S.C. Wang

A Model for the Yield Strength of Overaged Al-Zn-Mg-Cu Alloys

A model for the yield strength of multi-component alloys is presented and applied to overaged Al–Zn–Mg–Cu alloys (7xxx series). The model is based on an approximation of the strengthening due to precipitate bypassing during precipitate coarsening and takes account of ternary and higher order systems. It takes account of the influence of supersaturation on precipitation rates and of volume fraction on coarsening rates, as well crystallographic texture and recrystallisation. The model has been successfully used to fit and predict the yield strength data of 21 Al–Zn–Mg–Cu alloys, with compositions spread over the whole range of commercial alloying compositions, and which were aged for a range of times and temperatures to produce yield strengths ranging from 400 to 600 MPa. All but one of the microstructural and reaction rate parameters in the model are determined on the basis of microstructural data, with one parameter fitted to yield strength data. The resulting accuracy in predicting unseen proof strength data is 14 MPa. In support of the model, microstructures and phase transformations of 7xxx alloys were studied by a range of techniques, including differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD) in an SEM with a field emission gun (FEG-SEM).

Effect of self-accommodation on α/α boundary populations in pure titanium

An analysis of the shape strains produced by the martensitic ??? transformation in pure titanium indicates that there are three likely slip systems which could operate to give the complementary shear. The greatest degree of self-accommodation of the shape strains was given by clusters of three variants of ? grains and two kinds of clusters were identified. In one kind of cluster, the high-angle grain boundaries (HAGBs) formed between adjacent laths of different variants would all be [ 1 1 2 0 ] / 60o -type, and in the other kind they would all be [ 10 5 5 3 ] / 63.3o. -type. Analysis of the frequency distributions for misorientation angles in pure Ti using electron backscattered diffraction confirms a strong preference for the formation of these two types of HAGBs, with 78% of the HAGBs being classed as such, as compared to 36% which one would expect for a random distribution of variants. A statistical analysis of nucleation, which accounts for the observed frequencies of the different types of HAGBs, is proposed.

Estimation of dislocation densities in cold rolled Al-Mg-Cu-Mn alloys by combination of yield strength data, EBSD and strength models

Al‐Mg‐Cu‐Mn alloys have been developed for the packaging industry, in which large cold‐working deformations are normally applied that can produce high dislocation densities. In this study, we present a simplified model for the yield strength contributions and apply that to obtain the dislocation densities by determining the orientation factors, which can be obtained via the crystal information of electron backscatter diffraction (EBSD). One alloy subjected to three cold‐rolling reductions (10%, 40% and 90%) has been analysed by EBSD, and the density of dislocations are estimated using the strengthening model. This assessment suggests that dislocation densities by the Taylor model are roughly consistent but slightly lower than those determined by transmission electron microscopy.

Tailoring microstructure and phase segregation for low friction carbon-based nanocomposite coatings

Friction has a direct relation with the energy efficiency and environmental cleanliness in all moving mechanical systems. To develop low friction coatings is extremely beneficial for preserving not only our limited energy resources but also the earth’s environment. This study proposes a new design for low friction carbon-based nanocomposite coatings by tailoring the microstructure and phase segregation, and thereby it contributes to better controlling the mechanical and tribological properties. Experimental findings and theoretical calculations reveal that high-hardness (18.2 GPa), high-adhesion strength (28 N) as well as low-internal stress (−0.8 GPa) can be achieved by a nanocrystallite/amorphous microstructure architecture for the nc-WC/a-C(Al) carbon-based nanocomposite coating; in particular low friction (∼0.05) can be acquired by creating a strong thermodynamic driving force to promote phase segregation of graphitic carbon from the a-C structure so as to form a low shear strength graphitic tribo-layer on the friction contact surfaces. This design concept is general and has been successfully employed to fabricate a wide class of low friction carbon-based nanocomposite coatings.

Microstructure and precipitation in Al-Li-Cu-Mg-(Mn, Zr) alloys

Abstract Hot rolled Al–6Li–1Cu–1Mg–0·2Mn (at.-%) (Al–1·6Li–2·2Cu–0·9Mg–0·4Mn, wt-%) and Al–6Li–1Cu–1Mg–0·03Zr (at.-%) (Al–1·6Li–2·3Cu–1Mg–0·1Zr, wt-%) alloys developed for age forming were studied by tensile testing, electron backscatter diffraction (EBSD), three-dimensional atom probe (3DAP), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For both alloys, DSC analysis shows that ageing at 150°C leads initially to formation of zones/clusters, which are later gradually replaced by S phase. On ageing at 190°C, S phase formation is completed within 12 h. The precipitates identified by 3DAP and TEM can be classified into (a) Li rich clusters containing Cu and Mg, (b) a plate shaped metastable precipitate (similar to GPB2 zones/S″), (c) S phase and (d) δ′ spherical particles rich in Li. The Zr containing alloy also contains β′ (Al3Zr) precipitates and composite β′/δ′ particles. The β′ precipitates reduce recrystallisation and grain growth leading to fine grains and subgrains.

Structure characterization and tribological study of magnetron sputtered nanocomposite nc-TiAlV(N,C)/a-C coatings

Grown by reactive unbalanced magnetron sputtering in a mixed N2 and CH4 gaseous medium, heterogeneous nanocomposite coatings in the Ti–Al–V–N–C system show extraordinarily excellent tribological performance of coated machining tools. Using analytical high-resolution TEM, EELS, FEG-SEM, XRD, and Raman spectroscopy, this paper reports detailed structural and chemical characterization of the coatings grown at various CH4 : N2 ratios. Meanwhile, the mechanical and tribological properties were also measured, including hardness, Youngs modulus, residual stress and the dry-sliding friction and wear at varying environmental humidity. When CH4 gas was introduced in the deposition, the structure of the coatings has been found to experience a change from nano-scale TiAlN/VN multilayer architecture to a complex mixture of columnar grains of nc-TiAlV(N,C)/a-C nanocomposites and inter-column network of sp2-type amorphous carbon. Carbon incorporation and segregation also shows remarkable influence on the columnar growth model by leading to finer grain size. As compared to the carbon-free nitride coating, the nanocomposite coatings showed substantially reduced residual stress owing to the free-carbon precipitation, whereas the coatings maintained comparable hardness to the carbon-free TiAlN/VN. Their tribological properties were found to be strongly dependent on the environment. In humid air at RH > 30%, the coatings showed low friction coefficient less than 0.4 and extremely low wear rate at a scale of ∼10−17 m3N−1 m−1.

One-step electrodeposition of a self-cleaning and corrosion resistant Ni/WS2 superhydrophobic surface

Superhydrophobic surfaces have been intensively investigated for applications requiring self-cleaning and corrosion resistance. The techniques used to fabricate such a coating tend to be costly, time and energy consuming; further surface modification steps are often needed. In this study, a superhydrophobic composite electrodeposit based on tungsten disulphide nanoparticles dispersed in nickel on a mild steel substrate was successfully developed. At room temperature, the deposit showed a water contact angle of 158.3 deg and a sliding angle of 7.7 deg. The effects of operational parameters on surface morphology and superhydrophobicity are discussed. Compared to the substrate, the robust surface of the as-prepared coatings exhibited good self-cleaning and corrosion resistance, providing potential for industrial applications.

Electrodeposition of nanocrystalline nickel and cobalt coatings

Abstract The importance of nanostructured deposits and methods for achieving them are considered. The scope for nickel and cobalt nanocrystalline single metal deposits is summarised and developments in the last decade are highlighted. In each case, chemical composition, phase composition and crystal structure are included. Nanostructure, physical and mechanical properties (including corrosion resistance) v. bath type and composition (including pH and electrolyte additives) together with plating conditions, including current density, temperature and agitation are summarised. While the majority of published studies have featured nickel, the more expensive cobalt has also been considered due to its better corrosion and high temperature oxidation resistance in some environments. An overall comparison between the two metals is made. The major applications of nanocrystalline nickel have been in wear and corrosion resistant coatings, particularly as a hard chromium replacement, but speciality emerging uses include catalytic electrode surfaces in water electrolysis. Future research needs are briefly listed.

The monitoring of coating health by in situ luminescent layers

The monitoring of coating health is crucial for the assessment of wear life in service. Electrodeposition is one of most common methods used for producing coatings including composite layers by electrophoretic deposition of particles into a growing metal electrodeposit. A new luminescent Ni coating containing an embedded, blue-emitting rare-earth mixed metal oxide (BaMgAl11O17:Eu2+) phosphor has been electrodeposited successfully from an aqueous electrolyte. Two types of surfactants were utilised to study the effective co-deposition of these particles into the nickel matrix. The surfactants of non-ionic PEG (polyethylene glycol) and cationic CTAB (cetyl trimethylammonium bromide) were observed to increase the particle content in the deposit from zero to 4.6% and 11.5%, respectively. A mixture of these two surfactants produced the highest particle embedded coverage (15.6%). The feasibility of luminescent layers in monitoring the wear of coatings has also been verified in the final part of the manuscript.

Low temperature growth of hybrid ZnO/TiO2 nano-sculptured foxtail-structures for dye-sensitized solar cells

ZnO/TiO2 nano-sculptured foxtail-structures were fabricated using two sequential low-temperature processes combining hydrothermal growth of ZnO nanorods (NRs) and plasma ion assisted evaporation of crystalline TiO2 nanostructures. The ZnO NRs were homogeneously covered with a thin layer of anatase TiO2 nanostructure to form nano-sculptured foxtail-like patterns. Power conversion efficiency of dye-sensitized solar cells made from these hybrid ZnO/TiO2 structures was improved from 0.3% to 1.8% after using the ZnO/TiO2 hybrid structure. Measurement using electrochemical impedance spectroscopy and photocurrent decay proved that the hybrid structures have good electron transport capability, because the ZnO NRs can provide a direct pathway for charge transport; the TiO2 layer can improve charge injection and prevent the formation of Zn2+/dye complex (thus reducing the recombination); the hybrid structures can further increase surface area (thus higher dye loading).