Internal oxidation of Fe-Si alloys in γ-phase region

Internal oxidation measurements of Fe-0.070, 0.219, 0.483, and 0.920 wt % Si alloys were made in the γ-phase region in order to discuss kinetics of internal oxidation, to evaluate the diffusion coefficient of oxygen in the internal oxidation layer, and to determine the diffusion coefficient of oxygen in γ-iron. Internal oxidation of these alloys was conducted at temperatures between 1223 and 1323 K using a powder mixture of iron and Fe₂O₃. The internal oxidation front in Fe-Si alloys with between 0.070 and 0.483 wt % Si advances in parallel to the specimen surface. The internal oxidation in these alloys obeys a parabolic rate law, which indicates that the internal oxidation is controlled by an oxygen diffusion process in the alloy. The diffusion coefficient of oxygen, D _O ^IO , in the internal oxidation layer where SiO₂ particles disperse was determined by using the thermodynamic data for the solution of oxygen in γ-iron. D _O ^IO increases with the increase of the volume fraction of the oxide, f ^IO, in the oxidation layer at a given temperature. The diffusion coefficient of oxygen, D _O, in γ-iron was evaluated by extrapolating D _O ^IO to f ^IO=0. D _O may be given by the following equation: $$D_O = \left( {6.42\begin{array}{*{20}c} { + 4.37} \\ { - 2.60} \\ \end{array} } \right) \times 10^{ - 5} exp \left[ { - \frac{{159 \pm 5(kj mol^{ - 1} )}}{{RT}}} \right]m^2 \sec ^{ - 1} $$ .     

The effect of nickel content on the fracture behaviour of Cu-Al-Niβ-phase alloys

The fracture behaviour of Cu-14 wt% Al alloys has been studied as a function of nickel content varying from 0 to 10 wt%. It was found that the presence of a brittle phase ₂ at the grain boundaries is responsible for intergranular fracture in low nickel alloys. Severe intergranular embrittlement exhibited by high nickel alloys in not associated with any precipitate at the grain boundaries. In fact when high nickel alloys are cooled slowly, a ductile phase () forms along the grain boundaries that resists the propagation of crack through grain boundaries and the fracture is transgranular.     

Recent progress in the high-cycle fatigue behaviour of γ-TiAl alloys

The high-cycle fatigue (HCF) properties of γ-TiAl (gamma titanium aluminide) alloys are reviewed, particularly with regards to the deformation mechanisms active in the near-threshold cyclic loading regime. By examining the influence of lamellar orientation and thickness on the HCF threshold, in addition to more conventional microstructural considerations such as the grain size or the volume fraction of lamellar colonies, factors to improve the γ-TiAl microstructure for HCF are assessed. Finally, experimental methods and loading strategies are surveyed to identify techniques for improving HCF testing of γ-TiAl alloys. In this, we consider both the conservativism of differing approaches and the possibility to measure with suitable resolution the local mechanical behaviour under HCF of the lamellar γ-TiAl microstructure.

This review was submitted as part of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

Structural behaviour of earthen corbelled domes in the Aleppo’s region

The paper illustrates the results of a study aimed at understanding the structural behaviour of earthen corbelled domes, used as roofs in several villages in the Aleppo??s region. The materials and the shape of the domes suggested the hypothesis of a ??monolithic?? structural behaviour, justified by the very good bond between earthen bricks and earthen mortar, and by the use of very low values of thickness, which would have otherwise lead to collapse during the construction. Thanks to this hypothesis, although the dome is built with corbelled layers (??false dome??), the membrane theory can be applied to analyze the stresses in the domes under investigation. The results of this analysis, that were compared also to those obtained by a finite element analysis with shell elements, show that the dome is subjected to biaxial compression only; moreover, the dome shape seems close to the funicular surface due to its own weight, i.e. a dome shape is the optimal geometrical choice for that material.     

Grain boundary embrittlement in Cu-AI-Niβ-phase alloys

The grain boundary embrittlement in Cu-AI-Ni-phase alloys has been investigated. The study included both the bulk alloys and rapidly solidified ribbons. It was observed that the fracture characteristics and the phase transformations in rapidly solidified ribbons were similar to those in the bulk alloys. Various factors responsible for intergranular fracture were considered. It was found that intrinsic or extrinsic precipitates at the grain boundaries are not responsible for the embrittlement. It was further observed that the segregation of impurities does not occur at the grain boundaries and hence is not a factor contributing to the embrittlement. The intrinsic characteristics, however, appear to play an important role. These may include high elastic anisotropy, ordered structure and plastic incompatibility. Severe embrittlement in high-nickel alloys is associated with spinodal decomposition occurring in these alloys. The large grain size exhibited by these alloys is found not to be a significant factor contributing to the embrittlement.     

The ε-phase in rapidly solidified ferrous alloys

An Fe-1.9 wt% Si-4.2 wt% C alloy has been rapidly quenched from the melt in a controlled-atmosphere gun splat-cooling device to produce material consisting predominantly of the hexagonal close-packed-phase. The techniques of X-ray diffraction, scanning and transmission electron microscopy, and differential thermal analysis have been used to study the nature of the-phase and its decomposition during heat treatment. The presence of the metastable-phase could not be accounted for as the transformation product of a low stacking-fault energy austenite, and it appears to be formed directly from the liquid state. It has a complex and variable microstructure, which ranges from elongated, relatively featureless grains in the most rapidly-cooled areas to highly-faulted and twinned structural units containing precipitate particles. The-phase undergoes a two-stage decomposition process upon heating, but the first stage does not appear to involve the formation of martensite and-carbide, which has been the hitherto accepted reaction. As an alternative it is suggested that the-phase may decompose by a spinodal reaction.     

Effects of the variation in α-phase volume fraction on the thermal stability of TiAl alloys with a lamellar microstructure

The thermal stability of two-phase ( + ) lamellar microstructure in Ti-Al-Mo PST (polysynthetically twinned that has single colony) crystals, containing C or Si, was investigated. In addition, the variation of -phase volume fraction in Ti-Al-Mo-(C,Si) systems was investigated at several temperatures. Ti-46Al-1.5Mo-0.2C and Ti-46Al-1.5Mo-1.0Si alloys did not recrystallized (stable in this paper) during heat treatments at various heating rates and temperatures. Moreover, the -phase volume fractions of Ti-46Al-1.5Mo-0.2C and Ti-46Al-1.5Mo-1.0Si alloys which were stable compositions, changed less than those of Ti-47Al and Ti-46Al-1.5Mo alloys which were unstable compositions. The instability of the latter alloys was caused by their relatively higher variation of -phase volume fraction during heating. Therefore, it is suggested that the variation of -phase volume fraction is an important factor in controlling the thermal stability of lamellar microstructure.     

The intergranular embrittlement of Cu-AI-Ni β-phase alloys

Intergranular embrittlement in the Cu-Al-Ni -phase alloys has been investigated. Results of various experiments, which included an Auger electron spectroscopic analysis of freshly fractured grain boundary surfaces, lead to the conclusion that impurities do not play any significant role in the intergranular embrittlement of these alloys.     

Deformation behaviour of retained β phase inβ-eutectoid Ti-Cr alloys

Plastic deformation mode and its relation to tensile properties were investigated in retained phase of-eutectoid type Ti-Cr alloys. A plate-like single variant phase is induced during deformation of the most unstable phase having a minimum chromium content required to suppress martensitic transformation. A selected area electron diffraction pattern taken from a boundary region of the stress-induced phase plate can be explained by the idea that a single variant of phase is induced in a {3 3 2} 1 1 3 twin produced during deformation. Anisotropy in population of four phase variants decreases with increasing chromium content. On further increasing chromium content, deformation occurs by slip. Enhanced ductility is obtained in as-quenched Ti-Cr alloys accompanied by phase transformation or {332} 1 1 3 twinning during deformation, phase of as-quenched Ti-Cr alloys changes continuously from commensurate structure with sharp reflections to incommensurate structure with diffuse reflections with increasing chromium content. The obtained results in-eutectoid type Ti-Cr alloys are quite similar to those in-isomorphous type Ti-V alloys.     

Effects of oxide stringers on the β-phase depletion behaviour in thermally sprayed CoNiCrAlY coatings during isothermal oxidation

This paper details an investigation of the effects of oxide stringers on the β-phase depletion behaviour in thermally sprayed CoNiCrAlY coatings. Vacuum Plasma Sprayed(VPS) Co NiCrAlY coatings, which are free of oxide stringers, are used as the reference materials in comparison with High-Velocity Oxy-Fuel(HVOF)sprayed CoNiCrAlY coatings during isothermal oxidation at 1100℃. An outer layer of spinel oxides and an inner layer of alumina are formed in the as-sprayed coatings, while only a single alumina scale is found in the heat-treated coatings. Less β-phase depletion occurred in the HVOF coatings than in the VPS coatings. It was found that the β phases tend to coalesce at the oxide stringers in the HVOF coatings,which is likely due to the internal oxide particles and stringers acting as short diffusion barriers to tie up the β phase and inhibit the β-phase depletion.     

Effect of ultrasonic treatment on the mechanical behaviour of Al–Ni alloys

Applications of Al–Ni alloys are limited because their matrix is weaker than other binary aluminium alloys. Ultrasonic treatment (UST) is an effective tool for grain refinement that can strengthen the matrix phase. It not only reduces the grain size and porosity but also refines and uniformly distributes the secondary phase, which can influence the mechanical properties of Al–Ni alloys. Varying the amount of nickel (1, 2, 3, and 5?wt-%) in molten aluminium along with ultrasonication of the melt is investigated through grain-structure, mechanical properties, and fractography. Mechanical properties of the alloys subjected to UST are superior to respective as-cast alloys. UST also altered the fracture behaviour from dominant ductile fracture in as-cast alloys to dominant mixed mode fracture.     

Electrochemical behaviour of iron—chrome alloys in relation to pitting corrosion

The polarization behaviour and pitting corrosion of Fe-Cr alloys of 7, 13, 18, 24 wt% Cr were studied. Potentiodynamic and galvanostatic measurements were performed in the absence and presence of Cl^–. As the Cr content increases the active dissolution current densities decrease while the passive range and transpassive current densities increase. Polarization parameters gave for the passive transition of the alloys a Cr concentration of -13%. An increase of Cl^– concentration causes the progressive destruction of passivity. It interfered with O₂ evolution, and then destroyed the transpassive region. Still higher Cl^– concentrations initiated pitting corrosion as shown by the oscillations in potential of the galvanostatic polarization curves supported by visual observation. Results are discussed on the basis of competitive adsorption between the aggressive and inhibitor anions for the active sites on the alloys' surface.     

The effect of notch plasticity on the behaviour of fatigue cracks emanating from edge-notches in high-strength β-titanium alloys

The present paper is aimed at investigating the behaviour of fatigue cracks emanating from edge-notches for two different microstructures of the Ti-6246 alloy, produced by two specific thermo-mechanical treatments and defined as β-annealed and β-processed, respectively. Pulsating four point bending tests were performed on double-edge-notched specimens. The initiation and early propagation of fatigue cracks were investigated at two relatively high nominal stress levels corresponding to 88 and 58% of the 0.2% material’s yield stress. Plastic deformation at the notch tip initially produced a local stress redistribution followed by elastic shake down due to the high cyclic strain hardening rates exhibited by both microstructures, as confirmed by finite element modelling. Crack closure effects, measured by an extensometric technique, and variations in crack aspect ratio were considered in the ΔK calculation. The obtained crack growth rate data were compared with those of long cracks measured on standard C(T) specimens as well as of microcracks measured on round, unnotched S-N type of specimens to evaluate the intrinsic fatigue crack propagation resistance of the two microstructures. The contribution of notch plasticization to crack closure was estimated by finite element modelling.     

The stress dependence of superconducting parameters in pure In and Sn and in α-phase In-Sn alloys

We have measured the length change due to the transition from the super-conducting to the normal state in pure In and Sn and in In-Sn alloys. The measurements were made on single-crystal specimens. The alloys had Sn content ranging from zero to 12 at %. Crystals parallel and perpendicular to the tetragonal axis were grown at each alloy content. We find that the relative length changes l/l are very sensitive to alloy content. From l/l we have calculated the stress derivatives H _c/ and T _c / and we find that T _c / changes from +59 to –92 mK/bar for stress along the tetragonal axis, and from –9 to 46 mK/kbar for stress perpendicular to the tetragonal axis. We suggest that these large changes are due to Fermi surface topology changes upon increasing the Sn content in In.     

Precipitation characteristics of μ-phase in wrought nickel-base alloys and its effect on their properties

Thermal exposures consisting of 1–16000 h at 540, 650, 760, and 870°C were used to study the susceptibility of selected nickel-base alloys to precipitation of -phase and its effect on mechanical strength and corrosion resistance. Analytical electron microscopy and X-ray diffraction were used to characterize the -phase. A -phase of the type Mo₆Ni₇ in nickel-base alloys was found to be stabilized by critical concentrations of iron in an excess of about 3 wt%. Generally, the -phase had a characteristic defect structure consisting of twins and stacking faults, and it exhibited a preferential tendency for precipitation at existing molybdenum-rich carbide particles within the alloy matrix and at grain boundaries. Precipitation of -phase was found to produce a moderate loss of room-temperature tensile ductility; however, it resulted in a considerable degradation of impact toughness and corrosion resistance. In contrast, it had no significant effect on elevated temperature tensile properties. A correlation was found to exist between the Ni/Fe + Co ratio as well as the Mo + W content of the alloy and susceptibility to precipitation of -phase.     

The investigation of metastable ζ-hcp phase in the Ag-rich f c c region of Ag-In alloys rapidly quenched from the melt

Stacking faults, which were detected in the Ag-In system, have been examined by transmission electron microscopy and electron diffraction. An enhanced concentration of stacking faults in splat cooled specimens as well as the formation of a metastable hexagonal phase in the f c c region of the alloy was observed. As far as we know this is the first case of the terminal solid solubility being reduced by rapid quenching. Terminal solid solubility is reduced because of the high concentration of structural defects introduced by quenching, e.g. dislocations and stacking faults, which serve as the nuclei for the transformation from the f c c to the h c p structure. Our measurements and calculations show that the stacking fault energy minimum is shifted to lower electron concentrations with respect to the stacking fault energy minimum corresponding to the equilibrium phase boundary for the f c c-h c p transformation. The new metastable phase boundary for this transformation was confirmed by X-ray examinations. We explain this earlier h c p phase appearance in rapid quenched specimens as the consequence of enhanced interaction of the Fermi surface and contracted Brillouin zone. The Brillouin zone contraction we attributed to quenched-in vacancies.     

The behaviour of commercial FeCrAlRE alloys in nitrogen-oxygen – water vapour bioxidant environments

Abstract

The corrosion behaviour of a range of commercial FeCrAlRE alloys (MA956, ODM751, PM2000, Kanthal AF, Kanthal APM and Aluchrom YHf) have been examined in nitrogen–oxygen-H₂O or N₂–H₂–H₂O bioxidant environments, at temperatures between 1100°C and 1350°C. The corrosion behaviour is governed by the competition between oxidation leading to protective alumina formation/ maintenance and nitrogen ingress leading to nitridation of the matrix alloy. Key issues addressed by four series of experiments, have included: the influence of a pre-formed protective alumina scale; the oxidant level required to form/reheal a protective oxide scale; the role of mechanical failure of the scale above the critical thickness for cracking/spallation in oxygen rich environments; chemical failure of the protective oxide scale leading to breakaway (non-protective) attack and in particular, the potential roles in such failure processes of nitridation concurrent with, and following defective oxide scale formation, and of oxidation following nitridation.

Detailed characterisation of the chemical composition and physical microstructure of the attack of the respective alloys was undertaken using a range of surface analytical techniques, including X-ray diffraction, optical and scanning electron microscopy and energy dispersive X-ray analysis.