Austenite Grain Growth in a 2.25Cr-1Mo Vanadium-Free Steel Accounting for Zener Pinning and Solute Drag: Experimental Study and Modeling

Metallurgical and Materials Transactions A - Austenite grain size has been experimentally determined for various austenitization temperatures and times in a 2.25Cr-1Mo vanadium-free steel. Three...     

Numerical modeling of oxygen diffusion in the wall thickness of Low-Tin Zircaloy-4 fuel cladding tube during high temperature (1100-1250 °C) steam oxidation

A new numerical tool has been developed by coupling the diffusion code “EKINOX” (Estimation Kinetics Oxidation) and the thermodynamic database “ZIRCOBASE” using TQ (ThermoCalc program interface). The aim of this tool is to calculate the oxygen diffusion during oxidation of Zr based alloys. The simulation results of the oxide growth kinetics and the induced oxygen diffusion profiles in the [1100-1250 °C] temperature range, at different times, are presented and compared to previous experimental results. An estimation of the diffusion coefficient in the α phase is deduced from this comparison. The results are discussed in comparison to previous models based on an analytical treatment and the influence of the choice of the thermodynamic data set on the final oxygen diffusion profile is explored.Finally, it is shown that the present modeling is able to predict quite accurately the “critical” oxidation time corresponding to the overall ductile-to-brittle transition of the high temperature (HT) oxidized clad.     

Carbide Precipitation in 2.25 Cr-1 Mo Bainitic Steel: Effect of Heating and Isothermal Tempering Conditions

The effect of the tempering heat treatment, including heating prior to the isothermal step, on carbide precipitation has been determined in a 2.25 Cr-1 Mo bainitic steel for thick-walled applications. The carbides were identified using their amount of metallic elements, morphology, nucleation sites, and diffraction patterns. The evolution of carbide phase fraction, morphology, and composition was investigated using transmission electron microscopy, X-ray diffraction, as well as thermodynamic calculations. Upon heating, retained austenite into the as-quenched material decomposes into ferrite and cementite. M₇C₃ carbides then nucleate at the interface between the cementite and the matrix, triggering the dissolution of cementite. M₂C carbides precipitate separately within the bainitic laths during slow heating. M₂₃C₆ carbides precipitate at the interfaces (lath boundaries or prior austenite grain boundaries) and grow by attracting nearby chromium atoms, which results in the dissolution of M₇C₃ and, depending on the temperature, coarsening, or dissolution of M₂C carbides, respectively.     

Contribution to Modeling of Hydrogen Effect on Oxygen Diffusion in Zy-4 Alloy During High Temperature Steam Oxidation

Previous studies have shown that the numerical model EKINOX-Zr was able to simulate with accuracy oxide growth and oxygen diffusion into the matrix during high-temperature oxidation of Zy-4. In this study, the aim of the development was to evaluate if the observed effect of hydrogen cladding content on the increase of oxygen solubility in the high-temperature β_Zr was only a thermodynamic effect. Previous experimental studies have shown that hydrogen induces an evolution of equilibrium oxygen concentration at the α_Zr/β_Zr interface. The present work showed that EKINOX-Zr linked with the thermodynamic database Zircobase reproduced the evolution induced by hydrogen during the high-temperature steam oxidation. However, the results showed also that additional studies are necessary to better understand hydrogen behavior during high-temperature oxidation of Zr.     

Modeling Two- and Three-Stage Oxygen Tracer Experiments during High-Temperature Oxidation of Metals with a High Oxygen Solubility

The numerical tool EKINOX-Zr has been upgraded to simulate oxygen tracer experiments during the high-temperature oxidation of a metal with a high oxygen solubility limit. The penetration of ¹⁸O tracer is calculated during the dynamic evolution of the ZrO_2-x/α_{Zr(O )}/β_zr system. The numerical approach allows to explicitly take into account the variation of the tracer diffusion coefficient through the oxide scale as a function of the vacancy concentration. A classical two-stages ¹⁶O₂/¹⁸O₂ tracer experiment has been simulated. It is shown that a classical fitting procedure on the ¹⁸O concentration profile obtained for short-time experiments leads to the identification of the oxygen chemical diffusion coefficient. The second type of tracer experiment is proposed using a three-stages ¹⁶O₂/¹⁸O₂/¹⁶O₂ oxidation. It allows the direct estimation of the diffusion coefficient from the transport of ¹⁸O peak in the growing oxide scale.     

The Zr–Sn binary system: New experimental results and thermodynamic assessment

The Zr–Sn binary system has been reinvestigated by several experimental techniques: X-ray diffraction, electron probe micro-analysis, mass density and calorimetry measurements. The existence of a miscibility gap inside the homogeneity domain of the η$\eta$ phase (Zr₅Sn₃–Zr₅Sn₄) has been confirmed. It has been also shown that Zr substitution on the Sn sublattice is responsible for the non-stoichiometry of the A$A$15 phase (Zr₄Sn). The temperature of the peritectoid reaction β_Zr+A15↔α_Zr has been determined to be at 1216 K that is 40^°$40^{°}$ below the temperature reported in the literature. All these new experimental data have been taken into account for a new thermodynamic assessment of this system.