Effect of Y for enhanced age hardening response and mechanical properties of Mg-Gd-Y-Zr alloys

In this study, compositional dependence of age hardening response and tensile properties were investigated for Mg-10G(d-x)Y-0.4Zr (x = 1, 3, 5 wt.%) alloys. With increasing Y content, the age hardening response of the alloys enhanced and tensile properties increased. The Mg-10Gd-5Y-0.4Zr alloy exhibited maximum tensile strength and yield strength at aged-peak hardness, and the values were 302 MPa and 289 MPa at room temperature, and 340 MPa and 267 MPa at 250 degrees C, respectively. The strong peak age hardening was attributed to the precipitation of prismatic beta' plates in a triangular arrangement. The cubic shaped beta phase was also observed at grain boundaries. The remarkable improvement in strength is associated with a uniform and high dense distribution of beta' and cubic shaped beta precipitate phases in Mg matrix. Elongation of Mg-10Gd-0.4Zr alloys decreased with increasing Y content, and the elongation of Mg-10Gd-5Y-0.4Zr alloy was less than 3% below 250 degrees C, whereas the alloys containing I wt.% and 3 wt.% Y exhibited higher elongation than 5% at room temperature.

LPCVD of tungsten by selective deposition on silicon

Beta-phase W, selectively grown at 440C had resistivity 20 micro-ohm cm and maximum layer thickness 100nm. Hydrogen passivation proved essential in this process. Higher deposition temperatures resulted in increased layer thickness but incorporated WSi2 and alpha- phase W. Self limiting W grown on polycrystalline and heavily doped silicon yielded reduced thickness. Boron is involved in the WF6 reduction reaction but phosphorus is not and becomes incorporated in the W layer. The paper establishes an optimised and novel CVD process suited to IC contact technology. A funded technology transfer contract with National Semiconductor Greenock (M Fallon) resulted from this work.

Pinning down the solid-state polymorphism of the ionic liquid [bmim][PF6]

The solid-state polymorphism of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], has been investigated via low-temperature and high-pressure crystallisation experiments. The samples have been characterised by single-crystal X-ray diffraction, optical microscopy and Raman spectroscopy. The solid-state phase behaviour of the compound is confirmed and clarified with respect to previous phase diagrams. The structures of the previously reported gamma-form, which essentially exhibits a G'T cation conformation, as well as those of the elusive beta- and alpha-forms, are reported. Crystals of the beta-phase are twinned and the structure is heavily disordered; the cation conformation in this form is predominantly TT, though significant contributions from other less frequently encountered conformers are also observed at low temperature and high pressure. The cation conformation in the alpha-form is GT; the presence of the G'T conformer at 193 K in this phase can be eliminated on cooling to 100 K. Whilst X-ray structural data are overall in good agreement with previous interpretations based on Raman and NMR studies, they also reveal a more subtle interplay of intermolecular interactions, which give rise to a wider range of conformers than previously considered.

The influence of high-temperature sintering on microstructure and mechanical properties of free-standing APS CeO-YO -ZrO coatings

In this study, ceria-yttria co-stabilized zirconia (CYSZ) free-standing coatings, deposited by air plasma spraying (APS), were isothermally annealed at 1315 °C in order to explore the effect of sintering on the microstructure and the mechanical properties (i.e., hardness and Young's modulus). To this aim, coating microstructure, before and after heat treatment, was analyzed using scanning electron microscopy, and image analysis was carried out in order to estimate porosity fraction. Moreover, Vickers microindentation and depth-sensing nanoindentation tests were performed in order to study the evolution of hardness and Young's modulus as a function of annealing time. The results showed that thermal aging of CYSZ coatings leads to noticeable microstructural modifications. Indeed, the healing of finer pores, interlamellar, and intralamellar microcracks was observed. In particular, the porosity fraction decreased from ~10 to ~5% after 50 h at 1315 °C. However, the X-ray diffraction analyses revealed that high phase stability was achieved, as no phase decomposition occurred after thermal aging. In turn, both the hardness and Young's modulus increased, in particular, the increase in stiffness (with respect to "as produced" samples) was equal to ~25%, whereas the hardness increased to up to ~60%. © 2010 Springer Science+Business Media, LLC.

Detection of ZnS phases in CZTS thin-films by EXAFS

Copper zinc tin sulfide (CZTS) is a promising Earthabundant thin-film solar cell material; it has an appropriate band gap of ~1.45 eV and a high absorption coefficient. The most efficient CZTS cells tend to be slightly Zn-rich and Cu-poor. However, growing Zn-rich CZTS films can sometimes result in phase decomposition of CZTS into ZnS and Cu2SnS3, which is generally deleterious to solar cell performance. Cubic ZnS is difficult to detect by XRD, due to a similar diffraction pattern. We hypothesize that synchrotron-based extended X-ray absorption fine structure (EXAFS), which is sensitive to local chemical environment, may be able to determine the quantity of ZnS phase in CZTS films by detecting differences in the second-nearest neighbor shell of the Zn atoms. Films of varying stoichiometries, from Zn-rich to Cu-rich (Zn-poor) were examined using the EXAFS technique. Differences in the spectra as a function of Cu/Zn ratio are detected. Linear combination analysis suggests increasing ZnS signal as the CZTS films become more Zn-rich. We demonstrate that the sensitive technique of EXAFS could be used to quantify the amount of ZnS present and provide a guide to crystal growth of highly phase pure films.

The diffusion of Co⠶溩 Zrâ â Tiâ â alloy /|nS. Kumar. -- 260 St Catharines [Ont.] : Dept. of Physics, Brock University,

The diffusion of Co60 in the body centered cubic beta phase of a ZrSOTi SO alloy has been studied at 900°, 1200°, and 1440°C. The results confirm earlier unpublished data obtained by Kidson17 • The temperature dependence of the diffusion coefficient is unusual and suggests that at least two and possibly three mechanisms may be operative Annealing of the specimen in the high B.C.C. region prior to the deposition of the tracer results in a large reduction in the diffusion coefficient. The possible significance of this effect is discussed in terms of rapid transport along dislocation network.

Retention at room temperature of the tetragonal t ``-form in Sc(2)O(3)-doped ZrO(2) nanopowders

Synchrotron X-ray powder diffraction was applied to the study of the effect of crystallite size on the crystal structure of ZrO(2)-10 mol% Sc(2)O(3) nanopowders synthesized by a nitrate-lysine gel-combustion route Nanopowders with different average crystallite sizes were obtained by calcination at several temperatures, ranging from 650 to 1200 degrees C The metastable t""-form of the tetragonal phase, exhibiting a cubic unit cell and tetragonal P4(2)/nmc spatial symmetry, was retained at room temperature in fine nanocrystalline powders, completely avoiding the presence of the stable rhombohedral beta phase. Differently, this phase was identified in samples calcined at high temperatures and its content increased with increasing crystallite size The critical maximum crystallite size for the retention of the mestastable t""-form resulted of about 35 nm (C) 2009 Elsevier B.V All rights reserved

The effects of temperature on the molecular orientation of zinc phthalocyanine films

Phthalocyanine compounds have been widely investigated as candidate materials for technological applications, which is mainly due to their thermal stability and possibility of processing in the form of thin films. In most applications, the controlled growth of thin films with high crystalline quality is essential. In this study, zinc phthalocyanine (ZnPc) thin films were prepared by evaporation on glass and Au-coated glass substrates with subsequent annealing at different temperatures in ambient atmosphere. The morphological and structural features of 80 nm thick zinc phthalocyanine films were investigated, evidencing an alpha -> beta phase transformation after annealing the films at 200 A degrees C, as indicated by UV-Vis spectroscopy and FTIR analyses. A better uniformity of the annealed films was also evidenced via AFM analysis, which may be of importance for applications where film homogeneity and excellent optical quality are required.

Crystallographic variant selection in α–β brass

The transformation texture of α/β brass with a diffusional Widmanstätten α growth morphology has been investigated. Electron micrographs and electron backscattered diffraction was used to determine that the orientation relationship between the β phase and the α associated with nucleation at β grain boundaries was (44.3°) left angle bracket1 1 6right-pointing angle bracket. Crystallographic variant selection was observed across those prior β/β grain boundaries, but this has little effect on the transformation texture due to the crystal symmetry. The effect of the crystallographic variant selection on texture is further weakened by nucleation of diffusional transformed α in the grain interior.

Crystallographic variant selection in Ti-6Al-4V

Transformation textures in the two-phase alloy Ti–6Al–4V have been studied. Samples were heated into the fully β phase condition and then slow cooled to allow diffusional transformation to α. This produced a microstructure of grain boundary α encircling colonies of Widmanstätten α. Electron backscattered diffraction (EBSD) texture measurements showed that the α texture was markedly sharper than that calculated on a basis of equal variant probability, indicating that significant variant selection was occurring during diffusional transformation. Investigation of the α variants produced across prior β grain boundaries has shown that the selection of variants during transformation is highly dependant on the crystallography of those boundaries. The effect of this crystallographic variant selection on the transformation texture has been modelled.

Microstructure evolution of TI-SN-NB alloy prepared by mechanical alloying

In the present study, Ti-16Sn-4Nb alloy was prepared by mechanical alloying (MA). Optical microscopy, scanning electron microscopy combined with energy dispersive X-ray analysis (SEM-EDX), and X-ray diffraction analysis (XRD) were used to characterise the phase transformation and the microstructure evolution. Results indicated that ball milling to 8 h led to the formation of a supersaturated hcp α-Ti and partial amorphous phase due to the solid solution of Sn and Nb into Ti lattice. The microstructure of the bulk sintered Ti-16Sn-4Nb alloy samples made from the powders at shorter ball milling times, i.e. 20 min- 2 h, exhibited a primary α surrounded by a Widmanstätten structure (transformed β); while in the samples made from the powders at longer ball milling times, i.e. 5- 10 h, the alloy evolved to a microstructure with a disordered and fine β phase dispersed homogeneously within the α matrix. These results contribute to the understanding of the microstructure evolution in alloys of this type prepared by powder metallurgy.

Synthesis of Ti–Sn–Nb alloy by powder metallurgy

The microstructural evolution and characteristics of the Ti–16Sn–4Nb powder particles and bulk alloys sintered from the powders ball-milled for various periods of time were studied. Results indicated that ball milling to 8 h led to the development of a supersaturated hcp α-Ti and partial amorphous phase due to the solid solution of Sn and Nb into Ti lattice. The bulk Ti–16Sn–4Nb alloy made from the powders ball milled for a short time, up to 2 h, exhibited a primary α and a Widmanstätten structure consisting of interlaced secondary α and β. With an increase in ball milling time up to 10 h, the microstructure evolved into a fine β phase dispersed homogeneously within α phase matrix. The microhardness values of the bulk alloy in both α- and β-phases increased with the increasing of the ball milling time and reached a plateau value at 8 h and longer, i.e. 687 and 550 HV for α- and β-phases, respectively. Likewise, the microhardness of the α phases was always higher than that of the β phases in the bulk alloys made from the powders ball milled for the same milling time.

Properties of a carbon-fibre composite modified by electrospun poly (vinylidene fluoride)

The interlaminar toughening of a carbon-fibre reinforced composite by incorporation of electrospun polyvinylidene fluoride (PVDF) nanofibrous membranes was explored in this work. The nanofibres were electrospun directly onto commercial pre-impregnated carbon fibre materials under optimised conditions and PVDF was found to primarily crystallise in its β phase polymorphic form. There is strong evidence from DMTA analysis to suggest that a partial miscibility between the amorphous phases of the PVDF nanofibres and the epoxy exists. The improved plastic deformation at the crack tip after inclusion of the nanofibres was directly translated to a 57% increase in the mode II interlaminar fracture toughness (in-plane shear failure). Conversely, the fracture toughness in mode I (opening failure) was slightly lower than the reference by approximately 20%, and the results were interpreted from the complex micromechanisms of failure arising from the changes in polymorphism of the PVDF.

Quantification of sensitization in AA5083-H131 via imaging Ga-embrittled fracture surfaces

Sensitization of 5xxx series Al alloys involving precipitation of β phase (Mg2Al3) at grain boundaries was studied for different exposure times at 100°C upon AA5083-H131 (UNS A95083). In this work, we reveal that fracture surfaces prepared by liquid gallium embrittlement can yield a quantification of grain boundary β phase with significant statistics on β phase size and spacing. This information is a necessary first step toward development of quantitative damage models to describe inter-granular corrosion (IGC) and stress corrosion (IGSCC).

Ultrafine grain formation in a Ti-6Al-4V alloy by thermomechanical processing of a martensitic microstructure

In the current study, ultrafine equiaxed grains with a size of 150 to 800 nm were successfully produced in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. This was achieved through a novel mechanism of grain refinement consisting of several concurrent processes. This involves the development of substructure in the lath interiors at an early stage of deformation, which progressed into small high-angle segments with increasing strain. Consequently, the microstructure was gradually transformed to an equiaxed ultrafine grained structure, mostly surrounded by high-angle grain boundaries, through continuous dynamic recrystallization. Simultaneously, the supersaturated martensite was decomposed during deformation, leading to the progressive formation of beta phase, mainly nucleated on the intervariant lath boundaries.