Candidate materials for the sulfur electrode current collector—III. Al/SiC composite materials

It has been previously found that aluminum and some of its alloys can be used as component materials in high temperature sodium/sulfur cells. The present study describes the electrochemical behavior of composite aluminum/silicon carbide materials in the polysulfide/sulfur melts at 350°C. Some of the composites studied have been found suitable to serve as the material, coating or lining of the positive current collector in Na-S cells.     

Candidate materials for the positive current collector in sodium-sulfur cells—I. Ceramic oxides

Chromium oxides doped with various metal oxides and strontium doped lanthanum chromite perovskites were prepared, physically characterized and tested for their suitability as coating materials for positive current collectors in sodium-sulfur cells. All of the materials studied are corrosion resistant. Perovskites show the highest conductivities, whereas only the chromium oxides doped with lower valence metal cations indicated acceptably low resistivities. However, upon exposure to the melt the resistivity of all materials studied increased in time. This has been shown to be caused by the loss of electron hole carriers during sample equilibration with the low oxygen partial pressure in the melt. The rate of the loss of conductivity is sufficiently slow to make the perovskites and chromium oxides doped with lithia or magnesia usable as coating materials for positive current collectors in sodium-sulfur cells.     

Kinetics and mechanism of the nickel electrode—II. Acid solutions containing a high concentration of sulphate and nickel ions

The kinetics of the Ni electrode in acid solutions with a high sulphate and nickel ion concentration has been investigated in the range of 25–75°C. The active dissolution and the passive regions and the active-passive transition phenomena have been studied employing different potential perturbation techniques using both still and stirred solutions.Two electrochemical processes are competing within the prepassive film potential region, namely, the active Ni dissolution to Ni(II) ions and the Ni(OH)₂ film formation. Nickel passivation is explained by reaction pathways involving successive electrochemical and chemical steps implying the occurrence of hydroxo- and oxo-species as reactions intermediates. The advanced mechanism is based upon a positively charged Ni surface structure and attributes the onset of the complete passivity to a particular surface oxide species.     

Sulphidation of copper-nickel alloys—II. Scale growth kinetics

The growth rates of the individual layers in sulphide scales on Cu-Ni alloys have been studied in order to attempt to determine the controlling processes. Parabolic kinetics are obeyed by each layer in scales on Cu?20% Ni and Cu?50% at 718 K. The variations in the layer growth rates with alloy composition at 718 K and the variations for Cu?50% Ni between 653 K and 770 K are also assessed. The growth rate of the outer, CuS, layer decreases with increasing temperature, essentially because of its decreasing thermodynamic stability. The variation of the growth rate of each phase with alloy composition and temperature is interpreted in terms of cation diffusion, growth of both copper and nickel sulphides apparently being controlled by ionic diffusion across the NiS layer. The effects of alloy-scale separation on both scale morphologies and layer growth rates are examined.     

Filler materials: The current situation and future trends with particular reference to lead‐free soldering alloys

Summary

Over the past five years, the electronic assembly industry has been examining a variety of alloy systems as lead‐free alternatives to the conventional 63Sn/37Pb eutectic system. Many of the proposed systems are quite interesting from a technical standpoint but fall short when considering practical issues such as cost, availability and repeatability of manufacture.

The purpose of this paper is to provide the industry with a practical and technically feasible, lead‐free process which can be implemented or used as a baseline to compare other more complex systems.     

The effect of some alloying elements on the corrosion resistance of lead-antimony alloys — II. Silver

The effect of Ag on the stationary and non-stationary anodic corrosion rates of PbSbCd and PbSb alloys in H₂SO₄ has been studied. Anodic polarization curves were constructed under galvanostatic and potentiostatic conditions. Optical microscopic examination and microprobe analysis of the alloys were conducted. The beneficial effect of Ag was ascribed to a delay in closure of pores in the initial PbSO₄ film. The leaching out of Sb from the outermost layers and the simultaneous nucleation of PbSO₄ and Ag₂SO₄ from supersaturated solutions in the pores is thus made possible.     

A mathematical theory of plasticity for compressible powder metallurgy materials — Part II

A new mathematical equation for the calculation of the yield stress in the case of a simple upsetting-compression test is proposed for powder metallurgy-sintered preforms of material. A new yield function developed by Doraivelu et al. which takes into account of the hydrostatic stress, is considered for the development of the above equation. The ratio of the yield stress of the P/M porous material to the yield stress of cent percent dense material is found to be increasing with increase in deformation strain. Further, a theory for simple plastic instability and work hardening rate of porous P/M material has been developed.     

Galvanic corrosion of A1 alloys — II. Effect of solution composition

Galvanic corrosion of the A1 alloys 1100, 2024, 2219, 6061 and 7075 coupled to Cu, stainless steel 304L, Ti-6A1-4V, 4130 steel or zinc has been studied in 3·5% MaCl, tapwater and distilled water using electrochemical and weight loss data. In 3·5% NaCl the galvanic effect decreases in the order Cu > 4130 steel > SS304L > Ti-6A1-4V for A1 alloys coupled to one of these metals, while in tapwater and distilled water the ranking is Cu > SS304L ～ Ti-6A1-4V > 4130 steel. Zinc, although being the anode in all galvanic couples, can sometimes accelerate corrosion rates of A1 alloys. Dissolution rates of A1 alloys coupled to a given dissimilar material are higher in 3·5% NaCl than in tapwater and distilled water where they are found to be comparable. In assessing galvanic corrosion behaviour of a given A1 alloy as a function of environment, one has to consider the effect of the dissimilar metal. The dissolution rate of A1 6061 is, for example, higher in tapwater with Cu as cathode than in 3·5% NaCl with SS304L or Ti-6A1-4V as cathode.     

Flux cored arc welding — the state of the art and current applications

Abstract

The advantages and potential applications of flux cored welding are reported, together with a review of process developments and the situation internationally.     

Effect of welding speed and electrode extension on the approximate entropy of welding current in short-circuiting GMAW

Based on the phase space reconstruction of welding current in short-circuiting transfer arc welding under carbon dioxide, the approximate entropy of welding current and its standard deviation have been calculated and analyzed at different welding speeds and different electrode extensions respectively. The experimental and calculated results show that at a certain arc voltage, wire feeding rate and gas flow rate, welding speed and electrode extension have significant effects not only on the approximate entropy of welding current, but also on the stability of welding process. Further analysis proves that when the welding speed and electrode extension are in an appropriate range respectively, the welding current approximate entropy attains maximum and its standard deviation minimum. Just under such circumstances, the welding process is in the most stable state.     

A new approach to designing a grain refiner for Mg casting alloys and its use in Mg–Y-based alloys

A totally new grain refiner, Al₂Y, for cast Mg alloys has been predicted using the recently developed edge-to-edge matching (E2EM) crystallographic model. An addition of 0.6–1.0 wt.% Al into the Mg–10 wt.% Y melt promotes the in situ formation of Al₂Y, which reduces the average grain size from about 180 to 36 μm. Active nucleation Al₂Y particles were reproducibly observed at the centres of many refined grains. The excellent grain-refining efficiency is comparable to that of Zr for the same alloy, but the thermal stability of the grains refined by Al₂Y is much higher than those refined by Zr up to a temperature of 550 °C for 48 h. The mechanisms of grain refinement and the superior thermal stability of the refined grains due to Al addition in the Mg–Y alloy are discussed based on the current experimental results.     

Unidirectional solidification of Zn-rich Zn–Cu peritectic alloys—II. Microstructural length scales

Experimental results are presented of solidification microstructure length scales including η-phase cell spacing, primary ε secondary dendrite arm spacing, size of nonaligned dendrite of primary ε, and volume fraction of primary ε, as functions of alloy concentration (containing up to 7.37 wt% Cu) and growth velocity (ranging from 0.02 to 4.82 mm/s) in the unidirectional solidification of Zn-rich Zn–Cu peritectic alloys. Intercellular spacing (λ) of two-phase cellular structure decreases with increasing growth velocity (V) such that λV^1/2 is constant at a fixed alloy concentration in parametric agreement with the KGT and Hunt–Lu models. The value of λV^1/2 varies from 216±10 to 316±55 μm^3/2/s^1/2 with decrease in alloy concentration from 4.94 to 2.17 wt% Cu. These values are much greater than for normal eutectic systems but comparable with monotectic alloys. Dendritic secondary arm spacing (λ₂) of primary ε decreases with increasing V such that λ₂V^1/3 is constant ranging 14.9±0.9 to 75.6±8.1 μm^4/3/s^1/3 with increase in alloy concentration (C₀) from 2.17 to 7.37 wt% Cu, which is in parametric agreement with predictions of arm-coarsening theory. The volume fraction (f_ε) of primary ε increases with increasing V for Zn-rich Zn–3.37, 4.94 and 7.37 wt% Cu hyperperitectic alloys. Predictions of the Scheil and Sarreal–Abbaschian models show good agreement with the observed f_ε for Zn–4.94 wt% Cu at moderate V from 0.19 to 2.64 mm/s, but fail at low V of less than 0.16 mm/s and at high V of greater than 3.54 mm/s. The measured average size, Λ, of nonaligned dendrites of primary ε decreases with increasing V such that ΛV^1/2 is constant for a given alloy, increasing from (0.98±0.04)×10³ to (7.2±0.7)×10³ μm^3/2/s^1/2 with increase in alloy concentration from 2.17 to 4.94 wt% Cu.     

A new method to improve surface morphology of Ni-Fe-Mo-Co alloy electrode and its catalytic activity for HER

A new pretreatment method has been developed to improve the catalytic activity of the Ni-Fe-Mo-Co alloy elec- trode for hydrogen evolution reaction (HER). The procedure involves pre-electrolyzing the Ni-Fe-Mo-Co alloy electrode in 30% KOH solution containing 10% potassium sodium tartrate at 70°C for 2 h, until some of the Mo and Fe elements are leached out. The surface morphology of the Ni-Fe-Mo-Co alloy demonstrates a unique hive-like structure after the pre- treatment, which has the pore size in a nanometer range (about 50 nm), a very large real surface area, and good stability. The results of the electrochemical studies show that compared to other similar electrode materials and the treated Ni-Fe-Mo-Co electrode by leaching method, the pre-treated Ni-Fe-Mo-Co electrode has a much lower overpotential and much higher exchange current density for HER. In addition, a long-term continuous electrolysis test with a current interrup- tion shows that the Ni-Fe-Mo-Co alloy has excellent catalytic stability.     

TiAlSi intermetallic formation and its impact on the casting processing in Al–Si alloys

The effect of titanium additions on the formation of primary TiAlSi intermetallics and resulted cast microstructure in Al–Si alloys has been studied. The precipitation temperature, growth kinetic and morphology of TiAlSi intermetallics as well as the solubility limit of Ti in Al–Si alloys were experimentally evaluated. The impact of primary TiAlSi intermetallic particles on the casting process related problems was investigated on the large scale of industrial production. The influence of Ti in the occurrence of feeding blockage and wormhole defect during ingot casting was described. The perspectives of Ti addition in Al–Si cast alloys were discussed. It is proposed that the Ti level in the most Al–Si cast alloys should be controlled within 0.10 wt.%.     

The hot corrosion of nickel-based ternary alloys and superalloys for gas turbine applications—II. The mechanism of corrosion in SO2/O2 atmospheres

A stage-by-stage mechanism is proposed for the corrosion of Ni-20Cr-X ternary alloys in SO₂/O₂ (in equilibrium with SO₃) atmospheres. Stage 1 represents the building of the initial oxide layer which takes place immediately. Stage 2 elaborates on the build-up of external oxide layers and sulphide areas and can be applied to both NiSO₄ or non-NiSO₄ forming conditions. Stage 3 elucidates the development of the internal striated sulphide structure. The beneficial effect of the ternary additions of Al, V and Si is related to the mechanism.     

Mechanical properties and rapid sintering of nanostructured WC and WC–TiAl3 hard materials by the pulsed current activated heating

Tungsten carbides are primarily used as cutting tools and abrasive materials in the form of composites with a binder metal, such as Co or Ni. However, these binder phases have inferior chemical characteristics compared to the carbide phase and the high cost of Ni or Co. Therefore, low corrosion resistance of the WC–Ni and WC–Co cermets has generated interest in recent years for alternative binder phases. In this study, TiAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC–TiAl₃ with a relative density of up to 99% was obtained within 2 min by PCAS under a pressure of 80 MPa. The method was found to enable not only the rapid densification but also the inhibition of grain growth preserving the nano-scale microstructure. The average grain sizes of the sintered WC and WC–TiAl₃ were lower than 100 nm. The addition of TiAl₃ to WC enhanced the toughness without great decrease of hardness due to crack deflection and decrease of grain size.     

Application of the wavelet packet and its energy spectrum to identify nugget splash during the aluminum alloys spot welding

Nugget splash during aluminum alloys spot welding has a detrimental effect on weld nugget integrity, strength and durability of the welded joints. This investigation is performed to identify nugget splash from voltage signals became these are easily accessible on-line. In the present work, we propose a novel method based on the wavelet packet transform and its energy spectrum for pattern recognition of splash signal. The result demonstrates that this novel method is more accuracy and a useful way of monitoring the spot welding quality.     

Drilling model for cutting lip and chisel edge and comparison of experimental and predicted results. II — revised cutting lip model

The previous model assumed a series of elements across the cutting lip and underestimated the torque and thrust, and is modified here to account for the necessary integrity of the chip. Because the integral chip flows initially as a unit in one direction and is rotating about a point, there must be a linear variation in the chip velocity across the lip. This variation in chip velocity effects the shear angle across the lip, effectively reducing the range from that calculated for a series of elements. When this extra work of effectively holding the various elements together was taken into account, predictions much closer to the experimental results were obtained. The modified model accounts reasonably for variation of most of the drill and process variables.     

Dislocation configurations and stress-corrosion properties of alloys in the nickel—chromium—iron system

Abstract

Stress-corrosion tests have been performed in sodium chloride at 300° on commercially produced and laboratory-made nickel alloysand austenitic steels. It has been found that resistance to transgranular stress corrosion cracking depends upon the (Ni + Cr) content. Commercial high-Ni alloys are resistant, as are relatively pure Ni–Cr–Fe alloys with (Ni + Cr) ≥ 50% wt. The resistance of alloys with approximately 20% Cr and 20—40% Ni is markedly influenced by other alloying elements, for example, silicon and molybdenum.

Electron microscope studies indicate that resistance to cracking can be approximately correlated with the extent to which dislocation ‘tangling’ occurs at a particular deformation. Planar dislocation arrays appear to be a necessary, though not sufficient,condition for susceptibility. It is thought that the observed dislocation arrangements can be understood without recourse to hypotheses of ordering in these alloys. The influence of some alloying elements, e.g. chromium and silicon, cannot be interpreted simply in terms of their influenceon dislocation arrangements. The favourable effectof these elementson resistance to stress-corrosion cracking is probably associated withtheir ability to increase the protective properties of surface oxide films and thus to inhibit the crack initiation process.