The high temperature corrosion resistance of α-phase bronzes

The oxidation of Cu-Sn alloys containing up to 13 wt% Sn in oxygen (1 atm) has been studied in the temperature range 550 °C to 800 °C at 50 °C intervals; the investigation involved the study of the kinetics and of the morphology and structure of the scales by appropriate physical methods, with the aim of correlating the two aspects of the oxidation behaviour. The oxidation rate of a 3 wt% Sn alloy is lower than for pure Cu at 550 and 600 °C, but higher above 600°C. The effect of the tin addition is related to the interplay of many different factors affecting the reaction rate, the overall result depending on temperature. The rate of oxidation of the more concentrated alloys, however, is always lower than that of pure Cu. This result is attributed to the formation of an SnO₂ layer at the base of the scale which, although thin and usually discontinous, slows down the outward diffusion of copper. The highest tin concentration allowed by its high temperature solubility in copper is not sufficient to produce a continous thick healing layer of SnO₂ at the alloy surface, but nevertheless it produces a pronounced decrease of the oxidation rate.     

Relation between the magnetic transition temperature and magnetic moment for manganese nitrides MnNγ (O <γ < 1)

The magnetic properties of Mn₆N₅ and Mn₃N₂ prepared by nitridation of -Mn powder in NH₃ flow were studied by the measurement of magnetic susceptibilities in the temperature range from 100 to 958 K. It was revealed that both Mn₆N₅ and Mn₃N₂ exhibited antiferromagnetism and that the magnetic transition of Mn₆N₅ and Mn₃N₂ was accompanied by phase transitions at about 655 and 925 K respectively. The magnetic transition temperature of MnN_γ with γ ranging between 0 and 1 is discussed on the basis of both the shortest Mn—Mn distance and the magnetic moment per Mn atom.     

The π–π interaction induced secondary doping in conducting poly-3-alkylthiophenes

A new driven force of the secondary doping in polyalkylthiophene was reported. It was found that the mechanism of secondary doping in polyalkylthiophene was mainly due to the π–π interaction between the secondary dopant and the thiophene rings in polymer backbone. The secondary dopant altered the conformation of polymer chains from coiled to expanded form by π–π interaction induced face-to-face interchain stacking as well as dissolution and rearrangement of the alkyl side chains. Upon treating with secondary dopant, the intensity of bipolaron band, the IR reflectance, and the crystallinity of the polymer films all increased. Therefore, the conductivity of the AuCl₃-doped polyalkylthiophene films increased two times after being secondarily doped with benzene.     

Influence of composition and annealing conditions on the site-occupation in the σ-phase of Fe–Cr and Fe–V systems

Neutron diffraction technique was used to study the site-occupation in the σ-phase in the Fe–Cr and Fe–V systems. It was found that all five sites A, B, C, D and E are “mixed”, i.e. occupied by both elements. The occupation is neither random – the degree of randomness increases with Fe content – nor regular, i.e. sites A and D are predominantly occupied by Fe atoms while B, C and E sites are occupied preferentially by Cr or V atoms. For all five sites the increase of Cr(V) concentration results in an irregular decrease of the number of Fe atoms on each site, the rate of decrease being the smallest for the sites A and D. In the Fe–Cr system the population of A with Fe atoms, N_A, is similar to that of D, N_D, while for the system Fe–V N_D > N_A and the difference increases with V content. The lowest Fe population in both investigated systems has B, but N_B(V) < N_B(Cr). N_C = N_E for Fe–V, while N_C > N_E for Fe–Cr. The influence of sample preparation conditions (plastic deformation prior to phase transformation, annealing time, t_a, and temperature of annealing, T_a) were also tested on a σ-Fe_53.8Cr_46.2 sample. In general the influence of these conditions on the site population is small. The largest effect had the plastic deformation, and the smallest one the annealing time in case of non-deformed samples. The most insensitive sites were revealed to be D and E, and the most sensitive ones were B and A.     

Plating by electric explosion of conductors

The influence of the energy characteristics of the electric explosion of metal conductors on the structure and stability of a coating applied on a substrate has been studied experimentally. It is established that, at ω = ω_c, the nickel coating consists of chaotically placed hardened metal drops and is unsteady. The coating formed by fine-dispersed nickel particles deposited from the gaseous phase in the course of an electroexplosion at ω = 2ω_c is uniform and resistant to mechanical impacts.     

Relation between the magnetic transition temperature and magnetic moment for manganese nitrides MnNγ (O <γ < 1)

The magnetic properties of Mn₆N₅ and Mn₃N₂ prepared by nitridation of α-Mn powder in NH₃ flow were studied by the measurement of magnetic susceptibilities in the temperature range from 100 to 958 K. It was revealed that both Mn₆N₅ and Mn₃N₂ exhibited antiferromagnetism and that the magnetic transition of Mn₆N₅ and Mn₃N₂ was accompanied by phase transitions at about 655 and 925 K respectively. The magnetic transition temperature of MnN_γ with γ ranging between 0 and 1 is discussed on the basis of both the shortest Mn—Mn distance and the magnetic moment per Mn atom.     

Interkristalline Korrosion durch Bildung der σ-Phase

Intercrystalline corrosion by σ-phase formation Local corrosion (leakages) in a cooling tube for phosphate melts (800 °) are primarily attributed to incrustation obstructing the action of cooling water, so that at such points σ-phase eas precipitated and inter-crystalline corrosion set in; the propagation of this corrosion is enhanced by thermal stresses. Remedies may consist in design changes (to avoid local overheating), use of CrNi-steels with low carbon content and (eventually) of Mo-free steels.     

Ball-milling in vacuum of α and σ phases of near-equiatomic FeCr alloys

The structural changes induced by ball-milling of near-equiatomic σ-FeCr and α-FeCr in vacuum were followed. Besides the α-phase, an amorphous phase appears when milling the σ-phase for times longer than 20 h. An amorphous phase forms too, but at a slower rate than the latter, when milling the α-phase. The partial amorphisation of σ-FeCr and α-FeCr ball-milled in vacuum is concluded to be a phenomenon of intrinsic origin. The amorphous phase crystallizes into a bcc Cr-rich phase and a bcc Fe-rich phase during short annealing steps.     

Identification of σ and Ω phases in AA2009/SiC composites

The microstructure evolution during ageing treatment at 170 and 190 °C of AA2009/SiC composites, reinforced with 15 vol.% particulates and whiskers, was studied by transmission electron microscopy. Besides θ′ and S′ phases, the typical hardening precipitates on Al–Cu–Mg alloys, it was found the presence of Ω and σ (Al₅Cu₆Mg₂) phases in the matrix. σ phase was only found in the matrix of particulate composite, while Ω phase appeared in both. This phase has not been previously observed in Al matrix composites based on conventional Al–Cu–Mg alloys.     

Oxide phase development upon high temperature oxidation of γ‐NiCrAl alloys

The amount of each oxide phase developed upon thermal oxidation of a γ‐Ni‐27Cr‐9Al (at.%) alloy at 1353 K and 1443 K and a partial oxygen pressure of 20 kPa is determined with in‐situ high temperature X‐ray Diffractometry (XRD). The XRD results are compared with microstructural observations from Scanning Electron Microscope (SEM) backscattered electron images, and model calculations using a coupled thermodynamic‐kinetic oxidation model. It is shown that for short oxidation times, the oxide scale consists of an outer layer of NiO on top of an intermediate layer of Cr₂O₃ and an inner zone of isolated α‐Al₂O₃ precipitates in the alloy. The amounts of Cr₂O₃ and NiO in the oxide scale attain their maximum values when successively continuous Cr₂O₃ and α‐Al₂O₃ layers are formed. Then a transition from very fast to slow parabolic growth kinetics occurs. During the slow parabolic growth, the total amount of non‐protective oxide phases (i.e. all oxide phases excluding α‐Al₂O₃) in the oxide scale maintain at an approximately constant value. The formation of NiCr₂O₄ and subsequently NiAl₂O₄ happens as a result of solid‐state reactions between the oxide phases within the oxide scale.     

A phenalenyl-based neutral stable π-conjugated polyradical

The first neutral π-conjugated polyradical based on phenalenyl with the spin unit inside the main chain was prepared. Cyclic voltammetry studies showed that this polyradical had excellent redox reversibility and enhanced π-delocalization between the neighboring spin units, in consistent with the UV–vis results. Initial magnetic studies showed that a strong antiferromagnetic interaction existed at solid state. The conductivity of the polyradical was measured with a value σ_RT ≈ 10⁻⁸ S/cm.     

Internal friction of γ-TiAl alloys at high temperature

Intermetallic γ-TiAl based alloys of engineering interest with respect to high temperature applications are two-phase alloys consisting of γ-TiAl (ordered face-centered tetragonal structure) and ₂-Ti₃Al (ordered hexagonal structure). For this investigation a γ-TiAl based alloy with a composition of Ti-46.4 at%Al-4 at% (Cr, Nb, Ta, B) was studied utilizing a low frequency subresonance apparatus at frequencies between 0.01 and 10 Hz. Above 1000 K, the damping increases strongly up to Q⁻¹=0.2. The frequency and temperature dependence of the high-temperature background was analysed by applying a Maxwell rheological model for viscoelastic relaxation including a distribution of relaxation times. The activation enthalpy of H=3.9 eV agrees well with that from creep experiments (H=3.6 eV) carried out at temperatures between 973 and 1073 K at 200 MPa. The results are discussed in terms of diffusion controlled dislocation climb.     

Luminescence properties of La1−χTmχTa7O19

The luminescence properties of Tm³⁺ in La_1−χTm_χTa₇O₁₉ solid solutions were examined systematically. The substitution of Tm³⁺ for La³⁺ was carried out by a decomposition reaction of nitrates involving the corresponding constituents at 1200 °C in air. X-Ray diffraction patterns of the solid solutions indicated that the crystal structure consisted of a network of (La_1−χ³⁺Tm_χ^staggered|3+, Ta⁵⁺)—O²⁻ polyhedra interstratified with a double layer of Ta⁵⁺—O²⁻ polyhedra. According to the excitation and emission spectra, the most intense emission was found near 460 nm and quenched above χ=0.14 in La_1−χTm_χTa₇O₁₉. Also, lifetime results verified that the emission could be assigned not to the transition ¹G₄ å ³H₆, but to the transition ¹D₂ å³H₄. Upon cathode ray excitation some emissions of Tm³⁺ were superimposed by a broad emission due to the clusters of Ta⁵⁺—O²⁻ polyhedra. As a result, a low dimensional arrangement of Tm³⁺ was much more preferable for getting intense emission because it reduced the energy migration between Tm³⁺ ions.     

High temperature air oxidation behaviour of “poor man” high manganese-aluminum steels

The behaviour of austenitic high manganese (19.8%–32.5%)aluminum (7.1%–10.2%) steels in high temperature air has been examined. Tests have been carried out in the 600–900°C temperature range for durations up to 200 h. The steels form a high manganese scale at high oxidation rates; aluminum does not contribute to the formation of this scale. Only a 1.5% silicon bearing steel shows a good oxidation resistance up to 700°C, the oxidation rate being lower because as shown by XPS analyses a silicon-containing scale is formed. The oxidation resistance of these steels is always lower than that of conventional grades of austenitic stainless steels.     

σ-Phase in Lean Duplex Stainless Steel Sheets

The precipitation kinetics of secondary phases in two austeno-ferritic lean duplex stainless steels (lean DSS) were examined after aging the materials at 800 °C. Owing to the instability of ferrite, all DSS are known to be sensitive to solid-state phase transformations in the critical temperature range 600-1,000 °C and different secondary phases may form, depending on composition and microstructure. The performed thermodynamic simulations revealed the proneness to the precipitation of such phases also have been done in lean DSS, but only information on the equilibrium microstructures were achieved. Therefore, the materials were aged at various times, in order to verify the simulations and determine the precipitation kinetics. The occurred structural modifications were observed and quantified by scanning electron microscope and X-ray diffraction measurements, determining phase type, composition and volumetric fraction. At 800 °C, grade 2101 was found to be only affected by Cr₂N nitrides precipitation, whereas a significant amount of σ-phase was found to form in LDX 2404 for treatment longer than 1 h, almost totally replacing ferrite after 50 h. Up to now, the intermetallic σ-phase has been observed only in the high alloyed DSS, and the unexpected precipitation in grade 2404 highlighted that the increased content of molybdenum in this steel might be considered as determinant for the formation.