Defect and Diffusion Forum Vols. 312-315

Abstract: The superaustenitic stainless steel ASTM A 744 Gr. CN3MN (22Cr-25Ni-7Mo-0.2N) has as mainly characteristic high corrosion resistance in severe environment. As the corrosion resistance depends on the microstructure, it was investigated the phase transformations after a solution treatment at 1200°C. Thermocalc calculation for 53Fe-25Ni-22Cr alloy indicates austenitic phase between 1300 and 800°C and austenite + sigma phase below 800°C. The as-cast steel studied presented 2.7 % of precipitates volume fraction and the precipitates were located on the grain boundaries and inside the austenitic grains. X-ray diffraction confirmed the presence of sigma phase in as-cast sample. Scanning electron microscopy showed that the level of Cr and Mo was higher in the precipitates than in the austenitic matrix and the Ni content was higher in matrix compared to precipitates. After heating at 1200°C during 90 minutes, the precipitate volume fraction was reduced to 2.1 % and the grain boundaries precipitates were dissolved. The microstructural analyses made through transmission electron microscopy and X-ray diffraction showed the presence sigma phase and M6C carbide.

Abstract: Diffusion of Uranium in compacted clay was studied through the non-steady state diffusion method for the safety assessment of the radioactive waste in the storage and disposal practices. Since the permeability of the compacted clay is very low, the main mechanism for radionuclide transport is governed by the diffusion phenomenon. The diffusion process of uranium in compacted clay as a porous medium has been modeled by taking into account the effect of sorption. An increase in temperature, caused by the radiation from the canisters, takes place in the repository. Then, the effect of temperature on textural and structural properties of clay and coefficient diffusion has been examined. First, we examined the structure changes of the clay pellets under the influence of the heat in the temperature range 25–100°C. These changes are studied in the region of mesopores and macropores using the method of the mercury porosimetry. Second, the effect of temperature on the diffusion behavior of uranium was simulated. In these simulations, the experimental values of apparent diffusion coefficients of uranium were used to determine the concentration profiles versus time, depth and temperature. As conclusion, it was shown that the temperature plays an important role in the uranium transport through the compacted clay.

Abstract: The 32CrMoV13 low alloy steel was gas nitrided at 550°C, for three time durations (6.5, 13 and 20 h) and under a variable nitriding potential (1, 2.2 and 6 atm-0.5). The generated nitride layers were characterized by SEM observations, XRD and GDOS analyses as well as microhardness testing. The XRD analysis indicates that the compound layer was composed of and iron nitrides and CrN phase. The surface hardness (inside the compound layer) was found to be dependent on the nitriding potential value, its value increases as rises. It was shown by GDOS analysis that the upper and lower nitrogen concentrations at the (compound layer / diffusion zone) interface are approximatively: 4 and 0.88 wt. % N, respectively.

Abstract: The formation of self-aligned titania and zirconia nanotubes is achieved by the anodisation of Ti and Zr in a fluorine contained electrochemical bath. The anodic oxidation was performed at 30 V for 60 min in a two-electrode glycerol (15% water) bath containing varying amount of NH4F. Despite the fact that a self-aligned nanotubular structure is formed on both titanium and zirconium, the dimensions of zirconia and titania nanotubes are different under the same anodisation parameters. It appears that by using 30 V as the anodisation voltage, the diameter of zirconia nanotubes (30-60 nm) is much smaller compared to that of titania nanotubes (80-100 nm). The length of zirconia nanotubes in the bath consisting of 0.7 g NH4F is 3 µm whereas titania nanotubes formed in the same bath have a length of ~700 nm. The fundamental difference between the nanotubes formed on titanium and zirconium may be related to the rate of oxidation, initial oxide formation during anodisation, pits formation and rate of pits growth for pores formation and stabilisation. Moreover, investigation on the crystallinity of the nanotubes reveals that titania nanotubes are weakly crystalline with crystallite sizes of <5 nm. Whereas, zirconia nanotubes are much more crystalline in cubic modification. The stabilisation of the high temperature phase is thought to originate from the size of the nanotubes walls and the deficiency in oxygen during the growth of the anodic oxide by anodisation.

Abstract: In this paper, length of grain-to-grain contact, number of contacts per grain and ratios between the length of contacts and geometric characteristics of grains in heterogeneous Berea sandstone and Indiana limestone are studied. It is shown how hydrostatic pressure up to 40 MPa influences these grain boundary parameters. Observations show that there are different types of influence of hydrostatic pressure on grain boundary parameters in the studied sandstone and limestone.

Abstract: The mass transfer coefficients at different axial positions in a bubble column have been measured using a conductivity method. Measurements of CO2 absorption in methyldiethanolamine solutions were carried out in a prismatic column 0.06 m side and 1.03 m height. Five electrode pairs were placed at various axial positions in the column. Measurements were conducted at different gas flow rates and amine concentration. It was found that the local mass transfer varied in the axial direction. An experimental model was developed to obtain the absorption rate values using the electrochemical method data. The model agrees quite well with the experimental obtained data.

Abstract: Using a simulation program, an absorption process has been simulated of an air stream contaminated with acetone using water as absorbent and the subsequent recovery of the solvent by distillation. After verifying the effectiveness of the simulated process, the next step was to carry out an economic analysis at the level of equipment to assess the economic viability of the simulated recovery plant.

Abstract: A wet chemical approach, originating from sol-gel preparation, was adopted with the intention to develop a low-temperature benign method of preparation. ZnO nanorods are successfully grown in an aqueous medium. The precursor, zinc nitrate hexahydrate (Zn(NO3)2.6H2O), is stabilized by hexamethylene tetraamine (HMTA). The effect of changing the molarity of HMTA to the structural orientation of ZnO nanorods is investigated. X-ray diffraction of the synthesized ZnO shows hexagonal zincite structure. The structural features of the nanocrystalline ZnO were studied by SEM. Structural features, surface morphology and differences in lattice orientation are seemingly influenced by varying the Zn2+: HMTA molar ratio. The formation of ZnO nanorods with blunt and sharp tips is found to be significantly affected by this ratio.

Abstract: We report an optimisation of reaction conditions leading to successful growth of aligned ZnO nanorods by a simple aqueous immersion method. Si substrates were immersed in aqueous Zn2+ solution at various temperatures ranging between 50 - 90°C and at various immersion times. Degree of crystallisation, thermal decomposition, surface morphology and crystallite size were analysed by FT-IR, TGA, SEM and XRD. Seeded noble metals like gold, platinum and nickel on Si substrate were found to improve the vertical alignment of rods’ growth and reduce the diameter of ZnO nanorods.

Abstract: To improve the strength and toughness of AISI 4340 steel, different microstructures, containing full bainite, bainite-ferrite, martensite-ferrite and full martensite were produced by different heat treatment cycles. Tensile, impact and hardness tests were carried out at room temperature. The ductile-brittle transition temperature was determined from impact data at different temperatures. The results showed that steel with bainite - 0.34 ferrite microstructure has the highest elongation and charpy impact energy, while its tensile strength and yield stress decreased in comparison to other microstructures. This increment was noticeable when bainite - 0.34 ferrite steel was tempered. The ductile-brittle transition temperature decreased with tempering of bainite -0.34 steel. The fracture surface analysis of charpy specimens also showed an increase in toughness of tempered bainite-ferrite in comparison to other microstructures.