Combination of surface nanocrystallization and co-rolling: creating multilayer nanocrystalline composites

International audienceThis paper presents a combination of the surface mechanical attrition treatment (SMAT) and the corolling process performed at 550 °C. This duplex method leads to the formation of a semi-massive multilayer structure of alternating nanocrystalline layers, transition layers and coarse grain layers. Transmission electron microscope observations correlated with nanoindentation hardness measurements demonstrated that grain size in the nano- and sub-nanocrystalline layers is preserved after the process. Tensile tests showed improved yield and ultimate strengths, and acceptable ductility

Characterization of mechanical behavior of nanocrystalline layer induced by SMAT using micro-pillar compression technique coupled with finite element analysis

Micro-pillar compression tests were used to study the mechanical behavior of a stainless steel that has undergone SMAT (Surface Mechanical Attrition Treatment). Micro-pillars were machined using a Focused Ion Beam (FIB) on the cross-section of a SMATed specimen at different distances from the treated surface. These micro-pillars were thus located in different areas more or less affected by the SMAT. They were then compressed with a flat head mounted on a nanoindenter to obtain loading-displacement curves. These compression tests can give information on the mechanical gradient present from the top surface down to the bulk material after SMAT: a superficial nanocrystalline layer (from 10 to 50 micrometers thick and composed of grains with a diameter ranging from 10 to 50 nm) is indeed generated as well as a transition layer (between 200 and 300 micrometers thick and characterized by a grain size gradient from the nanometer to the micrometer scale as the distance from the surface increases) just below the nanocrystalline layer. These compression tests coupled with finite element analysis (FEA) can provide precious information at the mesoscopic scale on the mechanical behavior of the different layers present in the SMATed steel. FEA was used to study the effect of experimental parameters including taper angle (the angle between the tangent of wall and the axis of pillar), aspect ratio (the ratio of height and diameter of the pillar), and misalignment between the pillar axis and the compression direction. Based on the results of FEA, the constitutive behavior in the form of stress-strain curve was identified for the different layers beneath the treated surface including the nanocrystalline layer. According to the obtained stress-strain curves, the mechanical strength of the stainless steel is significantly improved after SMAT

High Strength Nanocrystallized Multilayered Structure Obtained by Smat and Co-rolling

International audienceIn the present study, a method is presented combining surface nanocrystalline treatment (SMAT) and the co-rolling process. The aim of this duplex treatment is the development of a 316L stainless steel semi-massive multilayered bulk structure with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure by optimizing the volume fraction and distribution of the nano-grains in the laminate. To characterize this composite structure, tensile tests as well as sharp nanoindentation tests were carried out to follow the local hardness evolution through the cross-section of the laminate. Furthermore, transmission electron microscope (TEM) observations were carried out to determine the correlation between the microstructure, the local hardness and the mechanical response of the structure

Effect of interfacial oxidation occurring during the duplex process combining surface nanocrystallisation and co-rolling

International audienceThis paper presents an investigation of the interface quality of nanocristallised 316L stainless steel multilayer structures. They were produced by a duplex process, combining the Surface Mechanical Attrition Treatment (SMAT) and the co-rolling process at two different annealing temperatures (550°C and 650°C). Oxide layers were observed at the interfaces between the sheets and their morphology was characterised by optical microscopy. Their chemical composition was determined by Energy Dispersive X-ray spectrometry. The microstructure near the interfaces was analysed by Transmission Electron Microscopy (TEM). In the laminate co-rolled at 550°C, the presence of ultrafine grains was demonstrated. Additional tensile tests have shown an influence of the annealing temperature on the yield strength, as well as on the resistance of the interfaces of the co-rolled multilayer structures

Interfacial oxidation in processing of nanocrystallised metallic materials using duplex technique - experimental and modelling aspects

Duplex techniques are attempted to be developed combining nanocrystallisation processes with a subsequent thermomechanical processing in order to produce multilayered bulk structures with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure. However, bonding imperfections at the interfaces due to interfacial oxidation among other reasons during the duplex process can significantly influence the final properties. Moreover, the interface oxidation occurring during duplex processes influences microstructure development around the interfaces depending on whether the oxide scale is a continuous layer or a layer of discontinuous oxide clusters with heterogeneous thicknesses. Effectively the oxide scale becomes a part of microstructure development in such nano-crystallised multilayered structures. This paper deals with understanding of the underlying events around the highly reactive interfaces explaining the microstructure evolution applying advanced experimental and numerical modelling techniques. The research encompassed surface mechanical attrition treatment followed by constrained compression testing and hot rolling of the assembly of steel strips supported by multilevel numerical analysis using combined finite element (FE) and cellular automata (CA) methods. Shear banding has been observed near metal-metal contact between the oxide clusters at the interfaces. The shear banding can be considered as bonding enhancement creating channels for the base metal of the different laminates to come into contact through the oxidised interface. Temperature, texture and grain refining are among the factors influencing the shear banding. In the simulations, the meso-level of the developed multi-level FE-based model is combined with the advanced 3D frontal CA numerical model allowing for both appearance of the new boundaries and rotation of dislocation cells (sub-grains and grains) simultaneously

Structure-mechanical property relationship in Hot-Dip Galvanized S355k2+N steel plates

Abstract. The aim of this study is the characterization of hot-dip galvanized S355K2+N steel plates used as components for engineering civil structures. Two thin zinc coatings with a thickness of 145+/-14 µm and 329+/-23 µm, were developed at the surface of the plates. Several experimental techniques were performed to study the microstructure, the chemical composition at the surface of the galvanized plates. The residual stress field was also evaluated in the coatings and the top surface of the plates using the incremental hole drilling method, in the rolling and the transverse directions. The results show the presence of tensile stresses and compressive stresses respectively in the coating and the substrate

Ein Modell für verantwortungsvolles Handeln in der IT-Organisation

In der Betriebswirtschaft ist verantwortungsvolles Handeln, repräsentiert durch Konzepte wie Corporate Social Responsibility (CSR), Corporate Citizenship (CC) oder Nachhaltigkeit/Sustainability, eine in Forschung und Praxis breit diskutierte Thematik. Eine umfassende Übertragung der Problematik auf das IT-Management hat jenseits von isolierten Einzelthemen wie „Green IT“ etc. jedoch noch nicht stattgefunden. In diesem Beitrag wird deshalb ein Modell vorgestellt, welches auf der einen Seite einen ganzheitlichen Rahmen für verantwortungsvolles Handeln in der IT-Organisation eröffnet, und auf der anderen Seite Stellschrauben für die aktive, verantwortungsvolle Beeinflussung des Wettbewerbsumfelds aufzeigt. Konkretisiert wird das aufgezeigte Modell anhand verschiedener Maßnahmenfelder zur Umsetzung verantwortungsvollen Handelns

Inhomogeneity of plastic deformation in austenitic stainless steel after surface mechanical attrition treatment

Inhomogeneity of microstructure evolution in cold-rolled austenitic stainless steel after surface mechanical attrition treatment (SMAT) was investigated. A characteristic deformation pattern was obtained for all studied specimens. Selected areas were examined through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Estimation of the α′-martensite volume fraction below the treated surfaces showed, that some of the studied areas are characterised by significant different amount of this phase (from 10 to 22% - BB, from 19 to 31% - GIXD). The performed finite element (FE) numerical analysis showed, that the reason for this may be the presence of an air gap between the impacted material and fixation and also relatively short high stress time duration due to surface inclination during the surface treatment. Annealing at 550 and 650 °C greatly increased the volume fraction of α′-martensite (up to 47% - BB) and formed Fe2O3 as well as Fe3O4, whereas annealing at 700 °C resulted in both disappearance of α′-martensite and in reduction of oxides

Numerical simulations of impacts during Surface Mechanical Attrition Treatment using crystal plasticity model in finite element method

Surface Mechanical Attrition Treatment (SMAT) is a process, which transforms the top surface layer of materials from coarse grains to nano-sized grains by severe plastic deformation. SMAT is based on multidirectional mechanical impacts between shot and the surface of material. As the strain rate is high and the accumulated plastic strain is large, a great number of defects such as dislocations and deformation twins can be generated at the top surface, which progressively lead to the formation of a nanostructured layer. Simultaneously, high compressive residual stresses may be introduced in the SMAT affected layer. This nanostructured layer coupled with compressive residual stresses induced by SMAT allow to significantly improve the mechanical properties of materials. Due to excellent mechanical properties of SMATed materials, it is necessary and useful to investigate the SMAT process both experimentally and numerically in order to obtain a better understanding and a better control of the process. Experimental studies, extensively performed previously and recorded in the literature, have shown that the mechanical properties of SMATed materials are highly influenced by the microstructure such as grain size and work hardening. From a modeling perspective, it would be highly beneficial to establish accurate numerical models of SMAT in order to consider the influence of the different parameters of this process at the different scales. In this work, a crystal plasticity model introduced in finite element analysis, taking into account the microstructure, was used to investigate the plastic activity due to the impacts between shot and the surface of material. To do this, the shape-controllable 3D Voronoï geometries as well as meshes were first generated using Neper software. A phenomenological crystal plasticity model implemented through user-defined ABAQUS subroutines was used in this work to perform numerical simulations. A number of parameters are studied such as shot size, impact velocity, incident angle, etc. The influences of these different parameters on slip systems and stress fields were analyzed. The first results demonstrated the interests of numerical simulations for this specific process