Benoît Vieille
email: benoit.vieille@insa-rouen.fr

Clément Keller

M Mokhtari

H Briatta

T Breteau

Mouldi Josiane Nguejio

Ben Azzouna

Fabrice Barbe

E Baustert

M Ben Azzouna

Investigations on the fracture behavior of Inconel 718 superalloys obtained from cast and additive manufacturing processes

Keywords: fracture toughness, ductility, cast, Additive Manufacturing, microstructure

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

Introduction

Inconel 718 alloys are high-strength nickel-base superalloys which are widely used as structural components under high-temperature and relatively high stress conditions. A potential premature failure exists if crack-like defects of sufficient size and shape are present at critical locations [START_REF] Mills | Variations in Fracture Toughness for Alloy 718 Given a Modified Heat Treatment[END_REF]. The fracture behaviors of conventional cast and forged Inconel 718 alloys have been extensively studied since the seventies [START_REF] Mills | The Deformation and Fracture Characteristics of Inconel X-750 at Room Temperature and Elevated Temperatures[END_REF][START_REF] Mills | Fracture toughness variations for alloy 718 base metal and welds[END_REF][START_REF] Pettit | Flaw Growth Behavior of Inconel 718 at Room and Cryogenic Temperatures[END_REF][START_REF] Stout | Structural Property/Continuum Synthesis of Ductile Fracture in Inconel 718[END_REF][START_REF] Menke | An Elastic-Plastic Toughness Characterization of Alloy 718 Product Forms[END_REF]. Due to their complex metallurgical microstructure, involving precipitation of several phases, their fracture behavior is strongly influenced by heat treatment [START_REF] Mills | Variations in Fracture Toughness for Alloy 718 Given a Modified Heat Treatment[END_REF], alloy composition and manufacturing processes [START_REF] Mills | Fracture toughness variations for alloy 718 base metal and welds[END_REF]. Depending on the microstructure (hence the heat treatments), significant differences in the J-R curves were observed. Fracture toughness tests are conventionally performed using linear-elastic (KIc) and elastic-plastic (JIc) fracture mechanic approaches. Most of the references available in the literature have specifically discussed the relationship between key microstructural features, fracture mechanisms and macroscopic properties based on metallographic and fractographic analyses [START_REF] Mills | The Deformation and Fracture Characteristics of Inconel X-750 at Room Temperature and Elevated Temperatures[END_REF] [START_REF] Begley | The J Integral as a Fracture Criterion[END_REF][START_REF] Ritchie | Macroscopic and Microscopic Analyses for Crack Initiation and Crack Growth Toughness in Ductile Alloys[END_REF][START_REF] Gustafsson | Fatigue crack growth behaviour of Inconel 718 with high temperature hold times[END_REF].

With the emergence of additive manufacturing parts in cutting-edge applications, it is necessary to better understand the Inconel 718 fracture behavior and properties. These are expected to significantly differ from the cast materials. Additive manufacturing (AM) by PBF is of great interest given its ability to produce complex and non-conventional geometries; however, microstructures generated by this process can differ significantly from their conventionally developed counterparts (e.g. cast and forged materials). Depending on the manufacturing parameters (e.g. laser velocity and power), this process is based on a spatially and temporally mobile heat source. This heat source may cause the development of significant microstructural heterogeneity: a mixture of columnar (elongated) and equiaxed (isotropic) grains, often with a visible periodicity corresponding to the layer height, the hatch distance and scanning pattern used [START_REF] Zhang | Macroscopic finite element thermal modelling of laser beam melting of Inconel 718[END_REF][START_REF] Carter | The influence of the laser scan strategy on grain structure and cracking behaviour in PBF powder-bed fabricated nickel superalloy[END_REF]. To these microstructural heterogeneities are associated crystallographic textures that influence the mechanical behaviour of the parts [START_REF] Debroy | Additive manufacturing of metallic components -Process, structure and properties[END_REF][START_REF] Lewandowski | Metal Additive Manufacturing: A Review of Mechanical Properties[END_REF]. The choice of trajectories, and also specific manufacturing parameters (thickness of the powder to be fused, the power and speed of the laser) define the construction strategy of a part. The control of the heat source is therefore essential to control the solidified grain structure, and obtain optimal microstructures for the targeted applications.

Microstructure and mechanical behavior

The AM-PBF process involves high solidification rates and high temperature gradients.

Cooling rates are therefore also very important, causing the formation of microstructures that can deviate significantly from the thermodynamic equilibrium. In the case of nickel-based superalloy industrial alloys (e.g. Inconel 718), this may cause undesirable sequences. Indeed, the precipitation of intermetallic phases from the liquid state, to hardened solid solutions of composition very far from the thermodynamic equilibrium, without or with few precipitated phases, are observed. Unexpected primary solidification structures or morphologies significantly different from those produced by more conventional processes may also appear [START_REF] Zhang | Macroscopic finite element thermal modelling of laser beam melting of Inconel 718[END_REF] [START_REF] Zhang | Tensile properties of laser additive manufactured Inconel 718 using filler wire[END_REF][START_REF] Zhang | Comparison of microstructures and mechanical properties of Inconel 718 alloy processed by selective laser melting and casting[END_REF][START_REF] Li | Microstructural evolution and mechanical properties of IN718 alloy fabricated by selective laser melting following different heat treatments[END_REF][START_REF] Wang | The microstructure and mechanical properties of deposited-IN718 by selective laser melting[END_REF]. Most of the studies available in the literature underline the importance of heat treatments on both microstructure and mechanical properties of Inconel 718. These references illustrate the capability of the PBF-process to produce parts with mechanical properties better than forged and cast material at room temperature and equal properties to forged material at elevated temperatures with a high proportion of intragranular δ Laves phases in the PBFprocessed material [START_REF] Trosch | Microstructure and mechanical properties of selective laser melted Inconel 718 compared to forging and casting[END_REF]. More specifically, the fatigue cracks stopped in front or detoured around the Laves phases. This means that the unbroken Laves phases play an important role in hindering crack propagation [START_REF] Sui | The influence of Laves phases on the high-cycle fatigue behavior of laser additive manufactured Inconel 718[END_REF].

Most references on Inconel 718 obtained from PBF address the relationship between its microstructure and the common mechanical properties (hardness, tensile properties) [START_REF] Moussaoui | Effects of Selective Laser Melting additive manufacturing parameters of Inconel 718 on porosity, microstructure and mechanical properties[END_REF][START_REF] Schneider | Effect of heat treatment variations on the mechanical properties of Inconel 718 selective laser melted specimens[END_REF][START_REF] Holland | Multiscale characterization of microstructures and mechanical properties of Inconel 718 fabricated by selective laser melting[END_REF][START_REF] Amato | Microstructures and mechanical behavior of Inconel 718 fabricated by selective laser melting[END_REF][START_REF] Chlebus | Effect of heat treatment on the microstructure and mechanical properties of Inconel 718 processed by selective laser melting[END_REF][START_REF] Deng | Microstructure and mechanical properties of Inconel 718 produced by selective laser melting: Sample orientation dependence and effects of post heat treatments[END_REF][START_REF] Jia | Selective laser melting additive manufacturing of Inconel 718 superalloy parts: Densification, microstructure and properties[END_REF][START_REF] Tucho | Microstructure and hardness studies of Inconel 718 manufactured by selective laser melting before and after solution heat treatment[END_REF][START_REF] Zhang | Effect of standard heat treatment on the microstructure and mechanical properties of selective laser melting manufactured Inconel 718 superalloy[END_REF] and fatigue crack growth [START_REF] Sui | The influence of Laves phases on the high-cycle fatigue behavior of laser additive manufactured Inconel 718[END_REF] [START_REF] Brynk | Fatigue crack growth rate and tensile strength of Re modified Inconel 718 produced by means of selective laser melting[END_REF][START_REF] Puppala | Fatigue and fracture toughness characteristics of laser rapid manufactured Inconel 625 structures[END_REF][START_REF] Poulin | Long fatigue crack propagation behavior of Inconel 625 processed by laser powder bed fusion: Influence of build orientation and postprocessing conditions[END_REF][START_REF] Konečná | Long fatigue crack growth in Inconel 718 produced by selective laser melting[END_REF][START_REF] Riemer | Crack propagation in additive manufactured materials and structures.21 st confrenece onf fracture ECF21[END_REF][START_REF] Popovich | Creep and Thermomechanical Fatigue of Functionally Graded Inconel 718 Produced by Additive Manufacturing[END_REF][START_REF] Pei | Assessment of mechanical properties and fatigue performance of a selective laser melted nickel-base superalloy Inconel 718[END_REF][START_REF] Aydinöz | On the microstructural and mechanical properties of post-treated additively manufactured Inconel 718 superalloy under quasi-static and cyclic loading[END_REF][START_REF] Wan | Effect of scanning strategy on mechanical properties of selective laser melted Inconel 718[END_REF]. From the comparison with cast and forged Inconel 718, these references indicate a trend regarding the changes in the mechanical properties in tension. Due to the presence of interdentritic precipitates and the development of γˊ and γ〞 phases, the as-fabricated AM samples are characterized by finer dislocated cellular structures, higher ductility and transgranular ductile fractures in tension [START_REF] Li | Microstructural evolution and mechanical properties of IN718 alloy fabricated by selective laser melting following different heat treatments[END_REF] [START_REF] Zhang | Effect of standard heat treatment on the microstructure and mechanical properties of selective laser melting manufactured Inconel 718 superalloy[END_REF]. After standard solution and ageing treatments of PBF manufactured 718 superalloys, a transgranular ductile fracture is also observed. Their tensile properties are about the same with respect to cast and forged Inconel 718; however, their ductility dramatically decreased resulting from the precipitation of fine γˊ and γ〞phases in the austenitic matrix and needle-like δ phases inside the grain and at the grain boundaries [START_REF] Sui | The influence of Laves phases on the high-cycle fatigue behavior of laser additive manufactured Inconel 718[END_REF] [START_REF] Zhang | Effect of standard heat treatment on the microstructure and mechanical properties of selective laser melting manufactured Inconel 718 superalloy[END_REF].

Microstructure and fracture behaviour

Very few studies have specifically addressed the fracture toughness of metallic materials produced by AM [START_REF] Quénard | Measurement of fracture toughness of metallic materials produced by additive manufacturing[END_REF][START_REF] Puppala | Evaluation of fracture toughness and impact toughness of laser rapid manufactured Inconel-625 structures and their co-relation[END_REF][START_REF] Solberg | Notch-defect interaction in additively manufactured Inconel 718[END_REF]. As reviewed in literature [START_REF] Lewandowski | Metal Additive Manufacturing: A Review of Mechanical Properties[END_REF], orientation-dependent microstructures, texture, and defects result in different tensile properties and even more significant differences in the fracture-critical properties (e.g. fatigue crack growth, fracture toughness). Indeed, the PBF process may induce microstructures with columnar architectures, parallel to the building z-direction (along the heat flow direction). In the x-y plane, a unique structural morphology was observed using different cooling rates resulting from laser beam overlapping. Post-processing (e.g., heat treatment and/or Hot Isostatic Pressing) can reduce or eliminate process-induced defects (e.g. lack of fusion and porosities) but conversely influence the additive manufactured materials cost [START_REF] Mostafa | Structure, texture and phases in 3D Printed IN718 Alloy Subjected to Homogenization and HIP Treatments[END_REF]. These post-processing treatments also contribute to the reorganization of the highly textured columnar structure into fine columnar and/or equiaxed grains (whose average grain size is about a hundred µm) of random orientations. Based on Single-Edge-Notch Bending (SENB) tests conducted by means of a Charpy impact pendulum, the fracture toughness of AM specimens appear to be close to that reported in Inconel weld metal ones [START_REF] Puppala | Evaluation of fracture toughness and impact toughness of laser rapid manufactured Inconel-625 structures and their co-relation[END_REF].

Objectives of the study

This study is expected to be helpful to engineers willing to design Inconel 718 parts, processed by Additive Manufacturing, for structural applications. Considering the lack of knowledge on the fracture behavior and toughness of additive manufactured nickel-base superalloys 718, this study proposes an original contribution to the comparison of cast and PBF materials via: (i) an evaluation of the mode I fracture toughness at room temperature -(ii) an accurate in situ monitoring of the crack propagation based on digital picture analyses -(iii) an insight into the fracture mechanisms associated with fractographic examinations.

Ultimately, the purpose is to better understand how the microstructure induced by additive manufacturing is related to the ductile fracture behavior of Inconel 718. In order to focus on the fracture properties and behavior of cast and AM materials, all specimens were tested as received, with no prior heat treatment.

The paper is organized as follows. After a brief introduction of the studied materials, the experimental procedure will be detailed. A particular attention will be paid to the presentation of the digital image analysis from which the in situ crack growth was evaluated during mechanical loading. Then, the J-R curves were obtained from the image analysis and the computation of the fracture toughness from the ASTM E1820-01 standard test method [START_REF]Standard Test Method for Measurement of Fracture Toughness[END_REF].

Finally, these curves are discussed along with the fractographic examination of fracture surfaces.

Materials and experimental procedure

Materials

Cast and additive manufactured Inconel 718 alloys are studied. The chemical compositions of this alloy are given by the providers in Tables 1 and2 for cast and AM materials, respectively.

The cast round bars of IN718 are made by means of Vacuum Induction Melting (VIM) followed by Vacuum Arc Remelting (VAR) in order to produce the desired alloy composition.

The VIM process insures the entire bulk of the alloy to be liquid. The VIM process can accomplish a degree of refining by removing oxides and nitrides.

Their tensile mechanical properties are presented in Table 3. Cast round bars and AM specimens were tested as received (with no heat treatment). It is therefore expected that the mechanical behavior and properties significantly differ between cast and AM specimens. AM Inconel 718 blocks (35x100x100mm 3 ) were manufactured by Company VolumE with an EOS M400-4 PBF machine under argon from a powder whose particles average diameter is about 30µm. The laser scanning speed and power, as well as the hatch distance used for blocks manufacturing are indicated in Table 4. The porosity rate of AM specimens is about 0.27%. It was calculated from the specimen density measured by means of a Sartorius Secura densimeter whose accuracy is about 1 mg. Both types of materials were machined into parallelepipedic samples (Fig. 1) in agreement with the ASTM E1820-01 standard test method [START_REF]Standard Test Method for Measurement of Fracture Toughness[END_REF]. The edge notch was machined by means of a precision endless diamond wire saw whose radius is 0.085 mm.

Experimental procedure

Mechanical testing

Tensile tests

Tensile tests were conducted in displacement-controlled mode (2 mm/min) at room temperature using a 100kN capacity load cell of a MTS electro-mechanical testing machine.

These conditions insure a quasi-static loading with a strain rate of about 10 -3 s -1 . The axial strain was measured by means of a blade extensometer whose gage length is 12mm. The dimensions of the tensile specimens are specified in Fig. 2a. According the build direction, three types of tensile specimens (A, B and C) were machined to evaluate the influence of the AM process on the tensile properties (Fig. 2b). The anisotropy of cast samples has not been studied by mechanical testing in different directions as cast specimens were machined from as-received round bars.

Single-edge-notch bending tests SENB tests (Fig. 3) were performed at room temperature using a 100kN capacity load cell of an MTS 810 servo-hydraulic testing machine in quasi-static displacement-controlled mode (1mm/min). The mechanical properties in bending (modulus, strain and stress) were determined according to the following relationships:

= and = (1)
Where is specimen's thickness, is the span between the support points, and is the displacement applied to the upper part of the specimen (Fig. 3).

Fracture toughness estimation

Inconel 718 alloys have a very ductile behavior [START_REF] Stout | Structural Property/Continuum Synthesis of Ductile Fracture in Inconel 718[END_REF][START_REF] Menke | An Elastic-Plastic Toughness Characterization of Alloy 718 Product Forms[END_REF]. To assess the structural integrity of the material in the presence of preexisting defects, it is necessary to quantify their fracture behavior by means of the J-integral vs. crack growth resistance (J-R) curve [START_REF] Mills | Variations in Fracture Toughness for Alloy 718 Given a Modified Heat Treatment[END_REF][START_REF] Mills | The Deformation and Fracture Characteristics of Inconel X-750 at Room Temperature and Elevated Temperatures[END_REF][START_REF] Mills | Fracture toughness variations for alloy 718 base metal and welds[END_REF]. These curves represent the energy per unit of fracture area required to initiate and propagate the crack. The value of the fracture toughness at the initiation is called . This value is associated with the mechanical energy required for the fracture to occur in the material [START_REF] Begley | The J Integral as a Fracture Criterion[END_REF].

The procedure for obtaining the J-R curve is described in ASTM E1820-01 standard test method. Depending on the fracture mode, this standard recommends the use of specimens with different geometries on which are machined notches for stress concentration and crack initiation purposes. Thus, fracture toughness tests were conducted on SENB specimens whose dimensions are given in Fig. 1. In order to estimate the influence of possible anisotropy induced by PBF process on the fracture behavior, notches were machined with different plane orientations with respect to the lasing planes (Fig. 4). Three specimens were tested in each AM material type and four in cast material.

In materials with ductile fracture behaviors, the mode I fracture toughness is computed from the elastic-plastic fracture mechanics concepts proposed by [START_REF] Rice | Some further results of J-integral analysis and estimates. Progress in flaws growth and fracture toughness testing[END_REF][START_REF] Sumpter | Method for laboratory determination of Jc. Cracks and fracture[END_REF][START_REF] Turner | The ubiquitous η factor. In: Fracture mechanics: twelfth conference[END_REF][START_REF] Roos | A procedure for the experimental assessment of the Jintegral by means of specimen of different geometries[END_REF] :

= + (2)
With computed from the mode I critical stress intensity factor [START_REF] Tada | The stress analysis of cracks handbook[END_REF] and the elastic constants of the materials (Young modulus ! and Poisson ratio "):

= # $% ('() ) + (3)
And the plastic component is obtained from the mechanical energy dissipated , corresponding to the area under the force-displacement curve:

( ) = - ( (') + . /0 (123) (123)
?9

where @ ( (') = 1.9 and = ( (') = 0.9 are plastic factors in agreement with ASTM E1820 standard test method in the case of SEN specimens loaded in bending. . C D is the net specimen thickness. E ( ) and ( ) = F -E ( ) are the crack length and the unbroken ligament at iteration (G), respectively. These values are computed for each iteration (G) by means of the experimental data obtained from the digital image analysis detailed in the section 2.2.3.

In notched specimens, the triaxiality factor HI is defined from the mean stress J = 1 3 ⁄ ( ' + + ) and the equivalent Von Mises stress M as follows:

HI = N O N P (5)

In situ location of the crack tip

A high-speed monochrome Grasshopper® camera was used to record digital images of the outer surface during thermomechanical loading. By means of an algorithm based on a binarization method and implemented in the free source numerical computation software Scilab, digital images were thresholded in order to track the location of the crack tip during mechanical loading. To know the position of crack tip, images are computed by the algorithm from top to bottom in search of a pixel value change, therefore indicating the crack tip. The physical meaning of the crack tip is also verified by comparing its location between two successive images (Fig. 5).

Microstructure characterization and fractographic analyses

The microstructure was characterized by means of a Keyence VHX-1000 digital microscope and a JEOL 7900F Scanning Electron Microscopy (SEM) coupled with Electron BackScattered Diffraction (EBSD). For all microstructure observations, acquisitions were done using EDAX OIM analysis software with a 0.7 µm step on samples electro-polished with an A2 solution at 24V for 30 seconds (Struers Lectropol-5). For melting pools characterization, chemical attack was performed at 10V during 5 seconds using the same electrolyte. Fracture mechanisms were analyzed and discussed by means of microscopic (Optical and SEM) observations.

Results and discussion

EBSD microstructure analyses

The microstructure was investigated by SEM-EBSD to reveal the grains crystallography and grain morphology. The XY and YZ/XZ EBSD maps reveal an anisotropy on the grain's morphology of PBF samples. For the XY map, the individual laser scan tracks are identified within many single tracks, and the width of the tracks was measured to be about 100 μm. This corresponds to the hatch distance applied during the process. The laser scan tracks were composed of small equiaxed grains and inter-tracks large grains causing a large grain size distribution (cf. insert in Fig 6a). For the cross-section maps parallel to the building direction (YZ and XZ), the microstructure was dominated by cellular dentrites elongated along the building direction. It causes a grain shape aspect ratio lower than that measured in (XY) plane. Grains were longer than the layer thickness and pool depth resulting from an epitaxial growth occurring during the PBF process. As shown in Tab. 5, the maximum of pole figure <5 implies a random texture of PBF samples and the low value of the Grain Orientation Spread parameter (GOS), indicated homogenous grain orientation for these samples.

In literature, similar microstructure has been observed for the PBF IN718 [22][48-49];

however, these samples are often textured. Here, the low texture observed may result from the chosen scanning strategy (67°) [START_REF] Sun | Effect of scanning strategy on texture formation in Ni-25at.%Mo alloys fabricated by selective laser melting[END_REF]. In Fig. 6b, the microstructure of cast sample presents an equiaxed grains with some annealing twins, typical for Ni-based superalloys [START_REF] Shi | Twin boundary characters established during dynamic recrystallization in a nickel alloy[END_REF][START_REF] Mortezaie | An assessment of microstructure, mechanical properties and corrosion resistance of dissimilar welds between Inconel 718 and 310S austenitic stainless steel[END_REF][START_REF] Song | Effects of different heat treatments on the dynamic shear response and shear localization in Inconel 718 alloy[END_REF]. The presence of these twins affects the aspect ratio value for cast sample (cf. Tab. 5). Similar average grain size range (20-40 µm) is found for both cast and PBF samples. A homogeneous texture can also be observed in cast samples. But the GOS value is higher than that of PBF samples revealing partial microstructure recovery. As shown in Fig. 2b and Fig. 6, microstructures of Type A (YZ) and Type B (XZ) samples with respect to the loading direction are different. There is a rotation angle of 90° of the elongated grains, where the loading is perpendicular and parallel to the building direction for samples A and B, respectively.

Mechanical behaviour

Monotonic tensile tests

From the macroscopic standpoint, all specimens are characterized by an elastic-ductile behavior (see Fig. 7). The tensile response of AM specimens is similar, but cast specimens are much more ductile with an elongation at break (about 40%, cf. Tab. 6). This is virtually 50% higher than the value observed in AM specimens, suggesting that AM significantly modifies plastic deformation mechanisms. From the tensile properties standpoint, type C specimens significantly differ from other specimens as their yield and ultimate tensile strengths are 20% and 10% lower, respectively (Tab. 6). As far the axial stiffness is concerned, there are significant differences between the AM specimens as type C value is 1.5 times that of type A for which the average value is similar to the one of cast specimens. This result suggests that lasing planes are somehow equivalent to a laminated material such as the formation of columnar grains through the sample thickness (Fig. 6c) may provide good adhesion between the layers. Ultimately, it may contribute to a significant improvement of the axial stiffness with respect to other AM specimens.

SENB tests

From the definitions given in Eq. ( 1), the bending stress-strain curves are drawn from loaddisplacement data. These curves clearly imply that both cast and AM specimens are characterized by large plastic deformations (up to about 6%) followed by ductile fracture behaviors (Fig. 8). The manufacturing process significantly influences the mechanical behavior (plasticity and fracture) of Inconel 718 Single Edge Notch specimens subjected to bending loading at ambient temperature. With respect to cast specimens, both yield and maximum strengths of AM specimens decrease (see Tab. 7) whereas the maximum strain is much higher (Fig. 8).

It is worth noticing that cast specimens reach ultimate failure. This is not observed in AM samples at the end of test. In type A specimens, the decrease in yield and maximum strengths is about -24% and -13%, respectively. In type B specimens, though a more significant dispersion of experimental data is observed, the decrease is about -8% and -10%, respectively.

The decrease in yield strength and the increase in maximum strain observed in AM samples suggest that plastic deformation mechanisms are more extensive in these samples. It also indicates that ductility will be play a prominent role into the fracture initiation and propagation as pointed out in Fig. 8. Initiation point is determined from the surface crack onset based on the examination of digital pictures. In addition, displacement-controlled mechanical tests imply that the crack initiation starts slightly earlier in cast specimens. This feature seems to indicate that earlier local plastic deformation (resulting from lower yield strength) near the crack tip may contribute to delaying the onset of the crack.

The crack propagations also differ between the cast and AM specimens (Fig. 9). Indeed, the rate of cracking appears to be steady until ultimate failure in cast samples (at about 6mm of transverse displacement). Nevertheless, it is slightly slower and gradually decreases from an applied displacement of about 6mm in AM specimens. These observations also suggest that more extensive plastic deformation mechanisms will take place within these specimens, therefore resulting in slowing down the crack growth. Finally, the value of the crack length is slightly lower in AM specimens in which the ultimate failure is not observed at the end of the bending test.

Evaluation of the fracture toughness

As far fracture toughness and behavior are concerned, the notches plane orientations with respect to the lasing planes (Fig. 4) are expected to have a significant influence resulting from the anisotropy induced by the additive manufacturing process (Fig. 10).

As indicated in the previous section, all specimens experience ductile fracture with a slow depending on the manufacturing process from which the specimens are obtained (Fig. 11).

Crack blunting is clearly observed in cast specimens with two different slopes in J-R curves, but not in AM specimens. This observation is rather unexpected as the yield strength of AM specimens is lower (-10 to 24%) with respect to cast specimens. It therefore suggests that early plastic deformations do not promote extensive plasticity near the crack tip. It also indicates that the onset of plastic mechanisms within the materials does not necessarily mean lower fracture toughness at initiation. Finally, the slope of AM specimens J-R curves is steady and virtually the same as the one of cast specimens during the stable ductile tearing phase.

As commented previously when discussing the mechanisms of crack blunting, another major difference between cast and AM specimens is the value of fracture toughness at crack initiation (Fig. 12). This value is deduced from the value of the strain energy release rate when crack length starts growing. It clearly appears that fracture toughness at crack initiation significantly increases in AM specimens, as their value is about 70% and 100% higher, in types A and B specimens, respectively. The question is therefore to know why fracture toughness at initiation is higher in AM specimens.

These previous results tend to illustrate that cast and AM specimen fracture behaviour primarily differ in the early stage of fracture resulting from different fracture mechanisms.

These mechanisms are addressed by means of fractographic analyses based on macroscopic (Optical) and SEM observations of fracture surfaces. They also result from specific microstructure features that will be further investigated in the section 3.4.

Fractographic analyses

Three point bending loading conditions applied to SEN specimens are expected to cause a mode I fracture mechanism. Even for pure mode I macroscopic loading, local mixed-mode (I and III modes) is observed depending on material ductility or specimen thickness [START_REF] Madi | Mixed mode ductile fracture of an anisotropic 2024 Al-Cu alloy[END_REF][START_REF] Mahgoub | Three-dimensional stress and deformation fields around flat and slant cracks under remote Mode I loading conditions[END_REF]. To evaluate the influence of microstructure (depending on manufacturing process and heat treatments) on fracture mechanisms, fractographic analyses were conducted at macroscopic scale via digital microscope observations, and at microscopic scale via SEM observations.

Macroscopic observations of fracture surfaces

As expected, the macroscopic observations of fracture surfaces are relevant to draw conclusions on both the fracture modes and the corresponding fracture energies which are associated with fracture toughness. All Inconel specimens are characterized by ductile fracture behaviors. In specimen core, the fracture surface is primary flat (plane strain conditions). On the outer surfaces of specimens, fracture surfaces form shear lips resulting from a ductile mixed mode failure [START_REF] Belan | The fractography analysis of IN718 alloy after three-point flexure fatigue test[END_REF]. The material is in state of plane stresses, plastic deformation mechanisms occur by shearing along the 45° direction towards the lateral surface (Fig. 13). In addition, shear lips open freely on the surface. Plasticity is prominent near the outer surfaces where the triaxiality factor remains low. The shear lips are not very large in cast and type B specimens. The ratio of flat surface over lips surface is a rough approximate of the differences in triaxiality factors between tested materials. This ratio is higher (about 2.3) in cast and type B specimens with respect to type A specimens (about 1.5). These results suggest that the failure modes differ from one manufacturing process to another. In all specimens tested, the ductile fracture processes either occur in mode I (specimen core) or mixed-mode (outer surfaces) depending on the triaxiality rate. The critical fracture energy Jc is low in specimen core (which is in plane strain state); however, the fracture energy is much higher in the shear lips (which are in plane stresses states) because plasticity is more extensive on the outer free surfaces. The fracture energy is therefore minimal in plane strain and is called the critical fracture toughness in mode I (denoted JIc). It is independent from specimen thickness as it is sufficient for the plane strain state to be achieved. JIc is therefore an intrinsic characteristic of the material. When the shear lips are large (type B specimen), the mixed-mode fracture requires more mechanical energy to be dissipated than the energy required in mode I fracture.

Hence, the corresponding fracture toughness is expected to be higher in type B specimens.

In cast and type A specimens, the shear lips are confined in the vicinity of the outer surfaces (Fig. 14a and14b). Fracture primarily occurs in mode I and the fracture energy is referred to as JIc. In type B specimens, the shear lips in plane stress occupy an important part of the fracture surface (Fig. 14c). This is the same effect as that observed when specimen thickness decreases. It therefore suggests that the position of the lasing planes perpendicular to the initial machined notch contribute to the increase in the fracture energy resulting from mode I (specimen core) and mixed-mode fractures. In addition to the macroscopic observations of fracture surfaces from different point of view, the influence of the manufacturing process is also clearly noticeable when considering the surface crack paths. The crack path is obtained from the digital image analysis (described in section 2.2.4). This allows the in situ location of the crack tip during the mechanical loading (Fig. 15). In cast specimens, crack growth is primarily driven by a mode I fracture as the crack virtually remains in the initial notch plane what is also known as self-similar fracture. In AM specimens, the crack path on the outer surface does not remain in the initial notch plane (also known as non-self-similar), therefore confirming a mixed-mode failure mode.

Microscopic observations of fracture surfaces

SEM observations provide insights into the influence of the manufacturing process on the mechanisms associated with crack growth at microscopic scale (Fig. 16). The fracture surfaces of cast and additively manufactured specimens present a dimpled surface. This is a feature of a transgranular ductile failure mode. Dimples result from the debonding between the f.c.c. matrix and the secondary phase inclusions. It appears that the size of the dimples significantly depends on the triaxiality factor. Thus, the SEM observations of flat surfaces (high triaxiality factor) in Inconel 718 SENB specimens imply dimples with similar size in all specimens (Fig. 17); however, the SEM observations of shear lips (low triaxiality factor) present dimples only in cast specimens (Fig. 16). In AM specimens, extensive channel fracture (striations) surrounding small primary dimples with large walls are observed along with secondary cracks (Fig. 17b and17c). Striations are marks produced on the fracture surface that reveal the incremental growth of cracks. Striations are more specific to AM specimens and their formation may result in delaying the advance of the crack front, hence causing higher fracture toughness with respect to cast specimens. SEM observations of the core area at high magnification (x1000) also reveal that fracture surfaces are characterized by micro-voids nucleated at the inclusions (Fig. 18). Micro-voids coalescence is usually observed in materials experiencing a ductile fracture mode.

In cast specimens, the fracture process involves the formation of primary microvoids at failed inclusions (Fig. 18a). Subsequent separation of the ligaments between primary micro-voids then caused the crack front growth. As pointed out by Mills et al, ligament separation in the cast specimens involved three possible mechanisms: coalescence of micro-voids, void sheet formation initiated by the y" phase, and channel fracture whereby localized tearing occurred along planar slip bands [START_REF] Mills | Variations in Fracture Toughness for Alloy 718 Given a Modified Heat Treatment[END_REF]. The latter two mechanisms usually occur during the final stages of fracture as primary micro-voids were about to impinge; hence, they did not necessarily degrade fracture resistance. As indicated by the macroscopic observations of fracture surfaces (previous section), the failure mode is usually referred to as mode I fracture in cast specimens.

It is dominated by void initiation occurring at a second phase particle or inclusions, and followed by void growth in the crack plane. SEM observations at higher magnification reveal that fracture surfaces consist of small dimples each being nucleated at a precipitate whose size and shape are typical of the δ Laves phases as commonly observed in the literature [START_REF] Stout | Structural Property/Continuum Synthesis of Ductile Fracture in Inconel 718[END_REF].

Indeed, the primary fracture mechanism is a slow tearing process within large pockets of inhomogeneous carbides and nitrides. In cast specimens, this fracture is associated with fracture toughness values ranging from about 400kJ/m² at initiation to values of about 810kJ/m² at ultimate failure. As pointed out by many authors, the fracture mechanism involves fracture initiation at carbides and nitrides followed by off crack plane void sheet growth nucleated at the Laves intermetallic phases [START_REF] Mills | Variations in Fracture Toughness for Alloy 718 Given a Modified Heat Treatment[END_REF][5] [START_REF] Zhang | Comparison of microstructures and mechanical properties of Inconel 718 alloy processed by selective laser melting and casting[END_REF].

In AM specimens, SEM observations with high magnification of core fracture surfaces also reveal micro-voids coalescence and a large number of dimples. These features are representative of a transgranular failure mode [START_REF] Wan | Effect of scanning strategy on mechanical properties of selective laser melted Inconel 718[END_REF]. A few cracks may initiate from processinduced porosities. SEM observations also suggest that the growth of the cracks initiated from the secondary phases may be delayed by the presence of striations (Fig. 18b). Indeed, the dimples are characterized by large walls as forming striations. The formation of these striations is commonly observed in specimens subjected to fatigue loadings resulting from the incremental crack growth. These striations interact with the secondary phases in the interdendritic regions [START_REF] Wan | Effect of scanning strategy on mechanical properties of selective laser melted Inconel 718[END_REF]. In cast specimens, due to the inhomogeneous deformation between the austenitic matrix and the secondary phases, these phases are preferential sites for cracks nucleation (Fig. 18a).

Finally, the coarse grain microstructure of AM specimens is characterized by columnar grains (Fig. 6) resulting in intergranular fracture with secondary cracks initiating at the grain boundaries [START_REF] Popovich | Creep and Thermomechanical Fatigue of Functionally Graded Inconel 718 Produced by Additive Manufacturing[END_REF]. In AM specimens, depending on the location of the initial notch with respect to the lasing planes, the energy required to initiate cracking from secondary phases ranges from 666-790kJ/m² in type A and B specimens. The energy required to grow cracks until ultimate failure is about 750kJ/m² and 936kJ/m² in type A and type B specimens, respectively.

The presence of secondary cracks on core fracture surfaces (Fig. 17) may result from the different crystallographic textures (Fig. 6) or from higher triaxiality factors in AM specimens associated with the spatial distribution of lasing planes [START_REF] Algarni | A study of Inconel 718 dependency on stress triaxiality and Lode angle in plastic deformation and ductile fracture[END_REF]. In other words, the formation of secondary cracks could be fostered between lasing tracks. The presence of these secondary cracks may also dissipate the mechanical energy, hence contributing to the increase in the fracture toughness in AM specimens. In type B specimens, the crack growth through two successive layers (or lasing planes -see Fig. 10) is expected to be more difficult to propagate than in type A specimens in which the crack propagates more easily at the interface between two layers. As a result, the mechanisms responsible for these crack growths may reflect on the fracture toughness and explain why higher values are observed in type B specimens.

Conclusion

The influence of manufacturing processes (cast and additive manufacturing) on the microstructure and mechanical behavior of Inconel 718 alloys is addressed. The tensile behavior and the ductile fracture were investigated along with their underlying deformation and damage mechanisms. A few major conclusions are drawn from this study:

• Without heat treatment, cast and AM specimens have different microstructures. Cast specimens are characterized by significant grain disorientations whereas AM samples imply a morphological texture.

• Significant changes in axial stiffness suggest that lasing planes are equivalent to a laminated material such as the formation of columnar grains through the sample thickness may provide good adhesion between the layers.

• All specimens experience a transgranular ductile fracture at macroscopic scale, and the analysis of fracture surfaces reveal different fracture mechanisms at microscopic scale.

• With respect to cast specimens, the critical fracture toughness (at crack initiation) of AM specimens is about 70% and 100% higher, in types A and B specimens, respectively.

• AM specimens do not experience a purely mode I fracture contrary to cast specimens.

They are characterized by curved crack paths, resulting in larger fracture surfaces.

These differences may explain why AM specimens have much higher fracture toughness at initiation.

• The J-R curves, obtained from the ASTM standard E1820-01, are different depending on the manufacturing process. Once the crack is initiated, the energy required to grow the crack, during the stable ductile tearing phase, is the same in all specimens.

• Due to different microstructures and depending on the location of the initial notch with respect to the lasing planes, the crack growth through two successive lasing planes (type B specimen) is more difficult to propagate than in type A specimens in which the crack propagates more easily at the interface between two layers. As a result, the mechanisms responsible for these crack growths may reflect on the fracture toughness and explain why higher values are observed in type B specimens.

In a forthcoming paper, the influence of testing temperature on the fracture toughness properties of cast and AM Inconel alloy 718 will be investigated. Tab. 6 -Influence of the manufacturing process (cast or PBF AM) on the tensile mechanical properties. Error bars are computed with a confidence index of 95%.

Figure captions

  Fig.6a-c-ddisplay the inverse pole figure (IPF) of normal direction XY, YZ and XZ planes for PBF samples, as described in Fig.2b. The IPF map of the cast sample is shown in Fig.6bfor comparison. EBSD results from these maps are summarized in Tab. 5.

  crack growth. The description of ductile fracture behaviour is classically based on the J-R curves. These curves require the evaluation of the fracture energy (or strain energy release rate) denoted by = + and the crack extension ∆a along with the mechanical loading. The strain energy release rate is computed from Eqs 3-5 and the experimental data resulting from SENB tests. The estimation of crack length is obtained from the in situ location of the crack tip by means of the optical-based technic described in section 2.2. In materials characterized by a ductile fracture, J-R curves are typically characterized by successive deformation mechanisms: crack blunting then stable plastic tearing. Crack blunting results from the dissipation of mechanical energy resulting from plastic deformation mechanisms near crack tip when crack initiates. In the early stage of loading, crack blunting is primarily observed at specimen surface where the triaxiality factor is low what facilitates local plastic deformations. It therefore causes the formation of a deformed area prior to material separation. While the lips of the initial notch open along with plasticity, the energy required by the crack to propagate increases. Usually, crack blunting is characterized by a momentary steep increase in the value of with little crack growth. The J-R curves are different

Fig. 1 -Fig. 2 -Fig. 3 -Fig. 4 -Fig. 5 -Fig. 7 -Fig. 8 -Fig. 9 -Fig. 10 -Fig. 11 -Fig. 12 -Fig. 13 -Fig. 14 -Fig. 15 -Fig. 16 -Fig. 17 -Fig. 18 -

 12345789101112131415161718 Fig.1-Single-edge-notch bending specimens for fracture toughness estimation -ASTM

Tab. 7 -

 7 Influence of the manufacturing process (cast or PBF AM) on the yield and maximum strengths (with corresponding standard deviation) of Inconel 718 Single Edge Notch specimens subjected to bending loading at ambient temperature

  .063 0.031 0.01 0.002 0.024 0.192 2.92 0.501 0.92 0.005 3.28 49.9 18.13 19.11 Tab. 2 -Chemical composition of Inconel 718 powder for AM (source: supplier) Tab. 5 -Results obtained from the SEM-EBSD maps: average grain size, grain shape aspect ratio, the maximum value of pole figure (Max PF) and average Grain Orientation Spread (GOS) for maps shown in Fig.6.

		Tables
					Grain size (µm)	Aspect ratio	Max PF		GOS (°)
		Cast		35±22		0.40		3.3			5.6±2.7
	C 0.043 0.32 Si PBF-XY Mn 0.18 0.009 0.002 0.07 0.063 2.96 P S Cu Co Mo 20±14 0.42 PBF-YZ 21±17 0.30	Al 0.68	Ti 0.85 0.005 4.93 B Nb 2.3 2.3	Ni 1.7±0.9 50.8 17.74 18.74 Cr Fe 1.9±0.8
		Tab. 1 -Chemical composition of cast Inconel 718 (source: supplier) PBF-XZ 40±20 0.30 3.3 2.0±0.9
	C	Si	Mn	P	S	Cu	Co	Mo	Al	Ti	B	Nb	Ni	Cr	Fe
	Young modulus (GPa) 174±1 153±1 211±3 0.032 0Young modulus Cast Type A Type B Tensile strength (MPa) Ultimate tensile strength (MPa) 1023±5 1035±7 1056±9 Type C 236±1 965±10	Yield strength 0.2% (MPa) 855±3 740±5 800±7 Yield strength 655±5	Elongation at break (%) 39±2 27±1 22±1 Elongation at break 27±2
			(GPa)						(MPa)			(%)
			192			1330				1085			17
		Tab. 3 -Room temperature tensile mechanical properties of cast round bars of Inconel 718
					(1010°C hardened tempered -source: supplier)
		Laser power		Laser scanning speed		Hatch distance		Hatch angle
			(W)			(mm/s)				(mm)			(°)
			400			1330				0.1			67
			Tab. 4 -Manufacturing parameters of AM Inconel 718 specimens


MD Regis Souche
email: fr-souche@chu-montpellier.fr

MD Antoine Coignac

MD Marie Dupuy

MD Martin Bertrand

MD Isabelle Raingeart

MD Boris Guiu

MD Astrid Herrero

MD Fabrizio Panaro

Fabienne Stéphane Obled

MD Stephane Obled

MD Fabienne Portales

MD Benjamin Riviere

MD Jeanne Ramos

MD Frederic Borie

MD Francois Quenet

MD Pierre-Emmanuel Colombo

MD Michel Prudhomme

MD Eric Assenat

MD Jean-Michel Fabre

Outcome after pancreatectomy for neuroendocrine neoplams according to the WHO 2017 grading system: A retrospective multicentric analysis of 138 consecutive patients

Keywords: pancreatectomy, surgery, neuroendocrine tumor, postoperative outcome, pancreas

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

INTRODUCTION

Neuroendocrine neoplams of the pancreas (panNET) are neoplasms that originate from the endocrine portion of the pancreas. These tumors, which newly affect 2-3 per 100,000 individuals per year, constitute only about 1% to 2% of all pancreatic neoplasms [START_REF] Marx | Multiple endocrine neoplasia type 1: clinical and genetic topics[END_REF][START_REF] Milan | Neuroendocrine tumors of the pancreas[END_REF]. PanNET can be classified as functioning and non-functioning [START_REF] Metz | Gastrointestinal neuroendocrine tumors: pancreatic endocrine tumors[END_REF]. At least 50% of panNET are non-functioning and hence asymptomatic. Prognosis of panNET depends on clinico-pathological factors including tumor size, differentiation and proliferative index that give an indication of the biological aggressiveness of the tumor and likelihood of lymph node involvement [START_REF] Scarpa | Pancreatic endocrine tumors: improved TNM staging and histopathological grading permit a clinically efficient prognostic stratification of patients[END_REF][START_REF] Martin | Analysis of 6,747 pancreatic neuroendocrine tumors for a proposed staging system[END_REF][START_REF] Toste | Nonfunctional pancreatic neuroendocrine tumors <2 cm on preoperative imaging are associated with a low incidence of nodal metastasis and an excellent overall survival[END_REF]. Complete surgical resection plays an important role in the curative treatment of patients with panNET. Several guidelines exist for the treatment of functioning and non-functioning panNET [START_REF] Falconi | ENETS Consensus Guidelines Update for the Management of Patients with Functional Pancreatic Neuroendocrine Tumors and Non-Functional Pancreatic Neuroendocrine Tumors[END_REF][START_REF] Kunz | Consensus guidelines for the management and treatment of neuroendocrine tumors[END_REF].

It is generally accepted that tumors > 2 cm and functioning tumors should be resected but the management of non-functioning tumors < 2 cm remains controversial. Further controversy exists as to the surgical approach (open vs. laparoscopic) and the lymphadenectomy. PanNET are associated with synchronous liver metastasis in 30-80% of patients and require dedicated multidisciplinary staff discussion [START_REF] Pavel | ENETS Consensus Guidelines for the management of patients with liver and other distant metastases from neuroendocrine neoplasms of foregut, midgut, hindgut, and unknown primary[END_REF]. The prognostic classification of NET has been challenging due to their relative rarity. The WHO 2010 grading system adopted the Ki-67 index and/or mitotic index to divide digestive system NEN into 3 main types: NET G1, NET G2 NEC G3. Several studies leading to the new WHO 2017 grading system had reported that NEC G3 is a heterogeneous category that can be separated into two subcategories with significantly different prognosis and more importantly, therapeutic strategies seem to be distinct [START_REF] Sorbye | Gastroenteropancreatic high-grade neuroendocrine carcinoma[END_REF][START_REF] Basturk | The high-grade (WHO G3) pancreatic neuroendocrine tumor category is morphologically and biologically heterogenous and includes both well differentiated and poorly differentiated neoplasms[END_REF][START_REF] Heetfeld | Characteristics and treatment of patients with G3 gastroenteropancreatic neuroendocrine neoplasms[END_REF][START_REF] Velayoudom-Cephise | Are G3 ENETS neuroendocrine neoplasms heterogeneous?[END_REF]. The new WHO 2017 classification (dividing G3 tumors in well differentiated NET G3 or poorly differentiated NEC G3) validation is ongoing for its ability to predict disease-free survival (DFS) and overall survival (OS) for panNET [START_REF] Inzani | The New World Health Organization Classification for Pancreatic Neuroendocrine Neoplasia[END_REF].

Therefore, we aimed to explore the clinicopathological characteristics, short-term and survival outcome on the basis of 138 consecutive patients with panNET, as well as to evaluate the prognostic value of the proposed new WHO 2017 classification.

PATIENTS AND METHODS

Patient selection and study design

This was a retrospective review of a prospectively maintained database including all consecutive patients requiring a pancreatic resection for panNET at 3 tertiary referral centers for pancreatic surgery in France, between January 1998 and December 2017. Patients with panNET that was symptomatic, functional, size ≥2 cm or with presence of aggressive features like pancreatic duct dilation underwent systematic pancreatic resection [START_REF] Falconi | ENETS Consensus Guidelines Update for the Management of Patients with Functional Pancreatic Neuroendocrine Tumors and Non-Functional Pancreatic Neuroendocrine Tumors[END_REF][START_REF] Janson | Nordic guidelines 2014 for diagnosis and treatment of gastroenteropancreatic neuroendocrine neoplasms[END_REF]. During the second part of the study period, small panNET (<2 cm) that were slow growing with indolent course were considered to observation regardless their location [START_REF] Bettini | Tumor size correlates with malignancy in nonfunctioning pancreatic endocrine tumor[END_REF][START_REF] Lee | Small, nonfunctioning, asymptomatic pancreatic neuroendocrine tumors (PNETs): role for nonoperative management[END_REF]. Patients with synchronous distant metastasis and G1 or G2 panNET, functioning tumor, resectable liver metastasis and absence of extra-hepatic metastasis were considered to surgery after systematic multidisciplinary discussion.

Only patients with well-controlled comorbidities and American Society of Anesthesiologist grade I, II, or III were candidates for surgery. All procedures were performed by at least one experienced (> 40 procedures) pancreatic surgeon.

Lymph node dissection was systematically discussed with the exception of some insulinomas. In case of a reassuring prognosis (panNET G1 at biopsy, no tumor growth over time, absence of suspicious lymph node on CT scan, positive 111 Inoctreotide scintigraphy and recently, the absence of FDG PET scan hypermetabolism, these last two suggesting a well-differentiated panNET), a parenchymal sparing pancreatectomy with lymph node picking was performed when feasible. During the first decade of the present study, lymph node pathological examination was not routine in case of G1 panNET.

After surgery, all patients were seen daily by a physician until hospital discharge.

Thoracoabdominopelvic CT scan with intravenous contrast injection was performed selectively in patients with suspected abdominal or thoracic complications.

Pancreatic fistula (PF) was defined and classified according to the ISGPF [START_REF] Bassi | Postoperative pancreatic fistula: an international study group (ISGPF) definition[END_REF]. Early postoperative hemorrhage was defined according to the ISGPS [START_REF] Wente | Postpancreatectomy hemorrhage (PPH): an International Study Group of Pancreatic Surgery (ISGPS) definition[END_REF]. Postoperative complications were stratified according to the Dindo-Clavien classification which defines major complications by a score of III or more [START_REF] Dindo | Classification of surgical complications: a new proposal with evaluation in a cohort of 6336 patients and results of a survey[END_REF]. Complications, readmissions, and operative mortality were considered as those occurring within 90 days of surgery, or at any time during the postoperative hospital stay [START_REF] Deoliveira | Assessment of complications after pancreatic surgery: A novel grading system applied to 633 patients undergoing pancreaticoduodenectomy[END_REF][START_REF] Mise | After Pancreatectomy, the "90 Days from Surgery" Definition Is Superior to the "30 Days from Discharge" Definition for Capture of Clinically Relevant Readmissions[END_REF].

Histopathologic analysis

Resection margin status was graded R0 (complete resection with no microscopic residual tumor), R1 (complete macroscopic resection but margins microscopically positive) or R2 (grossly residual tumor).

Histological grading and staging was based on the WHO-AJCC 2010 grading system and ENETS classification [START_REF] Bosman | WHO classification of tumours of the digestive system, 4th edn[END_REF]. Histological examination included regular haematoxylin and eosin staining, and additional immunolabelling using the standard avidin-biotin peroxidase method with antibodies against neuroendocrine markers (synaptophysin (27G12 -Novocastra), chromogranin A (LK2H10 -Ventana)), and Ki67/MIB1 (Dako). Tumor grade (G) was determined by mitotic countin and by Ki-67-labeling index. The mitotic rate was determined by counting mitotic figures in 50 high power fields (at 400x on an Leica microscope) and averaged to 10 high power fields. The Ki67 labeling index was determined by manual counting in the most intensely labeling regions (hot spots), by microscopic examination. At least 500 tumor cells were counted, for each case. All specimens were re-evaluated and regraded according to the WHO-AJCC 2017 grading system for PanNET by 2 expert TENpath pathologists [START_REF] Inzani | The New World Health Organization Classification for Pancreatic Neuroendocrine Neoplasia[END_REF][START_REF] Amin | AJCC cancer staging manual, Eight edition[END_REF].

Data collection and Follow-up

Demographic details, surgical treatment (enucleation, anatomic resection, multivisceral resection), pathological variables and follow-up information regarding survival and disease status were evaluated.

After pancreatic resection, all patients were followed-up every 3 months by clinical examination, chromogranin A levels determination, and chest/abdomen computed tomography scan for the first 2 years, and every 6 months thereafter. Follow-up data were collected during routine clinic visits or by contact with the referring physicians.

The end of follow-up was considered on December 31, 2017 or at the time of death.

During the follow-up, data regarding site, type, and treatment of panNET recurrence were recorded as well.

Statistical analysis

Categorical variables were expressed as percentages, and compared by Chi-square test or Fisher's exact test when appropriate. Continuous variables were expressed as median with range and compared using Student's t test or Mann-Whitney U-test as appropriate. Overall survival (OS) was calculated from the date of diagnosis to the date of death or of last follow-up. Disease-free survival (DFS) was calculated from date of surgery to time of first radiological evidence of local, regional, or distant relapse, or death due to panNET. OS were assessed using Kaplan Meier method and comparisons were performed using the log-rank test, for all characteristics of patients. Patients were censored as of their last follow-up visit if they were alive and/or disease-free throughout the study period. A P value <0.05 was considered statistically significant. Hazard ratios, with 95% confidence intervals (CI), were estimated using Cox proportional hazard models. For univariate and multivariate analyses, relevant clinicopathological variables were used to determine the association of each parameter with OS, DFS and N+ status. Only variables clinically or statistically relevant (p value of <0.05 at univariate analysis) were introduced in the multivariate model analysis. Data were analyzed with SPSS 21 (IBM, Armonk, USA).

RESULTS

Patient characteristics

A total of 138 patients underwent pancreatic resection for panNET in our 3 centers.

The age at surgery was 56 (13-77) years, and 48.5% (67/138) were female. ASA score at diagnosis was I/II in 86.5% of the patients. Diagnosis was incidental in 57 (41%) patients. panNET was sporadic in 132 patients (96%) and associated to Multiple Endocrine Neoplasia syndrome type 1 in 5 (3.5%) patients and Von Hippel-Lindau in 1 (0.5%) patients. Primary locations were equally distributed in the pancreas: head (32.5%), neck/body (32%) and tail (35.5%). Nineteen patients had synchronous metastasis at diagnosis (14%). Main metastatic sites were liver only (11%) and distant lymph nodes (2%). A preoperative biopsy was performed in 87 (63%) of the patients and was positive for panNET in 78 (89%). Key demographics, surgical and preoperative tumor characteristics are summarized in Table 1. Six patients (4%) received neoadjuvant treatment for locally advanced non metastatic lesions with cisplatine-etoposide (n=4) or doxorubicine-streptozocine (n=2). Seven metastatic patients (5%) with panNET G2 (n=5) or panNEC (n=2) received preoperative chemotherapy (doxorubicine-streptozocine (n=5) or cisplatine-etoposide (n=2)).

Surgical data

On 138 patients, surgical procedure was pancreaticoduodenectomy (29%), distal pancreatectomy (58%), enucleation (9%) and central pancreatectomy (4%). An open approach was used in 56% of patients and laparoscopy was used in 44% of patients with a rate of open conversion of 3%. Adjacent organ resection was required in 6 patients (4%): partial gastrectomy (n=1), small bowel resection (n=1), transverse colic resection (n=1) and liver resection (n=3; 1 right hepatectomy, 1 left lobectomy + wedge resection, 1 wedge resection).

In the 19 patients with synchronous metastasis, 4 symptomatic patients (aged of 49, 49, 39 and 43 years old) with preoperative jaundice due to a cephalic PanNET and potentially resectable liver metastasis underwent a pancreaticoduodenectomy after discussion in multidisciplinary meeting.

The 5 patients with PanNEC were under 65 years old and symptomatic. Two of them had potentially resectable liver metastasis and received a preoperative chemotherapy.

Three patients underwent a distal pancreatectomy and 2 a pancreaticoduodenectomy for panNEC.

Postoperative outcome

The postoperative outcome data are presented in Table 2. The overall complication rate was 44%. A postoperative pancreatic fistula (POPF) occurred in 23% of the patients and was mostly clinically relevant (grade B/C) (n=27/33). A major complication Clavien-Dindo ≥ 3 occurred in 14.5% of the patients and the reoperation rate was 10% essentially due to pancreatic fistula. The overall postoperative mortality rate was 0.7% (n=1). The cause of the postoperative death in one patient was grade C POPF after pancreaticoduodenectomy complicated by massive hemorrhage.

Pathological tumour characteristics

The pathology results are presented in Table 3. Resection of the panNET was complete (R0) in 92.5 % of cases. The median tumor size was 31 mm (+/-23, range: 2-140mm). The median number of harvested lymph nodes was 8 (range 1 -43) in 92 patients who underwent lymphadenectomy. Seven of 12 panNET graded NEC G3 according to the WHO 2010 grading system were well differenciated and re-graded as panNET G3 according to the new proposal WHO 2017 grading system. Of 24 asymptomatic patients with nonfunctional panNET smaller than 2 cm, 7 patients had tumors graded G2, and 5 of 10 patients who underwent lymphadenectomy had lymph node metastasis.

Factors associated with lymph node metastasis

In the sub group of patients with lymphadenectomy, factors associated with N+ status at univariate analysis were Ki67 > 2% (p = 0.028), tumor size > 2 cm (p = 0.044) and microvascular invasion (p = 0.005). None of them were independent predictors of N+ status after logistic regression.

Survival

The median follow-up, based on the Kaplan-Meier estimate of potential follow-up, was 79 months (range: 3-245 months). Overall (OS) and Disease free Survival (DFS) were 98, 89, 81, 68% and 93, 88, 79, 67% at 1, 3, 5 and 10 years, respectively (Figure 1).

The median OS was not reached. On 118 non metastatic patients, recurrence was experienced by 22.9 % of patients (n=27/118), with a median time interval until disease recurrence not reached (range: 12-245 months). OS and DFS according to WHO 2017 classification are displayed in Figure 2A and 2B respectively. Five-year overall survival differed according to WHO 2017 grading system: 95% among 58 patients with panNET G1, 82% in 68 patients with NETG2, 35% in 7 patients with NETG3 and 0% in 5 patients with pancreatic neuroendocrine carcinomas NECG3 (p<0.0001).

Prognostic factors analysis

Overall survival As shown in Table 4 resuming all significant variables at univariate analysis in the whole study cohort, independent factors associated with overall survival were age > 60 y.o (p=0.014), synchronous metastasis (p=0.005) and WHO-AJCC 2017 grading system with significant differences between NETG1 versus NETG2 (p=0.005), NETG3 (p<0.001) and NECG3 (p<0.001).

Disease-free survival Independent factors associated with Disease free survival were symptomatic NET (p=0.038), pN+ status (p=0.027) and WHO-AJCC 2017 grading system with significant differences between NETG1 versus NETG3 (p=0.014) and NECG3 (p=0.009). No significant difference was found between NETG1 and NETG2 in multivariate analysis (p=0.064).

DISCUSSION

This study, which involved three centers with the same clinical RENATEN network, has many strengths: only panNET patients were included, standardization of work-up across our centers and systematic pathological review by 2 TENpath labeled experts. Pancreatic neuroendocrine neoplasms were majority nonfunctioning, sporadic, single lesion of 31 mm in median size, non-metastatic with homogenous stage distribution. Our findings confirm several specific issues related to the management of panNET and the prognostic value of the recent WHO-AJCC 2017 grading system in a real-life setting, even with a relative limited sample size.

Surgery for panNET provides good results on OS and DFS, both higher than 80% at 5 years. Median recurrence time was not reached in the present study. However, pancreatic resection remains a challenging procedure and its morbidity must be systematically considered in therapeutic strategy with benefits/risks evaluation.

Morbidity after pancreatectomy is closely related to pancreatic fistula (23%) which is classically high due to (i) the soft texture of pancreas, (ii) an usual small diameter of main pancreatic duct and (iii) a larger number of patients who benefits of parenchymal sparing pancreatectomy [START_REF] Sauvanet | Medial pancreatectomy: a multi-institutional retrospective study of 53 patients by the French Pancreas Club[END_REF][START_REF] Cherif | Parenchyma-sparing resections for pancreatic neuroendocrine tumors[END_REF].

The present study found that WHO-AJCC 2017 grading system provides good discrimination between panNET G1, NETG2, NETG3 and NEC for OS and DFS. In line with the Rindi et al international cohort study, we found that OS and DFS following the resection of panNET G3 vs PNEC G3 significantly differed, thereby suggesting distinct aggressive behavior [START_REF] Rindi | Competitive Testing the Who 2010 Vs the Who 2017 Grading of Pancreas Neuroendocrine Neoplasia: Data from a Large International Cohort Study[END_REF]. DFS following the resection of panNET G3 vs G2 was statistically different whereas OS was not probably due to the low statistical power resulting from the relatively low number of panNET G3. These data reinforces this new subgroup of panNET G3 is the most aggressive form of well differentiated pancreatic neuroendocrine neoplasms, and as such should be carefully diagnosed and benefit from a dedicated treatment [START_REF] Rindi | Competitive Testing the Who 2010 Vs the Who 2017 Grading of Pancreas Neuroendocrine Neoplasia: Data from a Large International Cohort Study[END_REF]. These findings could suggest that resection may be considered as a treatment option for patients with panNET G3 contrary to panNEC G3. Progress in diagnosis tools to distinguish panNET G3 vs. panNEC are required in order to provide a better tailoring in the management of patients for personalized therapy. Larger prospective series dedicated to G3 tumors are required to explore this question.

In addition to the WHO-AJCC grading system, age and distant metastasis were independent predictors of worse survival in the present study; symptomatic panNET, lymph node metastasis were independently associated with disease recurrence.

We found that age over 60 years at diagnosis was an independent factor associated with poor survival as in the National Cancer Data Base study by Bilimoria et al. including 3851 patients. The group of patients aged 55 to 75 years and those over 75 had a worse survival than patients younger than 55 years (HR 1.57 and 3.04 respectively, p <0.0001) [START_REF] Bilimoria | Prognostic score predicting survival after resection of pancreatic neuroendocrine tumors: analysis of 3851 patients[END_REF].

The incidence of panNET has increased in recent years, particularly for small lesions [START_REF] Kuo | Population-level analysis of pancreatic neuroendocrine tumors 2 cm or less in size[END_REF][START_REF] Gratian | Impact of extent of surgery on survival in patients with small nonfunctional pancreatic neuroendocrine tumors in the United States[END_REF]. In multivariate analysis, we found that a symptomatic lesion (59% of cases) was an independent factor of poor DFS compared to incidental panNET. In the same way, the study by Cheema et al. reports a significant difference in 5-year disease-free survival rate between incidental panNET (86%) and symptomatic panNET (59% , p = 0.007) [START_REF] Cheema | Incidental detection of pancreatic neuroendocrine tumors: an analysis of incidence and outcomes[END_REF].

The present study included 86% of nonfunctional panNET in line with previous data [START_REF] Halfdanarson | Pancreatic neuroendocrine tumors (PNETs): incidence, prognosis and recent trend toward improved survival[END_REF]. In asymptomatic patients with nonfunctional panNET smaller than 2 cm, some patients had tumors graded G2, and 50% of patients who underwent lymphadenectomy had lymph node metastasis. These findings results are in line with previous studies that found a potential malignancy for small lesions [START_REF] Gratian | Impact of extent of surgery on survival in patients with small nonfunctional pancreatic neuroendocrine tumors in the United States[END_REF][START_REF] Sallinen | Prognosis of sporadic resected small (</=2 cm) nonfunctional pancreatic neuroendocrine tumors -a multi-institutional study[END_REF].

Considering we did not find any influence of tumor size on OS or DFS, this data highlights that a 'watch-and-wait' strategy for nonfunctional panNET < 2 cm should be confronted to a strong benefits/risks discussion in order to avoid under/overtreatment. The presence of liver metastases was an independent factor of poor prognosis [START_REF] Bilimoria | Prognostic score predicting survival after resection of pancreatic neuroendocrine tumors: analysis of 3851 patients[END_REF][START_REF] Chu | Predictive factors associated with long-term survival in patients with neuroendocrine tumors of the pancreas[END_REF][START_REF] Fischer | Outcome of surgery for pancreatic neuroendocrine neoplasms[END_REF]. Unlike other pancreatic carcinomas, patients with metastatic panNET can benefits of multimodal therapy and these patients justify expert multidisciplinary staff discussion.

The question of lymphadenectomy in modern parenchymal sparing era remains a major concern in patients with panNET. Although controversial, some studies have highlighted the impact of lymph node invasion on survival [START_REF] Fischer | Outcome of surgery for pancreatic neuroendocrine neoplasms[END_REF][START_REF] Curran | Importance of lymph node involvement in pancreatic neuroendocrine tumors: impact on survival and implications for surgical resection[END_REF][START_REF] Tomassetti | Endocrine pancreatic tumors: factors correlated with survival[END_REF][START_REF] Partelli | Pattern and clinical predictors of lymph node involvement in nonfunctioning pancreatic neuroendocrine tumors (NF-PanNETs)[END_REF][START_REF] Hashim | Regional lymphadenectomy is indicated in the surgical treatment of pancreatic neuroendocrine tumors (PNETs)[END_REF]. pN+ status was independantly asociated with poor DFS. Lymph node invasion was significantly associated with Ki67 > 2%, tumor size > 2 cm and microvascular invasion but logistic regression failed to identify independent predictors in the present study. A comprehensive analysis is difficult because of a high rate of surgery with unknown lymph node pNx status (46%) due to the selective lymphadenectomy and initial lymph node pathology examination policies of involved centers. The management and monitoring of N+ panNET remain a major concern such as they currently do not benefit from adjuvant treatment.

We are, of course, aware of some limitations of the present study. First, it is limited by its retrospective nature. Second, we have data only on resected patients, and it is unknown how many patients have been under surveillance during the same time period in our 3 centers. Third, the patient sample size, particularly in panNET with Ki67 > 20%, remains limited to provide meaningful subgroup survival analysis. However, our results were obtained from a homogeneous cohort of consecutive patients treated in the same clinical network and reflect real life. Missing data in our database were inferior to 5%.

In conclusion, the WHO 2017 grading system is a useful tool for patient survival prognosis after pancreatic resection and the tailoring of therapeutic strategy. PanNET G3 has an intermediate prognosis between panNET G2 and PNEC on DFS indicating that panNET G3 is the most aggressive panNET. Progress in diagnosis tools to distinguish panNET G3 vs. PNEC are required in order to provide a better management of patients in the era of personalized multimodal therapy.

FIGURES AND TABLES

The 3

 3 centers were Digestive Surgery Units of Montpellier, Nimes and Institute of Cancer of Montpellier. The University Institutional Review board approved the study (N°: 2019_IRB-MTP_01-02), which was registered on Clinicaltrials.gov registry (NCT03791346). The inclusion criteria were patients with histologically proven panNET who underwent pancreatic resection (by open, laparoscopic or robotic approach) with no previous or concurrent malignancy. Specimens of patients with incomplete pathological records were re-analyzed by pathologists. Patients with tumors diagnosed as adenocarcinoma with scattered neuroendocrine cells or with focal neuroendocrine component were excluded of this study. Surgical management and postoperative outcomes In each centers, indications for surgery were established during NET dedicated multidisciplinary meeting (Réseau National de Référence pour la prise en charge des Tumeurs neuro-Endocrines Malignes Rares Sporadiques et Héréditaires -RENATEN) including surgeons, gastroenterologists, radiologists, and TENpath labeled pathologists (TENpath: national network of expertise for the pathological diagnosis of adult and sporadic NET). Functional and hereditary lesions were diagnosed on the basis of the distinct clinical syndromes, serum elevation and positive immunohistochemistry of the relevant hormones. Computed tomography, magnetic resonance imaging, somatostatin receptor scintigraphy and endoscopic ultrasound were used for preoperative assessment of tumor location as deemed necessary.

Figure 1 -Figure 2

 12 Figure 1 -(A) Overall survival of 138 patients after surgery for pancreatic neuroendocrine neoplasm and (B) Disease-free survival of 118 patients after surgery for non-metastatic pancreatic neuroendocrine neoplasm (color should be used)

Table 1 -

 1 Demographic and clinicopathological characteristics of patients with pancreatic neuroendocrine neoplasms (n = 138)

Table 2 -

 2 Surgical data and postoperative outcome of patients with pancreatic neuroendocrine neoplasms (n = 138)

Table 3 -

 3 Pathologic results of patients with pancreatic neuroendocrine neoplasms (n = 138)

Table 4 -

 4 Multivariable Cox regression analysis of variables associated with overall survival and disease free survival after surgery for pancreatic neuroendocrine neoplasm

Table 4 -

 4 Multivariable Cox regression analysis of variables associated with overall survival and disease free survival after surgery for pancreatic neuroendocrine neoplasm HR : hazard ratio; *Variables with p value < 0.100 in univariate analysis; **statistical reference, WHO : World Heatlh Organisation

	Variables*	HR	95% CI	P value
	Overall survival
	Gender (male)	1.660	0.785 -3.510	0.185
	Age > 60 y.o	2.554	1.207 -5.405	0.014
	Synchronous metastasis	3.365	1.430 -7.916	0.005
	Microvascular invasion	1.633	0.671 -3.973	0.280
	Surgical margin (R1)	1.214	0.145 -10.193	0.858
	WHO 2017			<0.001
	NET G1**	_	_	_
	NET G2	3.084	1.048 -9.075	0.041
	NETG3	10.758	2.863 -40.422	<0.001
	NECG3	29.747	6.236 -141.889	<0.001
	Disease free survival
	Symptomatic NET	3.044	1.065 -8.703	0.038
	Functional tumor	0.161	0.020 -1.267	0.083
	pN			0.015
	N0**	_	_	_
	Nx	0.354	0.074 -1.704	0.195
	N1	2.739	1.123 -6.682	0.027
	Tumor size	0.		0.702
	< 20 mm**	_	_	_
	20-39 mm	1.721	0.371 -7.975	0.488
	> 40 mm	1.935	0.410 -9.129	0.404
	Microvascular invasion	1.962	0.809 -4.757	0.136
	Surgical margin (R1)	1.834	0.470 -7.159	0.383
	WHO 2017			0.031
	NET G1**	_	_	_
	NET G2	3.042	0.935 -9.892	0.064
	NETG3	5.703	1.429 -22.767	0.014
	NECG3	15.139	1.998 -114.718	0.009
	CI: confidence interval;

All authors declare that has participated sufficiently in the work to take public responsibility for appropriate portions of the content, as defined in to the guidelines of the International Committee of Medical Journal Editors (ICMJE).

Authors' contributions: RS, AC, MD, JR, BR, MP, EA and JMF were responsible for study concept and design. RS, AC, MD, MB, IR, BG, AH, SO, FP, BR, FB, JR, FQ, PEC, MP, EA and JMF contributed for acquisition and interpretation of data. RS, AC, MD, MB, IR, BG, AH, SO, BR, FB, JR, FQ, PEC, MP, EA and JMF performed drafting of the manuscript. RS, AC, MD, MB, IR, BG, AH, SO, FP, BR, FB, JR, FQ, PEC, MP, EA and JMF contributed for critical revision. All authors have viewed and approved the final version of the manuscript.

AKNOWLEDGMENTS

We thank RENATEN LR and the University of Montpellier for its support. We thank Lorenzo Ferre, Cyprien Toubert, John Chauvat and Françoise Guillon for data collection and their advice. We are grateful to Jacqueline Butterworth for editorial assistance. We thank the residents, fellows, and nurses of our division for the care they provided to these patients. The data will not be made publicly available, but is available from the authors on request.

The authors have no source of funding to disclose for this study.

DISCLOSURE

The authors declare no conflict of interest.