Materials Science Forum Vol. 762

Abstract: Metal matrix nanocomposites have been produced by powder metallurgy route. Al and nanoAl₂O₃ powders were grinded through high energy ball milling. Then, the composite powders were sintered by Equal Channel Angular Pressing (ECAP). 12 ECAP passes were carried out in order to improve the dispersion of the hard particles. SEM analysis was performed to investigate the distribution of the ceramic nanoparticles within the matrix. Hardness tests were executed to evaluate the mechanical behavior of the nanocomposites. Finally, mechanical strength values obtained by numerical models were compared with those estimated from hardness measurements. High energy ball milling followed by ECAP process revealed to be a suitable route for the production of metal matrix composites reinforced with well dispersed nanoparticles.

Abstract: A two dimensional model was employed to simulate the densification process of thick-walled hollow cylinder preform of C/C composite. Using the hydrogen inhibition model, carbon deposition processes under different partial pressures were compared and the efficiency of densification was analyzed. Then the density distribution of some definite time was analyzed to explain the mechanism of hydrogen inhibition. The results showed that with the adjunction of hydrogen, density distribution in preform was improved while the deposition process took a longer time.

Abstract: The 8.33at% Mn-doped TiFeSb half-heusler thermoelectric materials were studied by first-principles in this paper. The space occupying of Mn atoms in Mn-doped TiFeSb system was studied according to thermodynamic stability, mechanical stability, and density of states at the Fermi level. The results show that Mn atoms would substitute Ti atoms preferentially at 8.33at% doping amount. The electronic and phonon transport properties were calculated in TiFeSb and (Ti_0.75Mn_0.25)FeSb to characterize their electronic and thermal conductivity. The results indicate that Mn-doping can increase the power factor due to improving the electronic conductivity while reducing the lattice thermal conductivity. Therefore, the (Ti_0.75Mn_0.25)FeSb are expected to show better thermoelectric properties than TiFeSb.

Abstract: Aluminum metal matrix composites (Al-MMCs) are new promising materials for aviation, aerospace and automotive industries. However, due to the poor weldability they have very limited applications. In this paper, the authors present the welding achievements of Al-MMCs developed by their scientific research team in recent years. Laser welding, liquid phase impact diffusion welding and vacuum brazing were utilized. Based on analysis of microstructure, good joints can be achieved by using these welding methods.

Abstract: Shape memory alloys exhibit a peculiar property, shape memory effect that is the result from the structural changes in microscopic scale. These alloys return to previously defined shapes when they are subjected to variation of temperature after deformation of the low temperature phase. Shape-memory effect is based on martensitic transformation, with which the material changes its internal crystalline structure. The ordered structure or super lattice structure is essential for the shape memory effect of the material. Copper based alloys exhibit this property in the β-phase field, which possesses the simple bcc-structure at high temperature austenite phase. As the temperature is lowered, austenite phase undergoes martensitic transition following two ordering reactions, and microstructural changes in microscopic scale govern this transition. In the present work, Cu alloys were investigated by transmission electron microscope, TEM, and x-ray diffraction techniques.

Abstract: The aim of the present study is first to describe an original process, the so called duplex process, whose feature is the coupling between the well-known SMAT (Surface Mechanical Attrition Treatment) and the traditional co-rolling. The first step of this process consists of SMA-Treatment of 316L stainless steel sheets to generate nanocrystalline layers on their top surfaces according to the grain refinement mechanism of austenitic steels which is well described in the literature. During the second step, three treated sheets are co-rolled at 550°C to obtain a semi-massive nanocrystallised multilayer structure with improved mechanical strength alternating nanocrystalline, transition and coarse grain layers. The second part of this work deals with the mechanical and the microstructural characterization of the as-obtained structures. Thus, sharp nanoindentation tests performed over the cross section of the laminates coupled with Transmission Electron Microscopy (TEM) confirm the presence of nanograins after the thermomechanical treatment. In addition, the enhanced yield strength demonstrated by tensile tests correlate well with the theoretical volume fractions of nanoand transition layers. The interface cohesion between the sheets is tested by three-point bending tests and the interface bonding is evaluated by microstructural observations.

Abstract: Fouling of surfaces is a major challenge in design and operation of many industrial heat transfer equipment. Fouling causes significant energy, material and production losses, which increase the environmental impact and decrease economic profitability of processes. Even small improvements in prevention of fouling would lead to significant savings in a wide range of heat transfer applications. In this study, crystallization fouling of aqueous calcium carbonate solutions on a heated stainless steel surface is used to investigate the prevention of fouling in heat transfer equipment by physical surface modifications. Fouling behaviour of different surface patterns are studied experimentally in a laboratory scale fouling test apparatus. CFD modelling is used to study hydrodynamic and thermal conditions near surfaces with different patterns. In addition, the effect of surface pattern on the removal of particles is studied numerically. Surface patterning is found to affect the hydrodynamic and thermal conditions near the wall, and therefore to change the conditions for fouling layer build-up and removal, when compared to a flat heat transfer surface. The most promising surface pattern includes curved shapes, and it seems to create flow conditions in which improved convective heat transfer decreases the driving force for crystallization fouling. In addition, curved surfaces increase the shear forces at the wall, which prevents adhesion of the foulants to the surface and increases resuspension.

Abstract: Thick coatings for heavy duty applications in many industrial fields are produced by generating hard layers of Fe-based complex alloys on the critical surfaces. The composition and structure of these coatings have to be carefully tuned in order to generate a defect-free layer after solidification and to meet the expected service conditions. The present study is aimed at investigating and developing improved Fe-based alloys for the above described thick coatings by a careful definition of the microstructural features required to bear service conditions and by defining the optimal compositions to achieve the desired properties. Primary carbide distribution and size as well as steel matrix composition are the main microstructural features that will be evaluated in order to fine tune coating performance in terms of toughness and hardness. The experimental part of the investigation here presented will be mainly concerned to laboratory casting the above alloys and to assess their microstructure and properties.

Abstract: Si_5.4Al_0.6O_0.6N_7.4 (β-Sialon)/ Y_1.75Si_2.625Al_1.0O_7.5N_1.25 (oxynitride glass) composite solders with different ratio were designed using SiO₂-Al₂O₃-AlN-Y₂O₃-Si₃N₄ mixture powders to join Si₃N₄ ceramic to itself. It was found that both solder composition and bonding temperature have strong influence on the microstructure and strength of the joints. As far as using a pure oxynitride glass solder was concerned, the thickness of the seam turned narrow with the increase of temperature, and the strength had a peak value at 1550 °C. When increasing the designed content of β-Sialon, serious composition separation occurred in the seam at low bonding temperatures, and the strength was also lower than as using pure oxynitride glass. Increasing bonding temperature, the strength of the joints was improved and reached the maximum value of near 80 MPa at 1600 °C for the designed 20%β-Sialon. In this instance, large amounts of fine elongated β-Sialon grains were uniformly distributed in the seam. However, for the 60%β-Sialon containing solder, some pores appeared in the seam and its strength was very low at bonding temperatures.

Abstract: This paper aims to study the catalytic effects of phosphotungstic acid on the degradation behaviour of E-51/DDM and E-51/MeTHPA epoxy resins. The results show that phosphotungstic acid promotes the decomposition of E-51/DDM and E-51/MeTHPA in different degree. The decomposition rate of E-51/MeTHPA is relatively higher. According to the GC-MS results, the composition of products when phosphotungstic acid is used as the catalyst is complicated, indicating that the decomposition mechanism is complicated.