Defect and Diffusion Forum Vols. 312-315

Abstract: Mechanical degradation and ductile failure in metal forming operations can be successfully modelled using fully coupled damage models. In addition, it has been largely reported in the literature that temperature variations affect material behaviour, especially thermal softening. This paper presents a numerical discussion of the coupled effects between ductile damage and temperature evolution based on the simulation of tensile tests of notched specimens.

Abstract: Composite electrodeposition technology is one of the methods used to fabricate metal matrix composite coating materials. One of the most challenging issues concerns the addition of homogeneously suspending oxide powders, such as TiO2 and ZrO2 in the electrolyte, in order to result in the uniform dispersion of the oxide powders in the metal matrix [1]. Moreover, the application of a current periodically varied by time during electrodeposition, i.e. pulse current (p.c.) instead of the commonly used direct current (d.c.) has been concluded that leads to the preparation of surfaces with well-defined properties [2]. It is also well known that composite electrodeposits of Ni/metal oxides have remarkable mechanical and anticorrosion properties [3]. In this work nickel matrix composite electrocoatings including ZrO2 micron–sized particles were produced from an additive free Watts type bath under strong agitation conditions applying d.c. and p.c. regime. The synergetic action of the pulse application, the incorporation of the particles into the nickel matrix and the applied hydrodynamic conditions on the crystalline development, the morphological characteristics, the microhardness and the roughness of the composite electrocoatings, were studied.

Abstract: A numerical simulation study, using finite element method, was carried out to examine the temperature and concentration fields in the dissolution process of silicon into germanium melt. This work utilized a simplified configuration which may be considered to be similar material configuration to that used in the Vertical Bridgman growth methods. The concentration profile for the Si-Ge sample processed using this technique shows increasing transport silicon into the melt with time, moreover, a flat stable interface is observed. The mass and momentum equations for fluid flow, the energy and the solute mass transport were numerically solved. Results showed good agreements with experiments.

Abstract: In this work, the effects of mechanical milling on the extent of reduction were investigated to give an overview of potential improvements in the preparation of WC. A mixture of graphite and tungsten oxide (WO3) was mechanically milled together for 10 h. The as-milled powder and un-milled powder were investigated by thermal analysis, isothermal treatment, and X-ray diffraction to determine the effect of milling on the carbothermal reduction of WO3 to tungsten carbide (WC). The as-milled powder underwent a rapid reduction reaction at about 150°C lower than the un-milled powder. The reduction sequence to WC was illustrated to differ for the two powders. The milled powder showed complete reduction to WC in 1 h at 1215°C whereas the un-milled powder was incompletely reduced. Finally, WC powder was readily achieved by carbothermic reduction of mechanical activated WO3 and graphite, leaving its grains nano-sized.

Abstract: This paper addresses the effect of thermosolutal convection in the formation of defects in directionally solidified alloys. The numerical model is based on a bi-dimensional solution consisting of an implicit time integration scheme to couple thermal and solutal fields, which is supported by a finite volume numerical modeling technique. In this article, the macrosegregation phenomenon under a static magnetic field effect is analyzed numerically by a computer code developed and validated with experimental data. The numerically obtained results have been widely discussed in dependence of the characteristic parameters of the studied problem.

Abstract: We consider non-stationary convective mass transfer in a binary system comprising a stationary dielectric two-dimensional fluid drop embedded into an immiscible dielectric liquid under the influence of a constant uniform electric field. The partial differential equation of diffusion is solved by means of a similarity transformation, and the solution is obtained in a closed analytical form. Dependence of Sherwood number vs. the strength of the applied electric field is analyzed. It is shown that an electric field can be used for enhancement of the rate of mass transfer in terrestrial and reduced gravity environments.

Abstract: The Raman effect has been widely used for years for carbon layers analysis for qualitative analysis of samples as well as quantitative. Two main lines of attitude to the quantitative analysis of the Raman spectra have been de¬ve¬lo¬ped. The first one is applied mainly in forensic investigations when one has pre¬pa¬red comparative samples which consist of known substances with known concentration. This is the ba¬se for comparison of spectra from unknown sample. A tool used in these in¬ves¬ti¬ga¬tions is the Voigt profile based on Faddeeva function, allowing a very precise com¬pa¬rison of spectra with elimination of individuality of me¬a¬s¬u¬re¬ment equipment. This at¬ti¬tu¬de seems not to be the best solution when one has not comparative samples and the sam¬p¬le un¬der test is unique – the situations often meet in scientific investigation. Using Voigt profiles may cause a loss of in¬for¬ma¬tion car¬¬ried with the spec¬trum, especially related with small quantities of nanotubes or ful¬le¬renes generated in the process of imposing diamond layers in plas¬ma arc. Another function for approximation shape of the Raman peak based on Breit-Figner-Fano curve is frequently used but difficulties arise when iden¬ti¬fy¬ing physical meaning of parameters and subtracting idle pedestal from the ex¬pe¬ri¬men¬tal spec¬t¬rum. A different attitude based on simple Gaussian peak approximation is pre¬sen¬ted as better solution. Use of this procedure as a metrological tool has been dis¬cus¬sed leading to the conclusion that engaging more advanced and so¬p¬hi¬s¬ti¬ca¬ted me¬thods does not provide automatically better results.

Abstract: The demand for weight reduction in automobiles has generated strong interest in research and development of magnesium applications, especially for powertrain components. Ca-alloyed magnesium alloys have excellent creep properties and can be potentially used for powertrain applications. In order to better understand their solidification behavior, the grain structure evolution of Ca-alloyed magnesium alloys AM60 has been investigated in this study. The results of thermal analysis and microstructure evaluation indicate the occurrence of heterogeneous nucleation of primary magnesium during solidification. The crystal structural analysis suggests that the nucleation behavior of Ca-alloyed AM60 may be attributed to a good crystallographic match of // .

Abstract: Magnesium matrix composites have great potential for aerospace and automotive applications due to its low density and superior specific stiffness. The magnesium composites can often be reinforced by either particles or/and fibers. There were certain studies on solidification behaviors of particle-reinforced magnesium composites in the past. However, development of grain structure during the solidification of fibre-reinforced magnesium is barely investigated. In this work, an Al2O3 fiber reinforced magnesium (AM60) matrix composite (AM60/Al2O3,f) was cast. The solidification behavior of the cast AM60/Al2O3,f composite was investigated by computer-based thermal analysis. Optical and scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) were employed to examine the occurrence of nucleation and grain refinement involved in solidification of the composite. The results show that the addition of Al2O3 fibers leads to the formation of fine grain structure in the matrix of the AM60/Al2O3,f composite. The refinement of grain structure should be primarily attributed to the restriction of grain growth by the limited cellular space formed in the skeleton of the fiber preform structure instead of the nucleation of primary -Mg phase directly on Al2O3 fibers.

Abstract: The corrosion behavior of X33CrNiMn23-8, X50CrMnNiNbN21-9 and X53CrMnNiN20-8 steels utilized in automobile industry has been studied in oxidizing atmosphere, containing water vapor and acetic acid, usually present in combustion gases of biofuels in car engines. It has been found that the mechanism of corrosion under these conditions is rather complex. After early stages of the reaction, not exceeding 40 hours, the process follows approximately parabolic kinetics, being thus diffusion controlled. It has been shown that the presence of acetic acid highly increases corrosion rate of X33CrNiMn23-8 steel, containing highest chromium content, but has virtually no influence on the corrosion rate of X50CrMnNiNbN21-9 steel with lowest chromium concentration. These differences have been explained in terms of phase and chemical composition of corrosion products.