Materials Science Forum Vol. 879

Abstract: The properties and surface appearance of aluminium extrusion are critically dependent on the microstructure and texture of the extruded profiles, and the requirements with respect to these aspects may vary with applications. Moreover it is often a challenge to produce extrusions with a consistent and homogenous grain structure and texture along as well as through the cross section of the profiles. It is thus vital to understand and be able to predict (model) how different microstructures and textures are formed and how they evolve during and after extrusion. In the present work a model framework has been implemented which includes a FEM model to account for the strain, strain rate and temperature along a set of particle paths during extrusion. From these the deformation texture and grain structure are calculated with an appropriate deformation texture model and a sub-structure evolution model, respectively. The sub-structure model have in the present work been coupled to a crystal plasticity model to provide an orientation dependent subgrain size and dislocation density during deformation which provides the driving force for the post-extrusion recovery and possible recrystallization behaviour. The post-extrusion microstructure and texture evolution is calculated with a recovery and recrystallization model, which is accompanied by a recrystallization texture model. The framework and its constituent models and their interplay are presented, and some preliminary results when applying this modelling framework to Al-Mg-Si extrusions are presented and discussed in view of corresponding experimental results.

Abstract: Fe-Al alloys with Al contents between 5 and 40 at.% Al were oxidised for 1000 h in synthetic air at 700 and 900 °C to determine their oxidation behaviour. The minimum Al content which is necessary for the formation of protective Al₂O₃ scales decreases with increasing temperature from about 17 at.% Al at 700 °C to about 12 at.% Al at 900 °C. Established parabolic rate constants for the steady state growth of Al₂O₃ indicate formation of γ-Al₂O₃ at 700 °C while at 900 °C α-Al₂O₃ + Θ-Al₂O₃ scales form. At lower Al contents scales are predominantly formed by Fe₂O₃ as revealed by GI-XRD. It is also found that the oxidation behaviour is independent of the crystallographic order of the alloys, i.e. whether they are disordered A2 or ordered B2.

Abstract: Nickel-base superalloys are usually employed for large forged parts in aerospace industry. A comprehensive understanding of their mechanical behavior during hot working is required, especially for manufacturers in order to enhance the in-service properties. In this context, the first part of the work aims at investigating the mechanical behavior of nickel during hot deformation, with particular emphasis on the influence of niobium additions in solid solution. For this purpose, a series of wrought model alloys including pure nickel and Ni-Nb alloys (Ni-0.01, 0.1, 1, 2, 5 and 10 wt. % Nb) were prepared and deformed by hot torsion at temperatures ranging from 800 to 1000 °C degrees and at three (von Mises equivalent) strain rates of 0.03, 0.1 and 0.3 s^-1. Afterwards, the key rheological parameters that characterize strain hardening and dynamic recovery were determined through a simple analytical method based on the classical Laasraoui-Jonas constitutive equation, allowing reasonable fit for the flow curves for all studied Ni-Nb alloys. In this way, the effect of niobium solutes on the fundamental mechanisms of deformation was well highlighted. In the second part, three usual models describing strain hardening and dynamic recovery, referred to as the Laasraoui-Jonas (LJ), Kocks-Mecking (KM), and power law (PW) equations are compared within the range of moderate strains. Transformation formulae are derived, allowing the parameters of one law to be computed from the parameters of any of the two others. The theoretical derivations are illustrated by the specific case of a Ni-Nb alloy in the solid solution domain.

Abstract: Electronics devices consist of silicon chips, copper leads, resin or ceramics substrates and which are jointed to each other with solder, conductive adhesive or other materials. Each coefficient of thermal expansion is different and it causes strain concentration and cracks. The solder easily deformed by the difference of the thermal expansion and it relieved the stress on the devices however the epoxy resin of the conductive adhesives are harder. So we suggested the composed joint including the relaxation layers of low elastic material. The shear strength and elongation of the epoxy resin joint, silicone rubber joint and the composite joint of the two materials were investigated. The analytical study was carried out to clarify the stress reduction effect of the design of the relaxation layer in the composite joints. The parameters such as the width, height, pitch and the distance of the relaxation layer from the joint edge are investigated. The high relaxation layer close to the joint edge effectively reduced the stress of the joint. The stress reduction effect appeared in the different pitch of the layers.

Abstract: Mg alloy chips produced by machining processes are fine and active, and thus difficult to recycle by melting. The treatment of Mg chips is one of the concerns for expansion of the application of Mg alloys. Therefore, suitable processing for Mg chips is necessary. Mg alloys have the disadvantage of poor corrosion resistivity, and high reactivity with water, leading to the hydrogen and Mg hydroxide. This hydrolysis reaction is enhanced by the presence of NaCl in water. In this study, hydrogen was produced by the hydrolysis reaction of Mg chips generated by machining and seawater. Furthermore, ball milling was performed to enhance the formation of hydrogen. The hydrolysis reaction combined with the ball milling produced 795mL of hydrogen for 1g of Mg chips after 120 min. However, only 180 mL of hydrogen was obtained by the reaction without ball milling; a notable improvement in hydrogen formation was observed. A similar result was obtained for AZ91 chips. It is believed that the combined process of hydrolysis and ball milling is useful for the production of hydrogen with the disposal of Mg chips.

Abstract: Natural sulfated glycosaminoglycans (GAGs) play a crucial role as components of the extracellular matrix (ECM). They participate in the regulation of important cellular functions including cell growth, differentiation and signalling. The generation of artificial ECM mimicking selected functions of the native ECM is a promising approach to improve the biological acceptance of materials which are in direct contact to living tissue. In this context we developed synthesis routes for polysaccharide and GAG derivatives bearing both bioactive sulfate and reactive (meth) acrylate functions of different degrees of substitution within the sugar repeating unit. In addition, we studied the photochemically initiated cross-linking of these biopolymer derivatives to form biodegradable hydrogels usable as coatings for biomaterials or scaffolds in tissue engineering.

Abstract: Zeolites posses a high stability, high specific surface area and pores tridimensional system that make them useful to formation of inorganic membranes. During membranes synthesis different parameters should be considered such as nature substrate and the method used in order to obtain a membrane according to its application field. In the present work the formation of a zeolitic layer on the functionalized surface of zirconia substrates was studied. Zirconia disks of ten millimeters of diameter were prepared. They were submitted a chemical functionalization with three different chemical linkers: polyethylenimine (PEI), polydialildimethylamine chloride (PDDA) and 3-aminopropyltriethoxysilane (γ-APS). Subsequently the substrates were submitted to a seeding process, where their surface was grafted with zeolitic crystals corresponding to W zeolite. In order to promote the formation of zeolitic layer the substrates were submitted a hydrothermal treatment with a batch composition similar to that used in the W zeolite synthesis, at 150°C for 48 h. The crystallization products were characterized by XRD and SEM techniques. The results indicated that the chemical linker enhances the formation of a homogeneous zeolitic layer on the substrate and besides acts as structural directing agent allowing to crystallization of a different zeolitic phase to that used in the seeding process, the merlinoite. The morphology, crystalline phase and thickness of zeolitic layer formed on the surface of the substrate depend of the nature of chemical linker used and its interaction with the substrate.

Abstract: To fulfill the industrial demand of forged steels with high mechanical and microstructural requirements coupled with reduced cost, the possibility to decrease the content of Mo and other elements has been evaluated. In order to do that, the effect of boron addition (up to 30 ppm) on the steel hardenability has been investigated on two steels with different chemical composition at laboratory scale. In particular, the steel chemical composition has been designed in order to make effective the B addition in terms of hardenability. Two 80 kg ingots cast by a vacuum induction melting plant have been hot rolled by a pilot mill. The effect of B addition on hardenability has been evaluated and compared to that of steel for same application but without B. Results show an improvement of hardenability if 30 ppm B are added even if a Mo reduction is performed.

Abstract: This paper reports on a preliminary investigation into the elaboration, by the additive process known as laser cladding, of composite coatings with a matrix of stainless steel 316L reinforced with varying contents of tungsten (WC) or silicon carbides (SiC) particles. Laser cladding is characterised by ultra-fast solidification and cooling rates, thus giving rise to ultra-fine out-of-equilibrium microstructures and potentially enhanced mechanical properties. Both types of composite coatings – i.e. with SiC or WC ‒ are compared in terms of their microstructures and hardness. Special attention is given to the dissolution of the carbides particles and to interfacial reactions taking place between the particles and the metallic matrix.

Abstract: We have studied the diffusive mobility of hydrogen molecules confined in different size cages in clathrate hydrates. In clathrate hydrate H₂ molecules are effectively stored by confinement in two different size cages of the nanoporous host structure with accessible volumes of about 0.50 and 0.67 nm diameters, respectively. For the processes of sorption and desorption of the stored hydrogen the diffusive mobility of the molecules plays a fundamental role. In the present study we have focused on the dynamics of the H₂ molecules inside the cages as one aspect of global guest molecule mobility across the crystalline host structure. We have found that for the two cage sizes different in diameter by only 34 % and in volume by about a factor of 2.4, the dimension can modify the diffusive mobility of confined hydrogen in both directions, i.e. reducing and surprisingly enhancing mobility compared to the bulk at the same temperature. In the smaller cages of clathrate hydrates hydrogen molecules are localized in the center of the cages even at temperatures >100 K. Confinement in the large cages leads to the onset already at T=10 K of jump diffusion between sorption sites separated from each other by about 2.9 Å at the 4 corners of a tetrahedron. At this temperature bulk hydrogen is frozen at ambient pressure and shows no molecular mobility on the same time scale. A particular feature of this diffusive mobility is the pronounced dynamic heterogeneity: only a temperature dependent fraction of the H₂ molecules was found mobile on the time scale covered by the neutron spectrometer used. The differences in microscopic dynamics inside the cages of two different sizes can help to explain the differences in the parameters of macroscopic mobility: trapping of hydrogen molecules in smaller pores matching the molecule size can to play a role in the higher desorption temperature for the small cages.