V.S. Raghunathan

Electron microscopic study of cr2n formation in thermally aged 316ln austenitic stainless steels

The formation of dichromium nitride phase in low nitrogen austenitic stainless steels has been studied by transmission electron microscopy (TEM). The electron diffraction patterns unambiguously confirm the formation of Cr2N phase on aging these steels in the range of 1023 to 1123 K, for time durations up to 100 hours. It is found that while no perceptible microstructural features could be recognized, formation of dislocation pairs is an important characteristic that could be associated with the nitrogen ordering in the matrix. The precipitation sequence processes have been discussed on the basis of stress-induced interactions that are predominant in interstitial alloys. Further, certain aspects of me-chanical behavior are explained on the basis of our study.

A study of ternary element site substitution in Ni3Al using pseudopotential orbital radii based structure maps

It is well known that ternary alloying additions are usually added in small quantities to structural intermetallics with a view to optimize their structural, physical, chemical and mechanical properties. Since structural intermetallics are highly ordered alloys, the added ternary solutes often reveal a preponderant tendency to substitute preferentially one of the sublattices. A knowledge of the basic factors governing the preferential site substitution behavior is useful in elucidating the role of ternary additions in controlling the materials properties. In this paper, the authors apply the concept of structure maps for explaining the site substitution behavior of ternary alloying additions in Ni{sub 3}Al.

Structure and vibrational properties of carbon tubules

The structure of multilayered carbon tubules has been investigated by electron microscopy and X-ray diffraction. The structure of tubules is characterized by disorder in the stacking of cylindrical graphene sheets. Raman scattering measurements have been carried out in tubules and compared with graphite. The observed features in the Raman spectra in tubules can be understood in terms of the influence of disorder. The additional Raman modes predicted for single layer carbon tubules have not been observed.

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.

Nano-quasicrystalline phase formation in Mg-Cd-Yb alloys

Abstract The microstructure of Mg 60+ x Cd 25− x Yb 15 , x =0, 5, 10, Mg 50 Cd 30 Yb 20 and Mg 25 Cd 65 Yb 10 as-cast alloys has been examined by transmission electron microscopy. While the Mg 60 Cd 25 Yb 15 alloy showed only an icosahedral quasicrystalline phase, the magnesium rich alloys also showed a Zn 2 Mg type (Mg,Cd) 2 Yb Laves phase. The alloys showed nanosized particles of icosahedral phase. Eutectic microstructures involving the icosahedral phase were observed. A lamellar (50 nm) structure of icosahedral phase and Mg was observed, exhibiting a definite orientation relationship with the hexagonal axis along one of the twofold axes of the icosahedral phase. In the Cd-rich alloys the cubic Cd 6 Yb approximant phase occurred, along with the icosahedral phase and intermediate structures in nanocrystalline form.

Theoretical analysis of interstitial ordering and precipitation processes in the Ti-N system

Abstract The energetics of interstitial ordering was analysed using the quasi-chemical approach incorporating interactions up to second nearest-neighbours. It was found that the interstitial ordering is required because of strong repulsive interactions between interstitial atoms situated along the c -axis. A nitrogen occupation scheme with a view to minimizing this strong coulombic repulsion and to offer minimum lattice disturbance is proposed. Keeping in accord with the occupation scheme, the combined effect of first and second nearest-neighbour interactions was found to give rise to the formation of ordered superstructures. A martensitic transformation of the ordered domains as a result of the stress field associated with the interstitial atom is predicted to occur around the composition of 20 at. % nitrogen. The nature of the phase separation of the ordered superstructures and the crystallography of the lattice distortion are briefly discussed.

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.

Structure property correlations in superconducting Ti-Nb alloys

Titanium-rich transition metal alloys are metastable in their quenched boc β phase. The instability is relieved by low temperature structural transformations. We have investigated this in a series of Ti-Nb alloys, through the measurements of electrical resistivity (ρ), superconducting transition temperature and upper critical field. Supporting structural evidence has been obtained from transmission electron microscopy (tem) and x-ray studies. It is shown that both ρ and dρ/dT can be used as useful indices of this instability. The enhanced value of resistivity on account of the instability results in the enhancement of upper critical field as shown from dHc2/dT measurements.

Energetics of point defects in γ-TiAl

{gamma}TiAl has been receiving a great deal of attention in recent times owing to its industrial importance. This structural intermetallic is a candidate material for high temperature aerospace applications. Therefore, a study of point defect properties is useful in elucidating its physical metallurgy. In this brief communication, the authors discuss the vacancy and antisite defect properties of {gamma}-TiAl.

Application of the macroscopic atom model of cohesion to structural systematics of L10 compounds

Abstract The structural systematics of L1 0 compounds is studied using Miedemas macroscopic atom model (MAM) of cohesion. A characteristic L1 0 structure stability map is obtained which brings out the possibility of alternate metastable structures for certain L1 0 compounds. The degree of tetragonality of the L1 0 structure is discussed on the basis of the key parameters of the model and the role of ternary additions in modifying the L1 0 structure is also discussed by considering TiAl as an example.