Dilip Kumar Saha

Short-range order in Cu-Pd alloys

The X-ray diffuse scattering intensity was measured at room temperature from disordered Cu-Pd alloys containing 8.0, 9.5, 13.0, 21.8, 28.5 and 42.0 at.% Pd. Twofold and fourfold splittings of diffuse scattering due to short-range order (SRO) were observed at 100, 110 and their equivalent positions respectively from alloys with more than 13.0 at.% Pd, where the separation of the split diffuse maxima increases monotonically with increasing Pd content. No splitting of diffuse scattering was observed for alloys containing 13.0 at.% Pd or less. The diffuse intensities measured in Laue units increase with increasing Pd content except for 42.0 at.% Pd alloy, which shows a much lower intensity compared with that for 28.5 at.% Pd alloy. The SRO parameters were determined from all the six alloys. One representative set of the local atomic arrangements of Pd atoms in the FCC lattice which has been derived from computer simulation of the observed SRO parameters is shown. The authors have calculated the Pd-Pd atom pairs of the first- and second-nearest neighbours for all the six alloys, in both the random and the SRO case. Although the ordered structure for Cu-42.0 at.% Pd alloy is of the CsCl type, no different feature in the SRO structure was observed for this alloy in comparison with the other alloys.

Stable icosahedral nanostructure in gold-copper alloy clusters

Abstract Different formation rates of stable icosahedral nanostructures were observed for the clusters of four different Au- 13, 25, 39 and 57 at.% Cu alloys. The rate was found to be maximum, 80%, for Au-25 at.% Cu alloy clusters and decrease with increasing and decreasing the Cu content from 25 at.%. The icosahedral structures along 5-, 3- and 2-fold axes were identified clearly by high resolution transmission electron microscope. The critical size for the Ic formation was found to be 6 nm in diameter for all the alloys. Cu content in the alloys plays an important role to form the stable Ic and controls the formation rate.

Short-range order in Cu-Pt alloys

The X-ray diffuse scattering intensity from disordered Cu-Pt alloys whose composition were Cu-7.0, 10.0, 16.0, 24.5, 32.0 and 75.0 at.% Pt was measured at room temperature. Twofold and fourfold splittings of diffuse scattering due to short-range order (SRO) were observed at 100, 110 and their equivalent positions (X points), respectively, for Cu-16.0, 24.5 and 32.0 at.% Pt alloys, where the separation of split diffuse maxima increases monotonically with increasing Pt content. On the other hand, no splitting of diffuse scattering was observed at Cu-<16.0 at.% Pt alloys. Extra diffuse maxima were observed at 1/21/21/2 and its equivalent positions (R points) for all samples, the intensity being related to the concentration. The SRO parameters were determined from all six alloys. The sign and magnitude of alpha 200 strongly depend on the ratio of R- to X-point diffuse intensities. Synthesized data indicated that the first few SRO parameters dominate the R-point maxima, though the sign and magnitude of higher-order parameters play an important role in causing the appearance of split diffuse maxima. It is proposed that there are two types of correlation in the disordered state of the Cu-Pt alloy system, i.e. between the consecutive (111) atomic layers and due to Fermi surface imaging.

X-ray Diffraction Study of Short-range Order and Long-range Ordered Structure in Cu–87.5 at.% Pt Alloy

X-ray diffraction study was performed for the atomic arrangements in Pt-rich (87.5 at.%) Cu–Pt alloy with the cubic structure both in the disordered and ordered states using single crystal samples....

Phonon dispersion curves in disordered Cu0.715Pd0.285 alloy

Both the atomic short-range order diffuse intensities and the phonon frequencies along the high-symmetry directions for disordered Cu0.715Pd0.285 alloy were measured using neutron experiments, to investigate the relationship between the diffuse maxima due to the atomic short-range order and the Kohn anomalies. The observed phonon dispersion curves were compared with the curves calculated from the second-nearest-neighbour Born-von Karman model. There were no anomalies in the phonon dispersion curves themselves or their derivatives under the authors experimental conditions. An improvement in energy resolution would be required to search further for the Kohn anomalies in this alloy system.

X-Ray Diffraction Study of the Atomic Short-Range Order in Pt–12.5 at.% Mn Alloy

Atomic diffuse scattering intensity from a single crystal of fcc Pt–12.5 at.% Mn alloy was measured with the use of X-ray diffraction. Diffuse intensity maxima at 1/2 1/2 1/2 and its equivalent pos...

X-Ray Diffraction Study of Atomic Short-Range Order in Pt–15.0 at.% Cr Alloy

The X-ray diffuse scattering intensities were measured at room temperature for the Pt rich Pt–15.0 at.% Cr single crystal quenched from 800°C three-dimensionally. The atomic short-range order (ASRO) intensity component from the total diffuse ones was separated with the use of Borie–Sparks method [Acta Cryst. A 27 (1971) 198]. The ASRO intensity maxima were observed at the 100, 110 and their equivalent positions (X-point) on the reciprocal lattice plane, which are widely diffused. We did not find any ASRO diffuse scattering intensity at \(\frac{1}{2}\frac{1}{2}\frac{1}{2}\) and its equivalent positions (L-point) for the alloy. The diffuse maxima at the X-point are due to the Cu 3 Au type ASRO in the present alloy case. The ASRO parameters were determined from the data and the required number of ASRO parameters, to reconstruct the peaks, indicates that the maxima are widely diffused. The correlation length is deduced to be about 10 A. Any superlattice reflections were not found from the specimen, which was ...

Short-range order in Ni-Pt alloys

Electron and X-ray diffraction patterns were taken from four different Ni1-xPtx alloys, with x=0.25, 0.35, 0.44 and 0.50, to understand the existence and shape of the diffuse intensity, which gives us information about structural fluctuations. A peculiar shape of diffuse scattering has been observed at 100 and 110 and their equivalent positions in the patterns, the shape of the diffuse intensity at the 100 position changing with Pt content in the alloys. The shape of the diffuse intensity at the 100 position for the x=0.25 alloy is rod-like, but the shape is nearly spherical for the x=0.50 alloy. The X-ray diffuse scattering intensities were measured from the x=0.50 single-crystal alloy at room temperature, and the Warren-Cowley short-range order parameters were determined after analysing the data. The correlation length has been deduced to be about 19 AA from estimating the inverse of the full width at half-maximum of the diffuse peak.

Structure and stability of nano-metallic particles

The electron microscopic studies have been done for Au and Au-25 at.% Cu nanoparticles (1–10 nm) prepared by the inert-gas vapor-aggregation technique, and those annealed on amorphous carbon films at 723 K under vacuum. The Au-Cu particles with complex structures have been restructured into icosahedron (1–6 nm in diameter) and polycrystalline (>6 nm) particles by annealing, despite coalescence taken place between the particles. The Au particles, however, did not show such drastic structural changes by annealing. The results suggest that the Au-Cu binary state produces a global minimum for the icosahedral structure on the configurational free energy surface.The electron microscopic studies have been done for Au and Au-25 at.% Cu nanoparticles (1–10 nm) prepared by the inert-gas vapor-aggregation technique, and those annealed on amorphous carbon films at 723 K under vacuum. The Au-Cu particles with complex structures have been restructured into icosahedron (1–6 nm in diameter) and polycrystalline (>6 nm) particles by annealing, despite coalescence taken place between the particles. The Au particles, however, did not show such drastic structural changes by annealing. The results suggest that the Au-Cu binary state produces a global minimum for the icosahedral structure on the configurational free energy surface.