Ádám Révész

The contrast factors of dislocations in cubic crystals: the dislocation model of strain anisotropy in practice

It has been shown recently that in many cases strain anisotropy in powder diffraction can be well accounted for by the dislocation model of the mean square strain. The practical application assumes knowledge of the individual contrast factors C of dislocations related to particular Burgers, line and diffraction vectors or to the average contrast factors C¯. A simple procedure for the experimental determination of C¯ has been worked out, enabling the determination of the character of the dislocations in terms of a simple parameter q. The values of the individual C factors were determined numerically for a wide range of elastic constants for cubic crystals. The C¯ factors and q parameters were parametrized by simple analytical functions, which can be used in a straightforward manner in numerical analyses, as e.g. in Rietveld structure refinement procedures.

Dislocations and grain size in ball-milled iron powder

The microstructure of ball-milled iron powder has been investigated by high resolution X-ray line profile analysis. Analysis of line breadths suggested that the contrast factors related to dislocations have to be taken into account in the Williamson-Hall procedure. This concluded to a modified Williamson-Hall plot which provided, in a straightforward manner, the grain size refinement of nanocrystals ball-milled for different periods of time. At the same time it has been shown that strain broadening, even in these nanoscale small-grain particles, is caused by the presence of dislocations. The line profiles were also studied by the method of Fourier analysis, which gave the absolute values of dislocation densities.

Dislocations and grain size in electrodeposited nanocrystalline Ni determined by the modified Williamson-Hall and Warren-Averbach procedures

Electrodeposited nanocrystalline Ni foils were studied by high-resolution X-ray diffractometry. The full width at half-maximum and Fourier coefficients were found to vary rather anisotropically as a function of diffraction order. The modified Williamson–Hall plot and the modified Warren–Averbach analysis, developed recently by taking into account the dislocation contrast in peak broadening, have been applied to interpret this anisotropic behaviour in terms of grain size, dislocation densities and twin boundaries. The average grain size has been found to range between 50 and 12 nm, in good agreement with transmission electron microscopy observations. The average dislocation density has been found to be 4.9 (5) × 1015 m−2 and the dislocations are of the screw character.

Partial amorphization of a Cu-Zr-Ti alloy by high pressure torsion

High pressure torsion was applied to produce disk-shape specimen of Cu60Zr20Ti20 composition. Radial dependence of the microstructure was monitored by x-ray diffraction, scanning, and transmission electron microscopies. The disk consists of a top surface layer, homogeneous on a micrometer scale with an average thickness of 10–20μm, and an inhomogeneous bulk region of 200μm thickness. Calorimetric studies revealed that the disk contains detectable amount of amorphous phase. Characteristics of this amorphous content were compared to a fully amorphous melt-quenched Cu60Zr20Ti20 ribbon.

Structural anisotropy in a Zr57Ti5Cu20Al10Ni8 bulk metallic glass deformed by high pressure torsion at room temperature

Fully amorphous Zr57Ti5Cu20Al10Ni8 bulk metallic glass specimens were subjected to deformation by high pressure torsion at room temperature. Consecutive high resolution synchrotron x-ray diffraction mapping has revealed the variation of the average atomic bond length and shown no evidence of crystallization in this excellent glass former. Difference in the sign of the atomic distortion obtained in direction parallel to the sample surface and along the cross section indicates strong structural anisotropy.

Nanocrystallization Studies of an Electroless Plated Ni-P Amorphous Alloy

Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements were performed on an electroless plated Ni-P amorphous alloy to study the influence of different heat-treatments (linear heating and isothermal annealing). The phases formed after crystallization and the average grain size of the crystallization products were determined from XRD line broadening, and the heat evolved during the structural transformations were established from DSC measurements. A detailed study of the transformation products obtained along different heating routes was performed. From these studies, a scheme of the structural transformations and their energetics was constructed. The grain boundary energies in the different nanocrystalline states were also estimated.

High pressure torsion of amorphous Cu60Zr30Ti10 alloy

High pressure torsion was applied to produce a disk-shaped specimen of Cu60Zr30Ti10 composition. The dependence of the morphology, microstructure, and thermal behavior on the applied shear strain was monitored by scanning and transmission electron microscopies, synchrotron x-ray diffraction, and calorimetry. The disk consists of a gradient microstructure ranging from large homogeneous blocks (about 20 μm) to finely dispersed nanocrystals (about 20 nm) of two stable hexagonal phases and continuously decreasing amorphous content with increasing strain. The evolution of such microstructure was interpreted by using a model based on heat conduction generated by the extensive shear deformation.

Thermal stability, crystallization kinetics, and grain growth in an amorphous Al85Ce5Ni8Co2 alloy

Thermal stability and crystallization kinetics of the melt-quenched amorphous Al 8 5 Ce 5 Ni 8 Co 2 alloy were investigated by x-ray diffraction and differential scanning calorimetry (DSC). The glass transition was followed by a supercooled liquid region (21 °C) and then by a two-step crystallization process. The final microstructure contained Al 3 Ce, α-Al, Al 3 Ni, and Al 9 Co 2 phases. Isothermal annealing of the as-quenched samples in the range of 275-285 °C showed that both crystallization reactions occurred through a nucleation and growth process. Continuous heating DSC measurements following pre-anneals for different times were also carried out to study the crystallization kinetics and the stability of the material. The Avrami analysis of the isothermal DSC-curves revealed that the 3-dimensional nucleation and growth process became more dominant with increasing annealing temperature. The average specific grain boundary energy corresponded to high-angle grain boundaries and indicated independent nucleation events.

Thermal properties of ball-milled nanocrystalline Fe, Co and Cr powders

Abstract Ball-milled Fe, Co and Cr powders have been investigated by differential scanning calorimetry to study the stored enthalpy and the origin of the enhanced specific heat in these materials. The samples were prepared of high purity powder of 40–50 μm initial particle size and milling times up to 1 month have been applied. The results of calorimetric measurements have been correlated with structural parameters (average grain size and dislocation density) obtained by high resolution X-ray diffractometry.

Preparation and magnetic properties of nanosized amorphous ternary Fe-Ni-Co alloy powders

Nanosized amorphous alloy powders of Fe 25 Ni 13 Co 62 , Fe 38 Ni 23 Co 39 , Fe 40 ,Ni 24 ,Co 36 , and Fe 69 Ni 9 Co 22 were prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO) 5 , Ni(CO) 4 , and Co(NO)(CO) 3 in decalin, under an argon pressure of 100 to 150 kPa at 273 K. The amorphous nature of these particles was confirmed by various techniques, such as scanning electron microscopy, transmission electron microscopy, electron microdiffraction, and x-ray diffractograms. Magnetic measurements indicated that the as-prepared amorphous Fe–Ni–Co alloy particles were superparamagnetic. The observed magnetization measured up to a field of 1.5 kG of the annealed Fe–Ni–Co samples (75–87 emu g −1 ) was significantly lower than that for the reported multidomain bulk particles (175 emu g −1 ), reflecting the ultrafine nature of our sample.