Carbon Concentration Dependence of the Superconducting Transition Temperature and Structure of MgCxNi3

The crystal structure of the superconductor MgCxNi3 is reported as a function of carbon concentration determined by powder neutron diffraction. The single-phase perovskite structure was found in only a narrow range of carbon content, 0.88 < x < 1.0. The superconducting transition temperature was found to decrease systematically with decreasing carbon concentration. The introduction of carbon vacancies has a significant effect on the positions of the Ni atoms. No evidence for long range magnetic ordering was seen by neutron diffraction for carbon stoichiometries within the perovskite phase stability range.Comment: 4 figure

Quark Effects in the Gluon Condensate Contribution to the Scalar Glueball Correlation Function

One-loop quark contributions to the dimension-four gluon condensate term in the operator product expansion (OPE) of the scalar glueball correlation function are calculated in the MS-bar scheme in the chiral limit of $n_f$ quark flavours. The presence of quark effects is shown not to alter the cancellation of infrared (IR) singularities in the gluon condensate OPE coefficients. The dimension-four gluonic condensate term represents the leading power corrections to the scalar glueball correlator and, therein, the one-loop logarithmic contributions provide the most important condensate contribution to those QCD sum-rules independent of the low-energy theorem (the subtracted sum-rules).Comment: latex2e, 6 pages, 7 figures embedded in latex fil

Superplasticity of a nano-grained Mg-Gd-Y-Zr alloy processed by high-pressure torsion

While most of the reports on Mg-Gd-Y-Zr alloys report superplasticity after extrusion or friction stir processing, it is important to investigate superplasticity in these alloys after other severe plastic deformation processes having greater grain refinement capability. Accordingly, superplasticity was studied in an Mg–9Gd–4Y–0.4Zr (GW94) alloy after different high-pressure torsion (HPT) conditions. The HPT was performed at room temperature under an applied pressure of 6.0 GPa for up to 16 turns. TEM microstructural characterization revealed that the grain size was reduced from an initial value of ?8.6 ?m in the extruded condition to ?95±10 and ?85±10 nm after 8 and 16 turns, respectively. A shear punch testing method was used for evaluation of superplasticity at 573, 623, 673 and 723 K. Maximum strain rate sensitivities of ?0.51±0.05 and ?0.48±0.05 were obtained at 623 K for the material processed through 16 and 8 turns, respectively. This strain rate sensitivity and an activation energy of ?100±5 kJ mol–1 suggests the occurrence of grain boundary sliding in the superplastic regio

Effect of Long-Term Storage on Microstructure and Microhardness Stability in OFHC Copper Processed by High-Pressure Torsion

Tests are conducted to evaluate the effect of long-term storage on the microstructure and microhardness of an oxygen-free high conductivity (OFHC) copper after processing by high-pressure torsion (HPT) for various numbers of revolutions at ambient temperature. Results are presented for samples subjected to storage at room temperature through periods of either 1.25 or 7 years. The results show that an increase in storage time leads to a coarsening of the ultrafine-grained structure produced by HPT processing and a corresponding decrease in the microhardess where this is associated with the occurrence of recrystallization and grain growth. Plots of hardness against equivalent strain reveal a three-stage behavior with much lower hardness values over a range of equivalent strains of ~2-8. This behavior is similar after both storage periods but the hardness values are lower and the grain sizes are larger after storage for the longer time. The results demonstrate that long-term storage has a significantly detrimental effect on the microstructure and hardness of ultrafine-grained OFHC Cu

An investigation of the thermal stability of an Mg-Dy alloy after processing by high-pressure torsion

An Mg-0.41Dy (wt%) alloy was successfully processed by high-pressure tension (HPT) through 5 turns at room temperature. The evolution of the recrystallization microstructure and the texture and mechanical properties of the deformed alloy were investigated after annealing at 200 and 400 °C for 1 h using Electron Backscatter Diffraction (EBSD) and Vickers measurements. The recrystallization temperature and activation energy were evaluated using Differential Scanning Calorimetry (DSC). Processing by HPT led to significant grain refinement with an average grain size of ~0.5 ± 0.1 μm which increased to ~1.2 ± 0.8 μm after annealing at 400 °C. This slow increase in grain size at a high temperature demonstrates a good thermal stability of the microstructure. The alloy exhibited two main fiber textures after HPT processing: firstly a typical basal fiber (φ1 = 0–360°, Φ = 0° and φ2 = 0–60°) and secondly a fiber localized at φ1 = 180°, Φ = 60° and φ2 = 0–90°. These textures were retained after annealing at 400 °C. There was no change in the microhardness value after annealing at 200 °C (41 ± 1 Hv) and only a minor decrease after annealing at 400 °C (38.4 ± 0.5 Hv). The DSC results showed that the temperature associated with the recrystallization process increased with increasing heating rate and the activation energy for recrystallization was measured as ~25 kJ mol−1

Recrystallization in an Mg-Nd alloy processed by high-pressure torsion: a calorimetric analysis

Differential scanning calorimetry (DSC) was used to evaluate the recrystallization temperature and activation energy for an Mg-1.43Nd (wt.%) alloy after severe plastic deformation by high-pressure torsion (HPT) at room temperature up to 10 turns. The recrystallization kinetics were determined from DSC analysis. The results show that the recrystallization temperature increases with increasing heating rate and decreases with increasing numbers of HPT turns. Severe plastic deformation by HPT significantly reduces the recrystallization temperature. The estimated activation energy for recrystallization was in the range of ~ 84-89 kJ mol-1

An investigation of strain softening phenomenon in Al-0.1% Mg alloy during high-pressure torsion processing

An Al-0.1% Mg alloy was processed by high-pressure torsion (HPT) at room temperature. The Al-0.1% Mg alloy displays strain softening phenomenon through hardness evolution: the hardness values in the disc centre area are higher than at the disc edge area after 1/2, 1 and 3 turns, and the size of the hard region in the disc centre gradually reduces as the numbers of turns increases from 1/2 to 3 turns. The hardness values evolve towards homogeneity along the disc diameters after 5 and 10 turns. Electron backscatter diffraction (EBSD) and X-ray line profile analysis suggest that the lower hardness values at the disc edge area in the Al-0.1% Mg alloy are related to a recovery / recrystallization mechanism where the material is subjected to heavy straining

On the microstructure and mechanical properties of an Fe-10Ni-7Mn martensitic steel processed by high-pressure torsion.

High-pressure torsion (HPT) processing was applied to an Fe-10Ni-7Mn (wt.%) martensitic steel at room temperature and the grain size was reduced from an initial value of ~5.5 μm to an ultrafine value of ~185 nm for the ferritic phase and around 30 nm for the austenitic phase after 20 HPT turns. The microstructure and mechanical properties of the as-processed material were evaluated using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), field emission scanning electron microscopy (FESEM), microhardness measurements and tensile testing. In addition, annealing of an as-processed specimen was analyzed by differential scanning calorimetry (DSC). The results show that HPT processing increases the hardness and ultimate tensile strength to ~690 Hv and ~2230 MPa, respectively, but the ductility is decreased from ~16.5% initially to ~6.4% and ~3.1% after 10 and 20 turns, respectively. The hardness distributions and EBSD images show that a reasonably homogeneous microstructure is formed when applying a sufficient level of pressure and torsional strain. The DSC results demonstrate that processing by HPT reduces the start and finish temperatures of the reverse transformation of martensite to austenite and there is continuous re-crystallization after the recovery process

High temperature deformation in 2036 Al and 0.2 wt % Zr-2036 A1

The microstructure and high-temperature deformation of 2036 Al and a 0.2 wt % Zr modified 2036 Al were characterized. A particle-simulated- nucleation process was applied to refine grain structure in both alloys. Thermomechanically processed materials were tested from 450 to 500 C and strain rates from 2{times}10{sup {minus}1} to 2{times}10{sup {minus}4}s{sup {minus}1}. Strain rate sensitivity exponent, activation energy, and total elongation were measured, and the deformation mechanism was proposed. Effect of Zr on microstructure and deformation of 2036 Al at elevated temperatures was discussed

Wear resistant multilayer nanocomposite WC1−x/C coating on Ti–6Al–4V titanium alloy

A significant improvement of tribological properties on Ti–6Al–4V has been achieved by developed in this study multilayer treatment method for the titanium alloys. This treatment consists of an intermediate 2 μm thick TiCxNy layer which has been deposited by the reactive arc evaporation onto a diffusion hardened material with interstitial O or N atoms by glow discharge plasma in the atmosphere of Ar+O2 or Ar+N2. Subsequently, an external 0.3 μm thin nanocomposite carbon-based WC1−x/C coating has been deposited by a reactive magnetron sputtering of graphite and tungsten targets. The morphology, microstructure, chemical and phase compositions of the substrate material after treatment and coating deposition have been investigated with use of AFM, SEM, EDX, XRD, 3D profilometry and followed by tribological investigation of wear and friction analysis. An increase of hardness in the diffusion treated near-surface zone of the Ti–6Al–4V substrate has been achieved. In addition, a good adhesion between the intermediate gradient TiCxNy coating and the Ti–6Al–4V substrate as well as with the external nanocomposite coating has been obtained. Significant increase in wear resistance of up to 94% when compared to uncoated Ti–6Al–4V was reported. The proposed multilayer system deposited on the Ti–6Al–4V substrate is a promising method to significantly increase wear resistance of titanium alloys