Electrochemical equilibrium and the transfer of mass and charge through interfaces at the atomic scale are of fundamental importance for the microscopic understanding of elementary physicochemical processes. Approaching atomic dimensions, phase instabilities and instrumentation limits restrict the resolution. Here we show an ultimate lateral, mass and charge resolution during electrochemical Ag phase formation at the surface of RbAg(4)I(5) superionic conductor thin films. We found that a small amount of electron donors in the solid electrolyte enables scanning tunnelling microscope measurements and atomically resolved imaging. We demonstrate that Ag critical nucleus formation is rate limiting. The Gibbs energy of this process takes discrete values and the number of atoms of the critical nucleus remains constant over a large range of applied potentials. Our approach is crucial to elucidate the mechanism of atomic switches and highlights the possibility of extending this method to a variety of other electrochemical systems.
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 <111> 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.
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 pileup is the main responsible of the secondary pop-ins produced after certain cycles.
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 γ to e-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 <111> 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.
Experimental Section
Austenitic metastable stainless steels have outstanding mechanical properties. Their mechanical behavior comes from the combination of different deformation mechanisms, including phase transformation. The present work aims to investigate the main deformation mechanisms through the grain boundary under monotonic and cyclic tests at the micro- and sub-micrometric length scales by using the nanoindentation technique. Within this context, this topic is relevant as damage evolution at grain boundaries is controlled by slip transfer, and the slip band-grain boundary intersections are preferred crack nucleation sites. Furthermore, in the case of metastable stainless steels, the interaction between martensitic phase and grain boundaries may have important consequences.
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