Strain mapping by non destructive method: Laue microdiffraction

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New structural insight into interface-controlled α–σ phase transformation in fe-cr alloys

Synchrotron Laue microdiffraction scanning is used for the ex situ study of the body-centered, cubic-to-tetragonal phase transformation that occurs in equiatomic polycrystalline Fe-Cr alloys at temperatures between 550 and 800◦ C. Grain orientation and grain strains were scanned with a micron step resolution after annealing at 700◦ C for 12 h. Further microstructural details on the early stage of the transformation, and more particularly on the cubic-to-tetragonal phase interface, were achieved. Only the α and ordered σ phases were detected. The crystallographic relationships at the interface between the two phases did not follow the predicted rules; this result is discussed in relation to the measured microstrains

Where the atoms are Cation disorder and anion displacement in DIIXVI AIBIIIXVI2 semocinductors

Structural phase transitions tetragonal cubic dependent on composition as well as temperature were studied in ternary CuBIIIX2 chalcopyrites as well as 2 ZnX x CuInX2 1 x alloys B Ga, In; X S, Se, Te by neutron and synchrotron X ray powder diffraction. The transitions are due to a combination of disorder of the cation sublattice and displacement of the anion sublattice. The first goes along with a phase separation by chemical disorder, within a homologeous series depending on the size of the anion, for the same anion depending on the size of the cation B. The temperature dependent phase transition is clearly driven by a Cu B anti site occupancy. The order parameter of the phase transition, expressed as a difference in the number of electrons on the cation sites in the disordered and ordered phase, shows a critical behaviour with a critical exponent beta 0.35

Etude par microdiffraction Laue de la redistribution des déformations dans un superallliage gros grain à base de nickel après grenaillage, maintien isotherme et fatigue.

International audienceThe Laue microdiffraction technique was used to investigate the strain field caused by the shot-peening operation and its redistribution after thermal hold or fatigue in a model nickel-based superalloy with an average grain size of 40$\mu$m. Micrometer and millimeter size mappings showed that the plastic deformation introduced by shot-peening in the whole sample partially relaxes after a thermal exposure at 450∘C and was fully redistributed by the fatigue of the material, except in the hardened layer close to the sample edge. Diffraction patterns permitted to measure separately the strains related to the average alloy ($\gamma$+$\gamma$′) and to the γ′ phase. No difference was observed between the two deviatoric strain fields. Even if there were small stresses in the inner part of the samples, the sensitivity of the Laue microdiffraction method was large enough to quantitatively characterize the crystal misorientations and the deviatoric strain redistributions. Useful data were provided not only at the grain scale but also at the mesoscopic scale, thus bridging the gap between the sin$^2$$\psi$ and Ortner’s methods used to determine residual stresses, respectively, in fine and single-grain microstructures. The obtained results are also of first interest for a quantitative comparison with HR-EBSD measurements in the scanning electron microscope. Energy coupled measurements with an energy-dispersive point detector were also performed to determine the full elastic strain tensors associated with the $\gamma$ and $\gamma$′ phases. We demonstrated that, for Ni-based superalloys, the accuracy on strains and stresses was, respectively, of the order of 1×10$^{−3}$ and $250$ MPa for the diagonal components of tensors. The measurements suffered from the 150 eV resolution of the detector which made it difficult to the separate the energies of the $\gamma$ and $\gamma$′ phases. Owing to large crystal misorientations, the microdiffraction technique was not able to determine elastic strains and hardening in the highly deformed layer, where a large amount of plastic strain and a number of defects were accumulated. Some improvements are proposed to overcome these difficulties

Low temperature transformation in the b-metastable Ti-5553 alloy

9th European Symposium on Martensitic Transformations (ESOMAT 2012), St Petersburg, RUSSIA, SEP 09-16, 2012International audiencePhase transformation of the b metastable phase Was characterized at low temperature in the Ti-5553 alloy. Electrical resistivity and High Energy X-Ray Diffraction were utilized to characterize transformation kinetics along two thermal paths: i) direct quenching from stable b-domain to the isothermal transformation temperature of 325 degrees C, and ii) quenching from stable b-domain to room temperature and further ageing at 325 degrees C. Both treatments led to different transformation sequences with formation of w and/or a''. Mean cell parameters of b and a'' were obtained all along the transformation. Results suggest a transformation with poor solute partitioning and large stress building of in each phase