Alloying effects on sulphur embrittlement of the γ-γ′ interface of nickel-based single-crystal superalloys

Fully first-principles calculations have been performed to investigate the alloying effect on the sulphur embrittlement of the n - n ' interface of nickel-based single-crystal superalloys. The shear and cohesive strengths of the inter-face are calculated in terms of Mayer bond orders (BOs), and the ratio R BO of shear strength over cohesive strength is analysed as a function of alloying element substitution. The interface characteristics are also studied using the electron charge-density distribution. It is found that the interplay between shear and cohesive strengths has a significant influence on the interfacial embrittlement. Using the phenomenological theory of fracture as well as the calculated ratio R BO, we propose a new mechanism responsible for relieving sulphur embrittlement of the n - n ' interface through alloying substitution.     

Interactions between interfacial dislocations and γ/γ′ interface during high temperature – low stress creep in nickel-based single crystal superalloy

ABSTRACT

The interaction between interfacial dislocations and γ/γ′ interface is critical to the high temperature creep properties of single crystal superalloys. However, only a few studies have paid attention to the detailed structure such as local interfacial morphologies and the elemental distribution around interfacial dislocations. In this paper, the interfacial protrusions and related dislocations in a single crystal superalloy after creep at high temperature – low stress have been investigated in detail. It is found that the morphology and size of the interfacial protrusions remain almost the same during the early and middle stages of high temperature creep, which indicates a local equilibrium at the interfacial protrusions. Steps at different height are formed at the γ/γ′ interface at the initial stage of high temperature creep since dislocations could move along the γ/γ′ interface, which indicates that dislocation motion at different creep stage may affect the morphology of γ/γ′ interface.     

The surface core-level shift of the Nb(110) surface

The surface core-level shift (SCLS) of the Nb(110) surface has been determined by means of high-resolution core-level photoelectron spectroscopy of the Nb 3d5/2 core level. A SCLS of 180 +- 15 meV towards higher binding energy with respect to the bulk component was estimated by curve fitting the Nb 3d5/2 core-level spectra taken using several photon energies. The sign of the SCLS agrees with recent ab initio calculations including both initial- and finalstate effects, but the shift is significantly smaller than predicted in these calculations.     

First-principles pseudopotential study of an aluminium grain boundary containing sulphur atoms

The electronic structure of an aluminium grain boundary with segregated sulphur impurity atoms has been calculated by a first-principles pseudo-potential method. It is found that a sulphur atom bonds to only one of the neighbouring aluminium atoms. This bond is a mixed-character metallic-covalent bond which is stronger than the metallic Al-Al bonds. Electrons that participate in forming this bond are 3p electrons of sulphur but not its 3s electrons. Other Al--S bonds in the boundary contain no covalent character. From the nature of Al--S bonds in the boundary it cannot be decided whether the embrittlement promotion mechanism by sulphur segregation should be classified as a 'bond mobility model' or a 'decohesion model'.     

Structural analysis of 30RO–30Al2O3–40B2O3 ( R = Mg,Ca or Sr) glasses

Bond-compression and ring-deformation models are used to quantitatively analyze the acoustical properties (elastic moduli, Poisson's ratio, Debye temperature and longitudinal attenuation) of 30RO–30Al₂O₃–40B₂O₃ glasses (in wt%), where R?=?Mg, Ca or Sr. The network structure of the glass is quantified in terms of the four-fold coordinated formers, atomic ring size, cross-link density, first-order stretching force constant and bulk modulus. It is found that network strength increases in the order Mg?>?Ca?> Sr. Structural changes of the glasses are deduced by relating the elastic moduli, the ratio of bond compression to experimental bulk modulus and ultrasonic attenuation to structural parameters and oxygen density.     

Role of impurity elements in metal-metal bond strengths

Abstract

The influence of impurity elements on the strength of neighbouring metal–metal bonds is investigated using a simple tight-binding model. We show that the quantum-mechanical response function of the metal host plays a central role in determining whether the bond is weakened or strengthened. The Messmer–Briant model, which is based on classical concepts of charge flow due to differences in electronegativity, is not applicable to metals because of their perfect screening. Recent experiments on impurity core level and isomer shifts should not be taken as a direct measure of neighbouring metal–metal bond decohesion.     

Mechanical behaviour of an Al–matrix composite reinforced with nanocrystalline Al-coated B4C particulates

In metal–matrix composites (MMCs), interfacial bonding between the metal matrix and the ceramic reinforcement plays a crucial role in their mechanical performance. In the present study, B₄C particles were cryomilled with an Al alloy to produce a composite powder, in which the B₄C was uniformly distributed in nanocrystalline Al. The cryomilling developed a strong bond between the B₄C and the Al, allowing the nanocrystalline Al to act as a coating with a strong ceramic–metal interface. This cryomilled composite powder was then introduced, as a reinforcement, into a conventional Al alloy to strengthen the material. After consolidation, the result was a bulk Al–matrix composite reinforced with B₄C particles encapsulated in nanocrystalline Al. This composite exhibits greatly improved strength and stiffness.     

On the size dependence of the critical point of nanoscale interstitial solid solutions

It is shown that a size dependence of the critical temperature T_c of the miscibility gap in nanocrystalline and nanoscale particle interstitial solid solutions results from stress owing to elastic interaction of the bulk with layers of material at the grain boundaries or surfaces which have a small solute susceptibility (i.e. a weak dependence of the concentration on the chemical potential) at the phase transition. When the volume fraction occupied by the interfacial layers is not too large, then the changes in T_c and in the critical concentration x_c can be predicted on the basis of a series expansion of the solute chemical potential in the bulk about the critical point. The model can be extended to free-standing thin films and coherent multilayers. The dependence of the pressure on the hydrogen concentration in the crystal lattice of nanocrystalline palladium-hydrogen is measured on the basis of X-ray diffraction data. The result agrees with the predictions of the theory.     

Ab initio modelling of alumina

We have performed extensive ab initio modelling of alumina, calculating energy differences between various possible structures and highlighting the reasons why alumina forms in the corundum structure. We have also performed Harris-Foulkes calculations for these structures and demonstrate the remarkably accurate results that this method gives for these very complex structures.     

The inherent tensile strength of iron

The cohesive energy of Fe as a function of structure, strain and magnetic state has been computed using the full potential linearized augmented-plane-wave method within the framework of density functional theory and the generalized gradient approximation. Calculations corresponding to uniaxial stress in the <100> direction reveal that the ideal tensile strength of bcc Fe is about 14.2GPa and is determined by instability with respect to transformation into an unstable ferromagnetic fcc structure. The low-energy fcc phase is a modulatedantiferromagnetic fcc structure that is connected to the bcc phase via a first-order magnetic transformation and does not compromise its ideal strength.     

Assessment of tensile interphase profile of PVC/TiO2 polymer nanocomposites

The tensile properties of polymer nanocomposites depend dominantly on their interphase profile. The interfacial layer adhesion B-parameter, the relative interphase tensile strength (σ_IR) and the Z-interphase parameter for the central layer are quantified for assessment of tensile interphase properties of PVC/TiO₂ polymer nanocomposites. The redeveloped Pukanszky model is employed to determine the layer adhesion B-parameter of interphases. The study shows that the tensile strength of the interphase σ_I, as well as σ_IR and the B- and Z-parameters of the central layer of the interphase, are significantly correlated with the tensile strength of the nanocomposites.     

Energetics and kinetics of surfaces and interfaces in the Fe-Pb system

The energy and the diffusivity of interfaces in the solid Fe-liquid Pb system have been investigated in the temperature range 650-900°C. Grain-boundary grooves are formed at the solid Fe-liquid Pb interface and these have been studied by atomic force microscopy. From the topography of the grooves the relative interfacial energies and interfacial diffusivities are obtained. It is found that liquid Pb does not wet the grain boundaries in Fe. Possible mechanisms for the growth of the grain-boundary grooves are discussed.     

Bulk modulus of semiconductors and its pressure derivatives

Expressions for the bulk modulus and its first and second pressure derivatives for group I–VII, II–VI, III–V and IV–IV semiconductor binary compounds are derived using an ab initio pseudopotential approach to the total crystal energy within the framework of local density functional formalism. The computed results are very close to the available experimental data.     

Core structure of a〈100〉 superdislocations in a single-crystal superalloy during high-temperature and low-stress creep

A thorough TEM analysis has been carried out to study the dislocations cutting into γ′ phase in a single-crystal superalloy during uniaxial tensile creep under high-temperature and low-stress conditions. It is proved that the a〈100〉 edge superdislocation originates from the interfacial a〈100〉 dislocations and moves into the γ′ phase by pure climbing. And the dissociation of the a〈100〉 superdislocation core into two a/2〈101〉 superpartial dislocations during uniaxial tensile creep has been identified by HRTEM method for the first time.     

Study on the damping behaviour of eutectic high-entropy alloys with lamellar structures

ABSTRACT

Damping is the ability to reduce the energy produced by vibrations in a system. In this research, the eutectic high-entropy alloys (EHEAs), AlCo_xCrFeNi_2.1 (x?=?0.7, 1.0 and 1.5), are firstly studied for their damping behaviour. EHEA is a promising concept of high-entropy alloys. By combining the advantages of eutectic phases and HEAs, the EHEAs show better liquidity, castability and compositional homogeneity than conventional HEAs. Meanwhile, it can strike a great balance in mechanical strength and tensile ductility owing to the lamellar structures. The microstructures and crystal structure of two near-EHEAs are also studied. AlCo_0.7CrFeNi_2.1 is a hypo-eutectic HEA with primary body-centred cubic phases. AlCo_1.5CrFeNi_2.1 is a hyper-eutectic HEA with primary face-centred cubic phases. The results showed that these eutectic and near-eutectic alloys gave rise to greater tan δ values than conventional dual-phase HEAs, which might be attributed to their lamellar structures.     

Kinetics of formation of interfaces between immiscible polymers

The kinetics of the formation of the interfacial width between immiscible polymer pairs are studied using neutron reflectivity. Bilayers of deuterated polystyrene (d-PS) and poly(methyl methacrylate) of high molecular weight are used. The time dependence of the formation of the interface in both cases where the thickness of the top d-PS layer is thin (less than 200 Å) and thick (greater than 1000 Å) is investigated. Following an initial rapid increase, a logarithmic dependence of the interfacial width on time is observed. We interpret this result as arising from the slow equilibration of long-wavelength capillary-wave fluctuations; comparison of our results with theory yields a value of the interfacial tension in good agreement with the self-consistent field theory prediction.     

Dislocation core structures in GaN grown on Si(111) substrate

The Ga-N bond extension and compression at the cores of different types of dislocation in hexagonal GaN films grown on Si(111) substrates have been studied by high-resolution transmission electron microscopy. The relative bond extension and compression are about 6 4 and 7 4% respectively around the cores of mixed dislocations and about 9 6 and 12 7% respectively around the cores of pure edge dislocations. The core structure of a pure edge dislocation is an eightfold atom-column ring.     

Improved representation of metallic bonding in atomistic simulations

Previous use of the Sutton-Chen glue potential to model deformation processes in aluminium showed marked deviation from the results of an ab initio simulation prior to yield. The origin of this problem is identified, and a revised scheme is suggested and tested. It is based on the fact that the atomic volume is a key quantity in the metallic bonding of sp bonded elements, and our idea should therefore have wide applicability to interatomic bonding models. The scheme uses a new approximate but rapidly computable formula for the local atomic volume in an inhomogeneous system.     

The infrared absorption spectra of diamonds expected to contain voidites

Abstract

The infrared spectra of several type Ia diamond crystals expected to contain voidites have been examined in the wavenumber range 4000–2700 cm^?1. The absence of any absorption which could possibly be attributable to the stretching of N─H bonds makes unlikely recent suggestions that the voidites may contain a solid phase of NH₃.