The effect of grain size on the twin initiation stress in a TWIP steel

The influence of grain size on the twinning stress of an Fe-15Mn-2Al-2Si-0.7C Twinning Induced Plasticity (TWIP) steel has been investigated. Five grain sizes were obtained using a combination of cold rolling and annealing. Electron backscatter diffraction (EBSD) analysis revealed that the material exhibited a typical cold rolled and annealed texture. Tensile testing showed the yield stress to increase with decreasing grain size, however, the ductility of the material was not substantially affected by a reduction in grain size. Cyclic tensile testing at sub-yield stresses indicated the accumulation of plastic strain with each cycle, consequently the nucleation stress for twinning was determined. The twin stress was found to increase with decreasing grain size. Furthermore, the amount of strain accumulated was greater in the coarser grain material. It is believed that this is due to a difference in the twin thickness, which is influenced by the initial grain size of the material. SEM and TEM analysis of the material deformed to 5% strain revealed thinner primary twins in the fine grain material compared to the coarse grain. TEM examination also showed the dislocation arrangement is affected by the grain size. Furthermore, a larger fraction of stacking faults was observed in the coarse-grained material. It is concluded that the twin nucleation stress and also the thickness of the deformation twins in a TWIP steel, is influenced by the initial grain size of the material. In addition grain refinement results in a boost in strength and energy absorption capabilities in the material

Microscopic evidence for field-induced magnetism in CeCoIn5_5

We present NMR data in the normal and superconducting states of CeCoIn$_5$ for fields close to $H_{\rm c2}(0)=11.8$ T in the $ab$ plane. Recent experiments identified a first-order transition from the normal to superconducting state for $H> 10.5$ T, and a new thermodynamic phase below 290 mK within the superconducting state. We find that the Knight shifts of the In(1), In(2) and the Co are discontinuous across the first-order transition and the magnetic linewidths increase dramatically. The broadening differs for the three sites, unlike the expectation for an Abrikosov vortex lattice, and suggests the presence of static spin moments in the vortex cores. In the low-temperature and high-field phase the broad NMR lineshapes suggest ordered local moments, rather than a long wavelength quasiparticle spin density modulation expected for an FFLO phase.Comment: 4 pages, 4 figures. to appear in Phys. Rev. Let

Unconventional superconductors under rotating magnetic field II: thermal transport

We present a microscopic approach to the calculations of thermal conductivity in unconventional superconductors for a wide range of temperatures and magnetic fields. Our work employs the non-equilibrium Keldysh formulation of the quasiclassical theory. We solve the transport equations using a variation of the Brandt-Pesch-Tewordt (BPT) method, that accounts for the quasiparticle scattering on vortices. We focus on the dependence of the thermal conductivity on the direction of the field with the respect to the nodes of the order parameter, and discuss it in the context of experiments aiming to determine the shape of the gap from such anisotropy measurements. We consider quasi-two dimensional Fermi surfaces with vertical line nodes and use our analysis to establish the location of gap nodes in heavy fermion CeCoIn$_5$ and organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$.Comment: 17 pages, 13 figure

Supercomputer Simulations of Disk Galaxies

The time evolution of models for an isolated disk of highly flattened galaxies of stars is investigated by direct integration of the Newtonian equations of motion of N=30,000 identical stars over a time span of many galactic rotations. Certain astronomical implications of the simulations to actual disk-shaped (i.e. rapidly rotating) galaxies are explored as well.Comment: 3 pages, 2 figure Aat.sty, Aattable.sty, presented by E. Griv at the JENAM 2000, S02, Moscow, Russia, 200

Superelastic load cycling of Gum Metal

The superelastic beta titanium alloy, Gum Metal, has been found to accumulate plastic strain during tensile load cycling in the superelastic regime. This is evident from the positive drift of the macroscopic stress vs. strain hysteresis curve parallel to the strain axis and the change in its geometry subsequent to every load-unload cycle. In addition, there is a progressive reduction in the hysteresis loop width and in the stress at which the superelastic transition occurs. In situ synchrotron X-ray diffraction has shown that the lattice strain exhibited the same behaviour as that observed in macroscopic measurements and identified further evidence of plastic strain accumulation. The mechanisms responsible for the observed behaviour have been evaluated using transmission electron microscopy, which revealed a range of different defects that formed during load cycling. The formation of these defects is consistent with the classical mathematical theory for the bcc to orthorhombic martensitic transformation. It is the accumulation of these defects over time that alters its superelastic behaviour

Effect of annealing on the specific heat of Ba(Fe1-xCox)2As2

We report on the effect of annealing on the temperature and field dependencies of the low temperature specific heat of the electron-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ for under-(x = 0.045), optimal- (x = 0.08) and over-doped (x = 0.105 and 0.14) regimes. We observed that annealing significantly improves some superconducting characteristics in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. It considerably increases $T_{c}$, decreases $\gamma_{0}$ in the superconducting state and suppresses the Schottky-like contribution at very low temperatures. The improved sample quality allows for a better identification of the superconducting gap structure of these materials. We examine the effects of doping and annealing within a self-consistent framework for an extended s-wave pairing scenario. At optimal doping our data indicates the sample is fully gapped, while for both under and overdoped samples significant low-energy excitations possibly consistent with a nodal structure remain. The difference of sample quality offers a natural explanation for the variation in low temperature power laws observed by many techniques.Comment: 9 pages: added references, two figures and supplementary information; Accepted to Physical Review B (Jan 10, 2010