Roberto Ribeiro de Avillez

A comparison between the Warren-Averbach method and alternate methods for X-ray diffraction microstructure analysis of polycrystalline specimens

The fundamental parameters approach is used to simulate the instrument contribution to the X-Ray diffraction profile. This procedure eliminates the need to experimentally prepare a reference sample of the studied crystalline material when using the Warren-Averbach method to investigate microstrutural parameters. The Warren-Averbach method is also compared to the Balzar and Enzo methods, two other popular methods of size-strain analysis. The analysis was carried out using bohmite powder having two different nominal average crystallite sizes, 80 A and 200 A. A 50%-50% mixture of these materials was used as a third sample. The proposed simulation procedure provides good results and is much faster to implement than the usual procedure that includes the preparation of a reference. For larger crystallite sizes, the results calculated from the Warren-Averbach method for the volume-weighted average crystallite size differs significantly from the ones obtained from the Balzar and Enzo methods. The limitations of the Balzar and Enzo methods are also discussed.

Thermodynamic Evaluation of Carbide Precipitates in 2.25Cr ¾ 1.0Mo Steel for Determination of Service Degradation

The 2.25Cr¾1.0Mo steel is widely used in petroleum refining units, for example, in pressure vessels, working under severe operational conditions. Due to the high temperatures of some processes, many phenomena such as precipitation of carbides and their coarsening occur during service, resulting in changes of material microstructure. Such changes may modify the mechanical properties, which are responsible for the performance of the pressure vessels. Using a software for thermodynamic calculations, the thermodynamically stable carbides over the temperature range of interest (700-1040 K) were evaluated, supposing ferrite as the original microstructure. The theoretical carbide nucleation sequence was also established. Moreover, the theoretical amount of thermodynamically stable carbides and the fraction of Fe, Cr and Mo in the metallic sublattice of M23C6 were determined as a function of temperature. These theoretical calculations were compared to some experimental results on carbide precipitation in the 2.25Cr¾1.0Mo steel and two approaches for evaluating the service degradation were proposed. The first one is based on the amount of precipitated M23C6 and the other on the Fe/Cr ratio in the M23C6 carbide.

Development of Bulk Bi2+xSr3-yCa yCu 2O8+delta Superconductors by Partial-Melting Route for Fault Current Limiters Application

The production of bulk Bi2+xSr3-yCayCu 2O8+D (Bi-2212) superconductors for fault current limiter application was developed via a partial-melting route. Aiming high Ic (critical current), which is the essential superconducting characteristic for application of this material in the construction of Fault Current Limiters (FCL), the produced blocks have predominance of Bi-2212 phase (83 wt%), which characterizes with high values of zero and onset transport critical temperature of 92K and 97.5K, respectively. A relatively low transition width, DT, from the superconducting to the normal state of 5.5K, revealed a good intergrain connectivity. Consequently, current measurements on the blocks of Bi-2212 show promising Ic values of 230A and 850A for direct and alternate current, respectively. It is expected that further increases in the Ic values will depend on the elimination of an observed amorphous phase and further reduction of amount and grain sizes of secondary phases, still present in the blocks obtained by the proposed partial-melting route. This may be achieved by a further optimization of the partial-melting processing parameters.

Evaluation via multivariate techniques of scale factor variability in the rietveld method applied to quantitative phase analysis with X ray powder diffraction

The present work uses multivariate statistical analysis as a form of establishing the main sources of error in the Quantitative Phase Analysis (QPA) using the Rietveld method. The quantitative determination of crystalline phases using x ray powder diffraction is a complex measurement process whose results are influenced by several factors. Ternary mixtures of Al2O3, MgO and NiO were prepared under controlled conditions and the diffractions were obtained using the Bragg-Brentano geometric arrangement. It was possible to establish four sources of critical variations: the experimental absorption and the scale factor of NiO, which is the phase with the greatest linear absorption coefficient of the ternary mixture; the instrumental characteristics represented by mechanical errors of the goniometer and sample displacement; the other two phases (Al2O3 and MgO); and the temperature and relative humidity of the air in the laboratory. The error sources excessively impair the QPA with the Rietveld method. Therefore it becomes necessary to control them during the measurement procedure.

LIQUID IRON DEOXIDATION BY ALUMINUM: A BRIEF REVIEW OF EXPERIMENTAL DATA AND THERMODYNAMIC DESCRIPTION

The iron-oxygen-aluminum equilibrium has been a subject of interest for many years due to its importance in the steel industry. A great percentage of all flat rolled steel is aluminum killed “AK” (deoxidized) low-carbon steel, not only because of the low soluble oxygen contents that can be achieved but also because of the importance of the residual aluminum in forming aluminum nitride that is one of the main tools used to achieve crystallographic texture adequate for forming. The behavior both of oxygen and of aluminum in liquid iron are relatively well understood and quantified, as is the free energy of formation of alumina. Nonetheless, the Fe-Al-O behavior at liquid iron temperatures is not fully understood and considerable controversy still prevails. Thus, the objectives of this work are (a) to critically review the experimental work and assessments performed mainly on the Al-Fe-O–Al2O3 equilibrium, (b) to evaluate the current models used to describe this equilibrium and (c) to present a recommendation about the data, based on its evaluated reliability and consistency.

The effect of alloying elements on constrained carbon equilibrium due to a quench and partition process

Abstract The model proposed by Speer et al. to calculate the constrained carbon equilibrium is expanded to include any number of substitutional solutes and coded with the MatLab and Thermo-Calc programs. This model is used to evaluate the effect of Al, Cr, Cu, Mn, Mo, Ni and Si solutes on the final austenite carbon concentration of ternary Fe – X – C alloys. Comparison is also made with the carbon concentration measured experimentally in some Transformation Induced Plasticity steels.