Magnetism and solid solution effects in NiAl (40% Al) alloys

The solid solution effects of ternary additions of transition elements in intermetallic Ni–40% Al were investigated by both experimental studies and theoretical calculations. Co solute atoms when sitting at Ni sublattice sites do not affect the lattice parameter and hardening behavior of Ni–40Al. On the other hand, Fe, Mn, and Cr solutes, which are mainly on Al sublattice sites, substantially expand the lattice parameter and produce an unusual solid solution softening effect. First-principles calculations predict that these solute atoms with large unfilled d-band electrons develop large magnetic moments and effectively expand the lattice parameter when occupying Al sublattice sites. The theoretical predictions were verified by both electron loss-energy spectroscopy (EELS) analyses and magnetic susceptibility measurements. The observed softening behavior can be explained quantitatively by the replacement of Ni anti-site defects (potent hardeners) by Fe, Mn, and Cr anti-site defects with smaller atom size mismatch between solute and Al atoms. This study has led to the identification of magnetic interaction as an important physical parameter affecting the solid solution hardening in intermetallic alloys containing transition elements.     

Up-hill diffusion of hafnium in Ni3Al intermetallic alloys

An up-hill diffusion of hafnium was observed during the interdiffusion of Ni-Ni₃Al(Hf, B) couples. The Hf atoms diffused backward from the original interface to the Ni₃Al end, instead of moving forward to the Ni end under the concentration gradient. The observation confirms that the Hf atoms partition preferentially to the Ni₃Al phase by substituting for Al atoms in their sublattice sites.     

Effect of Ni-modified α-Al2O3 prepared by sol–gel and solvothermal methods on the characteristics and catalytic properties of Pd/α-Al2O3 catalysts

In the present study, Ni-modified α-Al₂O₃ with Ni/Al ratios of 0.3 and 0.5 were prepared by sol–gel and solvothermal method and then were impregnated with 0.3 wt.% Pd. Due to different crystallization mechanism of the two preparation methods used, addition of nickel during preparation of α-Al₂O₃ resulted in various species such as NiAl₂O₄, mixed phases between NiAl₂O₄ and α-Al₂O₃, and mixed phases between NiAl₂O₄ and NiO. As revealed by NH₃-temperature programmed desorption, formation of NiAl₂O₄ drastically reduced acidity of alumina, hence lower amounts of coke deposited during acetylene hydrogenation was found for the Ni-modified α-Al₂O₃ supported catalysts. For any given method, ethylene selectivity was improved in the order of Pd/Ni–Al₂O₃-0.5 > Pd/Ni–Al₂O₃-0.3 > Pd/Ni–Al₂O₃-0  Pd/α–Al₂O₃-commercial. When comparing the samples prepared by different techniques, the sol–gel-made samples showed better performances than the solvothermal-derived ones.     

Interaction between substitutional and interstitial solute atoms in α iron studied by isothermal mechanical spectroscopy

The interaction of interstitial nitrogen with substitutional alloying elements has been studied by measuring the Snoek relaxation in very dilute Fe–X–N ternary alloys (X: V, Cr, Mn, Mo). Isothermal measurements by the sub-resonance forced-vibration technique have revealed characteristic influence of substitutional solutes on the Snoek relaxation, which can be understood as trapping of N atoms at neighbouring sites of X atoms. The interaction energy has been evaluated by comparing the relaxation profiles observed experimentally with those calculated theoretically. The values thus obtained are of the order of 0.1 eV, with the magnitude being larger for elements of smaller group numbers, and are similar to those in the γ and liquid phases.     

Probing diffusional jumps in ordered compounds by anelastic relaxation spectroscopy

Intrinsic point defects may give rise to anelastic relaxation effects in ordered compounds in which some of the sublattice sites have lower symmetry than the host lattice. Detailed experiments have been carried out on one of such effects: the relaxation effect in Ni₃Al, which was interpreted to be due to stress‐induced reorientation of antisite Al atoms in the Ni‐sublattice (Numakura et al. 1999), focussing on the effects of deviation of composition from stoichiometry. The relaxation strength has been found to increase sensitively with increasing Al concentration, supporting firmly the earlier interpretation. The observed relaxation rates have been analysed on the basis of the mechanism of atomic diffusion (Numakura et al. 1998) to evaluate the diffusion coefficient of Al in Ni₃Al for various compositions.     

Plastic flow instabilities of L12 Ni3Al alloys at intermediate temperatures

The serrated plastic flow of L1₂ Ni₃Al alloys at intermediate temperatures was investigated using tensile tests. The effects of temperature, strain rate and composition were examined. The serrated plastic flow accompanied by the lowest (negative) strain-rate sensitivity was observed most strongly at 673 K and at a strain rate of 3.2 × 10^–4 s^–1. The serrated plastic flow became more significant as the alloy departed from a stoichiometric composition. The static strain aging at 673 K resulted in a reduced flow strength. The activation energy of the serrated plastic flow was estimated to be about 66 kJ/mol, which suggests that it is smaller than that for lattice diffusion of solutes in L1₂ lattices. The serrated plastic flow behavior of the Ni₃Al alloys was compared with that of the L1₂ Co₃Ti and Ni₃(Si,Ti) alloys, and is qualitatively explained on the basis of the dynamics of solutes in the core of dissociated screw dislocations.     

First-principles study of 4d solute diffusion in nickel

Diffusion of the 4d transition elements in Ni has been investigated within the five-frequency model framework using migration energy barriers calculated from the first principles. Agreement with counterintuitive experimental/calculated data is observed; atoms in the middle of 4d row have the smallest atomic radii while exhibiting the lowest diffusivity as compared to larger atoms at the beginning and the end of 4d row. We show that 4d solute diffusion is controlled mainly by the size misfit. The larger atoms have higher solute–vacancy binding energies and lower migration barriers. Both were shown to correlate with a displacement of the equilibrium solute position toward the adjacent vacancy. The difference in mechanisms controlling sp- and transition elements diffusion rates in Ni is discussed.     

Ab initio calculations of Fe–Ni clusters

The clusters of Fe, Ni, and Fe–Ni are investigated computationally using a density functional approach. The geometries of clusters are optimized under the constraint of well-defined point group symmetries at the UB3LYP/LanL2DZ level. The equilibrium geometries and binding energies are presented and discussed, together with natural populations and natural electron configurations. In addition, the binding energies of Fe_n−xNi_x clusters are found to generally decrease by successive substitutions of Ni atoms for Fe atoms. For Fe_n−xNi_x clusters, the comparisons on total energies between isomers indicate that Ni atoms energetically prefer clustering in the mixed Fe–Ni clusters. The calculations for Fe_n−xNi_x clusters show that the clustering leads to a segregation of Ni atoms from Fe atoms.     

Non-equilibrium grain boundary co-segregation of Boron and Magnesium in Ni3Al

The non-equilibrium grain boundary co-segregation of boron and magnesium in Ni₃Al–B–Mg alloys was determined by Auger electron spectroscopy (AES) in conjunction with ion sputtering in a cooling rate range of 0.05–269 K/s from temperatures of 1023, 1223 and 1373 K. The analytical expressions of diffusion rate equations describing non-equilibrium segregation process based on the concept that mobile solute-vacancy complexes migrating to grain boundaries is responsible for non-equilibrium grain boundary segregation of solute atoms in an alloy was used to simulate well the experimental results. The diffusion coefficients for boron atoms, boron-vacancy complexes, magnesium atoms and magnesium-vacancy complexes were determined, and the binding energy of boron-vacancy and magnesium-vacancy complexes was estimated.     

Microscopic phase-field study on atomic site occupation probability in L12-Ni3(Al1 − xFex) structure

The effect of Fe content on atomic site occupation in L1₂-Ni₃(Al_1 − xFe_x) structure at 1073 K was studied by using microscopic phase-field model. The results show that with increasing Fe, Fe atoms prefer to occupy the Al sites (β sites), and the site occupation probability (SOP) gradually increases, while the SOP of Al atoms on β sites decreases accordingly. Meanwhile, a small amount of Ni atoms also occupy β sites which is the so-called anti-site behavior; reversely, such behavior happens as a trace amount of Al and Fe atoms occupies α sites. Unlike Ni atoms, the anti-site behavior of Al and Fe atoms is strengthened with the increase of Fe content. Further, the precipitation mechanism, the size and the ordering degree of L1₂ structure also present obvious changes with the addition of Fe.     

Site preference of Zr in Ti<Subscript>3</Subscript>Al and phase stability of Ti<Subscript>2</Subscript>ZrAl

The site preference of Zr atoms in Ti ₃Al and the phase stability of Ti2ZrAl are examined using first-principles electronic structure total energy calculations. Of the sixteen possible ways in which Ti, Zr and Al atoms can be arranged, in the lattice sites corresponding to D0₁₉ structure of Ti₃Al, to obtain Ti₂ZrAl, it is shown that Zr atoms prefer to get substituted at the Ti sites. It is further shown that among the seven crystal structures considered, D0₁₉-like and L1₂-like are the competing ground-state structures of Ti₂ZrAl. The above results are in agreement with the experimental results reported in the literature. Calculated values of equilibrium lattice parameters, heat of formation and bulk modulus of Ti₂ZrAl are presented. The basis for the structural stability and bonding are analysed in terms of the density of states. Between the two possibleB2-like structures, Ti₂ZrAl shows enhanced stability for the one where Zr is substituted in the Ti sublattice, which again is in agreement with the experimental observation.     

Interaction of dissolved atoms and relaxation due to interstitial atoms in hcp metals

The literature data on the mechanism of internal friction maxima induced by O, N, and C in α-Ti, α-Zr, and α-Hf, are contradictory. They do not answer the question which kind of complexes induces relaxation: interstitial atoms or interstitial atoms with substitutional atoms. To clarify this question, modeling of the short-range order and atomic displacement fields around the solute atom clusters was carried out by the Monte-Carlo technique for typical Ti–O–Zr alloys. The energies of strain-induced (elastic) O–O and O–Zr interactions and displacement fields of host atoms around the solute atoms were calculated and used in modelling. The concentration dependence of relaxation strength due to diffusion under stress of oxygen atoms was evaluated using the values of local displacement around the solute atom complexes. It is shown that the developing short-range order cannot be described by the single O–O or O–Zr pair and the associated relaxation, as simple reorientation of any specific atomic pairs. It seems likely that in many cases the internal friction is caused by more complicated clusters constituted by interstitial and substitutional atoms.     

Study of the diffusion of Al–Li alloys subjected to an electric field

An experimental investigation of the homogenization treatment of 2091 Al–Li alloy in the presence of an electric field, has revealed the phenomena of reduced volume fraction, small size, spherical shape and random distribution of second-phase particles, which bring about an increase in ductility. The results show that the dissolution of second-phase particles is promoted by means of the vacancy mechanism, because of the larger diffusion coefficient of solute atoms than the vacancy–solute complexes at the beginning of the homogenization treatment. By increasing the homogenization time and applying an electric field, the diffusion coefficient of vacancy-solute complexes is raised, whereas that of solute atoms is reduced, because of the decreased potential energy of the second phase at grain boundaries. Therefore, the non-equilibrium segregation of magnesium and copper elements is generated near the surface of the ingot by a complex mechanism. An experimental study of the solution treatment under an electric field, revealed that the lithium non-equilibrium segregation is induced at grain boundaries responsible for the precipitates. © 1998 Chapman & Hall     

On the giga cycle fatigue behaviour of two-phase (α2+γ) TiAl alloy

Fatigue crack initiation behaviour is investigated at room temperature in the (α₂-Ti₃Al and γ-TiAl) alloy. High cycle fatigue tests ranging up to 10¹⁰ cycles are carried out on the powder metallurgy (P/M) bar specimens under different loading conditions with a stress ratio of R=0.1 and R=0.5. Microstructural characterization and fracture surface analysis are also investigated by optical (OM) and scanning electron microscopy (SEM). Ti–Al alloy studied here shows two phases in microstructure (nearly refined lamellar thickness) composed of α₂-Ti₃Al and γ-TiAl (hereafter called γ+α₂ alloys) and fracture mechanism is explained with different plastic incompatibilities between the two phases.     

Alloying element additions to Ni3Al: Site preferences and effects on elastic properties from first-principles calculations

First-principles calculations were performed to study the effects of alloying elements (Mo, Re, Ta, W, Ti, Co, Nb, Ru, Cr, Y) on the elastic properties of Ni₃Al. The site preferences of the alloying elements in Ni3Al at different temperature and concentrations were predicted. The influence of alloying elements on the lattice parameters of Ni₃Al were calculated and compared with the values fitted from experimental data. The effects of alloying elements on the elastic constants of Ni₃Al were present. The directional shear and Young’s moduli for single-crystal Ni₃Al alloys with alloying elements were estimated. The bulk, shear, and Young’s moduli of polycrystalline alloys were obtained. It is found that all the alloying elements occupy Al sites except Ru and Co, which may occupy both sites depending on concentration and temperature. All the elements increase shear and Young’s moduli of single-crystal Ni₃Al in all orientations except Cr, Co and Y. All the elements increase both bulk and shear moduli of polycrystalline Ni₃Al except Co and Y. The solute atoms with higher bulk modulus tend higher bulk modulus of Ni₃Al alloys, and the bulk modulus is related to the mole volume either.     

Rare earth elements in α-Ti: A first-principles investigation

The interaction energies between substitutional rare earth (RE) atoms, between RE and interstitial C, N, O, H atoms, as well as between RE and vacancies in α-Ti are calculated via first-principles density-functional theory with projector augmented-wave (PAW) pseudopotentials. The results show that the RE–vacancy and RE–RE interactions are attractive due to the weaker RE–Ti bond than the host Ti–Ti bond. All of the RE atoms investigated in this paper are repulsive to C and N, but attractive to H. RE–O interactions are repulsive for the light RE atoms, though the interactions are very weak for the heavy RE atoms. The mechanism underlying the interactions and their possible influence on the properties of Ti alloys are discussed.     

Effect of Cr and Ni on diffusion bonding of Fe3Al with steel

Microstructure at the diffusion bonding interface between Fe₃Al and steel including Q235 low carbon steel and Cr18-Ni8 stainless steel was analysed and compared by means of scanning electron microscopy and transmission electron microscopy. The effect of Cr and Ni on microstructure at the Fe₃Al/steel diffusion bonding interface was discussed. The experimental results indicate that it is favourable for the diffusion of Cr and Ni at the interface to accelerate combination of Fe₃Al and steel during bonding. Therefore, the width of Fe₃Al/Cr18-Ni8 interface transition zone is more than that of Fe₃Al/Q235. And Fe₃Al dislocation couples with different distances, even dislocation net occurs at the Fe₃Al/Cr18-Ni8 interface because of the dispersive distribution of Cr and Ni in Fe₃Al phase.     

Correlation of magnetic properties to oxygen and potassium stoichiometry in single-crystal Ba1?xKxBiO3?y

Recent theoretical calculations have suggested the coupling of electrons to high-energy oxygen phonons as an explanation of superconductivity in the Ba_1–x K _x BiO_3–y system. We have synthesized high-quality single crystals of the material and have examined the behaviors of critical field and critical current parameters as a function of changes in the oxygen content and in the Ba/K ratio. We have determined, via positron lifetime spectroscopy and singlecrystal X-ray measurements, that the oxygen stoichiometry in this system can be varied without significant impact on the metal atom sublattice. These results facilitate an investigation of the dependence of critical parameters on dopant and defect levels in this system.     

Electronic Structure of Non-oxide Perovskite Superconductor MgCNi3

The electronic structure of new superconducting perovskite MgCNi₃ has been studied using pure Hartree-Fock and density functional theory methods. The main peak of density of states is located below the Fermi level and it is dominated by Ni d. The results of total-energy calculations and electronic density calculations show that MgCNi₃ is not energy favorable in ferromagnetic state. Also we found that the bonding feature between C orbitals and Ni orbitals is covalent, but the bonding between Mg and Ni atoms is ionic or metallic.      

Influence of subgrain orientation on martensitic transformation in β1–Cu–Zn–Al-single crystals

In β₁–Cu–Zn–Al single crystals the course of cyclic martensititic transformation ‘β₁ parent phase↔γ′₁ martensite’ induced by tensile stress were studied with use of X-ray topography, light microscopy and etch pits. Two groups of single crystals were studied. The first one (OR) contained single crystals of subgrain boundaries parallel to the direction of elongation [001], the second one (RA) consisted of single crystals of random subgrain boundaries orientations. Single crystals from the RA group cracked after about 300 cycles of martensitic transformation; single crystals from the OR group did not crack even after 1200 cycles. In OR single crystals changes of dislocation density inside the subgrains caused by cycling occurred much more slowly than in the RA single crystals. This has been related to the dislocation movement from inside the subgrains to their boundaries.