Browsing by Author "P. Mandal"

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Approximant twins in a titanium-iron-silicon alloy
(Kluwer Academic Publishers, 1994) P. Mandal; R.K. Mandal; R.S. Tiwari; O.N. Srivastava
[No abstract available]
Arcs of diffuse scattering from the new Fe-Ti-Si rapidly solidified alloy
(1991) P. Mandal; R.S. Tiwari; O.N. Srivastava
A rapidly quenched Fe-Ti alloy has been investigated in relation to the stability of the icosahedral phase arising from the substitution of Fe by Si. For less than 6 at.% Si, a crystalline (f.c.c. of FeTi2) phase has invariably been found to be present. However, an X-ray diffraction study of the Fe-Ti-Si system clearly reveals that 6 at.% Si results in the formation of single-phase icosahedral quasicrystal. The changes in grain size and morphology resulting from Si substitution are reported. Electron diffraction studies of this alloy show arcs of diffuse intensity which are indicative of the structural disorder present in the quasicrytalline phase. Detailed investigations of the diffuse scattering in Fe-Ti-Si alloy are presented and the results are compared with those for other known alloy systems. © 1991 Taylor & Francis Group, LLC.
Bcc metastable phase coexisting with the icosahedral phase in rapidly quenched Fe28Ti68Si4 alloys
(1991) P. Mandal; R.S. Tiwari; O.N. Srivastava
The FeTi-Si quasicrystalline system has been investigated by systematically varying the alloy composition. A metastable bcc phase with the lattice parameter close to 1.31 nm is reported for the Fe28Ti68Si4 alloy. Its orientational relationship with the icosahedral phase has been established based on x-ray-diffraction and electron-microscopy experiments. Electron-microscopy studies have revealed the interrelationship between the icosahedral and the cubic variants; the twofold, threefold, and fivefold icosahedral (i) phase axes are parallel to 001, 111, and nearly 530 directions of the cubic phase. A possible growth mechanism for the formation of both the icosahedral and the metastable bcc phase has been suggested. Structural transformations originating because of annealing (at 700 and 900°C) in an argon atmosphere have been investigated. The transformation sequence has been found to correspond to (FeTi2)(i)long->700°C (FeTi2)(bcc,metastable)long->900°C(FeTi2) (fcc,equilibrium)-Ti. © 1991 The American Physical Society.
Commensurate ordering in rapidly solidified Ti-Fe-Si alloys
(1992) P. Mandal; R.K. Mandal; R.S. Tiwari; O.N. Srivastava
We report here the occurrence of a commensurate phase in a rapidly solidified Ti-Fe alloy. On the basis of energy-dispersive x-ray analysis, the composition of the alloy has been analyzed and found to correspond to Ti2Fe1-xSix (x0.030.06), within the limits of experimental error. The x-ray- and electron-diffraction studies have revealed commensurate ordering based on a metastable CsCl-type phase with lattice parameter close to 0.298 nm. Available diffraction evidence suggests that commensuration in this alloy takes place along 100 directions of the basic cell. The ordered cell corresponds to a tetragonal one with cell parameters a0.298 nm and c9×0.298 nm. All the three variants arising due to symmetry breaking of the basic cell have been observed to be related by a threefold symmetry operation. Coherent diffraction of these yields cubic symmetry. © 1992 The American Physical Society.
Hydrogenation behaviour of the new composite storage material Mg-x% FeTi
(1994) P. Mandal; O.N. Srivastava
Mg-x wt.% FeTi composite alloys have been successfully synthesized. The hydrogenation behaviour of these alloys has been extensively studied. The materials have been activated at 400 °C under a hydrogen pressure of approximately 30 kg cm-2 and their hydrogen storage capacities and kinetics have been evaluated. The new composite hydrogen storage material in contrast to the native ingredient FeTi, has been found to possess much higher storage capacity and exhibits favourable absorption-desorption kinetics. For Mg-40% FeTi, a storage capacity of ∼3-3.6 wt.% at room temperature (≈27 °C) has been found. This is the highest known capacity exhibited by any hydrogen storage material at ambient conditions. In order to unravel the hydrogenation behaviour of these materials, structural-microstructural characteristics, and chemical composition before and after hydrogenation through X-ray diffraction, scanning electron microscopy and energy-dispersive analysis of X-rays have been carried out. The ambient condition hydrogenation properties of these composite alloys have been found to be strongly correlated with the structural and microstructural characteristics. Based on the observed structural and microstructural characteristics, the details of hydrogenation behaviour have been outlined in terms of cracking of FeTi matrix on hydrogenation and provision of continued fresh surfaces for hydrogenation of FeTi-Mg, Mg-TiMg eutectic mixtures or FeTi-Mg complexes. The present composite alloy corresponds to a new hydrogen storage material with higher storage capacity (≈3-3.6 wt.%) and suitable kinetics (initial hydrogenation in 10 min, saturation hydrogenation in 40 min). © 1994.
Occurrence of a quasicrystalline phase in the Ti-Fe(Ni)-Si system
(1993) P. Mandal; R.S. Tiwari; O.N. Srivastava
Extensive studies have been carried out regarding the possible occurrence of the icosahedral phase in the rapidly solidified Ti-Fe(Ni)-Si alloy system in a wide range of compositions. Transmission electron microscopy and x-ray-diffraction data reveal the formation of the icosahedral phase in Ti68Fe26-xNixSi6 where x≤9. In addition to the sharp diffraction spots conforming to the icosahedral point group, subtle structural features manifested by spot shifts, asymmetry in the spot shape, and diffuse intensity have been observed in electron-diffraction patterns. A particular composition has been found, Ti56Fe5Ni23Si16, that exhibits a metastable fcc phase with lattice parameter close to 1.79 nm. We also report on structural transformations of the icosahedral and metastable crystalline phases originating as a result of annealing (icosahedral and metastable crystalline phases change to the equilibrium crystalline phase at 900°C). © 1993 The American Physical Society.
On the evidence of the existence of the decagonal phase in rapidly solidified Fe1-xTi2Six alloy
(1990) P. Mandal; R.S. Tiwari; O.N. Srivastava
The rapidly solidified FeTi2 alloy is known to exist in an icosahedral phase. Silicon substitution for iron in FeTi2 leading to Fe1-xTi2Six (x = 0.01 to 0.05) has been shown to lead to the formation of the decagonal phase. X-ray and electron diffraction evidences have been obtained to confirm the existence of the decagonal phase. The electron diffraction patterns have been shown to clearly reveal the simultaneous existence of aperiodic and periodic arrangements of diffraction spots embodied in the same reciprocal lattice net - a feature confirming the existence of the decagonal phase. Besides this the long and short lengths ratio in the aperiodic arrangement corresponds closely to the expected value τm (τ: the golden mean or (5√ + 1) 2 = 1.618..., m: integer). In addition the diffraction pattern corresponding to the pseudo-five and three fold symmetries have also been obtained. These provide further support for the existence of the decagonal phase in Fe1-xTi2Six. © 1990.
Quasicrystalline and related phases in titanium based alloy systems
(Elsevier Ltd, 1997) R.S. Tiwari; P. Mandal; O.N. Srivastava
Amongst the non-aluminium based quasicrystalline alloys, investigations of titanium containing alloys in regard to the occurrence and stability of quasicrystalline phases have aroused considerable interest in recent years. Employing X-ray and TEM techniques a systematic investigation of the influence of substitution for Fe by Si and Ni on the stability of icosahedral phase in rapidly quenched Fe-Ti alloy has been carried out. The occurrence of metastable phases including commensurately modulated phase and the decagonal phase in Fe-Ti-Si system have been found. In addition, the occurrence ofstructural disorder manifested by arcs of diffuse scattering in diffraction patterns and anisotropy in the shape of idffraction spots has also been pointed out. It has been observed that 6 at.% of Si in Fe-Ti-Si system results in the formation of single-phase icosahedral quasicrystals. We have shown that contrary to some earlier reported results on Ti2Ni, Ti2Ni(Si) and Ti56Ni23Fe5Si16 alloys do not possess the icosahedral phase. In Ti68Fe26-xNixSi6 alloy system, icosahedral phase formation ability is limited to the value of x< 9. The occurrence of icosahedral phase in these alloy systems has been analysed in terms of e/a ratio.
Synthesis, characterization and hydrogenation behaviour of Mg-ξwt.%FeTi(Mn) and La2Mg17-ξwt.%LaNi5-new hydrogen storage composite alloys
(1992) P. Mandal; K. Dutta; K. Ramakrishna; K. Sapru; O.N. Srivastava
New hydrogen storage materials with higher capacity and better suited for applications have been successfully synthesized. The hydriding behaviour of the new composite materials, Mg-ξwt.%FeTi(Mn) and La2Mg17-ξwt.%LaNi5, were studied for various values of ξ (ξ = 10, 20, 30, 40 and 50). The Mg-ξ-%FeTi(Mn) materials were activated under a hydrogen atmosphere (about 33 kgf cm-2) and an optimum storage capacity of about 3.5 wt.% corresponding to room temperature hydrogenation was established for ξ=40. This high storage capacity-almost double the storage capacity of the well-known FeTi(Mn)-has been observed under ambient conditions. The La2Mg17-ξ-%LaNi6 materials were activated at higher temperatures (about 360 °C) in a hydrogen atmosphere. An optimum storage capacity of 4 wt.% in terms of pressure and composition was observed for La2Mg17-20%LaNi5 at 350 °C. In comparison with the native ingredient La2Mg17, much faster (nearly three times) kinetics were found. In order to understand the hydrogenation behaviour and the high storage capacity, structural-microstructural and chemical analyses of these composite materials were carried out. From the structural investigations it has been found that all the synthesized materials are multiphase. The composite material Mg-FeTi(Mn) was found to contain FeTi1-ξ1 magnesium, titanium and Ti-Mg phases. The higher storage capacity (about 3.5 wt.%) in the case of Mg-40%FeTi(Mn) is probably due to FeTi-Mg complexes. The hydrogen molecule is split at the FeTi surface and diffuses into the magnesium matrix via FeTi. In the case of La2Mg17-ξ%LaNi5, the composite material consists of La2Mg17, MgNi2, nickel and LaNi3 phases. Because of the presence of nickel and nickel-containing phases (e.g. Mg-Ni), it is assumed that the dissociation of hydrogen is easier and hence the system La2Mg17-ξ%LaNi5 has better kinetics than its counterpart La2Mg17 alone. © 1992.
The synthesis and hydrogenation behaviour of some new composite storage materials: Mg-xwt% FeTi(Mn) and La2Mg17-xwt% LaNi5
(1994) K. Dutta; P. Mandal; K. Ramakrishna; O.N. Srivastava
The composite alloys La2Mg17-xwt% LaNi5 and Mg-xwt% FeTi(Mn) have been successfully synthesized and the hydriding behaviours of these materials were studied for various values of x. Both these systems have been found to possess characteristics which are superior to the individual storage systems alone. In contrast to La2Mg17, hydrogen storage capacities of 5.24wt% with 3-4 times faster kinetics have been obtained for La2Mg17-xwt% LaNi5. For Mg-40wt% FeTi(Mn), the optimum storage capacity in relation to their reversible characteristic has been found to be about 3.5wt% even at ambient conditions, which is almost double the storage capacity of well-known FeTi(Mn) (∼ 1.9). In order to understand the hydrogenation behaviour and high storage capacity, structural/microstructural characteristics and chemical analyses of these composite materials were carried out employing the XRD, SEM and EDAX techniques. © 1994.
X‐ray, electron microscopic studies, and electrical resistivity behaviour in rapidly solidified FeTi2
(1988) P. Mandal; A.K. Singh; O.N. Srivastava
[No abstract available]