Heterogeneous grain-scale response in ferroic polycrystals under electric field

Understanding coupling of ferroic properties over grain boundaries and within clusters of grains in polycrystalline materials is hindered due to a lack of direct experimental methods to probe the behaviour of individual grains in the bulk of a material. Here, a variant of three-dimensional X-ray diffraction (3D-XRD) is used to resolve the non-180?? ferroelectric domain switching strain components of 191 grains from the bulk of a polycrystalline electro-ceramic that has undergone an electric-field-induced phase transformation. It is found that while the orientation of a given grain relative to the field direction has a significant influence on the phase and resultant domain texture, there are large deviations from the average behaviour at the grain scale. It is suggested that these deviations arise from local strain and electric field neighbourhoods being highly heterogeneous within the bulk polycrystal. Additionally, the minimisation of electrostatic potentials at the grain boundaries due to interacting ferroelectric domains must also be considered. It is found that the local grain-scale deviations average out over approximately 10-20 grains. These results provide unique insight into the grain-scale interactions of ferroic materials and will be of value for future efforts to comprehensively model these and related materials at that length-scaleopen

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

GRAIN BOUNDARY WETTING PHASE TRANSFORMATIONS IN THE Zn-Sn AND Zn-In SYSTEMS

The wetting phase transition of grain boundaries (GBs) by the melt has been studied in the Zn - 6 wt.% Sn and Zn - 5.3 wt.% In polycrystals and Zn bicrystals (two [10 (1) over bar 0] tilt GBs with misorientation angles f = 19 degrees and 66 degrees, and one [11 (2) over bar 0] tilt GB with misorientation angle phi = 79 degrees, wetting by the In-rich melt). In the Zn - 6 wt.% Sn polycrystals, the fraction of the completely wetted GBs gradually increases from 20 to 70% with increasing temperature between 260 and 400 degrees C. In the Zn - 5.3 wt.% In polycrystals, the fraction of the completely wetted GBs gradually increases from 20 to 65% with increasing temperature between 215 and 395 degrees C. Temperature dependences of the contact angle theta(T) for bicrystals were measured using scanning electron microscopy and light microscopy. 0 decreases with increasing T and reaches zero (complete wetting) at a certain temperature T(w). It has been observed for the first time that temperature dependences of contact angle intersect (at about 350 degrees C). The wetting transformation for Zn GBs is discontinuous (first order): q(T) dependence is convex, d theta/d T has a break at T(w) and theta similar to ((T-T(w))/T(w))(1/2). Tilt GBs in Zn-Sn and Zn-In systems become completely wetted when about 60-70% of GBs with higher energy are already completely wetted in a polycrystal

On incipient plasticity in the vicinity of grain boundaries in aluminum bicrystals: Experimental and simulation nanoindentation study

International audienceThe local mechanical behavior near symmetric tilt grain boundaries in aluminum bicrystals were studied by the nanoindentation technique as well as by computer simulations. Experimentally, grain boundaries with misorientation angles 8.7°, 13.8° and 18.8° were examined. Two well pronounced pop-in events were observed on the load – penetration depth curves measured during indentation in the close vicinity of the studied boundaries. The load of the pop-ins, observed close to the boundaries, practically did not differ from that obtained in the grain interior. This was interpreted as evidence that the boundaries with misorientations in the examined angular range do not represent specific sites for sources of lattice dislocations. The load, at which the second pop-ins took place, substantially increased with increasing misorientation angle of the examined boundaries. Quasistatic molecular dynamics simulations were performed to identify the details of the interaction between grain boundary and dislocations generated during indentation. For this purpose, the bicrystals with similar geometry and misorientation angles, as investigated experimentally, were computed. The simulation results showed that the direct transmission of incoming dislocations across the grain boundary was the primary mechanism for the plastic flow transfer past the boundary. The analysis of the results of both experiments and simulations provided evidence that the capability of grain boundaries to act as a barrier for the motion of incoming dislocations depends crucially on grain boundary structure

Interpreting slip transmission through mechanically induced interface energies: a Fe–3%Si case study

© 2018, Springer Science+Business Media, LLC, part of Springer Nature. Nanoindentation experiments are performed at the vicinity of grain boundaries, in Fe–Si tricrystals, to illustrate the existence of a critical stress at which slip transmission occurs across grain boundaries. Such a critical stress can be considered as a grain boundary yield stress and can be quantified within the framework of conventional gradient plasticity theory, enhanced by introducing a new mechanically induced “interface energy” term. The present study takes a first step in trying to provide a physical interpretation for this “far from thermodynamic equilibrium” interface energy term by conducting nanoindentation tests in three Fe–3wt%Si tricrystals, each of which had three distinct types of grain boundary misorientations, namely 22.5°, 42.0° and 44.6°. By relating the experimentally measured grain boundary yield stress to the predictions of interfacial gradient plasticity, it is possible to determine the interface parameter (ξ), which provides a measure of the resistance to slip transmission for each grain boundary examined. In particular, microscopic arguments from standard dislocation theory reveal that ξ depends on both the grain interior properties and the grain boundary structure. The internal length is shown to depend on multiple characteristic lengths of the microstructure, while a new expression is deduced for relating the Hall-Petch slope to both the interface parameter and internal length