenges that are inter-related: a lack of standards and a lack of reproducibility and reliability.

What are the approaches in public policy?

Central is the issue of complexity in biological systems. Small changes in the cellular or environmental context of a biological system have large implications for reproducibility in experimentation [START_REF] Plant | Improved reproducibility by assuring confidence in measurements in biomedical research[END_REF]. The engineering approach of synthetic or engineering biology makes this a particularly pertinent challenge as modern manufacturing and engineering rely on quantitative precision. To align with the rationale of the engineering design cycle, each new iteration of 'design-build-test" must be based on previous observations and data (Figure 1). But even small changes in cellular or environmental context, often caused by additional undefined layers of control [START_REF] Lechner | The need for integrated approaches in metabolic engineering[END_REF], may undermine learning from earlier iterations and developing reliable and replicable designs and construction with each new cycle.

Government research funding responds to the complexity challenge through multiple approaches. The wide-ranging approaches to public funding include: greater integration of artificial intelligence and machine learning, the need for changes to undergraduate and postgraduate education [START_REF] Delebecque | Education and training for industrial biotechnology and engineering biology[END_REF], even citizen science, and specific funding of contributing factors such as robustness and the need for minimal cells (see [START_REF]Meeting Policy Challenges for a Sustainable Bioeconomy[END_REF]).

At the same time, the burgeoning private sector interest in engineering biology to make bio-based medicines, fuels, chemicals and materials (such as plastics and textiles), has spurred new business models and investments, including the formation of numerous start-ups. But most start-ups lack the resources for fundamental research and access to sophisticated R&D infrastructure. And larger biotechnology or chemical companies may lack sufficient incentives to address these fundamental issues in the face of policy uncertainty and adequate incentives for creating new or enhanced markets for bio-based goods and services.

Public-private partnerships are needed to focus directly on solving problems of reproducibility, reliability and predictability. They also can help create business confidence in public sector commitments to transitioning to new types of manufacturing in the bioeconomy and overcome current barriers for realizing the significant promise of engineering biology.

What is a standard?

Standards are important tools to accelerate innovation. A standard is a consensus of good practice, preferably developed with full stakeholder engagement, open public consultation and independent governance, capturing the distilled wisdom of experts in the field. Standards (norms, codes) may describe the technical specifications of a specific process or product, support companies to improve the quality of their operations and business processes, or set out high level principles or frameworks. Standards underpin trade, accelerate routes to market, deliver competitive advantage and enhance consumer, investor and regulator confidence. According to research for the British Standards Institution (BSI), a large proportion of annual productivity growth can be attributed to standards [START_REF] Bsi | The Economic Contribution of Standards to the UK Economy, British Standards Institution[END_REF]. Industry experts participate in standards development as a strategic tool to influence best practice and enhance reputation.

Standards in various forms are needed in engineering biology. Technical standards of high priority are those that automate methods, the description and assembly of components, and documentation of the performance of engineered bacterial strains [START_REF] Hayden | Synthetic biology called to order. Meeting launches effort to develop standards for fast-moving field[END_REF]. De Lorenzo and Schmidt [START_REF] De Lorenzo | Biological standards for the knowledge-based bioeconomy: What is at stake[END_REF] argue that the adoption of standards will accelerate the transition to a future advanced bioeconomy. Additionally standards are required that enable companies to control their management systems or novel processes and show that working with them does not introduce excessive risks.

A reproducibility and reliability crisis ?

The lack of reproducibility in pre-clinical research has an estimated cost of USD 28 billion per annum in the United States alone [START_REF] Freedman | The economics of reproducibility in preclinical research[END_REF] (with caveats due to difficulties in obtaining data). These challenges extend to engineering biology, too. They include human error/variations, machine variations, biological variations and sensitivity, material variations, quantitative precision in models and measurements, communication of methods, and a lack of standards. Automation and robotics offer ways to overcome the challenges by removing human variation and 'craft', by allowing a much greater level of rational design, high-throughput in experiments, improving statistical analysis, and allowing programming with standardized protocols (see Figure 1).

Computer-aided biology

In computer science, high-level programming languages allow non-experts to programme and create systems as they distance the user from machine code, making them more or less independent. This higher level of abstraction enhances their ease of use. Another key goal of the current revolution in engineering biology is to make biology easier to engineer, bringing expert knowledge to the non-expert and making complex problems tractable. The core engineering principles of abstraction, modularity, and standardization increasingly enable workable solutions as sub-problems of design become routine.

The current biology approach of 'English language representation of biological processes' plus 'manual execution' plus 'artisanal insight' is not fit for purpose. Computer-aided biology (CAB) offers a promising approach that is more fit for purpose and can be represented by: 'Digital representation of biological processes' plus 'connected automation' plus 'active learning'.

CAB needs several elements in order to deliver a better approach: a high-level language for biology, libraries of optimised protocols (building blocks), interoperability across equipment, a simple and powerful user interface (UI), and compatibility with the other elements of CAB.

Foremost among emerging computing standards is the Synthetic Biology Open Language (SBOL) which provides a data format for the representation of genetic designs to a level of detail previously unattained [START_REF] Appleton | Design automation in synthetic biology[END_REF] and finds growing support among design software tools. Antha, developed by Synthace of London, represents an innovative approach for creating a programming language for general-purpose computation in biology [START_REF] Sadowski | Harnessing QbD, programming languages, and automation for reproducible biology[END_REF]. It embraces the departure from one-factor-at-a-time (OFAT) by enabling detection of interactions between different experimental factors.

How to advance both public and private sector needs ?

One commentator [START_REF] Burgard | Development of a commercial scale process for production of 1,4-butanediol from sugar[END_REF] has suggested that what is needed is "an integrated technology platform encompassing metabolic modeling, high-throughput (HT) pathway and strain construction, quantitative small scale screening, and systems biology, all of which are intimately linked to fermentation and process engineering". This aligns with the view of Lee and Kim [START_REF] Lee | Systems strategies for developing industrial microbial strains[END_REF] that "researchers often fail to consider a fully integrated industrial bioprocess when developing microbial strains with new activities". In other words, there is no one technology at fault, but it is the failure to integrate several technologies that defeats commercialization. The public biofoundry is one such facility where public and private sector needs intersect (Box 1), but there are very few of these world-wide. How these would interact with productionscale biorefineries to integrate an entire bioprocess is shown in Figure 1.

Policy makers, however, have been slow to embrace the critical role of public or public-private biofoundries. Recent experience at the MIT-Broad biofoundry underscores the benefits from biofoundries. Faced with uncertainties about the technology, this biofoundry was pressure-tested by the government funder to build organisms to produce ten molecules in three months. It produced the desired molecule or a closely related one for six out of ten targets and made advances toward production of the others [START_REF] Casini | A pressure test to make 10 molecules in 90 days: external evaluation of methods to engineer biology[END_REF]. This deserves close scrutiny in the range of chemicals involved, the range of production strains and the establishment of two cell-free systems.

Policy options in summary

As governments wrestle with budgets and prioritisations, there are many policy options to support engineering biology as an integral part of a bioeconomy. We argue that getting more success stories in engineering biology can be accelerated through policy. Important goals are working with experts in national and international standards organizations, researchers and the private sector to develop the necessary standards, technical and otherwise, in a cohesive manner; de-risking private sector investments in biofoundries through public-private initiatives; supporting cross-disciplinary research and education to embed computer-aided biology; and re-visiting engineering biology approaches that might provide the breakthrough to cost-effective lignocellulose conversion, e.g. consolidated bioprocessing (CBP).

Concluding remarks: a missing link

Due to foundations laid down in previous decades, biology, quite suddenly, finds itself in a data-rich era, and is in need of infrastructures to reflect this sudden change. Bioeconomy policy support, in a global context, has helped to establish new value chains, such as those associated with biorefineries in some countries. Large-scale biorefineries, however, only provide certain types of benefits and many have failed to live up to their original expectations, especially cellulosic biorefineries. From the farm and forest to finished products and end-of-life, the biofoundry may be the important missing link, technologically and in policy. This paper attempts to draw needs to the attention of policy makers. Policy must also give the private sector confidence that governments realise that the era of bio-based production is upon us and biofoundries are essential to realizing the promise of engineering biology in addressing pressing societal challenges and creating new economic value.

Disclaimer statement. The views expressed are those of the authors and not necessarily those of the OECD or of the governments of its member countries.

Acknowledgements. This manuscript has its basis in a joint Imperial College/OECD workshop entitled "Engineering biology: from the lab to products", held at Imperial College, London, on September 21, 2018. The work was partly funded by a SynbiCITE grant from the Engineering and Physical Science Research Council (EPSRC), UK, grant number EP/M006700/1.

Figure 1. Automating the design-build-test iteration cycle for engineering biology in the biofoundry.

Engineering biology seeks to increase reproducibility to enable the quantitative precision required for modern manufacturing. Standards, automation and machine learning are key to the success of this approach. It is an approach applicable to both research and industrial production. "Scale-down" refers to acquiring data at production scale and transferring the information back to the laboratory via scale-down simulators. If need be, the production strain can be re-engineered and/or new information is back-translated to the fermentation operation for its fine-tuning. The approach, however, is inhibited by a lack of rigorous modelling and computation. Additionally, the transition to multi-thousand litre bioreactor processes alters conditions greatly from those of the laboratory e.g. oxygen concentration gradients, changes in pH, shear forces on cells, even impurities in lower-cost culture media. Industrial scale production has its own specific and potentially expensive requirements which can be addressed by biofoundry operations at a distance from a production site. A feature of the biofoundry approach consistent with modern manufacturing is that the site of the design (the biofoundry) can be totally separated from the site of manufacturing (typically the biorefinery).

Box 1. Infrastructures to democratize engineering biology for an advanced manufacturing bioeconomy

Most molecular biology laboratories have historically resisted automation. There are several possible reasons. First of all, scaleability, as in industrial scaleability, has seldom been a requirement in biology.

Another is that problem solving in the biology wet laboratory is not necessarily applicable to automation, and thus the typical molecular biologist is not taught and trained with automation in mind. Married to the complexity of biology, it can be conjectured that combinations of such factors lead to the irreproducibility often discussed in biology and biotechnology.

Automation may also be perceived as expensive, although this is not necessarily the case. McClymont and Freemont [START_REF] Mcclymont | With all due respect to Maholo, lab automation isn't anthropomorphic[END_REF] argued that reproducible research can be enabled by individual laboratories that have inhouse, low-cost automation work cells but which can access more centralised biofoundries via cloud computing to carry out more complex experimental workflows. In this way, some of the drudgery of routine pipetting can be removed, allowing researchers to concentrate more on experimental design as the machines assume the task of undertaking experiments. A cost-benefit analysis that compares the time and costs involved in manual and automated experimentation is likely to show increasing cost-effectiveness of automation with increasing complexity of experimental design. An article in The Economist in 2018 [START_REF] Economist | Robotic labs for high-speed genetic research are on the rise. The design of synthetic lifeforms could become a new industry[END_REF] went as far as to suggest that biotechnology companies of the future would operate without the expense of owning and running their own laboratories as biofoundries could do this for them.

The approach has commonalities with the Industrial Biotechnology Innovation and Synthetic Biology Accelerator (IBISBA) project, coordinated by the Institut National de la Recherche Agronomique, France but with sixteen partners across nine EU member states. It is accessible to industrial biotechnology professionals, including academic researchers, SMEs and large companies. The shared facilities (Figure I) together represent an "R&D continuum in the range TRL 2 to 5 needed to build quality biomanufacturing processes for industrialization" (https://cordis.europa.eu/project/rcn/212584/factsheet/en). The facilities consist of hardware but also ICT to link all project events using the facilities. The project has a particular focus on interoperability to harmonize repeatable working practices within an Open Science framework.

Figure I .

 I Figure I. IBISBA workflow architecture (courtesy of Alan Williams and Carole Goble, School of Computer Science, University of Manchester, UK).


D Thiaudière

J Nicolai

M F Beaufort

M Drouet

P O Renault

J Drieu

La Rochelle

P Godard

C Mocuta

Study of uniaxial deformation behavior of 50 nm-thick thin film of gold single crystal using in situ X-ray pole figure measurements

Keywords: In situ deformation, Synchrotron X-ray diffraction, mechanical twins, mechanical properties, metallic thin films, single-crystal thin films

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

Introduction

Due to the increasing production and commercialization of various kinds of micro-and nanoelectro-mechanical systems, the interest on mechanical behavior of metallic thin films and nanowires has gained considerable attention in the past few decades. In this way, metallic thin films on flexible substrates have wide range of applications such as compliant microelectronic devices biosensors. The analyses of mechanical response, the knowledge of the mechanical properties and the identification of mechanical failure of such metallic nano-crystalline materials may help to find solutions for new design.

Deformation twinning is one of the important deformation mechanisms in face-centered cubic (fcc) metals with a low stacking fault energy (SFE) [START_REF] Christian | Deformation twinning[END_REF][START_REF] Remy | The Interaction Between Slip and Twinning Systems and the Influence of Twinning on the Mechanical Behavior of FCC Metals and Alloys[END_REF]. As nanotwinned films exhibit interesting mechanical properties, a fundamental understanding of the mechanisms of the twin nucleation and twin growth is essential to capture the mechanical behavior of fcc materials.

Twinning deformation is known to be responsible of high mechanical properties of some fcc metals [START_REF] Zhang | Enhanced hardening in Cu/330 stainless steel multilayers by nanoscale twinning[END_REF][START_REF] Lu | Ultrahigh strength and high electrical conductivity in copper[END_REF][START_REF] Lu | Nano-sized twins induce high rate sensitivity of flow stress in pure copper[END_REF][START_REF] Sansoz | An atomistic perspective on twinning phenomena in nano-enhanced fcc metals[END_REF][START_REF] Zhu | Deformation twinning in nanocrystalline materials[END_REF][START_REF]Twinning in metallic materials[END_REF]. In spite of a lot of efforts that have been undertaken to understand how the deformation process is associated with twinning from the 50s [START_REF] Christian | Deformation twinning[END_REF] up to the present years [START_REF]Twinning in metallic materials[END_REF],

there are still some needs to precisely quantify this deformation process. Twinning can occur more frequently in nanostructured metals and can play a more important role in plastic deformation than in their microcrystalline bulk counterpart. Experimentally, a strong effect of the crystal size on deformation twinning has been reported [START_REF] Yu | Strong crystal size effect on deformation twinning[END_REF]. Molecular dynamic simulations have shown that twins are mainly nucleated from free surface in thin films or nanowires [START_REF] Park | Deformation of fcc nanowires by twinning and slip[END_REF].

Very recently, Bejaud et al. have shown that steps at the surface of metallic thin films are privileged sites for twin nucleation at lower stresses [START_REF] Bejaud | The effect of surface step and twin boundary on deformation twinning in nanoscale metallic systems[END_REF]. H.S. Park et al. showed the important effect that surface orientation can have on the deformation mode via slip or twinning in nanowires [START_REF] Park | Deformation of fcc nanowires by twinning and slip[END_REF]. Effect of alloying can also have a significant effect on the twin-based plasticity mechanisms as shown by Ye et al. [START_REF] Ye | In situ TEM compression testing of Mg and Mg-0.2 wt.%Ce single-crystals[END_REF]. They conducted in situ uniaxial compression experiments on single crystalline Mg and Mg-0.2%Ce nano-pillars inside a Transmission Electron Microscope (TEM) and revealed the presence of strong size effects associated with two distinct mechanisms: basal plane sliding and extension twinning in both materials with a dramatic reduction in critical stress for extension twinning between these two materials.

Studies on single crystals are obviously interesting to understand the deformation mechanisms and, more specifically, single-crystalline films have a favorable microstructure for analyzing the scaling behavior of the mechanical properties. It is relatively easy to fabricate few nanometer thick metallic single crystals. During the 1960s and 1970s, a lot of research was carried out on epitaxial growth of metallic thin films evaporated on alkali halide surfaces [START_REF] Ino | Epitaxial growth of metals on rocksalt faces cleaved in vacuum. I[END_REF][START_REF] Ino | Epitaxial Growth of Metals on Rocksalt Faces Cleaved in Vacuum. II. Orientation and Structure of Gold Particles Formed in Ultrahigh Vacuum[END_REF][START_REF] Matthews | Growth of Face-Centered-Cubic Metals on Sodium Chloride Substrates[END_REF][START_REF] Matthews | Influence of substrate steps on arrangement of defects in evaporated films[END_REF]. In their work, Ino and collaborators [START_REF] Ino | Epitaxial growth of metals on rocksalt faces cleaved in vacuum. I[END_REF][START_REF] Ino | Epitaxial Growth of Metals on Rocksalt Faces Cleaved in Vacuum. II. Orientation and Structure of Gold Particles Formed in Ultrahigh Vacuum[END_REF] studied the effect of the substrate temperature and vacuum conditions on the preferred orientation of the deposits of different metals. For another example, Matthews and Allison [START_REF] Matthews | Influence of substrate steps on arrangement of defects in evaporated films[END_REF] studied the formation of twins of evaporated fcc metals on NaCl. Much more recently, Dehm et al. [START_REF] Dehm | Strain compensation by twinning in Au thin films: experiment and model[END_REF] analyzed the density of nanotwins in gold single-crystals 40-160 nm thick. They observed an increase in twin density with decreasing film thickness that was explained by kinematical and thermodynamical arguments.

Single-crystal foils may contain growth twins depending on the growth parameters that can affect the mechanical behavior and induce specific deformation processes like those reported in nanocrystalline materials [START_REF] Zhang | Enhanced hardening in Cu/330 stainless steel multilayers by nanoscale twinning[END_REF][START_REF] Lu | Ultrahigh strength and high electrical conductivity in copper[END_REF][START_REF] Lu | Nano-sized twins induce high rate sensitivity of flow stress in pure copper[END_REF][START_REF] Sansoz | An atomistic perspective on twinning phenomena in nano-enhanced fcc metals[END_REF][START_REF] Zhu | Deformation twinning in nanocrystalline materials[END_REF][START_REF]Twinning in metallic materials[END_REF]. Even twinning in a high SFE material such as Pd [START_REF] Wang | Advanced TEM investigation of the plasticity mechanisms in nanocrystalline freestanding palladium films with nanoscale twins[END_REF] or Al [START_REF] Chen | Deformation twinning in nanocrystalline aluminum[END_REF] has been observed; it is attributed to emission of twinning partials from the grain boundaries because the nanograins are almost dislocation-free. However, the detailed microstructural aspects as the quantitative development of twins in ultrafine structure such as a thin film in the tens of nanometer range are not well understood.

The study of plastic deformation and fracture of single-crystal films of fcc metals started in 1960 with Catlin and Walker [START_REF] Catlin | Mechanical properties of thin single-crystal gold films[END_REF] who reported that one of the most interesting of their results was the appearance of bands due to mechanical twinning in single-crystal gold films 100-300 nm thick. Matthews [START_REF] Matthews | Role of deformation twins in the fracture of single-crystal films[END_REF] reported the role of mechanical twins in the fracture of single fcc crystals such as gold. More recently, Oh et al. [START_REF] Oh | In situ TEM straining of single crystal Au films on polyimide: change of deformation mechanisms at the nanoscale[END_REF] performed in situ TEM straining experiments on Au single crystal of a few tens of nanometer thick deposited on polyimide substrate. They reported a very interesting transition in the mechanisms of plastic deformation at a thickness of about 80 nm. The films thinner than 80 nm deform through the glide of perfect threading dislocations and Shockley partials on {111} planes while films thicker than 80 nm deform through the glide of perfect dislocations. If several mechanisms of plastic deformation dislocation are proposed, an experimental relationship between the applied deformation and the related mechanism is still lacking. Gruber et al. [START_REF] Gruber | Strong single-crystalline Au films tested by a new synchrotron technique[END_REF] have performed an in situ synchrotronbased tensile testing technique to study the mechanical behavior of the same gold single-crystal films. Interestingly, they reported a transition of the dominant plastic deformation mechanism from full to partial dislocations in films thinner than 160 nm. However, the thinner films have a higher density of voids that may facilitate the nucleation of these partial dislocations.

The present experimental study focuses on the deformation mechanisms of a gold single-crystal foil studied by x-ray diffraction. The aim was to add quantitative values to the twinning deformation mechanisms in situ at relatively small applied strains. The 50 nm thick foil is deposited onto polyimide substrate and submitted to tensile loading-unloading. Indeed metallic film on polymer substrates can be stretched well beyond the fracture strain of their freestanding metal film counterpart [START_REF] Lacour | Stretchable gold conductors on elastomeric substrates[END_REF][START_REF] Li | Delocalizing strain in a thin metal film on a polymer substrate[END_REF]. The tensile loading has been performed in the [110] direction of gold up to a true strain of about 4%. The microstructure of gold single crystal and the x-ray pole figures of {111} planes are presented and discussed. Up to a tensile deformation of about 1%, no significant change appears on the in situ Au (111) pole figures. The gold foils remains in the elasto-plastic regime, certainly based on dislocation activity. Then, from tensile deformations from 1% up to 4%, the diffracted intensity of two of the three measured peaks related to {111} twins drastically increases by a value of the order of 450% unlike the intensity of the third peak which only decreases by 25%. In this strain range, the gold foil exhibits a mechanical deformation that is based on the relative increase of the initial twinning volume related to the (111) and 1 1 1 twins. The true strain-twin volume curves exhibit a hysteresis that could reveal a de-twinning during unloading.

Experimental

Sample elaboration and characterization

The gold thin films with a (001) orientation were prepared by ion beam sputtering on a single crystal of sodium chloride. The NaCl substrates were cleaved along (001) plane just before installation in the vacuum deposition chamber. The NaCl surface is 10 x 10 mm². The cleaving produces a relatively flat and clean surface that is composed of very flat terraces and cleavage steps that can be observed by optical microscopy. When the pressure had been reduced to about 7x10 -5 Pa, the NaCl substrate was raised to a temperature of 400°C for 1 hour. Then the temperature was maintained at 400 ° C to perform the deposit of gold with an Ar+ ion gun sputtering beam at 1.2 keV and a working pressure of approximately 10 -2 Pa. Growth rate measurement of 0.15 nm/s was performed by determining the film thickness using ex situ xray reflectometry measurement. Finally, for the in situ synchrotron experiment, a NaCl substrate had been dissolved in water, and the gold thin film that floats on the surface of the water was simply caught at the center of the polyimide substrate. The 50 µm thick polyimide susbtrate (kapton HN®) has a cruciform shape with arms of 200 mm long and 20 mm wide (see Geandier et al. for the details of the cruciform geometry [START_REF] Geandier | Development of a synchrotron biaxial tensile device for in situ characterization of thin films mechanical response[END_REF]). Interestingly, even if no specific surface treatment nor adhesion layer nor glue is used, the adhesion is rather good. For example, the gold film is difficult to extract even using a sticky tape; thus the deformations will be considered as fully transmitted through metal-polyimide interface. Metallic thin film on polymers are much easier to handle and can be stretched well beyond the fracture strain of their free-standing counterpart [START_REF] Lacour | Stretchable gold conductors on elastomeric substrates[END_REF][START_REF] Li | Delocalizing strain in a thin metal film on a polymer substrate[END_REF].

The in-plane orientation and the epitaxial relationships of the gold film on NaCl were determined via x-ray pole figures on another gold film. The pole figures have been performed on a Seifert diffractometer (3003 PTS-HR) with a four-circle diffractometer equipped with a copper anode (punctual focus) with a 0.5 mm diameter circular collimator. The x-ray diffracted signal was recorded using a point detector with a curved graphite crystal monochromator in front (selecting only Kα1 and Kα2 of Cu lines) and a 3 mm slit. The step widths for scans of φ (0 ≤ φ ≤ 359°) and ψ (0 ≤ ψ≤80°) were ∆ψ=0.5° and ∆φ=1°.

Conventional, High-Resolution TEM and Scanning TEM (STEM) images were carried out using a JEOL 2200 FS (Schottky-FEG, 200 kV). To realize plane view TEM (Fig. 5 Top) the samples were immersed in deionized water to dissolve NaCl. Then, the gold film was transferred to a copper grid. Cross-sectional TEM (XTEM) samples were prepared by Focused Ion Beam (FIB) using an FEI-HELIOS dual-beam using the standard lift-out method [START_REF] Langford | In situ lift-out using a FIB-SEM system[END_REF]. Pt was deposited on the top surface of the sample, then thin foil has been extracted using Ga at 30kV. Thin foil has been thinned using Ga at 30kV and finished at 5kV.

Tensile loading and in situ pole figure measurements

We have used the DIFFABS-SOLEIL biaxial tensile tester working in the synchrotron environment for in situ diffraction measurements for thin crystalline films mechanical response [START_REF] Geandier | Development of a synchrotron biaxial tensile device for in situ characterization of thin films mechanical response[END_REF]. The biaxial loading device was installed on the 6 circle diffractometer (kappa geometry) at the DiffAbs beamline of the SOLEIL Synchrotron (Gif-sur-Yvette, France). A pole figure measurement is performed as follows (Fig. 1): two axes of the diffractometer are used to set the Bragg angle: the detector is positioned (fixed) at the corresponding chosen diffraction angle 2θ and the incident angle is set (fixed) to half of this value (with the sample normal to surface being in the vertical plane). Then, the x-ray diffracted intensity is recorded while the sample is rotated around its surface normal (azimuth angle φ) and around the incident x-ray beam direction projected on the sample surface, F1 direction (i.e. elevation angle ψ, defined as the angle between the normal to the surface and the vertical scattering plane) (Fig. 1). Thus a pole figure probes the distribution of orientations in the sample of a crystallographic plane family.

In this way, the different orientations of the crystal plane under investigation can be brought into Bragg diffraction condition; the diffraction intensity scales with the crystalline volume with that specific orientation. This tensile tester allows measuring x-ray diffraction in reflection mode from 0° up to high elevation angles (ψ); there is almost no shadowing edge. Two couples of motors and force sensors are fixed to the device frame. The four motors can be actuated separately in order to keep the studied area at a fixed position in the goniometer. The cruciform substrates were gripped by a cam rotating in a cylindrical fixation. In addition, the in-plane macroscopic strains (ε11 and ε22) of the polyimide substrate (opposite surface) were measured by Digital Image Correlation (DIC) [START_REF] Besnard | Finite-Element" Displacement Fields Analysis from Digital Images: Application to Portevin-Le Châtelier Bands[END_REF][START_REF] Djaziri | Combined synchrotron X-ray and image-correlation analyses of biaxially deformed W/Cu nanocomposite thin films on Kapton[END_REF] at the center of the cruciform shape substrate by using images of about 6 × 9 mm². Noteworthy, the applied strain is homogeneous over this area.

The energy of the incoming monochromatic x-ray beam was 9 keV corresponding to a wavelength of λ = 0.13776 nm using a double-crystal monochromator. The beamsize was of 316 × 271 μm² (horizontal and vertical full width at half maximum, FWHM, respectively). A monitor detector was measuring the intensity of the incident x-ray beam during the in situ experiment. A hybrid pixel area detector (XPAD detector, acronym for x-ray Pixel chip with Adaptable Dynamics [START_REF] Pangaud | XPAD3-S: A fast hybrid pixel readout chip for X-ray synchrotron facilities[END_REF]) was used at a sample detector distance of 621 mm. It contains 240 × 560 pixels, with a pixel size of 130 × 130 µm². The in situ pole figure measurements have been performed on the {111} planes of gold with an incident ω angle fixed at 17° and the center of the 2D detector fixed at a 2θ angle of 34°. As the long side of the detector was mounted in the 2θ direction (see Fig. 1), the geometrical diffraction conditions resulted in a 2θ angular acceptance determined to be 6.8°, and around 5.1° in the perpendicular direction (Fig. 2); this perpendicular direction mainly corresponds to ψ in the pole figure. So, in order to have a significant overlap between two sequential ψ steps, the sample elevation angle (ψ) moved by steps ∆ψ of 3° from ψ= 0° to 81°. Moreover, the area detector acquired a snapshot every 100 ms during a continuous in-plane rotation of the sample azimuth (denoted as the φ angle), rotating at a speed of 8° s -1 and covering 210° in φ. In other word, a diffraction pattern was continuously recorded with a step ∆φ of 0.8°. The ψ and φ ranges have been chosen by exploiting the four-fold symmetry of the Au (001) single-crystal, the two-fold symmetry of the tensile test and the fact that the main interesting poles are situated at ψ angles of about 15° and 55°. This set-up enables the measurement of a set of pole figures within 29 min. Thanks to a homemade python code, the raw data were re-calculated to usual angular (ψ, φ) space and the resulting datasets are presented as a standard pole figure [START_REF] Mocuta | Fast pole figure acquisition using area detectors at the DiffAbs beamline -Synchrotron SOLEIL[END_REF]. In principle, for a quantitative study, the measured intensity of the gold thin film should be corrected for the defocusing and background effects and the volume of diffracting material and absorption. But these corrections are not necessary as the presented results only focus on the relative quantification of the same poles.

The measurements of x-ray pole figure were performed while subjected to a quasi-uniaxial tension deformation state at progressively higher applied load and then unloaded. Indeed, the component ε22 is kept roughly constant around zero while the component ε11 increases up to 4% (see Fig. 3). The loading of the sample was controlled by displacement of the grips in the two perpendicular directions (labelled 1 and 2 in Fig. 1) with a ratio of 1/4 in order to have a quasi-uniaxial deformation at the center of the cruciform shape substrate. Fig. 3 shows the applied loads increments and the related applied strain increments; at each increment 10 images were taken before and again 10 after collection time for an x-ray pole figure (i.e. 29 minutes later). On the one hand, the true strain at the center of the cruciform remains constant during interrupted loading intervals for the first part of the test (up to approximately 2.5%). This observation makes it possible to design unique experiments for the study of the relaxation of thin films [START_REF] Godard | Relaxation Mechanisms in a Gold Thin Film on a Compliant Substrate as Revealed by X-Ray Diffraction[END_REF]. On the other hand, as emphasized in the inset of Fig. 3, from applied strain larger than 2.5%, there is an apparent decrease of the true strain ε11 but it is only because no DIC measurements are performed during x-ray measurements (i.e. 29 minutes). The apparent drop corresponds to a continuous relaxation of the substrate which rises up to about 0.001 for applied true strains in the range 0.035-0.04. Hence, the relaxation of the center of the cruciform substrate is negligible even for the largest applied strains. Noteworthy, the maximum strain of about 4% is measured at the center of the cruciform shape substrate, this corresponds to the elastic domain of polyimide (Kapton ®HN DupontTM). However, the deformations in other part of the sample are much larger than 4% and more specifically in the toe weld regions. So considering the whole sample, the deformation test is over the yield point and this is why the applied strain at the center of the cruciform shape substrate is not going back to zero when the sample is unloaded (see Fig. 3). Once unloaded, the applied true strain ε11 is equal to 1.8%.

Results and discussion

Microstructure of the as-deposited Au single crystal

The {111} x-ray pole figure shown in Fig. 4 exhibits the four-fold symmetry in {111} poles indicating that the sample is [001] oriented. As reported by many authors [START_REF] Ino | Epitaxial growth of metals on rocksalt faces cleaved in vacuum. I[END_REF][START_REF] Ino | Epitaxial Growth of Metals on Rocksalt Faces Cleaved in Vacuum. II. Orientation and Structure of Gold Particles Formed in Ultrahigh Vacuum[END_REF][START_REF] Matthews | Growth of Face-Centered-Cubic Metals on Sodium Chloride Substrates[END_REF][START_REF] Matthews | Influence of substrate steps on arrangement of defects in evaporated films[END_REF][START_REF] Dehm | Strain compensation by twinning in Au thin films: experiment and model[END_REF][START_REF] Catlin | Mechanical properties of thin single-crystal gold films[END_REF][START_REF] Matthews | Role of deformation twins in the fracture of single-crystal films[END_REF][START_REF] Oh | In situ TEM straining of single crystal Au films on polyimide: change of deformation mechanisms at the nanoscale[END_REF][START_REF] Gruber | Strong single-crystalline Au films tested by a new synchrotron technique[END_REF] despite the very large lattice mismatch (0.408 nm for gold and 0.564 nm for NaCl), gold grew with a cube-on-cube orientation relationships: Au (001)//NaCl (001) and Au [100]//NaCl [100]. The selection of a particular texture of the film depends on a number of kinetic and thermodynamic effects. In the 60s, Ino et al. [START_REF] Ino | Epitaxial growth of metals on rocksalt faces cleaved in vacuum. I[END_REF] studied the effect of temperature on the different texture components of fcc metals, Matthews [START_REF] Matthews | Growth of Face-Centered-Cubic Metals on Sodium Chloride Substrates[END_REF] reported the importance of contaminants. So in view of the experimental deposition parameters, the cube-on-cube growth of Au was well expected.

The {111} pole figure also shows other peaks of much smaller intensity at ψ angles of about 16° and 78°; these secondary peaks are attributed to {111} twins. Indeed a {111} twinning volume induces peaks at 15.8°, 54.7° and 78.9°. Due to the four-fold symmetry of gold, four different twins I, II, III and IV as labeled in Fig. 4 are planned giving rise each to 4 secondary peaks: one at 15.8°, one at 54.7° and two at 78.9° (see dashed circle in Fig. 4 for twins I).

Obviously, the intensity arising from the twinning volume at 54.7° is completely negligible compared to the intensity of the gold matrix. The fact that the measured twinning peaks are about 3 orders of magnitudes weaker than the epitaxial peaks of the matrix indicates that this 50 nm Au/NaCl layer grown at T=400°C is primarily an epitaxial layer with a relatively low (but clearly detectable) density of twins. As the diffraction intensity scales with the crystalline volume, the volume of twins can be estimated in the order of 0.1%. The presence of nanotwins was confirmed by a TEM observation on a plane view sample. The Bright Field micrograph on Fig. 5 clearly shows numerous {111} nanotwins regularly distributed. Their length is ranging between 20 and 80 nm and their thickness is less than 10 nm. Note that the thickness of the twins has been estimated as the size of black domains. Indeed, we assume that, due to lamellae orientation and to crystallographic considerations, the width of black domains can be assigned as twins' thickness. As in other fcc metals with a low SFE, many authors [START_REF] Chen | Deformation twinning in nanocrystalline aluminum[END_REF][START_REF] Catlin | Mechanical properties of thin single-crystal gold films[END_REF][START_REF] Matthews | Role of deformation twins in the fracture of single-crystal films[END_REF] have already observed those {111} twins in gold single crystal. {111} twins have also been reported by Miller et al. in polycrystalline gold thin films elaborated by electron beam evaporation on silicon [START_REF] Miller | Thermomechanical evolution of multilayer thin films: Part II. Microstructure evolution in Au/Cr/Si microcantilevers[END_REF]. Noteworthy our gold single crystal seems to have twins that have similar geometrical dimensions than the smaller ones observed by Matthews [START_REF] Matthews | Role of deformation twins in the fracture of single-crystal films[END_REF] while the twins reported by Dehm et al. [START_REF] Dehm | Strain compensation by twinning in Au thin films: experiment and model[END_REF] and Gruber et al. [START_REF] Gruber | Strong single-crystalline Au films tested by a new synchrotron technique[END_REF] were much longer (a few microns). While out of the scope of the present paper, the presence of twins may enable an important interfacial energy dissipation mechanism and can be formed during crystal growth (growth twins).

However, the twin nucleation and growth might happen during cooling from fabrication temperature to room temperature and, the thickness of a twin might change in response to an applied loading. Dehm et al. [START_REF] Dehm | Strain compensation by twinning in Au thin films: experiment and model[END_REF] justify the density of twins by a kinematical and thermodynamical approach. The twins would nucleate during the cooling to room temperature to relax the strain induced by the large difference of coefficient of thermal expansion between NaCl substrate and gold foil. Similar {111} pole figures exhibiting such secondary peaks attributed to twins have been reported on low SFE materials such as Cu [START_REF] Wang | Orientation modulated epitaxy of Cu nanorods on Si(001) substrate[END_REF] and Ag [START_REF] Chawla | Epitaxial Ag(001) grown on MgO(001) and TiN(001): twinning, surface morphology, and electron surface scattering[END_REF]. The latter authors attributed the nucleation of twins to the low energy of {111} facets at the growth front while Wang et al. [START_REF] Wang | Orientation modulated epitaxy of Cu nanorods on Si(001) substrate[END_REF] observed that the growth of twins was directly related to the flux of incident atoms. In our case, even if the gold growing condition are different from the one of Cu and Ag, the assumption based on strain relaxation proposed by Dehm et al. [START_REF] Dehm | Strain compensation by twinning in Au thin films: experiment and model[END_REF] seems to be not appropriate as we experienced that smaller temperatures of growth induced larger diffraction intensities of peaks related to the twins.

Evolution of twinned volume during in situ deformation test

As mentioned in section 2.2, the pole figures are acquired with a XPAD 2D detector. It is worth noting that the diffraction patterns corresponding to matrix and to {111} twins exhibit streaks (see Fig. 2). Diffraction streaks derive from the loss of translation invariance; this phenomenon is well-known along the surface normal direction for example. In our gold film, those streaks are induced by the presence of the surface and the interface with the substrate and also by the presence of the twins. For the (111) Bragg peak related to the gold matrix, the streaks are completely negligible. But for the (111) Bragg peak related to the twin volume, this streak has an intensity as high as the diffracting volume of the twins itself because the twins are very thin.

Theoretically, the streak is perpendicular to the twin boundaries and induces an apparent variation of 2θ angle along ψ in the pattern of Fig. 2. As the intensity of the streak is of the same order as the intensity of the peak itself, it is difficult to measure precisely the 2θ position of the Bragg peak related to the twins and to extract microstructural plastic features from the shape of the peak [START_REF] Budrovic | Plastic deformation with reversible peak broadening in nanocrystalline nickel[END_REF]. Hence, the present study only focuses on the intensity evolution of the peak, i.e. evolution of the integrated intensity of the peaks on the pole figures. As mentioned previously, the diffraction intensities of a pole figure scale with the crystalline volume with the specific orientation associated to φ and ψ, i.e. with the volume of twins. The integrated intensity is determined from the two-dimensional intensity distribution on the pole figure considering a large φ and ψ ranges to determined and subtract the background (φ range of 32° and ψ range of 24°). Fig. 6 shows a few pole figures that have been chosen for 4 applied true strain increments during loading, namely 0.2%, 1.1%, 2.1%, 3%, 4% and unloading 1.8%. Only a small ψ range of the recorded pole figures is shown in Fig. 6 to clearly highlight the main features that change during the deformation test. The 3 peaks at about ψ~16° are related to the {111} throughthickness twins I, II and III. The horizontal tensile axis lay near the direction [110] of gold single-crystal at an angle of about 7°. Defining [110] as the tensile direction, twins I and III correspond to twins with twin-boundaries parallel to (111) and 1 1 1 planes respectively and the twins II have their interfaces parallel to 1 11 planes. In other words, twins II may be described as parallel to the tensile direction. The intensity distribution corresponding to twins I and III is drastically increasing while the one corresponding to twins II is slightly decreasing. This observation is clearly shown in Fig. 7 where the integrated intensity of those peaks is plotted versus the applied true strain ε11. The integrated intensity is not exactly the same for twins I and III at the beginning of the tensile test, but the increase is quite similar for both twins: the intensity is multiplied by a factor of 4 to 5 during uniaxial straining up to 4%. At the same time, the integrated intensity of the twins II peak decreases by about only 25% in the perpendicular direction. During unloading, after a small plateau at the beginning of the unloading, the integrated intensity of the twins I and III is decreasing by 35-50% while nothing significant is observed for twins II.

The curves in Fig. 7 may be interpreted in term of plastic features as the following: (i) for an applied uniaxial tensile straining, the volumes of through-thickness {111} twins I, II and III are constant up to an applied strain of about 1%; (ii) then, as the applied strain increases from 1% up to 4%, the volume of (111) and 1 1 1 twins is drastically increasing of about 400-500% while the volume of 1 11 twins that are parallel to the straining direction is slightly decreasing (of about 25%). In other words, the elasto-plastic behavior of the Au single-crystal foil is mainly based on slip mechanisms up to an applied uniaxial strain of 1%. The present experiment did not allow capturing the yield point, so the exact amount of plastic deformation is not known. Then, for applied uniaxial true strains from 1 up to 4%, the plastic deformation mechanism is partly based on twinning-mediated plastic deformation mode; our results cannot quantify the relative proportion of slip and twinning mechanisms. Obviously, the twinning deformation mode requires dislocation activity as slip mechanism. A review of the mechanical twinning has been provided by Christian and Mahajan [START_REF] Christian | Deformation twinning[END_REF].

As mentioned in the introduction, the fact that the single crystal of gold undergoes plastic deformation through the appearance of mechanical twin bands has already been reported a long time ago, in 1960, by Catlin and Walker [START_REF] Catlin | Mechanical properties of thin single-crystal gold films[END_REF] and, more precisely studied by Matthews ten years later [START_REF] Matthews | Role of deformation twins in the fracture of single-crystal films[END_REF]. The deformed gold films contained numerous very long narrow deformation twins (several microns long) with high density of small twins in between. Matthews [START_REF] Matthews | Role of deformation twins in the fracture of single-crystal films[END_REF] concluded that the comparison of the growth and deformation twins suggests that the deformation twins were formed by elongation of the growth twins. Although this mechanism could be applied in the present experiment to explain the large increase of the intensity related to the twins, it is necessary to notice that the 2 deformation experiments are certainly carried out under very different deformation conditions, namely the deformation range and deformation rate. In his experiments, Matthews either cleaved the substrate with the film attached to it either glue the film to a tape that is stretched to about 1%. Much more recently, in their experiment, Oh et al. [START_REF] Oh | In situ TEM straining of single crystal Au films on polyimide: change of deformation mechanisms at the nanoscale[END_REF] did not report any growth of twins (or increase of twin density) in their gold single crystal during the in situ TEM straining. They mainly observed dislocation activity and, for a 40 nm thick gold film, they observed large partial dislocation spacings and creation of stacking fault. This visible stacking fault was erased by a second partial that was emitted at the same position. Noteworthy, the microstructure of their gold films seems to be different from the one shown in Fig. 4 of the present paper. Indeed the growth twins were very long (a few microns) in their case, and these authors did not report any density of small twins as in the present case. Only the thickness of twins is of the same order (less than 10 nm). Gruber et al. observed by post mortem TEM analysis that the twin density was higher after deformation, but they noted that their 40 nm thick gold single crystal contains a relatively high density of holes [START_REF] Gruber | Strong single-crystalline Au films tested by a new synchrotron technique[END_REF]. The observed {111} twins before and after deformation were very long compared to the one present in our sample and most of them started and ended at holes or on another twin. To compare our results with these two in situ experiments, independently of the difference of initial microstructure, it is also important to note that the tensile tests were with the same resolved shear and a highest Schmid factor of 0.47. It is probable that the initial growth {111} nanotwins are suitable nuclei for deformation twins and the huge increase of intensity could be justified by the elongation or thickening of these initial {111} nanotwins.

Further experimental evidence should be necessary to prove this comment. Indeed, if nanotwins may be considered as precursors for deformation twinning, the enhancement of the twin volume fraction can be dominated by the nucleation of new lamellar structures rather than by the expansion of previously existing ones as it has been reported in TWIP steels [START_REF] Steinmetz | Revealing the Strain-Hardening Behavior of Twinning-Induced Plasticity Steels: Theory, Simulations, Experiments[END_REF]. In this material, twins appear in stacks of thin, nanoscale lamellae with small thickness.

During the loading up to 4%, the Au single-crystal foil undergoes elastic and plastic deformation if we assume no crack regime. Indeed, as mentioned in section 2, the fact that the gold thin film is supported by a polymer delays the necking and cracking processes [START_REF] Lacour | Stretchable gold conductors on elastomeric substrates[END_REF][START_REF] Li | Delocalizing strain in a thin metal film on a polymer substrate[END_REF].

Cracking happens during tensile testing for metallic thin films on polymer substrates at relatively low deformation of about 3% for Ni [START_REF] Faurie | In situ x-ray diffraction analysis of 2D crack patterning in thin films[END_REF] or Cu [START_REF] Cordill | Fragmentation testing for ductile thin films on polymer substrates[END_REF]. Even if cracks may be difficult to observe, no significant crack evidence has been reported for deformation up to 15% in very ductile materials such as gold [START_REF] Putz | Improved electromechanical performance of gold films on polyimide without adhesion layers[END_REF]. We can therefore consider that there is no cracking at tensile straining of 4% in the present experiment. The exact amount of plastic strain has not been evidenced experimentally but it is obviously between 3 to 4%. So, whatever the mechanism of dislocation-based plasticity or twinning-based plasticity, such amount of plastic deformation should induce a compressive stress state during unloading [START_REF] Hommel | A new method to study cyclic deformation of thin films in tension and compression[END_REF][START_REF] Renault | Damage mode tensile testing of thin gold films on polyimide substrates by X-ray diffraction and atomic force microscopy[END_REF]. Indeed the polyimide substrate which has an elastic limit much higher than the one of gold single-crystal contracts to smaller length (larger than its initial length because of a given amount of plastic deformation in the cruciform shape substrate as mentioned in the section 2.1) when the external applied load is removed from the film-substrate composite. So the gold single crystal may deform: (i) elastically at the beginning of the unloading, (ii) then plastically in compression because of the contraction of the polyimide. These 2 deformation regimes should justify the shape of the curves in Fig. 7 during unloading. The unloading part starts with a plateau as the applied true strain decreases from 4 to approximately 3%. Then, a decrease of the intensity of the two peaks related to twins I and III that can be explained by detwinning during compressive stress state.

Noteworthy the rate of detwinning during unloading as a function of applied strain is roughly similar to the one of twinning during loading. Detwinning has been experimentally observed in fcc metals. For example, Li et al. reported that the primary plastic deformation transits from inclined dislocation gliding to detwinning in nano-twinned polycrystalline Cu when twin thickness decreases from 25 nm to 4 nm [START_REF] Li | Dislocation nucleation governed softening and maximum strength in nano-twinned metals[END_REF]. Detwinning has also been reported during straining in a nano-polycrystalline Pd films containing a high density of twins while twinning is observed for the same materials with a lower density of twins [START_REF] Kobler | Deformation-induced grain growth and twinning in nanocrystalline palladium thin films[END_REF]. In gold nanowires, successive tension-compression test revealed that the compressive strain was accommodated by a detwinning process [START_REF] Lee | Reversible cyclic deformation mechanism of gold nanowires by twinning-detwinning transition evidenced from in situ TEM[END_REF].

Conclusions

We have determined some mechanical characteristic of gold single-crystal thin film supported by a polymer sample. The as-deposited gold single crystal contains small amount of {111} twins. These small through-thickness {111} twins, probably formed during film crystal growth, are about 5-10 nm thick and about 20-80 nm long. The 50 nm thick gold single crystal has been submitted at room temperature to an uniaxial deformation test in the Au [110] direction from 0% up to tensile strain of 4% and then unloaded to a tensile strain of 1.8%.

In situ x-ray pole figure measurements evidenced the strong evolution of the diffracting volumes related to the (111) and 1 1 1 twins which increase of about 450% as the applied strain increases from 1 to 4%. At the same time, the diffracting volume related to the twins that are parallel to the tensile direction, i.e. 1 11 twins, decreases of only 25%. The deformation seems to start via traditional crystallographic slip with no evidence of twinning before an uniaxial strain of 1%. Then x-ray pole figure measurements identify deformation twinning as an important deformation mechanism for uniaxial strain of 1% up to 4%, however these measurements cannot capture any changes induced by slip mechanism. In other words, the plastic deformation mechanisms are partly based on twinning-mediated plastic deformation mode. The present results can quantify precisely the relative increase of twinning volume but not the relative amount of slip and twinning mechanisms for applied uniaxial true strains from 1 up to 4%.

During unloading, i.e. uniaxial reverse straining from 4% down to 1.8%, x-ray pole figure measurements evidenced also a significant evolution of the diffracting volumes related to the (111) and 1 1 1 twins which decrease of about 40%. Concomitantly the diffracting volume related to the twins that lie in the parallel direction to the tensile direction, i.e. 1 11 twins, did not evolve within the experimental uncertainty. The decrease of intensity is attributed to a decrease of diffracting volume related to the (111) and 1 1 1 twins, i.e. to a detwinning process. This detwinning happens during unloading if we consider the whole sample, i.e. the composite gold/polyimide; but it is in fact a reverse loading for the gold single crystal. In other words, the detwinning happens at small applied compressive strains in the gold single-crystal film; it is due to reversible compression in the same direction. As the twinning process the detwinning affects only the two twin variants, i.e. (111) and 1 1 1 twins. The 1 11 twins that lie in the parallel direction to the tensile direction are not affected by the deformation test. This work demonstrates the interest of x-ray texture analysis by proving the existence of deformation twinning, by quantifying the relative evolution of the twin volume for the [110] tensile or compressive directions. On the right: corresponding 2D diffraction pattern recalculated in the angular space (2θ, ψ).

List of figure captions

Noteworthy the distribution of intensity corresponding to gold is not constant as a function of 2θ as underlined by the dashed line. In other word, the 2D pattern exhibits a streak. As the twins are very thin, this streak has intensity as high as the diffracting volume of the twins itself and, theoretically, it is perpendicular to the twin boundaries. versus number of images. The corresponding applied loads are given for the two arms of the substrate. The inset shows the strain relaxation that happens for the larger loads only: a decrease of about 0.1% during x-ray collection time for an applied true strain of about 3.8%.

  performed along the [100] direction and not in the [110] as in the present paper. Obviously the deformation mechanisms are strongly dependent on the tensile direction in relation to the crystallographic direction. A tensile deformation along [110] may activate only two twinning systems contrary to the direction [100] for which the four twinning systems are equivalent.Considering the [110] direction, the two activated systems are 111 [1 1 2] and 11 1 [112]

Figure 1 :

 1 Figure 1: Schematic of experimental set-up for the definition of diffraction angles, namely ψ inclination angle of diffraction planes relatively to the normal surface, φ the azimuthal angle

Figure 2 :

 2 Figure 2: On the left: as-recorded typical diffraction pattern recording with the XPAD 2D

Figure 3 :

 3 Figure 3: True strains (ε11, ε22) measured by DIC at the center of the cruciform shape substrate

Figure 4 :

 4 Figure 4: X-ray pole figure constructed from the (111) diffraction peak of the sample grown at

Figure 5 :

 5 Figure 5: Top: Bright field TEM micrograph of a 50 nm thick Au single-crystal. General view

Figure 6 :

 6 Figure 6: Selection of x-ray {111} pole figures acquired during uniaxial deformation test. Only the part of the pole figure that evolves during the test is shown. The true strain ε11 whose value

Figure 7 :

 7 Figure 7: Integrated intensity of the secondary peaks at ψ~16° related to {111} twins versus the true strain ε11. Defining [110] as the tensile direction, twins I and III correspond to (111)

Acknowledgements

This work partially pertains to the French Government program "investissements d'avenir" (LABEX INTERACTIFS, reference ANR-11-LABX-0017-01) and has been partially supported by "Nouvelle Aquitaine" Region and by European Structural and Investment Funds (ERDF reference: P-2016-BAFE-94/95). Synchrotron SOLEIL is acknowledged for allocated beamtime.