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The growth kinetics of ferrite during intercritical annealing of steel has been investigated using in situ high energy X-ray diffraction with a specially designed furnace allowing highly quantitative measurements of phase volume fractions. Kinetics have been obtained at 730 ∘ C, 750 ∘ C and 775 ∘ C in different ternary Fe-C-X (where X : Mn, Ni, Mo and Cr) alloys and in a quaternary Fe-C-1Mn-1Cr (wt.%) alloy. The obtained results were compared with the predictions of classical Local Equilibrium and Para-Equilibrium models as well as an improved version of a three-jump solute drag model, where the interactions between the solute elements and the moving interface are described comprehensively. Good agreement was obtained between the measured ferrite growth kinetics and the predictions of the solute drag model for the different Fe-C-X systems using only one fitting parameter, namely the Fe-X interaction parameter at the interface. The interactions found here qualitatively match those reported in literature for all solutes except for Ni, which displayed an attractive interaction with the interface. For the quaternary Fe-C-Mn-Cr system, the solute drag model succeeded in predicting ferrite growth kinetics using the same interaction values as used for the ternary Fe-C-Mn and Fe-C-Cr systems.

Materials characterization 169, 110654 (2020). doi:10.1016/j.matchar.2020.110654, Phase transformations in near Ti2AlNb alloy occurring on cooling of 10 ��C min$^{���1}$ from 1020 ��C were studied in situ using high energy X-ray diffraction and electrical resistivity measurements coupled with simultaneous dilatometric analysis. From Rietveld refinement of X-ray data, it was established that high temperature range from about 870 to 1020 ��C contained partially ordered $��$ phase. The degree of long-range order was of 0.6 at 1020 ��C and decreased slowly with temperature decrease and fast when O-phase was formed. The formation of O-Ti$_2$AlNb phase started from about 870 ��C and consisted of two stages: (i) fast transformation of partially ordered $��$ into O phase at higher temperatures followed by (ii) slow transformation of fully disordered $��$ into O-phase at lower temperatures. The appearance of O-phase was accompanied by important volume decrease of the alloy as recorded using dilatometry as well as by a decrease of electrical resistivity. The transformations temperatures revealed by high energy X-ray diffraction, dilatometry and electrical resistivity were in good agreement. The obtained results were discussed using thermodynamic simulations carried out with Thermo-Calc software and TCTI2 database., Published by Elsevier, New York, NY

Le nombre d’etudes associant l’utilisation de la diffraction des rayons X synchrotron haute energie pour l’etude des transformations de phases est en constante augmentation. Cette technique offre l’avantage d’analyser le materiau en volume avec des temps d’acquisition courts. Combine a un instrument permettant de realiser des cycles en temperature et eventuellement sous charge, ce dispositif devient un outil puissant pour caracteriser precisement les sequences de transformations de phases, les cinetiques de transformations de phases, et dans certains cas bien specifiques, d’atteindre des informations sur la germination et la croissance. Une analyse simultanee des evolutions des parametres de maille des phases en presence au cours du processus de transformation peut egalement nous fournir des informations sur les mecanismes de transformation. L’etude proposee a pour objectif de contribuer a accroitre nos connaissances sur la transformation martensitique dans les aciers. Pour ce faire, nous avons selectionne deux aciers (i.e. un acier Maraging avec une faible teneur en carbone et un acier 35NiCrMo16 avec une forte teneur en carbone). Pour les deux aciers, nous avons entrepris une campagne d’analyses in situ par diffraction des rayons X synchrotron haute energie afin de suivre les cinetiques de la transformation martensitique, les evolutions de parametres de maille ainsi que la largeur a mi-hauteur de la fraction des phases en presence. L’effet d’une contrainte sur la cinetique de la transformation martensitique dans le cas de l’acier Maraging a egalement ete etudie. L’ensemble de ces resultats est discute et l’evolution du parametre de maille moyen de l’austenite est comparee a un calcul micromecanique de type Mori Tanaka.

13th International Conference on Martensitic Transformations (ICOMAT), Osaka, JAPAN, SEP 04-09, 2011; International audience; Thanks to time resolved high energy X-ray diffraction, isothermal decomposition of beta metastable phase was studied, directly after solution treatment in the beta temperature range, for temperatures ranging from 300 to 450 degrees C for two beta metastable alloys (Ti 17 and Ti 5553). The formation of an orthorhombic alpha'' phase is clearly identified at the beginning of the transformation whatever the alloy studied. If transformation occurs at the higher temperature an evolution of alpha'' is observed toward the hexagonal alpha phase. The phase amounts and the mean cell parameters of each phase were quantified by the Rietveld refinement method. The obtained cell parameters evolutions and the orthorhornbicity of alpha'' are discussed. Moreover, the orthorhombicity of alpha'' compared to that obtained for stress induced martensite may indicate a slight partitioning of solutes in isothermal alpha''.