Volume 107, Issue 2

The preparation of negative electrodes for nickel–metal hydride batteries using LaNi 4.3 Co 0.4 Al 0.3 alloy is presented. The constant current discharge technique is employed to determine the discharge capacity, the exchange current density and the hydrogen diffusion coefficient of the studied electrodes. The electrochemical performance of metal hydride electrode is strongly affected by preparation conditions. The results are compared and the advantages and disadvantages of preparation methods of the electrodes are also discussed.

This paper demonstrates the hydrogen-induced phase transformation and the associated pitting nucleation sites of 2507 duplex stainless steel using scanning Kelvin probe force microscopy and magnetic force microscopy. The low potential sites in Volta potential images, which are considered as the pitting nucleation sites, are strongly dependent on the hydrogen-induced phase transformation. They firstly initiate on the magnetic martensite laths in the austenite phase or at the ferrite/austenite boundaries, and then appear near the needle-shaped microtwins in the ferrite phase, because of the difference in physicochemical properties of hydrogen-induced phase transformation microstructures.

In this study, NiTi coatings were deposited onto AISI 316L stainless steel substrates by closed field unbalanced magnetron sputtering. The microstructure and properties of the coatings were characterized by means of X-ray diffraction, scanning electron microscopy, and nano-indentation. The tribocorrosion resistance and corrosion behavior of the stainless steel substrates and NiTi coatings were investigated in Hanks' solution. The experimental results indicated the NiTi coatings show higher corrosion polarization resistance and a more stable corrosion potential in the Hanks' solution than the uncoated stainless steel substrate. The NiTi coatings also exhibited excellent wear resistance and chemical stability in sliding tests with an Si 3 N 4 ball in the Hanks' solution. The tested samples showed different wear mechanisms in the sliding tests. Compared to the SS substrates, the NiTi coatings were more compatible with the Si 3 N 4 ball.

The semi-empirical cleavage fracture stress model (CFS), based on the microscopic cleavage fracture stress, σ f , for estimating the ASTM E1921 reference temperature ( T 0 ) of ferritic steels from instrumented impact testing of unprecracked Charpy V-notch specimens is further confirmed by test results for additional steels, including steels highly embrittled by thermal aging or irradiation. In addition to the ferrite–pearlite, bainitic or tempered martensitic steels (which was examined earlier), acicular or polygonal ferrite, precipitation-strengthened or additional simulated heat affected zone steels are also evaluated. The upper limit for the applicability of the present CFS model seems to be T 41J ∼160 to 170 °C or T 0 or T Qcfs ( T 0 estimate from the present CFS model) ∼100 to 120 °C. This is not a clear-cut boundary, but indicative of an area of caution where generation and evaluation of further data are required. However, the present work demonstrates the applicability of the present CFS model even to substantially embrittled steels. The earlier doubts expressed about T Qcfs becoming unduly non-conservative for highly embrittled steels has not been fully substantiated and partly arises from the necessity of modifications in the T 0 evaluation itself at high degrees of embrittlement suggested in the literature.

When the conventional power law creep equation is applied to rationalise the creep data of creep resistant steels, its parameters depend strongly on stress and temperature and hence cannot be used to predict long term creep properties. Here, it is shown that this problem can be resolved if it is modified to satisfy two boundary conditions, i. e. when σ (stress) = 0, ε˙min${\dot \varepsilon _{{\rm{min}}}}$ (minimum creep rate) = 0, and when σ = σ TS (tensile stress at creep temperature T ), ε˙min${\dot \varepsilon _{{\rm{min}}}}$ = ∞. This can be achieved by substituting the reference stress σ 0 in the conventional equation by the term (σ TS – σ). The new power law creep equation describing the stress and temperature dependence of minimum creep rate can then be applied to predict long term creep strength from data of short term measurements. This is demonstrated using the creep and tensile strength data measured for 11Cr-2W-0.4Mo-1Cu-Nb-V steel (tube).

The effect of delamination or cavitation of particles on the toughness of spherical and layered particulate composites is discussed using the Mori–Tanaka model. Both spherical and spheroidal particles are considered. The model is compared with other models reported in literature and shows fair agreement. In general, at the onset of delamination both stress and bulk modulus drop and this drop continues until delamination is complete. The debonding toughness increases with volume fraction but more effectively with particle aspect ratio. Rather surprising is the fact that the debonding toughness does not increase for high ratios of particle bulk modulus to matrix bulk modulus (>20). This may be a consequence of the sharp decrease in the composite bulk modulus as delamination progresses. The study shows marginal increase in toughness especially from debonding of spherical particles.

Carbon foams were produced from mixtures of mesophase pitch and mesocarbon microbeads (MCMBs) and the effects of MCMB addition and the MCMB particle size on the structure and properties of the foams were investigated. Experimental results showed that the addition of MCMBs raised the compressive strength due to the reduction of micro-cracks as well as the increase of bulk density. The thermal diffusivity increased initially and then decreased when the amount of MCMBs was raised. Compared to the particle size of MCMB, concentration makes a larger contribution to pore size and properties under the current experimental conditions.

Vaporisation studies over (Ni 2 Te 3 O 8 + NiTe 2 O 5 ) in the Ni–Te–O system were carried out by means of transpiration thermogravimetry (TTG) and Knudsen effusion mass spectrometry (KEMS) in the temperature ranges of 950 – 1 060 K and 850 – 950 K respectively. The transpiration measurements were performed for the first time. Comparison of total pressures obtained by TTG with that deduced using partial pressures of vaporising species from KEMS showed a good agreement providing reliable vapour pressures over this phase region. From vapour pressures, enthalpies of solid–gas and gas-phase equilibria and subsequently enthalpy and Gibbs free energies of formation of NiTe 2 O 5 (s) were derived. A thermochemical calculation was performed to assess the possibility of formation of the ternary NiTe 2 O 5 (s) phase on stainless steel clad of mixed-oxide fuelled fast breeder nuclear reactors.

Al-5Ti-0.2C master alloy was prepared via a simple combustion synthesis approach. By means of different smelting parameters, changes in its microstructure and refining performance were studied. Under short holding conditions (5 – 10 min) increase of Al content in the preform from 10 % to 40 % results in more numerous carbon-rich particles in the master alloy and better refinement performance. Nevertheless, preparation of refiner with satisfactory refining ability requires longer holding conditions (30 min). With prolonged holding time after the combustion, alloys possess similar microstructure and refining efficacy regardless of the chemical composition of the preform. Decrease of smelting temperature from 1 250 °C to 950 °C results in noticeable deterioration of the master alloy refining performance. The reasons, which impede successful utilization of primary benefits provided by combustion reaction, are discussed.

Multilayer graphene films were synthesized on copper foil by means of low pressure chemical vapor deposition and characterized using Raman spectroscopy. Low energy nitrogen bombardment was performed to form N-doped graphene, which is a metal catalyst method to induce nitrogen disorder in the carbon network. In order to investigate the effect of the nitrogen bombardment on graphene, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy are used. Analysis before and after the bombardment reveals that nitrogen doping of graphene is performed successfully by this method.

Two dimensional fracture features (i. e., dimple diameter, extent of tearing ridges etc.) quantified from the tensile fractographs are investigated to predict the nature of variation in mechanical properties with strain rates in zirconium alloys tested under ambient temperature where the initial inclusion or other second phase particle contents were kept unaltered. It has been possible to reasonably estimate the strength and ductility properties of an alloy from a systematic analysis of fractographic features.