GLOBAL BEHAVIOR OF A NANOSTRUCTURED MULTILAYERED COMPOSITE MATERIAL PRODUCED BY SMAT AND CO-ROLLING

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1 GLOBAL BEHAVIOR OF A NANOSTRUCTURED MULTILAYERED COMPOSITE MATERIAL PRODUCED BY SMAT AND CO-ROLLING L. Waltz a,*, T. Roland a, D. Retraint a, A. Roos a, P. Olier b, J. Lu c a Institut Charles Delaunay, FRE CNRS 2848, LASMIS, University of Technology of Troyes, Troyes, France b DRT/LITEN/DTNM/LTMEX, Commissariat à l Energie Atomique de Saclay, Gif-sur-Yvette, France c Department of Mechanical Engineering, Hong Kong Polytechnic University, Hung Hom Kowloon, Hong Kong *laurent.waltz@utt.fr ABSTRACT The aim of the present work is to produce a nanostructured multilayer composite structure with enhanced mechanical properties by assembling three 316L surface nanostructured stainless steel plates using the co-rolling process. The SMAtreatment is first used to generate nanocrystalline layers on the elementary plates so that their mechanical properties are improved. They are then assembled through co-rolling. A composite structure where nanostructured layers of high strength alternate with more ductile layers is thus obtained. As a consequence, the material exhibits a global high strength and conserves a good ductility. In order to quantify the influence of different experimental parameters, such as shot diameter, reduction ratio, rolling temperature and number of SMATed sides, several tests were carried out. In order to follow the evolution of the microhardness through the section of samples, and consequently the evolution of the average grain size, nanoindentation experiments and microscopy observations were carried out. In parallel, tensile tests were performed to show the correlation between the microstructure evolution and the global mechanical response of the specimen. Introduction Nowadays, due to its excellent anticorrosion properties and formability characteristics, 316L austenitic stainless steel is a widely used engineering material in a large number of applications such as in the chemical, petrochemical and food industries. However, this material presents a relatively low mechanical strength which limits its use in applications which request materials with enhanced mechanical properties. In order to remedy this, in recent years, many studies relating to nanocrystallization methods by electrochemical and mechanical techniques such as inert gas condensation, electrodeposition, severe ball-milling and severe plastic deformation (SPD) have been carried out. The challenge was to improve the mechanical properties of materials without changing the chemical composition. Accordingly to this, Tao et al [1], Liu et al [2], Roland et al [3], Chen et al [4] and Lu et al [7] have shown that a recently developed Surface Mechanical Attrition Treatment (SMAT) technique may induce a grain refinement up to the nanometer scale in the top surface layer of metallic materials and alloys, based on mechanisms of severe plastic deformation. In the Ultrasonic Assisted Surface Mechanical Attrition Treatment (UASMAT), a technique developed recently at the Charles Delaunay Institute, spherical shot placed in a reflecting chamber is put in motion through an ultrasonic generator (20KHz) [7]. With the high frequency of the system, the entire surface of the sample is peened with a very high number of impacts over a short period of time. The main parameters affecting the results of this treatment are the shot diameter, the number of shot, the treatment time, the amplitude of vibration of the sonotrode and the temperature. The application of this process on a 316L austenitic stainless steel [3, 4], has shown a considerable enhancement of the strength and of the surface hardness of these materials. A 1mm thick tensile sample treated by SMAT on both sides for 15 minutes with 3 mm diameter shot reaches a yield strength as high as 525 MPa. In spite of the benefit of the yield stress, mechanical strength or the microhardness, this treatment leads to a notable decrease of the ductility of the treated samples, as shown in figure 1.

2 Figure 1. Effect of the treatment time on the tensile properties of 316L stainless steel treated by SMAT [3]. In addition, currently, none of the techniques developed allows the possibility to obtain a bulk nanocrystalline structure free of contamination, free of porosity and presenting a mechanical strength similar to the thin nanocrystalline structure. In the present work, a method is presented combining the superficial nanocrystalline treatment (SMAT) and the co-rolling process for the development of a bulk structure with improved yield and ultimate tensile strength, while conserving an acceptable elongation to failure. To characterize this new material, several tests have been carried out on plates treated with ultrasonic shot peening with different parameters, and on co-rolled composite structures. For that reason, a surface characterization of treated samples which has been performed simultaneously with a local hardness analysis will be presented. In addition, the influence of the SMA-Treatment parameters, the co-rolling parameters and annealing temperature will be studied. Experimental Superficial nanocrystalline structures were created by using the ultrasonic SMAT technique. The samples are sheets with dimension 120x120x1mm 3 and the as-received material was the 316L austenitic stainless steel, whose chemical composition is given in table 1. Elements C Si Mn P S Cr Mo Ni N Cu B Co Mass % Table 1. Chemical composition of the 316L stainless steel samples. In the as-received state, the 316L stainless steel is face-centered cubic (fcc) with an austenitic microstructure, with grain sizes ranging between 10µm and 50µm. After 30 minutes ultrasonic shot peening with perfectly spherical shot of 3 mm and with average amplitude of vibration of 25µm, the grain sizes observed by transmission electron microscopy (TEM) are in the nanometer scale and is ranging between 20 and 50nm in the top surface layer. The thickness of the nanocrystalline layer reaches a value of about 30µm [4,5,6,7]. The grain refinement leads to a considerable enhancement of the yield tensile strength, which is in agreement with the Hall-Petch relationship in equation 1: where : k y σ y = σ 0 + (1) d σ y = yield stress

3 σ 0 et k y = constant d = grain size However, below a critical grain size, estimated at 25nm, the relationship presented in equation (1) is not verified anywhere, and some authors have shown that the evolution of the yield stress according to the grain size from that limit on is with a negative slope [8, 9]. As shown in table 2, the treatment has been carried out with two different shot diameters (2mm and 3mm) in order to quantify the influence of the shot dimension on the final behavior of the co-rolled sheet. Firstly, noting that the dimensions of the rectangular sonotrode are relatively small, samples have been SMATed 30 minutes at three different spots on both sides. After that, the treated areas have been obtained by laser cutting. The second step after ultrasonic-assisted shot peening was to put three treated samples in a co-rolling sheath especially developed for this application. In fact, the use of co-rolling sheaths leads to a very good junction at the interfaces between the different plates during the rolling process, and avoids their relative motion. Furthermore, considering the large deformation of the samples after treatment, this technique of co-rolling ensures a large contact area between the samples which leads to a good welding of the final composite structure. The co-rolling has been realized on a semi-industrial STANAT reversible quarto rolling equipment at the Laboratoire des Technologies et Milieux Extrêmes (LTMEX) of the Commissariat à l Energie Atomique de Saclay (CEA). Because of the maximum load limitation of 3500KN for this rolling machine and in order to favour a good adhesion on the different faces of the composite structure, an annealing treatment has been applied before co-rolling. Annealing and rolling the sheaths at high temperature decreases considerably the load to apply in the process. So, for a same load level, one can reach a very high reduction rate, higher than for cold rolling. In order to quantify the impact of the annealing temperature and annealing time on the final composite structure, several co-rolling cycles were performed at 500 C (trial test), 550 C and at 650 C. The effect of the global imposed reduction rate and the number of passes to reach it have been studied as well. Figure 2 gives a global view of the different steps of the preparation of a co-rolled sample. Figure 2. Global view of the preparation of a co-rolled sample.

4 Vickers hardness has been measured directly on the ultrasonic shot peened surfaces of the samples by using a Mitutoyo hardness machine. For the experiments, a load of 3kg was imposed during a time of 15s. Six measurements per sample have been made from which average hardness has been calculated. Referring to previous studies and publications [1,3,4,5,6,7], it is well known that a sample treated with a superficial nanocrystallization technique resembles a stratified composite material with different sublayers having a specific average grain size and a particular mechanical behavior. In order to quantify the thickness and the evolution of the different sublayers, local hardness measurements have been made on the cross-section of different SMATed samples and on a co-rolled specimen. The local hardness variation has been determined using a Nano Indenter XP TM fitted with a Berkovich diamond indenter. Furthermore, in order to obtain a precise and complete hardness profile, six indents were put at the same depth from the treated surface. In that way, on the ultrasonic shot peened samples, the nano-indentations tests have been carried out at the following depths from the treated surface: 5µm, 10µm, 15µm, 20µm, 30µm, 40µm, 50µm, 75µm, 100µm, 150µm and 250µm. In addition, to avoid erroneous hardness measurements due to the affected plastic zone around an indent, the distance between two indentations has been fixed to at least 10µm. The indentation profile on the co-rolled sample was slightly different and nanoindentations have been performed on the whole cross section of the sample with a spacing of 100µm between two indentation depths. Near the free surfaces, the indentation scheme given for a SMA-Treated sample was imposed. The depth of the indentations themselves was fixed at 300nm. In order to measure the thickness evolution and the geometrical aspect of co-rolled samples, a NORELEM device has been specifically developed which premising a palpation on both sides in the laminate (length X ) and in the perpendicular (width Z ) directions with a LH54 WENZEL three dimensional measuring machine. The program developed in this study imposes an acquisition of a grid of 13x7 points per face. Figure 3. Photography of the NORELEM device. In the co-rolling process, an important factor of a satisfactory adhesion of the different SMATed plates is the surface roughness. In fact, results of a co-rolling process are better if the interface surfaces of samples present a certain roughness. In order to quantify the roughness of our treated samples, roughness measurements have been performed on ultrasonic-assisted shot peened sheets with a Surtronic 3+ machine from Taylor Hobson (palpation length: 4,2mm, cutt-off length: 0,8mm and scratch tip: 5µm). The investigated samples have been treated with 2 and 3 mm diameter shot during 30 minutes, and measurements performed on an un-treated sample were taken as reference. In addition, images obtained with an optical microscope give an idea of the evolution of the surface aspect after treatment. Results Three-dimensional measurements The figures below (figures 4, 5 and 6) show the measured geometries of three co-rolled plates at 550 C obtained with the three-dimensional measuring machine. The differences between the three samples are the global reduction rate and the number of passes performed. We can note by analysing the different results that the number of passes performed to reach a fixed global reduction rate is a primordial parameter, having an important influence on the final geometry of the co-rolled composite structure. In fact, figures 4a and 6a give a perfect example of this phenomenon and show that for three passes instead of only one, and for the same reduction rate of about 65-68%, the co-rolled sample is quasi plane. Contrary to this, the composite structure co-rolled with only one pass presents a high curvature in the laminate direction (of about 4mm), and a smaller curvature of around 0,8mm in the perpendicular direction (width of the sample).

5 In addition, figure 5a indicates that the number of passes alone cannot explain the good results obtain with three passes. To have a better analysis, one has to check the couple reduction ratio, number of passes. Effectively, performing only two passes during the rolling process, with a global reduction ratio of 54%, which is smaller than the ratio in figure 4, shows that the geometry of the sample after co-rolling is similar to the one of the sample obtained with only one pass. The curvature in the laminate direction is quite the same. However, one may notice that for a number of passes higher than one, the curvature in the perpendicular direction (width of the sample) is absent, even for a small reduction ratio. Figures 4b, 5b and 6b show the thickness evolution of the co-rolled samples as a function of the length and the width Z of the composite structures. It can be noted that the thickness of the specimens oscillates slightly around an average value. Furthermore, the thickness of the side of the co-rolled samples put in first in the rolling machine is smaller than the thickness of the opposite side. This phenomenon is well known and typical for rolling processes. Nevertheless, this aspect is less visible on figure 5b, which shows the thickness evolution of a sample co-rolled with two passes for a reduction ratio of 54%. This could be attributed to a manipulation mistake in the co-rolling process during the second pass. The explication could be that the side of the co-rolling sheath charged at first during the first pass, was not charged as first during the second pass, but the opposite side, so that effect of the thickness induced by the first pass would be annihilated. a) b) Figure 4. a) Composite structure co-rolled at 550 C in one pass with a reduction ratio of about 63% ; b) Evolution of the thickness as a function of the length and the width of the structure. a) b) Figure 5. a) Composite structure co-rolled at 550 C in two passes with a reduction ratio of about 54% (about 27% per pass); b) Evolution of the thickness as a function of the length and the width of the structure.

6 b) a) Figure 6. a) Composite structure co-rolled at 550 C in three passes with a reduction ratio of about 68% (about 23% per pass) ; b) Evolution of the thickness as a function of the length and the width of the structure. Characterization of the surface roughness of treated samples The histogram of figure 7 shows that the ultrasonic-assisted shot peening enhances slightly the roughness R a of the treated samples in comparison with the un-treated one, for both shot diameters. In spite of this enhancement, the average roughness for the two kinds of shot after a treatment of 30 minutes reaches about 0,67 µm and remains around this value. In addition, the increasing of the roughness induced by SMAT may probably have a beneficial effect of the final result of co-rolled samples. In fact, as mentioned, an average roughness leads to a better adherence and welding between the different interfaces of the corolled sheets. Figure 7. Roughness on SMATed samples treated with shot diameters of 2 and 3mm during 30 minutes.

7 Figure 8 shows the results of optical microscopy imaging performed on a crude sample and on an ultrasonic-assisted shot peened sample during 30 minutes with 3mm shot. The SMA-Treatment induces a notable modification of the surface morphology on treated sheets, even if the roughness remains relatively constant between these two cases. Recent works [5,6] have shown that with SMA-Treatment, after a while, the sample roughness diminishes, and then after 30 minutes, the roughness becomes stable. So, in our case, with a 30 minutes treatment an optimum state with respect to roughness and grain size has been obtained. a) b) Figure 8. a) Surface of a raw 316L stainless steel sample; b) Surface of a treated sample during 30 minutes with 3mm shot. Vickers hardness measurements Figure 9 gives the results of Vickers hardness measurements performed on the surface of SMA-Treated samples during 30 minutes with two shot diameters. As expected, the hardness of the crude sample is lower than the hardness of treated ones, and reaches about 170Hv3. Furthermore, the mechanical treatment enhances considerably the surface hardness of the treated samples, which reaches about 320Hv3. This effect could be attributed to a high strengthening rate and probably a defect storage in the top surface layers of the material. However, the surface hardness of treated samples with 2 and 3mm shot is quite similar. This result shows that for a 30 minutes SMA-Treatment, the influence of shot diameter (in the range 2-3mm) on the surface hardness is negligible. Contrary to this, a recent study has shown that the treatment time has a considerable effect on the surface hardness [5,6]. Figure 9. Vickers hardness measurements.

8 Local hardness measurements by nanoindentation Nanoindentation measurements have been performed on the cross section of three samples as given below : - sample SMATed during 30 minutes with 2mm diameter shot. - sample SMATed during 30 minutes with 3mm diameter shot. - sample SMATed during 30 minutes with 3mm diameter shot followed by co-rolling of three sheets at the temperature of 500 C and a reduction rate of 60%. Before proceeding to local hardness measurements by nanoindentation, a rigorous mechanical polishing has been carried out to minimize the roughness. The result of an instrumented nanoindentation test is the load-displacement curve which contains a lot of information. In the literature, one can find techniques for characterizing a variety of mechanical properties based on load-displacement curve, but the technique used in this work gives only the local contact hardness and the Young s modulus of the sample. The important quantities which can be calculated with these data are the maximum load P max corresponding to a maximal displacement h max, the residual depth after unloading, and the elastic stiffness of the contact corresponding to the slope of the initial portion of the unloading curve ' S = dp '. Based on this curve, the local hardness can be calculated with equation (2): dh H P max = (2) A where : H = hardness of the surface P = load A = projected contact area In otherwise, the Young s modulus of the material E is determined with equation (3): eff 2 2 ( 1 ν ) ( 1 i ) ( π S ) = + with 1 ν E E E i E eff = (3) 2β A where : E and ν = respectively the Young s modulus et the Poisson ratio of the tested material Ei and ν i = respectively the Young modulus and the Poisson ratio of the Berkovich tip material ( E i=1141gpa and ν i =0,07 for diamond). β = a constant and equal to 1,034 for a Berkovich tip. E eff = reduced Young modulus. The contact depth is estimated using the following equation: h c Pmax = hmax ε (4) S where :

9 ε : is a constant equal to 0,75 for a Berkovich tip. Finally, the widely used method developed by Oliver and Pharr [9, 10] can be used to calculate the projected contact area which is a function of the contact depth. Results of the three nanoindentation experiments are presented in figures 10 and 11. Figure 10 shows the local hardness variation along the depth of the specimen which has been treated during 30 minutes with 2 and 3mm shot. It can be observed that the ultrasonic-assisted shot peening induces a considerable enhancement of the local hardness. Furthermore, this hardness is very high in the top surface nanostructured layers and decreases with the depth from the treated surface. The local hardness profile for a SMATed sample with 2mm shot is similar to the local hardness profile for a SMATed sample with 3mm shot, and the average local hardness recorded in the top surface layer reaches around 5,70GPa. Values proposed in the literature are much lower than the value recorded in this work [5,6,7]. A partial explanation for this is the grain refinement to the nanometer scale in the top surface layers. This refinement process follows a Hall-Petch trend combined with a strain hardening phenomenon of sublayers, and gives a simple explanation of the local hardness evolution. However, as pointed out by T. Roland [5], one has to take into account the martensite phase transformation during SMAT, so that a part of the high hardness could be attributed to the presence of this hard phase. Figure 10. Local hardness measurements by nanoindentation tests on SMATed samples (30 minutes with shot of diameter 2 and 3mm). Figure 11 shows the co-rolling effect on the local hardness evolution. Actually, after co-rolling, the local hardness through the cross section of the composite structure varies around an average value of 4,5GPa, and the hardness in the top surface layers is reduced in comparison to the only SMATed sample. This local hardness curve trend could be explained by the high strain hardening rate induced by the co-rolling process in the nanocrystalline layers, the sub layers and the coarse grain layers. In that way, the hardness of ultrasonic-assisted shot peened samples increases by a strengthening effect combined with a grain refinement, at least for the coarse grain layers.

10 Figure 11. Local hardness evolution in the cross section of a co-rolled sample at 500 C with a reduction rate of 60%. Conclusion The global co-rolling process of SMAT nanocrystallised samples described in figure 2 is very difficult to set up. However, this process seems to be very promising for the development of semi-massive composite structures exhibiting enhanced mechanical strength with acceptable ductility. This paper presents the first investigations on the co-rolled composite structures concerning the surface roughness characterization of SMATed samples, the Vickers hardness measurements and strengthening induced by SMAT, the effect of co-rolling parameters on the geometry of sandwich structures, and the evaluation of the local hardness through the cross section by nanoindentation. It has been shown that the shot diameter does not have a significant influence on the local hardness profile in the cross section of the samples. In addition, there is practically no difference in term of Vickers hardness. However, a slight enhancement of the roughness of samples treated with 3mm shots can be observed with respect to the roughness of samples treated with 2mm shots. In this way, one can expect better adherence and welding of co-rolling of treated sheets SMATed with 3mm shots. Finally, a co-rolling of three SMATed plates with three passes for a fixed reduction rate leads to a relatively planar composite structure. References 1. Tao, N.R., Sui, M.L., LU, J, and LU, K, Surface nanocrystallization of iron induced by ultrasonic shot peening, Acta Metallurgica, vol. 11, , Liu, G., Lu, J., and LU, K., Materials Science and Engineering, A 286, 91-95, Roland, T., Retraint, D., Lu, K., Lu, J., Materials Science Forum, edited by Trans Tech Publications, Switzerland, 2005, Chen, X.H., Lu, J., Lu, L., Lu K., Tensile properties of a nanocrystalline 316L austenitic stainless steel, Scripta Materialia, vol. 52, , Roland, T., Ya, M., Retraint, D., Lu, K., Lu J., New multilayered nanostructured composite material produced by assembling SMA-treated thin plates, Journal of Materials Science and Technology 20, pp , Roland, T., Retraint, D., Lu, K., Lu J., Enhanced mechanical behavior of a nanocrystallised stainless steel and its thermal stability, Materials Science and Engineering A , pp , Lu, J., Lu, K., Surface Nanocrystallization (SNC) of Materials and its Effects on Mechanical Behavior, Comprehensive Structural Integrity, Fracture of Materials from Nano to Macro, vol. 8, , Chokshi, A.H., Rosen, A., Karch, J., Gleiter, H., On the validity of the Hall-Petch relationship in Nanocrystalline materials, Scripta. Mater., 23 (1989) Weertman, J.R., Hall-Petch strengthening in Nanocrystalline metals, Mater. Sci. Eng., A166 (1993) J.L. Hay, G.M. Pharr, Instrumented Indentation Testing. 11. Tsui, T.Y., Oliver, W.C., Pharr, G.M., Influence of stress on the measurement of mechanical properties using nanoindentation: Part I. Experimental studies in an aluminum alloy, J. Mater. Res., Vol. 11, 1996.