R.K. Mandal

Structure of orthorhombic martensitic phase in binary Ti–Nb alloys

The structure of orthorhombic (α″) martensitic phase of Ti–8Nb, Ti–12Nb, and Ti–16Nb alloys has been investigated using Rietveld refinement of x-ray diffraction data. The chemical analysis data have been used to determine the site occupancy of initial models. A limit of the Wyckoff positions has been proposed based on the symmetry of the Cmcm space group, which allows the movement of atoms without breaking the space group symmetry. This has also been incorporated in the initial models of experimental alloys, and accordingly Wyckoff positions have been refined. It has been observed that the atoms move along the Y-axis (parallel to the b-axis) and the movement of atoms increases with increase in Nb concentration. The formation of orthorhombic (α″) phase has been explained based on the movement of atoms along the Y-axis. This in turn breaks the hexagonal symmetry and forms an orthorhombic phase.

Microstructure and mechanical properties of Al–Si alloys produced by spray forming process

Abstract The microstructural characteristics and mechanical properties of Al–6.5Si and Al–18Si alloys have been investigated. The alloys were spray-deposited and hot extruded at 480°C with an area reduction ratio of 6:1. The microstructure of the spray-deposit of Al–6.5Si alloy showed spherical morphology of the primary α-phase with a globular shape of the eutectic Si phase in the inter-particle boundaries. On the other hand, a fine particulate morphology of primary Si phase uniformly dispersed in the matrix of the α-phase was observed in spray-deposit of Al–18Si alloy. Microstructural refinement was further increased in the hot extruded alloys. The room temperature tensile tests of spray formed and extruded alloys showed considerable increase in their strength and ductility over that of the as cast alloys. The improvement in mechanical properties of spray formed alloys is discussed in light of the microstructural modifications induced during spray deposition process.

Negative regulation of fibrin polymerization and clot formation by nanoparticles of silver

Thrombolytic therapy in acute stroke has reduced ischemia; however, it is also associated with increased incidence of intracerebral hemorrhage and expanding stroke. Platelets and fibrin are the major components of thrombi. Since fibrin is available in large concentration at lesion sites and in all types of thrombi, it is an obvious target for majority of antithrombotic therapies. Previously we have demonstrated innate antiplatelet properties with nanosilver. It can effectively prevent platelet activation in response to physiological agonists, under both in vitro as well as ex vivo conditions, and immobilize and stabilize proteins. Here we report for the first time that nanosilver can significantly retard fibrin polymerization kinetics both in pure and plasma-incorporated systems and hence can impede thrombus formation. We also discuss the conformational changes ensued upon fibrinogen following interaction with nanosilver. Together with its inherent antiplatelet and antibacterial properties, capacity to inhibit fibrin polymerization can open up possibilities of newer biomedical application and research potential involving silver nanoparticles.

Dielectric behaviour of glasses and glass ceramics in the system BaO-PbO-TiO2-B2O3-SiO2

Glasses with varying molar ratios of PbO/BaO in the system BaO-PbO-TiO2-B2O3-SiO2 were prepared keeping (BaO + PbO)/TiO2 ratio equal to one. The glasses were ceramized by two-stage heat treatment. X-ray diffraction indicates that PbTiO3 crystallizes in lead-rich glasses while BaTiO3 precipitates in barium-rich compositions. Solid solution (Ba, Pb)TiO3 does not seem to crystallize over the entire range of compositions. Simultaneous presence of PbO and BaO in the initial glass composition reduces the yield of ferroelectric phase. Dielectric properties have been interpreted in terms of microstructural features.

Synthesis and microhardness measurement of Ti–Zr–Ni nanoquasicrystalline phase

Abstract The Ti–Zr–Ni forms one of the interesting systems of quasicrystals and related structures. It is the only system in Ti-based quasicrystalline phases which belongs to Bergman class and at the same time gives rise to a stable quasicrystalline structure. We report the formation of nanoquasicrystalline phase directly from melt spinning of molten Ti 53 Zr 27 Ni 20 alloy. Such a phase has been obtained at an optimum copper wheel speed (40 m/s) and jet pressure of 90 atm. We have also measured the mechanical response of this material by the microhardness technique. It shows much better ductility, strength and fracture toughness than the usual alloy having micron-sized quasicrystalline phase. Even at higher load (200 g) no crack seems to appear. We do observe shear bands suggesting the better toughness of bulk material containing nanoquasicrystalline/nanocrystalline phase.

Non-equilibrium solidification of undercooled droplets during atomization process

Thermal history of droplets associated with gas atomization of melt has been investigated. A mathematical model, based on classical theory of heterogeneous nucleation and volume separation of nucleants among droplets size distribution, is described to predict undercooling of droplets. Newtonian heat flow condition coupled with velocity dependent heat transfer coefficient is used to obtain cooling rate before and after nucleation of droplets. The results indicate that temperature profile of droplets in the spray during recalescence, segregated and eutectic solidification regimes is dependent on their size and related undercooling. The interface temperature during solidification of undercooled droplets rapidly approaches the liquidus temperature of the alloy with a subsequent decrease in solid-liquid interface velocity. A comparison in cooling rates of atomized powder particles estimated from secondary dendrite arm spacing measurements are observed to be closer to those predicted from the model during segregated solidification regime of large size droplets.

Avrami exponent under transient and heterogeneous nucleation transformation conditions

Abstract The Kolmogorov–Johnson–Mehl–Avrami model for isothermal transformation kinetics is universal under specific assumptions. However, the experimental Avrami exponent deviates from the universal value. In this context, we study the effect of transient heterogeneous nucleation on the Avrami exponent for bulk materials and also for transformations leading to nanostructured materials. All transformations are assumed to be polymorphic. A discrete version of the KJMA model is modified for this purpose. Scaling relations for transformations under different conditions are reported.

Hot rolling of binary Ti―Nb alloys Part II: mechanical properties anisotropy

Abstract This work describes the nature of anisotropy in mechanical properties of three alloys, namely Ti-8Nb, Ti-12Nb and Ti-16Nb in the hot rolled condition. The work hardening curves of the alloys Ti-8Nb and Ti-12Nb exhibit a single slope and follow the Ludwik relation. The alloy Ti-16Nb displays two slopes in work hardening curves. This indicates that two different mechanisms are operative during tensile testing. The work hardening curves follow a modified Ludwik equation. The alloy Ti-16Nb exhibits the lowest anisotropy index and percentage in plane anisotropy. The alloy Ti-12Nb exhibits the highest values of both the anisotropy index and percentage in plane anisotropy. The low percentage in plane anisotropy of the alloy Ti-16Nb has been attributed to the presence of fibre textures. The yield loci of the hot rolled alloys have been determined using the Knoop hardness method. The results show that hot rolling induces anisotropy in yield strength and changes the shape and size of yield locus plots.

On the six-dimensional lattice of the decagonal phase

Abstract A general formulation for the six-dimensional lattice of the decagonal phase has been utilized for systematically deducing its geometrical consequences on the three-dimensional structures obtained by taking a section of the hyperlattice. Indexing of diffraction patterns has been accomplished on this basis. A zone axis rule and an interplanar spacing formula have been derived. Structures have been proposed for the decagonal phases with different reciprocal space periodicities and having P105/mcm symmetry.

Effect of Ti addition on the quasicrystalline phase formation and indentation characteristics of Zr69.5Al7.5Cu12Ni11 alloy

The crystallisation behaviour and indentation characteristics of melt-spun (Zr69.5Al7.5Cu12Ni11)100− x Ti x (x = 0–16 at. %) metallic glasses was investigated. Differential scanning calorimetry traces revealed changes in crystallisation behaviour with addition of Ti. Formation of a nanoquasicrystalline phase by annealing the glass was found up to x = 12. Further increase of Ti content gives rise to formation of a Zr2Ni-type crystalline phase. In addition to this, the size of nanoquasicrystals decreases with increasing addition of Ti. The load-dependent hardness behaviour of the metallic glasses and nanoquasicrystal–glass composites is reported in detail. The value of the yield strength of the materials was estimated with the help of hardness values based on the empirical correlations available in literature. The formation of shear bands around the indentation periphery was observed. It was found that microstructural and morphological changes alter the indentation characteristics of glasses as well as composites formed after controlled crystallisation.