Defect and Diffusion Forum Vols. 273-276

Abstract: Non-destructive and on-line Li diffusion experiments in Li ionic conductors are conducted using the short-lived !-emitting radiotracer of 8Li. The radiotracers produced as an energetic and pulsed ion beam from TRIAC (Tokai Radioactive Ion Accelerator Complex) are implanted into a structural defect mediated Li ionic conductor of NaTl-type intermetallic compounds ("-LiGa and "-LiIn). The experimental time spectra of the yields of !-particles are compared with simulated results and Li diffusion coefficients in the intermetallic compounds are extracted with an accuracy of ±10%. The diffusion coefficients obtained for "-LiGa with Li content of 43-54 at.% are discussed in terms of the interaction between Li-ion and the structural defects in the specimen, compared with the cases of "-LiAl and "-LiIn. The nonlinear Li-content dependency of Li diffusion coefficients for "-LiGa suggests that the Li diffusion with the Li-deficient region is obstructed by the defect complex composed of vacancies at the Li sites.

Abstract: Orthogonal metal cutting process involves large plastic deformation accompanied by excessive heat generation. This work addresses the thermal-mechanical responses of the workpiece material at the tool-workpiece contact. In this respect, the orthogonal cutting process of Ti-6Al-4V using CVD diamond tool is simulated using finite element method. The cutting condition consists of cutting speed, V=180 m/min, feed rate, t=0.125 mm/rev and width of cut of 1.25 mm. Eulerian formulation with coupled thermal-mechanical analysis is employed in the model. The Johnson- Cook constitutive equation is employed for Ti-6Al-4V workpiece material to accurately simulate the formation of shear bands. The stick-slip friction condition is modeled at the tool-chip interface. The sliding coefficient of friction of 0.8 and the limiting shear stress of 700 MPa for stick-slip condition are determined experimentally. Results show that high temperature and temperature gradient concentrate in the primary shear zone and the contact area between the tool rake face and the chip. A primary shear band is predicted in the workpiece ahead of the tool-workpiece contact face while the secondary shear band is formed in the chip. This highly-deformed shear band is revealed in the microstructure of etched chips. The predicted high strain rate results in build-up edge at tool cutting edge-chip contact. Low cutting condition of V=150 m/min, t=0.125 mm/rev promotes stagnant zone formation that helps preserve the cutting edge of the tool. The maximum predicted temperature at the cutting edge is in excess of 700 °C. Such high temperature level facilitates diffusion of carbon elements into the chips and conversely, elements of titanium into the CVD diamond tool.

Abstract: The RH process is a secondary refining process that can simultaneously attain significant levels of removal of interstitial elements, such as carbon, nitrogen and hydrogen, from liquid steel. In the RH process, the decarburization rate plays a very important role in determining the productivity of the equipment. The kinetics of this reaction is controlled by mass transfer in the liquid phase. In the present work, a physical model of a RH degasser has been built and used in the study of the kinetics of decarburization. The effects of the gas flow rate and of the configurations of the nozzles used in the injection of the gas have been analyzed. The decarburization reaction of liquid steel was simulated using a reaction involving CO2 and caustic solutions. The concentration of CO2 in the solution was evaluated using pH measurements. Based on the experimental results, it was possible to estimate the reaction rate constant. A volumetric mass transfer coefficient was then calculated based on these rate constants and on the circulation rate of the liquid. The logarithm of the mass transfer coefficient showed a linear relationship with the logarithm of the gas flow rate. The slope of the line was found to vary according to the relevance of the reaction at the free surface in the vacuum chamber. A linear relationship between the volumetric mass transfer coefficient and the nozzle Reynolds number was also observed. The slopes of the lines changed according to the relative importance of the two reaction sites, gas-liquid interface in the upleg snorkel and in the vacuum. At higher Reynolds number, the reaction in the vacuum chamber tends to be more significant.

Abstract: The effect of the SiO2/Si interface on Si self-diffusion in SiO2 during thermal oxidation was investigated using silicon isotopes. Samples with natSiO2/28Si heterostructures were oxidized at 1150 ~ 1250 °C and the 30Si diffusion in 28SiO2 during the thermal oxidation was investigated by secondary ion mass spectrometry (SIMS) measurements. Near the SiO2/Si interface, a significant profile broadening of the 30Si isotope from natSiO2 toward the newly grown 28SiO2 was observed. This 30Si self-diffusivity sharply decreases with oxidation time and hence with increasing distance between 30Si diffusion region and the interface. This distance-dependent 30Si self-diffusion was simulated taking into account the effect of Si species generated at the interface upon oxidation and diffusing into SiO2 to enhance Si self-diffusion. The simulation fits the SIMS profiles and these results indicate that Si species, most likely SiO, are emitted from the SiO2/Si interface upon Si thermal oxidation to release the oxidation-induced stress, as has been predicted by recent theoretical studies. Furthermore, combined with our recent results on O self-diffusion, the diffusion behavior of the emitted SiO near the SiO2/Si interface is discussed.

Abstract: The current trend in nuclear industry to extend the burnup limits up to high burnup requires the analysis of the phenomenology related to the nuclear fuel, specifically the diffusion and fission gas release (FGR) related to this phenomenon plays an important role. In this paper, the diffusion of stable fission gases (xenon and krypton) in nuclear fuel UO2 is addressed through the predictions for FGR to rod void volumes obtained with FRAPCON-3 fuel performance code. The theoretical base of diffusion model in FRAPCON-3 code is shown and some modifications are proposed, such as the removing of empirical correlations not related to diffusion phenomenon and fitting parameters included in the model. Besides, the resolution process in the proximities of grain boundaries is considered in a different way, and the grain growth mechanism from a specific temperature threshold is implemented into the code. The database applied for evaluation is presented and the results with the original and modified model are shown.

Abstract: Thermal properties of honeycomb structures with different cell shapes are investigated in this paper. The influence of cell shape, relative density and pore gases on the macroscopic honeycomb thermal properties is investigated by means of transient dynamic computational simulations. The ANSYS CFX code is used to evaluate the heat conduction trough the base material and the filler gas, as well as the convection in gas filler. The computational results clearly show a strong influence of the filler gas on heat conduction and macroscopic thermal properties of analyzed honeycomb structures, which is attributed to low relative density of the cellular structure. Additionally, the influence of considered relative densities is more prominent than the influence of cell shape. The evaluated results are valuable for further development of homogenization models of heat transfer in honeycomb structures accounting for gaseous pore fillers.

Abstract: Using nuclear microanalysis (NRA) and electron probe microanalysis (EPMA), concentrations of carbon, oxygen, nitrogen, aluminum, and vanadium were measured on a large group of macrodefects formed in the course of smelting titanium alloys. A remarkable enrichment of the defect material in oxygen and nitrogen atoms was detected; histograms of defect distribution over the concentrations of oxygen, nitrogen, aluminum, and vanadium were obtained. The above results agree with the concepts according to which the defects are formed from the particles that have the melting temperature higher then the temperature of smelting.

Abstract: The presence of the residual element copper in recycled steels causes a surface cracking phenomenon during thermo-mechanical processing which is known as “hot shortness”. The cracks result from a copper-rich liquid that forms at the oxide/metal interface and subsequently embrittles austenite grain boundaries. Minimizing formation of the liquid phase would reduce or eliminate cracking. Thus, the evolution of the liquid layer is an important consideration when designing an optimal thermomechanical processing cycle in scrap-based steel plants. The time evolution of the liquid phase is dependent on the competing processes of enrichment rate due to iron oxidation and the rate of copper back-diffusion into the steel. This paper presents a fixed grid finite difference model that predicts the evolution of the enriched region as a result of a given oxidation kinetics and solution of Fick’s 2nd law. The model predictions are in agreement with measured data for the case of an iron alloy containing 0.3 wt% copper oxidized in air at 1150°C. Model predictions indicate that initial copper content, oxidation kinetics, and alloy microstructure (i.e. grain boundary diffusion) have the most significant influence on the copper-rich layer whereas the solubility increase due to nickel additions was not found to have an appreciable influence.

Abstract: Studies of dilatometry, DSC, hardness and XRD have shown that a low temperature cryogenic treatment immediately after supersaturation affects changes of structure and properties taking place during ageing after supersaturation. The third peak visible on DSC diagrams for samples conventially treated disappears on DSC diagrams for deep cryogenically treated samples, and the intensity of the two first peaks on DSC diagrams of deep cryogenically treated samples is a few times lower than on diagrams for conventionally treated samples. The dissolution process for precipitation of Al2Cu in conventionally treated samples progresses faster. Hardness of samples aged after a deep cryogenic treatment within the range of 175-220 oC for more than 60 minutes was higher than of those conventionally treated; at higher ageing temperatures (275-350 °C) the differences in hardness were smaller.

Abstract: The study analyses of influence of cryogenic treatment on the resultant structures, the development of changes during tempering and the charcteristic obtained. It was observed that cryo treatment affects the changes through: a significant reduction in retained austenite content, gradual reduction in value of martensite tetragonality and an increase in the number of carbon atoms in transformations during tempering. The consequence of the transformations taking place during cryogenic treatment is precipitation of η – carbide during low tempering instead of ε – carbide. The number of η – carbides precipitated is higher than that of ε – carbide. This affects the functional properties of steel products. Key words: cryogenic processing, supersaturation, dilatometry, DSC, ageing.