Accessing the phase transformation and deformation behavior of metastable stainless steels through cyclic nanoindentation

Austenitic metastable stainless steels are a materials group distinguished by their excellent mechanical properties, offering high potential for further improvement by thermo-mechanical treatments. Under deformation, these steels undergo a complex deformation and phase transformation. Their mechanical properties at macroscale, such as strength, ductility or fatigue behavior, have been largely investigated, yet they are not always predictable, as they highly depend on the microstructural characteristics of the material. In order to achieve a better understanding at the microstructural level, this work aims at the investigation of the deformation mechanisms in metastable stainless steels at sub-grain level and the interaction between grains. Therefore, monotonic and cyclic nanoindentation tests were performed in order to increase the cumulative deformation in a controlled way with the number of cycles. The emerging deformation mechanisms under the indents and on the surface, as well as the resulting morphology and mechanical and magnetic properties of the different phases, were characterized through different advanced microscopy techniques. It was found that, even after a high number of nanoindentation cycles, a loading-unloading hysteresis is present, indicating a reversible plastic behavior (which is believed to be due to formation of unstable dislocations at maximum load). The apparent hardness of the material drops with increasing cycles due to the high plasticity of austenitic stainless steels. Gradual phase transformation was triggered and the load–displacement curves exhibited features, such as pop-ins and changes in the slope and hysteresis size, probably related to the propagation of the induced martensite to the neighboring grains and the resulting stress relaxation. This behavior was found to be highly dependent on the crystalline orientation of the respective indented austenite grains. Nucleation of martensite at shear band intersections was detected by TEM investigation of a horizontal lamella, as well as by MFM, while a FIB tomography revealed the shape and location of the nucleated martensitic zones

Etude de l'angle de propagation d'une fissure carrée dans un milieu tridimensionnel à comportement élastique non local

On s'intéresse au comportement d'une fissure dans un milieu tridimensionnel dont le comportement est décrit par un modèle élastique non local tel que celui proposé par Erigen. On sait que pour ce type de comportement, les contraintes en pointe de fissure sont finies. Nous étudions la possibilité de prendre avantage de ce fait pour s'affranchir de l'utilisation des facteurs d'intensité de contrainte dans la prédiction du développement de la fissure

Nanoindentation and tribological tests – Suitable tools for modelling the nanostructure of sheet nacre

1. Introduction Nacre (the pearly internal layer of seashells) is a natural nanocomposite currently studied for the design of new organic/inorganic hybrid materials by mimicking biomineralization processes. It is a bioceramic formed at ambient temperature and pressure [1] which displays an exceptional high strength, stiffness and toughness [2] to weight ratio, as well as a natural biocompatibility with human bones [3]

Reversible phase transformation in polycrystalline TRIP steels induced by cyclic indentation performed at the nanometric length scale

Metastable austenitic stainless steels are an interesting group of materials, which exhibit the Transformation Induced Plasticity effect. In this regard, phase transformation from austenite to martensite enhances the work hardening of the metastable austenitic stainless steels affecting the deformation dynamics and mechanical properties including fatigue properties. Within this context, the reversible load-induced phase transformation from ¿ to ¿-martensite is investigated at the local scale under cyclic indentation. This reversible phase transformation is manifested itself by a combination of hysteresis loops, elbow formation, and reversible pop-ins in the loading curve. The initial cyclic achieved through the nanoindentation technique allows to identify three different deformation regimes for the austenitic grains. Firstly, a softening effect takes place due to the dislocation activation; subsequently the phase transformation induces a hardening effect and finally, the load deformation curve reaches a plateau where no more plastic deformation is observed.Peer ReviewedPostprint (author's final draft

Influence of testing mode on the fatigue behavior of <111> austenitic grain at the nanometric length scale for TRIP steels

Metastable austenitic stainless steels, and in particular the TRansformation Induced Plasticity steels, rely on a phase transformation from a ductile austenite to a slightly harder martensite. These materials under fatigue testing conditions at the macrometric length scale can induce a softening or hardening effect as a function of the underlying deformation feature activated. Thus, given the interaction of single grains in polycrystalline materials, the collective response to macroscopic fatigue testing is not trivial to interpret. Within this context, small scale tests are required to obtain a more in detail understanding of the fatigue properties at the local level of those materials. In this regard, cyclic nanoindentation tests represent a suitable technique to give insight on the local fatigue of metastable stainless steels for a certain crystallographic orientation. In this experimental work, the influence of the testing mode (loading and/or displacement control mode) on the fatigue behavior of austenitic grains as a function of their micromechanical properties as well as their deformation features was investigated in detail. It was found that the experiments done under loading control mode could be compared to conventional low cycle fatigue tests. In contrast when experiments were performed under displacement control mode they may be compared to high cycle fatigue tests. Furthermore, the microstructural observation by transmission electron microscopy allowed to observe the formation of shear bands. This phenomenon preceded the apparition of martensitic laths during the cyclic indentation process.Peer ReviewedPreprin

Dynamic deformation of metastable austenitic stainless steels at the nanometric length scale

Cyclic indentation was used to evaluate the dynamic deformation on metastable steels, particularly in an austenitic stainless steel, AISI 301LN. In this work, cyclic nanoindentation experiments were carried out and the obtained loading-unloading (or P-h) curves were analyzed in order to get a deeper knowledge on the time-dependent behavior, as well as the main deformation mechanisms. It was found that the cyclic P-h curves present a softening effect due to several repeatable features (pop-in events, ratcheting effect, etc.) mainly related to dynamic deformation. Also, observation by transmission electron microscopy highlighted that dislocation pile-up is the main responsible of the secondary pop-ins produced after certain cycles.Peer ReviewedPostprint (author's final draft

Unsteady flow in cone and plate geometry: how computation can help rheometry

The use of rheometers for the evaluation of rheological properties and the establishment of behaviour laws for fluids requires the knowledge of the shear rate at any moment and everywhere in the region between the cone and the plate, referred to as “the gap” throughout this paper. However, the accurate determination of the shear rate supposes that the constitutive equation of the fluid is known beforehand. In order to avoid this paradox, rheometers are generally built such that the shear rate is supposed to be approximately constant throughout the gap. This approximation is realistic for steady flow but may be crude for other types of fluid motion. The aim of the present work is to determine the limits of validity of such an approximation when testing complex fluids in a cone and plate geometry. In this paper, only purely viscous properties are taken into account. The numerical solution is based on the control-volume method. When non-linear and time-dependent effects occur, it is shown that the flow cannot be represented by simple shearing conical surfaces. This result is especially important for the characterisation of time-dependent fluids (as thixotropic fluids), typical of unsteady flow. Finally the abilities of the proposed model which is named “RHEOUTIL

Deformation mechanisms induced by nanoindentation tests on a metastable austenitic stainless steel: a FIB/SIM investigation

Metastable austenitic stainless steels are materials that can undergo austenite to martensite phase transformation when subjected to deformation and thus they represent a multiphase material with interesting mechanical properties. Different electron microscopy techniques are widely applied for the characterization of their deformation mechanisms at micrometric length scale. In doing so, Scanning Ion Microscopy (SIM) imaging, performed with a Focused Ion Beam (FIB), can be useful to evaluate microstructural features induced by different stress fields and, in certain cases, may substitute the conventional Transmission Electron Microscopy (TEM) technique. In this work, nanoindentation experiments (both monotonic and cyclic) were carried out on AISI 301LN metastable steel in order to induce localized deformation of individual austenitic grains. The activated plastic deformation mechanisms were evaluated by using different advanced characterization techniques (Electron BackScattered Diffraction (EBSD) and TEM), but mainly by FIB/SIM. FIB/SIM 3D-tomography was also conducted to reconstruct the deformation structure under the residual imprint. These observations, surprisingly, showed the existence of a good correlation between SIM and TEM images, concerning phase transformation and plastic zone development.Peer ReviewedPreprin

Influence of testing mode on the fatigue behavior of <111> austenitic grain at the nanometric length scale for TRIP steels

Metastable austenitic stainless steels, and in particular the TRansformation Induced Plasticity steels, rely on a phase transformation from a ductile austenite to a slightly harder martensite. These materials under fatigue testing conditions at the macrometric length scale can induce a softening or hardening effect as a function of the underlying deformation feature activated. Thus, given the interaction of single grains in polycrystalline materials, the collective response to macroscopic fatigue testing is not trivial to interpret. Within this context, small scale tests are required to obtain a more in detail understanding of the fatigue properties at the local level of those materials. In this regard, cyclic nanoindentation tests represent a suitable technique to give insight on the local fatigue of metastable stainless steels for a certain crystallographic orientation. In this experimental work, the influence of the testing mode (loading and/or displacement control mode) on the fatigue behavior of austenitic grains as a function of their micromechanical properties as well as their deformation features was investigated in detail. It was found that the experiments done under loading control mode could be compared to conventional low cycle fatigue tests. In contrast when experiments were performed under displacement control mode they may be compared to high cycle fatigue tests. Furthermore, the microstructural observation by transmission electron microscopy allowed to observe the formation of shear bands. This phenomenon preceded the apparition of martensitic laths during the cyclic indentation process.Peer Reviewe

Influence of testing mode on the fatigue behavior of <111> austenitic grain at the nanometric length scale for TRIP steels

Metastable austenitic stainless steels, and in particular the TRansformation Induced Plasticity steels, rely on a phase transformation from a ductile austenite to a slightly harder martensite. These materials under fatigue testing conditions at the macrometric length scale can induce a softening or hardening effect as a function of the underlying deformation feature activated. Thus, given the interaction of single grains in polycrystalline materials, the collective response to macroscopic fatigue testing is not trivial to interpret. Within this context, small scale tests are required to obtain a more in detail understanding of the fatigue properties at the local level of those materials. In this regard, cyclic nanoindentation tests represent a suitable technique to give insight on the local fatigue of metastable stainless steels for a certain crystallographic orientation. In this experimental work, the influence of the testing mode (loading and/or displacement control mode) on the fatigue behavior of austenitic grains as a function of their micromechanical properties as well as their deformation features was investigated in detail. It was found that the experiments done under loading control mode could be compared to conventional low cycle fatigue tests. In contrast when experiments were performed under displacement control mode they may be compared to high cycle fatigue tests. Furthermore, the microstructural observation by transmission electron microscopy allowed to observe the formation of shear bands. This phenomenon preceded the apparition of martensitic laths during the cyclic indentation process.Peer Reviewe