Brigitte Bacroix

Experimental observation of microstructure evolution under strain-path changes in low-carbon IF steel

The microstructure of a Ti-killed interstitial free (IF) steel has been examined through TEM observations after simple-shear/simple-shear strain-path changes, in connection with the crystallographic orientation of the examined grains. A partial disappearance of prestrain microstructures is shown to cause the stagnation of work hardening at earlier stages of reversed loading during Bauschinger simple shear sequences. Under progressing reversed deformation, a fragmentation of the grains of unstable orientations still slows down the work-hardening rate. A strong localization of plastic flow within microbands following an orthogonal strain-path change is shown to occur within the grains containing well-developed prestrain dislocation boundaries and belonging to certain orientation groups.

Thermal creep of Zr-Nb1%-O alloys: experimental analysis and micromechanical modelling

Abstract Zirconium alloys present a large variability of their mechanical behaviour with respect not only to their chemical composition but also to their microstructure. We analyze here the creep behaviour at 400 °C of two Zr–Nb1%–O alloys presenting identical chemical composition, crystallographic texture, grain size and grain shape. Both alloys only differ by the thermal cycles imposed during the fabrication process, either below (alloy A) or alternatively above and below (alloy B) the monotectoid transition. This sole difference gives rise to creep rates varying by a factor of about 4 between the two alloys. From a microstructural point of view, alloys A and B differ by the precipitates distribution and the thermodynamical state (alloy B is in a metastable equilibrium state). Our experimental analysis based on mechanical tests, transmission electron microscopy (TEM) observations and phase analysis by X-ray diffraction strongly suggests an hardening effect of Nb in solid solution to explain the differences between alloys A and B. This result is confirmed by TEM X-ray spectrometry which gives a weight content of Nb in solid solution differing by about 0.1% between the two alloys. A predictive micromechanical model, based on the self-consistent affine scheme, is then applied. This model well captures the anisotropy of the specimens, and describes accurately both transient and secondary creep regimes. As a result of the identification procedure, identical hardening laws are obtained for the two alloys at the grain scale, and the saturating reference stress for prismatic slip is found to be higher for alloy B by about 30 MPa with respect to alloy A.

Texture evolution induced by strain path changes in low carbon steel sheets

Two-stage sequences of simple shear performed on low carbon steel sheets were supplemented by texture measurements. A detailed analysis of the texture evolution during the simple shear sequences was realized by studying the orientation distribution functions. The calculation of the orientation stability map (using the classical viscoplastic Taylor model) allows explanation of the observed texture development: the initial texture is shown to be partly stable and hence the measured textures differ from the usually reported ones. The influence of the texture evolution on the shape of the stress-strain curves, as well as on the remaining symmetries of the material, is also discussed. In particular, it is demonstrated that both intragranular and geometrical contributions have to be considered to explain the shape of the stress-strain curves after a strain path change.

A bioactive polymer grafted on titanium oxide layer obtained by electrochemical oxidation. Improvement of cell response

The anchorage failure of titanium implants in human body is mainly due to biointegration problem. The proposed solution is to graft a bioactive polymer at the surface of the implant in order to improve and control the interactions with the living system. In this paper, we describe the grafting of poly sodium styrene sulfonate on titanium surface by using a silanization reaction. The key point is to increase the TiOH content at the surface of the implant which can react with methoxy silane groups of 3-methacryloxypropyltrimethoxysilane (MPS). Two procedures were used: chemical oxidation and electrochemical oxidation. The last oxidation procedure was carried out in two different electrolytes: oxalic acid and methanol. These different oxidation methods allow controlling the roughness and the depth of the oxide layer. The methacryloyl group of MPS grafted at the titanium surface by silanization reaction is copolymerized with sodium styrene sulfonate using a thermal initiator able to produce radicals by heating. Colorimetric method, ATR-FTIR, XPS techniques and contact angle measurements were applied to characterize the surfaces. MG63 osteoblastic cell response was studied on polished, oxidized and grafted titanium samples. Cell adhesion, Alkaline Phosphatase activity and calcium nodules formation were significantly enhanced on grafted titanium surfaces compared to un-modified surfaces.

Simulation of the orientation dependence of stored energy during rolling deformation of low carbon steels

In order to furnish some input data to Monte Carlo codes developed for the simulation of static recrystallization in low carbon steels, two polycrystalline models are used in conjunction with four different hardening laws to estimate numerically the stored energy within individual grains, which is due to the increase in dislocation density during rolling. Three quantities are calculated as a function of final orientation, which are believed to be good estimates of this energy: these are the average dislocation density (linked to the square of an average reference shear stress), the total plastic work and the final plastic work rate. It is thus found that the three selected parameters present the same variation trends for a given model, whatever the hardening law. However, the Taylor and VPSC models lead to opposite conclusions: at the end of the simulated rolling process, the (respectively ) orientations are the hardest (respectively softest) with the Taylor model and the softest (respectively hardest) with the VPSC one; thus, the present data cannot be used in the present state to perform recrystallization simulations but may be used to validate the different polycrystalline models, since they are more sensitive to the interaction law than the texture evolution or macroscopic response.

Recrystallization textures-two types of modelling

Dislocation density is assumed to be the stored energy in a deformed material, i.e, the driving force in recrystallization. It can be estimated in diffraction experiments and it can also be predicted. Model calculations for b.c.c. structure give higher dislocation density for the orientations of the γ fibre compared with those of the α one. This explains the observed increase of γ fibre intensity (and decrease of α intensity) in the recrystallization texture of low-carbon steels, because nuclei appear preferentially in high stored energy regions. Hence, the oriented nucleation behaviour explains the texture change in this case. In other materials the oriented growth behaviour dominates. Phenomenological laws state that only these nuclei grow intensively which have a given misorientation with the deformed matrix. This description is frequently verified in f.c.c. metals and generally reported misorientations are 30°–50° rotations around the <111> axis. The above approach leads to good predictions of recrystallization textures in copper, brass and aluminium. The predicted results are still improved assuming that only these nuclei which are able to consume many deformed grains simultaneously (with different crystal orientations) grow effectively. Consequently, so-calledcompromise criterion andcompromise functions are defined.

Influence of microstructures and particle concentrations on the development of extrusion textures in metal matrix composites

Abstract Crystallographic textures and microstructures induced by extrusion in aluminium alloys reinforced by silicon carbides are analysed in some detail. It is shown that the crystallographic texture and the morphology of the Al phase vary with the SiC percentage and distribution. When the SiC concentration is low (i.e. less than 10%), the particles are dispersed randomly within the matrix and the texture intensity is greater than that of the Al alloy deformed under the same conditions. For high concentrations (i.e. greater than 10%) the particle distribution is no longer uniform and the Al texture tends towards isotropy. Simultaneously, the Al grains, which are very elongated in the particle-free alloy, break into subgrains during deformation in the composite materials. To interpret these various observations, extrusion textures have been simulated with a viscoplastic self-consistent model, introducing several concentrations of SiC and the specific morphology of the composite material. The experimental trends can thus be quantitatively reproduced and explained: the influence of the Al morphology and the particle distribution on the Al texture is clearly shown.

Torsion texture development of zirconium alloys

Abstract In order to study texture formation during high temperature torsion tests of Zirconium alloys, results from experiments and simulations are presented and compared in this paper. Simulations put in evidence that the Viscoplastic Selfconsistent model (VPSC), in spite of some limitations for the description of plastic deformation at high temperatures, gives a reasonable description of the experimental textures. Predictions show that prismatic and basal 〈a〉 slip are necessary to reproduce the main texture components and that pyramidal 〈c+a〉 slip must have a low activity.

Grafting of bioactive polymers onto titanium surfaces and human osteoblasts response

In this paper, the adhesion of human osteoblast- like cells (line MG63) onto functionalized pure Titanium (Ti cp) has been studied. The Titanium surfaces were functionalized by grafting bioactive polymers bearing anionic groups such as sodium sulfonate. The grafting was achieved under inert atmosphere, by radical polymerization of sodium styrene sulfonate NaSS after the activation of the surface. ATR/FTIR and XPS were used to analyse the chemical composition of the grafted and non grafted titanium surfaces. The efficiency of the grafting was evidenced by the high amounts of grafted polyNaSS (5mug/cm2), measured by Toluidin Blue colorimetric method. Biological tests have been investigated to highlight the influence of the grafting polymer on the cell response. Human osteoblast-like cells were cultured on titanium surfaces. Differences in the adhesion strength of cells were observed. Mineralization of osteoblast-like cells was studied after 28 days of culture and the amount of calcium formed were evaluated. Surface modification by bioactive polymers bearing anionic groups appears as an effective way to stimulate the bone regeneration over that, as provided by titanium as suggested by basic studies and in vitro results.

Electron backscatter diffraction investigation of local misorientations and orientation gradients in connection with evolution of grain boundary structures in deformed and annealed zirconium. A new approach in grain boundary analysis

The main aim of the present work is to study the relation between microstructural features – such as local misorientations, grain orientation gradients and grain boundary structures – and thermomechanical treatment of hexagonal zirconium (Zr702α). Electron backscatter diffraction (EBSD) topological maps are used to analyze the aforementioned material parameters at the early stages of plastic deformation imposed by channel-die compression, as well as at a partial recrystallization state achieved by brief annealing. The evolution of local misorientations and orientation gradients is investigated using the so-called kernel average misorientation (KAM) and grain orientation spread (GOS) statistics implemented in the TSL OIM data analysis software [TexSEM Laboratories (2004), Draper, UT, USA]. In the case of grain boundaries (GBs) a new method of analysis is presented. As an addition to the classical line segments method, where the grain boundary is represented by line segments that separate particular pairs of neighboring points, an approach that focuses on grain boundary areas is proposed. These areas are represented by sets of EBSD points, which are specially selected from a modified calculation procedure for the KAM. Different evolution mechanisms of intragranular boundaries, low-angle grain boundaries and high-angle grain boundaries are observed depending on the compression direction. The observed differences are consistent with the results obtained from KAM and GOS analysis. It is also concluded that the proposed method of grain boundary characterization seems to be promising, as it provides new and interesting analysis tools such as textures, absolute fractions and other EBSD statistics of the GB areas. This description may be more compatible with a real deformed microstructure, especially for grain boundaries with very small misorientation, which are indeed clustered areas of lattice defect accumulation.