The bonding charge density of β′NiAl

The (100), (110), (111) and (200) low-angle structure factors of β′NiAl have been accurately measured by high energy electron diffraction (HEED) and it was found that, except for the (200) reflection, these agreed closely with the values deduced by interpolation between the best pure element crystal atomic scattering (form) factors; the (200) structure factor was found to be about 2% lower than the best equivalent pure element value. The trend of these results is somewhat similar to those obtained from powder X-ray diffraction measurements made on β′NiAl over 25 years ago and indicates that there is no ionic component in the bonding scheme for this alloy. This was confirmed by the generation of deformation electron density distributions (DEDDS) from the structure factor data which showed that bonding is this alloy is achieved by the depletion of electrons from both the nickel and aluminum atom sites with extensive charge build-up at the mid-points between nearest-neighbor (n.n.) NiAl atoms (〈111〉 directions). This shows that bonding in this intermetallic compound is essentially covalent with some metallic character and no ionic component. The DEDDs also showed a large depletion of electrons between second n.n. AlAl atoms (〈100〉 directions) which also occurred, but to a lesser extent second n.n. NiNi atoms. This suggests that as well as the atomic size difference, there are electronic reasons why n.n. and second n.n. AlAl bonds are not favored in this B2 alloy type. This explains why β′FeAl, β′CoAl and β′NiAl are usually fully ordered and do not easily tolerate Al antistructure atoms on their transition metal sublattices. The build-up of electrons between n.n. atoms along 〈111〉 and depletion between second n.n. NiNi and AlAl atoms along 〈100〉 also gives indications of the electronic origins of the ω phase (collapse of atoms along 〈111〉 directions towards vacancy or antistructure atom defects) and the elastic softening associated with shear along [011] on (011), which occurs in β′NiAl containing more than about 53 at.% Ni.     

Interfacial structure and crystallographic studies of transformations in βt$́and β CuZn alloys—II. martensitic formation of α1′ plates in β′

A TEM study of the interphase boundary structure of 9R orthorhombic α₁' martensite formed in β′ CuZn alloys shows that it consists of a single array of dislocations with Burgers vector parallel to 〈110〉_β′ and spaced about 3.5 nm apart. This Burgers vector lies out of the interface plane; hence the interface dislocations are glissile. Unexpectedly, though, the Burgers vectors of these dislocations are not parallel when referenced to the matrix and the martensite lattices. This finding is rationalized on published hard sphere models as a consequence of relaxation of a resultant of the Bain strain and lattice invariant shear displacements within the matrix phase.     

Theory of bonding charge density in β′ NiAl

Fox and Tabbernor [Acta metall. mater.39, 669 (1991)] have recently measured the four lowest structure factors F(G) of NiAl using highly accurate high energy electron diffraction. We present here a systematic comparison of their results with ab initio band theory, in the context of the local density formalism. We find very good agreement for the three of the four lowest measured structure factors, while our F(200) is ∼0.4 e/cell higher. We tentatively attribute this difference to uncertainties in the treatment of the temperature factors in non-monoatomic compounds. Indeed, comparing with experiment our calculation for the monoatomic Si crystal (where the temperature term factors out), we find that theory reproduces the measured structure factors to within a very small deviations of ∼0.02 e/atom. We have also examined the effect of high Fourier components that are not currently amenable to measurements on the ensuing NiAl deformation electron density distribution (DEDD) maps. We find that the truncation of the Fourier series after four structure factors misses the directional d-like charge lobes near the Ni sites. We show that static and dynamic DEDD give a similar picture of the bonding.     

Kinetics and mechanism of oxidation of β-sialons

Conclusions We examined the behavior of two materials during high-temperature oxidation: the singlephase -sialon and the material based on -sialon containing up to 8% residual -Si₃N₄ and the glass phase produced by reaction sintering of the charge based on -modification of silicon nitride. The results show that the maximum oxidation resistance in the temperature range 800–1300°C is exhibited by the single-phase -sialon Si_5.5Al_0.5O_0.5N_7.5. The oxidation resistance decreases in the presence of the residual -Si₃N₄ and the glass phase in the material. The oxidation kinetics are also determined by the structure and composition of the oxide film. The kinetic curves are governed by a parabolic law which indicates that oxidation of the sialons is limited by the diffusion processes. No marked changes were detected in the lattice spacing of the sialon after oxidation.Translated from Poroshkovaya Metallurgiya, No. 6(306), pp. 69–73, June, 1988.     

Deformation and fracture behavior of a directionally solidified β/γ′ Ni-30 At. pct Al alloy

The effect of a ductile γ′-Ni₃Al phase on the room-temperature ductility, temperature-dependent yield strength, and creep resistance of β-NiAl was investigated. Room-temperature tensile ductility of up to 9 pct was observed in directionally solidified β/γ′ Ni-30 at. pct Al alloys, whereas the ductility of directionally solidified (DS), single-phase [001] β-NiAl was negligible. The enhancement in ductility was attributed to a combination of slip transfer from the ductile γ′ to the brittle β phase and extrinsic toughening mechanisms such as crack blunting, deflection, and bridging. As in single-phase Ni₃Al, the temperature-dependent yield strength of these two-phase alloys increased with temperature with a peak at approximately 850 K. The creep strength of the β/γ′ alloys in the temperature range 1000 to 1200 K was found to be comparable to that of monolithic β-NiAl. A creep strengthening phase needs to be incorporated in the β/γ′ microstructure to enhance the elevated temperature mechanical properties.     

Equilibria and microstructural evolution in the β/β′/γ′ region of the NiAlTi system: Modelling and experiment

As part of a comprehensive study of 3-phase alloys in the NiAlTi system incorporating the β (B2, NiAl type), β′ (L2₁, Ni₂AlTi type) and γ′ (L1₂, Ni₃Al type) phases, the phase equilibria for this part of the ternary diagram were modelled by CALPHAD methods. The approach used and the results obtained are presented, and compared with an experimental determination by analytical electron microscopy of the 3-phase equilibrium triangle at 900°C. The calculated/experimental 3-phase triangle differs significantly from earlier estimates. The theoretical model is also used to predict phase formation sequences for several alloys in the 3-phase triangle and these are critically compared with observed microstructures. An unusual graded antiphase domain structure seen in one alloy is interpreted.     

Synthesis and dissolution of slag minerals - a study of β-dicalcium silicate,pseudowollastonite and monticellite

Leaching of slag must be limited when slag is used in other areas than landfill. As slag is composed of different minerals, the properties of slag, inter alia the dissolution, depend on these minerals. If the leaching properties of each mineral are known, counter measurements can be taken to prevent the leaching of unwanted elements. In this study, the dissolution of three common slag minerals will be examined. The three minerals; boron-stabilised β-dicalcium silicate, pseudowollastonite and monticellite were synthesised. The dissolution was measured by setting pH to 4, 7 or 10 and adding 0.05 g mineral in the size range 20-38 mm and the HNO₃ consumption required to maintain the pH level was recorded during 40 h. As expected, the dissolution increases for all minerals when pH decreases. The boron-stabilised β-dicalcium silicate was the only mineral fully dissolved at all pH levels, while pseudowollastonite and monticellite only dissolved completely at pH 4.

On doit limiter le lessivage de la scorie lorsqu'elle est utilisée en d'autres endroits que la decharge publique. Puisque différents minéraux composent la scorie, ses proprieties, parmi d'autres la dissolution, dependent de ces minéraux. Si l'on connaTt les propriétés de lessivage de chaque mineral, on peut utiliser des contre-mesures pour empecher le lessivage d’éléments indésirables.

Dans cette étude, on a examine la dissolution de trois minéraux communs de la scorie. On a synthétisé les trois minéraux, soit le silicate β-dicalcique stabilise au bore, la pseudowallonite et la monticellite. On a mesuré la dissolution en établissant le pH à 4,7 ou 10 et en ajoutant 0.05 g de minéral dans la gamme de taille de 20 a 38 mm. On a ensuite enregistré pendant 40 heures la consommation d'HNO₃ requise au maintien du niveau de pH.

Comme on s'y attendait, la dissolution de tous les minéraux augmente avec la diminution du pH. Le silicate β-dicalcique stabilise au bore était le seul minéral complétement dissous a tous les niveaux de pH, alors que la pseudowallonite et la monticellite étaient complétement dissoutes seulement au pH de 4.     

Vacancy properties in β-NiGa and β-NiAl

The recently formulated theory of vacancy formation in CsCl-type ordered alloys has been applied to β-NiGa and β-NiAl. The observed Ni-sublattice vacancy concentrations in these compounds is well described by the theory. For the first time it is shown that structural vacancies most likely exist in Ga-rich NiGa and Al-rich NiAl at absolute zero temperature. It is also shown for the first time that in some ordered alloys, vacancy concentrations can decrease with increasing temperature.     

Crystallography of the ζ to β′ massive transformation in Ag-50 at.% Cd

The orientation relationship between massive β′ precipitates and the large grained ζ phase Ag-50 at.% Cd containing them were determined by the Laue method, and the orientation of planar interphase boundaries between the parent and product phases were determined by two-surface trace analyses. Computer assisted analysis of the orientation relationships demonstrated that many of them had recognizable low index orientation relationships with their matrix grains. All of the precipitates had orientation relationships which could be characterized in terms of nearly parallel low index planes and directions, but many of these orientation relationships were observed only once, and these may not have mechanistic significance. A total of 37 planar interphase boundaries were examined by two surface trace analysis and their orientations compared with low index lattice planes in both the parent and product phases. Few of the planar interfaces corresponded to low index lattice planes in either the parent or the product phases, and only one planar boundary corresponded to low index planes of both phases. These results show that the mechanism of massive transformations is more complex than has been previously assumed.     

Concurrent size and shape coarsening of γ′ in AlAg

The coarsening of γ′ precipitates in aluminum-silver has been investigated. Thesplate-shaped precipitates have coherent broad faces, and a hexagonal close packed crystal structure and develop on ∝111∝ planes of the aluminum matrix. During growth, a dispersion of nonequilibrium shaped γ′ precipitates develops. To understand the subsequent size and shape coarsening of these γ′ precipitates, several microstructural parameters have been experimentally determined as a function of aging time through quantitative electron microscopy. The measured thickening rate and nterledge spacing confirm that precipitate thickening takes place by a ledge mechanism and that the overall coarsening process of these precipitates is related to the average interledge spacing which remains relatively constant throughout the coarsening stage. The change in precipitate diameter does not appear to obey any temporal law and most likely accomodates the precipitate thickening and local supersaturation.     

Crystal Structure and Luminescence Property of Lanthanide Complexes with 2-Fluorobenzoic Acid and 2,2′-Bipyridine

Lanthanidemixed ligandscarboxylatecomplexes containing2,2′bipyridineor1,10phenanthrolinehavebeenwidelystudied[1～12],becausethesecom plexesexhibitvariouskindsofstructuraltypes,high stabilityandintensefluorescencecharacter.Inpartic ular,europiumcomplexeso…     

The defect arrangement in (non-stoichiometric) β′-NiAl

X-ray scattering similar to that for the diffuse omega state in Ti and Zr alloys has been detected with Ni_1+(−)xAl_1±x, and is strongest for non-stoichiometric compositions. A new method of analyzing diffuse X-ray scattering has been applied to the data to separate the effects of local order and atomic displacements. The defects (Ni vacancies in Al-rich alloys, Ni atoms on the AI sublattice for Ni-rich compositions) are locally ordered, exhibiting a strong tendency to avoid the same species at the first neighbor distance of their sublattice.The proposed theories of homogeneous nucleation of omega phase do not satisfactorily explain the data. Heterogeneous nucleation by collapse around a point defect appears to be more appropriate (that is, the trapping of phonon states by such defects). This is similar to the proposal by Kuan and Sass for Ti and Zr alloys, but their requirement of one per cent quenched in vacancies is eliminated because it is shown that substitutional atoms provide suitable nucleation sites.