Study of the effects of the hydrostatic pressure on the plasticity mechanisms in semi-conductors (the indium antimony (InSb) case)

La plasticité d InSb est étudiée entre -176C et 400C, i.e. de part et d autre de la température de transition fragile-ductile, Tf-g située autour de 150C. Les techniques de déformation utilisées sont la compression uniaxiale sous confinement gazeux (machine de Paterson), la micro-indentation et la compression sous forte pression dans une cellule multi-enclumes. Pour l analyse des microstructures de déformation, trois techniques sont utilisées : l annihilation de positrons, la MET conventionnelle et la technique du LACBED. L étude macroscopique révèle un changement du comportement mécanique vers 150C, i.e. proche de Tf-g. La technique d annihilation de positrons montre que les défauts formés à 20C et à 300C sont de natures différentes. L analyse par MET confirme la modification des mécanismes dislocationnels en fonction de la température : il est observé, à -176C, des dislocations parfaites vis non dissociées ; à 20C, une majorité de dislocations partielles en interaction et au dessus de 150C, des dislocations parfaites non dissociées en interaction. Ces résultats mettent en évidence un changement de mécanisme de déformation, autour de Tf-g, par glissement dans le système glide de dislocations parfaites à haute température et de dislocations partielles à basse température. L observation de dislocations parfaites non dissociée à très basse température suggère une transition supplémentaire, en dessous de la température ambiante, vers un glissement des dislocations dans le système shuffle. L apparition de la transition fragile-ductile pourrait donc être liée à ces transitions successives de mécanismes de déformation à basse et très basse températureThe plasticity of indium antimony InSb is studied between -176C and 400C, i.e. above and below the brittle to ductile temperature transition BDTT, situated around 150C. The deformation techniques which are used are the uniaxial compression under gaseous pressure (Paterson press), the deformation under localized load (micro-indentation) and the compression under high pressure in a multi-anvils cell. To analyse the deformation microstructures, three techniques are used: positron annihilation, conventional TEM and the LACBED technique. The macroscopic study reveals a change of the mechanical behaviour arround 150C, i.e. close to BDTT. The positron annihilation technique shows that the defects formed at 20C and at 300C have different nature. The TEM microstructural analysis confirms the modification of the dislocation mechanisms according to the temperature: only non dissociated perfect screw dislocations are observed at -176C; a majority of partial dislocations in interaction are observed at 20C and only non dissociated perfect dislocations in interaction above 150C. These results show a change of deformation mechanism around BDTT, by perfect dislocations gliding in the glide set at high temperature and partial dislocations with glide set at low temperature. The observation of non dissociated perfect dislocations at very low-temperature suggests an additional transition, below the ambient temperature, related to the gliding of dislocations in the shuffle set. The appearance of the brittle to ductile transition could be thus connected to these successive transitions of deformation mechanisms at low and very low temperaturePOITIERS-BU Sciences (861942102) / SudocSudocFranceF

Effect of aging heat treatment at 400-600°C on the microstructure and tensile properties of 15Cr-15Ni Ti-stabilized steel cladding

International audienceThis study investigates the effect of thermal aging on the microstructure and tensile properties of a 15-15Ti austenitic stainless steel in the baseline operating conditions of a sodium fast reactor, in the range between 400°C and 600°C.Samples that were aged at up to 600°C for 1000 hours exhibit no evidence of material recovery. Thus, after aging heat treatments, micro-hardness measurements do not decrease, and TEM analyses do not show any modification of the dislocation network. However, TEM examinations have indicated a new threshold for the precipitation of nanometric titanium carbides after an isothermal treatment at 500°C for about 5000 hours. Concerning the tensile properties, the aged states present a gain both in strength and in ductility compared to the initial cold-worked state. The large gain in ductility is observed for all of the temperatures tested (between 20°C and 400°C) and occurs concomitantly with an increase in the strain hardening rate of the material. One plausible hypothesis to explain this improvement of the mechanical behaviour relies on the nanometric titanium carbides formed during the aging process. These precipitates could act as obstacles that impede the motion of existing dislocations, thereby contributing an additional strain hardening mechanism, which would lead to greater strength and also delay the onset of strain localization

Comparison of 15Cr-15Ni austenitic steel cladding tubes obtained by HPTR cold pilgering or by cold drawing

International audienceDue to their high void swelling resistance, work-hardened titanium stabilized austenitic steels have been chosen as cladding material for sodium cooled fast reactor. In this study, HPTR cold pilgering process is compared to cold drawing at the last shaping step of the tube processing. The effects of the cold work accumulation and heat treatments are studied in connection with the microstructure (grain size), the hardness and the texture. The following results were found:-As larger amount of cold work can be applied by HPTR cold pilgering, a lower number of intermediate heat treatments are required. In addition, the bending of the tube is significantly reduced after each pass for this process allowing for a limitation of the straightening operations.-For both processes, optical micrographs show micrometric titanium carbide precipitates and the presence of deformation twins on a few grains on the final tube. A significant grain size refinement from $\approx$45 \mu^m to $\approx$17 \mu^m can be obtained by reducing annealing temperature from 1403K down to 1353K. For the latter, the precipitated mass fraction measured by selective dissolution of the alloy matrix is the largest, revealing a possible negative impact on swelling under irradiation.-Through-wall Vickers hardness profiles show an increase of the hardness at the outer diameter for HPTR cold pilgering whereas the hardness profile remains continuous for cold drawing. It is found that the Q-factor has an influence on these wall-thickness hardness profiles.-For each process, neutron diffraction measurements on finished tubes reveal two main fiber texture components and parallel to the tube axis with differences in their relative intensities

Fast X-ray reflectivity measurements using an X-ray pixel area detector at the DiffAbs beamline, Synchrotron SOLEIL

International audienceThis paper describes a method for rapid measurements of the specular X-ray reflectivity signal using an area detector and a monochromatic, well collimated X-ray beam (divergence below 0.01°), combined with a continuous data acquisition mode during the angular movements of the sample and detector. In addition to the total integrated (and background-corrected) reflectivity signal, this approach yields a three-dimensional mapping of the reciprocal space in the vicinity of its origin. Grazing-incidence small-angle scattering signals are recorded simultaneously. Measurements up to high momentum transfer values (close to 0.1 nm$^{-1}$ , also depending on the X-ray beam energy) can be performed in total time ranges as short as 10 s. The measurement time can be reduced by up to 100 times as compared with the classical method using monochromatic X-ray beams, a point detector and rocking scans (integrated reflectivity signal)

Loss of ductility in optimized austenitic steel at moderate temperature: A multi-scale study of deformation mechanisms

International audienceA Ti-stabilized cold-worked 15Cr-15Ni steel, called AIM1 (Austenitic Improved Material #1), has been selected as a candidate for the fuel cladding tubes of sodium-cooled fast reactors. This steel exhibits an unusual loss of ductility between 20 and 200 °C for both solution-annealed and cold-worked conditions, which is similar to that observed for Twinning Induced Plasticty steels and for the 200 and 300 series stainless steels. Therefore, a multi-scale study has been carried out to determine the deformation mechanisms that are active at different temperatures. Tensile tests have been performed to characterize the macroscopic material behavior, and Electron Backscattered Diffraction and Transmission Electron Microscopy characterization techniques have been used to investigate the meso and micro-scale phenomena, such as the deformation microstructures and the evolution of the lattice defects. The parameters governing the deformation mechanisms have been examined, with particular attention paid to the conditions for mechanical twinning activation. This work required an original study of the variation of Stacking Fault Energy with temperature, based on the measurement of the dissociation extension of dislocation nodes. An increase in the SFE was observed between 20 and 200 °C. After reviewing the existing models for predicting twinning, the present study proposes an approach based on the minimization of the total energy of the material to explain why twinning is not favorable at high temperatures. At 20 °C, both dislocation slip and twinning are active and efficient mechanisms to release the strain energy. However, at 200 °C, only dislocation slip is favorable and is often associated with dislocation cross-slip