Solid State Phenomena Vols. 172-174

Abstract: Solution-treated Al-4wt%Cu was strain-cycled at ambient temperature and above and the precipitation behavior investigated by TEM. In the temperature range 100°C to 200°C precipitation of Θ´´ appears to have been suppressed and precipitation of Θ´ promoted via cyclic strain. Anomalously rapid growth of precipitates appears to have been facilitated by a vacancy super-saturation generated by dislocation motion, with a diminishing effect observed at higher temperatures due to the faster recovery of non-equilibrium vacancy concentrations. Θ´ precipitates generated under cyclic strain are considerably smaller and more finely dispersed than those typically produced via quench-aging due to their heterogeneous nucleation on dislocations, and possess a low aspect ratio and rounded edges of the broad faces due to the introduction of ledges into the growing precipitates by dislocation cutting. Frequency effects indicate that dislocation motion, rather than the extremely small precipitate size, is responsible for the observed reduction in aspect ratio. Accelerated formation of grain boundary precipitates appears partially responsible for rapid inter-granular fatigue failure following cycling at elevated temperatures, producing fatigue striations and ductile dimples coexistent on the fracture surface.

Abstract: Direct hot extrusion of powder is the standard consolidation process to transform mechanically alloyed Oxide Dispersion Strengthened (ODS) steels into fully dense bars. It is a complex process including several steps. In this study, ODS steel bars were extruded and the material microstructure was characterized by TEM observations associated to SANS after each step. It was shown that the nano-cluster nucleation occurs during the powder pre-heating before hot-extrusion. During extrusion, the ferritic matrix undergoes a dynamic recovery but no further change of the nano-cluster distribution is observed.

Abstract: Composites of Fe-C₆₀ and Al C₆₀ produced by mechanical milling and sinterized by Spark Plasma Sintering are investigated with special attention to the mechanical properties of the products. The processing involves phase transformations of the fullerenes that are interesting to follow and characterize. This involves formation of tetragonal/rhombohedral diamond and carbides during sintering and milling. Transmission Electron Microscopy (TEM) and Raman Spectroscopy techniques are also used to confirm preliminary results of X Ray Diffraction (XRD) related to the formation of nanostructures i.e., grain size of the crystals during mechanical milling and after sintering, spatial distribution of phases and the different phases that are developed during processing.

Abstract: Characterization of the phase transformation of Nb-micro-alloyed steels has to be performed taking into account the effect of deformation and precipitation. In the present investigation, the austenite to ferrite phase transformation is characterized in continuous cooling experiments after deformation at high temperature. The resulting phase transformation kinetics and microstructure showed an influence of the soaking temperature. Detailed investigations of the possible causes of the change of mechanism of phase transformation indicate that the amount of niobium in solution correlates with the slowing down in phase transformation kinetics.

Abstract: In this work we compare and contrast the stability of retained austenite during tensile testing of Nb-Mo-Al transformation-induced plasticity steel subjected to different thermomechanical processing schedules. The obtained microstructures were characterised using optical metallography, transmission electron microscopy and X-ray diffraction. The transformation of retained austenite to martensite under tensile loading was observed by in-situ high energy X-ray diffraction at 1ID / APS. It has been shown that the variations in the microstructure of the steel, such as volume fractions of present phases, their morphology and dimensions, play a critical role in the strain-induced transition of retained austenite to martensite.

Abstract: The phase transformation kinetics on cooling and resulting microstructures of steel-based matrix composites (MMC) reinforced with TiC particles by powder metallurgy were studied. In addition, the phase transformation kinetics of the MMC were compared to those of the same steel without TiC and consolidated in the same conditions. The presence of TiC particles strongly favors the diffusive transformations in the steel matrix of the MMC. Different complementary techniques (XRD, SEM, TEM/EDX, atom probe tomography, in situ synchrotron XRD) were performed to analyze the chemical reactivity between TiC particles and the steel powders occurring during consolidation process and further heat treatments. Composition changes in the TiC as well as in the matrix were characterized. The chemical composition after treatment in the TiC particles tends toward the thermodynamic calculations with ThermoCalc. The effect of changes in chemical composition and the role of TiC particles acting as new favorable nucleation sites are discussed in regards to the obtained results.

Abstract: Due to their adequate properties, zirconium alloys are the reference materials for the nuclear fuel cladding tubes of Light Water Reactors (LWR). During some hypothetical accidental High Temperature (HT) transients, the materials should experience heavy steam oxidation and deep metallurgical evolutions. This promotes Alpha-Beta phase transformations and an associated strong partitioning of oxygen/hydrogen and of the main chemical alloying elements (Nb, Sn, Fe and Cr). Moreover, it has been shown quite recently that such chemical elements partitioning during on-cooling Beta-to-Alpha transformation can strongly impact the residual mechanical properties of HT oxidized materials. Thus, it appeared that it was important to better quantify and, if possible, to compute the quite complex phase equilibrium that occurs in multi-alloyed zirconium materials in the presence of both oxygen and hydrogen. For that, systematic studies have been performed on industrial alloys, charged with oxygen and/or hydrogen. After applying different heating/cooling scenarii, both Electron Microprobe using Wave Dispersive Spectrometry (WDS) and Nuclear Microprobe using Elastic Recoil Detection Analysis (ERDA) have been applied. Finally, to support the observed chemical elements partitioning between the Alpha and Beta allotropic phases, some thermodynamic calculations have been performed thanks to the development and the use of a specific thermodynamic database for zirconium alloys called “Zircobase".

Abstract: In the present study we focus on the precipitation processes during heating and ageing of β-metastable phase in the near β Ti-5553 alloy. Transformation processes have been studied using continuous high energy X-Ray Diffraction (XRD) and electrical resistivity for two different states of the β-metastable phase. Microstructures have been observed by electron microscopy. Different transformation sequences are highlighted depending on both heating rate and chemical composition of the β-metastable phase. At low temperatures and low heating rates, the hexagonal ω_iso phase is first formed as generally mentioned in the literature. Increasing the temperature, XRD evidences the formation of an orthorhombic phase (α’’), which evolves toward the hexagonal pseudo compact α phase. For higher heating rates or for richer composition in β-stabilizing elements of the β-metastable phase, ω phase may not form and α’’ forms directly and again transforms into α phase. A direct transformation from β-metastable to a phase is observed for the highest heating rate. The formation of the metastable ω_iso and α’’ phases clearly influences the final morphology of α.

Abstract: High temperature operation of power plant increases their efficiency and reduces emissions. Low gamma-prime fraction nickel-based superalloys, such as IN617, are becoming increasingly important in substituting for conventional steel components due to their better performance at higher temperatures. The aim of this work is to model the microstructural evolution in this alloy under typical operating conditions and relate this to its creep response. Although classified as solid solution strengthened alloys, they contain a range of different precipitates which contribute to the creep resistance, the most influential of these being gamma prime. The model is developed in three stages. Firstly, a precipitate-level variational model is derived to describe the nucleation and growth of individual particles and their interactions. The results are then passed up to a grain-level simulation where the response of a statistically significant particle ensemble is simulated via the evolution of particle distributions. The resulting model incorporates the effects of heat treatment and stress to give the simultaneous size and number evolution of the different phases in the material. This information is then used to construct plastic strain-temperature-time diagrams in order to estimate the creep rupture life of the material.

Abstract: The effect of the metallurgical state of austenite (undeformed vs. deformed vs. deformed + recrystallised) on the properties of the austenite to bainite transformation were investigated thanks to thermal (Gleeble simulations) and thermomechanical (hot torsion) treatments. No obvious influence of the state of austenite was found, using electron backscatter diffraction, on the resulting microtexture. Advantages and drawbacks of using misorientation angle histograms vs. axis-angle pair distribution are discussed regarding investigations of local variant selection. For an austenite grain size higher than about 50 µm, a strong effect of the transformation temperature was evidenced, bainite formed at lower temperature (530°C) exhibiting a microtexture close to that of lath martensite in the same steel.