Search

We have investigated the electronic structure of the off-stoichiometric Fe 2 V 1+ x Al 1− x alloys by high-resolution synchrotron radiation photoelectron spectroscopy (SR-PES) measurements in order to clarify the origin of the improvement in their thermoelectric properties. SR-PES results show that, for the Al-rich (hole-doping: x x > 0) case, the electronic structure is modified and the pseudo-gap widens with increasing V/Al substitution. The stability of the Heusler-type crystal structure and the modification of the electronic structure due to strongly correlated effects are possible factors contributing to the improvement in the thermoelectric properties of the n-type off-stoichiometric Fe 2 V 1+ x Al 1− x alloys.

A new automated crystallographic orientation mapping tool in a transmission electron microscope technique, which is based on pattern matching between every acquired electron diffraction pattern and precalculated templates, has been used for the microstructural characterization of nondeformed and deformed aluminum thin films and twinning-induced plasticity steels. The increased spatial resolution and the use of electron diffraction patterns rather than Kikuchi lines make this tool very appropriate to characterize fine grained and deformed microstructures.

The evolution of the orientation relationship between mechanical twins and the surrounding matrix with the degree of plastic deformation has been characterized at the nanoscale in twinning-induced plasticity steel. The recently developed automated crystal orientation mapping in transmission electron microscopy revealed that the ideal twin relationship is retained up to large levels of strain, while large orientation gradients are built up within the matrix. This particular evolution undoubtedly plays a role in the large work hardening rate of these steels.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

High-manganese austenitic TWIP steels exhibit very high strength and elongation before necking. The peculiarity of these steels is that mechanical twins form during straining due to their low stacking fault energy (SFE). These twins are usually thought to have a huge impact on the outstanding properties of the materials, either by bringing about a dynamic Hall & Petch effect and/or a composite effect. In this study, the appearance of mechanical twins during tensile straining is investigated for a Fe-20%Mn-1.2%C TWIP steel. The twinning rate was estimated by means of point counting analysis on EBSD micrographs at different strain levels. The reliability of this method is first thoroughly discussed. It is then shown that there exists a first order relationship between this twinning rate and the work hardening rate. © 2012 Elsevier B.V..

When strained in tension, high-manganese austenitic twinning induced plasticity (TWIP) steels achieve very high strength and elongation before necking. The main hypotheses available in the literature about the origin of their excellent work hardening include deformation twinning and dynamic strain ageing. In order to provide some answers, various experiments at different scales were conducted on Fe-Mn-C steels and the Fe-28wt%Mn-3.5wt%Al-2.8wt%Si alloy. At a macroscopic scale, tensile tests were performed on all the studied grades. It was shown that, though the Fe-Mn-Al-Si based alloy retains very high elongation, the Fe-Mn-C steels properties are even more extraordinary. Tensile tests at different strain rates with the help of digital image correlation were also performed on the Fe-20wt%Mn-1.2wt%C steel to study the PLC effect occurring in this type of steel. It is suggested that supplementary hardening could come from reorientation of Mn-C pairs in the cores of the dislocations. At a microscopic scale, the Fe-20wt%Mn-1.2wt%C TWIP steel and the Fe-Mn-Al-Si grade were thoroughly investigated by means of in situ TEM analysis. In the Fe-Mn-C steel, the formed twins could also lead to a composite effect, since they contain plenty of sessile dislocations. In the Fe-Mn-Al-Si alloy, mechanical twins are thicker and contain fewer defects, leading to a lower work hardening than the other grade. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fe-Mn-C and Fe-Mn-Si-Al TWIP steels deformed under the same conditions exhibit different work-hardening rates. The present study investigates the microstructure of plastically deformed Fe-Mn-C and Fe-Mn-Si-Al samples, particularly the internal structure of the mechanically generated twins and their topology at the grain scale. Twins in the Fe-Mn-C steel are finer and full of sessile dislocations, rendering this material distinctly stronger with an improved work-hardening rate. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

When strained in tension, high-manganese austenitic TWIP steels achieve very high strength and elongation before necking. They also present serrated flow within a certain range of temperatures and strain rates. A consequence of this jerky flow is the appearance of strain localisation in the form of narrow deformation bands apparent on the surface of the sample. This phenomenon arises from the dynamic interaction between solute atoms and mobile dislocations, otherwise known as dynamic strain ageing (DSA). In this study, the heterogeneous deformation of a Fe-20wt.%Mn-1.2wt.%C grade has been investigated at room temperature and at different strain rates by means of digital image correlation (DIC) for spatially resolved strain measurements made in situ during tensile tests. Simple tensile tests have also been conducted at different temperatures in order to investigate the evolution of the type of serrations and their influence on the bulk mechanical properties. The results of tests performed at room temperature and at two different strain rates, indicate that the plasticity is entirely governed by the appearance of localised deformation bands, similar to those observed in materials that exhibit the Portevin-Le Chatelier (PLC) effect. However, no critical strain for the onset of the phenomenon could be determined. The origin of these observations is discussed in this paper. It is proposed that the mode of propagation of the bands is dependent on the strain rate and the strain level. Moreover, it is shown that the band propagation is well correlated with the different types of serrations appearing on the stress-strain curve. These results match the characteristics of a classical DSA effect and help to shed light on the remarkable properties achieved by this material. (C) 2010 Elsevier B.V. All rights reserved.

We have developed a new reliable method combining template synthesis and nanolithography-based contacting technique to elaborate current perpendicular-to-plane giant magnetoresistance spin valve nanowires, which are very promising for the exploration of electrical spin transfer phenomena. The method allows the electrical connection of one single nanowire in a large assembly of wires embedded in anodic porous alumina supported on Si substrate with diameters and periodicities to be controllable to a large extent. Both magnetic excitations and switching phenomena driven by a spin-polarized current were clearly demonstrated in our electrodeposited NiFe/Cu/ NiFe trilayer nanowires. This novel approach promises to be of strong interest for subsequent fabrication of phase-locked arrays of spin transfer nano-oscillators with increased output power for microwave applications.

The microstructure of a Fe-Mn-Si-Al twinning-induced plasticity (TWIP) steel exhibiting remarkable work hardening rate under uniaxial tensile deformation was investigated using transmission electron microscopy to uncover the mechanism(s) controlling the nucleation and growth of the mechanically induced twins. The results show that the stair-rod cross-slip deviation mechanism is necessary for the formation of the twins, while large extrinsic stacking faults homogenously distributed within the grains could act as preferential sources for the activation of the deviation process. The influence of such features on the thickness and strength of the twins and the resulting mechanical behaviour is discussed and compared to similar works recently performed on Fe-Mn-C TWIP steels. © 2013 © 2013 Taylor & Francis.

The influence of the stress state on the twinning rate and work hardening is studied in the case of an Fe-Mn-C TWIP steel strained in uniaxial tension, simple shear and rolling. The resulting stress-strain responses exhibit marked differences. The twinning rate, number of activated twinning systems in each grain, twin thickness and transmission of twins across grain boundaries are dependent on the imposed stress state during straining. Relationships between twin features and macroscopic work hardening rate are established. © 2012 Acta Materialia Inc.

A new crystal plasticity model for face-centered cubic metals exhibiting deformation twinning is presented and assessed based on a comprehensive and original experimental dataset for twinning-induced plasticity (TWIP) steel. Various scale transition schemes are tested, including a finite-element representation of the polycrystalline aggregate and a simplified Taylor-type modeling of pairwise interactions of adjacent grains. The same constitutive law is used at the grain level, relying on a dislocation-density-based description of work hardening and accounting for the latent hardening due to the presence of twins. At the macroscopic scale, the model provides improved predictions of the development of texture and hardening in uniaxial tension and during rolling. At the grain level, electron backscatter diffraction measurements confirm that the model properly estimates the volume fraction of twins inside individual grains as well as the average disorientation relative to the grain mean lattice orientation.

Deformation twinning is known to play a major role in the huge work hardening and formability of TWinning Induced Plasticity steels (TWIP steels). Twins carry a part of the plastic deformation and they act as barriers to dislocation motion. However, their interaction with their parent grain is already anisotropic and they strongly influence the development of texture and macroscopic anisotropy of the material. Twinning in TWIP steels is investigated in this study by means of experimental observations as well as an advanced crystal plasticity model. Individual twins, as they are formed, are treated as separate crystal entities co-deforming with the parent grain. The behavior of the polycrystalline aggregate is assessed by means of a multisite model of short-range grain interactions in comparison with the crystal plasticity finite element method (CPFEM) and the Taylor FC model. This approach allows validating the occurrence of twinning in grains with preferred orientations as well as the orientation relation between a twin and its parent grain with evolving deformation. Moreover, the results demonstrate that a combined prediction of macroscopic work hardening, texture and twin volume fraction requires both an appropriate description of strain heterogeneity in the polycrystal and a proper account of the anisotropic twin-grain interactions at the crystal level.

Although it is well known that Fe-Mn-C TWIP steels exhibit high work-hardening rates, the elementary twinning mechanisms controlling the plastic deformation of these steels have still not been characterized. The aim of the present study is to analyse the extended defects related to the twinning occurrence using transmission electron microscopy. Based on these observations, the very early stage of twin nucleation can be attributed to the pole mechanism with deviation proposed by Cohen and Weertman or to the model of Miura, Takamura and Narita, while the twin growth is controlled by the pole mechanism proposed by Venables. High densities of sessile Frank dislocations are observed within the twins at the early stage of deformation, which can affect the growth and the stability of the twins, but also the strength of these twins and their interactions with the gliding dislocations present in the matrix. This experimental evidence is discussed and compared to recent results in order to relate the defects analysis to the macroscopic behaviour of this category of material. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

It is known for a long time that mechanical twinning occurring in FCC metal can bring about a very large work hardening rate. It is believed that deformation twins increase the work-hardening rate by acting as obstacles for gliding dislocations. This mode of plastic deformation is active, among others, in the Hadfield steel known for a very long time [1]. Fe-Mn-C grades exhibiting mechanical twinning present a renewed interest for some years since some applications are expected in the automotive industry. Several kinds of TWIP (Twinning Induced Plasticity) steels are now intensively studied.

This study focuses on the thermoelectric properties of the Heusler-type Fe2VTaxAl1−x alloys (0≤x≤0.12). By means of Rietveld analyses on synchrotron X-ray diffraction patterns, it is shown that the Ta atoms enter sites occupied by V atoms in the stoichiometric Fe2VAl alloy, while the ejected V atoms are transferred to the vacant Al sites. This Ta substitution leads to an improvement of the n-type thermoelectric properties owing to two mechanisms. On the one hand, the atoms position in the structure leads to an off-stoichiometric effect such as already observed in V-rich Fe2V1+yAl1−y compounds: the Seebeck coefficient is increased towards negative absolute values and the electrical resistivity is decreased, with a large shift of their peak temperature towards higher temperature. The maximum power factor is 6.5 × 10−3 W/mK2 for x = 0.05 at 340 K. On the other hand, the heavy element Ta substitution combined with this off-stoichiometric effect leads to a large decrease of the thermal conductivity, owing to a...