Characterisation of the Portevin-Le Chatelier effect affecting an austenitic TWIP steel based on digital image correlation

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

Al/stainless-invar composites with tailored anisotropy for thermal management in light weight electronic packaging

Composite plates in which the low CTE phase has the shape of a honeycomb are anticipated to present optimum anisotropy of thermal expansion and thermal conductivity for baseplates in electronic packaging. This design is explored by choosing an invar alloy for the low CTE phase. In order to allow the formation of a passivation layer protecting from reaction with liquid Al during squeeze casting, the honeycomb is made of the Cr-rich alloy commonly called “stainless-invar”. Composite plates containing 20 vol.% and 38 vol.% stainless invar were processed using honeycombs with the same thickness over cell side ratio. Experimental CTE values are significantly lower than the predictions of three different thermo-elastic models. The very limited amplitude of the strain hysteresis precludes the occurrence of global plastic yielding in the matrix. It appears that, owing to the high contiguity of the lowCTE phase, the lowvalue of the experimental CTE results fromvoid closing and opening by localised plastic flow. A honeycomb volume fraction of 38% is necessary for bringing the average CTE down to the level suitable for packaging applications. The ratio of transverse thermal conductivity to density then amounts to about half of the performance of the best Al/SiC composites. © 2015 Elsevier Ltd. All rights reserved

Hydrogen-induced pinning of dislocations in tungsten probed by spherical nanoindentation

Intense hydrogen plasma interactions with tungsten occur in the divertor region of ITER and the effect of hydrogen plasma on the mechanical behavior of tungsten is an important open question [1]. Such a study is unfortunately not feasible with most conventional mechanical testing techniques since low energy hydrogen plasma initiates modifications at shallow depths (typically less than a few µm). Nanoindentation probes the right length scale, but the resulting information is generally limited if a conventional analysis methodology is used [2]. In this study, a dedicated spherical nanoindentation data protocol is applied to extract more meaningful indentation stress-strain curves other than what the conventional technique provides [3]. For clarity, a recrystallized tungsten sample was used, which was exposed to deuterium plasma at a surface temperature of ~ 800 oC and ion energy of ~ 5 eV for 100 seconds using Magnum-PSI [4]. Spherical nanoindentation has been performed on reference and plasma exposed samples, while also exploring areas with a similar local environment (grain orientation, distance to grain boundaries, assured with the help of electron backscatter diffraction). We found that plasma-induced damage yields an increase of the local flow stress by more than 40%. This effect may be attributed to the obstruction of dislocation movement by plasma-induced defects. Besides the intensive sub-surface plastic deformation generated by high flux plasma loading reported earlier [5], it is hypothesized that plasma-originated hydrogen segregates at dislocation cores (of edge type dislocations). This renders the hardening of tungsten to be controlled by immobilized edge dislocations, which entails an increase of the flow stress as measured by nanoindentation. In addition, focused ion beam cross section examination and thermal desorption spectroscopy are carried out to relate the observed mechanical modifications to the underlying microstructures. The results reported here may have an impact on the fundamental understanding of the role of hydrogen plasma on the mechanical behavior of tungsten, which would be invaluable in defining the ITER divertor operational lifetime

On the control of the residual porosity in iron aluminides processed by reactive squeeze-infiltration of aluminium into a preform of steel fibres

An original approach for the synthesis of iron aluminides of the type FeAl1-x is presented, based on the use of a squeeze-casting equipment for inducing the reactive infiltration of Al into a porous preform made by sintering continuous Fe-based fibres. The casting is subsequently homogenised by heat treatment in the solid state. Preforms with fibre volume fractions ranging between 40 and 80% (fractions that correspond to stoichiometries close to FeAl and Fe3Al. respectively) are prepared using steel-wool fibres with various diameters. Small fibre diameters and high processing temperatures are found to be beneficial for obtaining, after squeeze casting a low-porosity material with a large amount of uniformly distributed reaction compounds. However, the preform thickness that can be properly infiltrated is limited to about 6 mm. The homogenisation of the intermetallic phase is complete after only 5 h at 1000 degreesC. A major problem to be solved is the heterogeneity of the porosity distribution in the sample after homogenisation. This heterogeneity results from an insufficiently uniform distribution of the fibre volume fraction. (C) 2002 Elsevier Science B.V. All rights reserved

Advanced characterization of the 3D morphology and mechanical preoprties of the enthesis: optimization study

status: accepte

Nano-indentation of biological tissues: opportunities and challenges for the bone-tendon interface

status: accepte

Determination of the morphological texture of the fibres in a composite material made from a textile of AISI 316L fibres using a mixed deconvolution/positivity method

The orientation distribution or 'morphological texture function' (MTF) of the fibres is determined from measurements of the crystallographic texture of the fibres themselves. From neutron diffraction pole-figure measurements of the fibres in the composite and X-ray diffraction pole figures of the individual fibres, the MTF is determined by a deconvolution. To account for errors in the expansion coefficients due to measurement uncertainties, the deconvolution is performed simultaneously with a positivity criterion in the form of a set of linear equations which may be solved by a least-squares method. This approach is validated using a model system of AISI 316L stainless-steel fibres in an Al-13 wt% Si eutectic casting alloy. The fibres had been spun into yarns and subsequently woven to form a textile. The composite was made by infiltrating a 'brick' containing several layers of the textile with the Al alloy. The results of the deconvolution procedure show that the method indeed yields the expected morphological texture. The so-determined MTF is resolved to an angular resolution not attainable by other methods. It also provides a very good estimate of the volume fractions of the different texture components

Friction Melt Bonding of steels to aluminium: on the importance of the welding parameters control to obtain performant joints

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Mean-Field Model Analysis of Deformation and Damage in Friction Stir Processed Mg-C Composites

Friction Stir Processing (FSP) is an attractive manufacturing technique to produce Mg matrix composites since it avoids the problem of excessive reactivity between reinforcement and matrix encountered in liquid-phase processing routes. However, the strength of the interface in C-reinforced Mg matrix composites produced by FSP remains to be assessed. A short fibre composite has been fabricated by FSP a stack of a C-fabric between two Mg-AZ91D alloy sheets. In order to elucidate the interplay between matrix hardness and interface bonding strength, the work investigates the influence of heat treatment on the mechanical properties of the composites. An incremental Mori-Tanaka model is developed to analyse the relative roles of heat treatment and C-fibre reinforcement on the flow strength and ductility of the composites in tension and compression. The mean-field model provides an estimate of the stress at the matrix/fibre interface, from which a simple debonding criterion can be derived. Comparison between model predictions and experimental data indicates that damage in the FSP composites is triggered by early interfacial debonding. Based on Finite Element simulations of a tensile test carried out in-situ in a scanning electron microscope, the critical interfacial stress for debonding was identified to be 435 MPa in simple traction but only 250 MPa when damage is governed by shear. This explains the limited strengthening by C fibres observed in heat treated composites