The annealing and re-oxidation of oxidized CuNi alloys

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu10%Ni and Cu24%Ni at 800°C and Cu80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques.Oxidation of Cu10%Ni and Cu24%Ni produces external scales consisting of a thick, inner Cu₂O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu₂O and oxygen. With Cu80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface.In Cu10%Ni and Cu24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu₂O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu-80%Ni, although to a lesser extent than for the copper-rich alloys.During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu-80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.     

Corrosion behaviour of the extruded and unidirectionally solidified AlAl4Ca eutectic alloy in a neutral sodium chloride solution

The corrosion behaviour of extruded and unidirectionally solidified AlAl₄Ca eutectic alloy in a neutral sodium chloride solution has been compared with the behaviour of commercially pure Al and AlCu alloy. From both anodic and cathodic cyclic linear potentiodynamic measurements and from S.E.M. observations it was possible to point out that the corrosion behaviour of unidirectionally solidified alloy is substantially equal to that of the extruded alloy, slightly different from that of commercially pure Al and better than AlCu alloy. Therefore, the use of this new lamellae-reinforced AlAl₄Ca composite is promising, even though from the corrosion point of view further studies involving different techniques and conditions (solutions, temperature, surface state, stress, etc.) should be undertaken.     

The effect of electronic delocalization in organic groups R in substituted thiocarbamoyl RCSNH2 and related compounds on inhibition efficiency

Polarization of mild steel in 1.OM hydrochloric acid containing a substituted thiocarbamoyl RCSNH₂ and related compounds has been studied at various inhibitor concentrations. The inhibition efficiency of the compound is related qualitatively to the electronic resonance in the R group.     

Manufacture of a new kind diamond grinding wheel with Al-based bonding agent

A new kind diamond grinding wheel with Al-based bonding agent was prepared in this paper. The influence of sintering temperature to the relative density (R.D.), hardness and service life of diamond grinding wheels with AlSnTi, AlSnTiNiCo, AlSnTiNi and AlSnNiCo bonding agent was studied. The microstructure of different bonding agent sintered at different temperature was observed. The service life of the Al-based grinding wheels was compared with Cu-based or resin-based ones. The results showed that the AlSnTiNiCo is the best composition system in this research. The best sintering temperature is 300 °C. The sample has a high relative density after sintered at 300 °C. The retention of Al-based bonding agent to diamond grit is strong. The service life of this Al-based diamond grinding wheel is about three times as long as that of resin-bonded grinding wheel.     

The role of mercury in the dissolution of aluminium sacrificial anodes: A study using ion implantation

Implantation of Hg⁺ ions into Al was used to produce, on Al, surface alloy layers consisting of a metastable AlHg solid solution. The anodic behaviour of these surface alloys was examined using potentiodynamic polarization, current-time transients at constant potential and galvanic coupling with steel. The electrochemical studies were supplemented by scanning electron microscopy and glancing-angle Rutherford backscattering measurements. The results obtained indicated that the presence of Hg in Al, even when homogeneously distributed in metastable solid solution, produced an alloy which was extremely reactive, electrochemically, and which had little immediate tendency to passivate. The reactivity was not associated with the grain boundary regions; the whole alloy dissolved more or less uniformly. The effects of implanted Hg on the dissolution of Al were relatively persistent, although they were eventually lost. The implications of these results are considered in the context of the role of mercury in the dissolution of commercial AlZnHg sacrificial anodes. In the latter materials the mercury is thought to be located primarily as a second phase in the grain boundaries but, although they exhibit some preferential grain boundary corrosion, their grain interiors also dissolve readily.     

Oxidation of FeCr alloys in O2/3% H2O

A range of FeCr binary alloys containing 2–100%Cr was exposed in O₂/3%H₂O at 1023–1223K to determine whether the presence of the water had a significant effect on oxidation. Oxidation rates were not significantly different from those in dry oxygen and decreased with increasing time of oxidation. The kinetics of oxidation and the oxide structures both suggested that the early stages of oxidation were controlled by lattice diffusion through layers of FeCr spinel for Fe-2 1/4Cr and Fe-8 1/2Cr, or through (Cr, Fe)₂O₃ scale for alloys containing 16%Cr or more.     

Nitridation of Ti6Al4V alloy under ultrasonic impact treatment in liquid nitrogen

X-ray photoelectron spectroscopy and scanning electron microscopy with energy dispersive X-ray microanalysis were employed to study the effect of mechanical treatment on the surface chemical state, composition and morphology of commercial Ti6Al4V alloy. It has been demonstrated that ultrasonic impact treatment of Ti6Al4V in a liquid nitrogen (LN2) environment results in substantial grain refinement and formation of nitrides and oxynitrides of all the alloy components. The mechanochemical synthesis of the nitrides and oxynitrides from LN2 was found to efficiently occur not only within the impacted area but outside it as well. The layer with a high content of incorporated nitrogen and average atomic ratio N/Ti = 0.94 was estimated to be at least ～2 μm thick. As a result of the combined effect of cryogenic deformation, nanoscale grain refinement and nitriding, a 3-fold increase in the microhardness of Ti6Al4V alloy is obtained. The impact treatment in LN2 was observed to be accompanied by material transfer from the hardened steel pin to the alloy followed by incorporation of the pin components into Ti6Al4V and formation of the FeN, FeC and TiC bonds.     

Influence of alloy composition and thermal history on carbide precipitation in γ-based TiAl alloys

Carbide precipitation in TiAl alloys with different alloying element additions and thermal history was investigated by transmission electron microscopy and high energy X-ray diffraction. The results reveal that Nb addition in TiAl alloys does not increase the carbon solubility in the γ matrix significantly. With increasing carbon concentration in the alloys, a splitting of the P-type carbides takes place earlier in the course of ageing at 800 °C and the formation of H-type carbides is promoted. For a nearly single γ phase alloy Ti51Al5Nb0.5C, H-type carbides are the thermodynamically stable phase. However, with decreasing Al concentration, P-type carbides become thermodynamically stable at low carbon concentration (below 1%) in Ti45Al and Ti45Al5Nb alloys. It is feasible to achieve high thermodynamical stability of the P-type carbides in TiAl alloys through controlling alloying elements.     

Thermomechanical modeling and simulation of aluminum alloy behavior during extrusion and cooling

The purpose of this work is the modeling and simulation of aluminum alloys during extrusion processes. In particular, attention is focused here on aluminum alloys of the 6000 series (AlMg Si) and 7000 series (Al Zn Mg). In the current paper, a number of aspects of the structural simulation as well as that of extrusion as a thermomechanical process are considered. These aspects include contact and adaptive mesh refinement, heat transfer inside the billet, heat transfer between the workpiece and the container, frictional dissipation, mechanical energy and surface radiation. The friction is considered to model the so called “dead material zone”. The radiation constant has been estimated so that the results are close to the experimental results.     

The polarization behaviour of FeCr alloys in acidic sulphate solutions in the active region

The polarization behaviour of six FeCr alloys was studied in sulphate solution of variou pH, with emphasis on the hydrogen evolution, active dissolution, and evaluation of active corrosion current, in comparison with results on the individual components. The exchange current densities of hydrogen evolution on FeCr alloys were found to increase with increasing Cr content up to 30Cr%. The rate of the anodic dissolution of low Cr alloys (< 10%Cr) strongly depended on pH and exhibited a Tafel behaviour with the slope of 40 mV/decade. That of high Cr alloys (> 10%Cr) was independent of pH and the Tafel slope was 60 mV/decade, similar to that of pure Cr, thus there is a threshold Cr content above which the dissolution of the alloy is controlled by the Cr component. The peak current at the active-passive transition decreased with increasing pH or Cr content. It is highly probable that the pre-passive state commences with the formation of chromic hydroxide, which promotes the formation of a ferrous oxide to shift the Flade potential in the negative direction.     

Effect of boron on the thermodynamic stability of amorphous polymer-derived Si(B)CN ceramics

The reason for the higher thermal persistence of amorphous polymer-derived SiBCN ceramics (T ～ 1700–2000 °C) compared to SiCN ones (T ～ 1500 °C) has been a matter of debate for more than a decade. Despite recent experimental results which indicate a major kinetic effect of boron on the thermal persistence of the ceramics, no experimental investigation of the thermodynamic stability of the materials has been reported. In this work, we present measured energetics of a series of the amorphous ceramics with various boron contents (0–8.3 at.%) using high-temperature oxidative drop-solution calorimetry. Through measurement of the drop-solution enthalpies in molten sodium molybdate at 811 °C, the formation enthalpies of the amorphous ceramics from crystalline components (SiC, BN, Si₃N₄, C) at 25 °C were obtained and found to be between ?1.4 and ?26.6 kJ g-atom^?1. The determined enthalpy data plus the estimated positive entropy of formation values point to the thermodynamic stability of the amorphous ceramics relative to the crystalline phases, but such stabilization diminishes with increasing boron content. In contrast, the higher boron content increases the temperature of Si₃N₄ crystallization despite less favorable energetics for the amorphous phase, implying more favorable energetics for crystallization. Thus the so-called “stability” of SiBCN ceramics in terms of persistence against Si₃N₄ crystallization appears to be controlled by kinetics rather than by thermodynamic stability.     

Study on the formation of core–rim structure in Ti(CN)-based cermets

Ti(CN)-based cermets were synthesized from Ti(CN)WCMo₂CTaCNiCo composite powders by vacuum-low pressure sintering. The phase evolution and the formation of core–rim structure in Ti(CN)-based cermets were systemically investigated during difference reaction stages at 950–1450 °C. The results show that the secondary carbides such as Mo₂C and TaC are begun to dissolve at 950 °C, finished at 1150 °C, and the solution temperature of WC phase is range from 1150 to 1300 °C, which are result in increase of the cermets lattice constant. At the same time, the inner rim is also formed, and Ti(CN)-based cermets are composed of (Ti, W, Mo, Ta)(CN) and Ni/Co solid solution phase. While at 1350 °C, it was found that the outer rim began to precipitate from the liquid phase with the metal binder. With increase of sintering temperature, mechanical properties of cermets improved obviously were related intimately to the increase of outer rim thickness.     

Thermophysical properties of TiB2SiC ceramics from 300 °C to 1700 °C

In the present work, high-frequency induction heating is used to fabricate TiB₂SiC ceramics and the relative density was more than 97%, and then the thermophysical properties of TiB₂SiC ceramics were investigated in detail. The specific heat showed the weak dependence on the test temperature due to the presence of the interface gap because the relative density was not 100%. As the sintering temperature increased, the thermal diffusivity of TiB₂SiC ceramics increased, which was due to the increase of relative density and grain growth. The thermal conductivity of TiB₂SiC ceramics showed a marked increase with increasing grain size and relative density. This could be attributed to a reduction in the number of grain boundaries that interrupt the heat flow path, resulting in an increase in the mean free path of the phonons. Larger grains led to an increase of mean free path of the phonons and thus contributed to a further increase in thermal conductivity.     

The dissolution and passivation of Fe and FeCr alloys in acidified sulphate medium: Influences of pH and Cr content

The steady state polarization curves of ferritic FeCr alloys, containing 7–12% Cr and immersed in 1M H₂SO₄, exhibit two current maxima, i.e. two passivating processes take place at the electrode interface within two different voltage ranges. The origin of these two passivation phenomena was investigated by plotting the steady state polarization curves and also by measuring the electrode impedance along with different polarization points. These results are then compared to those obtained with Fe electrode immersed in sulphate solutions of various pH value (0–5) since the steady state polarization curves of this electrode exhibit also two current maxima in a weakly acid medium, i.e. at solution pH greater than 4. However, even in a strongly acid solution in which only one current maximum is observed for the Fe electrode, the electrode impedance showed two passivation processes are occurring at the electrode interface. Therefore, the Fe dissolution involves at least two dissolution paths and the current at which one dissolution path overtakes the other is dependent upon the solution pH. The impedance diagrams of FeCr alloy electrode are similar to those of Fe electrode especially when the comparison is made with the Fe electrode immersed in a less acid solution, i.e. the addition of Cr in Fe enhances the appearance of two passivation phenomena with increase of solution pH.     

Effects of deep cryogenic treatment on the microstructure and properties of WCFeNi cemented carbides

The effects of deep cryogenic treatment on microstructure and properties of WCFeNi cemented carbides were investigated in this paper. The WCFeNi cemented carbides were deep cryogenically treated at about − 196 °C for 2, 12 and 24 h. In order to study the phase composition and quantitative analysis of the deep cryogenically treated specimens, a selective electrolytic corrosion was used. The internal friction, mechanical properties, wear properties and corrosion behavior of the deep cryogenically treated WCFeNi cemented carbides were measured. The results show that with an increase in deep cryogenic treatment time, martensitic phase transformation of binder phase from γ → α has taken place, the α-(Fe,Ni) phase content in the binder phase gradually increases from 12.7% to 86.8% (wt.%) and W particles precipitate from the binder phase. Compared with the as-sintered specimens, the maximum hardness and transverse rupture strength of the specimens treated by cryogenic environment increase by almost 20% and 7.7%, respectively. The fracture toughness decreases from 25.7 MPam^½ for as-sintered specimens to 19.6 MPam^½ for the alloy after deep cryogenic treatment. The wear rate and friction coefficient decrease by almost 56% and 17.2%, while the corrosion resistance slightly decreases compared with the alloys without deep cryogenic treatment. The change of the properties can be primarily attributed to the fact that the martensite phase transformation from γ-(Fe,Ni) to α-(Fe,Ni) and precipitation of W particles in the binder phase improve the hardness and strengthen the binder phase during the deep cryogenic treatment.     

Atom site preference and γ′/γ mismatch strain in NiAlCoTi superalloys

The structure and chemistry of NiCoAlTi quaternary superalloys are investigated at the microstructural and atomic scale. Atom probe tomograghy (APT) is used to quantify phase compositions and elemental partitioning behavior. Using aberration corrected electron microscopy, the site occupancy of the atoms within the A₃ B structure (L1₂–type) γ′ phase is determined via atomic resolution energy dispersive X-ray spectroscopy (EDX). Ni is observed to preferentially occupy the A sub-lattice positions for all alloys, while Al and Ti occupy the B sub-lattice. In contrast, Co's site preference changes from a random distribution to the A sub-lattice as the alloy composition changes, in agreement with theoretical predictions from literature. Finally, the lattice strain across the γ′/γ interfaces is measured as a function of alloy composition.     

Effect of consolidation and oxide dispersoid addition on phase formation and mechanical properties of WTi ODS alloy

WTi alloys are presently considered promising candidates for plasma facing components in advanced nuclear energy systems. The mechanically alloyed WTi model alloys consolidated by different techniques were investigated. The effect of different amounts of the dispersed oxide particles on characteristics and properties of the WTi materials was also discussed. The results show that a homogeneous fine grain structure without formation of Ti-rich oxides is obtained in the model alloy sintered by spark plasma sintering. The (W,Ti)C solid solution and α″-Ti martensite phase are also found in the alloy confirmed by TEM investigation, which can be contributed to the effect of the carbon contamination and cooling rate during the process. In this work, it is clear to demonstrate that the presence of different carbides and Ti phases plays an important role in determining hardness and elastic modulus of the materials. The microstructure homogeneity and mechanical properties of the alloys can be further improved by increasing numbers of oxide dispersoids.     

Effect of CuCe alloy addition on the microstructure and mechanical performance of brazed diamonds with NiCr alloy

This study was aimed to reduce unwanted thermal damage to diamond grains during brazing. To achieve this goal, the diamonds were brazed using NiCr alloy with the addition of CuCe alloy (i.e. NiCr composite solder). Surface morphology and interfacial microstructure of the brazed samples using NiCr composite solder was characterized. The thermally induced residual stresses of brazed diamond grains were examined by Raman spectroscopy. The static pressure strength and impact toughness of the brazed diamond grains were examined. Constitutional phases of NiCr composite solder after brazing were detected. The results show that the thermal residual stresses values of the brazed diamond grains using NiCr composite solder, in which 2 wt% and 5 wt% CuCe alloy were used respectively, were decreased by 6.4% and 9.7% respectively, meanwhile, the static pressure strength values increased by 4.9% and 13.4% respectively as well as impact toughness values increased by 4.0% and 9.2% respectively, compared with that of the brazed diamond grains using NiCr alloy. Chromium carbides (Cr₃C₂ and Cr₇C₃) were obtained at the bonding interface between diamonds and composite solder. The constitutional phases containing Ce₂Ni₇, which could be beneficial to reduce the thermal damage, were formed in the solder alloy after brazing.     

Designed fabrication of hard CrCr2O3Cr7C3 nanocomposite coatings for anti-wear application

Nanocrystalline CrCr₂O₃Cr₇C₃ composite coatings were fabricated by electrodeposition followed by thermal treatment. The structures of coatings were investigated using high-resolution transmission electron microscopy and X-ray diffraction analysis. The composition, elemental chemical state, mechanical properties and wear resistance of coatings were determined using energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, nanoindentation and oscillating friction-wear testing, respectively. Wear tracks were observed by scanning electron microscopy. The results show that the as-deposited coating exhibits amorphous structure. The subsequent thermal treatment at 600 °C induces the crystallization and the generation of nanoscale Cr₂O₃ and Cr₇C₃ particles in the Cr-matrix, which results in the hardness of the coating increasing to 21 GPa with slight increase in elastic modulus. Owing to the compromise between high hardness and low elastic modulus, the obtained CrCr₂O₃Cr₇C₃ composite coating exhibits excellent wear resistance.     

Photoluminescence and electroluminescence of oligoacetylenic silanes and germanes

A series of light-emitting oligoacetylenic silanes and germanes containing fluorene units was investigated. It was found that the higher oligomers display lower band gaps and that the CSi(Ge)C bond permits π-electron delocalization. The π-d_Si or π-d_Ge interactions aid the π-electrons in passing through the Si (Ge) atoms. The observed differences in fluorescence spectra may also result from a substituent effect of the silyl groups. The SiSi σ bond also permits π-electron delocalization and thus there is a through-bond interaction between π orbitals and the SiSi bond. The conjugation-interrupting Ge sp³ link aids π-electron delocalization along the oligomer backbone less efficiently than does a Si link, thus leading to relatively shorter effective conjugation lengths and higher band gaps. In the higher oligomers, multiple π-d_Si or π-d_Ge interactions permit the vibrational levels of the excited state to split to form sublevels observable as new transitions in the respective absorption spectra. FWHM values of the PL spectra increase with increase of the oligomer backbone length which relates to the increase of the intermolecular interaction between the chromophore units. With increasing oligomer backbone length, the EL efficiency decreases, which can be attributed to the enhanced probability of interchain exciton annihilation in LEDs using higher oligomers.