Browsing by Author "S. Lele"

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An ordered metastable phase in rapidly quenched Al-Nb alloys
(Kluwer Academic Publishers, 1986) S. Singh; S. Lele; C. Suryanarayana
[No abstract available]
Calculation of phase diagrams of binary fcc and ideal hcp alloys undergoing ordering transitions
(Springer India, 1994) Ananya Ghosh Moulic; G.V.S. Sastry; S. Lele
The tetrahedron approximation of the cluster variation method (CVM) has been employed to investigate phase diagrams having fcc-based ordered and disordered phases. This approximation is also applicable to the binary hcp ordered structures with ideal axial ratio. The CVM developed by Kikuchi consists of calculating approximate expressions for the number of configurations and hence entropy of a crystal lattice having definite distribution of clusters (points, pairs, triangles, tetrahedra, etc.) of lattice points which in general may be occupied by one of a given set of atomic species. Tetrahedral multi-atom interactions denoted by α and β are utilized for expressing the configurational energy. The equilibrium cluster distribution is then found by minimizing the free energy by utilizing the natural iteration method developed by Kikuchi. The effect of α and β parameters on the topology of the resulting phase diagrams is observed by assigning several negative and positive values to them. The invariant reactions were also determined in each case. Finally a study was made on the Cd-Mg diagram. © 1994 The Indian Academy of Sciences.
Correction to "geometrical interpretation of X-ray peak displacements due to stacking faults in powder patterns of close-packed structure"
(1970) S. Lele; P. Rama Rao
[No abstract available]
Decomposition behaviour of rapidly quenched Al–Nb alloys– TEM study
(1986) S. Singh; S. Lele; C. Suryanarayana
Two binary Al-rich Al–Nb alloys containing 4·5 and 7·7 wt-%Nb have been rapidly quenched from the liquid state using the ‘gun’ technique. Transmission electron microscopy and diffraction techniques have been extensively employed to characterize the as quenched as well as externally heat treated alloy foils. Each alloy in the as-quenched state shows only a supersaturated solid solution, indicating a substantial increase in the solid solubility limit of Nb in Al by rapid quenching. Elongated solid solution grains and defect structures have also been observed in the as-quenched foils. While no precipitation was found to occur on annealing the quenched dilute alloy (Al–4·5Nb) foils for 1 h at 673 K, precipitation along grain boundaries was observed in the concentrated alloy (Al–7·7Nb). Higher temperature annealing (≥773 K) resulted in the formation of a new metastable phase having an ordered fcc GeCa7 type structure with a lattice parameter a 0·8 nm. This phase forms predominantly with a rodlike morphology and is arranged in a Widmanstätten pattern inside the grains, although fine precipitate particles have also been observed. On prolonged annealing at or above 773 K, the metastable phase transforms with the formation of the equilibrium tetragonal Al3Nb phase. © 1986 The Institute of Metals.
Diffraction effects of antiphase boundaries in ordered structures
(1997) S. Lele; P. Ghosal
Ordered structures and order-disorder transitions were first detected through the presence of superlattic reflections. Frequently these reflections are diffuse along one direction in reciprocal space. This diffuseness usually arises from the presence of antiphase domain boundaries in the ordered structure. A systematic study of the diffraction effects of antiphase boundaries in L12 ordered structures was first undertaken by Wilson over fifty years ago. We present in this review the types of antiphase boundaries possible in the common ordered structures, namely Strukturbericht types L12, B2 and D019 which are based respectively on fcc(A1), bcc(A2) and cph(A3) fundamental cells. Classification of the boundaries into shear and climb types is indicated. This is important as the former type of boundaries are likely to occur in deformation processes while the latter are generally asociated with order-disorder transformations. Diffraction effects of these boundaries are then systematically outlined. The diffraction effects for the D019 structure have become available only recently and thus have been highlighted.
Entropy and free energy at the hcp-bcc transformation temperature in metals and alloys
(Springer-Verlag, 1980) P. Ramachandrarao; S. Lele
It has been shown that a plot of the Φ function, defined by - (G tr - Go)/Ttr where Gtr and G o are respectively the Gibb's free energies at the hcp → bcc allotropie transformation temperature (T tr ) and at absolute zero, vs the entropy (Str) of the hcp phase at Ttr is linear. This concept of the Φ function and its relation to S tr can be extended to alloys with the aid of the 'central atoms' solution model. Its use has been demonstrated with respect to the Ti-Zr system where the estimated transformation temperatures are shown to be within the (hcp + bcc) phase field. © 1980 American Society for Metals and The Metallurgical Society of AIME.
Entropy catastrophe and superheating of crystals
(1988) S. Lele; P. Ramachandra Rao; K.S. Dubey
Kauzmann1 has pointed out that at a temperature (Tk) far below the melting temperature (Tm) of a crystal, the entropies of an undercooled liquid and its corresponding equilibrium crystal tend to become equal. At temperatures below Tk, the crystal would have a greater entropy than its liquid. Such a catastrophe is avoided through a glass transition of the liquid above Tk. Fecht and Johnson have shown that such a transition should occur for aluminium at 0.23 Tm. A similar entropy catastrophe can also arise at a temperature, TiS, above Tm . Above TiS, the crystalline solid is once again expected to have an entropy greater than that of the liquid. Cahn3 has considered the implications of this idea with respect to the superheating of a solid. Here we present an alternative evaluation of the two temperatures of instability, Tk and TiS, from experimentally measurable parameters. Results for alkali metals show that the Kauzmann-type entropy catastrophe occurs at about half the absolute melting temperature, whereas we find that the entropy catastrophe as described by Fecht and Johnson2 occurs at twice the absolute melting temperature. For most solids, vaporization will probably intervene before the entropy catastrophe temperature above the melting temperature is reached. © 1988 Nature Publishing Group.
Estimation of complex concentration in a regular associated solution
(Springer-Verlag, 1981) S. Lele; P. Ramachandrarao
Novel procedures have been developed to estimate the concentration of A pB -type clusters in liquid alloys with the aid of regular associated solution model utilizing activity coefficients of the components at infinite dilution and activity data at any one other composition. The equilibrium constant for the dissociation of clusters as well as the nature and magnitude of pairwise interaction energies between unassociated atoms and clusters have been evaluated without making any assumptions whatsoever. These enable estimation of thermodynamic properties of the liquid alloys at any other desired composition. The utility of the procedures has been demonstrated with respect to the thermodynamics of association in molten Mg-Sn and In-Sb alloys. The calculated activities, enthalpies and free energies are shown to be in excellent agreement with experimental values. © 1981 American Society for Metals and the Metallurgical Society of AIME.
Estimation of the free energy of undercooled oxide melts
(1989) K.S. Dubey; P. Ramachandrarao; S. Lele
The difference in free energy between a liquid and a solid phase (ΔG) is an important parameter in nucleation theory. We show that a novel expression derived by a Taylor series expansion of ΔG around its value at the melting temperature describes accurately the ΔG of oxide melts as a function of degree of undercooling. This is demonstrated for a number of pure (SiO2, GeO2, and B2O3) and compound oxides.
ESTIMATION OF THE MODEL PARAMETERS OF A REGULAR ASSOCIATED SOLUTION: PART I THEORY.
(1985) S. Lele; G.V.S. Sastry; P. Ramachandrarao
The basic equations for the thermodynamic properties of regular associated solutions are presented. Equations for the partial excess free energies and enthalpies are considered at infinite dilution and shown to be related to the interchange energies and equilibrium constant for the formation of the complex. Procedures are indicated for evaluation of model parameters and complex concentration from known thermodynamic data. The detailed procedure for analysis of the data depends on stoichiometry and an accurate analysis is possible only for AB or A//2B associates.
Extrinsic deformation faults in double hexagonal close-packed crystals
(1970) S. Lele; B. Prasad; P. Rama Rao
[No abstract available]
Free energies of undercooled phases
(1989) P. Ramachandrarao; K.S. Dubey; S. Lele
A novel approach for the estimation of the free energies of metastable liquid and allotropie phases has been presented. The method is based on the Taylor series expansion of the free energy différence between stable and metastable phases. The procedure has been applied to arrive at allotropie transformation temperatures in iron and also the isentropic temperature for any two coexisting phases. Results obtained strongly validate the expressions derived which involve only the use of equilibrium heat capacity and entropy data. © 1989.
Geometrical interpretation of X-ray peak displacements due to stacking faults in powder patterns of close-packed structures
(1970) S. Lele; P. Rama Rao
[No abstract available]
Ground state structures in binary h.c.p. alloys using multiatom interactions
(1994) B. Nageswara Sarma; N. Ravisankar; M. Gopal; A.K. Singh; R. Prasad; S. Lele
The cluster method is applied to binary (A-B) h.c.p. alloys having a non-ideal axial ratio in the tetrahedral approximation. Eight distinct cluster configurations are identified by populating A and B atoms on the distorted tetrahedral motif sites. The fractions of these clusters are the corresponding cluster variables. The configurational energy of the alloy is expressed as a function of four tetrahedral multiatom (four-body) interaction energy parameters, an effective pair interchange energy parameter and the cluster variables. The energy of the alloy is minimized for all possible triplets of clusters using the linear programming method. From the inequalities that minimize the energy and thus define the ground state, the permissible values of the multiatom interaction parameters in each case are clearly specified by a suitable geometric representation in a four-dimensional hyperspace spanned by the multiatom interaction parameters. Several ground state structures are obtained and the observed superstructures are shown to be ground state structures. Limitations of the linear programming method are examined and the effect of considering multiatom interactions is compared with that of increasing interaction distances in the pair approximation. © 1994 Taylor & Francis Group, LLC.
Ground state structures in ordered binary H.C.P. alloys with pairwise third neighbour interactions
(1991) A.K. Singh; V. Singh; S. Lele
Ground state structures in binary h.c.p. (non-ideal axial ratio) alloys have been studied using the cluster method proposed by Allen and Cahn [Acta metall. 20, 423 (1972)]. The method has been suitably modified and pairwise interactions up to third neighbour have been considered. Nine ground state structures have been identified. The structural details such as lattice, number of atoms per unit cell, equivalent positions of atoms and their respective symmetries along with energy have been worked out. For off-stoichiometric compositions, the relative stabilities of solid solutions and two phase mixtures are also discussed. In the latter case, miscibility gaps arise. Three types of degeneracy have been found to occur. Two of these are associated with the possibility of arranging the same set of different types of atomic clusters in distinct ordered patterns. The third arise for h.c.p. alloys havingg ideal axial ratio. © 1991.
Impact of substrate temperature on rapid solidification of an Al-Cu eutectic alloy
(Kluwer Academic Publishers, 1982) R.K. Singh; K. Chattopadhyay; S. Lele; T.R. Anantharaman
An aluminium-copper eutectic alloy has been subjected to rapid solidification, utilising the "gun" technique, with different substrate temperatures to evaluate its influence on the cooling rate and microstructure. At all temperatures, the microstructure is essentially non-uniform and consists of metastable solid solution (with decomposition products) as well as eutectic regions. The solid state decomposition of the metastable solid solution is strongly dependent on the substrate temperature and leads to the formation of different metastable precipitates and consequent changes in microstructure. The origin of the different microstructures is discussed. © 1982 Chapman and Hall Ltd.
Influence of incident beam divergence on the intensity of diffuse streaks
(Springer India, 1985) Dhananjai Pandey; S. Lele; Lalita Prasad; J.P. Gauthier
The influence of incident beam divergence on the length of the streak intercepted by the Ewald sphere is considered, as a relp HK·L of a faulted hexagonal crystal, mounted about its c-axis on the goniometer head attached to the ø-circle, is brought into diffracting condition for the bisecting setting of a 4-circle diffractometer. For the special crystal mounting correction factors required to convert the measured intensities corresponding to a fixed length of the streak are derived. A procedure for experimentally verifying the mathematical approach employed in these derivations is also presented. © 1985 Indian Academy of Sciences.
Measurement of intensity of directionally diffuse streaks on a four-circle diffractometer: divergence correction factors for bisecting setting
(Wiley-Blackwell, 1987) D. Pandey; L. Prasad; S. Lele; J.P. Gauthier
Because of the inherent beam divergence, point intensities measured using a four-circle diffractometer at equispaced points along the diffuse streak correspond to variable lengths of the streak. Correction factors required to convert the measured intensities into those corresponding to a fixed length at each equispaced point along the diffuse streak are derived for two different crystal mountings under bisecting geometry. A procedure for experimentally verifying the mathematical approach employed in these calculations is also described. © 1987, Wiley-Blackwell. All rights reserved.
Mechanism of the 2H → 6H solid state transformation in SiC
(1977) Dhananjai Pandey; S. Lele; P. Krishna
Single crystals of 2H SiC transform directly into the 6H (ABCACB) structure when the transformation nucleates at temperatures above 2000°C. The 2H close-packed structure may be transformed to the 6H structure by displacing every third layer. The theory of X-ray diffraction from one-dimensionally disordered crystals undergoing the 2H to 6H structural transformation by such a layer displacement mechanism has been developed. The fact that all the observed solid state transformations in SiC crystals commence with the random insertion of stacking faults and then proceed further to create a statistically ordered structure, permits such a theory to be developed. Exact expressions for the diffracted intensity from such crystals have been obtained and the different diffraction effects observable on single crystal X-ray photographs predicted. A comparison of the theoretically predicted diffraction effects with those visible on the X-ray photographs of SiC crystals undergoing the 2H to 6H transformation shows that the structural transformations in SiC occur by the layer displacement mechanism. © 1977.
Microstructural characterization of rapidly solidified Al-Ta alloys
(1987) S. Singh; S. Lele; C. Suryanarayana
Two Al-rich Al-Ta alloys containing by weight 3 and 6 pct Ta have been rapidly solidified from the melt using the 'gun' technique. The microstructures and the crystal structures of the phases in the as-solidified as well as those formed on subsequent decomposition of the supersaturated solid solution have been characterized. A supersaturated solid solution was obtained in both the alloys in the as-solidified condition indicating a solid solubility extension of Ta in Al to almost 6 wt pct. The supersaturated solid solutions formed in both the alloys have been found to be quite stable up to 673 K (for 1 hour). Annealing at higher temperatures resulted in the formation of rod-shaped precipitates inside the grains and massive precipitates along grain boundaries. The rod-shaped precipitates arranged in a regular pattern constitute a new metastable intermediate phase Al 7Ta having an ordered structure. The massive precipitates which form along grain boundaries constitute the equilibrium Al 3Ta phase with a tetragonal crystal structure. The transformation behavior and the morphology of the transformation products are detailed in this paper. © 1987 The Metallurgical of Society of AIME.