Dynamic strain ageing in Ni-base superalloy 720Li

An experimental investigation into the dynamic strain ageing (DSA) of a wrought Ni-base superalloy 720Li was conducted. Characteristics of jerky, flow have been studied at intermediate temperatures of 350, 400 and 450 degrees C at strain-rates between 10(-3) and 10(-5) s(-1). Serrations of Type C are predominant within the temperature/strain-rate range explored. The major characteristics of the serrations-i.e. (a) critical plastic strain for onset of serrations, epsilon(c); (b) average stress decrement, Delta sigma(avg); and (c) strain increment between serrations. Delta epsilon(BS)-have been examined at selected temperatures and strain-rates. Negative strain-rate sensitivity was observed in the DSA regime. However. temperature did not influence tensile properties such as yield strength, ultimate strength. elongation, reduction in area, and work hardening rate or fracture features in DSA regime. Analysis of the results Suggests that locking of the mobile dislocations by substitutional alloying elements is responsible for the DSA in alloy 720Li.

The Broad-Line Type Ic Supernoava SN 2007ru: Adding To The Diversity Of Type Ic Supernovae

Photometric and spectral evolution of the Type Ic supernova SN 2007ru until around 210 days after maximum are presented. The spectra show broad spectral features due to very high expansion velocity, normally seen in hypernovae. The photospheric velocity is higher than other normal Type Ic supernovae (SNe Ic). It is lower than SN 1998bw at similar to 8 days after the explosion, but is comparable at later epochs. The light curve (LC) evolution of SN 2007ru indicates a fast rise time of 8 +/- 3 days to B-band maximum and postmaximum decline more rapid than other broad-line SNe Ic. With an absolute V magnitude of -19.06, SN 2007ru is comparable in brightness with SN 1998bw and lies at the brighter end of the observed SNe Ic. The ejected mass of Ni-56 is estimated to be similar to 0.4 M-circle dot. The fast rise and decline of the LC and the high expansion velocity suggest that SN 2007ru is an explosion with a high kinetic energy/ejecta mass ratio (E-K/M-ej). This adds to the diversity of SNe Ic. Although the early phase spectra are most similar to those of broad-line SN 2003jd, the [O I] line profile in the nebular spectrum of SN 2007ru shows the singly peaked profile, in contrast to the doubly peaked profile in SN 2003jd. The singly peaked profile, together with the high luminosity and the high expansion velocity, may suggest that SN 2007ru could be an aspherical explosion viewed from the polar direction. Estimated oxygen abundance 12 + log(O/H) of similar to 8.8 indicates that SN 2007ru occurred in a region with nearly solar metallicity.

Preparation, structure, and magnetic properties of isostructural La3MAlS7 and La3MFeS7 (M = Mg, Mn, Fe, Co, Ni, or Zn)

A series of quaternary metal sulfides of the general formula La3MM′S7 (M = Mn, Fe, Co; M′ = Al and M = Mg, Mn, Fe, Co, Ni; M′ = Fe) consisting of linear chains of face shared MS6 octahedra and isolated M′S4 tetrahedra has been prepared and studied. The aluminium compounds La3MAlS7 (M = Mn, Fe, Co) exhibit linear chain antiferromagnetism. Magnetic behavior of other La3MFeS7 sulfides has been examined in detail. The magnetic susceptibility of La3MgFeS7 shows that tetrahedral site Fe3+ undergoes a transition from Image to S = 2 spin state around 150 K.

O2- and O1- types of oxygen species on Ni and barium-dosed Ni and Cu surfaces

He II UPS and XPS study of oxygen adsorption on Ni and barium-dosed Ni and Cu surfaces at 300 K show two types of oxygen species which are assigned to O2- and O1- (ad).

Ce CORE level X-ray photo electron spectroscopy of CeX2Si2 (X = Fe, Co, Ni and Cu)

Ce(3d) and (4d) core level XPS spectra of CeX = Fe, Co, Ni and Cu) suggest that the mean valence of Ce was as well as 4f hybridization strength decrease systematically from Fe to Cu. This observation is in agreement with the results of Bremstrahlung Isochromat Spectroscopy (BIS), but in disagreement with LIII-edge data reported earlier.

Low-temperature stabilization of pure Ni(IT)O

Ordinary nickel (II) oxide prepared by any of the usual methods is always contaminated with some higher valency states of nickel. Such nickel oxide however can be converted to pure nickel (II) oxide by soaking for some hours at 800–850°C in a closed system under a static inert atmosphere and in the presence of pure MnO as oxygen-getter. The resulting Ni(IT)O is highly resistant to oxidation.

A Theoretical Investigation of the Ni(II)-Catalyzed Hydrovinylation of Styrene

We report a detailed and full computational investigation on the hydrovinylation reaction of styrene with the Ni(II)-phospholane catalytic system, which was originally presumed to proceed through a cationic mechanism involving a nickel hydride intermediate. The following general features emerge from this study on a specific catalyst complex that was found to give quantitative yield and moderate selectivity: (a) the activation barrier for the initiation (18.8 kcal/mol) is higher than that for the reaction due to a low-lying square-planar pentenyl chelate intermediate originating from a Ni(II)-allyl catalyst precursor. Consequently there is an induction period for the catalysis; (b) the exit of product from the catalyst is via a β-H-transfer step instead of the usual β-H elimination pathway, which has a very high activation energy due to a trans effect of the phospholane ligand; (c) the turnover-limiting and enantio- determining transition state is also the β-H-transfer; (d) because of the absence of a hydride intermediate, the unwanted isomerization of the product is prevented; (e) since the enantio-discrimination is decided at the H-transfer stage itself, the configuration of the product in a catalytic cycle influences the enantioselectivity in the subsequent cycle; (f) the trans effect of the sole strong ligand in the d8 square-planar Ni(II), the stability of the η3-benzyl intermediate, and the availability of three coordination sites enable regioselective hydrovinylation over the possible oligomerization/polymerization of the olefin substrates and linear hydrovinylation. This work has also confirmed the previously recognized role of the hemilabile group at various stages in the mechanism.

Small nickel metal particles in Ni---Al2O3 metal-ceramic composites

Magnetic measurements have been used in combination with transmission electron microscopy to investigate small nickel metal particles in metal-ceramic composites. Estimates of the average number of atoms in the particles are given for nonmagnetic samples with low Ni content.

Stress-induced phase transformation and pseudo-elastic/pseudo-plastic recovery in intermetallic Ni-Al nanowires

Extensive molecular dynamics (MD) simulations have been performed in a B2-NiAl nanowire using an embedded atom method (EAM) potential. We show a stress induced B2 -> body-centered-tetragonal (BCT) phase transformation and a novel temperature and cross-section dependent pseudo-elastic/pseudo-plastic recovery from such an unstable BCT phase with a recoverable strain of similar to 30% as compared to 5-8% in polycrystalline materials. Such a temperature and cross-section dependent pseudo-elastic/pseudo-plastic strain recovery can be useful in various interesting applications of shape memory and strain sensing in nanoscale devices. Effects of size, temperature, and strain rate on the structural and mechanical properties have also been analyzed in detail. For a given size of the nanowire the yield stress of both the B2 and the BCT phases is found to decrease with increasing temperature, whereas for a given temperature and strain rate the yield stress of both the B2 and the BCT phase is found to increase with increase in the cross-sectional dimensions of the nanowire. A constant elastic modulus of similar to 80 GPa of the B2 phase is observed in the temperature range of 200-500 K for nanowires of cross-sectional dimensions in the range of 17.22-28.712 angstrom, whereas the elastic modulus of the BCT phase shows a decreasing trend with an increase in the temperature.

Low cycle fatigue behaviour of a low interstitial Ni-base superalloy

The low cycle fatigue behaviour of precipitation strengthened nickel-base superalloy 720Li containing a low concentration of interstitial carbon and boron was studied at 25, 400 and 650 degrees C. Cyclic stress response at all temperatures was stable under fully reversed constant total strain amplitude (Delta epsilon/2) when Delta epsilon/2 <= 0.6%. At Delta epsilon/2 > 0.6%, cyclic hardening was followed by softening, until fracture at 25 and 650 degrees C. At 400 degrees C, however, cyclic stress plateaued after initial hardening. Dislocation-dislocation interactions and precipitate shearing were the micromechanisms responsible for the cyclic hardening and softening, respectively. The number of reversals to failure vs. plastic strain amplitude plot exhibits a bilinear Coffin-Manson relation. Transmission electron microscopy substructures revealed that planar slip was the major deformation mode under the conditions examined. However, differences in its distribution were observed to be the cause for the bilinearity in fatigue lives. The presence of fine deformation twins at low Delta epsilon/2 at 650 degrees C suggests the role of twinning in homogenization of cyclic deformation.

A kinetic study of the methanation of CO2 over Ni-Al2O3 catalyst

CO2 methanation has been studied over Ni-Al2O3 and the intrinsic kinetic data obtained are related by a powerlaw type rate expression, which established a good agreement between calculated and experimental values of conversion.

Mixed valency in the high-temperature phases of transition metal molybdates,AMoO4 (A=Fe, Co, Ni)

Transition metal molybdates of the formulaAMoO4 whereA=Fe, Co or Ni exhibit a first-order phase transition between 670K–970K. An investigation of the lowtemperature (lt) and high-temperature (ht) phases by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, magnetic susceptibility and other physical methods shows that the phase transition is associated with a valence change of the typeA 2++Mo6+αA 3++Mo5+ in the cases of iron and cobalt molybdates.