Phase stability of Al3Nb as a function of nickel additions

Abstract

Phase stability of the intermetallic compound Al₃Nb is investigated as a function of nickel additions by band structure calculations based on an extended Hueckel tight-binding method. With this method, the electronic structure and total energies of the Al_6-n -Ni _n Nb₂ compounds (where n is an integer from 0 to 6) are calculated for both D0₂₂ and L1₂ structures under the assumption that nickel substitutes for aluminium. The electronic total energies obtained from integration of the energy states of all electrons considered have shown that the D0₂₂ structure is stable in the binary Al₃Nb compound as compared with the L1₂ structure; conversely, the L1₂ structure is stable for compounds with n values greater than 1. These calculations are in good agreement with X-ray diffraction results reported by Schubert and co-workers. Differences in the electronic total energy per atom between D0₂₂ and L1₂ are - 0·79 eV and 0·56 eV for n equal to 0 and 2, respectively.     

Friction and wear behaviour of Inconel 718 alloy fabricated by selective laser melting after heat treatments

ABSTRACT

The effect of heat treatments (solution and double aging) on friction and wear behaviour of Inconel 718 (IN718) alloy fabricated by selective laser melting (SLM) were studied. After heat treatment of solution and double aging, the friction and wear of the alloy have been improved. The worn surfaces of heat-treated IN718 alloy became smoother, and micro-ploughing became shallower. After solid solution hearting, a portion of the Laves phase ((Ni, Fe, Co)₂(Nb, Ti, Mo)) dissolves into the matrix, after which the γ″ phase (Ni₃Nb) andthe equilibrium phase of the δ phase (Ni₃Nb) precipitates during double ageing. After solution and double aging, nano-scale γ′ (Ni₃(Al, Ti)) and γ″ phases distribute homogeneously in the matrix, which improves the material hardness and wear resistance of the SLMed IN718 alloy.     

Thermomagnetic gravimetry study of crystallization behaviour in amorphous Fe-Si-B-Nb alloys

The effect of the Nb content on the crystallization process of amorphous Fe84- xSi6B10Nbx alloys has been investigated using thermomagnetic gravimetry (TMG). In the TMG measurement, the sample weight was electronically zerobalanced, and then an apparent weight (AW) applied by placing a magnet beneath the sample pan. The heating treatment was carried out over temperature range between 373 and 1093 K, in order to detect the formation of a ferromagnetic crystallization-induced phase. The form of the TMG curve changed markedly in the vicinity of 2-3 at.% Nb. In the alloys containing less than 2 at.% Nb, a two-stage increase in AW was observed with increasing temperature, but only a single-stage increase occurred for the alloys containing more than 3 at.% Nb. On the basis of these results, it is inferred that the Nb content plays a key role in the structure of the amorphous phase, supporting our recent claim for a mechanism of nanocrystallization.     

Atomistic calculation of the interaction between an edge dislocation and a free surface

By using molecular statics, we compute the variation in the excess energy associated with the gradual approach of an edge dislocation towards the free surface of a crystal. The calculations rely on a phenomenological potential adapted to aluminium and an appropriate constraint procedure that allows investigations of both the extended and the perfect configurations of the dislocation core. Thereby, an estimation of the energy required for the introduction of a dislocation in a thin film is obtained.     

Positive and negative affectivity,stress, and well-being in African-Americans: Initial demonstration of a polynomial regression and response surface methodology approach

Objective: The extent to which positive (PA) and negative (NA) affect conjointly impact well-being is not yet well understood. Additionally, research investigating the role of affectivity in maintaining well-being among ethnic and racial minorities is scant. The current research demonstrates how polynomial regression and response surface methodology (PR and RSM) may be used to better understand how PA and NA jointly influence stress and well-being.

Design: In Study 1, 291 African-American undergraduates (M_age = 22.91, SD = 6.91; 67% female) completed measures of affectivity and psychological well-being. In Study 2, a community sample of 117 African-Americans (M_age = 31.87, SD = 13.83; 69% female) completed affectivity measures and a laboratory-based social stressor task to assess links between affectivity and salivary cortisol reactivity.

Main outcome measures: Study 1 included life satisfaction, perceived stress and self-reported depressive symptoms. Study 2 included salivary cortisol reactivity.

Results: Across both studies, PA ascendency (i.e. high PA combined with low NA) was associated with better well-being, while NA ascendancy (i.e. high NA combined with low PA) was associated with poorer outcomes.

Conclusion: PR and RSM may provide new insight into the conjoint influence of PA and NA on health and well-being. We discuss potential implications for affectivity research, including race-related explorations.     

Shear horizontal surface waves in piezoelectric materials with surface stress

ABSTRACT

The propagation of shear horizontal (SH) surface waves in piezoelectric semi-infinite bodies is investigated theoretically. The surface stress exerting on the boundary is taken into account through surface piezoelectricity theory, and the velocity ranges and existence conditions of such surface waves for both electrically open and shorted cases are derived in detail. Analysis results show that the SH surface waves occurring with surface stress are dispersive in contrast to the classical situation, and the propagation characteristics strongly depend on the relative value of surface elasticity, surface piezoelectricity and surface density.     

Low-energy boron implants into silicon: A molecular dynamics simulation study of implants into a stepped surface

Molecular dynamics simulations have been conducted to study implantation of boron with a low kinetic energy (E _k in the range 5 eV) into the silicon surface. The bombarded surface has a realistic stepped structure and the geometry of the step edge is taken from the theory of Chadi (1987, Phys. Rev. Lett., 59, 1691). It is found that reflection is the more common event. However subsurface implants and adatom formations are also observed. The effect of the step morphology is important and different trajectories are observed if the ions hit steps of different shapes.     

Identifying the character of intrinsic stacking faults in the A15 compound Nb3Al

The character of intrinsic stacking faults on {001} in the A15 intermetallic compound Nb₃Al has been investigated using energy-dispersive X-ray spectroscopy in a field-emission-gun transmission electron microscope. Line scans were obtained across edge-on faults with an electron beam of approximate diameter 0.5 nm and it was found that the Al concentration is enhanced at the fault. Taken in conjunction with crystallographic data obtained previously, this implies that a {004} layer composed of Nb atoms only is removed to form these intrinsic stacking faults in Nb₃Al. The configuration of these faults implies that they may form by Nb vacancy coalescence and could act as embryonic nuclei for the precipitation of the D8_bcompound Nb₂Al.     

The surface topography of cracks in strained In0.72Ga0.28P films

Cracks in a 2% tensile strained In_0.72Ga_0.28P film grown on an InP substrate by molecular-beam epitaxy have been studied by cross-section transmission electron microscopy and scanning probe microscopy. A dislocation analogue (i.e. replacing the crack by an array of equivalent infinitesimal edge dislocations) is employed to account for the ratio of the crack-opening displacement to the normal surface displacement associated with the crack.     

High-angle annular dark field scanning transmission electron microscopy of the antiphase boundary in a rapidly solidified B2 type TiPd compound

High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with Z-contrast is applied to characterize the antiphase boundary (APB) of the B2 structure in a rapidly solidified TiPd melt-spun compound. The atomic shift associated with the R ?=?(1/2) a ₀ ?111? type displacement vector is directly observed at the boundary. A microstructure modification of the melt-spun compound with the cooling rate during solidification is also described.     

Energetic effects of dopants on the eutectoid decomposition of Nb–Si in situ composites

The influences of doping elements (Ti, Zr, Hf, V, Ta, Cr, Mo, W and Al) on the heats of formation of Nb, Nb₅Si₃ and Nb₃Si have been studied using first-principles pseudopotential plane-wave method based on density functional theory. Site preferences of dopants in Nb₅Si₃ and Nb₃Si intermetallics are first determined by comparing the heats of formation of the systems with different site occupations of the doping atoms. The partitionings of dopants between the equilibrium phases Nb and Nb₅Si₃ of Nb–Si in situ composites are then discussed and compared with experimental results. Phase partitioning behaviours of the alloying elements are found to depend strongly on the number of their valence electrons. By calculating the enthalpy of reaction regarding the eutectoid decomposition of Nb₃Si phase, we conclude that Ta is a Nb₃Si stabiliser while other dopants destabilise Nb₃Si at the investigated concentration.     

High-resolution in situ electron microscopy of a silicon surface modification by molten aluminium at high temperatures

High-resolution in situ heating experiments in a transmission electron microscope have been applied to a study of the surface modification of Si by molten Al in a vacuum of 1 10^-5 Pa. Transformation from an atomically rough to an atomically flat surface was induced by wetting of molten Al atoms. Si {110}, {112}, {115} and {773} surfaces show sawtooth-like structures composed of nanofacets. It is concluded that molten Al atoms remove a native amorphous oxide layer which existed on the original Si surface, making the surface 'clean' even in a non-ultra-high vacuum such as 10^-5Pa.     

Thermomagnetic study of devitrification in Fe-Si-B-Cu-Nb(-X) alloys

Thermomagnetic measurements have been used to study the magnetic and structural changes occurring at the two steps of the crystallization process of Fe_73.5Si_13.5B₉Cu₁Nb₁X₂ (X = Zr, Nb, Mo and V) alloys. Alloying raises the thermal stability of the amorphous phase against nanocrystallization in the order V < Mo < Nb < Zr and some differences in the final crystalline phases are found. The Curie temperature of the amorphous phase increases (about 15K) during structural relaxation. In the course of nanocrystallization a further increase of about 30K in the Curie temperature of the amorphous intergranular is observed for samples with X = Zr, Nb and Mo, but only of about 15K for samples containing V. The observed increase in the Curie temperature of the Fe-Si phase between the end of the first crystallization process and the end of the second crystallization process is associated with a reduction in the Si content, in agreement with X-ray diffraction results.     

Hexagonal surface structure during the first stages of niobium growth on sapphire (1120)

A detailed reflection high-energy electron diffraction study of the first stages of the niobium growth on (1120)s sapphire is presented for several substrate temperatures. It is shown that the niobium film exhibits an hexagonal surface structure when the deposited thickness is smaller than a critical value, which, depending on the substrate temperature, varies between 5 and 15 A. For thicknesses larger than this critical thickness, the surface hexagonal structure relaxes to the (110) bcc niobium structure. The hexagonal surface structure is observed for high substrate temperatures (820-410oC) but does not appear when the substrate temperature is 270oC. The epitaxial relationships between the substrate, the surface hexagonal structure of niobium and the cubic niobium phase are presented.     

Temperature-dependent crack surface tension

The fracture surface energy is calculated for a one-dimensional, exactly solvable model of a crack. The temperature dependence of the crack surface tension is determined on the basis of a self-consistent Einstein approach. It is found that lattice vibrations result in a strong reduction in the crack surface tension.     

Long-range interaction of radiation-generated self-interstitial atoms and adatoms on tungsten surface

ABSTRACT

The morphological modification of the W surface induced by specimen irradiation with 5?keV He atoms was studied at the atomic level using low-temperature ?eld ion microscope techniques. The major impact-induced damage mode on W {110} surface was found to be the formation of adatom sets produced by radiation-generated self-interstitial atoms emerging at the surface. The long-range interaction was revealed by evaluating the experimental probability density of finding two radiation-generated adatoms at a given separation in comparison with the probability density corresponding to a random distribution of adatoms. The statistically significant difference of these distributions consists of the absence or severe reduction of observed adatom pairs with separations in the range 5–11?Å. The experimental distribution was characterised by a relatively open arrangement of adatoms with the closest separation of about 11?Å. Our field ion microscope observations also reveal the self-organisation of radiation-generated W adatoms into atomic chains with the lateral interatomic separation substantially larger than the nearest-neighbor distance. These effects indicate the existence of long-range interactions between radiation-generated adatoms and radiation-generated self-interstitial atoms in the subsurface region.     

What do pre-adolescents and adolescents understand about HIV/AIDS?

Abstract

Pre-adolescents and adolescents (N= 80, aged 10, 12, 15, and 18 years) were tested on their knowledge of HIV/AIDS. Knowledge was assessed using a self-report questionnaire (examining levels of factual knowledge) and a semi-structured interview (examining cognitive developmental levels of understanding). Examination of the developmental shifts in understanding of HIV/AIDS revealed an increase in conceptual complexity on causal and prevention dimensions. The main conceptual changes included a shift from single cause to multiple cause solutions and a shift from identifying surface or immediate features to implicating less salient features. Additionally, with age, concepts about HIV/AIDS were increasingly organised around internal, physiological principles. Questionnaire responses yielded significant age-related increases in knowledge. Level of knowledge, as measured by the questionnaire, was higher than that assessed by the interview for the three youngest age groups. Thus, factual information acquired about HIV/AIDS is not automatically accompanied by understanding. In light of these findings the past conclusion that adolescents have high levels of knowledge of HIV/AIDS is questioned.     

Influence of In substitution and plastic deformation on AsGa-related photoluminescence in GaAs

Abstract

Low-temperature photoluminescence experiments have been carried out on semi-insulating GaAs crystals undoped or containing ～5 × 10¹⁹ In atoms cm^?3. A broad band peaking around 0·8eV is observed which is generally related to the antisite defect As_Ga. The effect of In substitution or plastic deformation is to shift this band towards higher energies by 10–25 meV. This positive energy shift is quantitatively accounted for by considering the stress fields induced by the incorporation of indium or the creation of dislocations.     

Observation of planar stacking faults in a Ti-rich two-phase Ti-Al alloy after deformation at elevated temperatures

It is a common observation that in two-phase Ti-Al-based binary alloys, deformation of the gamma phase occurs by 1/2<110]-type ordinary dis-location activity and twinning associated with 1/6<112] type partials. In the present study the microstructure of a new Ti-Al-based alloy (Ti-47at.% Al-2at.%Mn-2at.%Nb+ 0.8 vol.% TiB₂) with a duplex microstructure consisting of primary equiaxed gamma grains and lamellar alpha₂+ gamma colonies was studied by transmission electron microscopy (TEM) after deformation at elevated temperatures. Planar stacking faults were found in the gamma laths. A detailed contrast analysis by TEM shows that these planar stacking faults lying on {111} planes are bound by all the fcc variants of the Shockley partial dislocations of type 1/6<121>, in contrast with the observations in stoichiometric binary TiAl alloys, where only 1/6<112]-type Shockley partials are found to be associated with the true twins. It is proposed that the addition of ternary and quaternary elements such as Mn and Nb promotes the other variants of the fcc-like dissociations (not common in L1₀structure) in the present alloy.