Defect and Diffusion Forum Vols. 297-301

Abstract: In this study, a two-dimensional (2-D) mathematical model was developed to simulate forced and natural convection, heat transfer and solidification occurring during the squeeze casting process. The model was based on the control-volume finite difference approach and on the enthalpy method. The computation was performed to understand the effect of applied pressures on the solidification and cooling behavior of a cylindrical squeeze casting of magnesium alloy AM50A which could have extensive usage in automobiles. The model computed the melt flow fields, the temperature distributions, the cooling curves, the shape and position of the phase front, and total solidification time of the casting. The predicted results show that high applied pressures result in high heat transfer across the casting/die interface, and consequently increase solidification and cooling rates.

Abstract: Dissolution of secondary phases during thermal treatment in cast magnesium alloys influences their engineering properties. In this study, a kinetic model based on a Kissinger-type method has been developed for describing dissolution of secondary phases in the high pressure die cast magnesium alloy AM50 during a thermally activated heating process. Also, differential scanning calorimetry (DSC) was effectively used for investigating the dissolution kinetics of secondary phases in the AM50 alloy. By fitting a kinetic model to the DSC results, the activation energy of the dissolution of the secondary phases can be determined. In parallel, the microstructure of the alloy was analyzed by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). It was found that the distribution of secondary phases and the concentration of alloying elements both at the grain boundaries and in the grains play an important role in the solid-state transformation kinetics of die cast magnesium alloy AM50.

Abstract: In this work, we have theoretically analyzed the heat convection process in a porous medium under the influence of spontaneous wicking of a non-Newtonian power-law fluid, trapped in a capillary element, considering the presence of a temperature gradient. The capillary element is represented by a porous medium which is initially found at temperature and pressure . Suddenly the lower part of the porous medium touches a reservoir with a non-Newtonian fluid with temperature and pressure . This contact between both phases, in turn causes spontaneously the wicking process. Using a one-dimensional formulation of the average conservation laws, we derive the corresponding nondimensional momentum and energy equations. The numerical solutions permit us to evaluate the position and velocity of the imbibitions front as well as the dimensionless temperature profiles and Nusselt number. The above results are shown by considering the physical influence of two nondimensional parameters: the ratio of the characteristic thermal time to the characteristic wicking time, , the ratio of the hydrostatic head of the imbibed fluid to the characteristic pressure difference between the wicking front and the dry zone of the porous medium, , and the power-law index, n, for the non-Newtonian fluid. The predictions show that the wicking and heat transfer process are strongly dependent on the above nondimensional parameters, indicating a clear deviation in comparison with and n = 1, that represents the classical Lucas-Washburn solution.

Abstract: Multifilamentary Nb3Sn-based superconducting composites manufactured by an internal-tin method have been studied by transmission (TEM) and scanning (SEM) electron microscopy. The main goal of this study is to reveal the effect of diffusion annealing regimes as well as the external diameter of the wires on the structure of nanocrystalline Nb3Sn layers (average grain size, grain size distribution, layer thickness, amount of Sn, etc.). It is demonstrated that multistep diffusion annealing results in quite a complete transformation of Nb filaments into Nb3Sn though some amount of the residual Nb remains in the filaments center. With an external diameter decrease the superconducting layers structure has been found to refine and get somewhat more uniform. An additional high-temperature annealing results in marked growth of Nb3Sn grain sizes and their scattering in sizes, which may negatively affect the current-carrying capacity of a wire.

Abstract: Atomic diffusion in metals is invariably enhanced by dissolution of hydrogen, in some cases by many orders of magnitude. This is a consequence of the formation of superabundant vacancies (SAVs), one of the general properties of M-H systems. Some examples of H-induced enhancement of interdiffusion in alloys and a detailed investigation of self-diffusion in Nb hydride, NbHx, are described, together with accelerated atomic migration observed in electrodeposited metals, also a consequence of SAV formation in the electrodeposition process.

Abstract: The growth of iron borides over the surface of different steels is of high anisotropy. It was determined that the anisotropy of borided phases reveals a significant instability of properties in service. One of the techniques to determine the effect of anisotropy on the mechanical properties of boride layers is the induced-fracture by Vickers microindentation. During the present work, the fracture toughness (KC) of the Fe2B hard coatings has been estimated at the surface of AISI 4140 borided steels. The force criterion of fracture toughness was determined from the extent of brittle cracks originating at the tips of an indenter impression. The indentation loads were established between 1.9 to 9.8 N at three different distances from the borided surface. The KC values were expressed as a function of temperature, treatment time and the indentation distances from the surface. Likewise, the adherence of the coated system was evaluated by Rockwell-C indentation, where the borided steel showed sufficient adhesion.

Abstract: The paper deals with the stability of centerline inhomogenity of continuously cast slabs and hot rolled products. The centerline segregation is a disadvantageous failure of slabs which can affect the quality properties of the final products. During hot rolling of slabs the centerline segregation pattern will become thin and stretch and it can also be detected in the middle part of heavy plates and coils. It is a common experience that the centerline segregation of heavy plates can not be easily decreased by post heat treatment. The pattern of the centerline segregation was modeled physically by preparing a sandwich structure of steel plates with different levels of carbon and alloying elements. Homogenization experiments were performed and the samples were examined metallographically. Diffusional calculations proved the governing role of carbon activity which is influenced by the distribution of alloying elements.

Abstract: Heat sinks enable the storage of energy that would otherwise be lost, thus ensuring significant energy-savings and fewer greenhouse gas emissions. Heat sinks also play the major role in the efficient temperature control of devices such as batteries. In principle, any material can act as a heat sink – traditionally, copper is used for many applications. However, copper is relatively expensive, has a high density and only a limited energy storage capacity. In contrast, a phase-change material (PCM) allows in effect an additional storage of energy through its phase change thus greatly increasing the achievable energy density. The aim of this work is the numerical analysis of the transient heat transfer in composite heat sinks containing phase-change materials. For the first time, a recently formulated Lattice Monte Carlo Method is applied to determine temperature distributions and the amount of energy transferred versus time in phase change materials.

Abstract: The paper presents some preliminary investigations on the sintering of alumina ceramics to translucency in a low thermal mass furnace. It is found that extremely fast sintering occurs in a forming gas atmosphere. Rapid densification occurs in minutes rather than hours normally encountered in conventional sintering of ceramics.

Abstract: There are now new legislations emerging or being contemplated to restrict the use of Pb in electronic devices. This development has provided the impetus for the development of Pb- free solder alloys and efforts are now geared towards characterizing their operational and functional properties. The most common alloys being recommended and investigated are those primarily based on the Sn-Ag-Cu (SAC) system. These SAC alloys generally have higher melting points than conventional Pb-Sn alloy. Additionally they are susceptible to microstructural evolution of inter-metallic compounds that have been implicated in thermal fatigue life, mechanical strength and fracture toughness of the soldered joints. We have studied the Sn rich corner of the Sn-Ag-Cu system with minor additions aimed at minimizing detrimental microstructural development and improving the solderability and the mechanical strength of soldered joints. Some of the SAC alloys with minor additions showed some interesting properties. Their shear strength measured ranged from 30 – 60 MPa. The combined properties of strength and conductivity recorded compared favorably with that of traditional Pb-Sn solders.