Volume 101, Issue 10

Interactions between dislocations and grain boundaries contribute significantly to plastic behaviour in polycrystalline metals. But a full understanding of the processes and of their influence on plastic response has yet to be achieved. In the present paper, the elementary interaction mechanisms, from the entrance of dislocations in grain boundaries to the relaxation of the resulting intergranular stresses, are briefly reviewed. They are examined for two types of deformation tests and two types of materials: low temperature deformation and relaxation in copper and creep behaviour of alumina. Only the comparison between the response at different scales (nanoscopic, microscopic and mesoscopic) allows us to analyse the response of grain boundaries to an applied stress and to propose an interpretation of their role in macroscopic behaviour. This work may be considered as a preliminary step towards “Interface Engineering”.

This work attempts to give a unified picture of the microscopic mechanisms which control the plastic deformation in yttria-stabilized cubic zirconia single crystals, particularly for the soft orientation (uniaxial compression along the crystallographic axis) at low, intermediate and high temperatures while also reviewing and updating the existing data for different yttria contents (9.4 – 32 mol.%). The controlling deformation mechanisms are: internal friction within the crystal lattice (Peierls mechanism at low temperatures), pinning of dislocations by localized obstacles and long-range interaction between dislocations (intermediate temperatures) and dislocation viscous glide and climb (recovery creep) at higher temperatures. New aspects of the Portevin–le Chatelier phenomenon during viscous glide, due to the pinning and unpinning of dislocations from their defect clouds (yttrium substitutional atoms), are included.

Superplasticity in ceramics has been the subject of intense research activity for the last two decades. Quite recently, the fabrication of fully dense nanocrystalline oxides with grain size below 100 nm enabled examination of their superplastic behaviour. This work presents a critical analysis of the plasticity of two important nanostructured oxide systems: MgO and yttria tetragonal zirconia polycrystals. A thorough comparison of their plastic deformation reveals that nano-structuring may be a necessary, but not a sufficient condition for superplasticity in ceramics as commonly assumed. Instead, the changes in the chemical composition and the transport properties, through the bulk and at grain boundaries, versus temperature and grain size can induce a rich variety of mechanical responses.

Experimental data on thermodynamic properties and phase relations of the ternary Mn–Ni–O system were reviewed, and by use of the CALPHAD method, a consistent set of model parameters was optimised. The solid oxide phases were modelled using the compound energy formalism. The model allows representation of non-stoichiometry and solid solution in the phases. The distribution of cations between different lattice sites in the two spinel phases is also modelled. The liquid phase is described using the ionic two-sublattice model. The same model is used both for the metallic and the oxide liquid. Good agreement between calculated and experimental values is achieved.

The effects of physical parameters on the total dissipation of Gibbs energy inside migrating ferrite/austenite interfaces have been re-investigated using Odqvist's solute drag model. It is shown that the mobility of the ferrite/austenite phase interface and the diffusivity of niobium in the interface have a strong effect on the Gibbs energy dissipation. As the phase interface mobility was recently reassessed, it is clear that the Gibbs energy of niobium segregation and the diffusivity of niobium in the interface must also be reassessed. In this paper both parameters are reassessed in the Fe–C–Nb system using new physical parameters and experimental results from ultra-low carbon steels.

In-situ Al 2 O 3 rod-like whiskers were synthesized by pressure-less sintering at 1 493 K for 30 min. The whiskers were characterized by X-ray diffraction and scanning electron microscopy combined with energy dispersive X-ray spectroscopy. The results indicate that the diameter distribution of the Al 2 O 3 rod-like whiskers ranges from 20 to 90 nm and the lengths are up to 20 μm. The growth mechanism of the Al 2 O 3 nanostructured whiskers is considered to be a vapor–liquid–solid process. Al liquid acts as both a catalyst and a key component of forming Al 2 O 3 whiskers. Nb 2 O 5 additive results in a lot of small diameter whiskers distributed more evenly in the substrate.

Depending on the contents of alloying elements (Nb, up to 5.4 at.%; W, up to 7.4 at.%; Re, up to 4.1 at.%; Mo, up to 9.5 at.%; V, up to 10.1 at.%; Mn, up to 22.1 at.%; Al, 12.0 at.%; and Cr, up to 22.0 at.%) texture, structure, and physical and mechanical properties are studied in thin tapes that serve as substrates for epitaxial deposition of a high-temperature superconductor on the nickel-based binary alloys. The upper limit of alloying element content is established at which sharp cube texture of primary recrystallization is feasible to be formed. It is shown that such boundary concentration of alloying elements is associated with a certain quantitative ratio between the deformation texture components and with a certain magnitude of the concentration expansion of lattice parameter. Some mechanical and physical properties of alloys with cube texture are determined.

The microstructural, textural evolution and changes in grain boundary character distribution during annealing of a prior cold worked (30 %, 50 % and 80 %) ferrite–bainite dual phase steel have been studied and correlated with mechanical properties. It has been shown that submicron sized subgrains can be obtained by selecting the appropriate amount of cold rolling and annealing cycle. Increasing the annealing temperature in all the materials produces the expected results, namely decrease in strength with a simultaneous increase in ductility. Although reasonably sharp γ-fibres were obtained in 80 % cold rolled and its 500 °C annealed counterpart, the very low values (< 1.0) make the steel unsuitable for the purpose of deep drawing. It is envisaged that grain boundary engineering may lead to better strength–ductility combinations in this steel for an enhanced range of applications.

Profiles of the magnesium alloy AZ31 were produced by conventional indirect extrusion and by indirect extrusion into counterpressure with variations of the billet temperature T B from 200 °C up to 350 °C. Microstructural examination by optical microscopy showed that for extrusion trials carried out with billet temperatures T B ≤ 250 °C fine dynamically recrystallized grains are found while elongated grains of the former cast structure remain. Above T B ≥ 300 °C most grains show higher dynamic recrystallization kinetics and additional static recrystallization and increase to grain sizes of d = 30 – 50 μm. At T B ≤ 250 °C the typical extrusion fiber texture is found, at T B ≥ 300 °C a double fiber texture is formed due to static recrystallization. The results of the quasistatic mechanical tests are due to these differences in the microstructure.

The present work was driven by the need to replace the source of talc (hereafter referred to as standard) used in the manufacturing of steatite ceramics at Rauschert Portuguesa Lda. For this purpose, several talc powders were characterised in terms of pH, mean particle size (d 50 ), morphology and crystalline phases, as well as the rheological behaviour of the resulting slurries as functions of the type of clay, type of deflocculant and pH of the slurry. The main goal was to attain density at around 1.70 and viscosities ranging between 200 and 800 mPa · s suitable for spray-drying. Slurries resulting from the mixture of two types of talc and two types of clays were spray-dried and the powders obtained were characterised. The spray-dried powders were then pressed and sintered under typical processing conditions. The results show that that the addition of a dispersant (deflocculant) considerably decreased the viscosity of the prepared slurries. Furthermore, the increase in pH did promote deflocculation. Steatite slurries were also found to be thixotropic. It was only possible to achieve the density level 1.70 by combining the deflocculant with dry steatite powder additions. D1, an aqueous solution of an anionic acrylic polymer sodium salt was found to be the most efficient deflocculant.

The objective of the present study was to investigate the properties of porous chitosan–gelatin scaffolds. The modification of samples was done by incorporation of chondroitin-6-sulphate in bulk phase through cross linking with glutaraldehyde. Scaffolds were prepared using the freeze-drying method. An in vitro cell culture of fibroblasts L-929 was employed to study the growth and attachment of cells to the scaffolds. Optical microscopy showed high cell density for the sample supplemented with chondroitin-6-sulphate. Scanning electron microscopy was used to analyze the growth and attachment of cells. Results of the tests revealed that the sample with a 65: 35 ratio of gelatin and chitosan was suitable for both cell attachment and proliferation in fibroblast cultures. The cell viability 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-bromide (MTT) assay with human mesenchymal stem cells on the previous sample showed about 80 % increase in cell viability. To analyze the mechanical properties of scaffolds, the tensile strengths of the samples were measured and all samples showed stress at break above 0.1 MPa. Optimum substratum composition for porous scaffolds would involve a compromise between the desired attributes of biocompatibility, promotion of cell growth and mechanical strength.

Two low carbon steels with the same manganese content (approximately 1.6 wt.%) but different silicon contents (0.34 and 2.26 wt.%) with the initial microstructures consisting of ferrite and pearlite were intercritically annealed at five different temperatures followed by water quenching to obtain dual-phase microstructures. The modified Crussard–Jaoul analysis was employed to describe the work hardening behaviour of the dual-phase steels. Both steels exhibited two stages of work hardening in the range of plastic deformation. It is shown that for each stage the work hardening exponent is not constant and varies with volume fraction of martensite and silicon content. The dependence of the transition stress σ k and strain ∊ k of dual-phase steels on the annealing temperature is discussed for both steels. It is concluded that ∊ k increases with increasing silicon content and decreases with annealing temperature, while the reverse is true for σ k .

Prussian blue nanoparticles were synthesized by employing polymyxin B/sodium bis(2-efhylexyl) sulfosuccinate/water/isooctane reverse microemulsion. Fourier transform infrared spectroscopy, scanning electron microscopy and electrochemical methods were used to characterize the resultant nanoparticles, that were subsequently used to fabricate prussian blue-based glucose biosensors. Prussian blue nanoparticles were assembled onto a cysteine-modified Au electrode surface and then glutaraldehyde was used to cross-link glucose oxidase on the Prussian blue modified electrode. Both the electrochemical properties of Prussian blue and biocatalytical performance of glucose oxidase were investigated using cyclic voltammetry. After the biosensors were optimized, the glucose biosensors had a linear range from 6.7 μM to 2.0 mM and a relative standard deviation of 4.38%.

Phase equilibria in the Ti–Dy–Sn system below 40 at.% Sn were studied using differential thermal analysis, X-ray diffraction, metallography and electron microprobe. The partial liquidus and solidus projections and the melting diagram (liquidus + solidus) were constructed. A new ternary compound τ with composition Ti 4.2–4.3 Dy 0.8–0.7 Sn ≲3 , found by us previously, melts congruently above 1 543 °C and coexists with all the phases based on the binary compounds of the boundary binaries in the concentration interval studied. The liquidus surface is characterized by primary crystallization regions of (βTi), (βDy), (αDy), (Ti 3 Sn), (Ti 2 Sn), (Ti 5 Sn 3 ), (Dy 5 Sn 3 ) and τ. Five three-phase fields in the solidus surface result from three eutectic and two transition type invariant four-phase equilibria: L E 1 ⇄ (βTi) + (Ti 3 Sn) + (Dy 5 Sn 3 ), L E 2 ⇄ (Ti 3 Sn) + τ + (Dy 5 Sn 3 ), L E 3 ⇄ (βTi) + (αDy) + (Dy 5 Sn 3 ), L U 1 + (Ti 2 Sn) ⇄ (Ti 3 Sn) + τ and L U 2 + (Ti 2 Sn) ⇄ (Ti 5 Sn 3 ) + τ at 1 524, 1 500, 1 150, 1 543 and 1 498 °C, respectively. In the two-phase areas (βTi) + (Dy 5 Sn 3 ), (Ti 3 Sn) + (Dy 5 Sn 3 ), (Ti 2 Sn) + τ and τ + (Dy 5 Sn 3 ) the solidus surface has the temperature maxima at 1 620, 1 540, > 1 543 and > 1 500 °C, respectively.

TiO 2 thin film coated onto glass was prepared by a spin coating method without being calcined at high temperature. X-ray diffraction showed that the TiO 2 film consisted of a single anatase phase. Good adhesion was obtained between the film and the substrate. UV-vis spectra showed that the TiO 2 film was transparent in the visible range and opaque in the ultraviolet range. The transmittance decreased and the absorption edge shifted as the layers increased. The photocatalytic performance of the TiO 2 film was evaluated by degradation of methylene blue. The results indicated that the degradation of TiO 2 film spun for 3 times (each time for 30 s) at a rotation speed of 2 500 revolutions per minute reached 93.3 %. The TiO 2 film could be reused for more than 5 times without great decrease in photocatalytic activity.