R Divakar

Interface structures in nanocrystalline materials

Grain boundary and triple junction structures in nanocrystalline palladium and titanium thin films and in nanocrystalline thorium dioxide powder and sintered pellets have been studied. Amorphous packets with bright contrast are seen in many of the grain boundary junctions and are considered to form as a result of the relaxation of triple junction disclinations. The varying degrees of relaxation of disclinations in NCs lead to a variety of grain boundary structures and are also responsible for the random variation in its lattice parameter in the different nanocrystalline grains. Nanocrystalline state has been suggested to be a desirable microstructural criterion for solid state amorphization. Further it is to be pointed out that no special features which could be attributed to the nature of bonding in the two materials were seen in this study.

Microstructural features of a type 304L stainless steel deformed at 1473 K in the strain rate interval 10−3 s−1 to 102 s−1

Deformation processing of materials in continuously being refined by dynamic materials modeling procedures to establish a safe window for the manufacture of engineering components. Microstructure development during the processing and its correlation with the mechanical properties is inevitable for better understanding of the materials. On this basis, microstructural examination of the dynamically processed type 304L austenitic stainless steels has been carried out. The samples that have been deformed at 1,473 K under various strain rates, ranging from 10[sup [minus]2]s[sup [minus]1] to 10[sup 2]s[sup [minus]1], were observed by transmission electron microscopy, to corroborate the energy efficiency of the process. The details of the energy efficiency contours and their implications are reported elsewhere. In this report the authors present some of the unusual microstructural features that, in general, are not desirable for the safe processing of materials.

Al-Cu-Fe quasicrystals: Stability and microstructure

AlCuFe quasicrystal has been considered a novel icosahedral quasicrystal in as much as it can be produced in both thermodynamically stable and metastable state. It is also recognised to be a face centred structure. The system has been studied extensively in the recent past and several contentious results have been obtained. For example, evidences have been presented for both reversible and irreversible transformation of the quasicrystalline phase to the crystalline phases. Similarly, distinct transition metal sites have been proposed based on the Mossbauer studies. A subsequent study of this in fact shows an absence of such distinct sites. It has been shown that in this system, vacancy type of defects of very large concentrations exist. Many of these factors bring into question, the various structural models that have been postulated to explain the icosahedral structure obtained in the system. A critical examination of these various aspects of the AlCuFe quasicrystal is presented.

Structural characterization of electrodeposited boron

Structural characterization of electrodeposited boron was carried out by using transmission electron microscopy and Raman spectroscopy. Electron diffraction and phase contrast imaging were carried out by using transmission electron microscopy. Phase identification was done based on the analysis of electron diffraction patterns and the power spectrum calculated from the lattice images from thin regions of the sample. Raman spectroscopic examination was carried out to study the nature of bonding and the allotropic form of boron obtained after electrodeposition. The results obtained from transmission electron microscopy showed the presence of nanocrystallites embedded in an amorphous mass of boron. Raman microscopic studies showed that amorphous boron could be converted to its crystalline form at high temperatures.

Studies of interfaces in Al65Cu20Fe15

The study of interfaces in quasicrystalline alloys is relatively new. Apart from the change in orientation, symmetry and chemistry which can occur across homophase and heterophase boundaries in crystalline materials, we have the additional, exciting possibility of an interface between quasicrystalline and its rational approximant. High resolution electron microscopy is a powerful technique to study the structural details of such interfaces. We report the results of a HREM study of the interface between the icosahedral phase and the related Al13Fe4 type monoclinic phase in melt spun and annealed Al65Cu20Fe15 alloy.

Interfaces In Quasicrystalline Systems

Structural relations between quasicrystalline and related crystalline rational approximant phases have been of interest for some time now. Such relations are now being used to understand interface structures. Interfaces between structural motif - wise related, but dissimilarly periodic phases are expected to show a degree of lattice match in certain directions. Our earlier studies in the Al-Cu-Fe system using the HREM technique has shown this to be true. The structural difference leads to well defined structural ledges in the interface between the icosahedral Al-Cu-Fe phase and the monoclinic Al13Fe4 type phase. In the present paper we report our results on the HREM study of interfaces in Al-Cu-Fe and Al-Pd-Mn systems. The emphasis will be on heterophase interfaces between quasiperiodic and periodic phases, where the two are structurally related. An attempt will be made to correlate the results with calculated lattice projections of the two structures on the grain boundary plane.